Increase of neurofilament-H protein in sensory neurons in antiretroviral neuropathy: Evidence for a neuroprotective response mediated by the RNA-binding protein HuD.

Nucleoside reverse transcriptase inhibitors (NRTIs) are key components of HIV/AIDS treatment to reduce viral load. However, antiretroviral toxic neuropathy has become a common peripheral neuropathy among HIV/AIDS patients leading to discontinuation of antiretroviral therapy, for which the underlying pathogenesis is uncertain. This study examines the role of neurofilament (NF) proteins in the spinal dorsal horn, DRG and sciatic nerve after NRTI neurotoxicity in mice treated with zalcitabine (2',3'-dideoxycitidine; ddC). ddC administration up-regulated NF-M and pNF-H proteins with no effect on NF-L. The increase of pNF-H levels was counteracted by the silencing of HuD, an RNA binding protein involved in neuronal development and differentiation. Sciatic nerve sections of ddC exposed mice showed an increased axonal caliber, concomitantly to a pNF-H up-regulation. Both events were prevented by HuD silencing. pNF-H and HuD colocalize in DRG and spinal dorsal horn axons. However, the capability of HuD to bind NF mRNA was not demonstrated, indicating the presence of an indirect mechanism of control of NF expression by HuD. RNA immunoprecipitation experiments showed the capability of HuD to bind the BDNF mRNA and the administration of an anti-BDNF antibody prevented pNF-H increase. These data indicate the presence of a HuD - BDNF - NF-H pathway activated as a regenerative response to the axonal damage induced by ddC treatment to counteract the antiretroviral neurotoxicity. Since analgesics clinically used to treat neuropathic pain are ineffective on antiretroviral neuropathy, a neuroregenerative strategy might represent a new therapeutic opportunity to counteract neurotoxicity and avoid discontinuation or abandon of NRTI therapy.

Spinal RyR2 pathway regulated by the RNA-binding protein HuD induces pain hypersensitivity in antiretroviral neuropathy.

The antiretroviral toxic neuropathy, a distal sensory polyneuropathy associated with antiretroviral treatment, is a frequently occurring neurological complication during treatment of patients with AIDS and often leads to discontinuation of antiretroviral therapy. The mechanisms by which antiretroviral drugs contribute to the development of neuropathic pain are not known. Using drugs that reduce intracellular calcium ions (Ca(2+)), we investigated the hypothesis that altered cytosolic Ca(2+) concentration contributes to the 2',3'-dideoxycytidine (ddC)-evoked painful neuropathy. Administration of ddC induced mechanical and cold allodynia, which were abolished by intrathecal administration of TMB-8, a blocker of Ca(2+) release from intracellular stores, and by ryanodine, a RyR antagonist. Treatment with the IP3R antagonist heparin prevented mechanical allodynia with no effect on thermal response. To further clarify the pathway involved, we investigated the role of HuD, a RNA binding protein involved in neuronal function. HuD silencing reverted both mechanical and cold allodynia inducing, a phenotype comparable to that of ryanodine-exposed mice. HuD binding to the RyR2 mRNA, the most abundant RyR isoform in the spinal cord, was demonstrated and RyR2 silencing prevented the ddC-induced neuropathic pain. A positive regulation of gene expression on CaMKIIα by HuD was also observed, but sequestration of CaMKIIα had no effect on ddC-induced allodynia. The present findings identify a spinal RyR2 pathway activated in response to ddC administration, involving the binding activity on RyR2 mRNA by HuD. We propose the modulation of the RyR2 pathway as a therapeutic perspective in the management of antiretroviral painful neuropathy.

The RNA-binding protein HuD promotes spinal GAP43 overexpression in antiretroviral-induced neuropathy.

Nucleoside reverse transcriptase inhibitors (NRTIs) are known to produce painful neuropathies and to enhance states of pain hypersensitivity produced by HIV-1 infection in patients with AIDS leading to discontinuation of antiretroviral therapy, thus limiting viral suppression strategies. The mechanisms by which NRTIs contribute to the development of neuropathic pain are not known. In the current study, we tested the hypothesis that HuD, an RNA binding protein known to be an essential promoter of neuronal differentiation and survival, might be involved in the response to NRTI-induced neuropathy. Antiretroviral neuropathy was induced by a single intraperitoneal administration of 2',3'-dideoxycytidine (ddC) in mice. HuD was physiologically expressed in the cytoplasm of the soma and in axons of neurons within DRG and spinal cord and was considerably overexpressed following ddC treatment. ddC up-regulated spinal GAP43 protein, a marker of neuroregeneration, and this increase was counteracted by HuD silencing. GAP43 and HuD colocalize in DRG and spinal dorsal horn (SDH) axons and administration of an anti-GAP43 antibody aggravated the ddC-induced axonal damage. The administration of a protein kinase C (PKC) inhibitor or the PKCγ silencing prevented both HuD and GAP43 increased expression. Conversely, treatment with the PKC activator PDBu potentiated HuD and GAP43 overexpression, demonstrating the presence of a spinal PKC-dependent HuD-GAP43 pathway activated by ddC. These results indicated that HuD recruitment and GAP43 protein increase are mechanistically linked events involved in the response to antiretroviral-induced neurodegenerative processes.

PKC-mediated HuD-GAP43 pathway activation in a mouse model of antiretroviral painful neuropathy.

Patients treated with nucleoside reverse transcriptase inhibitors (NRTIs) develop painful neuropathies that lead to discontinuation of antiretroviral therapy thus limiting viral suppression strategies. The mechanisms by which NRTIs contribute to the development of neuropathy are not known. In order to elucidate the mechanisms underlying this drug-induced neuropathy, we have characterized cellular events in the central nervous system following antiretroviral treatment. Systemic administration of the antiretroviral agent, 2',3'-dideoxycytidine (ddC) considerably increased the expression and phosphorylation of protein kinase C (PKC) γ and ɛ, enzymes highly involved in pain processes, within periaqueductal grey matter (PAG), and, to a lesser extent, within thalamus and prefrontal cortex. These events appeared in coincidence with thermal and mechanical allodynia, but PKC blockade did not prevent the antiretroviral-induced pain hypersensitivity, ruling out a major involvement of PKC in the ddC-induced nociceptive behaviour. An increased expression of GAP43, a marker of neuroregeneration, and decreased levels of ATF3, a marker of neuroregeneration, were detected in all brain areas. ddC treatment also increased the expression of HuD, a RNA-binding protein target of PKC known to stabilize GAP43 mRNA. Pharmacological blockade of PKC prevented HuD and GAP43 overexpression. Silencing of both PKCγ and HuD reduced GAP43 levels in control mice and prevented the ddC-induced GAP43 enhanced expression. Present findings illustrate the presence of a supraspinal PKC-mediated HuD-GAP43 pathway activated by ddC. Based on our results, we speculate that antiretroviral drugs may recruit the HuD-GAP43 pathway, potentially contributing to a response to the antiretroviral neuronal toxicity.