Synergistic control of keratinocyte adhesion through muscarinic and nicotinic acetylcholine receptor subtypes

A Critical Interpretive Synthesis of the Role of Arecoline in Oral Carcinogenesis: Is the Local Cholinergic Axis a Missing Link in Disease Pathophysiology?

Arecoline is the primary active carcinogen found in areca nut and has been implicated in the pathogenesis of oral squamous cell carcinoma (OSCC) and oral submucous fibrosis (OSF). For this study, we conducted a stepwise review process by combining iterative scoping reviews with a post hoc search, with the aim of identifying the specific mechanisms by which arecoline initiates and promotes oral carcinogenesis. Our initial search allowed us to define the current trends and patterns in the pathophysiology of arecoline-induced OSF and OSCC, which include the induction of cell proliferation, facilitation of invasion, adhesion, and migration, increased collagen deposition and fibrosis, imbalance in immune and inflammatory mechanisms, and genotoxicity. Key molecular pathways comprise the activation of NOTCH1, MYC, PRDX2, WNT, CYR61, EGFR/Pl3K, DDR1 signaling, and cytokine upregulation. Despite providing a comprehensive overview of potential pathogenic mechanisms of OSF, the involvement of molecules functioning as areca alkaloid receptors, namely, the muscarinic and nicotinic acetylcholine receptors (AChRs), was not elucidated with this approach. Accordingly, our search strategy was refined to reflect these evidence gaps. The results of the second round of reviews with the post hoc search highlighted that arecoline binds preferentially to muscarinic AChRs, which have been implicated in cancer. Consistently, AChRs activate the signaling pathways that partially overlap with those described in the context of arecoline-induced carcinogenesis. In summary, we used a theory-driven interpretive review methodology to inform, extend, and supplement the conventional systematic literature assessment workflow. On the one hand, the results of this critical interpretive synthesis highlighted the prevailing trends and enabled the consolidation of data pertaining to the molecular mechanisms involved in arecoline-induced carcinogenesis, and, on the other, brought up knowledge gaps related to the role of the local cholinergic axis in oral carcinogenesis, thus suggesting areas for further investigation.

The α9α10 acetylcholine receptor: a non-neuronal nicotinic receptor

Within the superfamily of pentameric ligand-gated ion channels, cholinergic nicotinic receptors (nAChRs) were classically identified to mediate synaptic transmission in the nervous system and the neuromuscular junction. The α9 and α10 nAChR subunits were the last ones to be identified. Surprisingly, they do not fall into the dichotomic neuronal/muscle classification of nAChRs. They assemble into heteropentamers with a well-established function as canonical ion channels in inner ear hair cells, where they mediate central nervous system control of auditory and vestibular sensory processing. The present review includes expression, pharmacological, structure-function, molecular evolution and pathophysiological studies, that define receptors composed from α9 and α10 subunits as distant and distinct members within the nAChR family. Thus, although α9 and α10 were initially included within the neuronal subdivision of nAChR subunits, they form a distinct clade within the phylogeny of nAChRs. Following the classification of nAChR subunits based on their main synaptic site of action, α9 and α10 should receive a name in their own right.

The Role of Non-Neuronal Acetylcholine in the Autoimmune Blistering Disease Pemphigus Vulgaris

The importance of acetylcholine (ACh) in keratinocyte adhesion and acantholysis has been investigated over the last three decades, particularly in the pathophysiology of autoimmune blistering dermatoses. Pemphigus vulgaris (PV) is an autoimmune blistering skin disease where autoantibody-mediated suprabasilar intraepidermal splitting causes flaccid blisters and non-healing erosions of the oral mucosa and sometimes also of the skin. Historically, acantholysis in PV was thought to be driven by anti-desmoglein (Dsg) antibodies. Herein, we describe the role of autoantibodies against keratinocyte muscarinic and nicotinic acetylcholine receptors, as well as the annexin-like molecule pemphaxin that also binds ACh, in the immunopathogenesis of PV. The identification of targets in this disease is important, as they may lead to novel diagnostic and therapeutic options in the future for this potentially deadly disease.

Sympathetic System in Wound Healing: Multistage Control in Normal and Diabetic Skin

In this review, we discuss sympathetic regulation in normal and diabetic wound healing. Experimental denervation studies have confirmed that sympathetic nerve endings in skin have an important and complex role in wound healing. Vasoconstrictor neurons secrete norepinephrine (NE) and neuropeptide Y (NPY). Both mediators decrease blood flow and interact with inflammatory cells and keratinocytes. NE acts in an ambiguous way depending on receptor type. Beta2-adrenoceptors could be activated near sympathetic endings; they suppress inflammation and re-epithelialization. Alpha1- and alpha2-adrenoceptors induce inflammation and activate keratinocytes. Sudomotor neurons secrete acetylcholine (ACh) and vasoactive intestinal peptide (VIP). Both induce vasodilatation, angiogenesis, inflammation, keratinocytes proliferation and migration. In healthy skin, all effects are important for successful healing. In treatment of diabetic ulcers, mediator balance could be shifted in different ways. Beta2-adrenoceptors blockade and nicotinic ACh receptors activation are the most promising directions in treatment of diabetic ulcers with neuropathy, but they require further research.

A growing understanding of the role of muscarinic receptors in the molecular pathology and treatment of schizophrenia

Pre-clinical models, postmortem and neuroimaging studies all support a role for muscarinic receptors in the molecular pathology of schizophrenia. From these data it was proposed that activation of the muscarinic M1 and/or M4 receptor would reduce the severity of the symptoms of schizophrenia. This hypothesis is now supported by results from two clinical trials which indicate that activating central muscarinic M1 and M4 receptors can reduce the severity of positive, negative and cognitive symptoms of the disorder. This review will provide an update on a growing body of evidence that argues the muscarinic M1 and M4 receptors have critical roles in CNS functions that are dysregulated by the pathophysiology of schizophrenia. This realization has been made possible, in part, by the growing ability to visualize and quantify muscarinic M1 and M4 receptors in the human CNS using molecular neuroimaging. We will discuss how these advances have provided evidence to support the notion that there is a sub-group of patients within the syndrome of schizophrenia that have a unique molecular pathology driven by a marked loss of muscarinic M1 receptors. This review is timely, as drugs targeting muscarinic receptors approach clinical use for the treatment of schizophrenia and here we outline the background biology that supported development of such drugs to treat the disorder.

The M3 Muscarinic Acetylcholine Receptor Promotes Epidermal Differentiation

The M3 muscarinic acetylcholine receptor is predominantly expressed in the basal epidermal layer where it mediates the effects of the autocrine/paracrine cytotransmitter acetylcholine. Patients with the autoimmune blistering disease pemphigus develop autoantibodies to M3 muscarinic acetylcholine receptor and show alterations in keratinocyte adhesion, proliferation, and differentiation, suggesting that M3 muscarinic acetylcholine receptor controls these cellular functions. Chmr3^-/- mice display altered epidermal morphology resembling that seen in patients with pemphigus vulgaris. In this study, we characterized the cellular and molecular mechanisms through which M3 muscarinic acetylcholine receptor controls epidermal structure and function. We used single-cell RNA sequencing to evaluate keratinocyte heterogeneity and identify differentially expressed genes in specific subpopulations of epidermal cells in Chmr3^-/- neonatal mice. We found that Chmr3^-/- mice feature abnormal epidermal morphology characterized by accumulation of nucleated basal cells, shrinkage of basal keratinocytes, and enlargement of intercellular spaces. These morphologic changes were associated with upregulation of cell proliferation genes and downregulation of genes contributing to epidermal differentiation, extracellular matrix formation, intercellular adhesion, and cell arrangement. These findings provide, to our knowledge, previously unreported insights into how acetylcholine controls epidermal differentiation and lay a groundwork for future translational studies evaluating the therapeutic potential of cholinergic drugs in dermatology.

