Critical role of Brg1 member of the SWI/SNF chromatin remodeling complex during neurogenesis and neural crest induction in zebrafish

Brg1 is a member of the SWI/SNF chromatin-remodeling complex, and in some organisms Brg1 has been shown to interact with beta-catenin and positively control the TCF/LEF transcription factor that is located downstream of the Wnt signal transduction pathway. During development, TCF/LEF activity is critical during neurogenesis and head induction. In zebrafish, Brg1-deficient embryos exhibit retinal cell differentiation and eye defects; however, the role of Brg1 in neurogenesis and neural crest cell induction remains elusive. We used zebrafish deficient in Brg1 (yng) or Brg1 specific-morpholino oligonucleotide-mediated knockdown to analyze the embryonic requirements of Brg1. Our results indicate that reduction in Brg1 expression leads to the expansion of the forebrain-specific transcription factor, six3, and marked reduction in expression of the mid/hind-brain boundary and hind-brain genes, engrailed2 and krox20, respectively. At 12 hpf, the expression of neural crest specifiers are severely affected in Brg1-morpholino-injected embryos. These results suggest that Brg1 is involved in neural crest induction, which is critical for the development of neurons, glia, pigment cells, and craniofacial structures. Brg1 is a maternal factor, and brg1-deficient embryos bearing the yng mutation derived from heterozygote intercrosses exhibit lesser effects on neural crest-specific gene expression, but show defects in neurogenesis and neural crest cell differentiation. This is exhibited by the aberrant brain patterning, a reduction in the sensory neurons, and craniofacial defects. These results further elucidate the critical role for Brg1 in neurogenesis, neural crest induction, and differentiation.