The Therapeutic Effects And Mechanisms Of IGF-1 On Gp120 And Dideoxycytidine-induced Peripheral Neuropathy

Human immunodeficiency virus (HIV)-associated sensory neuropathy is the most frequent neurological complication of HIV infection affecting up to 50% of HIV-positive individuals or acquired immunodeficiency syndrome (AIDS) patients. HIV-associated sensory neuropathy is caused not only by HIV infection itself, but also by antiretroviral therapy (ART) or highly active antiretroviral therapy (HAART). ART has greatly extended the lifespan of people living with HIV infection. In the era of HAART, HIV-associated sensory neuropathies have increased in prevalence. HIV-related neuropathic pain is a debilitating chronic condition that is severe and unrelenting and severely affects the quality of life of HIV infected patients. HIV-associated sensory neuropathy is one of the main causes of morbidity for HIV patients. Despite extensive research, the detailed neuropathological mechanisms remain unknown, which hinders our ability to develop effective treatments. It hence behooves us to find new approaches to mitigate the neuropathic pain and residual neurological morbidity that still impacts the quality of life of that population. In the present study, both primary cultured dorsal root ganglion (DRG) neurons with gp120 and/or 2’,3’-dideoxycytidine (zalcitabine, ddC) induced neurotoxicity or neuropathic pain rat model with gp120 and ddC were used to determine the neurotoxicity of gp120 and/or ddC. The neuroprotective effects of insulin-like growth factor-1 (IGF-1) on DRG neurons with gp120- and/or ddC-induced neurotoxicity were also determined.Part I The protective effects of IGF-1 on DRG neurons with neurotoxicity induced by gp120HIV envelope glycoprotein gp120 is the main protein that causes HIV-associated sensory neuropathy. However, the underlying mechanisms of gp120-induced neurotoxicity are still unclear. There are lack effective treatments for relieving HIV-related neuropathic symptoms caused by gp120-induced neurotoxicity. In the present study, the expression of distinct tyrosine kinase receptor (Trk)A, TrkB, and TrkC in primary cultured DRG neurons with gp120-induced neurotoxicity was investigated. The neuroprotective effects of IGF-1 on distinct Trk-positive DRG neurons with gp120-induced neurotoxicity were also determined. The results showed that gp120 caused a dose-dependent decrease of TrkA, TrkB, and TrkC expression. IGF-1 could partially reverse the decrease of TrkA and TrkB, but not TrkC, expression after gp120 treatment. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 blocked the effects of IGF-1. These results suggested that the subpopulations of DRG neurons which express distinct TrkA, TrkB, and TrkC receptors were affected by gp120 treatment. IGF-1 might relieve the gp120-induced neurotoxicity of TrkA- and TrkB-, but not TrkC-positive DRG neurons. These data imply that IGF-1 only targets the subpopulations of DRG neurons expressing distinct TrkA and TrkB which may relate to pain signaling transmission.Part II The protective effects of IGF-1 on DRG neurons with neurotoxicity induced by dideoxycytidineThe use of nucleoside reverse transcriptase inhibitors (NRTIs) has revolutionized the treatment of HIV infection. NRTIs can suppress viral replication in the long-term, but possess significant toxicity that can seriously compromise treatment effectiveness. NRTIs are known to produce painful neuropathies and to enhance states of pain hypersensitivity produced by HIV infection patients leading to discontinuation of antiretroviral therapy, thus limiting viral suppression strategies. Antiretroviral toxicity is an increasingly important issue in the management of HIV-infected patients. ddC is one of the NRTIs used to treatment of HIV infection. It is particularly important to understand the toxic effects of ddC on different subpopulations of DRG neurons which express distinct Trk receptors and to find therapeutic factors for the prevention and therapy for ddC-induced peripheral sensory neuropathy. IGF-1 has been shown to have neurotrophic effects on DRG sensory neurons. However, little is known about the effects of ddC on distinct Trk (TrkA, TrkB, and TrkC) expression in DRG neurons and the neuroprotective effects of IGF-1 on ddC-induced neurotoxicity. Here, we have tested the extent to which the expression of TrkA, TrkB, and TrkC receptors in primary cultured DRG neurons is affected by ddC in the presence or absence of IGF-1. In this experiment, we found that ddC caused a dose-dependent decrease of the mRNA, protein, and the proportion of TrkA-, TrkB-, and TrkC-expressing neurons. IGF-1 could partially reverse the decrease of TrkA and TrkB, but not TrkC, expression with ddC exposure. The PI3K inhibitor LY294002 blocked the effects of IGF-1. These