[3H]Dopamine Uptake through the Dopamine and Norepinephrine Transporters is Decreased in the Prefrontal Cortex of Transgenic Mice Expressing HIV-1 Transactivator of Transcription Protein

The PM20D1-NADA pathway protects against Parkinson's disease

Parkinson's disease (PD) is characterized by the selective loss of dopaminergic neurons in the substantia nigra and the accumulation of α-synuclein (α-Syn) aggregates. However, the molecular mechanisms regulating α-Syn aggregation and neuronal degeneration remain poorly understood. The peptidase M20 domain containing 1 (PM20D1) gene lies within the PARK16 locus genetically linked to PD. Single nucleotide polymorphisms regulating PM20D1 expression are associated with changed risk of PD. Dopamine (DA) metabolism and DA metabolites have been reported to regulate α-Syn pathology. Here we report that PM20D1 catalyzes the conversion of DA to N-arachidonoyl dopamine (NADA), which interacts with α-Syn and inhibits its aggregation. Simultaneously, NADA competes with α-Syn fibrils to regulate TRPV4-mediated calcium influx and downstream phosphatases, thus alleviating α-Syn phosphorylation. The expression of PM20D1 decreases during aging. Overexpression of PM20D1 or the administration of NADA in a mouse model of synucleinopathy alleviated α-Syn pathology, dopaminergic neurodegeneration, and motor impairments. These observations support the protective effect of the PM20D1-NADA pathway against the progression of α-Syn pathology in PD.

Effects of SRI-32743, a Novel Quinazoline Structure-Based Compound, on HIV-1 Tat and Cocaine Interaction with Norepinephrine Transporter

Prolonged exposure to HIV-1 transactivator of transcription (Tat) protein dysregulates monoamine transmission, a physiological change implicated as a key factor in promoting neurocognitive disorders among people living with HIV. We have demonstrated that in vivo expression of Tat in Tat transgenic mice decreases dopamine uptake through both dopamine transporter (DAT) and norepinephrine transporter (NET) in the prefrontal cortex. Further, our novel allosteric inhibitor of monoamine transporters, SRI-32743, has been shown to attenuate Tat-inhibited dopamine transport through DAT and alleviates Tat-potentiated cognitive impairments. The current study reports the pharmacological profiles of SRI-32743 in basal and Tat-induced inhibition of human NET (hNET) function. SRI-32743 exhibited less affinity for hNET binding than desipramine, a classical NET inhibitor, but displayed similar potency for inhibiting hDAT and hNET activity. SRI-32743 concentration-dependently increased hNET affinity for [3H]DA uptake but preserved the Vmax of dopamine transport. SRI-32743 slowed the cocaine-mediated dissociation of [3H]Nisoxetine binding and reduced both [3H]DA and [3H]MPP+ efflux but did not affect d-amphetamine-mediated [3H]DA release through hNET. Finally, we determined that SRI-32743 attenuated a recombinant Tat1-86-induced decrease in [3H]DA uptake via hNET. Our findings demonstrated that SRI-32743 allosterically disrupts the recombinant Tat1-86-hNET interaction, suggesting a potential treatment for HIV-infected individuals with concurrent cocaine abuse.

HIV-1 Tat and morphine interactions dynamically shift striatal monoamine levels and exploratory behaviors over time

