Background
Infection of the central nervous system with the human immunodeficiency virus type 1 (HIV-1) can lead to cognitive, motor and sensory disorders. HIV-associated sensory neuropathy (HIV-SN) is one of the most common forms of peripheral neuropathy, affecting about 30% of people with acquired immune deficiency syndrome (AIDS) [
1,
2]. The symptoms of HIV-SN are dominated by neuropathic pain [
3,
4]. The mechanisms underlying HIV-SN remain unclear. Astrocytosis and subsequent neuron death are two hallmarks of HIV infection in the central nervous system[
5]. Direct infection of neurons by HIV is thought to be unlikely [
6,
7]; HIV-1 binds via the external envelope proteins (e.g., gp120) to the chemokine receptors CXCR4 and/or CCR5 (co-receptors of gp120) on the cells. Previous reports have suggested that gp120 application contributes to neurotoxicity in
in vitro and nociceptive behaviour in rodents [
8‐
11]. Indeed, it has been demonstrated that gp120 application is capable of producing pain when administered peripherally [
12] or centrally [
13]. Proposed mechanisms underlying gp120 application induced a chronic nociceptive effect included spinal gliosis [
8]. HIV gp120 application might produce such effects indirectly, via an action on glial cells, causing them to release inflammatory cytokines[
13].
Dysregulation of cytokines has been implicated in a variety of painful neurological diseases and in animal models of neuropathic pain. HIV-1 transgenic rats overexpressing gp120 induce reactive gliosis (in the brain), a marker for central nervous system damage [
14]. HIV virus infection is able to increase the production and utilization of several cytokines, such as TNFα and IL-1β [
15]. Cerebrospinal fluid from most of the patients with AIDS has increased levels of TNFα[
16]. A transgenic rat developed using an HIV-1 construct, with deleted gag and pol genes, shows a strikingly high expression of TNFα [
17]. A mouse model of systemic HIV-1 infection increases expression of IL-1β [
18]. The viral gp120 induces the release of TNFα and IL-1β whose interaction have synergistic activities [
19]; TNFα and IL-1β upregulate HIV-1 expression in cells infected by HIV [
20]. This may result in an HIV gp120-cytokines reciprocal amplification with potential deleterious effects (a positive feedback cycles) [
19]. An elevated baseline of TNFα level among HIV-1 positive individuals, may lead to additional neurodegeneration [
21]. However, the role of spinal cytokines in the neuropathic pain induced by gp120 is not clear. In the present study, we investigated the role of TNFα in the neuropathic pain induced by gp120 application into the sciatic nerve.
Discussion
Previous studies indicate that TNFα is involved in the development of chronic pain. There is growing evidence suggesting that glial activation plays an important role in the HIV-sensory neuropathy. The current study showed 1) that HIV gp120 application into the sciatic nerve induced neuropathic pain behavior, and upregulated the expression of spinal GFAP, Iba1, and TNFα; 2) that TNFα was colocalized with either GFAP or Iba1 in the spinal cord, suggesting that TNFα is released from the activated astrocytes or microglia; 3) that gp120 application also induced upregulation of TNFα in the DRG; and 4) that knockdown of TNFα with siRNA or recombinant soluble TNF receptor reversed mechanical allodynia induced by gp120 application.
Neuropathic pain is disorder resulting from damage or alteration to nerve structures in the absence of demonstrated tissue damage. HIV infection might influence the basic neurobiology, neurological morphology, and clinical management of neurological dysfunction [
31‐
33]. The entry of HIV into cells requires the sequential interaction of the viral exterior envelope glycoprotein, gp120 (cleavage of gp160), with the CD4 glycoprotein and chemokine receptors on the cell surface [
34‐
37], facilitating receptor signaling in both the peripheral nervous system and the CNS [
36,
38,
39]. In
in vitro studies, HIV-gp120 binding to Schwann cells through CXCR4 results in the release of RANTES, which induces TNFα production by DRG, and subsequent TNFR1-mediated neurotoxicity in an autocrine/paracrine fashion [
9]. In
in vivo studies, HIV-1 transgenic rats overexpressing gp120 induce reactive gliosis in brain [
14]. Astrocyte activation or astrocytosis may directly contribute to HIV-associated neurological disorders [
40]. Injection of gp120 into the hindpaws produces pain hypersensitivity by directly exciting primary nociceptive neurons [
41]. Intrathecal injection of gp120 recombinant protein induces an acute painful behavior and proinflammatory cytokine release in the spinal cord [
10]. Cerebrospinal fluid from most patients with AIDS shows an increase in TNFα [
16]. The HIV gp120 induces the release of IL-1β and TNFα whose interaction has synergistic activities [
19]. In clinic, TNFα has also been implicated in the pathogenesis of HIV-1 infection, promoting HIV replication in T cell lines and in lymphocytes in HIV-infected patients [
42]. Serum concentrations of TNFα have been shown to increase as HIV-1 infection progresses[
43], suggesting that TNFα may contribute to disease progression. Thus, inhibition of TNFα in the setting of HIV infection has been appealing, at least in theory. However, whether TNFα is involved in the development of neuropathic pain in the HIV/AIDS patients is not clear.
