Nicotinic acetylcholine receptor α7 subunit is time-dependently expressed in distinct cell types during skin wound healing in mice

B cell-derived acetylcholine mitigates skin inflammation in mice through α9 nicotinic acetylcholine receptor-mediated signaling

Chronic inflammatory skin disorders are characterized by keratinocyte hyperproliferation and hyperactivation as well as immune cell infiltration. We investigated whether immune cell-derived acetylcholine (ACh) is a modulator of skin inflammation in mice. Here, we identify skin epithelial B cells as a key source of ACh that damps down inflammation. We used imiquimod (IMQ) to induce inflammatory skin disease (ISD) in mice lacking ACh production specifically in B cells (ChATfl/fl;Mb1-Cre mice). Increased keratinocyte proliferation, epidermal thickening, and elevated levels of proinflammatory cytokines resulted. ACh binding to α9 nicotinic ACh receptor (encoded by Chrna9) expressed on wild-type mouse keratinocytes reduced their proliferation. Chrna9-deficient mice exhibited the same exacerbated ISD phenotype as ChATfl/fl;Mb1-Cre mice following IMQ induction. Our data suggest that B cell-derived ACh maintains skin homeostasis by modulating keratinocyte turnover and controlling immune-related inflammation. Therapeutic manipulation of this cholinergic pathway might mitigate both keratinocyte dysfunction and immune dysregulation in human patients, potentially pointing to treatments for ISDs such as psoriasis and related disorders.

Pathophysiological Roles of Ion Channels in Epidermal Cells, Immune Cells, and Sensory Neurons in Psoriasis

Psoriasis is a chronic inflammatory skin disease characterized by the rapid abnormal growth of skin cells in the epidermis, driven by an overactive immune system. Consequently, a complex interplay among epidermal cells, immune cells, and sensory neurons contributes to the development and progression of psoriasis. In these cellular contexts, various ion channels, such as acetylcholine receptors, TRP channels, Ca2+ release-activated channels, chloride channels, and potassium channels, each serve specific functions to maintain the homeostasis of the skin. The dysregulation of ion channels plays a major role in the pathophysiology of psoriasis, affecting various aspects of epidermal cells, immune responses, and sensory neuron signaling. Impaired function of ion channels can lead to altered calcium signaling, inflammation, proliferation, and sensory signaling, all of which are central features of psoriasis. This overview summarizes the pathophysiological roles of ion channels in epidermal cells, immune cells, and sensory neurons during early and late psoriatic processes, thereby contributing to a deeper understanding of ion channel involvement in the interplay of psoriasis and making a crucial advance toward more precise and personalized approaches for psoriasis treatment.

Alpha-7 nicotinic acetylcholine receptor agonist alleviates psoriasis-like inflammation through inhibition of the STAT3 and NF-κB signaling pathway

Psoriasis is a chronic inflammatory cutaneous disease; it has been discovered that stimulation of the nervous system increases susceptibility to psoriasis. Although the cholinergic anti-inflammatory pathway, which is mediated by the alpha-7 nicotinic acetylcholine receptor (α7nAChR), is critical for controlling multiple types of inflammation, its expression pattern and pathogenesis function in psoriatic lesioned skin tissue are unknown. We hereby analyzed the expression of α7nAchR in human and mouse psoriatic skin tissue. In vivo, PNU-282987 or Methyllycaconitine, a specific agonist or antagonist of α7nAchR, were administered to imiquimod (IMQ)-induced psoriatic mouse models. The macroscopic appearance and histopathological features of the psoriatic mice skin were evaluated. In addition, cell proliferation and differentiation markers were investigated. The level of pro-inflammatory cytokines released from the lesioned skin, as well as the activation of the relevant signaling pathways, were measured. Our findings indicated that psoriatic lesional skin expressed an increased level of α7nAChR, with its tissue distribution being primarily in skin keratinocytes and macrophages. In an IMQ-induced murine psoriasis model, α7nAChR agonist PNU-282987 treatment alleviated psoriasis-like inflammation by down-regulating the expression of multiple types of pro-inflammatory mediators and normalized keratinocyte proliferation and differentiation, whereas α7nAChR antagonist treatment exacerbated its effect. Mechanically, we observed that activation of the α7nAChR inhibited the activation of the STAT3 and NF-κB signaling pathways in in vitro cultured HaCaT cells induced by Th17-related cytokine IL-6/IL-22 or Th1-related cytokine TNF-α. Taken together, these findings demonstrate that attenuation of psoriatic inflammation via the cholinergic anti-inflammatory pathway is dependent on α7nAChR activation.