Emerging roles of keratinocytes in nociceptive transduction and regulation

Keratinocytes are the predominant block-building cells in the epidermis. Emerging evidence has elucidated the roles of keratinocytes in a wide range of pathophysiological processes including cutaneous nociception, pruritus, and inflammation. Intraepidermal free nerve endings are entirely enwrapped within the gutters of keratinocyte cytoplasm and form en passant synaptic-like contacts with keratinocytes. Keratinocytes can detect thermal, mechanical, and chemical stimuli through transient receptor potential ion channels and other sensory receptors. The activated keratinocytes elicit calcium influx and release ATP, which binds to P2 receptors on free nerve endings and excites sensory neurons. This process is modulated by the endogenous opioid system and endothelin. Keratinocytes also express neurotransmitter receptors of adrenaline, acetylcholine, glutamate, and γ-aminobutyric acid, which are involved in regulating the activation and migration, of keratinocytes. Furthermore, keratinocytes serve as both sources and targets of neurotrophic factors, pro-inflammatory cytokines, and neuropeptides. The autocrine and/or paracrine mechanisms of these mediators create a bidirectional feedback loop that amplifies neuroinflammation and contributes to peripheral sensitization.

Chrna5 is overexpressed in psoriasis patients and promotes psoriasis-like inflammation in mouse models

It is well known that psoriasis is closely related to smoking, and the cholinergic receptor nicotinic subunit alpha-5 (Chrna5) plays an important role in smoking-related diseases. However, studies on the relationship between Chrna5 and psoriasis are limited. This study aimed to examine the role of Chrna5 in psoriasis development and pathogenesis. Analysis in psoriatic tissues and imiquimod (IMQ)-induced mouse models showed that Chrna5 was highly expressed in psoriatic lesional skin. To further verify the function of Chrna5, we constructed Chrna5-knockout mice and induced the psoriasis model. We found that Chrna5 knockout significantly reduced the severity of psoriasis and could regulate inflammation via the mitogen-activated protein kinase kinase kinase 1 (MEKK1)/c-Jun NH(2)-terminal kinase (JNK)-MAPK/NF-κB pathway. The single-cell sequencing results revealed that after Chrna5 knockout, the keratinocyte subpopulation was significantly reduced and the related Janus kinase/signal transduction and activator of transcription (JAK/STAT) signaling pathway was downregulated, further indicating the importance of Chrna5 in psoriasis. Human keratinocytes were analyzed, and silencing Chrna5 inhibited keratinocyte proliferation and migration. In summary, Chrna5 played important roles in the development and pathogenesis of psoriasis, and targeting Chrna5 may be an effective strategy for the treatment of psoriasis.

Chronic exposure to the anti-m3 muscarinic acetylcholine receptor autoantibody in pemphigus vulgaris contributes to disease pathophysiology

Pemphigus vulgaris (PV) is a potentially lethal autoimmune mucocutaneous blistering disease characterized by binding of IgG autoantibodies (AuAbs) to keratinocytes (KCs). In addition to AuAbs against adhesion molecules desmogleins 1 and 3, PV patients also produce an AuAb against the M3 muscarinic acetylcholine (ACh) receptor (M3AR) that plays an important role in regulation of vital functions of KCs upon binding endogenous ACh. This anti-M3AR AuAb is pathogenic because its adsorption eliminates the acantholytic activity of PV IgG; however, the molecular mechanism of its action is unclear. In the present study, we sought to elucidate the mode of immunopharmacologic action of the anti-M3AR AuAb in PV. Short-term exposures of cultured KCs to PV IgG or the muscarinic agonist muscarine both induced changes in the expression of keratins 5 and 10, consistent with the inhibition of proliferation and upregulated differentiation and in keeping with the biological function of M3AR. In contrast, long-term incubations induced a keratin expression pattern consistent with upregulated proliferation and decreased differentiation, in keeping with the hyperproliferative state of KCs in PV. This change could result from desensitization of the M3AR, representing the net antagonist-like effect of the AuAb. Therefore, chronic exposure of KCs to the anti-M3AR AuAb interrupts the physiological regulation of KCs by endogenous ACh, contributing to the onset of acantholysis. Since cholinergic agents have already demonstrated antiacantholytic activity in a mouse model of PV and in PV patients, our results have translational significance and can guide future development of therapies for PV patients employing cholinergic drugs.

The Hair Cell α9α10 Nicotinic Acetylcholine Receptor: Odd Cousin in an Old Family

Nicotinic acetylcholine receptors (nAChRs) are a subfamily of pentameric ligand-gated ion channels with members identified in most eumetazoan clades. In vertebrates, they are divided into three subgroups, according to their main tissue of expression: neuronal, muscle and hair cell nAChRs. Each receptor subtype is composed of different subunits, encoded by paralogous genes. The latest to be identified are the α9 and α10 subunits, expressed in the mechanosensory hair cells of the inner ear and the lateral line, where they mediate efferent modulation. α9α10 nAChRs are the most divergent amongst all nicotinic receptors, showing marked differences in their degree of sequence conservation, their expression pattern, their subunit co-assembly rules and, most importantly, their functional properties. Here, we review recent advances in the understanding of the structure and evolution of nAChRs. We discuss the functional consequences of sequence divergence and conservation, with special emphasis on the hair cell α9α10 receptor, a seemingly distant cousin of neuronal and muscle nicotinic receptors. Finally, we highlight potential links between the evolution of the octavolateral system and the extreme divergence of vertebrate α9α10 receptors.

Characterization of HA-tagged α9 and α10 nAChRs in the mouse cochlea

Neurons of the medial olivary complex inhibit cochlear hair cells through the activation of α9α10-containing nicotinic acetylcholine receptors (nAChRs). Efforts to study the localization of these proteins have been hampered by the absence of reliable antibodies. To overcome this obstacle, CRISPR-Cas9 gene editing was used to generate mice in which a hemagglutinin tag (HA) was attached to the C-terminus of either α9 or α10 proteins. Immunodetection of the HA tag on either subunit in the organ of Corti of adult mice revealed immunopuncta clustered at the synaptic pole of outer hair cells. These puncta were juxtaposed to immunolabeled presynaptic efferent terminals. HA immunopuncta also occurred in inner hair cells of pre-hearing (P7) but not in adult mice. These immunolabeling patterns were similar for both homozygous and heterozygous mice. All HA-tagged genotypes had auditory brainstem responses not significantly different from those of wild type littermates. The activation of efferent neurons in heterozygous mice evoked biphasic postsynaptic currents not significantly different from those of wild type hair cells. However, efferent synaptic responses were significantly smaller and less frequent in the homozygous mice. We show that HA-tagged nAChRs introduced in the mouse by a CRISPR knock-in are regulated and expressed like the native protein, and in the heterozygous condition mediate normal synaptic function. The animals thus generated have clear advantages for localization studies.

Mechanisms of synergy of autoantibodies to M3 muscarinic acetylcholine receptor and secretory pathway Ca2+/Mn2+-ATPase isoform 1 in patients with non-desmoglein pemphigus vulgaris

Pemphigus vulgaris (PV) is a potentially lethal mucocutaneous blistering disease characterized by IgG autoantibodies (AuAbs) binding to epidermal keratinocytes and inducing a devastating blistering disease affecting oral and/or esophageal surfaces and, sometimes, also the skin. Anti-keratinocyte AuAbs developed by the desmoglein (Dsg) 1/3 AuAb-negative acute PV patients are pathogenic, as they induced acantholysis and epidermal split in the experimental models of PV in vitro and in vivo. These PV patients have various combinations of AuAbs to keratinocyte muscarinic acetylcholine receptor subtype M3 (M3AR), the secretory pathway Ca²⁺/Mn²⁺-ATPase isoform 1 (SPCA1), and desmocollin 3 whose relative concentrations correlate with the disease activity. In this study, we identified new molecular mechanisms of the synergistic cooperation of AuAbs to M3AR and SPCA1 in inducing acantholysis in the anti-Dsg 1/3 AuAb-negative PV patients. Anti-M3AR AuAb was found to play an important role in determining the level of intraepidermal split just above the basal cells, caspase to mediate early pro-apoptotic events triggered by anti-SPCA1 AuAb, and the neonatal Fc receptor (FcRn) to contribute to the pathobiological actions of both anti-M3AR and anti-SPCA1 AuAbs. Altogether, these novel results support our original hypothesis that pemphigus acantholysis is a complex disease process (also known as apoptolysis) initiated by AuAbs directed against different keratinocyte proteins that play important roles in supporting cell viability and regulating vital cell functions.

