Mitochondrial α7 nicotinic acetylcholine receptors are displaced from complexes with VDAC1 to form complexes with Bax upon apoptosis induction

Agonists or positive allosteric modulators of α7 nicotinic acetylcholine receptor prevent interaction of SARS-Cov-2 receptor-binding domain with astrocytoma cells

SARS-Cov-2, the virus causing COVID-19, penetrates host target cells via the receptor of angiotensin-converting enzyme 2 (ACE2). Disrupting the virus interaction with ACE2 affords a plausible mechanism for prevention of cell penetration and inhibiting dissemination of the virus. Our studies demonstrate that ACE2 interaction with the receptor binding domain of SARS-Cov-2 spike protein (RBD) can be impaired by modulating the α7 nicotinic acetylcholine receptor (α7 nAChR) contiguous with ACE2. U373 cells of human astrocytoma origin were shown to bind both ACE2-specific antibody and recombinant RBD in Cell-ELISA. ACE2 was found to interact with α7 nAChR in U373 cell lysates studied by Sandwich ELISA. Our studies demonstrate that inhibition of RBD binding to ACE2-expressing U373 cells were defined with α7 nAChR agonists choline and PNU282987, but not a competitive antagonist methyllicaconitine (MLA). Additionally, the type 2 positive allosteric modulator (PAM2) PNU120596 and hydroxyurea (HU) also inhibited the binding. Our studies demonstrate that activation of α7 AChRs has efficacy in inhibiting the SARS-Cov-2 interaction with the ACE2 receptor and in such a way can prevent virus target cell penetration. These studies also help to clarify the consistent efficacy and positive outcomes for utilizing HU in treating COVID-19.

G6PD maintains the VSMC synthetic phenotype and accelerates vascular neointimal hyperplasia by inhibiting the VDAC1-Bax-mediated mitochondrial apoptosis pathway

BACKGROUND

Glucose-6-phosphate dehydrogenase (G6PD) plays an important role in vascular smooth muscle cell (VSMC) phenotypic switching, which is an early pathogenic event in various vascular remodeling diseases (VRDs). However, the underlying mechanism is not fully understood.

METHODS

An IP‒LC‒MS/MS assay was conducted to identify new binding partners of G6PD involved in the regulation of VSMC phenotypic switching under platelet-derived growth factor-BB (PDGF-BB) stimulation. Co-IP, GST pull-down, and immunofluorescence colocalization were employed to clarify the interaction between G6PD and voltage-dependent anion-selective channel protein 1 (VDAC1). The molecular mechanisms involved were elucidated by examining the interaction between VDAC1 and apoptosis-related biomarkers, as well as the oligomerization state of VDAC1.

RESULTS

The G6PD level was significantly elevated and positively correlated with the synthetic characteristics of VSMCs induced by PDGF-BB. We identified VDAC1 as a novel G6PD-interacting molecule essential for apoptosis. Specifically, the G6PD-NTD region was found to predominantly contribute to this interaction. G6PD promotes VSMC survival and accelerates vascular neointimal hyperplasia by inhibiting VSMC apoptosis. Mechanistically, G6PD interacts with VDAC1 upon stimulation with PDGF-BB. By competing with Bax for VDAC1 binding, G6PD reduces VDAC1 oligomerization and counteracts VDAC1-Bax-mediated apoptosis, thereby accelerating neointimal hyperplasia.

CONCLUSION

Our study showed that the G6PD-VDAC1-Bax axis is a vital switch in VSMC apoptosis and is essential for VSMC phenotypic switching and neointimal hyperplasia, providing mechanistic insight into early VRDs.

Nicotinic acetylcholine receptors in cancer: Limitations and prospects

Nicotinic acetylcholine receptors (nAChRs) have long been considered to solely mediate neurotransmission. However, their widespread distribution in the human body suggests a more diverse physiological role. Additionally, the expression of nAChRs is increased in certain cancers, such as lung cancer, and has been associated with cell proliferation, epithelial-to-mesenchymal cell transition, angiogenesis and apoptosis prevention. Several compounds that interact with these receptors have been identified as potential therapeutic agents. They have been tested as drugs for treating nicotine addiction, alcoholism, depression, pain and Alzheimer's disease. This review focuses on nAChR-mediated signalling in cancer, presenting opportunities for the development of innovative nAChR-based anticancer drugs. It displays the differences in expression of each nAChR subunit between normal and cancer cells for selected cancer types, highlighting their possible involvement in specific cases. Antagonists of nAChRs that could complement existing cancer therapies are summarised and critically discussed. We hope that this review will stimulate further research on the role of nAChRs in cancer potentially leading to innovative cancer therapies.