Rivastigmine Regulates the HIF-1α/VEGF Signaling Pathway to Induce Angiogenesis and Improves the Survival of Random Flaps in Rats

Random skin flaps are frequently used to repair skin damage. However, the ischemic and hypoxic necrosis limits their wider application. Rivastigmine, a carbamate cholinesterase inhibitor (ChEI), has also been shown to reduce ischemia–reperfusion injury (IRI) and inflammation. This study was performed to examine the effect of rivastigmine on flap survival. Sixty male Sprague–Dawley rats with a modified McFarland flap were randomly divided into three groups: control group, 1 ml of solvent (10% DMSO + 90% corn oil); low-dose rivastigmine group (Riv-L), 1.0 mg/kg; and high-dose rivastigmine group (Riv-H), 2.0 mg/kg. All rats were treated once a day. On day 7, the skin flap survival area was measured. After staining with hematoxylin and eosin (H&E), the pathological changes and microvessel density (MVD) were examined. The expression of inflammatory factors IL-1β and IL-18, CD34, hypoxia-inducible factor-1α (HIF-1α), and vascular endothelial growth factor (VEGF) was examined by immunohistochemical staining. The malondialdehyde (MDA) content and superoxide dismutase (SOD) activity were examined to determine the degree of oxidative stress. Lead oxide/gelatin angiography showed neovascularization and laser Doppler blood flowmetry showed the blood filling volume. Rivastigmine significantly increased the flap survival area and improved neovascularization. CD34, VEGF, and HIF-1α expression were increased, These changes were more pronounced in the Riv-H group. Treatment with rivastigmine reduced the level of MDA, improved SOD activity, and reduced expression of IL-1β and IL-18. Our results indicate that Rivastigmine can increase angiogenesis and significantly improve flap survival.

Neuroendocrinology and neurobiology of sebaceous glands

The nervous system communicates with peripheral tissues through nerve fibres and the systemic release of hypothalamic and pituitary neurohormones. Communication between the nervous system and the largest human organ, skin, has traditionally received little attention. In particular, the neuro-regulation of sebaceous glands (SGs), a major skin appendage, is rarely considered. Yet, it is clear that the SG is under stringent pituitary control, and forms a fascinating, clinically relevant peripheral target organ in which to study the neuroendocrine and neural regulation of epithelia. Sebum, the major secretory product of the SG, is composed of a complex mixture of lipids resulting from the holocrine secretion of specialised epithelial cells (sebocytes). It is indicative of a role of the neuroendocrine system in SG function that excess circulating levels of growth hormone, thyroxine or prolactin result in increased sebum production (seborrhoea). Conversely, growth hormone deficiency, hypothyroidism, and adrenal insufficiency result in reduced sebum production and dry skin. Furthermore, the androgen sensitivity of SGs appears to be under neuroendocrine control, as hypophysectomy (removal of the pituitary) renders SGs largely insensitive to stimulation by testosterone, which is crucial for maintaining SG homeostasis. However, several neurohormones, such as adrenocorticotropic hormone and α-melanocyte-stimulating hormone, can stimulate sebum production independently of either the testes or the adrenal glands, further underscoring the importance of neuroendocrine control in SG biology. Moreover, sebocytes synthesise several neurohormones and express their receptors, suggestive of the presence of neuro-autocrine mechanisms of sebocyte modulation. Aside from the neuroendocrine system, it is conceivable that secretion of neuropeptides and neurotransmitters from cutaneous nerve endings may also act on sebocytes or their progenitors, given that the skin is richly innervated. However, to date, the neural controls of SG development and function remain poorly investigated and incompletely understood. Botulinum toxin-mediated or facial paresis-associated reduction of human sebum secretion suggests that cutaneous nerve-derived substances modulate lipid and inflammatory cytokine synthesis by sebocytes, possibly implicating the nervous system in acne pathogenesis. Additionally, evidence suggests that cutaneous denervation in mice alters the expression of key regulators of SG homeostasis. In this review, we examine the current evidence regarding neuroendocrine and neurobiological regulation of human SG function in physiology and pathology. We further call attention to this line of research as an instructive model for probing and therapeutically manipulating the mechanistic links between the nervous system and mammalian skin.

