Disruption of the K-Cl Cotransporter-3 Leads to Severe Peripheral Neuropathy

Abstract

Mutations in the human K-Cl cotransporter-3 (KCC3) gene lead to a severe neurological disorder called peripheral neuropathy associated with agenesis of the corpus callosum (ACCPN). At the same time as that discovery, the KCC3 knockout mouse was created by our laboratory and was found to drag its hind limbs. My dissertation research focused on the KCC3 knockout mouse as a model for the disease. First, through behavioral characterization of the KCC3 knockout mouse, I show that it is a relevant model of ACCPN. Then, I concentrated on the peripheral neuropathy component of KCC3 loss. In order to assess the role of KCC3 in peripheral axon and/or myelin development and maintenance, I determined its expression in wild-type sciatic nerves. This work is the first to show that sciatic nerves do express KCC3. It is expressed in sciatic nerves of juvenile, but not adult, wild-type mice, specifically by Schwann cells. I performed a detailed morphometric analysis of sciatic nerves at different ages to determine the underlying pathophysiology of the peripheral neuropathy and answer whether the lack of KCC3 leads to a developmental or neurodegenerative disorder. In the knockout, Schwann cell and myelin development appears normal at P3, but axons are swollen. At P8 and P30, abnormal periaxonal swelling occurs in some myelinated fibers. These initial swelling pathologies are followed by Wallerian-like degeneration in adult KCC3 knockout nerves. To test whether these changes in the adult mouse lead to neurophysiologial deficits, I assessed nerve conduction velocity and pain sensitivity in wild-type and knockout mice. Mutant mice exhibit a reduction in nerve conduction velocity and sensitivity to noxious pain. The evidence I show here for fluid-related axonopathy, which ultimately results in neurodegeneration, implicates cell volume regulation, likely in conjunction with potassium buffering, as a critical component of peripheral nerve maintenance.