Differential Brain-Region Specific Roles of CASK in Optic Nerve Hypoplasia and Pontocerebellar Hypoplasia

PROJECT SUMMARY Understanding how different regions of the central nervous system (CNS) are affected by genetic insults is critical to advancing the study of CNS pathologies which present a major public health burden in the United States. The cerebellum and optic nerve are two such regions that are disproportionately hypoplastic in the majority of cases

of CASK gene mutation in humans. CASK is an enigmatic multi-domain scaffolding protein which plays a vital role in organizing protein complexes at the pre-synapse through interactions with both active zone proteins and trans-synaptic adhesion molecules such as liprins-α and neurexins. Mutations in the X-linked CASK gene in

humans are largely post-natally lethal in the hemizygous condition and result in microcephaly with pontine and cerebellar hypoplasia (PCH) as well as optic nerve hypoplasia (ONH) in heterozygous mutations. While CASK has been traditionally regarded as playing a role in CNS development, recent data indicate

that it plays a continued role in the maintenance of the adult CNS. Specifically, acute global deletion of Cask in adult mice using a CreER-Tamoxifen system leads to progressive degeneration in motor coordination culminating in profound ataxia several months after tamoxifen injection. This coincides with a progressive

deterioration of cerebellar gross morphology. Thus, this presents a unique opportunity to understand the continued role of a gene, previously regarded as developmentally important, in the function of the adult CNS and how this function may differ in a region-specific manner. Recently, it has been demonstrated that progression of ONH in the context of CASK-loss is non-cell

autonomous in nature as deleting Cask from retinal ganglion cells themselves does not exacerbate ONH, but deleting it from fibrous astrocytes of the optic nerve does exacerbate ONH. However, when Cask is deleted specifically in granule cells (GCs) using a Calb2 promoter driven Cre recombinase, there appears to be a cell-

autonomous death of GCs. This provides an opportunity to examine how underlying etiopathology and functional loss differ in two regions given the same genetic insult. As granule cells are the major excitatory driver of the cerebellum, this project will investigate the progression of granule cell loss, the correlation of granule cell loss to

functional motor loss and ataxia, and the electrophysiological implications of loss of the major excitatory driver in an isolated brain region. Further, the project will elucidate whether aberrations in spike-timing and frequency among cerebellar Purkinje cells precede or follow anatomical reductions in granule cell number. The project will

also examine whether visual function correlates to optic nerve diameter in models which display ONH but not PCH and vice versa using visual evoked potential recordings from V1 of visual cortex and the dorsal lateral geniculate nucleus of the thalamus. Finally, the study will determine molecular mechanisms of non-apoptotic or

apoptotic cell death underlying loss of GCs. Thus, the study will investigate differing causes and functional consequences with region-specificity in the context of a single genetic insult using Cask deletion as a test-case.