Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

Efficient fabrication of 3D bioprinted functional sensory neurons using an inducible Neurogenin-2 human pluripotent stem cell line

Three-dimensional (3D) tissue models have gained recognition for their improved ability to mimic the native cell microenvironment compared to traditional two-dimensional models. This progress has been driven by advances in tissue-engineering technologies such as 3D bioprinting, a promising method for fabricating biomimetic living tissues. While bioprinting has succeeded in generating various tissues to date, creating neural tissue models remains challenging. In this context, we present an accelerated approach to fabricate 3D sensory neuron (SN) structures using a transgenic human pluripotent stem cell (hPSC)-line that contains an inducible Neurogenin-2 (NGN2) expression cassette. The NGN2 hPSC line was first differentiated to neural crest cell (NCC) progenitors, then incorporated into a cytocompatible gelatin methacryloyl-based bioink for 3D bioprinting. Upregulated NGN2 expression in the bioprinted NCCs resulted in induced SN (iSN) populations that exhibited specific cell markers, with 3D analysis revealing widespread neurite outgrowth through the scaffold volume. Calcium imaging demonstrated functional activity of iSNs, including membrane excitability properties and voltage-gated sodium channel (NaV) activity. This efficient approach to generate 3D bioprinted iSN structures streamlines the development of neural tissue models, useful for the study of neurodevelopment and disease states and offering translational potential.

Towards the development of sensation-enabled skin substitutes

Recent advances in cell and biofabrication technologies have contributed to the development of complex human organs. In particular, several skin substitutes are being generated using tissue engineering and regenerative medicine (TERM) technologies. However, recent studies mainly focus on the restoration of the dermis and epidermis layers rather than the regeneration of a fully functional innervated skin organ. Innervation is a critical step in functional tissue repair which has been overlooked in the current TERM studies. In the current study, we highlight the importance of sensation in the skin as the largest sensory organ in the human body. In large non-healing skin wounds, the skin sensation is severely diminished or completely lost and ultimately lead to chronic pain and wound healing process interruption. Current therapeutics for restoring skin sensation after trauma are limited. Recent regenerative medicine-based studies could successfully induce neural networks in skin substitutes, but the effectiveness of these technologies in enhancing sensory capability needs further investigation.

Sensory sentinels: Neuroimmune detection and food allergy

Food allergy is classically characterized by an inappropriate type-2 immune response to allergenic food antigens. However, how allergens are detected and how that detection leads to the initiation of allergic immunity is poorly understood. In addition to the gastrointestinal tract, the barrier epithelium of the skin may also act as a site of food allergen sensitization. These barrier epithelia are densely innervated by sensory neurons, which respond to diverse physical environmental stimuli. Recent findings suggest that sensory neurons can directly detect a broad array of immunogens, including allergens, triggering sensory responses and the release of neuropeptides that influence immune cell function. Reciprocally, immune mediators modulate the activation or responsiveness of sensory neurons, forming neuroimmune feedback loops that may impact allergic immune responses. By utilizing cutaneous allergen exposure as a model, this review explores the pivotal role of sensory neurons in allergen detection and their dynamic bidirectional communication with the immune system, which ultimately orchestrates the type-2 immune response. Furthermore, it sheds light on how peripheral signals are integrated within the central nervous system to coordinate hallmark features of allergic reactions. Drawing from this emerging evidence, we propose that atopy arises from a dysregulated neuroimmune circuit.

Aging is associated with impaired triggering of TRPV3-mediated cutaneous vasodilation: a crucial process for local heat exposure

Sensing temperature is vitally important to adapt our body to environmental changes. Local warm detection is required to initiate regulation of cutaneous blood flow, which is part of the peripheral thermoregulatory mechanisms, and thus avoid damage to surrounding tissues. The mechanisms mediating cutaneous vasodilation during local heat stress are impaired with aging. However, the impact of aging on the ability of the skin to detect subtle thermal changes is unknown. Among heat-activated cation channels, transient receptor potential vanilloid 3 (TRPV3) is a thermo-sensor predominantly expressed on keratinocytes and involved in local vascular thermoregulatory mechanisms of the skin in young mice. In the present study, using a murine in vivo model of local heat exposure of the skin, we showed that heat-induced vasodilation was reduced in old mice associated with reduced expression of TRPV3 channels. We also found a decrease in expression and activity of TRPV3 channel, as well as reduced TRPV3-dependent adenosine tri-phosphate release in human primary keratinocytes from old donors. This study shows that aging alters the epidermal TRPV3 channels, which might delay the detection of changes in skin temperature, thereby limiting the mechanisms triggered for local vascular thermoregulation in the old skin.

Flexible Sono-Piezo Patch for Functional Sweat Gland Repair through Endogenous Microenvironmental Remodeling

Remodeling the endogenous regenerative microenvironment in wounds is crucial for achieving scarless, functional tissue regeneration, especially the functional recovery of skin appendages such as sweat glands in burn patients. However, current approaches mostly rely on the use of exogenous materials or chemicals to stimulate cell proliferation and migration, while the remodeling of a pro-regenerative microenvironment remains challenging. Herein, we developed a flexible sono-piezo patch (fSPP) that aims to create an endogenous regenerative microenvironment to promote the repair of sweat glands in burn wounds. This patch, composed of multifunctional fibers with embedded piezoelectric nanoparticles, utilized low-intensity pulsed ultrasound (LIPUS) to activate electrical stimulation of the target tissue, resulting in enhanced pro-regenerative behaviors of niche tissues and cells, including peripheral nerves, fibroblasts, and vasculatures. We further demonstrated the effective wound healing and regeneration of functional sweat glands in burn injuries solely through such physical stimulation. This noninvasive and drug-free therapeutic approach holds significant potential for the clinical treatment of burn injuries.

Effects of EGFR-TKI on epidermal melanin unit integrity: Therapeutic implications for hypopigmented skin disorders

Epidermal melanin unit integrity is crucial for skin homeostasis and pigmentation. Epidermal growth factor (EGF) receptor (EGFR) is a pivotal player in cell growth, wound healing, and maintaining skin homeostasis. However, its influence on skin pigmentation is relatively unexplored. This study investigates the impact and underlying mechanisms of EGFR inhibitors on skin pigmentation. We evaluated EGF and EGFR expression in various skin cells using quantitative real-time PCR, Western blot, and immunofluorescence. EGF and EGFR were predominantly expressed in epidermal keratinocytes, and treatment with the EGFR tyrosine kinase inhibitors (EGFR-TKIs) gefitinib and PD153035 significantly increased stem cell factor (SCF) and endothelin-1 (ET-1) expression in cultured keratinocytes. Enhanced melanocyte migration and proliferation were observed in co-culture, as evidenced by time-lapse live imaging and single-cell tracking assays. Furthermore, topical application of gefitinib to guinea pig dorsal skin induced increased pigmentation and demonstrated efficacy in mitigating rhododendrol-induced leukoderma. Suppression of EGF signaling indirectly enhanced skin pigmentation by upregulating SCF and ET-1 in epidermal keratinocytes. This novel mechanism highlights the pivotal role of EGF signaling in regulating skin pigmentation, and topical EGFR-TKI therapy at an appropriate dose may be a promising approach for depigmentation disorder management.

