Human immunodeficiency virus infection, inducible nitric oxide synthase expression, and microglial activation: Pathogenetic relationship to the acquired immunodeficiency syndrome dementia complex

Direct role of plasma membrane-expressed gp120/41 in toxicity to human astrocytes induced by HIV-1-infected macrophages

OBJECTIVE

To compare astrocyte toxicity induced by plasma membrane-expressed gp120/41 and soluble gp120.

DESIGN

Analysis of morphological alterations and lactate dehydrogenase (LDH) release from astrocytes in culture with monocytes infected with HIV-1, microglia expressing Env of a macrophage-tropic HIV-1 isolate or soluble Env.

METHODS

Primary human embryonic astrocytes were cultured with: monocytes infected with two M-tropic HIV-1 isolates (HIV-1(9533), HIV-1(BX08)); human microglia infected with the defective Semliki Forest virus (SFV) vector coding for the env gene of HIV-1(BX08) isolate (SFVenvBX08); and soluble gp140 purified from baby hamster kidney cells transfected with the env gene of HIV-1(BX08) lacking the intracytoplasmic region of gp41 (SFVdelta envBX08). Gp120 mRNA levels were assessed by quantitative reverse transcriptase-polymerase chain reaction and the protein was detected by immunofluorescence in infected monocytes or microglia.

RESULTS

Contact of HIV-infected monocytes induced morphological changes in astrocytes and a 137% increase in LDH release at day 2 of co-culture compared with controls (uninfected monocytes). Gp120/41(BX08)-expressing microglia induced a 170% increase in LDH release (relative to SFVLacZ-infected microglia). Pretreatment of co-cultures with an anti-gp120 monoclonal antibody (mAb; NEA-9305) directed against the V3 loop inhibited LDH release. Soluble purified gp140 from BX08 isolate induced only a weak LDH release (104%). Finally, cytotoxicity was not blocked by treatment of the co-culture with Bordetella pertussis toxin, an inhibitor of Gi alpha protein-dependent receptors.

CONCLUSION

HIV envelope glycoprotein expressed at the plasma membrane induced astrocyte damage more efficiently than its soluble counterpart. The V3 loop was involved in toxicity induction through a pathway independent of the Gi alpha protein-coupled receptor.

Nitric oxide and virus infection

Nitric oxide (NO) has complex and diverse functions in physiological and pathophysiological phenomena. The mechanisms of many events induced by NO are now well defined, so that a fundamental understanding of NO biology is almost established. Accumulated evidence suggests that NO and oxygen radicals such as superoxide are key molecules in the pathogenesis of various infectious diseases. NO biosynthesis, particularly through expression of an inducible NO synthase (iNOS), occurs in a variety of microbial infections. Although antimicrobial activity of NO is appreciated for bacteria and protozoa, NO has opposing effects in virus infections such as influenza virus pneumonia and certain other neurotropic virus infections. iNOS produces an excessive amount of NO for long periods, which allows generation of a highly reactive nitrogen oxide species, peroxynitrite, via a radical coupling reaction of NO with superoxide. Thus, peroxynitrite causes oxidative tissue injury through potent oxidation and nitration reactions of various biomolecules. NO also appears to affect a host's immune response, with immunopathological consequences. For example, overproduction of NO in virus infections in mice is reported to suppress type 1 helper T-cell-dependent immune responses, leading to type 2 helper T-cell-biased immunological host responses. Thus, NO may be a host response modulator rather than a simple antiviral agent. The unique biological properties of NO are further illustrated by our recent data suggesting that viral mutation and evolution may be accelerated by NO-induced oxidative stress. Here, we discuss these multiple roles of NO in pathogenesis of virus infections as related to both non-specific inflammatory responses and immunological host reactions modulated by NO during infections in vivo.

Activation of human microglial cells by HIV-1 gp41 and Tat proteins

The viral proteins, Tat (HIV-1 nuclear protein) and gp41 (HIV-1 coat protein), detected in the brains of HIV-1-infected patients have been shown to be neurotoxic. We investigated the effects of HIV-1 Tat and gp41 proteins on cytokine, chemokine, and superoxide anion (O(-)(2)) production by microglia, the resident macrophages of the brain. Tat and gp41 dose-dependently stimulated cytokine and chemokine production by microglia. Peak production of these cytokines and chemokines differed in microglial cells treated with gp41 and Tat. Expression of cytokine and chemokine mRNA was also stimulated in gp41- and Tat-treated microglia. Neither gp41 nor Tat alone stimulated O(-)(2) production by microglia. Treatment of microglial cells with Tat but not with gp41 evoked an increase in intracellular Ca(2+). The results of this study suggest that HIV-1 Tat and gp41 proteins impact several key functions of microglial cells which could contribute to the neuropathogenesis of HIV-1.

