Previously, there was no conceptual framework for linking the diverse molecular and cellular disruptions seen in the leukodystrophies into a coherent model for the production of the generalized CNS white matter destruction that is the hallmark of these diseases. This lack of a coherent model was particularly problematic for molecular disruptions in astrocytes, which are far removed from the oligodendrocyte myelin that becomes sclerotic. We recently introduced the Gateway Hypothesis to account for the widespread destruction of CNS myelin that characterizes these diseases. We proposed that generalized myelin sclerosis is caused by mutation or immunological disruption of proteins comprising the primary transport pathways for K+ and water within and between cells of the panglial syncytium. We and others had identified proteins of the K+ and water transport pathway that, when mutated or destroyed, disrupt ionic homeostasis, primarily because K+ and water continue to enter the panglial syncytium but are blocked before they can exit. The Gateway Hypothesis suggests that pharmacological agents that reduce K+ entry into the panglial syncytium may provide new therapeutic approach to treating these diseases. Two abundant protein molecules of that pathway - KV1 (the voltage-gated channels of myelinated axons) and Cx29 (a poorly-understood oligodendrocyte connexin that does not form gap junctions) - are proposed to represent the gateway for entry of water and K+ into the panglial syncytium. As such, these tightly-associated proteins, as well as voltage-gated sodium channels at nodes of Ranvier, are proposed as potential targets for pharmacologic intervention in many of the leukodystrophies. The rationale is that by reducing axonal Na+ influx and/or K+ efflux and its coupled influx into the surrounding myelin, the osmotic burden that causes myelin swelling and sclerosis can be reduced sufficiently to allow undamaged K+ and water transport pathways to redistribute the pharmacologically-reduced osmotic load, thereby permitting normal cellular repair mechanisms to partially restore myelin function, allowing for longer-term treatments, potentially including glial stem cell replacement therapies. We propose to use: 1) ultrastructural and super-resolution light microscopic immunocytochemistry; 2) molecular pull-down assays; 3) expression of Cx29 and KV1 channels in cell culture, with monitoring by intracellular recording electrophysiology; and 4) recording from oligodendrocytes in acute slices of mouse corpus callosum of normal and exercised wildtype and Cx29 knockout mice. We will thus characterize KV1 channels in axon plasma membranes and Cx29 channels in the adjacent innermost layer of myelin and establish functional interaction of those molecules as the K+ gateway into the panglial syncytium. With these new data, the Gateway Hypothesis will provide the framework for identifying which of the leukodystrophies are immediately amenable to pharmacologic intervention and/or stem cell therapies, allowing medical resources to be delivered to the patients who can be most effectively treated.
The devastating diseases of CNS white matter that are called 'leukodystrophies' are caused by such a wide variety of molecular alterations that there has been no unifying approach to treat the swelling and sclerosis of the insulating layer of myelin around nerve fibers that characterize these diseases. Based on ten years of work on the protein molecules that normally provide for K+ and water transport away from the innermost layers of myelin, we proposed the 'Gateway Hypothesis', which provides a new model for understanding myelin swelling and sclerosis in the leukodystrophies. The proposed research to characterize the molecular pathways for K+ and water entry into myelin will provide the necessary data to develop new drugs that reduce or reverse white matter damage in many of the leukodystrophies.
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