Differential Effects of Pharmacologic and Genetic Modulation of NMDA Receptor Activity on HIV/gp120-Induced Neuronal Damage in an In Vivo Mouse Model

Restorative effect of NitroSynapsin on synaptic plasticity in an animal model of depression

In the search for new options for the pharmacological treatment of major depressive disorder, compounds with a rapid onset of action and high efficacy but lacking a psychotomimetic effect are of particular interest. In the present study, we evaluated the antidepressant potential of NitroSynapsin (NS) at behavioural, structural, and functional levels. NS is a memantine derivative and a dual allosteric N-methyl-d-aspartate receptors (NMDAR) antagonist using targeted delivery by the aminoadamantane of a warhead nitro group to inhibitory redox sites on the NMDAR. In a chronic restraint stress (CRS) mouse model of depression, five doses of NS administered on three consecutive days evoked antidepressant-like activity in the chronically stressed male C57BL/6J mice, reversing CRS-induced behavioural disturbances in sucrose preference and tail suspension tests. CRS-induced changes in morphology and density of dendritic spines in cerebrocortical neurons in the medial prefrontal cortex (mPFC) were also reversed by NS. Moreover, CRS-induced reduction in long-term potentiation (LTP) in the mPFC was found to be prevented by NS based on the electrophysiological recordings. Our study showed that NS restores structural and functional synaptic plasticity and reduces depressive behaviour to the level found in naïve animals. These results preliminarily revealed an antidepressant-like potency of NS.

Aberrant Synaptic Pruning in CNS Diseases: A Critical Player in HIV-Associated Neurological Dysfunction?

Even in the era of effective antiretroviral therapies, people living with Human Immunodeficiency Virus (HIV) are burdened with debilitating neurological dysfunction, such as HIV-associated neurocognitive disorders (HAND) and HIV-associated pain, for which there are no FDA approved treatments. Disruption to the neural circuits of cognition and pain in the form of synaptic degeneration is implicated in developing these dysfunctions. Glia-mediated synaptic pruning is a mechanism of structural plasticity in the healthy central nervous system (CNS), but recently, it has been discovered that dysregulated glia-mediated synaptic pruning is the cause of synaptic degeneration, leading to maladaptive plasticity and cognitive deficits in multiple diseases of the CNS. Considering the essential contribution of activated glial cells during the development of HAND and HIV-associated pain, it is possible that glia-mediated synaptic pruning is the causative mechanism of synaptic degeneration induced by HIV. This review will analyze the known examples of synaptic pruning during disease in order to better understand how this mechanism could contribute to the progression of HAND and HIV-associated pain.

Novel Therapeutic Approach for Excitatory/Inhibitory Imbalance in Neurodevelopmental and Neurodegenerative Diseases

Excitatory/inhibitory (E/I) balance, defined as the balance between excitation and inhibition of synaptic activity in a neuronal network, accounts in part for the normal functioning of the brain, controlling, for example, normal spike rate. In many pathological conditions, this fine balance is perturbed, leading to excessive or diminished excitation relative to inhibition, termed E/I imbalance, reflected in network dysfunction. E/I imbalance has emerged as a contributor to neurological disorders that occur particularly at the extremes of life, including autism spectrum disorder and Alzheimer's disease, pointing to the vulnerability of neuronal networks at these critical life stages. Hence, it is important to develop approaches to rebalance neural networks. In this review, we describe emerging therapies that can normalize the E/I ratio or the underlying abnormality that contributes to the imbalance in electrical activity, thus improving neurological function in these maladies.

