Time-Dependent, HIV-Tat-Induced Perturbation of Human Neurons In Vitro: Towards a Model for the Molecular Pathology of HIV-Associated Neurocognitive Disorders

Transactivator of Transcription (Tat)-Induced Neuroinflammation as a Key Pathway in Neuronal Dysfunction: A Scoping Review

The activity of HIV-1 and its viral proteins within the central nervous system (CNS) is responsible for a wide array of neuropathological effects, resulting in a spectrum of neurocognitive deficits defined as HIV-associated neurocognitive disorders (HAND). Amongst the various viral proteins, the transactivator of transcription (Tat) remains detectable even with effective antiretroviral therapy (ART) and suppressed viremia, highlighting the significance of this protein in the modern ART era. Tat has been extensively researched in both fundamental and clinical settings due to its role in neuroinflammation, neuronal damage, and neurocognitive impairment amongst people living with HIV (PLHIV). To date, numerous fundamental studies have explored Tat-induced neuroinflammation. However, there is no clear consensus on the most frequently studied inflammatory markers or the consistency in the levels of these Tat-induced inflammatory marker levels across different studies. Therefore, we conducted a scoping review of studies investigating Tat-induced neuroinflammation. We conducted searches in PubMed, Scopus, and Web of Science databases using a search protocol tailored specifically to adhere to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses for scoping reviews (PRISMA-ScR) guidelines. From the 22 included studies, findings suggest that the HIV-1 Tat protein amplifies levels of neuroinflammatory markers. Amongst the vast array of inflammatory markers explored in the included studies, consistent results point to higher levels of CCL2, IL-6, IL-8, and TNF-α in primary cells and cell lines exposed to or transfected with HIV-1 Tat. These markers are regulated by key inflammatory pathways, such as the extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein kinase (MAPK) pathway, the phosphatidylinositol 3-kinase (PI3K) pathway, the p38 MAPK pathway, and nuclear factor-kB (NF-kB). Furthermore, Tat has been shown to induce neuronal apoptosis, both directly and indirectly. With regards to study designs, utilizing full-length Tat101 at concentrations ranging from 100 to 1000 ng/ml and durations of 24 and 48 h appears optimal for investigating Tat-induced neuroinflammation. In this context, we highlight specific inflammatory markers and pathways that are potentially pivotal in Tat-induced neuroinflammation and subsequent neuronal damage. A deeper investigation into these markers and pathways is crucial to better understand their roles in the development of HAND.

Modulation of OPRM1 Alternative Splicing by Morphine and HIV-1 Nef

Clinically used opioids, such as morphine, activate the mu opioid receptor (MOR) encoded by Opioid Receptor Mu 1 (OPRM1) gene. Examination of the opioid receptor genes showed that the human OPRM1 pre-mRNA undergoes extensive alternative splicing events and capable of expressing 21 isoforms. However, characterization of OPRM1 signaling is generalized, and only one isoform (MOR-1) has been extensively studied. Compounding this issue is the increasing significance of intravenous drug abuse in HIV neuropathogenesis. Here, we investigated the molecular impact of morphine and HIV-1 on regulation of OPRM1 pre-mRNA splicing in in vitro and in vivo models. Our results suggested that morphine treatment specifically induces the alternative splicing of MOR-1X isoform among the other isoforms analyzed in neuronal cells. Interestingly, alternative splicing and expression of MOR-1X isoform was also induced in postmortem brain tissues obtained from people with HIV (PWH). Additionally, treatment of control rats with morphine induced alternative splicing of MOR-1X in the brain regions involved in the reward pathways. More interestingly, HIV-1 transgenic (HIV-1Tg) rats, showed an additive induction of MOR-1X isoform with the exposure to morphine. To further assess the possible role of HIV secretory proteins in alternative splicing of OPRM1 gene, we analyzed the impact of HIV-1 Tat, gp120 and Nef proteins on alternative splicing of MOR-1X isoform. While the Tat and gp120 had no visible effects, treatment of neurons with Nef induced MOR-1X alternative splicing that was comparable to treatment with morphine. Altogether, our results suggest that HIV-1 may alter MOR isoform expression with Nef protein by amplifying the rate of MOR-1X alternative splicing induced by morphine.

HIV-1 Establishes a Sanctuary Site in the Testis by Permeating the BTB Through Changes in Cytoskeletal Organization

Studies suggest that HIV-1 invades the testis through initial permeation of the blood-testis barrier (BTB). The selectivity of the BTB to antiretroviral drugs makes this site a sanctuary for the virus. Little is known about how HIV-1 crosses the BTB and invades the testis. Herein, we used 2 approaches to examine the underlying mechanism(s) by which HIV-1 permeates the BTB and gains entry into the seminiferous epithelium. First, we examined if recombinant Tat protein was capable of perturbing the BTB and making the barrier leaky, using the primary rat Sertoli cell in vitro model that mimics the BTB in vivo. Second, we used HIV-1-infected Sup-T1 cells to investigate the activity of HIV-1 infection on cocultured Sertoli cells. Using both approaches, we found that the Sertoli cell tight junction permeability barrier was considerably perturbed and that HIV-1 effectively permeates the BTB by inducing actin-, microtubule-, vimentin-, and septin-based cytoskeletal changes in Sertoli cells. These studies suggest that HIV-1 directly perturbs BTB function, potentially through the activity of the Tat protein.