Hydroxyurea interaction with α7 nicotinic acetylcholine receptor can underlie its therapeutic efficacy upon COVID-19

In this paper the authors provide evidence that hydroxyurea (hydroxycarbamide) interacts with α7 nicotinic acetylcholine receptor, exerts anti-inflammatory and pro-survival effect, prevents α7 nicotinic receptor interaction with angiotensin-converting enzyme-2 and stimulates IgM to IgG class switch upon immunization with SARS spike protein fragment 674-685. Hydroxyurea shifts immunoglobulin glycosylation profile to anti-inflammatory phenotype and prevents the appearance of anti-idiotypic α7(179-190)-specific antibodies, as well as memory impairment. According to these results, interaction with α7 nicotinic acetylcholine receptor may underlie positive therapeutic effects of hydroxyurea upon SARS-Cov-2 infection by interfering with virus penetration into the cell and providing anti-inflammatory and immunomodulatory effects.

Apoptosis in the neuroprotective effect of α7 nicotinic receptor in neurodegenerative models

The α7 subtype of nicotinic receptors (α7 nAChRs) is one of the most abundant nicotinic receptor subtypes in the central nervous system (CNS) and both neurons and nonneuronal cells express α7 nAChRs. When activated, α7 nAChRs become permeable to cations and promote cellular responses such as anti-apoptotic signaling by modulating the caspases and proteins of the Bcl-2 family. Neuroprotection is an important function of these receptors, promoting neuronal survival under pathological conditions, including situations of stress and neuronal degeneration. Studies have demonstrated the relationship between the activation of these receptors and the reduction of neuronal or glial cell injury, by controlling apoptotic processes in different models, including neurodegenerative diseases such as Alzheimer's disease. Therefore, one of the most important signaling pathways activated by α7 nAChRs is the PI3K/Akt signaling cascade, which promotes the stimulation of anti-apoptotic molecules of the Bcl-2 family, Bcl-2 and Bcl-xl, and reduces the expression of caspases and proapoptotic molecules, resulting in cell survival. In Alzheimer's models, the literature shows that α7 nAChR activation attenuates Aβ-induced neurotoxicity through modulation of different intrinsic apoptotic pathways via PI3K/Akt and mitogen-activated protein kinase (MAPK). In this review, we provide an up-to-date summary of the current evidence on the relationship between the activation of α7 nAChRs, a subtype of nicotinic acetylcholine receptor, and its role in neuroprotection by modulating apoptotic pathways.

α7 Nicotinic acetylcholine receptors regulate translocation of HIF-1α to the cell nucleus and mitochondria upon hypoxia

Nicotinic acetylcholine receptors (nAChRs), initially characterized as ligand-gated ion channels mediating fast synaptic transmission, are now found in many non-excitable cells and mitochondria where they function in ion-independent manner and regulate vital cellular processes like apoptosis, proliferation, cytokine secretion. Here we show that the nAChRs of α7 subtype are present in the nuclei of liver cells and astrocytoma U373 cell line. As shown by lectin ELISA, the nuclear α7 nAChRs are mature glycoproteins that follow the standard rout of post-translational modifications in Golgi; however, their glycosylation profile is non-identical to that of mitochondrial nAChRs. They are exposed on the outer nuclear membrane and are found in combination with lamin B1. The nuclear α7 nAChRs are up-regulated in liver within 1 h after partial hepatectomy and in H₂O₂-treated U373 cells. As shown both in silico and experimentally, the α7 nAChR interacts with hypoxia-inducible factor HIF-1α and this interaction is impaired by α7-selective agonists PNU282987 and choline or type 2 positive allosteric modulator PNU120596, which prevent HIF-1α accumulation in the nuclei. Similarly, HIF-1α interacts with mitochondrial α7 nAChRs in U373 cells treated with dimethyloxalylglycine. It is concluded that functional α7 nAChRs influence HIF-1α translocation into the nucleus and mitochondria upon hypoxia.

Universal nature of cholinergic regulation demonstrated with nicotinic acetylcholine receptors

Mammalian nicotinic acetylcholine receptors (nAChRs) were initially discovered as ligand-gated ion channels mediating fast synaptic transmission in the neuro-muscular junctions and autonomic ganglia. They were further found to be involved in a wide range of basic biological processes within the brain and in non-excitable tissues. The present review summarizes the data obtained in our laboratory during last two decades. Investigation of autonomic ganglia with the nAChR subunit-specific antibodies was followed by identification of nAChRs in B lymphocytes, discovery of mitochondrial nAChRs and their role in mitochondrial apoptosis pathway, and revealing the role of α7 nAChRs and α7-specific antibodies in neuroinflammation-related Alzheimer disease and COVID-19. The data obtained demonstrate the involvement of nAChRs in cell survival, proliferation, cell-to-cell communication and inflammatory reaction. Together with the ability of nAChRs to function in both ionotropic and metabotropic way, these data illustrate the universal nature of cholinergic regulation mediated by nAChRs.