Impact of double-strand breaks induced by uv radiation on neuroinflammation and neurodegenerative disorders

Exposure to UV affects the development and growth of a wide range of organisms. Nowadays, researchers are focusing on the impact of UV radiation and its underlying molecular mechanisms, as well as devising strategies to mitigate its harmful effects. Different forms of UV radiation, their typical exposure effects, the impact of UV on DNA integrity, and the deterioration of genetic material are discussed in this review; furthermore, we also review the effects of UV radiation that affect the biological functions of the organisms. Subsequently, we address the processes that aid organisms in navigating the damage in genetic material, neuroinflammation, and neurodegeneration brought on by UV-mediated double-strand breaks. To emphasize the molecular pathways, we conclude the review by going over the animal model studies that highlight the genes and proteins that are impacted by UV radiation.

Mapping Somatosensory Afferent Circuitry to Bone Identifies Neurotrophic Signals Required for Fracture Healing

The profound pain accompanying bone fracture is mediated by somatosensory neurons, which also appear to be required to initiate bone regeneration following fracture. Surprisingly, the precise neuroanatomical circuitry mediating skeletal nociception and regeneration remains incompletely understood. Here, we characterized somatosensory dorsal root ganglia (DRG) afferent neurons innervating murine long bones before and after experimental long bone fracture in mice. Retrograde labeling of DRG neurons by an adeno-associated virus with peripheral nerve tropism showed AAV-tdT signal. Single cell transcriptomic profiling of 6,648 DRG neurons showed highest labeling across CGRP+ neuron clusters (6.9-17.2%) belonging to unmyelinated C fibers, thinly myelinated Aδ fibers and Aβ-Field LTMR (9.2%). Gene expression profiles of retrograde labeled DRG neurons over multiple timepoints following experimental stress fracture revealed dynamic changes in gene expression corresponding to the acute inflammatory ( S100a8 , S100a9 ) and mechanical force ( Piezo2 ). Reparative phase after fracture included morphogens such as Tgfb1, Fgf9 and Fgf18 . Two methods to surgically or genetically denervate fractured bones were used in combination with scRNA-seq to implicate defective mesenchymal cell proliferation and osteodifferentiation as underlying the poor bone repair capacity in the presence of attenuated innervation. Finally, multi-tissue scRNA-seq and interactome analyses implicated neuron-derived FGF9 as a potent regulator of fracture repair, a finding compatible with in vitro assessments of neuron-to-skeletal mesenchyme interactions.

The Skin-Brain Axis: From UV and Pigmentation to Behaviour Modulation

The skin-brain axis has been suggested to play a role in several pathophysiological conditions, including opioid addiction, Parkinson's disease and many others. Recent evidence suggests that pathways regulating skin pigmentation may directly and indirectly regulate behaviour. Conversely, CNS-driven neural and hormonal responses have been demonstrated to regulate pigmentation, e.g., under stress. Additionally, due to the shared neuroectodermal origins of the melanocytes and neurons in the CNS, certain CNS diseases may be linked to pigmentation-related changes due to common regulators, e.g., MC1R variations. Furthermore, the HPA analogue of the skin connects skin pigmentation to the endocrine system, thereby allowing the skin to index possible hormonal abnormalities visibly. In this review, insight is provided into skin pigment production and neuromelanin synthesis in the brain and recent findings are summarised on how signalling pathways in the skin, with a particular focus on pigmentation, are interconnected with the central nervous system. Thus, this review may supply a better understanding of the mechanism of several skin-brain associations in health and disease.

The IL-31/CysLT2R axis is associated with itching in patients with allergic rhinitis

BACKGROUND

Itching is a troublesome symptom that disturbs patients with allergic rhinitis (AR). The molecular mechanisms underlying itching in AR need to be further illuminated. The aim of this study was to investigate the role of epithelial cell-derived interleukin-31 (IL-31) in nasal itching in AR.

METHODS

A total of 33 patients and 20 healthy control subjects were enrolled in this prospective study. The disease severity of patients with AR was assessed by the total visual analog scale score. The levels of IL-31, cysteinyl leukotriene receptor 1 (CysLT1R), and CysLT2R in the nasal brush specimens from the enrolled subjects were measured by quantitative real-time polymerase chain reaction (RT-PCR) and immunohistochemical staining. The expression of CysLT2R in a human nasal epithelial cell line (HNEpC) was assessed by immunofluorescence staining.

RESULTS

Compared with the control subjects, the protein and mRNA levels of IL-31 and CysLT2R were significantly increased in patients with AR. Higher levels of IL-31 and CysLT2R in nasal epithelial cells were associated with itching but not nasal congestion, rhinorrhea, or sneezing in AR. A significant relationship was found between IL-31 and CysLT2R in nasal epithelial cells, with a correlation coefficient of 0.93. Furthermore, RT-PCR and immunofluorescence staining revealed that IL-31 directly induced CysLT2R expression in HNEpCs. Nasal steroid treatment inhibited IL-31 and CysLT2R expression in 13 patients with AR in vivo.

CONCLUSIONS

Nasal epithelial cell-derived IL-31 might be associated with itching symptoms via CysLT2R in AR.

Epithelial-neuronal-immune cell interactions: Implications for immunity, inflammation, and tissue homeostasis at mucosal sites

The epithelial lining of the respiratory tract and intestine provides a critical physical barrier to protect host tissues against environmental insults, including dietary antigens, allergens, chemicals, and microorganisms. In addition, specialized epithelial cells communicate directly with hematopoietic and neuronal cells. These epithelial-immune and epithelial-neuronal interactions control host immune responses and have important implications for inflammatory conditions associated with defects in the epithelial barrier, including asthma, allergy, and inflammatory bowel diseases. In this review, we discuss emerging research that identifies the mechanisms and impact of epithelial-immune and epithelial-neuronal cross talk in regulating immunity, inflammation, and tissue homeostasis at mucosal barrier surfaces. Understanding the regulation and impact of these pathways could provide new therapeutic targets for inflammatory diseases at mucosal sites.

