_:b536423037 . _:b26176018 . _:b536422906 . . . _:b536423020 . _:b26176051 "Imp1\u03B2-geo/+ (Hansen et al., 2004) (MMRRC stock number 011720-UCD), APCflox/+ (Shibata et al., 1997), \u03B2-catenin flox/+ (Brault et al., 2001), human-GFAP Cre (Malatesta et al., >>2003<<), Lin28a transgenic (Zhu et al., 2010), ilet-7 transgenic (Zhu et al., 2011), Nestin-Cre (Tronche et al., 1999), Hmga2+/\u2212 mice (Zhou et al., 1995), and let-7b/c2 fl/+ mice were each backcrossed at least six times onto a C57BL/Ka" . _:b26176109 "IMP1 is known to increase the levels of some proteins by increasing the stability of their mRNAs (Noubissi et al., 2006) and to reduce the levels of other proteins by inhibiting translation (Hansen et al., >>2004<<; Atlas et al., 2007)." . _:b26176075 . . _:b26176031 "Imp1 deficient mice have a dwarf phenotype with some neonatal mortality (Hansen et al., >>2004<<). However, it is unknown if IMP1 regulates stem cells." . _:b26176060 "artificial chromosome (BAC) clones containing the let-7b/c2 genomic locus were purchased (Invitrogen, Grand Island, NY) and a targeting vector was constructed using bacterial recombineering (Copeland et al., 2001; Liu et al., >>2003<<). In this construct, the let-7b/c2 genomic locus was flanked by loxP elements (see Figure 7\u2014figure supplement 2)." . . _:b26176011 "HMGA2 also promotes hematopoietic stem cell self-renewal (Cavazzana-Calvo et al., >>2010<<; Ikeda et al., 2011) and myoblast proliferation (Li et al., 2012)." . _:b536422909 . . . _:b536422896 . _:b26176054 . _:b26176115 "let-7b also negatively regulates the self-renewal of adult neural stem cells by reducing the expression of Hmga2 (Nishino et al., >>2008<<) and TLX (Zhao et al., 2010)." . . . _:b536422999 . _:b26176114 "Recently, Imp (a fly ortholog of mammalian Imp1) was shown to non-cell-autonomously regulate the aging of germline stem cells in the fly testis (Toledano et al., >>2012<<). This raises the possibility that IMP1 might have non-cell-autonomous effects on mammalian stem cells, at least in certain contexts." . _:b536422883 . . _:b536422917 "3"^^ . _:b26176062 . _:b536422949 . _:b536422916 "3"^^ . _:b26176083 "(Machon et al., 2007; Mutch et al., 2010); furthermore, the Imp1 loss-of-function phenotype in the telencephalon (Figures 2\u20136) was reminiscent of the phenotype observed in mutants with reduced Wnt signaling (Mutch et al., >>2010<<). We therefore cultured E12.5 wild-type lateral telencephalon explants for 12 hr with or without recombinant Wnt3a, and examined Imp1 transcript levels by qPCR." . _:b536422928 . _:b536422919 "3"^^ . _:b536422918 "3"^^ . _:b26176113 "Recently, two genome-wide studies in HEK293 cells suggested the existence of putative IMP1 recognition sequences: 5\u2032-CAUH-3\u2032 (H = A, U, or C) (Hafner et al., 2010) and 5\u2032-CCYHHCC-3\u2032 (Y = C, U and H = A, C, U) (J\u00F8nson et al., >>2007<<). We found these sequences at least once in all of our IMP1 pulled-downed mRNAs, but since these sequences would be expected to occur by chance in the mRNAs, no conclusion can be drawn regarding the functional relevance of these binding" . _:b26176119 "Musashi proteins appear to promote the self-renewal of fetal and adult stem cells from multiple tissues by translational repression of target RNAs (Sakakibara et al., >>2002<<; Okano et al., 2005; Kharas et al., 2010)." . _:b536422913 "3"^^ . . _:b26176065 "Differences in RNA abundance were assessed using the CuffDiff algorithm in the CuffLinks software (Roberts et al., >>2009<<; Trapnell et al., 2010)." . _:b536422912 "3"^^ . _:b26175997 . _:b26176066 "Differences in RNA abundance were assessed using the CuffDiff algorithm in the CuffLinks software (Roberts et al., 2009; Trapnell et al., >>2010<<). Parameter settings were: fragment-bias-correct (mm9.fa), compatible-hits-norm, multi-read-correct, and upper-quartile-norm. Using a locally derived perl script, we selected genes as being differentially expressed if they showed test" . _:b536423051 . _:b536422915 "3"^^ . . _:b536422914 "3"^^ . _:b536422912 . _:b536422925 "3"^^ . _:b536422924 "3"^^ . . _:b26176093 . _:b536422927 "3"^^ . _:b26176004 . _:b536422926 "3"^^ . _:b26176005 . _:b536422921 "3"^^ . _:b26176006 . _:b26176023 . _:b536422920 "3"^^ . _:b26176007 . . _:b536422923 "3"^^ . _:b26176000 . . _:b536422986 . _:b536422922 "3"^^ . . _:b26176001 . _:b26176056 . _:b26176122 "Lin28 is preferentially expressed by embryonic cells and essential for the development of primordial germ cells (West et al., >>2009<<; Viswanathan and Daley, 2010)." . _:b536422933 "3"^^ . _:b536422880 . . _:b26176002 . _:b536422932 "3"^^ . _:b536422976 . _:b26176003 . _:b536422935 "3"^^ . _:b26176033 . . _:b26176012 . _:b26176111 "We showed previously that HMGA2 promotes, and p16Ink4a inhibits, stem cell self-renewal in the telencephalon (Nishino et al., >>2008<<). The postnatal lack of IMP1 expression in forebrain neural stem cells (Figure 1D, F, Figure 1\u2014figure supplement 1D,F,G) is therefore likely to contribute to their reduced HMGA2 expression and self-renewal potential." . _:b536422934 "3"^^ . _:b26176013 . . _:b536422929 "3"^^ . _:b26176014 . _:b536422928 "3"^^ . . _:b26176015 . _:b536422931 "3"^^ . _:b26176008 . _:b536422930 "3"^^ . _:b26176009 . _:b536422941 "3"^^ . _:b536422975 . _:b26176010 . _:b536422878 . _:b536422940 "3"^^ . _:b26176011 . _:b26176051 . _:b536422943 "3"^^ . _:b26176020 . _:b536422942 "3"^^ . _:b26176021 . _:b536422937 "3"^^ . _:b26176022 . _:b536422936 "3"^^ . _:b26176023 . . _:b536422939 "3"^^ . . _:b26176016 . _:b536422938 "3"^^ . _:b26176017 . . . _:b536422949 "2"^^ . _:b26176015 "let-7 microRNAs are evolutionarily conserved heterochronic genes that regulate developmental timing (Pasquinelli et al., 2000) and aging (Shen et al., >>2012<<) in Caenorhabditis elegans." . _:b26176018 . _:b536422948 "2"^^ . _:b26176019 . _:b536422932 . _:b536422951 "2"^^ . _:b26176028 . _:b536422950 "2"^^ . _:b26176094 . _:b26176029 . _:b536422945 "2"^^ . _:b26176030 . _:b536422944 "2"^^ . _:b26176031 . _:b536422947 "2"^^ . . _:b26176024 . _:b536422946 "2"^^ . . _:b26176025 . _:b536422957 "2"^^ . _:b26176041 . _:b26176026 . _:b536422956 "2"^^ . . _:b26176027 . _:b536422959 "2"^^ . _:b26176036 . _:b26176118 "The evolutionarily conserved Piwi family proteins that bind to Piwi-interacting RNAs are also required for the maintenance of germline stem cells (Juliano et al., >>2011<<). Musashi proteins appear to promote the self-renewal of fetal and adult stem cells from multiple tissues by translational repression of target RNAs (Sakakibara et al., 2002; Okano et al., 2005; Kharas et al., 2010). Lin28 is" . _:b536422958 "2"^^ . _:b536422978 . _:b536422879 . . . _:b26176037 . _:b536422953 "2"^^ . _:b536422903 . _:b26176038 . _:b536422952 "2"^^ . . _:b26176059 "flox/flox mice, bacterial artificial chromosome (BAC) clones containing the let-7b/c2 genomic locus were purchased (Invitrogen, Grand Island, NY) and a targeting vector was constructed using bacterial recombineering (Copeland et al., >>2001<<; Liu et al., 2003). In this construct, the let-7b/c2 genomic locus was flanked by loxP elements (see Figure 7\u2014figure supplement 2)." . _:b26176039 . _:b536422955 "2"^^ . _:b26176032 . _:b536422954 "2"^^ . _:b26176033 . _:b536422965 "2"^^ . _:b26176117 "FBF proteins and GLD proteins control germline stem cell maintenance and the timing of meiosis in C. elegans (Kimble and Crittenden, >>2007<<). The evolutionarily conserved Piwi family proteins that bind to Piwi-interacting RNAs are also required for the maintenance of germline stem cells (Juliano