The Role of CASK in Central Nervous System Function and Disorder
Understanding how different regions of the central nervous system (CNS) are affected by genetic insults is critical to advancing the study of CNS pathologies. The cerebellum is one such region which is 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) and also optic nerve hypoplasia (ONH) in heterozygous mutations. Herein, I used various molecular and genetic strategies to uncover the role of the CASK protein in brain function and pathogenesis of cerebellar hypoplasia associated with CASK mutations/deletions. First, using the face- and construct-validated heterozygous CASK knockout (Cask+/-) murine model, I conducted bulk RNA-sequencing and proteomics experiments from whole brain lysates to uncover changes in the Cask+/- brain. RNA-sequencing revealed the majority of changes to be broadly categorized into metabolic, nuclear, synaptic, and extracellular-matrix associated transcripts. Proteomics revealed the majority of changes cluster as synaptic proteins, metabolic proteins, and ribosomal subunits. Thus, absence of CASK in half of brain cells seems to affect synaptic protein content, cell metabolism, and protein homeostasis. Extending these observations, I conducted GFP-trap immunoprecipitation followed by tandem mass spectroscopy to reveal protein complexes in which CASK participates. Commensurate with proteomic changes, CASK was found to complex with synaptic proteins, metabolic proteins, cytoskeletal elements, ribosomal subunits, and protein folding machinery. Next, in order to investigate the pathogenesis of CASK-linked cerebellar hypoplasia, I utilized a human case of early truncation wherein the 27th arginine of CASK is converted to a stop codon. Immunohistochemical analysis of this brain revealed an upregulation of glial fibrillary acidic protein, a common marker for degenerative cell death. To mechanistically test the hypothesis that cerebellar hypoplasia results from cell death rather than developmental failure, I created a murine model wherein CASK is deleted from the majority of cerebellar cells post-development using Cre recombinase driven by the Calb2 promoter. Deleting CASK from all cerebellar granule neurons post-migration indeed leads to degeneration of the cerebellum via massive depletion of granule cells while sparing Purkinje cells. Overall, the cerebellum shrinks by approximately half in cross-sectional area and degeneration is accompanied by a collapsing of the molecular layer and of Purkinje cell dendrites. In addition, cerebellar degeneration presents with a profound locomotor ataxia. In conclusion, CASK seems to be affecting brain energy homeostasis and synaptic connections via interactions with metabolic proteins, synaptic proteins, and protein homeostatic elements. Further, alterations in brain volume associated with CASK-linked disorders is the result of degenerative cell death rather than developmental failure as previously posited.