The expression of the transcriptional repressor Hes1 oscillates in many cell types, including neural progenitor cells (NPCs), but the significance of Hes1 oscillations in development is not fully understood. To examine the effect of altered oscillatory dynamics of Hes1, we generated two types of Hes1 knock-in mice, a shortened (type-1) and an elongated (type-2) Hes1 gene, and examined their phenotypes focusing on neural development. While both mutations affected Hes1 oscillations, the type-1 mutation dampened Hes1 oscillations more severely, resulting in much lower amplitudes. The average levels of Hes1 expression in type-1-mutant NPCs were also lower than in wild-type NPCs but similar to or slightly higher than those in Hes1 heterozygous mutant mice, which exhibit no apparent defects. While type-2-mutant mice were apparently normal, type-1-mutant mice displayed smaller brains than wild-type mice and up-regulated proneural gene expression. Furthermore, proliferation of NPCs decreased and cell death increased in type-1-mutant embryos. When Hes3 and Hes5 were additionally deleted, neuronal differentiation was also accelerated, leading to microcephaly. Thus, robust Hes1 oscillations are required for maintenance and proliferation of NPCs and the normal timing of neurogenesis, thereby regulating brain morphogenesis.

Imprinted genes are expressed from only one allele in a parent of origin-specific manner. The cyclindependent kinase inhibitor p57 kip2 is encoded by an imprinted gene Cdkn1c, with the paternal allele being silenced. The possible expression and function of the paternal allele of Cdkn1c have remained little studied, however. We now show that the paternal allele of the Cdkn1c gene is expressed at a low level in the developing mouse neocortex. Surprisingly, the central nervous system-specific conditional deletion of the paternal allele (pat cKo) at the Cdkn1c locus resulted in a marked reduction in brain size. furthermore, pat cKO gradually reduced the number of neural stem-progenitor cells (NPCs) during neocortical development, and thus reduced the number of upper-layer neurons, which were derived from late-stage NPCs. Our results thus show that the paternal allele of the Cdkn1c locus plays a key role in maintenance of NPCs during neocortical development. Genomic imprinting refers to an epigenetic process that results in the inactivation of one of the two alleles of a gene in a parent of origin-dependent manner 1-3. It is achieved mainly by allele-specific DNA methylation at a subset of CpG islands, known as imprinting control regions (ICRs), during early developmental stages 4. Imprinted genes play essential roles in development, homeostasis, and behavior in mammals 5,6. Changes at imprinted gene loci in humans are associated with diseases such as Beckwith-Wiedemann syndrome, Prader-Willi syndrome, and Angelman syndrome 7,8 , many of which are characterized by altered growth and mental disorders 6. Although canonical genomic imprinting has been thought to result in the complete silencing of one allele of a gene, recent studies have shown that silencing of some imprinted genes appears to be incomplete or reversed to various extents in the brain 9. For instance, derepressed expression of the imprinted alleles of Igf2 and Dlk1 contributes to the regulation of adult neural stem cells in mice 10,11. However, the functions of the imprinted alleles of other genes remain mostly obscure. The Cdkn1c gene is imprinted, with the maternal allele being expressed, and is located in the distal region of mouse chromosome 7 and human chromosome 11p15 12,13. In mice, DNA methylation of two ICRs, KvDMR 14-16 and ICG5 17 , has been suggested to suppress expression of the paternal allele of the Cdkn1c. The Cdkn1c-encoded protein p57 kip2 is a cyclin-dependent kinase inhibitor (CKI) 18,19 that is highly expressed in neural and skeletomuscular tissues 20,21 during embryonic development. In humans, changes at the Cdkn1c gene locus are associated with Beckwith-Wiedemann syndrome, features of which include excessive growth and an increased risk of childhood cancer 22,23. Gene knockout (KO) studies have implicated p57 kip2 in regulation of fetal growth and placental development 20,24,25. In the central nervous system (CNS), p57 kip2 plays a key role in regulation of the proliferation and differentiation of embryonic neural stem-progenitor ce...

Different classes of non-coding RNA (ncRNA) influence the organization of chromatin. Imprinted gene domains constitute a paradigm for exploring functional long ncRNAs (lncRNAs). Almost all express an lncRNA in a parent-of-origin dependent manner. The mono-allelic expression of these lncRNAs represses close by and distant protein-coding genes, through diverse mechanisms. Some control genes on other chromosomes as well. Interestingly, several imprinted chromosomal domains show a developmentally regulated, chromatin-based mechanism of imprinting with apparent similarities to X-chromosome inactivation. At these domains, the mono-allelic lncRNAs show a relatively stable, focal accumulation in cis. This facilitates the recruitment of Polycomb repressive complexes, lysine methyltranferases and other nuclear proteins — in part through direct RNA–protein interactions. Recent chromosome conformation capture and microscopy studies indicate that the focal aggregation of lncRNA and interacting proteins could play an architectural role as well, and correlates with close positioning of target genes. Higher-order chromatin structure is strongly influenced by CTCF/cohesin complexes, whose allelic association patterns and actions may be influenced by lncRNAs as well. Here, we review the gene-repressive roles of imprinted non-coding RNAs, particularly of lncRNAs, and discuss emerging links with chromatin architecture.