α7-nAChR Activation Has an Opposite Effect on Healing of Covered and Uncovered Wounds

The α7 nicotinic acetylcholine receptor (α7-nAChR) is associated with inflammation, re-epithelialization, and angiogenesis in wound healing process. A recent study demonstrated that PNU-282987, a selective agonist of α7-nAChR, accelerates the repair of diabetic excisional wounds. Whether α7-nAChR activation promotes non-diabetic wounds healing is unknown. The aim of this study was to evaluate the effects of α7-nAChR activation on non-diabetic wound healing. The effects were evaluated in two wound models. In the first model, the wound was covered with a semi-permeable transparent dressing. In the second model, the wound was left uncovered. In both models, the mice were randomly assigned to two treatment groups: saline or PNU282987 (25 mice in each group). In covered wounds, we found that α7-nAChR activation inhibited re-epithelialization, angiogenesis, and epithelial cells proliferation, promoted neo-epithelial detachment, and suppressed neutrophil infiltration and the expression of interleukin-6 (IL-6) and vascular endothelial growth factor (VEGF). However, in uncovered wounds, we observed that α7-nAChR activation promoted re-epithelialization and angiogenesis, inhibited neutrophil infiltration and the expression of high mobility group box (HMGB)-1, epidermal growth factor (EGF), and VEGF. In conclusion, this data demonstrated that α7-nAChR activation inhibited wound healing in covered wounds but played an opposite role in uncovered wounds. The opposite effect might be primarily due to inhibition of inflammation.

Oxidative stress management in the hair follicle: Could targeting NRF2 counter age-related hair disorders and beyond?

Widespread expression of the transcription factor, nuclear factor (erythroid-derived 2)-like 2 (NRF2), which maintains redox homeostasis, has recently been identified in the hair follicle (HF). Small molecule activators of NRF2 may therefore be useful in the management of HF pathologies associated with redox imbalance, ranging from HF greying and HF ageing via androgenetic alopecia and alopecia areata to chemotherapy-induced hair loss. Indeed, NRF2 activation has been shown to prevent peroxide-induced hair growth inhibition. Multiple parameters can increase the levels of reactive oxygen species in the HF, for example melanogenesis, depilation-induced trauma, neurogenic and autoimmune inflammation, toxic drugs, environmental stressors such as UV irradiation, genetic defects and aging-associated mitochondrial dysfunction. In this review, the potential mechanisms whereby NRF2 activation could prove beneficial in treatment of redox-associated HF disorders are therefore discussed.

Activation of α7nAChR Promotes Diabetic Wound Healing by Suppressing AGE-Induced TNF-α Production

Diabetes frequently presents accumulation of advanced glycation end products (AGEs), which might induce excessive TNF-α production from macrophages to cause impaired wound healing. Recent studies have shown that activation of α7 nicotinic acetylcholine receptor (α7nAChR) on macrophages efficiently suppressed TNF-α synthesis. The aim of this study was to investigate the accumulation of AGEs in the wounds and determine whether PNU282987, an α7nAChR agonist, can improve wound repair by inhibiting AGE-mediated TNF-α production in a streptozotocin (STZ)-induced diabetic mouse model. Animals were assigned into four groups: wounded control group, wounded diabetic group, wounded diabetic group treated intraperitoneally with PNU282987, or wounded diabetic group treated intraperitoneally with vehicle. Compared with the non-diabetic control mice, the diabetic mice exhibited delayed wound healing that was characterized by elevated accumulation of AGEs, increased TNF-α level and macrophage infiltration, and decreased fibroblast number and collagen deposition at the late stage of repair. Besides, macrophages of diabetic wounds showed expression of α7nAChR. During late repair, PNU282987 treatment of diabetic mice significantly reduced the level of TNF-α, accelerated wound healing, and elevated fibroblast number and collagen deposition. To investigate the cellular mechanism of these observations, RAW 264.7 cells, a macrophage cell line, were incubated with AGEs in the presence or absence of PNU282987. TNF-α production from AGE-stimulated macrophages was significantly decreased by PNU282987 in a dose-dependent manner. Furthermore, PNU282987 significantly inhibited AGE-induced nuclear factor-κB (NF-κB) activation and receptor for AGE (RAGE) expression. These results strongly suggest that activating α7nAChR can promote diabetic wound healing by suppressing AGE-induced TNF-α production, which may be closely associated with the blockage of NF-κB activation in macrophages.

