Positive allosteric modulators of α7* or β2* nicotinic acetylcholine receptors trigger different kinase pathways in mitochondria

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.

α7 Nicotinic acetylcholine receptor potentiation downregulates chemotherapy-induced inflammatory overactivation by overlapping intracellular mechanisms

We studied, using a combination of animal and cellular models, the glial mechanisms underlying the anti-neuropathic and anti-inflammatory properties of PAM-2 [(E)-3-furan-2-yl-N-p-tolyl-acrylamide], a positive allosteric modulator of α7 nicotinic acetylcholine receptors (nAChRs). In mice, PAM-2 decreased the inflammatory process induced by the combination of oxaliplatin (OXA), a chemotherapeutic agent, and interleukin-1β (IL-1β), a pro-inflammatory molecule. In the brain and spinal cord of treated animals, PAM-2 reduced pro-inflammatory cytokines/chemokines by mechanisms involving mRNA downregulation of factors in the toll-like receptor 4 (TLR4)/nuclear factor (NF)-κB pathway, and increased the precursor of brain-derived neurotrophic factor (proBDNF). To determine the molecular mechanisms underlying the anti-inflammatory activity of PAM-2, both human C20 microglia and normal human astrocytes (NHA) were used. The results showed that PAM-2-induced potentiation of glial α7 nAChRs decreases OXA/IL-1β-induced overexpression of inflammatory molecules by different mechanisms, including mRNA downregulation of factors in the NF-κB pathway (in microglia and astrocyte) and ERK (only in microglia). The OXA/IL-1β-mediated reduction in proBDNF was prevented by PAM-2 in microglia, but not in astrocytes. Our findings also indicate that OXA/IL-1β-induced organic cation transporter 1 (OCT1) expression is decreased by PAM-2, suggesting that decreased OXA influx may be involved in the protective effects of PAM-2. The α7-selective antagonist methyllycaconitine blocked the most important effects mediated by PAM-2 at both animal and cellular levels, supporting a mechanism involving α7 nAChRs. In conclusion, glial α7 nAChR stimulation/potentiation downregulates neuroinflammatory targets, and thereby remains a promising therapeutic option for cancer chemotherapy-induced neuroinflammation and neuropathic pain.

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.

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.

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

Nicotinic acetylcholine receptors (nAChRs) mediate fast synaptic transmission in muscles and autonomic ganglia and regulate cytokine and neurotransmitter release in the brain and non-excitable cells. The α7 nAChRs localized in the outer membrane of mitochondria regulate cytochrome c release stimulated by apoptosis-inducing agents. However, the mechanisms through which nAChRs influence mitochondrial permeability remain obscure. Here we put an aim to explore the interaction of nAChRs with voltage-dependent anion channels (VDAC1) and pro-apoptotic protein Bax in the course of apoptosis induction. By using molecular modeling in silico, it was shown that both Bax and VDAC1 can bind within the 4th transmembrane portion (M4) of nAChR subunits. Experimentally, α7 nAChR-Bax and α7 nAChR-VDAC1 complexes were identified by sandwich ELISA in mitochondria isolated from astrocytoma U373 cells. Stimulating apoptosis of U373 cells by H₂O₂ disrupted α7-VDAC complexes and favored formation of α7-Bax complexes accompanied by cytochrome c release from mitochondria. α7-selective agonist PNU282987 or type 2 positive allosteric modulator PNU120596 disrupted α7-Bax and returned α7 nAChR to complex with VDAC1 resulting in attenuation of cytochrome c release. It is concluded that mitochondrial nAChRs regulate apoptosis-induced mitochondrial channel formation by modulating the interplay of apoptosis-related proteins in mitochondria outer membrane.