Muscarinic Acetylcholine Type 1 Receptor Activity Constrains Neurite Outgrowth by Inhibiting Microtubule Polymerization and Mitochondrial Trafficking in Adult Sensory Neurons

The muscarinic acetylcholine type 1 receptor (M₁R) is a metabotropic G protein-coupled receptor. Knockout of M₁R or exposure to selective or specific receptor antagonists elevates neurite outgrowth in adult sensory neurons and is therapeutic in diverse models of peripheral neuropathy. We tested the hypothesis that endogenous M₁R activation constrained neurite outgrowth via a negative impact on the cytoskeleton and subsequent mitochondrial trafficking. We overexpressed M₁R in primary cultures of adult rat sensory neurons and cell lines and studied the physiological and molecular consequences related to regulation of cytoskeletal/mitochondrial dynamics and neurite outgrowth. In adult primary neurons, overexpression of M₁R caused disruption of the tubulin, but not actin, cytoskeleton and significantly reduced neurite outgrowth. Over-expression of a M₁R-DREADD mutant comparatively increased neurite outgrowth suggesting that acetylcholine released from cultured neurons interacts with M₁R to suppress neurite outgrowth. M₁R-dependent constraint on neurite outgrowth was removed by selective (pirenzepine) or specific (muscarinic toxin 7) M₁R antagonists. M₁R-dependent disruption of the cytoskeleton also diminished mitochondrial abundance and trafficking in distal neurites, a disorder that was also rescued by pirenzepine or muscarinic toxin 7. M₁R activation modulated cytoskeletal dynamics through activation of the G protein (Gα13) that inhibited tubulin polymerization and thus reduced neurite outgrowth. Our study provides a novel mechanism of M₁R control of Gα13 protein-dependent modulation of the tubulin cytoskeleton, mitochondrial trafficking and neurite outgrowth in axons of adult sensory neurons. This novel pathway could be harnessed to treat dying-back neuropathies since anti-muscarinic drugs are currently utilized for other clinical conditions.

Fibroblast heterogeneity and its implications for engineering organotypic skin models in vitro

Advances in cell culture methods, multidisciplinary research, clinical need to replace lost skin tissues and regulatory need to replace animal models with alternative test methods has led to development of three dimensional models of human skin. In general, these in vitro models of skin consist of keratinocytes cultured over fibroblast-populated dermal matrices. Accumulating evidences indicate that mesenchyme-derived signals are essential for epidermal morphogenesis, homeostasis and differentiation. Various studies show that fibroblasts isolated from different tissues in the body are dynamic in nature and are morphologically and functionally heterogeneous subpopulations. Further, these differences seem to be dictated by the local biological and physical microenvironment the fibroblasts reside resulting in "positional identity or memory". Furthermore, the heterogeneity among the fibroblasts play a critical role in scarless wound healing and complete restoration of native tissue architecture in fetus and oral mucosa; and excessive scar formation in diseased states like keloids and hypertrophic scars. In this review, we summarize current concepts about the heterogeneity among fibroblasts and their role in various wound healing environments. Further, we contemplate how the insights on fibroblast heterogeneity could be applied for the development of next generation organotypic skin models.

Critical Role of the Neonatal Fc Receptor (FcRn) in the Pathogenic Action of Antimitochondrial Autoantibodies Synergizing with Anti-desmoglein Autoantibodies in Pemphigus Vulgaris

Pemphigus vulgaris (PV) is a life-long, potentially fatal IgG autoantibody-mediated blistering disease targeting mucocutaneous keratinocytes (KCs). PV patients develop pathogenic anti-desmoglein (Dsg) 3 ± 1 and antimitochondrial antibodies (AMA), but it remained unknown whether and how AMA enter KCs and why other cell types are not affected in PV. Therefore, we sought to elucidate mechanisms of cell entry, trafficking, and pathogenic action of AMA in PV. We found that PVIgGs associated with neonatal Fc receptor (FcRn) on the cell membrane, and the PVIgG-FcRn complexes entered KCs and reached mitochondria where they dissociated. The liberated AMA altered mitochondrial membrane potential, respiration, and ATP production and induced cytochrome c release, although the lack or inactivation of FcRn abolished the ability of PVIgG to reach and damage mitochondria and to cause detachment of KCs. The assays of mitochondrial functions and keratinocyte adhesion demonstrated that although the pathobiological effects of AMA on KCs are reversible, they become irreversible, leading to epidermal blistering (acantholysis), when AMA synergize with anti-Dsg antibodies. Thus, it appears that AMA enter a keratinocyte in a complex with FcRn, become liberated from the endosome in the cytosol, and are trafficked to the mitochondria, wherein they trigger pro-apoptotic events leading to shrinkage of basal KCs uniquely expressing FcRn in epidermis. During recovery, KCs extend their cytoplasmic aprons toward neighboring cells, but anti-Dsg antibodies prevent assembly of nascent desmosomes due to steric hindrance, thus rendering acantholysis irreversible. In conclusion, FcRn is a common acceptor protein for internalization of AMA and, perhaps, for PV autoantibodies to other intracellular antigens, and PV is a novel disease paradigm for investigating and elucidating the role of FcRn in this autoimmune disease and possibly other autoimmune diseases.

M2muscarinic receptors inhibit cell proliferation and migration in urothelial bladder cancer cells

The role of muscarinic receptors in several diseases including cancer has recently emerged. To evaluate the hypothesis that muscarinic acetylcholine receptors may play a role in bladder cancer as well as in other tumor types, we investigated their expression in bladder tumor specimens. All examined samples expressed the M1, M2 and M3 receptor subtypes. We also found that the level of M2 transcripts, but not those of M1 or M3, significantly increased with the tumor histologic grade. In view of these results, we proceeded to investigate whether the M2 agonist Arecaidine had any effect on in vitro cell growth and migration of T24 cells, a bladder tumor cell line expressing the muscarinic receptors, including the M2 subtype. We observed that Arecaidine significantly reduced T24 and 5637 cell proliferation and migration in a concentration dependent manner. The silencing of M2 receptor by siRNA in T24 and 5637 cell lines showed the inability of Arecaidine (100 μM) to inhibit cell proliferation after 48 hours, whereas the use of M1 and M3 antagonists in T24 appeared not to counteract the Arecaidine effect, suggesting that the inhibition of cell proliferation was directly dependent on M2 receptor activation. These data suggest that M2 muscarinic receptors may play a relevant role in bladder cancer and represent a new attractive therapeutic target.