Modulatory effects of α7 nAChRs on the immune system and its relevance for CNS disorders

The clinical development of selective alpha-7 nicotinic acetylcholine receptor (α7 nAChR) agonists has hitherto been focused on disorders characterized by cognitive deficits (e.g., Alzheimer's disease, schizophrenia). However, α7 nAChRs are also widely expressed by cells of the immune system and by cells with a secondary role in pathogen defense. Activation of α7 nAChRs leads to an anti-inflammatory effect. Since sterile inflammation is a frequently observed phenomenon in both psychiatric disorders (e.g., schizophrenia, melancholic and bipolar depression) and neurological disorders (e.g., Alzheimer's disease, Parkinson's disease, and multiple sclerosis), α7 nAChR agonists might show beneficial effects in these central nervous system disorders. In the current review, we summarize information on receptor expression, the intracellular signaling pathways they modulate and reasons for receptor dysfunction. Information from tobacco smoking, vagus nerve stimulation, and cholinesterase inhibition is used to evaluate the therapeutic potential of selective α7 nAChR agonists in these inflammation-related disorders.

Further proof of the existence of a non-neuronal cholinergic system in the human Achilles tendon: Presence of the AChRα7 receptor in tendon cells and cells in the peritendinous tissue

Human tendon cells have the capacity for acetylcholine (ACh) production. It is not known if the tendon cells also have the potential for ACh breakdown, nor if they show expression of the nicotinic acetylcholine receptor AChRα7 (α7nAChR). Therefore, tendon tissue specimens from patients with midportion Achilles tendinopathy/tendinosis and from normal midportion Achilles tendons were examined. Reaction for the degradative enzyme acetylcholinesterase (AChE) was found in some tenocytes in only a few tendinopathy tendons, and was never found in those of control tendons. Tenocytes displayed more regularly α7nAChR immunoreactivity. However, there was a marked heterogeneity in the degree of this reaction within and between the specimens. α7nAChR immunoreactivity was especially pronounced for tenocytes showing an oval/widened appearance. There was a tendency that the magnitude of α7nAChR immunoreactivity was higher in tendinopathy tendons as compared to control tendons. A stronger α7nAChR immunoreactivity than seen for tenocytes was observed for the cells in the peritendinous tissue. It is likely that the α7nAChR may be an important part of an auto-and paracrine loop of non-neuronal ACh that is released from the tendon cells. The effects may be related to proliferative and blood vessel regulatory functions as well as features related to collagen deposition. ACh can furthermore be of importance in leading to anti-inflammatory effects in the peritendinous tissue, a tissue nowadays considered to be of great relevance for the tendinopathy process. Overall, the findings show that tendon tissue, a tissue known to be devoid of cholinergic innervation, is a tissue in which there is a marked non-neuronal cholinergic system.

Regulation of gene expression by tobacco product preparations in cultured human dermal fibroblasts

Skin fibroblasts comprise the first barrier of defense against wounds, and tobacco products directly contact the oral cavity. Cultured human dermal fibroblasts were exposed to smokeless tobacco extract (STE), total particulate matter (TPM) from tobacco smoke, or nicotine at concentrations comparable to those found in these extracts for 1h or 5h. Differences were identified in pathway-specific genes between treatments and vehicle using qRT-PCR. At 1h, IL1α was suppressed significantly by TPM and less significantly by STE. Neither FOS nor JUN was suppressed at 1h by tobacco products. IL8, TNFα, VCAM1, and NFκB1 were suppressed after 5h with STE, whereas only TNFα and NFκB1 were suppressed by TPM. At 1h with TPM, secreted levels of IL10 and TNFα were increased. Potentially confounding effects of nicotine were exemplified by genes such as ATF3 (5h), which was increased by nicotine but suppressed by other components of STE. Within 2h, TPM stimulated nitric oxide production, and both STE and TPM increased reactive oxygen species. The biological significance of these findings and utilization of the gene expression changes reported herein regarding effects of the tobacco product preparations on dermal fibroblasts will require additional research.