et al., 2011). Musashi proteins appear to promote the self-renewal of fetal and" . _:b536422927 . _:b26176034 . _:b536422964 "2"^^ . _:b26176034 . _:b26176035 . _:b536422967 "2"^^ . _:b26176044 . _:b536422966 "2"^^ . _:b26176045 . _:b26176026 "Imp1 expression is elevated in several cancers (Ioannidis et al., >>2004<<; Yisraeli, 2005), partly as a consequence of Wnt signaling, which promotes Imp1 transcription (Noubissi et al., 2006; Gu et al., 2008)." . _:b536422961 "2"^^ . _:b536422893 . _:b26176046 . _:b536422960 "2"^^ . _:b536423035 . _:b536422963 "2"^^ . _:b26176047 . . _:b26176040 . _:b536422962 "2"^^ . _:b26176041 . _:b536422973 "2"^^ . . . _:b26176042 . _:b26176047 "materials and methods" . _:b536422972 "2"^^ . _:b26176043 . _:b536422975 "2"^^ . _:b26176052 . _:b536422974 "2"^^ . _:b536423014 . . . _:b26176053 . _:b536422969 "2"^^ . _:b536423073 . _:b26176054 . _:b536422968 "2"^^ . _:b26176055 . _:b536423032 . _:b536422971 "2"^^ . _:b26176005 . _:b536423030 . _:b26176048 . _:b536422970 "2"^^ . . . _:b26176049 . _:b26176049 . _:b536422981 "2"^^ . _:b536423059 . _:b26176050 . _:b536422980 "2"^^ . . . _:b26176051 . _:b536422983 "2"^^ . _:b536422931 . _:b26176060 . _:b26176021 . _:b536422982 "2"^^ . _:b536422918 . _:b536423003 . _:b26176061 . _:b536422977 "2"^^ . _:b26176073 "Consistent with an earlier report (Hansen et al., >>2004<<), Imp1 deficiency led to growth retardation in mice that was evident by late fetal development and persisted into adulthood (Figure 2\u2014figure supplement 1A,C)." . _:b26176062 . _:b536422976 "2"^^ . _:b26176063 . _:b536422979 "2"^^ . _:b536422892 . _:b536423046 . _:b26176056 . _:b536422978 "2"^^ . . _:b26176057 . _:b536422919 . . _:b536422989 "2"^^ . _:b26176058 . _:b536422988 "2"^^ . _:b26176045 "Wnt signaling also promotes the maintenance of stem cells in the adult forebrain (Kuwabara et al., >>2009<<; Qu et al., 2010)." . _:b26176041 "Wnt signaling prevents cell cycle exit and delays differentiation in these cells (Megason and McMahon, 2002; Chenn and Walsh, 2002; Machon et al., 2003; Zechner et al., 2003; Zhou et al., >>2006<<; Woodhead et al., 2006; Gulacsi and Anderson, 2008; Wrobel et al., 2007)." . _:b536422966 . _:b26176059 . _:b26176116 . _:b536422991 "2"^^ . _:b26176068 . _:b536422990 "2"^^ . _:b536422985 "2"^^ . _:b26176069 . . _:b26176061 "Cg-Tg(ACTFLPe)9205Dym/J mice (Rodriguez et al., >>2000<<). Conditional let-7b/c2 mutant mice (Nestin-Cre; let-7b/c2flox/flox) were obtained by breeding Nestin-Cre mice (Tronche et al., 1999) with let-7b/c2 flox/+ mice." . _:b26176067 "Quantitative RT-PCR was performed as described previously (Nishino et al., >>2008<<). Primers used for amplification are listed in Supplementary file 1B." . _:b26176070 . _:b536422984 "2"^^ . _:b26176071 . _:b536422987 "2"^^ . . _:b26176023 "IMP1 binds to target RNAs, post-transcriptionally regulating their localization, turnover, and translation (Doyle et al., 1998; Nielsen et al., 1999; Farina et al., 2003; Atlas et al., >>2004<<). Imp1 expression is widespread in fetal tissues but declines perinatally and is not detected in most adult tissues (Hansen et al., 2004; Hammer et al., 2005). Imp1 expression is elevated in several cancers (Ioannidis et al., 2004;" . _:b26176064 . _:b26176107 . _:b536422986 "2"^^ . _:b26176065 . _:b536422997 "2"^^ . _:b26176066 . _:b536422996 "2"^^ . . _:b26176067 . _:b536422999 "2"^^ . _:b26176076 . . _:b536422998 "2"^^ . _:b26175999 . . _:b26176077 . . _:b536422993 "2"^^ . _:b536422924 . _:b26176078 . _:b536422992 "2"^^ . _:b536422900 . _:b26176079 . _:b536422995 "2"^^ . _:b536422957 . _:b26176072 . _:b536422994 "2"^^ . _:b536422983 . _:b26176073 . _:b536423005 "2"^^ . . . . _:b26176074 . _:b536423004 "2"^^ . _:b536422877 . _:b26176075 . _:b536423007 "2"^^ . _:b26176030 . _:b536422876 . _:b26176084 . _:b536423006 "2"^^ . _:b536422879 . _:b26176085 . . _:b536423001 "2"^^ . _:b536422878 . _:b26176086 . _:b536423000 "2"^^ . _:b26176087 . _:b536423003 "2"^^ . _:b536422908 . _:b536422887 . _:b26175995 _:b26176008 . _:b26176080 . _:b536423002 "2"^^ . _:b26175995 _:b26176009 . _:b26175995 _:b26176010 . _:b26175995 _:b26176011 . _:b26175995 _:b26176012 . _:b26176081 . _:b536423013 "2"^^ . _:b26175995 _:b26176013 . _:b26175995 _:b26176014 . _:b26175995 _:b26176015 . _:b26175995 _:b26176000 . _:b26176082 . _:b26176075 "the telencephalon is regulated by the timing of cell cycle exit, such that over-expression of cyclin D prolongs the proliferation of undifferentiated cells and delays the onset of neurogenesis (Dehay and Kennedy, 2007; Lange et al., >>2009<<). We therefore examined the expression of cell cycle regulators in the dorsal telencephalon of Imp1\u03B2-geo/\u03B2-geo mice." . _:b536423012 "2"^^ . _:b26175995 _:b26176001 . _:b26175995 _:b26176002 . _:b26175995 _:b26176003 . _:b536422885 . _:b26176083 . _:b26175995 _:b26176004 . _:b536423015 "2"^^ . _:b26175995 _:b26176005 . _:b26175995 _:b26176006 . _:b26175995 _:b26176007 . _:b536422884 . _:b26176092 . _:b26175995 _:b26176024 . _:b26176035 . _:b26175995 _:b26176025 . _:b536423014 "2"^^ . . _:b26175995 _:b26176026 . . _:b26175995 _:b26176027 . _:b536422887 . _:b26176093 . _:b26175995 _:b26176028 . _:b536423009 "2"^^ . _:b26175995 _:b26176029 . _:b26175995 _:b26176030 . _:b26175995 _:b26176031 . _:b536422886 . _:b26176094 . _:b26175995 _:b26176016 . _:b536423008 "2"^^ . _:b26175995 _:b26176017 . _:b26175995 _:b26176018 . . _:b26175995 _:b26176019 . _:b536422881 . _:b26176095 . _:b26175995 _:b26176020 . _:b536423011 "2"^^ . _:b26175995 _:b26176021 . _:b26175995 _:b26176022 . _:b26175995 _:b26176023 . _:b536422880 . _:b26176088 . _:b26175995 _:b26176040 . _:b536423042 . _:b536423010 "2"^^ . _:b26175995 _:b26176041 . _:b26175998 . _:b26175995 _:b26176042 . _:b26175995 _:b26176043 . _:b536422883 . _:b26176089 . _:b26175995 _:b26176044 . _:b536423054 . _:b536423021 "2"^^ . _:b26176082 "The Imp1 expression pattern we observed (Figure 1D and 6A, Figure 1\u2014figure supplement 1B) was reminiscent of the gradient of Wnt signaling observed in the telencephalon (Machon et al., 2007; Mutch et al., >>2010<<); furthermore, the Imp1 loss-of-function phenotype in the telencephalon (Figures 2\u20136) was reminiscent of the phenotype observed in mutants with reduced Wnt signaling (Mutch et al., 2010)." . _:b26175995 _:b26176045 . _:b26175995 _:b26176046 . _:b536422882 . _:b26176090 . _:b26175995 _:b26176032 . _:b536423020 "2"^^ . _:b26175995 _:b26176033 . _:b536423055 . . _:b26175995 _:b26176034 . _:b26175995 _:b26176035 . _:b536422893 . _:b26176091 . _:b26175995 _:b26176036 . _:b536423023 "2"^^ . _:b26176066 . _:b26175995 _:b26176037 . _:b26175995 _:b26176038 . _:b26175995 _:b26176039 . _:b536422892 . _:b26176100 . _:b536423010 . _:b536423022 "2"^^ . _:b536422895 . _:b26176101 . _:b536423017 "2"^^ . _:b536422894 . _:b26176102 . _:b536423016 "2"^^ . _:b536423034 . _:b26176027 . _:b536422889 . _:b536423019 "2"^^ . _:b26176103 . _:b536422989 . _:b536422888 . _:b26176096 . . _:b536423018 "2"^^ . _:b536423074 . _:b536422891 . _:b26176097 . _:b536423029 "2"^^ . _:b536422890 . _:b26176098 . . _:b536423028 "2"^^ . _:b536423048 . _:b26176099 . _:b536422901 . _:b536423031 "2"^^ . _:b536422900 . _:b26176108 . _:b536423030 "2"^^ . . _:b536422903 . _:b26176109 . _:b26176094 "Lin28a negatively regulates the expression of all mature let-7 microRNAs (Heo et al., >>2008<<; Newman et al., 2008; Rybak et al., 2008; Viswanathan et al., 2008)." . _:b536423025 "2"^^ . _:b536422898 . _:b536422895 . _:b536422902 . _:b26176110 . _:b536423024 "2"^^ . _:b536422897 . _:b26176111 . _:b536423027 "2"^^ . _:b536422896 . _:b26176104 . _:b536423026 "2"^^ . _:b536422994 . . _:b536422899 . _:b26176105 . _:b536423009 . _:b536423037 "2"^^ . _:b536422898 . _:b26176106 . _:b536423036 "2"^^ . . _:b536422909 . _:b26176107 . _:b536423039 "2"^^ . . _:b536422908 . _:b26176116 . . _:b536423038 "2"^^ . . _:b536422911 . _:b26176117 . _:b536423033 "2"^^ . _:b536422910 . _:b26176118 . _:b536423032 "2"^^ . _:b536422905 . _:b26176119 . _:b536423035 "2"^^ . . . _:b536422904 . _:b26176112 . . _:b536423034 "2"^^ . _:b536422907 . _:b26176113 . _:b536423045 "2"^^ . _:b536422906 . _:b26176114 . _:b536423044 "2"^^ . _:b26176115 . . _:b536423047 "2"^^ . _:b536423011 . _:b536423046 "2"^^ . . . _:b536423041 "2"^^ . _:b536423040 "2"^^ . _:b536423043 "2"^^ . _:b26176120 . _:b536423042 "2"^^ . _:b26176121 . _:b536423053 "2"^^ . _:b26176122 . _:b536423052 "2"^^ . . _:b26176123 . _:b536423008 . _:b536423055 "2"^^ . _:b26176038 "Wnt signaling prevents cell cycle exit and delays differentiation in these cells (Megason and McMahon, 2002; Chenn and Walsh, >>2002<<; Machon et al., 2003; Zechner et al., 2003; Zhou et al., 2006; Woodhead et al., 2006; Gulacsi and Anderson, 2008; Wrobel et al., 2007)." . _:b536423009 . _:b536423010 . _:b536423011 . _:b26176084 "Addition of BMP4, which also regulates dorsoventral patterning in the telencephalon (Hebert et al., >>2003<<), did not affect Imp1 expression (Figure 6\u2014figure supplement 2A)." . _:b536423012 . _:b536423054 "2"^^ . _:b536423013 . _:b536423014 . _:b536423015 . _:b536423016 . _:b536423049 "2"^^ . _:b536423017 . _:b536423018 . . _:b536423019 . _:b536423020 . _:b26176043 . _:b536423048 "2"^^ . _:b536423021 . _:b536423022 . _:b536423023 . _:b536423024 . _:b536423051 "2"^^ . _:b536423025 . _:b536423026 . _:b536423027 . _:b536423028 . _:b536423050 "2"^^ . . _:b26176019 . _:b536423029 . _:b536423030 . _:b536423031 . _:b536422974 . _:b536423032 . _:b536422941 . _:b536423061 "2"^^ . . _:b26176029 "Imp1 expression is elevated in several cancers (Ioannidis et al., 2004; Yisraeli, 2005), partly as a consequence of Wnt signaling, which promotes Imp1 transcription (Noubissi et al., 2006; Gu et al., >>2008<<). Over-expression of IMP1 can promote tumorigenesis (Tessier et al., 2004). Imp1 deficient mice have a dwarf phenotype with some neonatal mortality (Hansen et al., 2004). However, it is unknown if IMP1 regulates stem cells." . . _:b536423033 . _:b536423034 . . _:b536423035 . _:b26176004 "However, the rate of neurogenesis, the frequency of stem cells, and their rate of proliferation all decline with age (Kuhn et al., 1996; Enwere et al., 2004; Maslov et al., 2004; Molofsky et al., 2006; Bonaguidi et al., >>2011<<; Encinas et al., 2011)." . _:b536423060 "2"^^ . _:b536423036 . _:b26176045 . _:b536423037 . _:b536423038 . _:b536423039 . _:b536422976 . _:b536423063 "2"^^ . _:b536422977 . _:b536422978 . _:b536422979 . _:b536422980 . _:b536423062 "2"^^ . _:b536422981 . _:b536422982 . _:b536422968 . _:b536423024 . _:b536422983 . _:b536422984 . _:b536423057 "2"^^ . _:b536422985 . _:b536422986 . _:b536422987 . _:b536422988 . _:b536423056 "2"^^ . _:b536422989 . _:b536422990 . _:b536422991 . _:b536422992 . _:b536423044 . _:b536423059 "2"^^ . _:b536422993 . _:b536422994 . _:b536423052 . _:b536422995 . . _:b536423058 "2"^^ . _:b536422996 . _:b536422997 . _:b536422998 . _:b536422999 . _:b536423000 . _:b536423069 "2"^^ . _:b536423001 . _:b26176114 . _:b536423002 . _:b536423003 . _:b536423004 . _:b536423068 "2"^^ . . _:b536423005 . _:b26176064 . _:b536423006 . _:b536423007 . _:b536422944 . _:b536423071 "2"^^ . _:b536422945 . _:b536422946 . _:b536422947 . _:b536422948 . _:b26176091 "To test whether let-7 regulates Imp1 expression in vivo we administered doxycycline for 5 days to wild-type and tetracyclin-inducible let-7 transgenic (ilet-7) mice (Zhu et al., >>2011<<). IMP1 protein expression was reduced in telencephalon cells freshly isolated from E13.5 ilet-7 mice relative to controls, indicating that elevated expression of let-7 negatively regulates IMP1 expression (Figure 7C)." . _:b536423070 "2"^^ . _:b536422949 . _:b536422950 . _:b536422951 . _:b26176000 . _:b536423065 "2"^^ . _:b536422952 . _:b536422953 . _:b536422954 . _:b26176019 "let-7 microRNAs negatively regulate the expression of a number of gene products, including Insulin-like growth factor two mRNA binding protein 1 (IMP1; also known as CRD-BP and VICKZ1) (Boyerinas et al., >>2008<<). IMP1 binds to target RNAs, post-transcriptionally regulating their localization, turnover, and translation (Doyle et al., 1998; Nielsen et al., 1999; Farina et al., 2003; Atlas et al., 2004)." . _:b536422955 . _:b536422956 . _:b536423064 "2"^^ . _:b536422957 . _:b536422958 . _:b536422959 . _:b536422915 . _:b536423067 "2"^^ . _:b536422960 . _:b536422961 . _:b536422962 . _:b536422963 . _:b536422964 . _:b536423066 "2"^^ . _:b536422965 . _:b536422966 . _:b536423057 . _:b26176123 . _:b536422967 . _:b536422968 . _:b536422969 . _:b536422970 . _:b536422971 . _:b536422894 . _:b536422972 . _:b536422972 . . _:b536422973 . _:b536422974 . _:b536422975 . _:b536422912 . _:b536422913 . _:b536422914 . _:b26176121 "Musashi proteins appear to promote the self-renewal of fetal and adult stem cells from multiple tissues by translational repression of target RNAs (Sakakibara et al., 2002; Okano et al., 2005; Kharas et al., >>2010<<). Lin28 is preferentially expressed by embryonic cells and essential for the development of primordial germ cells (West et al., 2009; Viswanathan and Daley, 2010). Indeed, the decline in Lin28 expression with time during fetal development" . _:b536422915 . _:b536422916 . _:b536422917 . _:b536422918 . _:b536422919 . _:b26176049 "Imp1\u03B2-geo/+ (Hansen et al., 2004) (MMRRC stock number 011720-UCD), APCflox/+ (Shibata et al., >>1997<<), \u03B2-catenin flox/+ (Brault et al., 2001), human-GFAP Cre (Malatesta et al., 2003), Lin28a transgenic (Zhu et al., 2010), ilet-7 transgenic (Zhu et al., 2011), Nestin-Cre (Tronche et al., 1999), Hmga2+/\u2212 mice (Zhou et al., 1995), and" . _:b536423073 "2"^^ . _:b536422920 . _:b536422921 . _:b536422922 . _:b26176047 _:b26176064 . _:b536422923 . _:b26176047 _:b26176065 . _:b536423072 "2"^^ . _:b536422924 . _:b26176047 _:b26176066 . _:b536422925 . _:b26176047 _:b26176067 . _:b536422926 . _:b26176047 _:b26176068 . . _:b536422881 . _:b536422927 . _:b536422928 . _:b536423075 "2"^^ . _:b26176030 "Over-expression of IMP1 can promote tumorigenesis (Tessier et al., >>2004<<). Imp1 deficient mice have a dwarf phenotype with some neonatal mortality (Hansen et al., 2004)." . _:b536422929 . . _:b536422930 . _:b536422931 . _:b536423004 . _:b536423074 "2"^^ . _:b536422932 . _:b536422933 . _:b26176017 . _:b536422934 . _:b536422935 . _:b536422936 . _:b536422937 . _:b536422938 . _:b536422939 . _:b536422940 . _:b536422941 . . _:b536422917 . _:b536422942 . _:b536422943 . _:b26176096 . . . . . . . _:b26175999 "persist into adulthood in the lateral wall of the lateral ventricle subventricular zone (SVZ) as well as in the dentate gyrus, where they give rise to new interneurons throughout adult life (Alvarez-Buylla and Lim, 2004; Zhao et al., >>2008<<). However, the rate of neurogenesis, the frequency of stem cells, and their rate of proliferation all decline with age (Kuhn et al., 1996; Enwere et al., 2004; Maslov et al., 2004; Molofsky et al., 2006; Bonaguidi et al., 2011; Encinas et" . . . _:b26176072 "This mouse therefore provided a loss of function allele that allowed us to monitor Imp1 expression by \u03B2-galactosidase activity (Hansen et al., >>2004<<)." . _:b536423018 . . _:b536423027 . _:b26176105 . _:b26176121 . _:b26176079 . _:b26176047 _:b26176056 . _:b26176042 "Wnt signaling prevents cell cycle exit and delays differentiation in these cells (Megason and McMahon, 2002; Chenn and Walsh, 2002; Machon et al., 2003; Zechner et al., 2003; Zhou et al., 2006; Woodhead et al., >>2006<<; Gulacsi and Anderson, 2008; Wrobel et al., 2007)." . _:b26176047 _:b26176057 . _:b26176047 _:b26176058 . _:b26176047 _:b26176059 . _:b26176047 _:b26176060 . _:b26176047 _:b26176061 . _:b536422990 . . _:b26176047 _:b26176062 . _:b26176047 _:b26176063 . _:b26176047 _:b26176048 . . _:b26176047 _:b26176049 . . _:b26176047 _:b26176050 . _:b26176047 _:b26176051 . _:b26176047 _:b26176052 . _:b26176047 _:b26176053 . _:b536423072 . _:b26176047 _:b26176054 . _:b536423073 . _:b26176047 _:b26176055 . _:b536423074 . _:b536423075 . _:b26176021 "IMP1 binds to target RNAs, post-transcriptionally regulating their localization, turnover, and translation (Doyle et al., 1998; Nielsen et al., >>1999<<; Farina et al., 2003; Atlas et al., 2004)." . _:b536422992 . _:b536422980 . . . . _:b536422973 . _:b536423040 . _:b26176086 "After mice were generated from correctly targeted ES clones, the neomycin resistance cassette was eliminated by mating the mice with Actin-Flpe mice (Rodriguez et al., >>2000<<), and confirmed by PCR with primer pairs (blue arrows)." . _:b536423041 . _:b536423042 . _:b536423043 . _:b536423044 . _:b536423045 . _:b536422977 . _:b536423046 . _:b536423047 . _:b536423048 . _:b536423049 . _:b536423050 . _:b536423051 . _:b536423052 . _:b26176099 "Although the major effects of Lin28a are mediated by let-7, there are also let-7-independent effects (Cho et al., >>2012<<; Wilbert et al., 2012)." . _:b536423053 . _:b536423054 . _:b536423055 . . _:b536423056 . _:b536423057 . _:b536423058 . _:b536423036 . _:b536423059 . _:b536423060 . . _:b536423061 . _:b536423062 . _:b536423063 . _:b536423064 . _:b26176044 "cycle exit and delays differentiation in these cells (Megason and McMahon, 2002; Chenn and Walsh, 2002; Machon et al., 2003; Zechner et al., 2003; Zhou et al., 2006; Woodhead et al., 2006; Gulacsi and Anderson, 2008; Wrobel et al., >>2007<<). Wnt signaling also promotes the maintenance of stem cells in the adult forebrain (Kuwabara et al., 2009; Qu et al., 2010)." . . _:b536423065 . _:b536423066 . _:b536423067 . _:b536423068 . _:b536423069 . _:b536423070 . _:b536423071 . _:b26176074 "cells in the telencephalon is regulated by the timing of cell cycle exit, such that over-expression of cyclin D prolongs the proliferation of undifferentiated cells and delays the onset of neurogenesis (Dehay and Kennedy, >>2007<<; Lange et al., 2009). We therefore examined the expression of cell cycle regulators in the dorsal telencephalon of Imp1\u03B2-geo/\u03B2-geo mice." . . _:b26176113 . _:b26176012 . . _:b26176122 . _:b536422929 . _:b536422964 . _:b26176032 "Canonical Wnt signaling promotes a rapid expansion in the number of undifferentiated stem cells during forebrain development (McMahon et al., >>1992<<; McMahon and Bradley, 1990; Ikeya et al., 1997; Dickinson et al., 1994; Wrobel et al., 2007)." . _:b26176036 . . _:b26176095 . . . _:b536422951 . _:b536422946 . _:b26176090 "observation that Wnt signaling promotes Imp1 expression in the dorsal telencephalon in vivo (Figure 7A), and the similarity in the phenotypic consequences of reduced Imp1 function (Figures 2\u20136) and reduced Wnt signaling (Mutch et al., >>2010<<) all suggest that IMP1 acts autonomously within stem cells in the dorsal telencephalon to potentiate the effects of Wnt signaling during pallial expansion." . _:b536423029 . _:b26176064 "uk/projects/fastqc/) and aligned to the mouse reference genome MM9 (build 37) using TopHat (Langmead et al., 2009; Trapnell et al., >>2009<<). Differences in RNA abundance were assessed using the CuffDiff algorithm in the CuffLinks software (Roberts et al., 2009; Trapnell et al., 2010). Parameter settings were: fragment-bias-correct (mm9.fa), compatible-hits-norm," . _:b26176102 "dorsal telencephalon in response to Wnt signaling is through IMP1-promoted HMGA2 expression, but that HMGA2 expression and neural stem cell expansion decline postnatally as a consequence of increased let-7 expression (Nishino et al., >>2008<<) and a loss of IMP1 expression (Figure 1A)." . _:b26176044 . _:b536422891 . . _:b536423060 . . _:b26176103 _:b26176120 . . _:b26176103 _:b26176121 . _:b26176103 _:b26176122 . _:b26176103 _:b26176123 . . _:b26176103 _:b26176112 . _:b26176103 _:b26176113 . . _:b26176103 _:b26176114 . _:b26176103 _:b26176115 . _:b536422952 . . _:b26176103 _:b26176116 . _:b26176103 _:b26176117 . _:b26176103 _:b26176118 . _:b26176103 _:b26176119 . _:b26176103 _:b26176104 . _:b26176103 _:b26176105 . _:b536422913 . _:b26176103 _:b26176106 . . _:b26176103 _:b26176107 . _:b26176103 _:b26176108 . _:b26176111 . _:b26176103 _:b26176109 . _:b26176103 _:b26176110 . _:b26176103 _:b26176111 . _:b536422960 . _:b536422940 . _:b26176089 . . . _:b26175995 . _:b26176090 . _:b536422917 . _:b536422916 . _:b26176101 . _:b26176034 "Canonical Wnt signaling promotes a rapid expansion in the number of undifferentiated stem cells during forebrain development (McMahon et al., 1992; McMahon and Bradley, 1990; Ikeya et al., >>1997<<; Dickinson et al., 1994; Wrobel et al., 2007)." . _:b536422919 . _:b536422991 . . _:b536422918 . _:b536422959 . _:b536422907 . _:b536422913 . _:b536423016 . _:b26176102 . . _:b536422912 . _:b26176080 . _:b536422915 . _:b26176083 . _:b536422914 . _:b536422905 . _:b536422993 . _:b26176082 . _:b26176042 . _:b26176105 "Wnt signaling promoted Imp1 expression in the dorsal telencephalon in a medial-lateral gradient (Figure 7A\u2013B, Figure 6\u2014figure supplement 2A\u2013D) similar to the gradient in canonical Wnt signaling (Machon et al., 2007; Mutch et al., >>2010<<). Imp1 deficiency reduced stem cell self-renewal potential (Figure 2F\u2013I) and caused premature neuronal and glial differentiation (Figure 4), leading to stem cell depletion (Figures 2G, 3A\u2013C) and reduced pallial expansion (Figure 2A,D)." . _:b536422925 . _:b536422924 . _:b536422927 . _:b26176120 . _:b536422926 . _:b536422921 . _:b536423063 . . _:b26176020 . _:b536422920 . . _:b26176123 "Lin28 is preferentially expressed by embryonic cells and essential for the development of primordial germ cells (West et al., 2009; Viswanathan and Daley, >>2010<<). Indeed, the decline in Lin28 expression with time during fetal development may contribute to the increase in let-7 expression and the decline in IMP1 expression." . _:b536422923 . _:b536422922 . _:b536422933 . . _:b26176119 . _:b536422932 . . _:b536422935 . . _:b536422981 . _:b536422934 . _:b536422929 . . _:b536422928 . _:b536422931 . _:b26176071 . _:b536422930 . _:b26176115 . _:b536422941 . _:b26176097 . _:b536422940 . _:b26176074 . _:b536422963 . _:b536422943 . _:b26176069 _:b26176096 . _:b26176069 _:b26176097 . _:b26176069 _:b26176098 . _:b26176069 _:b26176099 . _:b536422942 . . _:b26176116 "let-7b also negatively regulates the self-renewal of adult neural stem cells by reducing the expression of Hmga2 (Nishino et al., 2008) and TLX (Zhao et al., >>2010<<). This is consistent with our conclusion that a network of let-7 gene targets regulates stem cell function and that the regulation of different targets at different ages by let-7 contributes to temporal changes in stem cell properties." . _:b26176069 _:b26176100 . _:b536422911 . _:b26176069 _:b26176101 . _:b26176069 _:b26176102 . _:b536422937 . . _:b536422936 . _:b26176089 "The similarity between the Imp1 expression pattern (Figure 1D) and the Wnt signaling