Type 3 muscarinic acetylcholine receptor stimulation is a determinant of endothelial barrier function and adherens junctions integrity: role of protein-tyrosine phosphatase 1B

The main purpose of this study was to investigate whether type 3 muscarinic acetylcholine receptor (M3R) dysfunction induced vascular hyperpermeability. Transwell system analysis showed that M3R inhibition by selective antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and small interfering RNA both increased endothelial permeability. Using coimmunoprecipitation and Western blot assay, we found that M3R inhibition increased VE-cadherin and β-catenin tyrosine phosphorylation without affecting their expression. Using PTP1B siRNA, we found that PTP1B was required for maintaining VE-cadherin and β-catenin protein dephosphorylation. In addition, 4-DAMP suppressed PTP1B activity by reducing cyclic adenosine monophosphate (cAMP), but not protein kinase Cα (PKCα). These data indicate that M3R preserves the endothelial barrier function through a mechanism potentially maintaining PTP1B activity, keeping the adherens junction proteins (AJPs) dephosphorylation. [BMB Reports 2014; 47(10): 552-557]

Mechanisms of growth-promoting and tumor-protecting effects of epithelial nicotinic acetylcholine receptors

Although the role of nicotine as a carcinogen is debatable, it is widely accepted that it contributes to cancer by promoting growth and survival of mutated cell clones and protecting them from the chemo- and radiotherapy-induced apoptosis. On the cell membrane (cm), the nicotinic acetylcholine (ACh) receptors (nAChRs) implement upregulation of proliferative and survival genes. Nicotine also can permeate cells and activate mitochondrial (mt)-nAChRs coupled to inhibition of the mitochondrial permeability transition pore (mPTP) opening, thus preventing apoptosis. In this study, we sought to pin down principal mechanisms mediating the tumor-promoting activities of nicotine resulting from activation of cm- and mt-nAChRs in oral and lung cancer cells, SCC25 and SW900, respectively. Activated cm-nAChRs were found to form complexes with receptors for EGF and VEGEF via the α7 and β2 nAChR subunits, respectively, whereas activated mt-nAChRs physically associated with the intramitochondrial protein kinases PI3K and Src via the α7 and β4 subunits. This was associated with upregulated expression of cyclin D1/activation of ERK1/2 and inhibition of mPTP opening, respectively, as well as upregulated proliferation and resistance to H(2)O(2)-induced apoptosis. The molecular synergy between cm-nAChRs and growth factor receptors helps explain how one biological mediator, such as ACh, can modulate activity of the other, such as a growth factor, and vice versa. Establishment of functional coupling of mt-nAChRs to regulation of mPTP opening provides a novel mechanism of nicotine-dependent protection from cell death. Further elucidation of this novel mechanism of tumor-promoting activities of nicotine should have a strong translational impact, because extraneuronal nAChRs may provide a novel molecular target to prevent, reverse, or retard progression of both nicotine-related and unrelated cancers.

Pemphigus vulgaris antibodies target the mitochondrial nicotinic acetylcholine receptors that protect keratinocytes from apoptolysis

The mechanism of detachment and death of keratinocytes in pemphigus vulgaris (PV) involves pro-apoptotic action of constellations of autoantibodies determining disease severity and response to treatment. The presence of antibodies to nicotinic acetylcholine receptors (nAChRs) and the therapeutic efficacy of cholinomimetics in PV is well-established. Recently, adsorption of anti-mitochondrial antibodies abolished the ability of PVIgGs to cause acantholysis, demonstrating their pathophysiological significance. Since, in addition to cell membrane, nAChRs are also present on the mitochondrial outer membrane, wherein they act to prevent activation of intrinsic (mitochondrial apoptosis), we hypothesized that mitochondrial (mt)-nAChRs might be targeted by PVIgGs. To test this hypothesis, we employed the immunoprecipitation-western blot assay of keratinocyte mitochondrial proteins that visualized the α3, α5, α7, α9, α10, β2 and β4 mt-nAChR subunits precipitated by PV IgGs, suggesting that functions of mt-nAChRs are compromised in PV. To pharmacologically counteract the pro-apoptotic action of anti-mitochondrial antibodies in PV, we exposed naked keratinocyte mitochondria to PVIgGs in the presence of the nicotinic agonist nicotine ± antagonists, and measured cytochrome c (CytC) release. Nicotine abolished PVIgG-dependent CytC release, showing a dose-dependent effect, suggesting that protection of mitochondria can be a novel mechanism of therapeutic action of nicotinic agonists in PV. The obtained results indicated that the mt-nAChRs targeted by anti-mitochondrial antibodies produced by PV patients are coupled to inhibition of CytC release, and that nicotinergic stimulation can abolish PVIgG-dependent activation of intrinsic apoptosis in KCs. Future studies should determine if and how the distinct anti-mt-nAChR antibodies penetrate KCs and correlate with disease severity.

The acetylcholine signaling network of corneal epithelium and its role in regulation of random and directional migration of corneal epithelial cells

PURPOSE

Because cholinergic drugs are used in ophthalmology and cholinergic stimulation has been shown to facilitate epithelialization of mucocutaneous wounds, we performed a systematic analysis of components of the cholinergic network of human and murine corneal epithelial cells (CECs) and determined the role of autocrine and paracrine acetylcholine (ACh) in regulation of CEC motility.

METHODS

We investigated the expression of ACh receptors at the mRNA and protein levels in human immortalized CECs, localization of cholinergic molecules in normal and wounded murine cornea, and the effects of cholinergic drugs on CEC directional and random migration in vitro, intercellular adhesion, and expression of integrin αV and E-cadherin.

RESULTS

We demonstrated that corneal epithelium expresses the ACh-synthesizing enzyme choline acetyltransferase, the ACh-degrading enzyme acetylcholinesterase, two muscarinic ACh receptors (mAChRs), M3 and M4, and several nicotinic ACh receptors (nAChRs), including both α7- and α9-made homomeric nAChRs and predominantly the α3β2±α5 subtype of heteromeric nAChRs. Wounding affected the expression patterns of cholinergic molecules in the murine corneal epithelium. Constant stimulation of CECs through both muscarinic and nicotinic signaling pathways was essential for CEC survival and both directional and random migration in vitro. Both α7 and non-α7 nAChRs elicited chemotaxis, with the α7 signaling exhibiting a stronger chemotactic effect. Cholinergic stimulation of CECs upregulated expression of the integrin and cadherin molecules involved in epithelialization. We found synergy between the proepithelialization signals elicited by different ACh receptors expressed in CECs.

CONCLUSIONS

Simultaneous stimulation of mAChRs and nAChRs by ACh may be required to synchronize and balance ionic and metabolic events in a single cell. Localization of these cholinergic enzymes and receptors in murine cornea indicated that the concentration of endogenous ACh and the mode of its signaling differ among corneal epithelial layers. Elucidation of the signaling events elicited upon agonist binding to corneal mAChRs and nAChRs will be crucial for understanding the mechanisms of ACh signaling in CECs, which has salient clinical implications.

M3 muscarinic acetylcholine receptor dysfunction inhibits Rac1 activity and disrupts VE-cadherin/β-catenin and actin cytoskeleton interaction

The objective was to investigate whether M3 muscarinic acetylcholine receptor (mAChR) dysfunction disrupts the linkage between the vascular endothelial (VE)-cadherin in the adherens junctional complex and the actin-based cytoskeleton, increasing vascular permeability in atherosclerosis. Western blotting revealed that a selective M3 receptor antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), and M3 receptor siRNA decrease VE-cadherin and β-catenin in Triton X-100-insoluble fractions, indicating that M3 receptor inhibition weakens the linkage between the VE-cadherin/β-catenin complex and the actin cytoskeleton. Co-immunoprecipitation assays showed that M3 receptor inhibition reduces Rac1 activity and the association of IQ motif-containing GTPase-activating protein 1 (IQGAP1) with Ras-related C3 botulinum toxin substrate 1 (Rac1), while increasing the interaction between IQGAP1 and β-catenin. Using IQGAP1 siRNA, we found that IQGAP1 is required for stable interaction between VE-cadherin/β-catenin and the actin cytoskeleton in quiescent endothelial cells; IQGAP1 siRNA augments the M3 receptor inhibition-induced dissociation between them. Moreover, S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide (NO) donor, attenuates this disassociation and Rac1 activity inhibition. The M3 receptor facilitates interaction of the VE-cadherin-based adherens junctional complex and the actin-based cytoskeleton by maintaining Rac1 activity, which regulates the interaction between IQGAP1/Rac1 and IQGAP1/β-catenin, and may contribute to endothelial barrier function under physiological conditions.