The time-dependent expression of α7nAChR during skeletal muscle wound healing in rats

The study on time-dependent expression of α7 nicotine acetylcholine receptor (α7nAChR) was performed by immunohistochemistry, Western blotting, and real-time PCR during skeletal muscle wound healing in rats. Furthermore, co-localization of α7nAChR with macrophage or myofibroblast marker was detected by double immunofluorescence. A total of 50 Sprague-Dawley male rats were divided into control and contusion groups (3 h, 6 h, 12 h, 1 day, 3 days, 5 days, 7 days, 10 days, and 14 days post-injury). In the uninjured controls, α7nAChR positive staining was observed in the sarcolemma and sarcoplasm of normal myofibers. In wounded specimens, a small number of polymorphonuclear cells, a number of macrophages and myofibroblasts showed positive reaction for α7nAChR in contused zones. Morphometrically, the average ratios of α7nAChR-positive cells were over 50 % from 3 to 10 days after contusion, and exceeded 60 % at 5 and 7 days post-injury. Besides, the positive ratios of α7nAChR were <50 % at the other posttraumatic intervals. By Western blotting analysis, the average ratio of α7nAChR protein expression maximized at 7 days after injury, which was >2.13. Similarly, the relative quantity of α7nAChR mRNA expression peaked at 7 days post-wounding as compared with control by real-time PCR detection, showing a relative quantity of >2.65. In conclusion, the expression of α7nAChR is upregulated and temporally distributed in macrophages and myofibroblasts during skeletal muscle wound healing, which might be closely involved in inflammatory response and fibrotic repair after injury. Moreover, α7nAChR is promising as a useful marker for wound age determination of skeletal muscle.

Nicotine effect on inflammatory and growth factor responses in murine cutaneous wound healing

The aim of the current study was to investigate the effect of nicotine in an experimental mouse model of cutaneous injury and healing responses, during the inflammatory phase of repair. Nicotine injection in full-thickness excisional skin wounds minimally affected inflammatory mediators like TNF, IL-6 and IL-12 while it induced a down-regulation in the expression of growth factors like VEGF, PDGF, TGF-β1 and TGF-β2, and the anti-inflammatory cytokine IL-10. Analysis of wound closure rate indicated no significant differences between nicotine and saline injected controls. In-vitro studies using bone marrow derived macrophages, resident peritoneal macrophages and RAW 264.7 macrophages, indicated that nicotine down-regulates TNF production. Moreover, nicotine was shown to down-regulate VEGF, PDGF and TGF-β1 in both bone marrow derived macrophages and RAW 264.7 cells. Using an NF-κB luciferase reporter RAW 264.7 cell line, we show that nicotine effects are minimally dependent on NF-κB inhibition. Moreover, nicotinic acetylcholine receptor (nAChR) subunit expression analyses indicated that while β2 nAChR subunit is expressed in mouse macrophages, α7 nAChR is not. In conclusion, while skin inflammatory parameters were not significantly affected by nicotine, a down-regulation of growth factor expression in both mouse skin and macrophages was observed. Reduced growth factor expression by nicotine might contribute, at least in part, to the overall detrimental effects of tobacco use in wound healing and skin diseases.

Recent progress in histochemistry and cell biology

Studies published in Histochemistry and Cell Biology in the year 2011 represent once more a manifest of established and newly sophisticated techniques being exploited to put tissue- and cell type-specific molecules into a functional context. The review is therefore the Histochemistry and Cell Biology's yearly intention to provide interested readers appropriate summaries of investigations touching the areas of tissue biology, developmental biology, the biology of the immune system, stem cell research, the biology of subcellular compartments, in order to put the message of such studies into natural scientific-/human- and also pathological-relevant correlations.

Presence of ChAT mRNA and a very marked α7nAChR immunoreaction in the synovial lining layer of the knee joint

AIMS

The aim was to examine if there is evidence of acetylcholine (ACh) production within the synovial lining layer and to examine the pattern of α7nAChR expression in the layer. This layer is of relevance clinically as it becomes thickened in response to both rheumatoid arthritis (RA) and osteoarthritis (OA) and as it has been shown to produce proteases that are involved in the cartilage destruction.

MAIN METHODS

Synovial tissue specimens from the knee joint of patients with RA and OA undergoing prosthetic surgery were examined. In situ hybridization and immunohistochemistry were used for the evaluation of ChAT reaction patterns. Immunohistochemistry was utilized for demonstration of activity of α7nAChR.

KEY FINDINGS

There were ChAT mRNA reactions in the synovial lining layer of both patient categories. On the other hand, no ChAT immunoreactions were detected in the layer. There was a very marked α7nAChR immunoreaction.

SIGNIFICANCE

There is a potential for ACh production within the synovial lining layer as there are ChAT mRNA reactions. However, the level of ACh production is apparently very low. It is thus possible that there is a down-regulation of ACh production but an apparent upregulation in expression level of α7nAChR. Based on the knowledge that the non-neuronal cholinergic system can have anti-inflammatory effects, the low level of ACh production in the synovial lining layer can be a drawback for the arthritic joints.