The Role of Connexin32 in the Pathogensis of CMTX

University Of Pennsylvania, Philadelphia PA

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Abstract

DESCRIPTION (provided by applicant): Recessive mutations in GJC2, the human gene that encodes Cx47, cause Pelizaeus-Merzbacher-like disease (PMLD) and hereditary spastic paraparesis (HSP), presumably related their lack of Cx47 function in oligodendrocytes. How the loss of Cx47 function results in the clinical picture of PMLD is unknown. MRI paints the picture of profound dysmyelination, but this has yet to be confirmed in an autopsy or biopsy. In cellular and animal models, the mutants associated with PMLD impair GJ communication, but we do not yet know why GJ communication is essential for the proper functioning of oligodendrocytes. In this competing renewal, we will determine whether novel Cx47 mutants affect GJ coupling, explore whether O: O coupling a general feature of oligodendrocytes, and the functional significance of O: O coupling. 1. Investigate the molecular defects of hCx47 mutants causing PMLD or familial lymphedema. In the last grant cycle of this grant, we showed that 3 recessive Cx47 mutants associated with PMLD have defective trafficking and do not form functional channels with either Cx47 or Cx43. We will build on this observation by investigating 2 additional missense mutations, 2 frameshift mutations that affect the C-terminus of Cx47, deletion of the PDZ-binding domain (del433-437; a relevant, but not a naturally-occurring mutation), and 6 dominant mutations that cause a completely different phenotype, Familial Lymphedema. The ability of each mutant to form functional GJ plaques will be investigated - by immunostaining, scrape loading, fluorescence recovery after photobleaching (FRAP), and electrophysiology. Whether the dominant mutants have dominant-negative effects on WT Cx47 will also be investigated (by co-staining, co-immunoprecipitation, and electrophysiology). We will also generate lines of transgenic mice that express the Cx47del433-437 or R257C, one of the dominant mutations causing Familial Lymphedema. We will assess the ability of these mutants to rescue the phenotype of Gjc2-null (Gjc2-/-) mice, and of the R257C mutant to worsen the phenotype of Gjc2-heterozygous (Gjc2+/-) mice. 2. Is O: O coupling a general feature of oligodendrocytes? In the last grant cycle of this grant, we showed that O: A coupling in lost in mice lacking both Cx32 and Cx47. We also showed, expectedly, O: O coupling was prominent in the corpus callosum, that O: O coupling was also lost in mice lacking both Cx32 and Cx47, that the GJs directly join intrafascicular oligodendrocytes. The goal of this aim is to determine whether O: O coupling is found in other white matter tracts, and hence is a general feature of intrafascicular oligodendrocytes. To that end, we will measure the diffusion of sulforhodamine-B (SR- B), examine the ultrastructure of O: O junctions, and examine the expression of glial connexins (Cx30, Cx32, Cx43, Cx47) in two tracts - the optic nerve and the ventral funiculus of the cervical spinal cord. 3. The functional significance of O: O coupling. The function of O: A and O: O coupling is uncertain. The experimental results to date support two, non-mutually exclusive, functions - spatial buffering of K+ and metabolic cooperation. K+ clearance is modestly decreased in acute brain slices from mice in which astrocytes lack both Cx43 and Cx30. These mice also provide the best evidence for metabolic coupling, demonstrating that Cx30 and Cx43 are required for the intracellular diffusion of a fluorescent glucose analogue (2-NDBG) to areas of neuropil with increased neuronal activity. We reasoned that O: O coupling might serve a similar purpose for myelinated axons, and propose an experimental analysis of this possibility in this Aim. We will determine whether (a) activity increases the extent of 2-NDBG diffusion, and (b) whether glucose infused into a single oligodendrocyte can rescue axonal conduction in glucose-deprived conditions.