This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. GAP-43 (neuromodulin) is a component of the presynaptic membrane that is the major protein of axonal growth cones. Its primary functions include axonal growth, axonal pathfinding, synaptic plasticity, and regulating neurotransmitter release. GAP-43 expression has been shown to be up-regulated during development, neuronal plasticity and neuronal death, and it has been used to measure functional axonal recovery in various chronic inflammatory diseases such as multiple sclerosis via the EAE animal model, Alzheimer's disease, and stroke. GAP-43 expression has also been used to test the pathogenesis of antiretroviral-induced peripheral neuropathy and possible effects of neuroprotective drugs in the dorsal root ganglion in rats. The purpose of this study has been to assess neuronal and synaptic damage associated with simian immunodeficiency virus (SIV) encephalitis using markers to evaluate the pre-synaptic terminals (GAP-43 and SYN) and post-synaptic membranes (MAP-2). SIV has been well established as a model for HIV encephalitis (HIVE). It has also been documented that increased severity of SIVE inversely correlates with decreased synaptophysin and MAP-2 expression and with decreased N-acetylaspartate/creatine ratio (NAA/Cr), a marker of neuronal metabolism. Immunohistochemical examination of GAP-43 protein expression in frontal cortex (FC) and hippocampus (HI) was undertaken was evaluated in FC and HI from a cohort of CD8-depleted SIVmac251-infected rhesus with and without combined antiretroviral therapy (CART). We had previously demonstrated that NAA/Cr decline in CD8-depleted SIV-infected macaques is reversed after 28 days of antiretroviral therapy associated with near complete clearance of CD68+ SIV-infected perivascular macrophages from the brain. We hypothesized that part of the cause of the normalization of NAA in treated macaques was reversal of presynaptic membrane loss that would be represented by increased synaptophysin. We also hypothesized that GAP-43 would be upregulated as a repair mechanism with CART. We demonstrate here that although synaptophysin loss is not reversed in this time frame with CART, GAP-43 is upreglulated, but the upregulation occurs with initial SIV infection and is not augmented with CART. Increased GAP-43 represents an endogenous repair mechanism that occurs with initial injury and may serve as a potential marker of repair in the SIV model. We have also demonstrated that microglia and infiltrating macrophages activated during SIV-infection express the axonal guidance molecule ephrin B3. Ephrin B3 is a ligand for EphA4 and EphB3 and is involved in synapse and dendritic spine formation. It mediates demonstrated anti-apoptotic activity in neurons, but also functions as an inhibitor of axonal growth, a mechanism to maintain axonal stability. Although ephrin B3 can regulate axonal sprouting through repulsion, its action has been associated with induction of GAP-43. The discovery of ephrin B3 in macrophages and microglia in the brain was not unanticipated. Ephrin B3 and its receptors are expressed in peripheral T cells and monocytes/macrophages. This may provide a mechanism whereby macrophages and microglia could modulate neuron and astrocyte function, as both cell types express the ephrin B3 receptors, EphA4 and EphB3. Despite the documented significant improvement in neuronal metabolism with short-term CART evidenced by normalization of NAA/Cr in the treated animals, there was no significant difference in GAP-43 expression between SIV-infected untreated monkeys and those that received CART. We speculate that there is ongoing injury (and attempts at repair) that continues to induce GAP-43 expression despite the marked reduction in brain virus burden that is observed in the animals that received CART in this study. We observe a mismatch in marked reduction of brain virus burden with CART, but persistent levels of microglial activation and TNFa, a neurotoxic factor, in the treated animals (L. Annamalai personal communication). The mismatch in parameters of improvement, neuronal metabolism and GAP-43/ephrin B3, may also reflect differences in the time course of metabolic improvement, compared with the time course of induction and suppression of factors that promote neuroadaptive processes. GAP-43 mRNA is induced by nerve growth factor (NGF), which is dependent on Mst3b, a neuron-specific kinase central to regulation of axonal outgrowth. The role of ephrins as factors that can promote or hinder neuronal repair mechanisms is controversial, but it is clear that ephrins are involved in the intricate communication of neurons with astrocytes and microglia. We propose a model in which microglia activated by virus infection (and ephrin B3 expression) stimulate astrocytes to secrete NGF, which binds TrkA receptors on neurons, thus inducing neuroprotective signaling cascades that result in the induction and increased mRNA stability of GAP-43. Other ephrins in addition to ephrin B3 and their receptors are likely to be involved in this multicellular activation and signaling pathway. Therapeutic modulation of these extracellular axonal growth molecules, their receptors, or downstream signaling elements may augment axonal sprouting and regeneration in the adult CNS. The adult SIV-infected macaque model will be useful for exploring additional genes and ephrins that are associated with synaptic plasticity as well as for further characterization of endogenous neuroregenerative potential.
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