Methamphetamine and Cannabis: A Tale of Two Drugs and their Effects on HIV, Brain, and Behavior

HIV infection and drug use intersect epidemiologically, and their combination can result in complex effects on brain and behavior. The extent to which drugs affect the health of persons with HIV (PWH) depends on many factors including drug characteristics, use patterns, stage of HIV disease and its treatment, comorbid factors, and age. To consider the range of drug effects, we have selected two that are in common use by PWH: methamphetamine and cannabis. We compare the effects of methamphetamine with those of cannabis, to illustrate how substances may potentiate, worsen, or even buffer the effects of HIV on the CNS. Data from human, animal, and ex vivo studies provide insights into how these drugs have differing effects on the persistent inflammatory state that characterizes HIV infection, including effects on viral replication, immune activation, mitochondrial function, gut permeability, blood brain barrier integrity, glia and neuronal signaling. Moving forward, we consider how these mechanistic insights may inform interventions to improve brain outcomes in PWH. This review summarizes literature from clinical and preclinical studies demonstrating the adverse effects of METH, as well as the potentially beneficial effects of cannabis, on the interacting systemic (e.g., gut barrier leakage/microbial translocation, immune activation, inflammation) and CNS-specific (e.g., glial activation/neuroinflammation, neural injury, mitochondrial toxicity/oxidative stress) mechanisms underlying HIV-associated neurocognitive disorders.

Potential pharmacological approaches for the treatment of HIV-1 associated neurocognitive disorders

HIV associated neurocognitive disorders (HAND) are the spectrum of cognitive impairments present in patients infected with human immunodeficiency virus type 1 (HIV-1). The number of patients affected with HAND ranges from 30 to 50% of HIV infected individuals and although the development of combinational antiretroviral therapy (cART) has improved longevity, HAND continues to pose a significant clinical problem as the current standard of care does not alleviate or prevent HAND symptoms. At present, the pathological mechanisms contributing to HAND remain unclear, but evidence suggests that it stems from neuronal injury due to chronic release of neurotoxins, chemokines, viral proteins, and proinflammatory cytokines secreted by HIV-1 activated microglia, macrophages and astrocytes in the central nervous system (CNS). Furthermore, the blood-brain barrier (BBB) not only serves as a route for HIV-1 entry into the brain but also prevents cART therapy from reaching HIV-1 brain reservoirs, and therefore could play an important role in HAND. The goal of this review is to discuss the current data on the epidemiology, pathology and research models of HAND as well as address the potential pharmacological treatment approaches that are being investigated.

The Glutamate System as a Crucial Regulator of CNS Toxicity and Survival of HIV Reservoirs

Glutamate (Glu) is the most abundant excitatory neurotransmitter in the central nervous system (CNS). HIV-1 and viral proteins compromise glutamate synaptic transmission, resulting in poor cell-to-cell signaling and bystander toxicity. In this study, we identified that myeloid HIV-1-brain reservoirs survive in Glu and glutamine (Gln) as a major source of energy. Thus, we found a link between synaptic compromise, metabolomics of viral reservoirs, and viral persistence. In the current manuscript we will discuss all these interactions and the potential to achieve eradication and cure using this unique metabolic profile.

Scaling Synapses in the Presence of HIV

A defining feature of HIV-associated neurocognitive disorder (HAND) is the loss of excitatory synaptic connections. Synaptic changes that occur during exposure to HIV appear to result, in part, from a homeostatic scaling response. Here we discuss the mechanisms of these changes from the perspective that they might be part of a coping mechanism that reduces synapses to prevent excitotoxicity. In transgenic animals expressing the HIV proteins Tat or gp120, the loss of synaptic markers precedes changes in neuronal number. In vitro studies have shown that HIV-induced synapse loss and cell death are mediated by distinct mechanisms. Both in vitro and animal studies suggest that HIV-induced synaptic scaling engages new mechanisms that suppress network connectivity and that these processes might be amenable to therapeutic intervention. Indeed, pharmacological reversal of synapse loss induced by HIV Tat restores cognitive function. In summary, studies indicate that there are temporal, mechanistic and pharmacological features of HIV-induced synapse loss that are consistent with homeostatic plasticity. The increasingly well delineated signaling mechanisms that regulate synaptic scaling may reveal pharmacological targets suitable for normalizing synaptic function in chronic neuroinflammatory states such as HAND.