gradient in the dorsal telencephalon (Machon et al., 2007; Mutch et al., >>2010<<), the observation that Wnt signaling promotes Imp1 expression in the dorsal telencephalon in vivo (Figure 7A), and the similarity in the phenotypic consequences of reduced Imp1 function (Figures 2\u20136) and reduced Wnt signaling (Mutch et al." . _:b536422939 . _:b26176069 _:b26176080 . _:b26176069 _:b26176081 . . _:b26176069 _:b26176082 . _:b26176069 _:b26176083 . _:b536422938 . _:b26176069 _:b26176084 . _:b26176099 . _:b26176069 _:b26176085 . _:b26176069 _:b26176086 . _:b26176069 _:b26176087 . _:b536422949 . _:b26176069 _:b26176088 . _:b536422997 . _:b26176069 _:b26176089 . _:b26176069 _:b26176090 . _:b26176069 _:b26176091 . _:b26176110 "IMP1 is known to increase the levels of some proteins by increasing the stability of their mRNAs (Noubissi et al., 2006) and to reduce the levels of other proteins by inhibiting translation (Hansen et al., 2004; Atlas et al., >>2007<<). Consistent with this, IMP1 post-transcriptionally reduced the levels of some proteins associated with differentiation (Figure 8A\u2013C, Figure 8\u2014figure supplement 1A\u2013C) while promoting the expression of the self-renewal factor, HMGA2" . _:b536422948 . . . _:b26176022 . _:b26176069 _:b26176092 . _:b536423065 . _:b536423007 . _:b26176069 _:b26176093 . . _:b26176069 _:b26176094 . _:b26176069 _:b26176095 . _:b536422951 . . _:b536422933 . . . _:b26176068 . _:b26176040 "Wnt signaling prevents cell cycle exit and delays differentiation in these cells (Megason and McMahon, 2002; Chenn and Walsh, 2002; Machon et al., 2003; Zechner et al., >>2003<<; Zhou et al., 2006; Woodhead et al., 2006; Gulacsi and Anderson, 2008; Wrobel et al., 2007)." . . _:b536422950 . _:b26176069 _:b26176070 . _:b26176069 _:b26176071 . _:b536422945 . _:b26176069 _:b26176072 . _:b26176069 _:b26176073 . _:b26176069 _:b26176074 . _:b26176069 _:b26176075 . _:b536422944 . _:b26176069 _:b26176076 . _:b26176069 _:b26176077 . _:b26176069 _:b26176078 . _:b26176069 _:b26176079 . _:b536422947 . _:b536422946 . . _:b26176067 . _:b26176092 "Since there are 10 let-7 family members that are thought to act redundantly to repress target gene expression (Roush and Slack, >>2008<<), we used inducible Lin28a transgenic (Lin28a Tg) mice (Zhu et al., 2010) to test whether reduced expression of endogenous let-7s would increase IMP1 expression." . _:b536422957 . . _:b536422956 . _:b26176014 . . _:b536422959 . _:b536422958 . _:b536422987 . _:b536422953 . "10.7554%2FeLife.00924.025" . _:b536422952 . _:b536422955 . . _:b536422954 . _:b536422886 . . _:b536422965 . . _:b536422964 . _:b26176008 "p16Ink4a deficiency or overexpression of a let-7 insensitive form of Hmga2 partially rescues the declines in neural stem cell function and neurogenesis in aging mice (Molofsky et al., 2006; Nishino et al., >>2008<<). This pathway appears to be conserved among multiple mammalian tissues as p16Ink4a deficiency also increases the function of hematopoietic stem cells and pancreatic beta cells during aging (Janzen et al., 2006; Krishnamurthy et al.," . . . _:b536422967 . _:b26176037 . _:b536422966 . _:b536422984 . _:b536422961 . _:b536422960 . _:b536422963 . _:b536422877 . _:b536422962 . . _:b536422973 . . _:b536422972 . _:b26176027 "Imp1 expression is elevated in several cancers (Ioannidis et al., 2004; Yisraeli, >>2005<<), partly as a consequence of Wnt signaling, which promotes Imp1 transcription (Noubissi et al., 2006; Gu et al., 2008)." . _:b536422975 . . _:b536422974 . _:b536422955 . _:b536422969 . _:b536422934 . _:b536422968 . _:b536422971 . _:b536422970 . _:b536423040 . _:b536422981 . _:b536423053 . . _:b536422980 . _:b26176001 . . _:b536422983 . . _:b536422982 . _:b536422977 . _:b536422976 . _:b536423049 . . _:b536422979 . _:b536422978 . _:b26176117 . _:b536422989 . . _:b26176062 "Conditional let-7b/c2 mutant mice (Nestin-Cre; let-7b/c2flox/flox) were obtained by breeding Nestin-Cre mice (Tronche et al., >>1999<<) with let-7b/c2 flox/+ mice." . _:b536422988 . _:b536422971 . . _:b536422991 . _:b536422990 . _:b536422985 . _:b536422984 . _:b26176068 "Constructs for let-7b or Hmga2 were described previously (Nishino et al., >>2008<<). Viral supernatants were prepared by co-transfecting proviral plasmids and packaging vectors (pCL-Eco and pC1-VSVG) into 293T cells by standard calcium phosphate precipitation methods. The supernatant was collected after 72 hr and" . . _:b26176057 . _:b536422967 . _:b536422987 . _:b536422986 . _:b536423012 . . _:b536422997 . . . _:b536422996 . _:b536422999 . _:b26176028 . _:b536422998 . _:b536422939 . . _:b26176047 . . _:b536422993 . _:b536423038 . _:b536422992 . _:b536423045 . _:b536422995 . _:b26176013 "HMGA2 also promotes hematopoietic stem cell self-renewal (Cavazzana-Calvo et al., 2010; Ikeda et al., 2011) and myoblast proliferation (Li et al., >>2012<<)." . _:b536422994 . . _:b536423005 . _:b536423064 . _:b26176002 . _:b26176070 . _:b536423004 . . . _:b536423007 . . _:b536423006 . . _:b536423001 . _:b536423000 . _:b26175997 "During fetal development rapidly dividing neural stem cells expand in number before differentiating in precisely defined temporal windows, first to form neurons and then to form glia (Salomoni and Calegari, >>2010<<). Largely quiescent neural stem cells persist into adulthood in the lateral wall of the lateral ventricle subventricular zone (SVZ) as well as in the dentate gyrus, where they give rise to new interneurons throughout adult life" . _:b26176073 . _:b536423062 . _:b536423003 . _:b26176008 . _:b26176072 . _:b536423002 . _:b536423013 . _:b536422953 . _:b536422970 . _:b536423013 . _:b26176012 "HMGA2 also promotes hematopoietic stem cell self-renewal (Cavazzana-Calvo et al., 2010; Ikeda et al., >>2011<<) and myoblast proliferation (Li et al., 2012)." . _:b536423039 . _:b536423012 . _:b26176037 "Wnt signaling prevents cell cycle exit and delays differentiation in these cells (Megason and McMahon, >>2002<<; Chenn and Walsh, 2002; Machon et al., 2003; Zechner et al., 2003; Zhou et al., 2006; Woodhead et al., 2006; Gulacsi and Anderson, 2008; Wrobel et al., 2007)." . _:b26176103 . _:b536423015 . _:b26176058 "medium, supplemented with 20 ng/ml recombinant human bFGF (R&D Systems, Minneapolis, MN), 20 ng/ml epidermal growth factor (EGF) (R&D Systems), 10% chick embryo extract (CEE; made as described [Stemple and Anderson, >>1992<<]), 1% N2 supplement (GIBCO, Grand Island, NY), 2% B27 supplement (GIBCO), 50 mM 2-mercaptoethanol, and penicillin/streptomycin (BioWhittaker, Walkersville, MD)." . _:b536423043 . _:b536423014 . _:b26176057 "CNS progenitors were isolated as described in prior studies (Molofsky et al., 2005; Nishino et al., >>2008<<). For adherent cultures, CNS progenitors were plated at a clonal density of 0.66 cells/\u03BCl (1000 cells per 35 mm well) in 6-well plates (Corning, Tewksbury, MA) that had been sequentially coated with 150 \u03BCg/ml poly-d-lysine (Biomedical" . _:b26176071 "fusion protein that contained the IMP1 RNA recognition motif encoded by exons 1 and 2 but lacked the second RNA recognition motif and the four hnRNP K homology domains, which are essential for biological activity (Nielsen et al., >>2002<<). This mouse therefore provided a loss of function allele that allowed us to monitor Imp1 expression by \u03B2-galactosidase activity (Hansen et al., 2004)." . _:b536423071 . _:b536423009 . _:b26176006 . _:b536423008 . . _:b536423011 . _:b536423010 . . _:b26176086 . _:b536423021 . _:b26175995 "introduction" . _:b536423020 . _:b26176069 . _:b536423023 . _:b536423022 . _:b536423017 . _:b26176110 . _:b536423016 . _:b536423019 . _:b536423018 . _:b536422904 . _:b536423075 . _:b536423029 . _:b536423017 . _:b26176000 "However, the rate of neurogenesis, the frequency of stem cells, and their rate of proliferation all decline with age (Kuhn et al., >>1996<<; Enwere et al., 2004; Maslov et al., 2004; Molofsky et al., 2006; Bonaguidi et al., 2011; Encinas et al., 2011)." . . _:b536423028 . . _:b26176078 "the proliferation of undifferentiated Pax6+ stem cells in the pallial region of the telencephalon, preventing the premature generation of Tbr2+ intermediate progenitors and differentiated neurons (Chenn and Walsh, 2002; Machon et al., >>2007<<; Wrobel et al., 2007; Mutch et al., 2010)." . _:b536423031 . _:b536423026 . _:b536423030 . . . . . _:b536423025 . . _:b536423024 . _:b26176085 . . _:b536423027 . _:b536423026 . . _:b536423037 . _:b536423036 . _:b536423039 . . _:b536423038 . _:b26176026 . _:b536423033 . _:b26176061 . _:b536423032 . . _:b536423035 . . _:b536422962 . _:b536423034 . _:b536422950 . _:b536423050 . _:b536422961 . _:b536423045 . _:b26176095 "Lin28a negatively regulates the expression of all mature let-7 microRNAs (Heo et al., 2008; Newman et al., >>2008<<; Rybak et al., 2008; Viswanathan et al., 2008)." . _:b536423044 . _:b536422880 . _:b26176109 . _:b536422881 . . _:b536422882 . _:b536423047 . _:b536422883 . _:b536422884 . . _:b536422885 . _:b536422886 . _:b536423046 . _:b536422887 . _:b536422888 . _:b536422889 . _:b536422965 . _:b536422890 . _:b536423041 . . _:b536422891 . _:b536423072 . _:b536422892 . _:b536422893 . _:b26176006 "the chromatin-associated HMGA2 high mobility group protein, and the p16Ink4a cyclin-dependent kinase inhibitor: let-7b expression increases with age, reducing Hmga2 expression and increasing p16Ink4a expression (Nishino et al., >>2008<<). p16Ink4a deficiency or overexpression of a let-7 insensitive form of Hmga2 partially rescues the declines in neural stem cell function and neurogenesis in aging mice (Molofsky et al., 2006; Nishino et al., 2008)." . _:b536422894 . _:b536423040 . _:b536422895 . _:b536422896 . _:b536422897 . _:b536422898 . _:b536423043 . _:b536422899 . _:b536422900 . _:b536422901 . _:b536422902 . _:b536423042 . _:b536422903 . _:b536422904 . . _:b536422905 . _:b536422906 . _:b536423053 . _:b536422907 . _:b536422908 . _:b26176088 "The similarity between the Imp1 expression pattern (Figure 1D) and the Wnt signaling gradient in the dorsal telencephalon (Machon et al., >>2007<<; Mutch et al., 2010), the observation that Wnt signaling promotes Imp1 expression in the dorsal telencephalon in vivo (Figure 7A), and the similarity in the phenotypic consequences of reduced Imp1 function (Figures 2\u20136) and reduced Wnt" . _:b536422909 . _:b26176087 . _:b536422878 "6"^^ . _:b536422910 . _:b536423052 . _:b536422911 . _:b536423055 . . _:b536423054 . _:b536422876 "8"^^ . _:b536422879 "5"^^ . _:b536423049 . _:b536422877 "7"^^ . _:b536423048 . _:b536423041 . _:b26176033 "Canonical Wnt signaling promotes a rapid expansion in the number of undifferentiated stem cells during forebrain development (McMahon et al., 1992; McMahon and Bradley, >>1990<<; Ikeya et al., 1997; Dickinson et al., 1994; Wrobel et al., 2007)." . _:b536423006 . _:b536423051 . _:b26176024 . _:b536423050 . _:b26176104 "Wnt signaling promoted Imp1 expression in the dorsal telencephalon in a medial-lateral gradient (Figure 7A\u2013B, Figure 6\u2014figure supplement 2A\u2013D) similar to the gradient in canonical Wnt signaling (Machon et al., >>2007<<; Mutch et al., 2010)." . _:b26176031 . _:b536423061 . . _:b536422876 . . _:b536422877 . . _:b536423060 . _:b536422878 . _:b536422879 . _:b536422885 "4"^^ . _:b26176077 "signaling maintains the proliferation of undifferentiated Pax6+ stem cells in the pallial region of the telencephalon, preventing the premature generation of Tbr2+ intermediate progenitors and differentiated neurons (Chenn and Walsh, >>2002<<; Machon et al., 2007; Wrobel et al., 2007; Mutch et al., 2010)." . _:b536423063 . _:b536422884 "4"^^ . _:b536423025 . _:b536423062 . . _:b536422887 "4"^^ . _:b536423057 . _:b536422886 "4"^^ . _:b26176112 "Recently, two genome-wide studies in HEK293 cells suggested the existence of putative IMP1 recognition sequences: 5\u2032-CAUH-3\u2032 (H = A, U, or C) (Hafner et al., >>2010<<) and 5\u2032-CCYHHCC-3\u2032 (Y = C, U and H = A, C, U) (J\u00F8nson et al., 2007)." . _:b536423056 . _:b536422938 . _:b536422881 "5"^^ . _:b536423059 . _:b536422880 "5"^^ . _:b536423058 . _:b536422883 "5"^^ . _:b536422985 . _:b26176010 "This pathway appears to be conserved among multiple mammalian tissues as p16Ink4a deficiency also increases the function of hematopoietic stem cells and pancreatic beta cells during aging (Janzen et al., 2006; Krishnamurthy et al., >>2006<<). HMGA2 also promotes hematopoietic stem cell self-renewal (Cavazzana-Calvo et al., 2010; Ikeda et al., 2011) and myoblast proliferation (Li et al., 2012)." . _:b536423069 . _:b536423068 . _:b536422882 "5"^^ . _:b536423068 . _:b536422893 "4"^^ . _:b536423023 . _:b26176046 "Wnt signaling also promotes the maintenance of stem cells in the adult forebrain (Kuwabara et al., 2009; Qu et al., >>2010<<). However, it is unclear why Wnt signaling expands the number of neural stem cells during development but only maintains declining numbers of stem cells during adulthood." . _:b536423071 . . _:b536422892 "4"^^ . _:b536423070 . _:b536422895 "4"^^ . _:b536423065 . _:b536422956 . _:b536422894 "4"^^ . _:b536423058 . _:b536423064 . _:b536422889 "4"^^ . "PMC0" . _:b536423067 . _:b536422888 "4"^^ . _:b26176084 . _:b536423066 . _:b536422891 "4"^^ . _:b26176078 . _:b26176017 "In mammals, let-7 microRNAs are known to regulate embryonic stem cells (Melton et al., 2010), primordial germ cells (West et al., >>2009<<), and adult neural stem cells (Zhao et al., 2010) but it is unclear to what extent let-7 targets regulate developmental changes in mammalian stem cell function over time." . _:b536422890 "4"^^ . _:b536422901 "3"^^ . _:b26176100 . _:b536422900 "3"^^ . _:b26176011 . _:b26176108 . _:b536422903 "3"^^ . _:b26176107 "let-7 regulates the function of stem cells in the fetal and adult forebrains (Zhao et al., >>2010<<); however, let-7 microRNAs appear to regulate different targets at different ages in neural stem cells." . _:b536423073 . _:b536422902 "3"^^ . _:b536423072 . _:b536422897 "4"^^ . . _:b536423075 . . _:b536422896 "4"^^ . _:b26176048 . _:b536423074 . _:b536422899 "3"^^ . _:b536422943 . _:b536422936 . _:b26176081 "The Imp1 expression pattern we observed (Figure 1D and 6A, Figure 1\u2014figure supplement 1B) was reminiscent of the gradient of Wnt signaling observed in the telencephalon (Machon et al., >>2007<<; Mutch et al., 2010); furthermore, the Imp1 loss-of-function phenotype in the telencephalon (Figures 2\u20136) was reminiscent of the phenotype observed in mutants with reduced Wnt signaling (Mutch et al., 2010)." . . _:b536422898 "4"^^ . . _:b26176039 . _:b536422909 "3"^^ . _:b26176103 "discussion" . _:b536422884 . _:b536422942 . _:b26176096 "Lin28a negatively regulates the expression of all mature let-7 microRNAs (Heo et al., 2008; Newman et al., 2008; Rybak et al., >>2008<<; Viswanathan et al., 2008)." . _:b536422908 "3"^^ . _:b536422911 "3"^^ . _:b536422910 "3"^^ . . _:b536422905 "3"^^ . _:b536422904 "3"^^ . _:b26176081 . _:b536422907 "3"^^ . _:b536422885 . _:b536422906 "3"^^ . . _:b536422888 . . _:b536423033 . _:b26176004 . _:b536423008 . . . . _:b26176059 . _:b26176101 "We previously demonstrated that HMGA2 promotes neural stem cell self-renewal in the fetal