Molecular basis for the differential sensitivity of rat and human α9α10 nAChRs to α-conotoxin RgIA

The α9α10 nicotinic acetylcholine receptor (nAChR) may be a potential target in pathophysiology of the auditory system, chronic pain, and breast and lung cancers. Alpha-conotoxins, from the predatory marine snail Conus, are potent nicotinic antagonists, some of which are selective for the α9α10 nAChR. Here, we report a two order of magnitude species difference in the potency of α-conotoxin RgIA for the rat versus human α9α10 nAChR. We investigated the molecular mechanism of this difference. Heterologous expression of the rat α9 with the human α10 subunit in Xenopus oocytes resulted in a receptor that was blocked by RgIA with potency similar to that of the rat α9α10 nAChR. Conversely, expression of the human α9 with that of the rat α10 subunit resulted in a receptor that was blocked by RgIA with potency approaching that of the human α9α10 receptor. Systematic substitution of residues found in the human α9 subunit into the homologous position in the rat α9 subunit revealed that a single point mutation, Thr56 to Ile56, primarily accounts for this species difference. Remarkably, although the α9 nAChR subunit has previously been reported to provide the principal (+) binding face for binding of RgIA, Thr56 is located in the (-) complementary binding face.

Activation of muscarinic receptors by non-neuronal acetylcholine

The biological role of acetylcholine and the cholinergic system is revisited based particularly on scientific research early and late in the last century. On the one hand, acetylcholine represents the classical neurotransmitter, whereas on the other hand, acetylcholine and the pivotal components of the cholinergic system (high-affinity choline uptake, choline acetyltransferase and its end product acetylcholine, muscarinic and nicotinic receptors and esterase) are expressed by more or less all mammalian cells, i.e. by the majority of cells not innervated by neurons at all. Moreover, it has been demonstrated that acetylcholine and "cholinergic receptors" are expressed in non-neuronal organisms such as plants and protists. Acetylcholine is even synthesized by bacteria and algae representing an extremely old signalling molecule on the evolutionary timescale. The following article summarizes examples, in which non-neuronal acetylcholine is released from primitive organisms as well as from mammalian non-neuronal cells and binds to muscarinic receptors to modulate/regulate phenotypic cell functions via auto-/paracrine pathways. The examples demonstrate that non-neuronal acetylcholine and the non-neuronal cholinergic system are vital for various types of cells such as epithelial, endothelial and immune cells.

The channel physiology of the skin

During embryonic development, the skin, the largest organ of the human body, and nervous system are both derived from the neuroectoderm. Consequently, several key factors and mechanisms that influence and control central or peripheral nervous system activities are also present and hence involved in various regulatory mechanisms of the skin. Apparently, this is the case for the ion and non-ion selective channels as well. Therefore, in this review, we shall focus on delineating the regulatory roles of the channels in skin physiology and pathophysiology. First, we introduce key cutaneous functions and major characteristics of the channels in question. Then, we systematically detail the involvement of a multitude of channels in such skin processes (e.g. skin barrier formation, maintenance, and repair, immune mechanisms, exocrine secretion) which are mostly defined by cutaneous non-neuronal cell populations. Finally, we close by summarizing data suggesting that selected channels are also involved in skin diseases such as e.g. atopic dermatitis, psoriasis, non-melanoma cancers and malignant melanoma, genetic and autoimmune diseases, etc., as well as in skin ageing.

Muscarinic receptor agonists and antagonists: effects on keratinocyte functions

The stratified epithelium enveloping the skin and lining the surfaces of oral and vaginal mucosa is comprised by keratinocytes that synthesize, secrete, degrade, and respond to acetylcholine via muscarinic and nicotinic receptors. The two pathways may compete or synergize with one another, so that net biologic effect represents the biologic sum of the effects of distinct acetylcholine receptors expressed by a keratinocyte at a particular stage of its development. Keratinocytes express a unique combination of muscarinic receptor subtypes at each stage of their development. Experimental results indicate that muscarinic receptors expressed in human keratinocytes regulate their viability, proliferation, migration, adhesion, and terminal differentiation, hair follicle cycling, and secretion of humectants, cytokines, and growth factors. Learning the muscarinic pharmacology of keratinocyte development and functions has salient clinical implications for patients with nonhealing wounds, mucocutaneous cancers, and various autoimmune and inflammatory diseases. Successful therapy of pemphigus lesions with topical pilocarpine and disappearance of psoriatic lesions due to systemic atropine therapy illustrate that such therapeutic approach is feasible.

Pemphigus autoimmunity: hypotheses and realities

The goal of contemporary research in pemphigus vulgaris and pemphigus foliaceus is to achieve and maintain clinical remission without corticosteroids. Recent advances of knowledge on pemphigus autoimmunity scrutinize old dogmas, resolve controversies, and open novel perspectives for treatment. Elucidation of intimate mechanisms of keratinocyte detachment and death in pemphigus has challenged the monopathogenic explanation of disease immunopathology. Over 50 organ-specific and non-organ-specific antigens can be targeted by pemphigus autoimmunity, including desmosomal cadherins and other adhesion molecules, PERP cholinergic and other cell membrane (CM) receptors, and mitochondrial proteins. The initial insult is sustained by the autoantibodies to the cell membrane receptor antigens triggering the intracellular signaling by Src, epidermal growth factor receptor kinase, protein kinases A and C, phospholipase C, mTOR, p38 MAPK, JNK, other tyrosine kinases, and calmodulin that cause basal cell shrinkage and ripping desmosomes off the CM. Autoantibodies synergize with effectors of apoptotic and oncotic pathways, serine proteases, and inflammatory cytokines to overcome the natural resistance and activate the cell death program in keratinocytes. The process of keratinocyte shrinkage/detachment and death via apoptosis/oncosis has been termed apoptolysis to emphasize that it is triggered by the same signal effectors and mediated by the same cell death enzymes. The natural course of pemphigus has improved due to a substantial progress in developing of the steroid-sparing therapies combining the immunosuppressive and direct anti-acantholytic effects. Further elucidation of the molecular mechanisms mediating immune dysregulation and apoptolysis in pemphigus should improve our understanding of disease pathogenesis and facilitate development of steroid-free treatment of patients.

Neuronal nicotinic alpha7 receptors modulate early neutrophil infiltration to sites of skin inflammation

Background

A major site of initiation of inflammatory responses upon physical perturbation(s) and infection by invading organisms is the skin. Control of responses in this organ is, in part, modulated by the neuronal nicotinic acetylcholine receptor (nAChR) alpha7.

Methods

To further investigate the role of alpha7 in skin inflammatory responses, a local inflammatory response was induced by topical application of croton oil to the ear skin of wild-type (alpha7WT) and alpha7 knock-out (alpha7KO) mice. Cells infiltrating the inflamed tissue were characterized by flow cytometry and RNA analysis.

Results

Six hours following croton oil application, analysis of infiltrating cells showed that the alpha7KO mice exhibited a significantly enhanced number of cells, and specifically, of Ly6G positive neutrophils. Macrophage and lymphocyte infiltration was equivalent in the alpha7KO and alpha7WT mice. RNA analysis showed that IL-1β and IL-6 were increased significantly in the infiltrating cells of the alpha7KO mouse, although TNF failed to reach significance. In contrast, resident cells of the skin exhibited no differences in these cytokines between genotypes. Both resident and infiltrating cell populations from alpha7KO mice did show elevated message levels for the adhesion protein ICAM1. Measurement of chemokines revealed enhanced expression of the skin-related CCL27 by resident cells in alpha7KO mice. Further, we demonstrate that the population of Ly6G⁺ neutrophils at the croton oil-inflamed skin site expresses low levels of CCR10, a receptor for CCL27 normally associated with lymphocytes.

Conclusion

nAChRalpha7 in the skin can impact on early local inflammatory responses mediated through a novel population of neutrophils that are Ly6G⁺CCR10^lo.