Role of neurogenic inflammation in the pathogenesis of alopecia areata

Alopecia areata refers to an autoimmune illness indicated by persistent inflammation. The key requirement for alopecia areata occurrence is the disruption of immune-privileged regions within the hair follicles. Recent research has indicated that neuropeptides play a role in the damage to hair follicles by triggering neurogenic inflammation, stimulating mast cells ambient the follicles, and promoting apoptotic processes in keratinocytes. However, the exact pathogenesis of alopecia areata requires further investigation. Recently, there has been an increasing focus on understanding the mechanisms of immune diseases resulting from the interplay between the nervous and the immune system. Neurogenic inflammation due to neuroimmune disorders of the skin system may disrupt the inflammatory microenvironment of the hair follicle, which plays a crucial part in the progression of alopecia areata.

Skin-brain axis in Alzheimer's disease - Pathologic, diagnostic, and therapeutic implications: A Hypothetical Review

The dynamic interaction between the brain and the skin is termed the 'skin-brain axis.' Changes in the skin not only reflect conditions in the brain but also exert direct and indirect effects on the brain. Interestingly, the connection between the skin and brain is crucial for understanding aging and neurodegenerative diseases. Several studies have shown an association between Alzheimer's disease (AD) and various skin disorders, such as psoriasis, bullous pemphigoid, and skin cancer. Previous studies have shown a significantly increased risk of new-onset AD in patients with psoriasis. In contrast, skin cancer may reduce the risk of developing AD. Accumulating evidence suggests an interaction between skin disease and AD; however, AD-associated pathological changes mediated by the skin-brain axis are not yet clearly defined. While some studies have reported on the diagnostic implications of the skin-brain axis in AD, few have discussed its potential therapeutic applications. In this review, we address the pathological changes mediated by the skin-brain axis in AD. Furthermore, we summarize (1) the diagnostic implications elucidated through the role of the skin-brain axis in AD and (2) the therapeutic implications for AD based on the skin-brain axis. Our review suggests that a potential therapeutic approach targeting the skin-brain axis will enable significant advances in the treatment of AD.

Microbial perspective on the skin-gut axis and atopic dermatitis

Atopic dermatitis (AD) is a relapsing inflammatory skin condition that has become a global health issue with complex etiology and mounting prevalence. The association of AD with skin and gut microbiota has been revealed by virtue of the continuous development of sequencing technology and genomics analysis. Also, the gut-brain-skin axis and its mutual crosstalk mechanisms have been gradually verified. Accordingly, the microbiota-skin-gut axis also plays an important role in allergic skin inflammation. Herein, we reviewed the relationship between the microbiota-skin-gut axis and AD, explored the underlying signaling molecules and potential pathways, and focused on the potential mechanisms of probiotics, antimicrobial peptides (AMPs), coagulase-negative staphylococci transplantation, fecal microbiota transplantation, AMPs, and addition of essential fatty acids in alleviating AD, with the aim to provide a new perspective for targeting microbiota in the treatment of allergic skin inflammation.

Photo-neuro-immuno-endocrinology: How the ultraviolet radiation regulates the body, brain, and immune system

Ultraviolet radiation (UVR) is primarily recognized for its detrimental effects such as cancerogenesis, skin aging, eye damage, and autoimmune disorders. With exception of ultraviolet B (UVB) requirement in the production of vitamin D3, the positive role of UVR in modulation of homeostasis is underappreciated. Skin exposure to UVR triggers local responses secondary to the induction of chemical, hormonal, immune, and neural signals that are defined by the chromophores and extent of UVR penetration into skin compartments. These responses are not random and are coordinated by the cutaneous neuro-immuno-endocrine system, which counteracts the action of external stressors and accommodates local homeostasis to the changing environment. The UVR induces electrical, chemical, and biological signals to be sent to the brain, endocrine and immune systems, as well as other central organs, which in concert regulate body homeostasis. To achieve its central homeostatic goal, the UVR-induced signals are precisely computed locally with transmission through nerves or humoral signals release into the circulation to activate and/or modulate coordinating central centers or organs. Such modulatory effects will be dependent on UVA and UVB wavelengths. This leads to immunosuppression, the activation of brain and endocrine coordinating centers, and the modification of different organ functions. Therefore, it is imperative to understand the underlying mechanisms of UVR electromagnetic energy penetration deep into the body, with its impact on the brain and internal organs. Photo-neuro-immuno-endocrinology can offer novel therapeutic approaches in addiction and mood disorders; autoimmune, neurodegenerative, and chronic pain-generating disorders; or pathologies involving endocrine, cardiovascular, gastrointestinal, or reproductive systems.

Molecular and cellular pruritus mechanisms in the host skin

Pruritus, also known as itching, is a complex sensation that involves the activation of specific physiological and cellular receptors. The skin is innervated with sensory nerves as well as some receptors for various sensations, and its immune system has prominent neurological connections. Sensory neurons have a considerable impact on the sensation of itching. However, immune cells also play a role in this process, as they release pruritogens. Disruption of the dermal barrier activates an immune response, initiating a series of chemical, physical, and cellular reactions. These reactions involve various cell types, including keratinocytes, as well as immune cells involved in innate and adaptive immunity. Collective activation of these immune responses confers protection against potential pathogens. Thus, understanding the molecular and cellular mechanisms that contribute to pruritus in host skin is crucial for the advancement of effective treatment approaches. This review provides a comprehensive analysis of the present knowledge concerning the molecular and cellular mechanisms underlying itching signaling in the skin. Additionally, this review explored the integration of these mechanisms with the broader context of itch mediators and the expression of their receptors in the skin.

The anatomy, neurophysiology, and cellular mechanisms of intradental sensation

Somatosensory innervation of the oral cavity enables the detection of a range of environmental stimuli including minute and noxious mechanical forces. The trigeminal sensory neurons underlie sensation originating from the tooth. Prior work has provided important physiological and molecular characterization of dental pulp sensory innervation. Clinical dental experiences have informed our conception of the consequence of activating these neurons. However, the biological role of sensory innervation within the tooth is yet to be defined. Recent transcriptomic data, combined with mouse genetic tools, have the capacity to provide important cell-type resolution for the physiological and behavioral function of pulp-innervating sensory neurons. Importantly, these tools can be applied to determine the neuronal origin of acute dental pain that coincides with tooth damage as well as pain stemming from tissue inflammation (i.e., pulpitis) toward developing treatment strategies aimed at relieving these distinct forms of pain.