Despite the advent of combination anti-retroviral therapy (cART), nearly half of people infected with HIV treated with cART still exhibit HIV-associated neurocognitive disorders (HAND). HAND can be worsened by co-morbid opioid use disorder. The basal ganglia are particularly vulnerable to HIV-1 and exhibit higher viral loads and more severe pathology, which can be exacerbated by co-exposure to opioids. Evidence suggests that dopaminergic neurotransmission is disrupted by HIV exposure, however, little is known about whether co-exposure to opioids may alter neurotransmitter levels in the striatum and if this in turn influences behavior. Therefore, we assayed motor, anxiety-like, novelty-seeking, exploratory, and social behaviors, and levels of monoamines and their metabolites following 2 weeks and 2 months of Tat and/or morphine exposure in transgenic mice. Morphine decreased dopamine levels, but significantly elevated norepinephrine, the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin metabolite 5-hydroxyindoleacetic acid, which typically correlated with increased locomotor behavior. The combination of Tat and morphine altered dopamine, DOPAC, and HVA concentrations differently depending on the neurotransmitter/metabolite and duration of exposure but did not affect the numbers of tyrosine hydroxylase-positive neurons in the mesencephalon. Tat exposure increased the latency to interact with novel conspecifics, but not other novel objects, suggesting the viral protein inhibits exploratory behavior initiation in a context-dependent manner. By contrast, and consistent with prior findings that opioid misuse can increase novelty-seeking behavior, morphine exposure increased the time spent exploring a novel environment. Finally, Tat and morphine interacted to affect locomotor activity in a time-dependent manner, while grip strength and rotarod performance were unaffected. Together, our results provide novel insight into the unique effects of HIV-1 Tat and morphine on monoamine neurochemistry that may underlie their divergent effects on motor and exploratory behavior.

The Impact of Neurotransmitters on the Neurobiology of Neurodegenerative Diseases

Neurodegenerative diseases affect millions of people worldwide. Neurodegenerative diseases result from progressive damage to nerve cells in the brain or peripheral nervous system connections that are essential for cognition, coordination, strength, sensation, and mobility. Dysfunction of these brain and nerve functions is associated with Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic lateral sclerosis, and motor neuron disease. In addition to these, 50% of people living with HIV develop a spectrum of cognitive, motor, and/or mood problems collectively referred to as HIV-Associated Neurocognitive Disorders (HAND) despite the widespread use of a combination of antiretroviral therapies. Neuroinflammation and neurotransmitter systems have a pathological correlation and play a critical role in developing neurodegenerative diseases. Each of these diseases has a unique pattern of dysregulation of the neurotransmitter system, which has been attributed to different forms of cell-specific neuronal loss. In this review, we will focus on a discussion of the regulation of dopaminergic and cholinergic systems, which are more commonly disturbed in neurodegenerative disorders. Additionally, we will provide evidence for the hypothesis that disturbances in neurotransmission contribute to the neuronal loss observed in neurodegenerative disorders. Further, we will highlight the critical role of dopamine as a mediator of neuronal injury and loss in the context of NeuroHIV. This review will highlight the need to further investigate neurotransmission systems for their role in the etiology of neurodegenerative disorders.

Advancing the preclinical study of comorbid neuroHIV and substance use disorders: Current perspectives and future directions

Human immunodeficiency virus (HIV) remains a persistent public health concern throughout the world. Substance use disorders (SUDs) are a common comorbidity that can worsen treatment outcomes for people living with HIV. The relationship between HIV infection and SUD outcomes is likely bidirectional, making clear interrogation of neurobehavioral outcomes challenging in clinical populations. Importantly, the mechanisms through which HIV and addictive drugs disrupt homeostatic immune and CNS function appear to be highly overlapping and synergistic within HIV-susceptible reward and motivation circuitry in the central nervous system. Decades of animal research have revealed invaluable insights into mechanisms underlying the pathophysiology SUDs and HIV, although translational studies examining comorbid SUDs and HIV are very limited due to the technical challenges of modeling HIV infection preclinically. In this review, we discuss preclinical animal models of HIV and highlight key pathophysiological characteristics of each model, with a particular emphasis on rodent models of HIV. We then review the implementation of these models in preclinical SUD research and identify key gaps in knowledge in the field. Finally, we discuss how cutting-edge behavioral neuroscience tools, which have revealed key insights into the neurobehavioral mechanisms of SUDs, can be applied to preclinical animal models of HIV to reveal potential, novel treatment avenues for comorbid HIV and SUDs. Here, we argue that future preclinical SUD research would benefit from incorporating comorbidities such as HIV into animal models and would facilitate the discovery of more refined, subpopulation-specific mechanisms and effective SUD prevention and treatment targets.

