During development of the CNS, neurons and glia are generated in a sequential manner. The mechanism underlying the later onset of gliogenesis is poorly understood, although the cytokineinduced Jak-STAT pathway has been postulated to regulate astrogliogenesis. Here, we report that the overall activity of Jak-STAT signaling is dynamically regulated in mouse cortical germinal zone during development. As such, activated STAT1/3 and STAT-mediated transcription are negligible at early, neurogenic stages, when neurogenic factors are highly expressed. At later, gliogenic periods, decreased expression of neurogenic factors causes robust elevation of STAT activity. Our data demonstrate a positive autoregulatory loop whereby STAT1/3 directly induces the expression of various components of the Jak-STAT pathway to strengthen STAT signaling and trigger astrogliogenesis. Forced activation of Jak-STAT signaling leads to precocious astrogliogenesis, and inhibition of this pathway blocks astrocyte differentiation. These observations suggest that autoregulation of the Jak-STAT pathway controls the onset of astrogliogenesis.During embryonic development, the generation of three major neural cell types (neurons, astrocytes, and oligodendrocytes) in the CNS occurs sequentially, whereby almost all neurons are generated before the appearance of glial cells 1,2 , with the exception of a few COMPETING INTERESTS STATEMENTThe authors declare that they have no competing financial interests. NIH Public Access Author ManuscriptNat Neurosci. Author manuscript; available in PMC 2014 November 06. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript sites of postnatal and adult neurogenesis such as the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ) of the forebrain 3 . This strategy of building the CNS through sequential production of neurons and glia has become more comprehensible, as recent findings have demonstrated that glial cells are important in critical neuronal maturation processes such as axonal path finding and synapse formation [4][5][6] . It is conceivable that delayed or precocious production of glial cells may lead to inappropriate wiring, disorganization, and eventually, dysfunction of the CNS.The 'neurons-first, glia-second' differentiation theme for neural progenitors can be recapitulated in culture. Cortical neural progenitor stem cells isolated from relatively early embryonic stages (for example, mouse embryonic day (E) 10-11) give rise to neurons, not glial cells, after short-term culturing (fewer than 4 d), whereas cortical progenitors isolated from perinatal stages tend to differentiate into astrocytes under the same culture conditions 7 .In addition, both E10-11 cortical progenitors and embryonic stem cell-derived neural stem or progenitor cells (NSCs or NPCs) switch from being neurogenic to gliogenic over time in vitro 8,9 , suggesting that the molecular switch for the transition from neurogenesis to gliogenesis may be internally programmed in neural progenitors.S...

Mutations resulting in defective splicing constitute a significant proportion (30/62 [48%]) of a new series of mutations in the ATM gene in patients with ataxia-telangiectasia (AT) that were detected by the protein-truncation assay followed by sequence analysis of genomic DNA. Fewer than half of the splicing mutations involved the canonical AG splice-acceptor site or GT splice-donor site. A higher percentage of mutations occurred at less stringently conserved sites, including silent mutations of the last nucleotide of exons, mutations in nucleotides other than the conserved AG and GT in the consensus splice sites, and creation of splice-acceptor or splice-donor sites in either introns or exons. These splicing mutations led to a variety of consequences, including exon skipping and, to a lesser degree, intron retention, activation of cryptic splice sites, or creation of new splice sites. In addition, 5 of 12 nonsense mutations and 1 missense mutation were associated with deletion in the cDNA of the exons in which the mutations occurred. No ATM protein was detected by western blotting in any AT cell line in which splicing mutations were identified. Several cases of exon skipping in both normal controls and patients for whom no underlying defect could be found in genomic DNA were also observed, suggesting caution in the interpretation of exon deletions observed in ATM cDNA when there is no accompanying identification of genomic mutations.