| The neocortex is a unique six-layered brain region composed of an array of morphologically and functionally distinct subpopulations of primary projection neurons forming complex circuits across the central nervous system. The developmentally progressive specification, differentiation, and signaling of these distinct subpopulations of neocortical projection neurons is critical to mammalian cognitive and sensorimotor abilities. Recent research points to mRNA metabolism as a key regulator of this development and maturation process. Hu antigen D (HuD), an RNA binding protein has been implicated in the establishment of neuronal identity and neurite outgrowth in vitro. Therefore, we investigated the role of HuD loss of function on neuron specification and dendritogenesis in vivo using a mouse model. We found that loss of HuD early in development results in a defective early dendritic overgrowth phase as well as pervasive deficits in neuron specification in the lower neocortical layers, as well as defects in dendritogenesis in the CA3 region of the hippocampus. Subsequent behavioral analysis revealed a deficit in performance of a hippocampal dependent task: the Morris water maze. Further, HuD knockout (KO) mice exhibited lower levels of anxiety than wild type counterparts, and were overall less active. Last, we found that HuD KO mice are more susceptible to auditory-induced seizures, often resulting in death. I have also discovered that HuD itself is heavily regulated at the post-transcriptional level, and is expressed in four transcript variants which encode 4 functionally distinct protein isoforms. Specifically, my data indicate that HuD4 is translated during early neocortical neurogenesis when lower layers are formed, where HuD3 is specifically translated during late neocortical neurogenesis. Further, early HuD3 overexpression drives the production of upper layer neurons, where HuD4 overexpression drives the fate of lower layer neurons. Using a conditional transgenic line as well as in-vitro cell cycle analysis, I also determined that the translational regulation of HuD3 is dependent upon NT-3 arriving from the thalamic afferents to the neocortex. This trophic source appears to only affect those stem cells distal to the ventricle when they are in S-phase. |