NitroSynapsin therapy for a mouse MEF2C haploinsufficiency model of human autism

Transcription factor MEF2C regulates multiple genes linked to autism spectrum disorder (ASD), and human MEF2C haploinsufficiency results in ASD, intellectual disability, and epilepsy. However, molecular mechanisms underlying MEF2C haploinsufficiency syndrome remain poorly understood. Here we report that Mef2c +/-(Mef2c-het) mice exhibit behavioral deficits resembling those of human patients. Gene expression analyses on brains from these mice show changes in genes associated with neurogenesis, synapse formation, and neuronal cell death. Accordingly, Mef2c-het mice exhibit decreased neurogenesis, enhanced neuronal apoptosis, and an increased ratio of excitatory to inhibitory (E/I) neurotransmission. Importantly, neurobehavioral deficits, E/I imbalance, and histological damage are all ameliorated by treatment with NitroSynapsin, a new dual-action compound related to the FDA-approved drug memantine, representing an uncompetitive/fast off-rate antagonist of NMDA-type glutamate receptors. These results suggest that MEF2C haploinsufficiency leads to abnormal brain development, E/I imbalance, and neurobehavioral dysfunction, which may be mitigated by pharmacological intervention.

Transgenic mice expressing HIV-1 envelope protein gp120 in the brain as an animal model in neuroAIDS research

HIV-1 infection causes injury to the central nervous system (CNS) and is often associated with neurocognitive disorders. One model for brain damage seen in AIDS patients is the transgenic (tg) mouse expressing a soluble envelope protein gp120 of HIV-1 LAV in the brain in astrocytes under the control of the promoter of glial fibrillary acidic protein. These GFAP-gp120tg mice manifest several key neuropathological features observed in AIDS brains, such as decreased synaptic and dendritic density, increased numbers of activated microglia, and pronounced astrocytosis. Several recent studies show that brains of GFAP-gp120tg mice and neurocognitively impaired HIV patients share also a significant number of differentially regulated genes, activation of innate immunity and other cellular signaling pathways, disturbed neurogenesis, and learning deficits. These findings support the continued relevance of the GFAP-gp120tg mouse as a useful model to investigate neurodegenerative mechanisms and develop therapeutic strategies to mitigate the consequences associated with HIV infection of the CNS, neuroAIDS, and HAND.

Targeting nitric oxide and NMDA receptor-associated pathways in treatment of high grade glial tumors. Hypotheses for nitro-memantine and nitrones

Glioblastoma multiforme (GBM) is a devastating brain cancer with no curative treatment. Targeting Nitric Oxide (NO) and glutamatergic pathways may help as adjunctive treatments in GBM. NO at low doses promotes tumorigenesis, while at higher levels (above 300 nM) triggers apoptosis. Gliomas actively secrete high amounts of glutamate which activates EGR signaling and mediates degradation of peritumoral tissues via excitotoxic injury. Memantine inhibits NMDA-subtype of glutamate receptors (NMDARs) and induces autophagic death of glioma cells in vitro and blocks glioma growth in vivo. Nitro-memantines may exert further benefits by limiting NMDAR signaling and by delivery of NO to the areas of excessive NMDAR activity leading NO-accumulation at tumoricidal levels within gliomas. Due to the duality of NO in tumorigenesis, agents which attenuate NO levels may also act beneficial in treatment of GBM. Nitrone compounds including N-tert-Butyl-α-phenylnitrone (PBN) and its disulfonyl-phenyl derivative, OKN-007 suppress free radical formation in experimental cerebral ischemia. OKN-007 failed to show clinical efficacy in stroke, but trials demonstrated its high biosafety in humans including elderly subjects. PBN inhibits the signaling pathways of NF-κB, inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX). In animal models of liver cancer and glioblastoma, OKN-007 seemed more efficient than PBN in suppression of cell proliferation, microvascular density and in induction of apoptosis. OKN-007 also inhibits SULF2 enzyme, which promotes tumor growth via versatile pathways. We assume that nitromemantines may be more beneficial concomitant with chemo-radiotherapy while nitrones alone may act useful in suppressing basal tumor growth and angiogenesis.