Mitochondrial nicotinic acetylcholine receptors: Mechanisms of functioning and biological significance

Nicotinic acetylcholine receptors mediate fast synaptic transmission in neuro-muscular junctions and autonomic ganglia and modulate survival, proliferation and neurotransmitter or cytokine release in the brain and non-excitable cells. The neuronal-type nicotinic acetylcholine receptors are expressed in the outer mitochondria membrane to regulate the release of pro-apoptotic substances like cytochrome c or reactive oxygen species. In the intracellular environment, nicotinic acetylcholine receptor signaling is ion-independent and triggers intramitochondrial kinases, similar to those activated by plasma membrane nicotinic acetylcholine receptors. The present review will describe the data obtained during the last five years including, in particular, post-translational glycosylation as a targeting signal to mitochondria, mechanisms of mitochondrial nicotinic acetylcholine receptor signaling studied with subtype-specific agonists, antagonists, positive allosteric modulators and knockout mice lacking certain nicotinic acetylcholine receptor subunits, interaction of mitochondrial nicotinic acetylcholine receptors with Bcl-2 family proteins and their involvement in important pathologies like neuroinflammation, liver damage and SARS-CoV-2 infection.

SARS-Cov-2 spike protein fragment 674-685 protects mitochondria from releasing cytochrome c in response to apoptogenic influence

In spite of numerous studies, many details of SARS-Cov-2 interaction with human cells are still poorly understood. The 674–685 fragment of SARS-Cov-2 spike protein is homologous to the fragment of α-cobratoxin underlying its interaction with α7 nicotinic acetylcholine receptors (nAChRs). The interaction of 674–685 peptide with α7 nAChR has been predicted in silico. In the present paper we confirm this prediction experimentally and investigate the effect of SARS-Cov-2 spike protein peptide on mitochondria, which express α7 nAChRs to regulate apoptosis-related events. We demonstrate that SARS-Cov-2 spike protein peptide 674–685 competes with the antibody against 179–190 fragment of α7 nAChR subunit for the binding to α7-expressing cells and mitochondria and prevents the release of cytochrome c from isolated mitochondria in response to 0.5 mM H₂O₂ but does not protect intact U373 cells against apoptogenic effect of H₂O_2. Our data suggest that the α7 nAChR-binding portion of SARS-Cov-2 spike protein prevents mitochondria-driven apoptosis when the virus is uncoated inside the cell and, therefore, supports the infected cell viability before the virus replication cycle is complete.

Stimulation of α7-nAChRs coordinates autophagy and apoptosis signaling in experimental knee osteoarthritis

Osteoarthritis (OA) is the most common chronic joint disease in the elderly population. Growing evidence indicates that a balance between autophagy and apoptosis in chondrocytes plays a key role in OA’s cartilage degradation. Thus, drugs targeting the balance between apoptosis and autophagy are potential therapeutic approaches for OA treatment. In previous studies, we found that the activation of α7 nicotinic acetylcholine receptors (α7-nAChRs) alleviated monosodium iodoacetate (MIA)-induced joint degradation and osteoarthritis pain. To explore the potential functions of α7-nAChRs in autophagy and apoptosis signaling in knee OA, we compared the expression of α7-nAChRs in human knee articular cartilage tissues from normal humans and OA patients. We found that knee joint cartilage tissues of OA patients showed decreased α7-nAChRs and an imbalance between autophagy and apoptosis. Next, we observed that α7-nAChRs deficiency did not affect cartilage degradation in OA development but reversed the beneficial effects of nicotine on mechanical allodynia, cartilage degradation, and an MIA-induced switch from autophagy to apoptosis. Unlike in vivo studies, we found that primary chondrocytes from α7-nAChRs knockout (KO) mice showed decreased LC3 levels under normal conditions and were more sensitive toward MIA-induced apoptosis. Finally, we found that α7-nAChRs deficiency increased the phosphorylation of mTOR after MIA treatment, which can also be observed in OA patients’ tissues. Thus, our findings not only confirmed that nicotine alleviated MIA-induced pain behavior and cartilage degradation via stimulating the α7-nAChRs/mTOR signal pathway but found the potential role of α7-nAChRs in mediating the balance between apoptosis and autophagy.

Changing Functional Signatures of Microglia along the Axis of Brain Aging

Microglia, the innate immune cells of the brain, are commonly perceived as resident macrophages of the central nervous system (CNS). This definition, however, requires further specification, as under healthy homeostatic conditions, neither morphological nor functional properties of microglia mirror those of classical macrophages. Indeed, microglia adapt exceptionally well to their microenvironment, becoming a legitimate member of the cellular brain architecture. The ramified or surveillant microglia in the young adult brain are characterized by specific morphology (small cell body and long, thin motile processes) and physiology (a unique pattern of Ca²⁺ signaling, responsiveness to various neurotransmitters and hormones, in addition to classic “immune” stimuli). Their numerous physiological functions far exceed and complement their immune capabilities. As the brain ages, the respective changes in the microglial microenvironment impact the functional properties of microglia, triggering further rounds of adaptation. In this review, we discuss the recent data showing how functional properties of microglia adapt to age-related changes in brain parenchyma in a sex-specific manner, with a specific focus on early changes occurring at middle age as well as some strategies counteracting the aging of microglia.