Biophysical characterization of gp41 aggregates suggests a model for the molecular mechanism of HIV-associated neurological damage and dementia

In human immunodeficiency virus (HIV)-infected individuals, the level of the HIV envelope protein gp41 in brain tissue is correlated with neurological damage and dementia. In this paper we show by biochemical methods and electron microscopy that the extracellular ectodomain of purified HIV and simian immunodeficiency virus gp41 (e-gp41) forms a mixture of soluble high molecular weight aggregate and native trimer at physiological pH. The e-gp41 aggregate is shown to be largely alpha-helical and relatively stable to denaturants. The high molecular weight form of e-gp41 is variable in size ranging from 7 to 70 trimers, which associate by interactions at the interior of the aggregate involving the loop that connects the N- and C-terminal helices of the e-gp41 core. The trimers are predominantly arranged with their long axes oriented radially, and the width of the high molecular weight aggregate corresponds to the length of two e-gp41 trimers (approximately 200 A). Using both light and electron microscopy combined with immunohistochemistry we show that HIV gp41 accumulates as an extracellular aggregate in the brains of HIV-infected patients diagnosed with dementia. We postulate that the high molecular weight aggregates of e-gp41 are responsible for HIV-associated neurological damage and dementia, consistent with known mechanisms of encephalopathy.

Clinical course of cardiomyopathy in HIV-infected patients with or without encephalopathy related to the myocardial expression of tumour necrosis factor-alpha and nitric oxide synthase

OBJECTIVE

To define whether the development of encephalopathy influences the clinical course of HIV-associated cardiomyopathy (HIV-DCM) in relation to the myocardial expression of tumour necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS).

DESIGN

Prospective study.

SETTING

University hospitals and AIDS centres.

METHODS

115 HIV-infected patients with echocardiographic diagnosis of HIV-associated cardiomyopathy (34 with encephalopathy and 81 without encephalopathy) were followed for a mean of 24 +/- 3.2 months. All patients underwent endomyocardial biopsy for determination of myocardial immunostaining intensity of TNF-alpha and iNOS. Cerebrospinal fluid (CSF) from patients with encephalopathy was examined for the presence of viruses. Patients underwent clinical examination every 3 months and echocardiographic examination every 6 months. The intensity of TNF-alpha and iNOS immunostaining was also evaluated on postmortem cerebral tissue of patients who died of congestive heart failure (CHF).

RESULTS

A greater impairment of echocardiographic parameters was observed in patients with HIV-associated cardiomyopathy after development of encephalopathy. These parameters tended to worsen progressively during the follow-up period and were inversely correlated with HIV-1 viral load, CD4 cell count, mini mental status score and the intensity of myocardial and cerebral TNF-alpha and iNOS staining. CSF specimens were available in 29 patients with encephalopathy. HIV-1 sequences were detected in CSF of all these patients with cytomegalovirus sequences in two. The mortality rate for CHF was greater among patients with encephalopathy (73% versus 12%).

CONCLUSIONS

The development of encephalopathy has an adverse effect on the clinical course of HIV-associated cardiomyopathy. In the relationship between cardiomyopathy and encephalopathy, the activation of iNOS by TNF-alpha may have a significant pathogenetic role in HIV disease.

Implications of inducible nitric oxide synthase expression and enzyme activity

We summarize here our current knowledge about inducible nitric oxide synthase (NOS) activity in human diseases and disorders. As basic research discovers more and more effects of low or high concentrations of NO toward molecular and cellular targets, successful therapies involving inhibition of NO synthesis or application of NO to treat human diseases are still lacking. This is in part due to the fact that the impact of NO on cell function or death are complex and often even appear to be contradictory. NO may be cytotoxic but may also protect cells from a toxic insult; it is apoptosis-inducing but also exhibits prominent anti-apoptotic activity. NO is an antioxidant but may also compromise the cellular redox state via oxidation of thiols like glutathione. NO may activate specific signal transduction pathways but is also reported to inhibit exactly these, and NO may activate or inhibit gene transcription. The situation may even be more complicated, because NO, depending on its concentration, may react with oxygen or the superoxide anion radical to yield reactive species with a much broader chemical reaction spectrum than NO itself. Thus, the action of NO during inflammatory reactions has to be considered in the context of timing and duration of its synthesis as well as stages and specific events in inflammation.