Cholinergic signaling controls immune functions and promotes homeostasis

Acetylcholine (ACh) was created by nature as one of the first signaling molecules, expressed already in procaryotes. Based on the positively charged nitrogen, ACh could initially mediate signaling in the absence of receptors. When evolution established more and more complex organisms the new emerging organs systems, like the smooth and skeletal muscle systems, energy-generating systems, sexual reproductive system, immune system and the nervous system have further optimized the cholinergic signaling machinery. Thus, it is not surprising that ACh and the cholinergic system are expressed in the vast majority of cells. Consequently, multiple common interfaces exist, for example, between the nervous and the immune system. Research of the last 20 years has unmasked these multiple regulating mechanisms mediated by cholinergic signaling and thus, the biological role of ACh has been revised. The present article summarizes new findings and describes the role of both non-neuronal and neuronal ACh in protecting the organism from external and internal health threats, in providing energy for the whole organism and for the individual cell, controling immune functions to prevent inflammatory dysbalance, and finally, the involvement in critical brain functions, such as learning and memory. All these capacities of ACh enable the organism to attain and maintain homeostasis under changing external conditions. However, the existence of identical interfaces between all these different organ systems complicates the research for new therapeutic interventions, making it essential that every effort should be undertaken to find out more specific targets to modulate cholinergic signaling in different diseases.

Unusual properties of α7 nicotinic acetylcholine receptor ion channels in B lymphocyte-derived SP-2/0 cells

This study demonstrates the presence of α7 nicotinic acetylcholine receptors (nAChR) in B lymphocyte-derived SP-2/0 cells by means of flow cytometry and immunocytochemistry. According to lectin and sandwich ELISA, the α7 subunits expressed in SP-2/0 cells are more glycosylated compared to those expressed in the brain or normal B lymphocytes and are combined with β2 subunits. At zero and negative pipette potentials, either acetylcholine or α7-specific agonist PNU282987 stimulated the ion channel activity in SP-2/0 cells revealed by single channel patch-clamp recordings. The conductivity was within the range of 19 to 39 pS and reversal potential was between -17 mV and +28 mV, the currents were potentiated by α7-specific positive allosteric modulator PNU120596 and were partially blocked by α7-specific antagonist methyllicaconitine (MLA). However, they were oriented downwards suggesting that the channels mediated the cation outflux rather than influx. As shown by Ca2+ imaging studies, PNU282987 did not stimulate immediate Ca2+ influx into SP-2/0 cells. Instead, Ca2+ influx through Ca-release-activated channels (CRACs) was observed within minutes after either PNU282987 or MLA application. It is concluded that SP-2/0 express α7β2 nAChRs, which mediate the cation outflux under negative pipette potentials applied, possibly, due to depolarized membrane or negative surface charge formed by carbohydrate residues. In addition, α7β2 nAChRs may influence CRACs in ion-independent way.

Positive Allosteric Modulation of Alpha7 Nicotinic Acetylcholine Receptors Transiently Improves Memory but Aggravates Inflammation in LPS-Treated Mice

Neuroinflammation accompanies or even precedes the development of cognitive changes in many brain pathologies, including Alzheimer’s disease. Therefore, dampening inflammatory reactions within the brain is a promising strategy for supporting cognitive functions in elderly people and for preventing the development of neurodegenerative disorders. Nicotinic acetylcholine receptors containing α7 subunits (α7 nAChRs) are involved in regulating cell survival, inflammation, and memory. The aim of our study was to evaluate the efficiency of α7-specific therapy at different stages of inflammation and to compare the effects of orthosteric agonist PNU282987 and type 2 positive allosteric modulator (PAM) PNU120596 in mice after a single injection of lipopolysaccharide (LPS). The data presented demonstrate that PNU282987 protected mice from LPS-induced impairment of episodic memory by decreasing IL-6 levels in the blood, stabilizing the brain mitochondria and up-regulating the brain α7-, α3-, and α4-containing nAChRs. Such treatment was efficient when given simultaneously with LPS or a week after LPS injection and was not efficient if LPS had been injected 2 months before. PNU120596 also decreased IL-6, stabilized mitochondria and up-regulated the brain nAChRs. However, its memory-improving effect was transient and disappeared after the end of the injection cycle. Moreover, cessation of PNU120596 treatment resulted in a sharp increase in IL-1β and IL-6 levels in the blood. It is concluded that activating α7 nAChRs protects the mouse brain from the pathogenic effect of LPS in the early stages of inflammation but is not efficient when irreversible changes have already occurred. The use of a PAM does not improve the effect of the agonist, possibly potentiates the effect of endogenous agonists, and results in undesirable effects after treatment cessation.