E.R., unpublished data). This somewhat broader pattern of expression of Prdm8 relative to Bhlhb5 suggests that Prdm8 may have additional partners to which it can couple, and one attractive candidate in this regard is Bhlhb4: loss of function studies have

revealed that Bhlhb4 is required for the survival of rod bipolar cells and, furthermore, that this factor is expressed, like Prdm8, in the embryonic diencephalon and DRG (Bramblett et al., 2002 and Bramblett et al., 2004). Because Prdm8 contains a SET domain that is characteristic of histone methyltransferases, it is possible that it may directly mediate repression of target genes by methylating target gene-associated histones. Consistent find more with this idea, Prdm8 has been shown to methylate histone H3K9 in vitro (Eom et al., 2009), a modification associated with transcriptional repression. Likewise, the tumor suppressor Prdm2 and the meiotic recombination determinant Prdm9 also show intrinsic histone methyltransferase activity (Hayashi et al., 2005 and Kim et al., 2003). However, several other Prdm family members, including Prdm1,

Prdm5, and Prdm6, appear to mediate repression indirectly by recruiting the histone methyltransferase, G9A (Davis et al., 2006, Duan et al., 2005 and Gyory et al., 2004). Thus, MDV3100 solubility dmso it is not yet clear whether Prdm8 functions directly or indirectly to mediate transcriptional repression. In either case, however, Prdm8 appears to be required for Adenylyl cyclase the repression of Bhlhb5 target genes. A curious aspect of the Bhlhb5/Prdm8 repressor complex is that, while each requires the other to repress target gene expression, we do not observe a perfect coincidence the expression of Bhlhb5 and Prdm8. Indeed, in many cases, the expression of these two factors appears to be somewhat reciprocal—neurons with highest levels of Bhlhb5 tend to have low levels of Prdm8, and vice versa. This disparity in expression level implies that Bhlhb5 and Prdm8 do not always

exist as part of a  functional repressor complex, and furthermore suggests that the expression of these factors is very tightly controlled, possibly to limit the degree and/or the duration of gene repression mediated by Bhlhb5/Prdm8. In keeping with this idea, we find that the Bhlhb5/Prmd8 repressor appears to curb its own activity by restricting the expression of Prdm8, which is upregulated in Bhlhb5 knockout mice. These observations suggest that Bhlhb5 and Prdm8 are part of a complex regulatory network that needs to be precisely coordinated for proper development. One of the consequences of disrupting the function of the Bhlhb5/Prdm8 repressor complex is that Cdh11 is aberrantly overexpressed, and our findings suggest that this misexpression has detrimental consequences for neural circuit development.