NMDARs Adapt to Neurotoxic HIV Protein Tat Downstream of a GluN2A-Ubiquitin Ligase Signaling Pathway

HIV-associated neurocognitive disorder (HAND) affects approximately half of HIV-infected patients. Infected non-neuronal cells release neurotoxic factors such as the viral protein transactivator of transcription (Tat) that potentiate NMDAR function. NMDARs regulate synaptic changes observed after exposure to HIV proteins, which may underlie cognitive impairment in HAND patients. Here, we used patch-clamp recording to measure NMDAR-mediated currents in rat hippocampal cultures after exposure to Tat. Tat (4-16 h) potentiated NMDA-evoked whole-cell current and increased the NMDAR:AMPAR ratio of evoked EPSCs. Potentiated currents adapted back to baseline amplitudes after 24 h of exposure to Tat. Pharmacological inhibition of GluN2A-containing NMDARs prevented adaptation, but inhibition of GluN2B-containing NMDARs did not. Pharmacological and genetic approaches determined that potentiated NMDARs activated the kinase Akt, which then activated the E3 ubiquitin ligase Mdm2. Inhibition of protein synthesis prevented adaptation, suggesting that Mdm2 altered gene expression, possibly through its well known target p53. Expression of GFP-tagged GluN1 subunits resulted in fluorescent puncta that colocalized with synaptic markers. Tat (24 h) caused an Mdm2-dependent loss of NMDAR puncta on a timescale similar to adaption of NMDAR function. Activation of the Mdm2 pathway degrades PSD-95, a scaffolding protein that clusters NMDARs at the synapse and enhances their function. Adaptation to the continued presence of excitotoxins that potentiate NMDARs such as HIV Tat may protect from excessive NMDAR activation while also contributing to the synaptic loss that correlates with cognitive decline in HAND.

SIGNIFICANCE STATEMENT

Synaptodendritic damage correlates with cognitive decline in HIV-associated neurocognitive disorder (HAND). In a cell culture model, we show that the HIV protein transactivator of transcription (Tat) initially potentiates NMDARs that then adapt to the presence of the toxin. Adaptation of NMDAR function was mediated by a GluN2A/Akt/Mdm2 pathway not previously linked to neuroinflammatory disorders such as HAND. Activation of this pathway caused a loss of synaptic NMDAR clusters. Decreased NMDAR function may result from a homeostatic response gone awry and underlie impaired synaptic function in HAND.

Effects of acute memantine administration on MATRICS Consensus Cognitive Battery performance in psychosis: Testing an experimental medicine strategy

RATIONALE

Pro-cognitive agents for chronic psychotic disorders (CPDs) might be detected via experimental medicine models, in which neural targets engaged by the drug predict sensitivity to the drug's pro-cognitive effects.

OBJECTIVE

This study aims to use an experimental medicine model to test the hypothesis that "target engagement" predicts pro-cognitive effects of the NMDA antagonist, memantine (MEM), in CPDs.

METHODS

MATRICS Consensus Cognitive Battery (MCCB) performance was assessed in CPD (n = 41) and healthy subjects (HS; n = 41) in a double-blind, randomized cross-over design of acute (single dose) MEM (placebo vs. 10 or 20 mg p.o.). Measures of prepulse inhibition (PPI) and mismatch negativity previously reported from this cohort substantiated target engagement. Biomarkers predicting MEM neurocognitive sensitivity were assessed.

RESULTS

Testing confirmed MCCB deficits associated with CPD diagnosis, age, and anticholinergic exposure. MEM (20 mg p.o.) reduced MCCB performance in HS. To control for significant test order effects, an "order-corrected MEM effect" (OCME) was calculated. In CPD subjects, greater age, positive MEM effects on PPI, and SNP rs1337697 (within the ionotropic NMDA receptor gene, GRIN3A) predicted greater positive OCME with 20 mg MEM.

CONCLUSIONS

An experimental medicine model to assess acute pro-cognitive drug effects in CPD subjects is feasible but not without challenges. A single MEM 20 mg dose had a negative impact on neurocognition among HS. In CPD patients, age, MEM effects on PPI, and rs1337697 predicted sensitivity to the neurocognitive effects of MEM. Any potential clinical utility of these predictive markers for pro-cognitive effects of MEM in subgroups of CPD patients cannot be inferred without a validating clinical trial.