HIV-tat alters Connexin43 expression and trafficking in human astrocytes: role in NeuroAIDS

Abstract

BackgroundHIV-associated neurocognitive disorders (HAND) are a major complication in at least half of the infected population despite effective antiretroviral treatment and immune reconstitution. HIV-associated CNS damage is not correlated with active viral replication but instead is associated with mechanisms that regulate inflammation and neuronal compromise. Our data indicate that one of these mechanisms is mediated by gap junction channels and/or hemichannels. Normally, gap junction channels shutdown under inflammatory conditions, including viral diseases. However, HIV infection upregulates Connexin43 (Cx43) expression and maintains gap junctional communication by unknown mechanism(s).MethodsHuman primary astrocytes were exposed to several HIV proteins as well as to HIV, and expression and function of Connexin43- and Connexin30-containing channels were determined by western blot, immunofluorescence, microinjection of a fluorescent tracer and chromatin immunoprecipitation (ChIP).ResultsHere, we demonstrate that HIV infection increases Cx43 expression in vivo. HIV-tat, the transactivator of the virus, and no other HIV proteins tested, increases Cx43 expression and maintains functional gap junctional communication in human astrocytes. Cx43 upregulation is mediated by binding of the HIV-tat protein to the Cx43 promoter, but not to the Cx30 promoter, resulting in increased Cx43 messenger RNA (mRNA) and protein as well as gap junctional communication.ConclusionsWe propose that HIV-tat contributes to the spread of intracellular toxic signals generated in a few HIV-infected cells into surrounding uninfected cells by upregulating gap junctional communication. In the current antiretroviral era, where HIV replication is often completely suppressed, viral factors such as HIV-tat are still produced and released from infected cells. Thus, blocking the effects of HIV-tat could result in new strategies to reduce the damaging consequences of HIV infection of the CNS.

Mechanisms of HIV Neuropathogenesis: Role of Cellular Communication Systems

“…In addition, we showed that HIV infection of astrocytes leads to opening to connexin43 hemichannels, which results in secretion of ATP via purinergic receptors and aids in increasing neuroinflammation [36]. Interestingly, HIV-Tat can also lead to enhanced connexin43 expression in primary human astrocytes [37]. Hence, we postulated that gap junctional proteins, connexins as well as pannexins, play an important role in amplifying HIV associated toxicity in the CNS [7, 9, 10].…”

“…However, in case of HIV, the expression of Cx43 and subsequently intercellular communication by gap junction channels is maintained in astrocytes. It has been shown that HIV infection in astrocytes upregulates Cx43 expression in infected cells and functional gap junction channels are maintained to preserve active communication with nearby non-infected cells [9, 37]. Moreover, functional gap junction channels are required to spread apoptotic signals to uninfected cells (neurons as well as astrocytes), as blocking gap junction communication led to reduced cell death in response to HIV infection.…”

“…Recent findings from our laboratory suggest that HIV-Tat can also lead to enhanced expression of Cx43 in primary human astrocytes by directly binding to its promoter [37]. Whereas other viral proteins (Vif, Gag, Rev, Nef) did not alter Cx43 expression, gp120 negatively regulated Cx43 levels in astrocytes [37].…”

“…Whereas other viral proteins (Vif, Gag, Rev, Nef) did not alter Cx43 expression, gp120 negatively regulated Cx43 levels in astrocytes [37]. Furthermore, exposure of HIV-Tat led to increased gap junctional communication in astrocytes, suggesting that HIV-Tat can also contribute to the propagation of toxic stimuli generated within a few infected astrocytes to adjacent uninfected cells [37].…”

“…Whereas other viral proteins (Vif, Gag, Rev, Nef) did not alter Cx43 expression, gp120 negatively regulated Cx43 levels in astrocytes [37]. Furthermore, exposure of HIV-Tat led to increased gap junctional communication in astrocytes, suggesting that HIV-Tat can also contribute to the propagation of toxic stimuli generated within a few infected astrocytes to adjacent uninfected cells [37]. Since the production and secretion of HIV-Tat continues independent of effective viral replication even in the presence of anti-retrovirals, it is highly likely that HIV-Tat contributes to amplification of virus mediated neuropathology via gap junctions.…”