results suggested that the subpopulations of DRG neurons which express distinct TrkA, TrkB, and TrkC receptors were affected by ddC exposure. IGF-1 might relieve the ddC-induced toxicity of TrkA- and TrkB-, but not TrkC-expressing DRG neurons. These data offer new clues for a better understanding of the association of ddC with distinct Trk receptor expression and provide new evidence of the potential therapeutic role of IGF-1 on ddC-induced neurotoxicity.Part III The protective effects of IGF-1 on different subpopulations of DRG neurons with neurotoxicity induced by gp120 and dideoxycytidineHIV infection caused a frequent neurological complication that is HIV-associated sensory neuropathy. Although highly effective, current antiretroviral therapies are associated with a variety of side effects including HIV-associated sensory neuropathy. Peripheral neuropathy induced by HIV infection and antiretroviral therapy is not only difficult to distinguish in clinical practice, but also difficult to relieve the pain symptoms by analgesics because of the severity of the disease at the later stage. Improved management of these side effects or development of treatments with a better side effect profile may have a substantial humanistic benefit. Hence, to explore the mechanisms of HIV-related neuropathy and find new therapeutic options are particularly important for relieving neuropathic pain symptoms of the patients. In the present study, primary cultured embryonic rat DRG neurons were used to determine the neurotoxic effects of HIV-gp120 protein and/or antiretroviral drug ddC and the therapeutic actions of IGF-1 on gp120- or ddC-induced neurotoxicity. The results showed that gp120 and/or ddC caused neurotoxicity of primary cultured DRG neurons. Interestingly, the severity of neurotoxicity induced by gp120 and ddC was different in different subpopulation of DRG neurons. gp120 mainly affected large diameter DRG neurons (>25 μm), whereas ddC mainly affected small diameter DRG neurons (≤25μm). IGF-1 could reverse the neurotoxicity induced by gp120 and/or ddC on small, but not large, DRG neurons. These data provide new insights in elucidating the pathogenesis of HIV infection- or antiretroviral therapy-related peripheral neuropathy and facilitating the development of novel treatment strategies.Part IV The therapeutic effects of IGF-1 on gp120 and dideoxycytidine-induced peripheral neuropathyIt is now clear that HIV-associated neuropathy is caused predominantly by the neurotoxic effects of ART, and may also be caused by the HIV itself. With a sizeable morbidity, the neuropathic pain caused by HIV-associated neuropathy is very difficult to manage; it is often necessary to change the ART regimen before deciding upon the putative role of HIV infection itself. If the change does not improve the pain, there are few options available; the most common drugs used for neuropathic pain are usually not effective. Hence, to evaluate the toxicity profile of gpl20 and currently used antiretroviral drug ddC is particularly important for interpretation the mechanisms of HIV-associated neuropathy. In the present study, neuropathic rat animal model was established by a combination administration of gp120 and ddC. The effects of targeting substance P (SP) and its neurokinin 1 (NK1) receptor signaling on neuropathic pain behaviors were tested, and the effect IGF-1 signaling was also determined. The results showed that a combination administration of gp120 and ddC caused obvious mechanical allodynia. Intrathecal injection of NK1 inhibitor L-733060 or IGF-1 could relieve gpl20 and ddC-induced mechanical allodynia. A combination administration of gpl20 and ddC not only induced elevation of preprotachykinin (PPT) mRNA and SP levels, but also promoted neuronal apoptosis. Intrathecal injection of NKl inhibitor L-733060 partially reversed PPT mRNA and SP elevation, but did not inhibit neuronal apoptosis induced by gp120 and ddC. Intrathecal injection of IGF-1 not only partially reversed PPT mRNA and SP elevation, but also inhibited neuronal apoptosis induced by gpl20 and ddC. These results imply that intervention of SP and its NKl receptor signaling which affects the pain transmission signaling system leading to development of peripheral neuropathic pain and might provide new targets for future therapeutic interventions. Activation of IGF-1 related signaling benefits improving HIV-associated neuropathy. While this study does not directly indicate that IGF-1 may contribute to reducing human HIV-associated neuropathic symptoms, it does suggest that additional research is warranted along this line. Exploration of the mechanisms and manipulation on HIV-associated neuropathic pain sensitivity by interfering several sensory modalities is orientated for HIV-associated neuropathic pain relieving.