My laboratory is interested in neuroprotective approaches in neuroinflammatory disorders, including MS and SCI, where inflammation has been shown to occur within the injured spinal cord. In addition to studies that target axonal ion channels for axonal neuroprotection in disorders such as MS, we are interested in the contribution of voltage-gated Na channels within glial and immune cells to their functions in neuroinflammatory diseases, and in the effects of Na channel block in these cells. In this proposal, we focus on the following Specific Aims: I. Sodium Channel Blockers and Neuroprotection in Neuroinflammatory Disorders We have demonstrated that Na channel blockers have anti-inflammatory and neuroprotective effects in mice with EAE, but we observed acute worsening following Na channel blocker withdrawal. Several clinical studies of Na channel blockers in MS are ongoing. We will now determine whether exacerbation of EAE is unique to C57/BL6 mice and whether withdrawal of other Na channel blockers exacerbates EAE. II. Sodium Channel Nav1.5 and Functions of Astrocytes in Neuroinflammation We have demonstrated that human astrocytes express Na channels and that reactive astrocytes exhibit robust upregulation of Nav1.5 at the borders of, and within, MS lesions. We will now determine the contribution of Na channels to effector roles of astrocytes in neuroinflammatory disorders, using methods that include shRNA knock-down and Cre/lox knockouts. III. Sodium Channels and Functions of Microglia We have shown that Nav1.5 and Nav1.6 are present and functional within microglia / macrophages, and that Na channel blockade attenuates their migration and phagocytosis. However, the effects of Na channel blockade on other microglial/macrophage functions is less well understood, and there is a need to understand signaling pathways that control microglial/macrophage function. We have shown that ERK, p38 and JNK regulate migration of microglia, and that levels of phosphorylated ERK are attenuated in microglia following Na channel blockade. Recently, it was shown that induction of mitogen-activated protein kinase phosphatase (MPK), which dephosphorylates ERK, reduces migration of microglia. We will now examine the contribution of Na channels to multiple functions and signaling pathways of activated microglia. Injured axons in the CNS can undergo substantial retraction (die-back) from the initial site of axotomy. This can interfere with functional transmission along more proximal, initially uninjured axonal branches;or may lead to degeneration of sustaining collaterals and neuronal degeneration. We and others have demonstrated that microglia/macrophages can engulf transected axonal ovoids, but details of the role of microglia in axonal degeneration are not yet understood. We will therefore study the role of microglia/macrophages in axonal degeneration. IV. Studies in Human MS Tissue Building upon our prior studies on human MS tissue, we will extrapolate from our in vitro studies and studies with EAE to test the hypotheses in human MS lesions that astrocytes at the active edge of MS lesions show different properties in terms of Na channel expression compared to astrocytes in central parts of these lesions;that microglia/macrophages at the active border of MS lesions show different properties in terms of Na channel expression compared to microglia/macrophages in central parts of these lesions;and that microglia/ macrophages mediate axonal die-back in MS.
Multiple sclerosis (MS) and spinal cord injury (SCI) are major health challenges for the VA. There are currently 40,000 veterans with SCI and at least 23,000 veterans with MS. The inflammatory response in SCI and MS contributes significantly to demyelination and axonal degeneration, resulting in substantial disability. Na channels play pivotal roles in MS and SCI, both in the restoration of conduction following demyelination, and in a degenerative cascade that leads to axonal loss. Our recent work demonstrates that Na channels contribute to regulation of function in microglia/macrophages in neuroinflammatory disorders. Clinical studies of several Na channel blockers as potential neuroprotective agents in MS have been planned and/or launched. However, we have demonstrated that withdrawal of phenytoin and carbamazepine from mice with EAE results in acute exacerbation and inflammatory rebound associated with significant mortality. Thus, it is critically important to more fully understand the roles of Na channels, and of Na channel block, in neuro-inflammatory disorders.
|Dib-Hajj, Sulayman D; Geha, Paul; Waxman, Stephen G (2017) Sodium channels in pain disorders: pathophysiology and prospects for treatment. Pain 158 Suppl 1:S97-S107|
|Pappalardo, Laura W; Black, Joel A; Waxman, Stephen G (2016) Sodium channels in astroglia and microglia. Glia 64:1628-45|