Novel Allosteric Modulator Southern Research Institute-32743 Reverses HIV-1 Transactivator of Transcription-Induced Increase in Dopamine Release in the Caudate Putamen of Inducible Transactivator of Transcription Transgenic Mice

Development of neurocognitive disorder in human immunodeficiency virus (HIV)-infected patients has been linked to dysregulation of dopamine by the HIV-1 transactivator of transcription (Tat) protein, a negative allosteric modulator of dopamine transporter (DAT). Using fast scan cyclic voltammetry, the present study determined the effects of in vivo Tat expression on dopamine release in the caudate putamen of inducible Tat transgenic (iTat-tg) mice and the impact of a novel DAT allosteric modulator, Southern Research Institute (SRI)-32743, on the Tat effect. We found that 7- or 14-day doxycycline (Dox)-induced Tat expression in iTat-tg mice resulted in a 2-fold increase in phasic but not tonic stimulated baseline dopamine release relative to saline control mice. To determine whether the Tat-induced increase in dopamine release is mediated by DAT regulation, we examined the effect of an in vitro applied DAT inhibitor, nomifensine, on the dopamine release. Nomifensine (1 nM-10 µM) concentration-dependently enhanced phasic stimulated dopamine release in both saline- and Dox-treated iTat-tg mice, while the magnitude of the nomifensine-mediated dopamine release was unchanged between saline and Dox treatment groups. A single systemic administration of SRI-32743 prior to animal sacrifice reversed the increased dopamine release in the baseline of phasic dopamine release and nomifensine-augmented dopamine levels in Dox-treated iTat-tg mice, while SRI-32743 alone did not alter baseline of dopamine release. These findings suggest that Tat expression induced an increase in extracellular dopamine levels by not only inhibiting DAT-mediated dopamine transport but also stimulating synaptic dopamine release. Thus, DAT allosteric modulators may serve as a potential therapeutic intervention for HIV infection-dysregulated dopamine system observed in HIV-1 positive individuals. SIGNIFICANCE STATEMENT: HIV infection-induced dysregulation of the dopaminergic system has been implicated in the development of neurocognitive impairments observed in HIV positive patients. Understanding the mechanisms underlying HIV-1 Tat protein-induced alteration of extracellular dopamine levels will provide insights into the development of molecules that can attenuate Tat interaction with targets in the dopaminergic system. Here, we determined whether Tat alters dopamine release and how the novel DAT allosteric modulator, SRI-32743, impacts dopamine neurotransmission to attenuate Tat-induced effects on extracellular dopamine dynamics.

SRI-32743, a novel allosteric modulator, attenuates HIV-1 Tat protein-induced inhibition of the dopamine transporter and alleviates the potentiation of cocaine reward in HIV-1 Tat transgenic mice

Cocaine abuse increases the incidence of HIV-1-associated neurocognitive disorders. We have demonstrated that HIV-1 transactivator of transcription (Tat) allosterically modulates dopamine (DA) reuptake through the human DA transporter (hDAT), potentially contributing to Tat-induced cognitive impairment and potentiation of cocaine conditioned place preference (CPP). This study determined the effects of a novel allosteric modulator of DAT, SRI-32743, on the interactions of HIV-1 Tat, DA, cocaine, and [³H]WIN35,428 with hDAT in vitro. SRI-32743 (50 nM) attenuated Tat-induced inhibition of [³H]DA uptake and decreased the cocaine-mediated dissociation of [³H]WIN35,428 binding in CHO cells expressing hDAT, suggesting a SRI-32743-mediated allosteric modulation of the Tat-DAT interaction. In further in vivo studies utilizing doxycycline-inducible Tat transgenic (iTat-tg) mice, 14 days of Tat expression significantly reduced the recognition index by 31.7% in the final phase of novel object recognition (NOR) and potentiated cocaine-CPP 2.7-fold compared to responses of vehicle-treated control iTat-tg mice. The Tat-induced NOR deficits and potentiation of cocaine-CPP were not observed in saline-treated iTat-tg or doxycycline-treated G-tg (Tat-null) mice. Systemic administration (i.p.) of SRI-32743 prior to behavioral testing ameliorated Tat-induced impairment of NOR (at a dose of 10 mg/kg) and the Tat-induced potentiation of cocaine-CPP (at doses of 1 or 10 mg/kg). These findings demonstrate that Tat and cocaine interactions with DAT may be regulated by compounds interacting at the DAT allosteric modulatory sites, suggesting a potential therapeutic intervention for HIV-infected patients with concurrent cocaine abuse.