telencephalon, partly by negatively regulating the expression of Ink4a, which encodes a cyclin-dependent kinase inhibitor (Nishino et al., >>2008<<). Consistent with this, we detected significantly (p<0.05) increased Ink4a expression in neurospheres cultured from E18.5 Imp1\u03B2-geo/\u03B2-geo mice as compared to wild-type controls (Figure 8J)." . . _:b26176088 . _:b536422897 . _:b26176056 "CNS progenitors were isolated as described in prior studies (Molofsky et al., >>2005<<; Nishino et al., 2008)." . _:b536422937 . . _:b536423005 . . _:b536423019 . . . _:b26176087 "To test whether there is a positive feedback between Imp1 and canonical Wnt signaling (Gu et al., >>2008<<) in neural stem/progenitor cells, we assessed \u00DF-catenin expression in uncultured E13.5 telencephalon and in cultured neurospheres by qPCR and by western blot." . _:b26176016 "In mammals, let-7 microRNAs are known to regulate embryonic stem cells (Melton et al., >>2010<<), primordial germ cells (West et al., 2009), and adult neural stem cells (Zhao et al., 2010) but it is unclear to what extent let-7 targets regulate developmental changes in mammalian stem cell function over time." . _:b26176020 "IMP1 binds to target RNAs, post-transcriptionally regulating their localization, turnover, and translation (Doyle et al., >>1998<<; Nielsen et al., 1999; Farina et al., 2003; Atlas et al., 2004)." . . _:b26176039 "Wnt signaling prevents cell cycle exit and delays differentiation in these cells (Megason and McMahon, 2002; Chenn and Walsh, 2002; Machon et al., >>2003<<; Zechner et al., 2003; Zhou et al., 2006; Woodhead et al., 2006; Gulacsi and Anderson, 2008; Wrobel et al., 2007)." . _:b26176016 . . _:b536422889 . . _:b536423021 . _:b536422901 . . _:b26176104 . . _:b26176022 "IMP1 binds to target RNAs, post-transcriptionally regulating their localization, turnover, and translation (Doyle et al., 1998; Nielsen et al., 1999; Farina et al., >>2003<<; Atlas et al., 2004)." . . . . . . . _:b26176018 "In mammals, let-7 microRNAs are known to regulate embryonic stem cells (Melton et al., 2010), primordial germ cells (West et al., 2009), and adult neural stem cells (Zhao et al., >>2010<<) but it is unclear to what extent let-7 targets regulate developmental changes in mammalian stem cell function over time." . . . _:b26176040 . _:b26176029 . _:b26176009 "This pathway appears to be conserved among multiple mammalian tissues as p16Ink4a deficiency also increases the function of hematopoietic stem cells and pancreatic beta cells during aging (Janzen et al., >>2006<<; Krishnamurthy et al., 2006)." . _:b26176106 . _:b536422923 . _:b26176058 . . . _:b26176050 "Imp1\u03B2-geo/+ (Hansen et al., 2004) (MMRRC stock number 011720-UCD), APCflox/+ (Shibata et al., 1997), \u03B2-catenin flox/+ (Brault et al., >>2001<<), human-GFAP Cre (Malatesta et al., 2003), Lin28a transgenic (Zhu et al., 2010), ilet-7 transgenic (Zhu et al., 2011), Nestin-Cre (Tronche et al., 1999), Hmga2+/\u2212 mice (Zhou et al., 1995), and let-7b/c2 fl/+ mice were each backcrossed at" . _:b26175996 "Stem cell properties change throughout life in many tissues in response to changing growth and regeneration demands (He et al., >>2009<<). These changes are particularly evident in the central nervous system (CNS) forebrain, where neural stem cells persist throughout life." . . . _:b26176076 "the cell cycle during the BrdU pulse was estimated based on the frequency of BrdU+Ki67- cells and the fraction that remained in cycle was estimated based on the frequency of BrdU+Ki67+ cells, as described previously (Chenn and Walsh, >>2002<<). The frequency of BrdU+Ki67- cells was significantly (p<0.05) higher in the Imp1\u03B2-geo/\u03B2-geo as compared to control telencephalon (Figure 6C), suggesting that cell cycle exit is accelerated in neural stem/progenitor cells from the" . _:b26176010 . . . . . _:b26176009 . _:b26176007 "p16Ink4a deficiency or overexpression of a let-7 insensitive form of Hmga2 partially rescues the declines in neural stem cell function and neurogenesis in aging mice (Molofsky et al., >>2006<<; Nishino et al., 2008)." . _:b26176054 "011720-UCD), APCflox/+ (Shibata et al., 1997), \u03B2-catenin flox/+ (Brault et al., 2001), human-GFAP Cre (Malatesta et al., 2003), Lin28a transgenic (Zhu et al., 2010), ilet-7 transgenic (Zhu et al., 2011), Nestin-Cre (Tronche et al., >>1999<<), Hmga2+/\u2212 mice (Zhou et al., 1995), and let-7b/c2 fl/+ mice were each backcrossed at least six times onto a C57BL/Ka background and housed at the University of Michigan Unit for Laboratory Animal Medicine or the University of Texas" . _:b26176093 "Since there are 10 let-7 family members that are thought to act redundantly to repress target gene expression (Roush and Slack, 2008), we used inducible Lin28a transgenic (Lin28a Tg) mice (Zhu et al., >>2010<<) to test whether reduced expression of endogenous let-7s would increase IMP1 expression." . _:b26176085 "We did not detect TCF4 binding to exon 6 of Imp1 or to the promoter/enhancer of Lgi4 (negative controls) but did detect TCF4 binding to the first intron of Axin (a positive control [Jho et al., >>2002<<]) (Figure 6\u2014figure supplement 2C)." . _:b26176028 "Imp1 expression is elevated in several cancers (Ioannidis et al., 2004; Yisraeli, 2005), partly as a consequence of Wnt signaling, which promotes Imp1 transcription (Noubissi et al., >>2006<<; Gu et al., 2008)." . _:b26176003 . . _:b536422935 . _:b536422954 . . _:b536423002 . . . _:b26176048 "Imp1\u03B2-geo/+ (Hansen et al., >>2004<<) (MMRRC stock number 011720-UCD), APCflox/+ (Shibata et al., 1997), \u03B2-catenin flox/+ (Brault et al., 2001), human-GFAP Cre (Malatesta et al., 2003), Lin28a transgenic (Zhu et al., 2010), ilet-7 transgenic (Zhu et al., 2011), Nestin-Cre" . _:b536423067 . _:b536423066 . _:b26176007 . . . . _:b536422948 . _:b26175998 "neural stem cells persist into adulthood in the lateral wall of the lateral ventricle subventricular zone (SVZ) as well as in the dentate gyrus, where they give rise to new interneurons throughout adult life (Alvarez-Buylla and Lim, >>2004<<; Zhao et al., 2008). However, the rate of neurogenesis, the frequency of stem cells, and their rate of proliferation all decline with age (Kuhn et al., 1996; Enwere et al., 2004; Maslov et al., 2004; Molofsky et al., 2006; Bonaguidi et al." . _:b536422902 . . _:b536423056 . . . _:b26176100 "Although the major effects of Lin28a are mediated by let-7, there are also let-7-independent effects (Cho et al., 2012; Wilbert et al., >>2012<<). To ensure that the effects of Lin28a on Imp1 expression (Figure 7F) are really mediated by changes in the physiological levels of let-7 family microRNAs we independently addressed this issue by testing whether Imp1 expression is" . _:b536422969 . _:b26176055 "et al., 1997), \u03B2-catenin flox/+ (Brault et al., 2001), human-GFAP Cre (Malatesta et al., 2003), Lin28a transgenic (Zhu et al., 2010), ilet-7 transgenic (Zhu et al., 2011), Nestin-Cre (Tronche et al., 1999), Hmga2+/\u2212 mice (Zhou et al., >>1995<<), and let-7b/c2 fl/+ mice were each backcrossed at least six times onto a C57BL/Ka background and housed at the University of Michigan Unit for Laboratory Animal Medicine or the University of Texas Southwestern Medical Center Animal" . _:b536422944 . _:b26176069 "results" . . _:b26176052 "Imp1\u03B2-geo/+ (Hansen et al., 2004) (MMRRC stock number 011720-UCD), APCflox/+ (Shibata et al., 1997), \u03B2-catenin flox/+ (Brault et al., 2001), human-GFAP Cre (Malatesta et al., 2003), Lin28a transgenic (Zhu et al., >>2010<<), ilet-7 transgenic (Zhu et