Skin neuropathy and immunomodulation in diseases

Skin is a vital barrier tissue of the body. Immune responses in the skin must be precisely controlled, which would otherwise cause severe disease conditions such as psoriasis, atopic dermatitis, or pathogenic infection. Research evidence has increasingly demonstrated the essential roles of neural innervations, i.e., sensory and sympathetic signals, in modulating skin immunity. Notably, neuropathic changes of such neural structures have been observed in skin disease conditions, implicating their direct involvement in various pathological processes. An in-depth understanding of the mechanism underlying skin neuropathy and its immunomodulatory effects could help reveal novel entry points for therapeutic interventions. Here, we summarize the neuroimmune interactions between neuropathic events and skin immunity, highlighting the current knowledge and future perspectives of this emerging research frontier.

Cutaneous-immuno-neuro-endocrine (CINE) system: A complex enterprise transforming skin into a super organ

Skin is now emerging as a complex realm of three chief systems viz. immune system, nervous system, and endocrine system. The cells involved in their intricate crosstalk, namely native skin cells, intra-cutaneous immune cells and cutaneous sensory neurons have diverse origin and distinct functions. However, recent studies have explored their role beyond their pre-defined functional boundaries, such that the cells shun their traditional functions and adopt unconventional roles. For example, the native skin cells, apart from providing for basic structural framework of skin, also perform special immune functions and participate in extensive neuro-endocrine circuitry, which were traditionally designated as functions of cutaneous resident immune cells and sensory neurons respectively. At the cellular level, this unique collaboration is brought out by special molecules called neuromediators including neurotransmitters, neuropeptides, neurotrophins, neurohormones and cytokines/chemokines. While this intricate crosstalk is essential for maintaining cutaneous homeostasis, its disruption is seen in various cutaneous diseases. Recent study models have led to a paradigm shift in our understanding of pathophysiology of many such disorders. In this review, we have described in detail the interaction of immune cells with neurons and native skin cells, role of neuromediators, the endocrine aspect in skin and current understanding of cutaneous neuro-immuno-endocrine loop in one of the commonest skin diseases, psoriasis. An accurate knowledge of this unique crosstalk can prove crucial in understanding the pathophysiology of different skin diseases and allow for generation of targeted therapeutic modalities.

An axon-T cell feedback loop enhances inflammation and axon degeneration

Inflammation is closely associated with many neurodegenerative disorders. Yet, whether inflammation causes, exacerbates, or responds to neurodegeneration has been challenging to define because the two processes are so closely linked. Here, we disentangle inflammation from the axon damage it causes by individually blocking cytotoxic T cell function and axon degeneration. We model inflammatory damage in mouse skin, a barrier tissue that, despite frequent inflammation, must maintain proper functioning of a dense array of axon terminals. We show that sympathetic axons modulate skin inflammation through release of norepinephrine, which suppresses activation of γδ T cells via the β2 adrenergic receptor. Strong inflammatory stimulation-modeled by application of the Toll-like receptor 7 agonist imiquimod-causes progressive γδ T cell-mediated, Sarm1-dependent loss of these immunosuppressive sympathetic axons. This removes a physiological brake on T cells, initiating a positive feedback loop of enhanced inflammation and further axon damage.

Mechanisms and treatment of opioid-induced pruritus: Peripheral and central pathways

BACKGROUND AND OBJECTIVE

Pruritus (also known as itch) is defined as an unpleasant and irritating sensation of the skin that provokes an urge to scratch or rub. It is well known that opioid administration can cause pruritus, which is paradoxical as itch and pain share overlapping sensory pathways. Because opioids inhibit pain but can cause itching. Significant progress has been made to improve our understanding of the fundamental neurobiology of itch; however, much remains unknown about the mechanisms of opioid-induced pruritus. The prevention and treatment of opioid-induced pruritus remains a challenge in the field of pain management. The objective of this narrative review is to present and discuss the current body of literature and summarize the current understanding of the mechanisms underlying opioid-induced pruritus, and its relationship to analgesia, and possible treatment options.

RESULTS

The incidence of opioid-induced pruritus differs with different opioids and routes of administration, and the various mechanisms can be broadly divided into peripheral and central. Especially central mechanisms are intricate, even at the level of the spinal dorsal horn. There is evidence that opioid receptor antagonists and mixed agonist and antagonists, especially μ-opioid antagonists and κ-opioid agonists, are effective in relieving opioid-induced pruritus. Various treatments have been used for opioid-induced pruritus; however, most of them are controversial and have conflicting results.

CONCLUSION

The use of a multimodal analgesic treatment regimen combined with a mixed antagonist and κ agonists, especially μ-opioid antagonists, and κ-opioid agonists, seems to be the current best treatment modality for the management of opioid-induced pruritus and pain.

SIGNIFICANCE

Opioids remain the gold standard for the treatment of moderate to severe acute pain as well as cancer pain. It is well known that opioid-induced pruritus often does not respond to regular antipruritic treatment, thereby posing a challenge to clinicians in the field of pain management. We believe that our review makes a significant contribution to the literature, as studies on the mechanisms of opioid-induced pruritus and effective management strategies are crucial for the management of these patients.

Local and systemic mechanisms that control the hair follicle stem cell niche

Hair follicles are essential appendages of the mammalian skin, as hair performs vital functions of protection, thermoregulation and sensation. Hair follicles harbour exceptional regenerative abilities as they contain multiple somatic stem cell populations such as hair follicle stem cells (HFSCs) and melanocyte stem cells. Surrounding the stem cells and their progeny, diverse groups of cells and extracellular matrix proteins are organized to form a microenvironment (called 'niche') that serves to promote and maintain the optimal functioning of these stem cell populations. Recent studies have shed light on the intricate nature of the HFSC niche and its crucial role in regulating hair follicle regeneration. In this Review, we describe how the niche serves as a signalling hub, communicating, deciphering and integrating both local signals within the skin and systemic inputs from the body and environment to modulate HFSC activity. We delve into the recent advancements in identifying the cellular and molecular nature of the niche, providing a holistic perspective on its essential functions in hair follicle morphogenesis, regeneration and ageing.

Neural dependency in wound healing and regeneration

In response to injury, humans and many other mammals form a fibrous scar that lacks the structure and function of the original tissue, whereas other vertebrate species can spontaneously regenerate damaged tissues and structures. Peripheral nerves have been identified as essential mediators of wound healing and regeneration in both mammalian and nonmammalian systems, interacting with the milieu of cells and biochemical signals present in the post-injury microenvironment. This review examines the diverse functions of peripheral nerves in tissue repair and regeneration, specifically during the processes of wound healing, blastema formation, and organ repair. We compare available evidence in mammalian and nonmammalian models, identifying critical nerve-mediated mechanisms for regeneration and providing future perspectives toward integrating these mechanisms into a therapeutic framework to promote regeneration.