Antimitochondrial autoantibodies in pemphigus vulgaris: a missing link in disease pathophysiology

A loss of epidermal cohesion in pemphigus vulgaris (PV) results from autoantibody action on keratinocytes (KCs) activating the signaling kinases and executioner caspases that damage KCs, causing their shrinkage, detachment from neighboring cells, and rounding up (apoptolysis). In this study, we found that PV antibody binding leads to activation of epidermal growth factor receptor kinase, Src, p38 MAPK, and JNK in KCs with time pattern variations from patient to patient. Both extrinsic and intrinsic apoptotic pathways were also activated. Although Fas ligand neutralizing antibody could inhibit the former pathway, the mechanism of activation of the latter remained unknown. PV antibodies increased cytochrome c release, suggesting damage to mitochondria. The immunoblotting experiments revealed penetration of PVIgG into the subcellular mitochondrial fraction. The antimitochondrial antibodies from different PV patients recognized distinct combinations of antigens with apparent molecular sizes of 25, 30, 35, 57, 60, and 100 kDa. Antimitochondrial antibodies were pathogenic because their absorption abolished the ability of PVIgG to cause keratinocyte detachment both in vitro and in vivo. The downstream signaling of antimitochondrial antibodies involved JNK and late p38 MAPK activation, whereas the signaling of anti-desmoglein 3 (Dsg3) antibody involved JNK and biphasic p38 MAPK activation. Using KCs grown from Dsg3(-/-) mice, we determined that Dsg3 did not serve as a surrogate antigen allowing antimitochondrial antibodies to enter KCs. The PVIgG-induced activation of epidermal growth factor receptor and Src was affected neither in Dsg3(-/-) KCs nor due to absorption of antimitochondrial antibodies. These results demonstrated that apoptolysis in PV is a complex process initiated by at least three classes of autoantibodies directed against desmosomal, mitochondrial, and other keratinocyte self-antigens. These autoantibodies synergize with the proapoptotic serum and tissue factors to trigger both extrinsic and intrinsic pathways of cell death and break the epidermal cohesion, leading to blisters. Further elucidation of the primary signaling events downstream of PV autoantigens will be crucial for the development of a more successful therapy for PV patients.

Desmosomal interactome in keratinocytes: a systems biology approach leading to an understanding of the pathogenesis of skin disease

We provide the first description of the desmosome network in keratinocytes using a systems level approach. The desmo-adhesome consists of 59 proteins connected by 128 direct interactions and forms different functional subnets. Whilst the structure appears to be extremely robust against random perturbations, network fragmentation analysis suggests that the desmo-adhesome is susceptible to targeted attacks. To confirm this prediction, we applied this model to the autoimmune disease Pemphigus Vulgaris (PV), a paradigm of external perturbation of the desmosome. Our analysis showed that the adaptor protein plakophilin (Pkp) 3 was in the highest percentile group for both connectivity rate and gene expression changes in experimental PV. This observation led us to speculate that Pkp3 was crucial in desmosomal remodelling, and therefore we designed the experiments to verify this hypothesis. Our data demonstrate that, whilst Pkp3 is important in conferring adhesive strength to keratinocytes, it also acts as a central molecule mediating cell-cell detachment induced by PV IgG.

Alpha9 nicotinic acetylcholine receptors and the treatment of pain

Chronic pain is a vexing worldwide problem that causes substantial disability and consumes significant medical resources. Although there are numerous analgesic medications, these work through a small set of molecular mechanisms. Even when these medications are used in combination, substantial amounts of pain often remain. It is therefore highly desirable to develop treatments that work through distinct mechanisms of action. While agonists of nicotinic acetylcholine receptors (nAChRs) have been intensively studied, new data suggest a role for selective antagonists of nAChRs. alpha-Conotoxins are small peptides used offensively by carnivorous marine snails known as Conus. A subset of these peptides known as alpha-conotoxins RgIA and Vc1.1 produces both acute and long lasting analgesia. In addition, these peptides appear to accelerate the recovery of function after nerve injury, possibly through immune mediated mechanisms. Pharmacological analysis indicates that RgIA and Vc1.1 are selective antagonists of alpha9alpha10 nAChRs. A recent study also reported that these alpha9alpha10 antagonists are also potent GABA-B agonists. In the current study, we were unable to detect RgIA or Vc1.1 binding to or action on cloned GABA-B receptors expressed in HEK cells or Xenopus oocytes. We review the background, findings and implications of use of compounds that act on alpha9* nAChRs.(1).

Significance of autoimmunity to non-desmoglein targets in pemphigus

The pathogenesis of pemphigus vulgaris (PV) is a highly controversial, "hot" topic that has received considerable enrichment in recent years by both clinical and basic researchers. On the one hand, the classical view of desmogleins (Dsg) as main targets of this autoimmune disease is supported by the characterization of pathogenic anti-Dsg3 antibodies from both patients and animal models. On the other hand, fundamental doubt has been raised towards this monopathogenic view by several independent factors: (1) pemphigus lesions can be induced in Dsg3-knockout (KO) mice; (2) pemphigus sera contain multiple autoantibodies against different adhesion molecules and also cholinergic receptors; (3) experimental inhibition of PV IgG induced acantholysis can be obtained by interference with different signaling cascades regulating both calcium homeostasis and apoptosis; and (4) cholinergic agonists exhibit anti-acantholytic activity both in vitro and in vivo. The field is open for controlled clinical trials and further basic research to unfold the true story of the pemphigus enigma and provide the basis for a better treatment of pemphigus patients.

Defects in muscarinic receptor cell signaling in bladder urothelial cancer cell lines

INTRODUCTION

To explore muscarinic receptor signaling in 4 bladder cancer cell lines, bladder urothelial cells (BUC) have been shown to release and respond to various putative neurotransmitters.

METHODS

Reverse transcription-polymerase chain reaction was used to detect the presence of m1-m5 transcripts in the J82, RT4, T24, and 5637 lines of cancer BUC. Immunofluorescence was used to detect expression of m3 protein. Cancer and normal BUC were stimulated with carbachol (100 microM), a muscarinic agonist. Carbachol-evoked changes in intracellular calcium ([Ca(2+)](i)) levels were measured using fura-2 ratiometric microfluorimetry. Transfection of J82 cells with m3 plasmid was performed, and changes in carbachol-evoked [Ca(2+)](i) were re-examined.

RESULTS

None of the cancer cell lines expressed m3 transcripts, unlike normal BUC, which expressed m3. None of the 4 bladder cancer cell lines responded to carbachol. However, 47% of normal BUC responded to carbachol. The m3-transfected J82 cells expressed both m3 transcript and protein. Thirteen percent of m3-transfected J82 cells responded to carbachol.

CONCLUSIONS

This is the first description of altered muscarinic signaling in cancer BUC. Unlike normal BUC, bladder urothelial cancer cells neither expressed m3 transcript nor responded to carbachol, as measured by changes in [Ca(2+)](i). We could partially reverse this defect in one of the cancer cell lines, J82, by transfecting these cells with the m3 plasmid. Although the effects of muscarinic receptor signaling on urothelial cell are unknown, this signaling pathway may play a role in urothelial cell adhesion similar to that in keratinocytes.