Inflammation of peripheral nerves causes sustained increased electrical activity in the C/Aδ fibers, that leads to transcriptional and post-translational changes in second order neurons in the spinal dorsal horn, that are characteristic of chronic pain [
44]. Evidence indicates that peripheral nerve damage or inflammation, results in the activation of glia in the dorsal horn that plays an important role in the pathogenesis of neuropathic pain [
45‐
47]. After peripheral nerve injury or spinal cord injury TNFα in spinal microglia or astrocytes is increased [
28,
29,
46]. In the current study, we used the peripheral gp120 application model and also found similar results.
In the chronic constriction injury model of peripheral neuropathic pain, neutralizing antibodies to TNF and to TNFR1 reduce thermal hyperalgesia and mechanical allodynia[
48], and intrathecal administration of the recombinant soluble TNFR (sTNFR) peptide (etanercept), prior to selective spinal nerve ligation reduces mechanical allodynia [
40]. Administration of drugs that block the effects of these cytokines [
24,
49] or that block glial activation [
50] can be used to prevent or reverse neuropathic pain, which is consistent with our results. Previous studies have shown that overexpression of spinal TNFα released from microglia and/or astrocytes play an important role in the different neuropathic pain models [
28,
29,
51,
52]. Our current study showed that TNFα in the DRG might also involve neuropathic pain in this model, which is consistent with previous reports [
23,
24].
Transmembrane TNFα, a precursor of the soluble form of TNFα (sTNFα), is expressed on activated macrophages and lymphocytes as well as other cell types (e.g. glia in the CNS). After processed by TNF-alpha-converting enzyme, the soluble form of TNFα is cleaved from transmembrane TNFα and mediates its biological activities[
53]. Although many studies demonstrate increased TNFα mRNA and/or protein in neuropathic pain, to our knowledge, none of those reports demonstrate the release of sTNFα in the spinal cord in models of persistent pain. In our previous studies of neuropathic pain induced by spinal cord injury [
28], spinal nerve ligation[
29], and of inflammatory pain [
30], we have found by Western blot that there is an increase in full-length mTNFα (26 kD) without detectable sTNFα in the spinal dorsal horn. In the current study, we did not observe sTNFα either.
In summary, there is abundant evidence to suggest that one of the important elements is neuroimmune activation of glia and glial products in the spinal cord in the neuropathic pain state [
54‐
56]. While the mechanisms underlying HIV-related neuropathic pain are poorly understood, the results of the current investigation provide an important insight into the pathogenesis of chronic pain. Other targets (e.g., IL-1β, p-p38) will be addressed in the near future.
Competing interests
David Fink receives compensation for professional services from the University of Michigan and from the Department of Veterans Affairs. He also receives payments from the University of Pittsburgh for patents owned by the University on which he is a co-inventor. None of the other authors have received compensation for professional services or anticipate receiving such compensation in the near future.
Authors' contributions
WZ participated in RT-PCR, and Western blot. HO performed the surgery and behavioral testing. XZ was involved in siRNA and behavioral testing with HO. SL carried out sample collection and immunohistochemistry. MM and DF participated in the data analysis and interpretation. SH contributed to the experimental designs, the data analysis and interpretation, and wrote the manuscript. All authors reviewed and approved the final manuscript.