Road-traffic-related air pollution contributes to skin barrier alteration and growth defect of sensory neurons

The effects of air pollution on health are gaining increasing research interest with limited data on skin alterations available. It was suggested that air pollution is a trigger factor for sensitive skin (SS). However, this data was based on surveys with a lack of experimental data. SS is related to altered skin nerve endings and cutaneous neurogenic inflammation. TTe present study was to assess the in vitro effect of particulate matter (PM) on epidermis and nerve ending homeostasis. PM samples were collected according to a validated protocol. Reconstructed human epidermis (RHE, Episkin®) was exposed to PM and subsequently the supernatants were transferred to a culture of PC12 cells differentiated into sensory neurons (SN). Cell viability, axonal growth and neuropeptide-release were measured. The modulation of the expression of different inflammatory, keratinocytes differentiation and neurites growth markers was assessed. PM samples contained a high proportion of particles with a size below 1 μm and a complex chemical composition. Transcriptomic and immunohistochemical analyses revealed that PM altered keratinocytes terminal differentiation and induced an inflammatory response. While viability and functionality of the SN were not modified, their outgrowth was significantly decreased after incubation with PM-exposed Episkin® supernatants. This was closely related to the modification of nerve growth factor/semaphorin 3A balance. This study showed that air pollutants have negative effects on keratinocytes and sensory nerve endings including inflammatory responses. These effects are probably involved in the SS pathophysiology and might be involved in inflammatory skin disorders.

Cutaneous neurogenic inflammation mediated by TRPV1-NGF-TRKA pathway activation in rosacea is exacerbated by the presence of Demodex mites

BACKGROUND

Rosacea is a common chronic inflammatory skin condition that is often refractory to treatment, with frequent relapses. Alterations in the skin immunological response and Demodex mite infestation are the primary aetiologic factors targeted for treatment. Transient receptor potential cation channel subfamily V member 1 (TRPV1) is a nociceptive cation channel that plays a role in cutaneous neurogenic pain and can be activated by various rosacea triggers.

OBJECTIVES

We investigated the effects of TRPV1 modulation in rosacea, focussing on Demodex mite colonization and cutaneous neurogenic inflammation.

METHODS

We examined mRNA expression levels according to Demodex population counts. An in vitro study using capsazepine as a TRPV1 antagonist was performed to assess the influence of TRPV1 in keratinocytes. A rosacea-like mouse model was generated by the injection of the 37-amino acid C-terminal cathelicidin peptide (LL37), and changes in the skin, dorsal root ganglion (DRG) and ears were examined.

RESULTS

Increased Demodex mite population counts were associated with increased expression levels of TRPV1, tropomyosin receptor kinase A (TrkA) and nerve growth factor (NGF), and these levels could be reduced by capsazepine treatment in keratinocytes. In an in vivo study, the downstream effects of TRPV1 activation were investigated in the skin, DRG and ears of the rosacea-like mouse model.

CONCLUSIONS

The findings of this study are instrumental for understanding the underlying causes of rosacea and could potentially lead to the development of new treatments targeting the NGF-TrkA-TRPV1 pathway. The identification of this pathway as a therapeutic target could represent a major breakthrough for rosacea research, potentially resulting in more effective and targeted rosacea treatments. This study contributes to an improved understanding of rosacea pathophysiology, which may lead to the development of more effective treatments in the future.

Molecular mechanisms of pruritus in prurigo nodularis

Pruritus is the most common symptom of dermatological disorders, and prurigo nodularis (PN) is notorious for intractable and severe itching. Conventional treatments often yield disappointing outcomes, significantly affecting patients' quality of life and psychological well-being. The pathogenesis of PN is associated with a self-sustained "itch-scratch" vicious cycle. Recent investigations of PN-related itch have partially revealed the intricate interactions within the cutaneous neuroimmune network; however, the underlying mechanism remains undetermined. Itch mediators play a key role in pruritus amplification in PN and understanding their action mechanism will undoubtedly lead to the development of novel targeted antipruritic agents. In this review, we describe a series of pruritogens and receptors involved in mediating itching in PN, including cytokines, neuropeptides, extracellular matrix proteins, vasculogenic substances, ion channels, and intracellular signaling pathways. Moreover, we provide a prospective outlook on potential therapies based on existing findings.

Molecular and spatial landmarks of early mouse skin development

A wealth of specialized cell populations within the skin facilitates its hair-producing, protective, sensory, and thermoregulatory functions. How the vast cell-type diversity and tissue architecture develops is largely unexplored. Here, with single-cell transcriptomics, spatial cell-type assignment, and cell-lineage tracing, we deconstruct early embryonic mouse skin during the key transitions from seemingly uniform developmental precursor states to a multilayered, multilineage epithelium, and complex dermal identity. We identify the spatiotemporal emergence of hair-follicle-inducing, muscle-supportive, and fascia-forming fibroblasts. We also demonstrate the formation of the panniculus carnosus muscle (PCM), sprouting blood vessels without pericyte coverage, and the earliest residence of mast and dendritic immune cells in skin. Finally, we identify an unexpected epithelial heterogeneity within the early single-layered epidermis and a signaling-rich periderm layer. Overall, this cellular and molecular blueprint of early skin development-which can be explored at https://kasperlab.org/tools-establishes histological landmarks and highlights unprecedented dynamic interactions among skin cells.

Exposure time as an influencing factor among rheumatoid arthritis patients subjected to traditional Siwan therapy

Rheumatoid arthritis (RA) is a long-term autoimmune disease characterized by intra- and extra-articular manifestations. Sand therapy is traditionally indicated for RA, chronic pain, skin diseases, and musculoskeletal disorders. Many places in the world use sand therapy, including Siwa, which is a famous place in Egypt. This study investigated the exposure time to Siwan traditional therapy as a factor influencing central sensitization, pain severity, pain threshold, and kinesiophobia in RA by measuring the central sensory inventory (CSI), visual analogue scale, pressure algometer, and TAMPA kinesiophobia scale, respectively. Twenty-four patients with RA were recruited from 6 traditional healing centers, 24 RA patients were recruited and randomly assigned to 2 equal groups (GI and GII). The first received Siwan traditional therapy for 3 days, while the second received the same program for 5 days. The results revealed a significant difference in CSI between pre- and posttreatment within the GII (P = .038). The Tampa Scale score improved significantly in both groups (P = .004 and P = .014, respectively). Pain severity and pain threshold at all sites showed significant posttreatment improvements in the GII. Significant posttreatment changes were only found for GI in terms of pain severity and the most painful joint (P = .010 and P = .035, respectively). Significant changes were observed in kinesiophobia, pain severity, and pain threshold in the most painful joint 3 and 5 days after Siwan traditional therapy. Despite the nonsignificant differences in all parameters between the 2 groups, all the measured parameters produced favorable results after 5 days of treatment, suggesting the need for a long-term effect investigation.