al., 2011), Nestin-Cre (Tronche et al., 1999), Hmga2+/\u2212 mice (Zhou et al., 1995), and let-7b/c2 fl/+ mice were each backcrossed at least six times onto a C57BL/Ka background and housed at the University of" . . _:b536422930 . . _:b26176053 "et al., 2004) (MMRRC stock number 011720-UCD), APCflox/+ (Shibata et al., 1997), \u03B2-catenin flox/+ (Brault et al., 2001), human-GFAP Cre (Malatesta et al., 2003), Lin28a transgenic (Zhu et al., 2010), ilet-7 transgenic (Zhu et al., >>2011<<), Nestin-Cre (Tronche et al., 1999), Hmga2+/\u2212 mice (Zhou et al., 1995), and let-7b/c2 fl/+ mice were each backcrossed at least six times onto a C57BL/Ka background and housed at the University of Michigan Unit for Laboratory Animal" . _:b536422921 . . _:b26176065 . _:b26176043 "signaling prevents cell cycle exit and delays differentiation in these cells (Megason and McMahon, 2002; Chenn and Walsh, 2002; Machon et al., 2003; Zechner et al., 2003; Zhou et al., 2006; Woodhead et al., 2006; Gulacsi and Anderson, >>2008<<; Wrobel et al., 2007). Wnt signaling also promotes the maintenance of stem cells in the adult forebrain (Kuwabara et al., 2009; Qu et al., 2010)." . _:b26176001 "However, the rate of neurogenesis, the frequency of stem cells, and their rate of proliferation all decline with age (Kuhn et al., 1996; Enwere et al., >>2004<<; Maslov et al., 2004; Molofsky et al., 2006; Bonaguidi et al., 2011; Encinas et al., 2011)." . _:b536423022 . _:b26176098 . . . . . _:b26176118 . _:b536422998 . _:b26176079 "undifferentiated Pax6+ stem cells in the pallial region of the telencephalon, preventing the premature generation of Tbr2+ intermediate progenitors and differentiated neurons (Chenn and Walsh, 2002; Machon et al., 2007; Wrobel et al., >>2007<<; Mutch et al., 2010)." . _:b26176038 . . _:b536422916 . . _:b536422982 . . _:b26176063 "uk/projects/fastqc/) and aligned to the mouse reference genome MM9 (build 37) using TopHat (Langmead et al., >>2009<<; Trapnell et al., 2009)." . _:b26176002 "However, the rate of neurogenesis, the frequency of stem cells, and their rate of proliferation all decline with age (Kuhn et al., 1996; Enwere et al., 2004; Maslov et al., >>2004<<; Molofsky et al., 2006; Bonaguidi et al., 2011; Encinas et al., 2011)." . _:b26176046 . . . _:b536422996 . . _:b536423001 . _:b26176120 "Musashi proteins appear to promote the self-renewal of fetal and adult stem cells from multiple tissues by translational repression of target RNAs (Sakakibara et al., 2002; Okano et al., >>2005<<; Kharas et al., 2010)." . _:b26176035 "Canonical Wnt signaling promotes a rapid expansion in the number of undifferentiated stem cells during forebrain development (McMahon et al., 1992; McMahon and Bradley, 1990; Ikeya et al., 1997; Dickinson et al., >>1994<<; Wrobel et al., 2007)." . _:b536422922 . . . . _:b26176013 . _:b536422945 . . _:b26175996 . _:b536422914 . _:b536422890 . _:b26175997 . _:b26175998 . . _:b26176050 . _:b26175999 . _:b26176032 . _:b26176097 "Lin28a negatively regulates the expression of all mature let-7 microRNAs (Heo et al., 2008; Newman et al., 2008; Rybak et al., 2008; Viswanathan et al., >>2008<<). We cultured E18.5 dorsal telencephalon cells or P60 lateral ventricle SVZ cells from doxycycline-treated wild-type or inducible Lin28a Tg mice and observed a significant reduction in endogenous let-7b expression in Lin28a Tg as compared" . _:b536422958 . _:b536423000 . _:b26176052 . _:b26175995 . _:b26176024 "Imp1 expression is widespread in fetal tissues but declines perinatally and is not detected in most adult tissues (Hansen et al., >>2004<<; Hammer et al., 2005)." . . . . _:b536422979 . . . . _:b26176025 "Imp1 expression is widespread in fetal tissues but declines perinatally and is not detected in most adult tissues (Hansen et al., 2004; Hammer et al., >>2005<<). Imp1 expression is elevated in several cancers (Ioannidis et al., 2004; Yisraeli, 2005), partly as a consequence of Wnt signaling, which promotes Imp1 transcription (Noubissi et al., 2006; Gu et al., 2008). Over-expression of IMP1 can" . _:b536422876 . _:b26176092 . _:b26176108 "IMP1 is known to increase the levels of some proteins by increasing the stability of their mRNAs (Noubissi et al., >>2006<<) and to reduce the levels of other proteins by inhibiting translation (Hansen et al., 2004; Atlas et al., 2007)." . . . _:b536422995 . . . . _:b536423070 . _:b26176003 "However, the rate of neurogenesis, the frequency of stem cells, and their rate of proliferation all decline with age (Kuhn et al., 1996; Enwere et al., 2004; Maslov et al., 2004; Molofsky et al., >>2006<<; Bonaguidi et al., 2011; Encinas et al., 2011)." . . . _:b26176055 . _:b536423028 . . . _:b26176014 "let-7 microRNAs are evolutionarily conserved heterochronic genes that regulate developmental timing (Pasquinelli et al., >>2000<<) and aging (Shen et al., 2012) in Caenorhabditis elegans." . _:b536423015 . _:b536422947 . . _:b26175996 . . _:b536422899 . _:b26176080 "stem cells in the pallial region of the telencephalon, preventing the premature generation of Tbr2+ intermediate progenitors and differentiated neurons (Chenn and Walsh, 2002; Machon et al., 2007; Wrobel et al., 2007; Mutch et al., >>2010<<). The Imp1 expression pattern we observed (Figure 1D and 6A, Figure 1\u2014figure supplement 1B) was reminiscent of the gradient of Wnt signaling observed in the telencephalon (Machon et al., 2007; Mutch et al., 2010); furthermore, the Imp1" . . . . _:b26176005 "the rate of neurogenesis, the frequency of stem cells, and their rate of proliferation all decline with age (Kuhn et al., 1996; Enwere et al., 2004; Maslov et al., 2004; Molofsky et al., 2006; Bonaguidi et al., 2011; Encinas et al., >>2011<<). A fundamental question concerns the mechanisms that control these temporal changes in stem cell properties." . . . _:b536423069 . _:b26176053 . _:b536422925 . _:b536422920 . . . _:b26176070 "To systematically examine Imp1 expression we analysed a gene-trap mouse (Imp1\u03B2-geo/+) in which \u03B2-galactosidase-neomycin (\u03B2-geo) was inserted into the second intron of Imp1 (Hansen et al., >>2004<<). This led to the expression of an IMP1-\u03B2-geo fusion protein that contained the IMP1 RNA recognition motif encoded by exons 1 and 2 but lacked the second RNA recognition motif and the four hnRNP K homology domains, which are essential for" . . . . _:b536422926 . . . . . _:b536423061 . _:b26176060 . . _:b26176025 . _:b26176098 "This is consistent with the expectation that let-7 family members act redundantly to regulate gene expression (Roush and Slack, >>2008<<) such that deletion of let-7b/c2 is not sufficient to change Imp1 expression or neural stem cell function." . . . _:b26176106 "A decline in Wnt signaling during aging also contributes to these effects (Seib et al., >>2013<<)." . _:b26176112 . _:b26176036 "Canonical Wnt signaling promotes a rapid expansion in the number of undifferentiated stem cells during forebrain development (McMahon et al., 1992; McMahon and Bradley, 1990; Ikeya et al., 1997; Dickinson et al., 1994; Wrobel et al., >>2007<<). Wnt signaling prevents cell cycle exit and delays differentiation in these cells (Megason and McMahon, 2002; Chenn and Walsh, 2002; Machon et al., 2003; Zechner et al., 2003; Zhou et al., 2006; Woodhead et al., 2006; Gulacsi and" . . _:b536422910 . . . . _:b26175995 _:b26175996 . . _:b26175995 _:b26175997 . . _:b26175995 _:b26175998 . _:b26175995 _:b26175999 . . _:b26176015 . . . . . _:b536423031 . _:b536422988 . . _:b26176091 . _:b26176063 . _:b26176076 . . _:b536422882 . _:b536423047 . _:b26176077 . .