Neuromodulation of BAG co-chaperones by HIV-1 viral proteins and H2O2: implications for HIV-associated neurological disorders

Despite increasing numbers of aged individuals living with HIV, the mechanisms underlying HIV-associated neurological disorders (HANDs) remain elusive. As HIV-1 pathogenesis and aging are characterized by oxidative stress as well as altered protein quality control (PQC), reactive oxygen species (ROS) themselves might constitute a molecular mediator of neuronal PQC by modulating BCL-2 associated athanogene (BAG) family members. Present results reveal H2O2 replicated and exacerbated a reduction in neuronal BAG3 induced by the expression of HIV-1 viral proteins (i.e., Tat and Nef), while also causing an upregulation of BAG1. Such a reciprocal regulation of BAG3 and BAG1 levels was also indicated in two animal models of HIV, the doxycycline-inducible Tat (iTat) and the Tg26 mouse. Inhibiting oxidative stress via antioxidants in primary culture was capable of partially preserving neuronal BAG3 levels as well as electrophysiological functioning otherwise altered by HIV-1 viral proteins. Current findings indicate HIV-1 viral proteins and H2O2 may mediate neuronal PQC by exerting synergistic effects on complementary BAG family members, and suggest novel therapeutic targets for the aging HIV-1 population.

HIV-1C and HIV-1B Tat protein polymorphism in Southern Brazil

The transactivator of transcription (Tat) is a key HIV regulatory protein. We aimed to identify the frequency of key polymorphisms in HIV-1C compared with HIV-1B Tat protein, chiefly in the cysteine-, arginine-, and glutamine-rich domains and identify novel point mutations in HIV-1B and C sequences from Southern Brazil. This study was the first to investigate the genetic diversity and point mutations within HIV-1 Tat C in a Brazilian cohort. This was an observational, cross-sectional study, which included sequences of HIV-1B (n = 20) and HIV-1C (n = 21) from Southern Brazil. Additionally, 344 HIV-1C sequences were obtained from the Los Alamos database: 29 from Brazil and 315 from Africa, Asia, and Europe. The frequency of C31S substitution on HIV-1 Tat C in Brazil was 82% vs. 10% in the HIV-1B group (p < 0.0001). The frequency of the R57S substitution among the HIV-1C sequences from Brazil was 74% vs. 20% in HIV-1B (p = 0.004), and that of substitution Q63E in HIV-1C was 80% and 20% in HIV-1B (p < 0.0001). The mutation P60Q was more frequent in HIV-1B than in HIV-1C (55% and 6.12%, respectively, p < 0.0001)). Novel point mutations in the HIV-1C and B Tat functional domains were described. The frequency of C31S and other key point mutations in HIV-1 Tat C in Brazil were similar to those described in Africa, although lower than those in India. The Tat-B and C sequences found in Southern Brazil are consistent with biological differences and have potential implications for HIV-1 subtype pathogenesis.

Neurocytoskeleton Proteins in Cerebrospinal Fluid of People With HIV-1 Subtypes B and C

BACKGROUND

The objective was to compare the effect of HIV-1C and HIV-1B subtypes on neurofilament light (NfL) cerebrospinal fluid (CSF) levels and ratios of NfL to tau proteins. Additional comparisons were performed between people with HIV (PWH), participants with Alzheimer disease (AD), and HIV-negative controls (HIV-). We also calculated the diagnostic characteristics of CSF NfL and its ratios in HIV-associated neurocognitive disorder (HAND) diagnosis.

METHODS

CSF NfL, T-tau, and P-tau181 concentrations were measured using immunoassays in a total of 108 CSF samples, including PWH (n = 68), HIV- (n = 16), and participants with AD (n = 24). These proteins were compared between HIV-1B (n = 27) and HIV-1C (n = 26) using multiple linear regression adjusted for nadir CD4 and plasma viral load suppression. Comparisons between PWH, HIV-, and participants with AD were adjusted for gender and age.

RESULTS

CSF neurocytoskeleton proteins and their ratios were comparable in HIV-1B and HIV-1C. However, the HIV-1C group had a higher proportion of samples of CSF NfL above the reference value (n = 14, 53.85%) than the HIV-1B group (n = 8, 29.63%), P = 0.098. The values of CSF NfL were higher in the AD group [2578 (1864; 3500) pg/mL] than those in PWH [683 (500; 1197) pg/mL, P < 0.001] and control [660 (539; 802) pg/mL, P = 0.012] groups. The value of CSF NfL and its ratios for HAND diagnosis were poor.

CONCLUSION

The effects of HIV-1B and HIV-1C on CSF NfL and tau ratios were comparable. The differences in CSF neurocytoskeleton proteins between PWH and individuals with AD suggested they might not share the same mechanisms of impairment. Further research is necessary to evaluate CSF NfL on the differential diagnoses of HAND with AD.

HIV-1 Tat alters neuronal intrinsic excitability

Objective

In HIV+ individuals, the virus enters the central nervous system and invades innate immune cells, producing important changes that result in neurological deficits. We aimed to determine whether HIV plays a direct role in neuronal excitability. Of the HIV peptides, Tat is secreted and acts in other cells. In order to examine whether the HIV Tat can modify neuronal excitability, we exposed primary murine hippocampal neurons to that peptide, and tested its effects on the intrinsic membrane properties, 4 and 24 h after exposure.

Results

The exposure of hippocampal pyramidal neurons to Tat for 4 h did not alter intrinsic membrane properties. However, we found a strong increase in intrinsic excitability, characterized by increase of the slope (Gain) of the input–output function, in cells treated with Tat for 24 h. Nevertheless, Tat treatment for 24 h did not alter the resting membrane potential, input resistance, rheobase and action potential threshold. Thus, neuronal adaptability to Tat exposure for 24 h is not applicable to basic neuronal properties. A restricted but significant effect on coupling the inputs to the outputs may have implications to our knowledge of Tat biophysical firing capability, and its involvement in neuronal hyperexcitability in neuroHIV.