Even in the era of combined antiretroviral therapy, up to 50% of HIV-positive patients will demonstrate neurocognitive impairments in their lifetimes. These impairments are collectively known as HIV-associated neurocognitive disorders (HAND). While the neuropathology of HAND has been well-characterized, the specific mechanism by which HAND occurs remains to be clarified. Considerable experimental evidence indicates that HIV proteins, including the envelope protein gp120, cause neurological damage to a similar extent as the full virus. Thus, gp120 has emerged as potential agent underlying HIV neurotoxicity. However, the full mechanism of gp120- mediated neurotoxicity is still unknown. Therefore, it is imperative to investigate these mechanisms of neurotoxicity and elucidate targets for potential therapeutic intervention. I have established that gp120 is internalized into neurons via dynamin-dependent endocytosis and that internalized gp120 can bind to class-III ?-tubulin, a component of neuronal microtubules. Moreover, gp120 causes the deacetylation of tubulin, a post-translational modification that impairs the functionality of microtubules. Furthermore, tubulin deacetylation causes a dissociation of the motor proteins kinesin-1 and dynein from microtubules, which impairs axonal transport. Preliminary data indicate that intracellular trafficking of essential organelles, such as mitochondria, is greatly diminished in the presence of gp120. Therefore, I hypothesize that gp120 impairs axonal transport of organelles and cargo-containing vesicles through the deacetylation of tubulin. To confirm whether this deacetylation of tubulin underlies the neurotoxic effect of gp120, I first will inhibit the regulatory enzyme HDAC6 pharmacologically with tubacin (AIM 1) to prevent deacetylation of tubulin. I will confirm these results by utilizing siRNA for HDAC6. Using primary rat cortical neurons, I hypothesize that inhibition of HDAC6 will be neuroprotective, as shown in other neurodegenerative diseases. Secondly, I propose to establish whether gp120 causes decreased binding of kinesin-1 and dynein to microtubules (AIM 2A). To examine this, I will evaluate the binding of kinesin-1/dynein to tubulin using co-immunoprecipitation and sub-cellular fractionation to isolate microtubule associated proteins. Finally, using rat primary cortical neurons grown in a microfluidic chamber to isolate axons, I will evaluate axonal transport in the presence of gp120 (AIM 2B) using live imaging of quantum dot labeled brain-derived neurotrophic factor (BDNF). I hypothesize that gp120 will cause a decrease in kinesin-1/dynein binding to microtubules and therefore will impair both velocity and total distance travelled by the labeled BDNF. These studies aim to establish a new mechanism of gp120-mediated neurotoxicity that impairs axonal transport through tubulin deacetylation. Moreover, throughout the proposed training, I will gain expertise in a variety of molecular experimental approaches with emphasis on motor proteins and axonal transport.
HIV-associated neurocognitive disorders (HAND) occur in up to 50% of the HIV+ population even in the antiretroviral era. HIV-1 envelope protein, gp120 causes neuronal death, though the mechanism through which this toxicity occurs is not entirely clear; we have recently found that gp120 associates with neuronal microtubules and impairs their function. This proposal aims to establish whether the neurotoxic effect of gp120 is related to the alteration of microtubule function and could reveal a novel mechanism of HAND.