Mutations of tyrosine 467 in the human norepinephrine transporter attenuate HIV-1 Tat-induced inhibition of dopamine transport while retaining physiological function

Dysregulation of dopaminergic transmission induced by the HIV-1 transactivator of transcription (Tat) has been implicated as a central factor in the development of HIV-1 associated neurocognitive disorders (HAND). We have demonstrated that the tyrosine470 residue of the human dopamine transporter (hDAT) plays a critical role in Tat-hDAT interaction. Based on the computational modeling predictions, the present study sought to examine the mutational effects of the tyrosine467 residue of the human norepinephrine transporter (hNET), a corresponding residue of the hDAT tyrosine470, on Tat-induced inhibition of reuptake of dopamine through the hNET. Mutations of the hNET tyrosine467 to a histidine (Y467H) or a phenylalanine (Y467F) displayed similar kinetic properties of reuptake of [3H]dopamine and [3H]norepinephrine in PC12 cells expressing wild-type hNET and its mutants. Compared to wild-type hNET, neither of Y467H or Y467F altered Bmax and Kd values of [3H]WIN35,428 binding, whereas Y467H but not Y467F decreased the Bmax of [3H]nisoxetine binding without changes in Kd. Y467H also increased the affinity of nisoxetine for inhibiting [3H]dopamine uptake relative to wild-type hNET. Recombinant Tat1-86 (140 nM) induced a significant reduction of [3H]dopamine uptake in wild-type hNET, which was attenuated in both Y467H and Y467F. Compared to wild-type hNET, neither Y467H or Y467F altered [3H]dopamine efflux in CHO cells expressing WT hNET and mutants, whereas Y467F but not Y467H decreased [3H]MPP+ efflux. These results demonstrate tyrosine467 as a functional recognition residue in the hNET for Tat-induced inhibition of dopamine transport and provide a novel insight into the molecular basis for developing selective compounds that target Tat-NET interactions in the context of HAND.

Substance abuse and neurotransmission

The number of people who suffer from a substance abuse disorder has continued to rise over the last decade; particularly, the number of drug-related overdose deaths has sharply increased during the COVID-19 pandemic. Converging lines of clinical observations, supported by imaging and neuropsychological performance testing, have demonstrated that substance abuse-induced dysregulation of neurotransmissions in the brain is critical for development and expression of the addictive properties of abused substances. Recent scientific advances have allowed for better understanding of the neurobiological processes that mediates drugs of abuse and addiction. This chapter presents the past classic concepts and the recent advances in our knowledge about how cocaine, amphetamines, opioids, alcohol, and nicotine alter multiple neurotransmitter systems, which contribute to the behaviors associated with each drug. Additionally, we discuss the interactive effects of HIV-1 or COVID-19 and substance abuse on neurotransmission and neurobiological pathways. Finally, we introduce therapeutic strategies for development of pharmacotherapies for substance abuse disorders.

HIV-Associated Apathy/Depression and Neurocognitive Impairments Reflect Persistent Dopamine Deficits

Individuals living with human immunodeficiency virus type 1 (HIV-1) are often plagued by debilitating neurocognitive impairments and affective alterations;the pathophysiology underlying these deficits likely includes dopaminergic system dysfunction. The present review utilized four interrelated aims to critically examine the evidence for dopaminergic alterations following HIV-1 viral protein exposure. First, basal dopamine (DA) values are dependent upon both brain region andexperimental approach (i.e., high-performance liquid chromatography, microdialysis or fast-scan cyclic voltammetry). Second, neurochemical measurements overwhelmingly support decreased DA concentrations following chronic HIV-1 viral protein exposure. Neurocognitive impairments, including alterations in pre-attentive processes and attention, as well as apathetic behaviors, provide an additional line of evidence for dopaminergic deficits in HIV-1. Third, to date, there is no compelling evidence that combination antiretroviral therapy (cART), the primary treatment regimen for HIV-1 seropositive individuals, has any direct pharmacological action on the dopaminergic system. Fourth, the infection of microglia by HIV-1 viral proteins may mechanistically underlie the dopamine deficit observed following chronic HIV-1 viral protein exposure. An inclusive and critical evaluation of the literature, therefore, supports the fundamental conclusion that long-term HIV-1 viral protein exposure leads to a decreased dopaminergic state, which continues to persist despite the advent of cART. Thus, effective treatment of HIV-1-associated apathy/depression and neurocognitive impairments must focus on strategies for rectifying decreases in dopamine function.