Ccrl2-centred immune-related lncRNA-mRNA co-expression network revealed the local skin immune activation mechanism of moxibustion on adjuvant arthritis mice

BACKGROUND

Moxibustion is an important external therapy of traditional medicine that operates on some acupoints on the skin and is usually used for immune-related diseases. However, whether the immune function of the skin, especially the immune-related lncRNAs, contributes to the mechanism of moxibustion remains unclear.

METHODS

Adjuvant arthritis (AA) was induced by injection of Complete Freund's adjuvant (CFA) into the right hind paw of mice. Moxibustion was administered on the Zusanli (ST36) acupoint for 3 weeks. The alteration of foot volume and cytokine concentration in serum was used to evaluate the anti-inflammation effect of moxibustion. CD83 expression in the local skin of ST36 was measured by immunofluorescence staining. Transcriptome RNA sequencing (RNA-seq) and lncRNA-mRNA network analysis were performed to construct a moxibustion-induced Immune-related lncRNA-mRNA co-expression network. qRT-PCR was used to validate the RNA-seq data.

RESULTS

Moxibustion at ST36 relieved the foot swelling, decreased the TNF-α and IL-1β concentrations in serum, and obviously increased the CD83 expression at the local skin of ST36. A total of 548 differentially expressed lncRNAs and 520 linked mRNAs were screened out. The significantly and predominately enriched Go term was inflammatory and immune response, and the main pathways related to inflammatory and immune responses include Toll-like receptor, cytokine-cytokine receptor, and MAPK signaling. The immune-related lncRNA-mRNA co-expression network showed 88 lncRNAs and 36 mRNAs, and Ccrl2 is the central hub of this network.

CONCLUSION

Local immune activation is significantly triggered by moxibustion in ST36 of AA mice. The Ccrl2-centered immune-related lncRNA-mRNA co-expression network would be a promising target for decoding the mechanism of moxibustion for immune-related diseases.

IL-31-Pruritus Interleukin: Serum Values and Clinical Impact in Chronic Spontaneous Urticaria-A Romanian Retrospective Study

(1) Background: This study aimed to evaluate the implications of interleukin-31 (IL-31) in the pathogenesis of chronic spontaneous urticaria (CSU) and to assess the differences that occur between its serum values compared to controls. Additionally, the serum IL-31 levels were measured alongside other clinical and paraclinical parameters that were identified in the patients to understand its immunological importance in this skin disease and to determine if it could potentially serve as a therapeutic target in CSU in the future. (2) Methods: The serum levels of IL-31 were estimated in 50 patients diagnosed with CSU according to the accepted international guidelines. Additionally, 38 controls who had not experienced any episodes of urticaria during their lifetime were included. (3) Results: Significantly elevated serum IL-31 levels were observed in CSU patients compared to the controls (p < 0.0001). Although no direct correlations were found between IL-31 and inflammatory markers (erythrocyte sedimentation rate (ESR), C-reactive protein (CRP)), eosinophils, or total immunoglobulins E (IgE), significant differences in IL-31 levels were identified based on CSU severity, quality of life impact, itch intensity, and response to histamine H1 receptor antagonists (H1 antihistamines) (p < 0.05 for all). (4) Conclusions: Our findings underscore that IL-31 is not directly associated with general inflammation, eosinophilic response, or atopy in CSU. Nevertheless, its expression is influenced by key disease characteristics: severity, pruritus, and H1 antihistamine response. This investigation provides essential insights into CSU pathogenesis, potentially leading to novel therapeutic interventions. An enhanced understanding of these mechanisms is crucial due to the limitations of current treatment modalities in terms of fully managing CSU symptoms.

Calcitonin Gene-Related Peptide Systemic Effects: Embracing the Complexity of Its Biological Roles-A Narrative Review

The calcitonin gene-related peptide (CGRP) is a neuropeptide widely distributed throughout the human body. While primarily recognized as a nociceptive mediator, CGRP antagonists are currently utilized for migraine treatment. However, its role extends far beyond this, acting as a regulator of numerous biological processes. Indeed, CGRP plays a crucial role in vasodilation, inflammation, intestinal motility, and apoptosis. In this review, we explore the non-nociceptive effects of CGRP in various body systems, revealing actions that can be contradictory at times. In the cardiovascular system, it functions as a potent vasodilator, yet its antagonists do not induce arterial hypertension, suggesting concurrent modulation by other molecules. As an immunomodulator, CGRP exhibits intriguing complexity, displaying both anti-inflammatory and pro-inflammatory effects. Furthermore, CGRP appears to be involved in obesity development while paradoxically reducing appetite. A thorough investigation of CGRP's biological effects is crucial for anticipating potential side effects associated with its antagonists' use and for developing novel therapies in other medical fields. In summary, CGRP represents a neuropeptide with a complex systemic impact, extending well beyond nociception, thus offering new perspectives in medical research and therapeutics.

Environmental Air Pollutants Affecting Skin Functions with Systemic Implications

The increase in air pollution worldwide represents an environmental risk factor that has global implications for the health of humans worldwide. The skin of billions of people is exposed to a mixture of harmful air pollutants, which can affect its physiology and are responsible for cutaneous damage. Some polycyclic aromatic hydrocarbons are photoreactive and could be activated by ultraviolet radiation (UVR). Therefore, such UVR exposure would enhance their deleterious effects on the skin. Air pollution also affects vitamin D synthesis by reducing UVB radiation, which is essential for the production of vitamin D3, tachysterol, and lumisterol derivatives. Ambient air pollutants, photopollution, blue-light pollution, and cigarette smoke compromise cutaneous structural integrity, can interact with human skin microbiota, and trigger or exacerbate a range of skin diseases through various mechanisms. Generally, air pollution elicits an oxidative stress response on the skin that can activate the inflammatory responses. The aryl hydrocarbon receptor (AhR) can act as a sensor for small molecules such as air pollutants and plays a crucial role in responses to (photo)pollution. On the other hand, targeting AhR/Nrf2 is emerging as a novel treatment option for air pollutants that induce or exacerbate inflammatory skin diseases. Therefore, AhR with downstream regulatory pathways would represent a crucial signaling system regulating the skin phenotype in a Yin and Yang fashion defined by the chemical nature of the activating factor and the cellular and tissue context.

