Mechanisms of Fast Axonal Transport Impairment by the HIV Glycoprotein gp120

Distal sensory polyneuropathy (DSP) is a prevalent neurological complication directly caused by human immunodeficiency virus (HIV). DSP is characterized by progressive dying-back degeneration of long sensory axons at the distal extremities, which originate from dorsal root ganglion (DRG) neurons. Gp120, a neurotoxic HIV glycoprotein that is overproduced and shed by infected macrophages, has been linked to DSP. However, the role that gp120 plays in promoting degeneration of DRG axons remains uncertain. For my dissertation, I hypothesized that gp120 exerts its neurotoxic effects inside DRG neurons. My research consisted of 2 specific aims. The first was to evaluate the mechanisms of internalization and intracellular location of gp120. Using immunocytochemical and pharmacological experiments, I defined the endosomal pathway for gp120 internalization. Additional experiments using compartmentalized microfluidic chambers revealed retrograde transport of gp120 from DRG axons to their cell bodies. Since dying-back neurodegeneration has been linked to impaired fast axonal transport (FAT), I hypothesized that intracellular gp120 inhibits FAT. My second aim was to identify intra-axonal signaling pathways underlying gp120-induced alterations in fast axonal transport. Gp120 was perfused in isolated squid axoplasm, revealing axon-specific effects of gp120 on fast axonal transport (FAT). Coperfusion of gp120 with inhibitors of certain regulatory kinases and phosphatases for FAT delineated the signaling pathways that gp120 activated to impair anterograde and retrograde FAT. These results were confirmed with biochemical assays and microtubule binding assays in a mammalian cell line. The unique reliance of neurons on FAT mechanisms suggests that gp120-induced activation of phosphotransferases in the axonal compartment might represent a critical pathogenic event in DSP.