Chronic SSRI treatment reverses HIV-1 protein-mediated synaptodendritic damage

HIV-1 infection affects approximately 37 million individuals, and approximately 50% of seropositive individuals will develop symptoms of clinical depression and/or apathy. Dysfunctions of both serotonergic and dopaminergic neurotransmission have been implicated in the pathogenesis of motivational alterations. The present study evaluated the efficacy of a SSRI (escitalopram) in the HIV-1 transgenic (Tg) rat. Behavioral, neurochemical, and neuroanatomical outcomes with respect to HIV-1 and sex were evaluated to determine the efficacy of chronic escitalopram treatment. Escitalopram treatment restored function in each of the behavioral tasks that were sensitive to HIV-1-induced impairments. Further, escitalopram treatment restored HIV-1-mediated synaptodendritic damage in the nucleus accumbens; treatment with escitalopram significantly increased dendritic proliferation in HIV-1 Tg rats. However, restoration did not consistently occur with the neurochemical analysis in the HIV-1 rat. Taken together, these results suggest a role for SSRI therapies in repairing long-term HIV-1 protein-mediated neuronal damage and restoring function.

Binding Mode of Human Norepinephrine Transporter Interacting with HIV-1 Tat

The increase of HIV infection in macrophages results in HIV proteins being released, like HIV Tat which impairs the function of monoamine transporters. HIV-infected patients have displayed increased synaptic levels of dopamine (DA) due to reduced binding and function of monoamine transporters such as the norepinephrine transporter (NET) and the dopamine transporter (DAT). Development of a three-dimensional model of the HIV-1 Tat-human NET (hNET) binding complex would help reveal how HIV-1 Tat causes toxicity in the neuron by affecting DA uptake. Here we use computational techniques such as molecular modeling to study microscopic properties and molecular dynamics of the HIV-1 Tat-hNET binding. These modeling techniques allow us to analyze noncovalent interactions and observe residue-residue contacts to verify a model structure. The modeling results studied here show that HIV-1 Tat-hNET binding is highly dynamic and that HIV-1 Tat preferentially binds to hNET in its outward-open state. In particular, HIV-1 Tat forms hydrogen bond interactions with side chains of hNET residues Y84, K88, and T544. The favorable hydrogen bonding interactions of HIV-1 Tat with the hNET side chain residues Y84 and T544 have been validated by our subsequently performed DA uptake activity assays and site-directed mutagenesis, suggesting that the modeled HIV-1 Tat-hNET binding mode is reasonable. These mechanistic and structural insights gained through homology models discussed in this study are expected to encourage the pursuit of pharmacological and biochemical studies on HIV-1 Tat interacting with hNET mechanisms and detailed structures.

Mini review: Promotion of substance abuse in HIV patients: Biological mediation by HIV-1 Tat protein

Despite successful viral suppression by combinatorial anti-retroviral therapy, HIV infection continues to negatively impact the quality of life of patients by promoting neuropathy and HIV-Associated Neurocognitive Disorders (HAND), where substance use disorder (SUD) is highly comorbid and known to worsen health outcomes. While substance abuse exacerbates the progression of HIV, emerging evidence also suggests the virus may potentiate the rewarding effect of abused substances. As HIV does not infect neurons, these effects are theorized to be mediated by viral proteins. Key among these proteins are HIV-1 Tat, which can continue to be produced under viral suppression in patients. This review will recap the behavioral evidence for HIV-1 Tat mediation of a potentiation of cocaine, opioid and alcohol reward, and explore the neurochemical dysfunction associated by Tat as potential mechanisms underlying changes in reward. Targeting rampant oxidative stress, inflammation and excitotoxicity associated with HIV and Tat protein exposure may prove useful in combating persistent substance abuse comorbid with HIV in the clinic.