Sox9 regulates melanocytic fate decision of adult hair follicle stem cells

The bulge of hair follicles harbors Nestin⁺ (neural crest like) stem cells, which exhibit the potential to generate various cell types including melanocytes. In this study, we aimed to determine the role of Sox9, an important regulator during neural crest development, in melanocytic differentiation of those adult Nestin⁺ cells. Immunohistochemical analysis after conditional Sox9 deletion in Nestin⁺ cells of adult mice revealed that Sox9 is crucial for melanocytic differentiation of these cells and that Sox9 acts as a fate determinant between melanocytic and glial fate. A deeper understanding of factors that regulate fate decision, proliferation and differentiation of these stem cells provides new aspects to melanoma research as melanoma cells share many similarities with neural crest cells. In summary, we here show the important role of Sox9 in melanocytic versus glial fate decision of Nestin⁺ stem cells in the skin of adult mice.

Sleep disorders in dermatology - a comprehensive review

Sleep is a normal physiological process that accounts for approximately one third of a person's life. Disruption of the normal sleep cycle, which maintains physiological homeostasis, can lead to pathology. It is not known whether sleep disturbance causes skin disease or skin disease causes sleep impairment, but a bidirectional influence is suspected. We have compiled the data from published articles on "sleep disorders in dermatology" in PubMed Central from July 2010 to July 2022 (with the option "full text available") and provide an overview of sleep disorders associated with dermatological conditions and certain drugs used in dermatology as well as sleep disturbances for which some drugs used can cause itch or dermatological issues. Atopic dermatitis, eczema and psoriasis have been shown to be exacerbated by sleep problems and vice versa. Sleep deprivation, night-time pruritus and disrupted sleep cycles are often used to assess treatment response and quality of life in these conditions. Some medications used primarily for dermatological conditions have also been associated with alterations in the sleep-wake cycle. Addressing patients sleep disorders should be an integral part of the management of dermatological conditions. More studies are needed to further investigate the influence of sleep and skin disorders.

Repeated stress-induced crosstalk between the sympathetic nervous system and mast cells contributes to delayed cutaneous wound healing in mice

The study identifies a link between the neuroimmune interaction and the impairment of wound healing induced by repeated stress. Stress increased mast cell mobilization and degranulation, levels of IL-10, and sympathetic reinnervation in mouse wounds. In contrast to mast cells, macrophage infiltration into wounds was significantly delayed in stressed mice. Chemical sympathectomy and the blockade of mast cell degranulation reversed the effect of stress on skin wound healing in vivo. In vitro, high epinephrine levels stimulated mast cell degranulation and IL-10 release. In conclusion, catecholamines released by the sympathetic nervous system stimulate mast cells to secrete anti-inflammatory cytokines that impair inflammatory cell mobilization, leading to a delay in the resolution of wound healing under stress conditions.

Fractional ablative laser therapy for the treatment of severe burn scars: A pilot study of the underlying mechanisms

Ablative fractional resurfacing is clinically an efficient treatment for burn scar management. The aim of this pilot study was to investigate the poorly understood mechanisms underlying ablative fractional CO2 laser (AFL-CO2) therapy in relation to biomarkers S100 and 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). S100 stains for Langerhans cells and neuronal cells, potentially representing the pruritus experienced. 11β-HSD1 catalyses the interconversion of cortisol and cortisone in cells, promoting tissue remodelling. Immunohistochemical analysis of S100 and 11β-HSD1 protein expression in the dermis and epidermis of the skin was performed on normal skin, before and after AFL-CO2 therapy. Data assessing outcome parameters was collected concurrently with the skin biopsies. 13 patients were treated with AFL-CO2 therapy. Langerhans cells decreased by 39% after 2nd treatment. Neuronal cells were overexpressed before treatment in the scar tissue by 91% but levels returned to that resembling normal skin. 11β-HSD1 expression in keratinocytes was significantly higher after laser treatment compared to before in scar tissue (p <0.01). No clear correlation was found in dermal fibroblast numbers throughout the treatment course. Whilst the role of the explored mechanisms and their association with clinical outcomes cannot conclusively be stated, this pilot study demonstrates promising trends that encourages investigation into this relationship.

[Touch, a crucial sense in social interactions to improve homeostasis in aging and promote healthy longevity]

Aging is associated with the generalized deterioration of the organism, being of great relevance experienced by homeostatic systems such as the nervous, immune, and endocrine systems, which increases the risk of morbidity and mortality. Among the lifestyle strategies that have been researched to improve these systems and achieve greater healthy longevity, this review will focus on the social environment. In order to verify the effectiveness of these both in the improvement of homeostasis and in life expectancy, the research carried out with experimental animals that have allowed this to be done will be discussed. In addition, as it has been observed that physical contact is crucial for the positive outcomes of social interaction on homeostatic systems and longevity to occur, we will focus on that mechanism, as well as some of the possible molecular pathways underlying the effects found.

Skin Inflammation with a Focus on Wound Healing

Significance: The skin is the crucial first-line barrier against foreign pathogens. Compromise of this barrier presents in the context of inflammatory skin conditions and in chronic wounds. Skin conditions arising from dysfunctional inflammatory pathways severely compromise the quality of life of patients and have a high economic impact on the U.S. health care system. The development of a thorough understanding of the mechanisms that can disrupt skin inflammation is imperative to successfully modulate this inflammation with therapies. Recent Advances: Many advances in the understanding of skin inflammation have occurred during the past decade, including the development of multiple new pharmaceuticals. Mechanical force application has been greatly advanced clinically. Bioscaffolds also promote healing, while reducing scarring. Critical Issues: Various skin inflammatory conditions provide a framework for analysis of our understanding of the phases of successful wound healing. The large burden of chronic wounds on our society continues to focus attention on the chronic inflammatory state induced in many of these skin conditions. Future Directions: Better preclinical models of disease states such as chronic wounds, coupled with enhanced diagnostic abilities of human skin, will allow a better understanding of the mechanism of action. This will lead to improved treatments with biologics and other modalities such as the strategic application of mechanical forces and scaffolds, which ultimately results in better outcomes for our patients.

Development of Neuropeptide Hemokinin-1 Analogues with Antimicrobial and Wound-Healing Activity

Wound healing is a complex process that can be delayed in some pathological conditions, such as infection and diabetes. Following skin injury, the neuropeptide substance P (SP) is released from peripheral neurons to promote wound healing by multiple mechanisms. Human hemokinin-1 (hHK-1) has been identified as an SP-like tachykinin peptide. Surprisingly, hHK-1 shares similar structural features with antimicrobial peptides (AMPs), but it does not display efficient antimicrobial activity. Therefore, a series of hHK-1 analogues were designed and synthesized. Among these analogues, AH-4 was found to display the greatest antimicrobial activity against a broad spectrum of bacteria. Furthermore, AH-4 rapidly killed bacteria by membrane disruption, similar to most AMPs. More importantly, AH-4 showed favorable healing activity in all tested mouse full-thickness excisional wound models. Overall, this study suggests that the neuropeptide hHK-1 can be used as a desirable template for developing promising therapeutics with multiple functions for wound healing.

