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Methyl-CpG-Binding domain proteins and histone deacetylases in the stage-specific differentiation of olfactory receptor neurons MacDonald, Jessica

Abstract

DNA methylation-dependent gene silencing, catalyzed by DNA methyltransferases (DNMTs) and mediated by methyl binding domain proteins (MBDs) and histone deacetylases (HDACs), is essential for mammalian development, with the nervous system demonstrating particular sensitivity to perturbations. Little is known, however, about the role of DNA methylation in the stage-specific differentiation of neurons. In the olfactory epithelium (OE), where neurogenesis is continuous and the cells demonstrate a laminar organization with a developmental hierarchy, we identified sequential, transitional stages of differentiation likely mediated by different DNMT, MBD and HDAC family members. Biochemically, HDAC1 and HDAC2 associate with repressor complexes recruited by both MBD2 and MeCP2. HDAC1 and HDAC2, however, are divergently expressed in the OE, a pattern that is recapitulated in the brain. Rather than simultaneous inclusion in a complex, therefore, the individual association of HDAC1 or HDAC2 may provide specificity to a repressor complex in different cell types. Furthermore, distinct transitional stages of differentiation are perturbed in the absence of MBD2 or MeCP2. MeCP2 is expressed in the most apical immature olfactory receptor neurons (ORNs), and is up-regulated with neuronal maturation. In the MeCP2 null OE there is a transient delay in ORN maturation and an increase in neurons of an intermediate developmental stage. Two protein variants of MBD2 are expressed in the OE, with MBD2b expressed in cycling progenitor cells and MBD2a in the maturing ORNs. MBD2 null ORNs undergo increased apoptotic cell death. There is also a significant increase in proliferating progenitors in the MBD2 null OE, likely due, at least in part, to feedback from the dying ORNs, acting to up-regulate neurogenesis. Increased cell cycling in the MBD2 null is also observed post-lesion, however, in the absence of feedback back from the ORNs, a phenotype that is recapitulated by an acute inhibition of HDACs with valproic acid. Therefore, disruptions at both transitional stages of ORN differentiation are likely in the MBD2 null mouse. Together, these results provide the first evidence for a sequential recruitment of different MBD proteins and repressor complexes at distinct transitional stages of neuronal differentiation.

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