M3 muscarinic acetylcholine receptor is associated with beta-catenin in ventricular myocytes during myocardial infarction in the rat

1. The present study was designed to investigate whether the M(3) muscarinic acetylcholine receptors (mAChR) is associated with beta-catenin in the ventricular myocardium during ischaemic myocardial injury and to determine the possible mechanism/s involved. 2. Rat hearts were subjected to coronary artery ligation for 1 and 6 h or 1 month to establish a myocardial ischaemia (MI) model. In the acute MI model, 16 rats were randomized into four groups: (i) control; (ii) ischaemia (rats were subjected to 20 min coronary occlusion); (iii) choline (10 mg/kg, i.v., choline chloride, an M(3) receptor agonist, was administered 15 min before occlusion); and (iv) 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; 0.12 mg/kg 4-DAMP, an M(3) receptor antagonist, was administered 20 min before occlusion, followed 5 min later by 10 mg/kg, i.v., choline chloride). Immunochemistry, western blot analysis and immunoprecipitation were used to determine the expression and localization of beta-catenin and the M(3) mAChR. 3. Myocardial ischaemia caused a time-dependent increase in the expression of beta-catenin. Moreover, a physical association was found between beta-catenin and the M(3) mAChR in intercalated discs. This association was enhanced by prolonged ischaemia. Administration of choline before ischaemia not only increased beta-catenin expression, but also strengthened the association between beta-catenin and the M(3) mAChR. However, blockade of M(3) mAChR by 4-DAMP completely inhibited the effect of choline on the expression of beta-catenin. In addition, MI increased phosphorylation of the M(3) mAChR. 4. The results indicate that increased beta-catenin activity is associated with M(3) mAChR during MI. This association is likely to play a role in heart signal transduction during ischaemia between neighbouring ventricular myocardiocum.

Evidence of key role of Cdk2 overexpression in pemphigus vulgaris

The pathogenesis of pemphigus vulgaris (PV) is still poorly understood. Autoantibodies present in PV patients can promote detrimental effects by triggering altered transduction of signals, which results in a final acantholysis. To investigate mechanisms involved in PV, cultured keratinocytes were treated with PV serum. PV sera were able to promote the cell cycle progression, inducing the accumulation of cyclin-dependent kinase 2 (Cdk2). Microarray analysis on keratinocytes detected that PV serum induced important changes in genes coding for one and the same proteins with known biological functions involved in PV disease (560 differentially expressed genes were identified). Then, we used two different approaches to investigate the role of Cdk2. First, small interfering RNA depletion of Cdk2 prevented cell-cell detachment induced by PV sera. Second, pharmacological inhibition of Cdk2 activity through roscovitine prevented blister formation and acantholysis in the mouse model of the disease. In vivo PV serum was found to alter multiple different pathways by microarray analysis (1463 differentially expressed genes were identified). Major changes in gene expression induced by roscovitine were studied through comparison of effects of PV serum alone and in association with roscovitine. The most significantly enriched pathways were cell communication, gap junction, focal adhesion, adherens junction, and tight junction. Our data indicate that major Cdk2-dependent multiple gene regulatory events are present in PV. This alteration may influence the evolution of PV and its therapy.

Differential coupling of M1 muscarinic and alpha7 nicotinic receptors to inhibition of pemphigus acantholysis

The mechanisms mediating and regulating assembly and disassembly of intercellular junctions is a subject of intensive research. The IgG autoantibodies produced in patients with the immunoblistering skin disease pemphigus vulgaris (PV) can induce keratinocyte (KC) dyshesion (acantholysis) via mechanisms that involve signaling kinases targeting intercellular adhesion molecules, thus providing a useful model to study the physiologic regulation of KC cohesion. Previous studies showed that activation of Src and protein kinase C are the earliest events in the PV IgG-induced intracellular phosphorylation cascades and that cholinergic agonists are effective for treating patients with pemphigus. In this study, we sought to elucidate the molecular mechanisms allowing cholinergic agonists to inhibit PV IgG-induced acantholysis and phosphorylation of KC adhesion molecules. The extent of acantholysis in KC monolayers correlated closely with the degree of PV IgG-induced phosphorylation of p120- and beta-catenins, with classic isoforms of protein kinase C mediating serine phosphorylation of beta-catenin and Src-tyrosine phosphorylation of p120-catenin. The M(1) muscarinic agonist pilocarpine blocked phosphorylation of both catenins, which could be abolised by the M(1) antagonist MT7. The alpha7 nicotinic agonist AR-R17779 inhibited phosphorylation of P120-cateinin. The alpha7 antagonist methyllycaconitine abolished the effect of AR-R17779. Okadaic acid abrogated protective effects of agonists on phosphorylation of beta-catenin, and pervanadate, on that of p120-catenin. Stimulation of KCs with pilocarpine significantly (p < 0.05) elevated both serine/threonine and tyrosine phosphatase activities in KCs. AR-R17779 both stimulated tyrosine phosphatase and decreased PV IgG-induced Src activity. Methyllycaconitine released Src activity in intact KCs and caused acantholysis. Thus, downstream signaling from M(1) abolished PV IgG-dependent catenin phosphorylation due to activation of both serine/threonine and tyrosine phosphatases, whereas alpha7 action involved both activation of tyrosine phosphatase and inhibition of Src. These findings identified novel paradigm of regulation of signaling kinases associated with cholinergic receptors and provided mechanistic explanation of therapeutic activity of cholinomimetics in PV patients.

Muscarinic acetylcholine receptors: mutant mice provide new insights for drug development

Muscarinic acetylcholine receptors (mAChRs), M(1)-M(5), regulate the activity of numerous fundamental central and peripheral functions. The lack of small-molecule ligands that can block or activate specific mAChR subtypes with high selectivity has remained a major obstacle in defining the roles of the individual receptor subtypes and in the development of novel muscarinic drugs. Recently, phenotypic analysis of mutant mouse strains deficient in each of the five mAChR subtypes has led to a wealth of new information regarding the physiological roles of the individual receptor subtypes. Importantly, these studies have identified specific mAChR-regulated pathways as potentially novel targets for the treatment of various important disorders including Alzheimer's disease, schizophrenia, pain, obesity and diabetes.

Central role of alpha9 acetylcholine receptor in coordinating keratinocyte adhesion and motility at the initiation of epithelialization

Epithelialization, a major component of wound healing, depends on keratinocyte adhesion and migration. Initiation of migration relies upon the ability of keratinocytes to free themselves from neighboring cells and basement membrane. The local cytotransmitter acetylcholine (ACh) controls keratinocyte adhesion and locomotion through different classes of ACh receptors (AChR). In this study, we explored signaling pathways downstream of the alpha9 AChR subtype that had been shown to control cell shape and cytoplasm mobility. Inactivation of alpha9 signaling by pharmacologic antagonism and RNA interference in keratinocyte cultures and null mutation in knockout mice delayed wound re-epithelialization in vitro and in vivo, respectively, and diminished the extent of colony scattering and cell outgrowth from the megacolony. Although keratinocytes at the leading edge elongated, produced filopodia and moved out, most of them remained anchored to the substrate by long cytoplasmic processes that stretched during their migration instead of retracting the uropod. Since the velocity of keratinocyte migration was not altered, we investigated the role of alpha9 in assembly/disassembly of the cell-cell and cell-matrix adhesion complexes. Stimulation of alpha9 upregulated in a time-dependent fashion phosphorylation of the adhesion molecules comprising focal adhesions (FAK, paxillin) and intercellular junctions (beta-catenin, desmoglein 3) as well as cytokeratins. Stimulation of alpha9 was associated with activation of phospholipase C, Src, EGF receptor kinase, protein kinase C, Rac and Rho, whereas inhibition of this receptor interfered with phosphorylation of adhesion and cytoskeletal proteins, and also altered cell-cell cohesion. We conclude that signaling through alpha9 AChR is critical for completion of the very early stages of epithelialization. By activating alpha9 AChR, ACh can control the dynamics and strength of cell-cell cohesion, disabling of a trailing uropod and disassembly and reassembly of focal adhesions, thus facilitating crawling locomotion.

