The ATP-dependent BRG1/BRM associated factor (BAF) chromatin remodeling complexes are crucial in regulating gene expression by controlling chromatin dynamics. Over the last decade, it has become increasingly clear that during neural development in mammals, distinct ontogenetic stage-specific BAF complexes derived from combinatorial assembly of their subunits are formed in neural progenitors and post-mitotic neural cells. Proper functioning of the BAF complexes plays critical roles in neural development, including the establishment and maintenance of neural fates and functionality. Indeed, recent human exome sequencing and genome-wide association studies have revealed that mutations in BAF complex subunits are linked to neurodevelopmental disorders such as Coffin-Siris syndrome, Nicolaides-Baraitser syndrome, Kleefstra's syndrome spectrum, Hirschsprung's disease, autism spectrum disorder, and schizophrenia. In this review, we focus on the latest insights into the functions of BAF complexes during neural development and the plausible mechanistic basis of how mutations in known BAF subunits are associated with certain neurodevelopmental disorders.

BAF (Brg/Brm-associated factors) complexes play important roles in development and are linked to chromatin plasticity at selected genomic loci. Nevertheless, a full understanding of their role in development and chromatin remodeling has been hindered by the absence of mutants completely lacking BAF complexes. Here, we report that the loss of BAF155/BAF170 in double-conditional knockout (dcKO) mice eliminates all known BAF subunits, resulting in an overall reduction in active chromatin marks (H3K9Ac), a global increase in repressive marks (H3K27me2/3), and downregulation of gene expression. We demonstrate that BAF complexes interact with H3K27 demethylases (JMJD3 and UTX) and potentiate their activity. Importantly, BAF complexes are indispensable for forebrain development, including proliferation, differentiation, and cell survival of neural progenitor cells. Our findings reveal a molecular mechanism mediated by BAF complexes that controls the global transcriptional program and chromatin state in development.

Neurogenesis is a key developmental event through which neurons are generated from neural stem/progenitor cells. Chromatin remodeling BAF (mSWI/SNF) complexes have been reported to play essential roles in the neurogenesis of the central nervous system. However, whether BAF complexes are required for neuron generation in the olfactory system is unknown. Here, we identified onscBAF and ornBAF complexes, which are specifically present in olfactory neural stem cells (oNSCs) and olfactory receptor neurons (ORNs), respectively. We demonstrated that BAF155 subunit is highly expressed in both oNSCs and ORNs, whereas high expression of BAF170 subunit is observed only in ORNs. We report that conditional deletion of BAF155, a core subunit in both onscBAF and ornBAF complexes, causes impaired proliferation of oNSCs as well as defective maturation and axonogenesis of ORNs in the developing olfactory epithelium (OE), while the high expression of BAF170 is important for maturation of ORNs. Interestingly, in the absence of BAF complexes in BAF155/BAF170 double-conditional knockout mice (dcKO), OE is not specified. Mechanistically, BAF complex is required for normal activation of Pax6-dependent transcriptional activity in stem cells/progenitors of the OE. Our findings unveil a novel mechanism mediated by the mSWI/SNF complex in OE neurogenesis and development.

Fusion between membranes is mediated by specific SNARE complexes. Here we report that fibroblasts survive the absence of the trans-Golgi network/early endosomal SNARE vti1a and the late endosomal SNARE vti1b with intact organelle morphology and minor trafficking defects. Because vti1a and vti1b are the only members of their SNARE subclass and the yeast homolog Vti1p is essential for cell survival, these data suggest that more distantly related SNAREs acquired the ability to function in endosomal traffic during evolution. However, absence of vti1a and vti1b resulted in perinatal lethality. Major axon tracts were missing, reduced in size, or misrouted in Vti1a −/− Vti1b −/− embryos. Progressive neurodegeneration was observed in most Vti1a −/− Vti1b −/− peripheral ganglia. Neurons were reduced by more than 95% in Vti1a −/− Vti1b −/− dorsal root and geniculate ganglia at embryonic day 18.5. These data suggest that special demands for endosomal membrane traffic could not be met in Vti1a −/− Vti1b −/− neurons. Vti1a −/− and Vti1b −/− single deficient mice were viable without these neuronal defects, indicating that they can substitute for each other in these processes.endosome | neurite outgrowth | neuronal survival

SummaryDuring early cortical development, neural stem cells (NSCs) divide symmetrically to expand the progenitor pool, whereas, in later stages, NSCs divide asymmetrically to self-renew and produce other cell types. The timely switch from such proliferative to differentiative division critically determines progenitor and neuron numbers. However, the mechanisms that limit proliferative division in late cortical development are not fully understood. Here, we show that the BAF (mSWI/SNF) complexes restrict proliferative competence and promote neuronal differentiation in late corticogenesis. Inactivation of BAF complexes leads to H3K27me3-linked silencing of neuronal differentiation-related genes, with concurrent H3K4me2-mediated activation of proliferation-associated genes via de-repression of Wnt signaling. Notably, the deletion of BAF complexes increased proliferation of neuroepithelial cell-like NSCs, impaired neuronal differentiation, and exerted a Wnt-dependent effect on neocortical and hippocampal development. Thus, these results demonstrate that BAF complexes act as both activators and repressors to control global epigenetic and gene expression programs in late corticogenesis.