In vivo proton magnetic resonance spectroscopy detection of metabolite abnormalities in aged Tat-transgenic mouse brain

Most individuals living with HIV in the USA are over 45 years old and are vulnerable to the combined effects of HIV and aging. Antiretroviral therapies reduce HIV morbidity and mortality but do not prevent HIV trans-activator of transcription (Tat) protein expression or development of HIV-associated neurocognitive disorder (HAND), which may be caused by Tat. Tat-transgenic (Tat-tg) mice are used to study Tat's effects, typically after transgene induction with doxycycline. However, uninduced Tat-tg mice experience transgene leak and model aspects of HAND when aged, including neuroinflammation. We used in vivo 9.4-tesla proton magnetic resonance spectroscopy to compare neurochemistry in aged versus young female and male uninduced Tat-tg mice. Aged Tat-tg mice demonstrated measurable tat mRNA brain expression and had lower medial prefrontal cortex (MPFC) GABA, glutamate, and taurine levels and lower striatal GABA and taurine levels. Females had lower MPFC glutathione and taurine and lower striatal taurine levels. Brain testosterone levels were negatively correlated with age in aged males but not females. Aged mice had cortical abnormalities not previously reported in aged wild-type mice including lower MPFC GABA and taurine levels. As glutathione and taurine levels reflect inflammation and oxidative stress, our data suggest that Tat may exacerbate these processes in aged Tat-tg mice. However, additional studies in controls not expressing Tat are needed to confirm this point and to deconvolve individual effects of age and Tat expression. Sex steroid hormone supplements, which counter climacteric effects, increase taurine levels, and reduce inflammation and oxidative stress, could attenuate some of the brain abnormalities we identified in aged Tat-tg mice.

Astrocyte HIV-1 Tat Differentially Modulates Behavior and Brain MMP/TIMP Balance During Short and Prolonged Induction in Transgenic Mice

Despite effective antiretroviral therapy (ART), mild forms of HIV-associated neurocognitive disorders (HAND) continue to afflict approximately half of all people living with HIV (PLWH). As PLWH age, HIV-associated inflammation perturbs the balance between brain matrix metalloproteinases (MMPs) and their tissue inhibitors of metalloproteinases (TIMPs), likely contributing to neuropathogenesis. The MMP/TIMP balance is associated with cognition, learning, and memory, with TIMPs eliciting neuroprotective effects. Dysregulation of the MMP/TIMP balance was evident in the brains of PLWH where levels of TIMP-1, the inducible family member, were significantly lower than non-infected controls, and MMPs were elevated. Here, we evaluated the MMP/TIMP levels in the doxycycline (DOX)-induced glial fibrillary acidic protein promoter-driven HIV-1 transactivator of transcription (Tat) transgenic mouse model. The HIV-1 protein Tat is constitutively expressed by most infected cells, even during ART suppression of viral replication. Many studies have demonstrated indirect and direct mechanisms of short-term Tat-associated neurodegeneration, including gliosis, blood-brain barrier disruption, elevated inflammatory mediators and neurotoxicity. However, the effects of acute vs. prolonged exposure on Tat-induced dysregulation remain to be seen. This is especially relevant for TIMP-1 as expression was previously shown to be differentially regulated in human astrocytes during acute vs. chronic inflammation. In this context, acute Tat expression was induced with DOX intraperitoneal injections over 3 weeks, while DOX-containing diet was used to achieve long-term Tat expression over 6 months. First, a series of behavior tests evaluating arousal, ambulation, anxiety, and cognition was performed to examine impairments analogous to those observed in HAND. Next, gene expression of components of the MMP/TIMP axis and known HAND-relevant inflammatory mediators were assessed. Altered anxiety-like, motor and/or cognitive behaviors were observed in Tat-induced (iTat) mice. Gene expression of MMPs and TIMPs was altered depending on the duration of Tat expression, which was independent of the HIV-associated neuroinflammation typically implicated in MMP/TIMP regulation. Collectively, we infer that HIV-1 Tat-mediated dysregulation of MMP/TIMP axis and behavioral changes are dependent on duration of exposure. Further, prolonged Tat expression demonstrates a phenotype comparable to asymptomatic to mild HAND manifestation in patients.