Expression of muscarinic and nicotinic acetylcholine receptors in the mouse urothelium

Acetylcholine (ACh) and its receptors play a crucial role in bladder physiology. Here, we investigated the presence of muscarinic receptor subtypes (MR) and nicotinic acetylcholine receptor (nAChR) alpha-subunits in the mouse urothelium by RT-PCR and immunohistochemistry. With RT-PCR, we detected mRNAs coding for all of the five different MR subtypes and for the nicotinic receptor subunits alpha2, alpha4, alpha5, alpha6, alpha7, alpha9 and alpha10, whereas the alpha3-subunit was not expressed. Using immunohistochemistry, we localised a panel of acetylcholine receptors in the different layers of the murine bladder urothelium, with predominant appearance in the basal plasma membrane of the basal cell layer and in the apical membrane of the umbrella cells. M2R and subunit alpha9 were observed exclusively in the umbrella cells, whereas the MR subtypes 3-5 and the nAChR subunits alpha4, alpha7 and alpha10 were also detected in the intermediate and basal cell layers. The subunit alpha5 was localised only in the basal cell layer. In conclusion, the murine urothelium expresses multiple cholinergic receptors, including several subtypes of both MR and nAChR, which are differentially distributed among the urothelial cell types. Since these receptors have different electrophysiological and pharmacological properties, and therefore are considered to be responsible for different cellular responses to ACh, this differential distribution is expected to confer cell type-specificity of cholinergic regulation in the bladder urothelium.

Functional Characterization of the Epidermal Cholinergic System In Vitro

The aim of this study was to analyze the influence of cholinergic and anticholinergic drugs on epidermal physiology using organotypic cocultures (OTCs). Blocking of all acetylcholine receptors (AChRs) by combined treatment with mecamylamine and atropine or treatment with strychnine (blocking alpha9nAChR) for 7-14 days resulted in a complete inhibition of epidermal differentiation and proliferation. Blockage of nicotinic (n)AChR with mecamylamine led to a less pronounced delay in epidermal differentiation and proliferation than blockage of muscarinic (m)AChR with atropine, evidenced by reduced epithelial thickness and expression of terminal differentiation markers like cytokeratin 2e or filaggrin. In OTCs treated with atropine, mecamylamine, or strychnine, we could demonstrate intracellular lipid accumulation in the lower epidermal layers, indicating a severely disturbed epidermal barrier. In addition, we observed prominent acantholysis in the basal and lower suprabasal layers in mecamylamine-, atropine-, and strychnine-treated cultures, accompanied by a decreased expression of cell adhesion proteins. This globally reduced cell adhesion led to cell death via intrinsic activation of apoptosis. In contrast, stimulation of nAChR and mAChR with cholinergic drugs resulted in a significantly thickened epithelium, accompanied by an improved epithelial maturation. In summary, we show that epidermal AChR are crucially involved in the regulation of epidermal homeostasis.

Adrenergic and Cholinergic Control in the Biology of Epidermis: Physiological and Clinical Significance

The presence of an autocrine adrenergic and cholinergic intra/intercellular signal transduction network in the human epidermis contributes significantly to homeostatic and compensatory responses regulating vital functions in keratinocytes and melanocytes. The ligands produced control autocrine and paracrine loops to initiate responses through cognate receptors expressed within the same or adjacent cells. The epidermal adrenergic signal controls calcium homeostasis, cell growth, differentiation, motility, and pigmentation via the beta2 and alpha1 adrenoceptors. The cholinergic system is highly complex comprising both nicotinic and muscarinic receptors with multiple subtypes and this system plays an important role in keratinocyte cell cycle progression, differentiation, directional migration, adhesion, and apoptotic secretion. Moreover, lymphocytes also express adrenergic and cholinergic receptors. Both types of signal transduction receptors are coupled to classical intracellular second messenger pathways, including cAMP-, cGMP-, and calcium-mediated downstream responses. To date, it has been recognized that several dermatoses such as psoriasis, atopic dermatitis, Mal de Meleda, vitiligo, palmoplantar pustulosis, and pemphigus may be mediated, in part, by the non-neuronal adrenergic/cholinergic systems. A detailed understanding of the physiology and pathophysiology of the adrenergic/cholinergic network in the skin could offer the development of specific drugs for novel treatment modalities.

How does acantholysis occur in pemphigus vulgaris: a critical review

BACKGROUND

Pemphigus vulgaris is a life-threatening autoimmune blistering disease targeting skin and mucous membranes, characterized by disruption of keratinocytes' adhesion termed acantholysis. Today multiple classes of targets are considered to play a role in the genesis of the acantholysis; of these, the classical pemphigus antigens, desmosomal cadherins (desmoglein 1 and 3) are the best characterized and considered as the most important. Additional antigens include the novel epithelial acetylcholine receptors (alpha9 and pemphaxin). Thus, acantholysis in pemphigus seems to result from a cooperative action of antibodies to different keratinocyte self-antigens, but the mechanisms by which epithelial cleft occurs are not yet clearly understood. In fact, the binding of the autoantibodies to these targets generates a plethora of biological effects due, on one hand, to their direct interference with adhesive function and, on the other, to more complex events involving intracellular pathways that modify proteases activity or calcium metabolism, leading to loss of cell-cell adhesion.

Cholinergic control of epidermal cohesion

The non-neuronal cholinergic system of human epidermis includes the keratinocyte (KC) acetylcholine (ACh) axis composed of the enzymes mediating ACh synthesis and degradation, and two classes of ACh receptors, the nicotinic and muscarinic ACh receptors, mediating biological effects of the cutaneous cytotransmitter ACh. Regulation of KC cell-cell and cell-matrix adhesion is one of the important biological functions of cutaneous ACh. The downstream targets of ACh effects mediated by distinct ACh receptor subtypes include both the intercellular adhesion molecules, such as classical and desmosomal cadherins, and integrins mediating KC adhesion to a substrate. The signaling pathways include activation or inhibition of kinase cascades resulting in either up- or down-regulation of the expression of cell adhesion molecules or changes in their phosphorylation status, or both. The components of the KC ACh axis are involved in cutaneous blistering in patients with autoimmune pemphigus, junctional and dystrophic forms of epidermolysis bullosa, thermal burns, and mustard-induced vesication. Recent progress with the development of antiacantholytic therapies of patients with pemphigus using cholinomimetics indicates that cholinergic drugs may be a promising approach for other cutaneous blistering disorders.

Muscarinic receptor subtypes in neuronal and non-neuronal cholinergic function

1 Muscarinic M1-M5 receptors mediate the metabotropic actions of acetylcholine in the nervous system. A growing body of data indicate they also mediate autocrine functions of the molecule. The availability of novel and selective muscarinic agonists and antagonists, as well as in vivo gene disruption techniques, has clarified the roles of muscarinic receptors in mediating both functions of acetylcholine. 2 Selective M1 agonists or mixed M1 agonists/M2 antagonists may provide an approach to the treatment of cognitive disorders, while M3 antagonism, or mixed M2/M3 antagonists, are approved for the treatment of contractility disorders including overactive bladder and chronic obstructive pulmonary disease. Preclinical data suggest that selective agonism of the M4 receptor will provide novel anti-nociceptive agents, while therapeutics-based upon agonism or antagonism of the muscarinic M5 receptor have yet to be reported. 3 The autocrine functions of muscarinic receptors broadly fall into two areas - control of cell growth or proliferation and mediation of the release of chemical mediators from epithelial cells, ultimately causing muscle relaxation. The former particularly are involved in embryological development, oncogenesis, keratinocyte function and immune responsiveness. The latter regulate contractility of smooth muscle in the vasculature, airways and urinary bladder. 4 Most attention has focused on muscarinic M1 or M3 receptors which mediate lymphocyte immunoresponsiveness, cell migration and release of smooth muscle relaxant factors. Muscarinic M4 receptors are implicated in the regulation of keratinocyte adhesion and M2 receptors in stem cell proliferation and development. Little data are available concerning the M5 receptor, partly due to the difficulties in defining the subtype pharmacologically. 5 The autocrine functions of acetylcholine, like those in the nervous system, involve activation of several muscarinic receptor subtypes. Consequently, the role of these subtypes in autocrine, as well neuronal cholinergic systems, significantly expands their importance in physiology and pathophysiology.