Microglial content-dependent inhibitory effects of calcitonin gene-related peptide (CGRP) on murine retroviral infection of glial cells
Introduction
As more effective antiretroviral therapies become available, human immunodeficiency virus (HIV)-1 infection is no longer represent a symbol of imminent death, but rather a chronic disease associated with a wide range of complications, including painful, HIV-associated peripheral neuropathy (Cornblath and McArthur, 1988, Miller, 1994, Hewitt et al., 1997, Simpson et al., 2002, Ellis et al., 2010). Difficult to diagnose, HIV-associated peripheral neuropathy is often undertreated, which can, partially, be attributed to a lack of understanding of its pathophysiology (Cherry et al., 2005, Cettomai et al., 2013). Consequently, the FDA has not yet approved a treatment for HIV-associated peripheral neuropathy (Ellis et al., 2010).
In line with other studies, we have employed a murine retroviral isolate (LP-BM5) infection model to study HIV-1, as LP-BM5 induces a similar immunodeficiency syndrome (termed MAIDS) in susceptible C57BL/6 (B6) mice (Jolicoeur, 1991). Our current work has shown that B6 mice infected with LP-BM5 displayed behavioral and pathological signs of peripheral neuropathy, both of which were associated with tissue-specific cytokine expression, including the elevation of selected pro-inflammatory cytokines (such as IL-12) in the lumbar spinal cord, post-LP-BM5 infection (Cao et al., 2012).
Glial activation is a well-known contributor to HIV-associated neurological disorders (Kraft-Terry et al., 2009). Although few studies have explored the roles of glial cells in the development of HIV-associated peripheral neuropathy, animal studies have shown that spinal cord glial activation and the subsequent production of pro-inflammatory cytokines, can contribute to the development of HIV-1, gp120-induced sensory hypersensitivity (a behavioral sign of painful peripheral neuropathy) (Milligan et al., 2000, Herzberg and Sagen, 2001, Wallace et al., 2007, Zheng et al., 2011). As with HIV-1 in humans, the LP-BM5 virus can gain access to the central nervous system (CNS), leading to the direct infection of microglia and astrocytes (Sei et al., 1992). Upon viral stimulation, glia have been reported to produce pro-inflammatory cytokines and cytotoxic factors capable of contributing to LP-BM5-induced neuronal death and CNS symptoms (Kustova et al., 1996, Kustova et al., 1998, Suzumura et al., 1998).
Comprised of 37 amino acids, calcitonin gene-related peptide (CGRP) is a member of the calcitonin family and is predominantly produced by primary afferent neurons of small and medium diameters (Arulmani et al., 2004). Following peripheral nerve injury, these subsets of primary afferent neurons have been shown to increase both the release and expression of CGRP in the spinal cord (Gardell et al., 2003, Zheng et al., 2008). The administration of a CGRP8–37, a CGRP antagonist, has been reported to reduce peripheral nerve injury-induced sensory hypersensitivity (Bennett et al., 2000, Lee and Kim, 2007). Furthermore, CGRP receptors have been detected on both microglia and astrocytes. We and others have previously reported on the CGRP-induced production of selected pro-inflammatory cytokines by glia (Priller et al., 1995, Moreno et al., 2002, Malon et al., 2011). Therefore, the current investigation was designed to elucidate the involvement of CGRP in LP-BM5-induced peripheral neuropathy, more specifically, the potential influence of CGRP-induced glial responses in LP-BM5-induced peripheral neuropathy.
Section snippets
Mice
Adult (7 weeks old) male C57BL/6 (B6) mice were purchased from the National Cancer Institute (NCI, Frederick, MD) and were allowed to habituate to the animal facility at the University of New England (UNE, Biddeford, ME) for a minimum of one week prior to experimentation. All mice were group-housed with food and water ad libitum and maintained on a 12-hour light/dark cycle. The Institutional Animal Care and Use Committee at UNE approved all of the experimental procedures for this study.
LP-BM5 virus
LP-BM5
LP-BM5-induced lumbar spinal cord CGRP expression
Previously, we have shown that LP-BM5 infection induces peripheral neuropathy and that mice infected with LP-BM5 display neuropathic pain-like behaviors, i.e., increased hind paw sensitivities to mechanical and heat stimulations (Cao et al., 2012). Due to the pro-nociceptive effects of CGRP found in other animal models of neuropathic pain (Bennett et al., 2000, Gardell et al., 2003, Arulmani et al., 2004, Lee and Kim, 2007, Zheng et al., 2008), we examined CGRP expression in the lumbar spinal
Discussion
Often accompanied by pain, HIV-associated peripheral neuropathy is the most common neurological disorder linked to HIV-1 infection. Yet, the mechanisms responsible for this complication are still poorly understood (Cornblath and McArthur, 1988, Ellis et al., 2010). Previously, we have shown that B6 mice infected with LP-BM5 display sensory hypersensitivity (indicative of painful peripheral neuropathy), along with the development of peripheral immunodeficiency (Cao et al., 2012). It has been
Acknowledgments
The authors would like to thank Dr. William R. Green, Geisel School of Medicine at Dartmouth College, for providing the LP-BM5 viral stock and technical assistance with the establishment of viral stock in our laboratory and the XC plaque assay. The authors would also like to acknowledge Dr. Alejandro MS Mayer, Department of Pharmacology, Chicago College of Osteopathic Medicine for the technical help with obtaining microglia-rich mixed glial cells. Funding for this project is provided by the NIH
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