Molecular Mechanisms of Neurogenic Inflammation of the Skin

The skin, including the hypodermis, is the largest body organ and is in constant contact with the environment. Neurogenic inflammation is the result of the activity of nerve endings and mediators (neuropeptides secreted by nerve endings in the development of the inflammatory reaction in the skin), as well as interactions with other cells such as keratinocytes, Langerhans cells, endothelial cells and mast cells. The activation of TRPV-ion channels results in an increase in calcitonin gene-related peptide (CGRP) and substance P, induces the release of other pro-inflammatory mediators and contributes to the maintenance of cutaneous neurogenic inflammation (CNI) in diseases such as psoriasis, atopic dermatitis, prurigo and rosacea. Immune cells present in the skin (mononuclear cells, dendritic cells and mast cells) also express TRPV1, and their activation directly affects their function. The activation of TRPV1 channels mediates communication between sensory nerve endings and skin immune cells, increasing the release of inflammatory mediators (cytokines and neuropeptides). Understanding the molecular mechanisms underlying the generation, activation and modulation of neuropeptide and neurotransmitter receptors in cutaneous cells can aid in the development of effective treatments for inflammatory skin disorders.

Skin-to-Skin Contact: Crucial for Improving Behavior, Immunity, and Redox State after Short Cohabitation of Chronologically Old Mice and Prematurely Aging Mice with Adult Mice

(1) Background: Aging is characterized by a deterioration of the homeostatic systems, namely the nervous and immune systems. The rate of aging can be modified by lifestyle factors such as social interactions. Recently, improvements in behavior, immune function, and oxidative state were observed in adult prematurely aging mice (PAM) and chronologically old mice after cohabitation with exceptional non-PAM (E-NPAM) and adult mice, respectively, for 2 months. However, the cause of this positive effect is not known. The objective of the present work was to study whether skin-to-skin contact promotes these improvements both in chronologically old mice and in adult PAM. (2) Methods: Old and adult CD1 female mice were used as well as adult PAM and E-NPAM. After cohabitation for 15 min/day for 2 months (two old mice or PAM with five adult mice or E-NPAM, respectively, with both non- and skin-to-skin contact), several behavioral tests were performed and functions and oxidative stress parameters in peritoneal leukocytes were analyzed. (3) Results: This social interaction improved behavioral responses, immune functions, redox state, and longevity, but only if the animals had skin-to-skin contact. (4) Conclusions: Physical contact seems to be crucial to experiencing the positive effects of social interaction.

Co-culture of iNeurons with primary human skin cells provides a reliable model to examine intercellular communication

OBJECTIVE

The skin is a sensory organ, densely innervated with various types of sensory nerve endings, capable of discriminating touch, environmental sensations, proprioception, and physical affection. Neurons communication with skin cells confer to the tissue the ability to undergo adaptive modifications during response to environmental changes or wound healing after injury. Thought for a long time to be dedicated to the central nervous system, the glutamatergic neuromodulation is increasingly described in peripheral tissues. Glutamate receptors and transporters have been identified in the skin. There is a strong interest in understanding the communication between keratinocytes and neurons, as the close contacts with intra-epidermal nerve fibers is a favorable site for efficient communication. To date, various coculture models have been described. However, these models were based on non-human or immortalized cell line. Even the use of induced pluripotent stem cells (iPSCs) is posing limitations because of epigenetic variations during the reprogramming process.

METHODS

In this study, we performed small molecule-driven direct conversion of human skin primary fibroblasts into induced neurons (iNeurons).

RESULTS

The resulting iNeurons were mature, showed pan-neuronal markers, and exhibited a glutamatergic subtype and C-type fibers characteristics. Autologous coculture of iNeurons with human primary keratinocytes, fibroblasts, and melanocytes was performed and remained healthy for many days, making possible to study the establishment of intercellular interactions.

CONCLUSION

Here, we report that iNeurons and primary skin cells established contacts, with neurite ensheathment by keratinocytes, and demonstrated that iNeurons cocultured with primary skin cells provide a reliable model to examine intercellular communication.

Anatomical contacts between sensory neurons and epidermal cells: an unrecognized anatomical network for neuro-immuno-cutaneous crosstalk

Sensory neurons innervating the skin are conventionally thought to be the sole transducers of touch, temperature, pain and itch. However, recent studies have shown that keratinocytes - like Merkel cells - act as sensory transducers, whether for innocuous or noxious mechanical, thermal or chemical stimuli, and communicate with intraepidermal free nerve endings via chemical synaptic contacts. This paradigm shift leads to consideration of the whole epidermis as a sensory epithelium. Sensory neurons additionally function as an efferent system. Through the release of neuropeptides in intimate neuroepidermal contact areas, they contribute to epidermal homeostasis and to the pathogenesis of inflammatory skin diseases. To counteract the dogma regarding neurocutaneous interactions, seen exclusively from the perspective of soluble and spreading mediators, this review highlights the essential contribution of the unrecognized anatomical contacts between sensory neurons and epidermal cells (keratinocytes, melanocytes, Langerhans cells and Merkel cells), which take part in the reciprocal dialogue between the skin, nervous system and immune system.

Functional validation of co-culture model of human keratinocytes and neuronal cell line for sensitive skin by using transient receptor potential channel vanilloid subfamily member 1 antagonist

BACKGROUND

Sensitive skin is a subjective cutaneous hyper-reactivity that occurs in response to various innocuous stimuli. Keratinocytes have recently been shown to participate in sensory transduction by releasing many neuroactive molecules that bind to intra-epidermal free nerve endings and modulate nociception. In the literature, the characterization of these interactions has been based on the co-culture of keratinocyte and mammalian-origin neuronal cell lines. In this study, we established an in vitro model based on a co-culture of primary human keratinocytes and differentiated SH-SY5Y cells, a human neuronal cell line.

METHODS

Human epidermal keratinocytes and SH-SY5Y cells were monocultured and co-cultured. Changes in calcium influx, substance P, inflammatory cytokines, and neuropeptides between the monoculture and co-culture groups treated with capsaicin only and capsaicin with transient receptor potential channel vanilloid subfamily member 1 (TRPV1) antagonist, trans-4-tert-butylcyclohexanol (TTBC), together. In addition, the difference in stinging sensation was evaluated by applying it to the volunteers.

RESULTS

When SH-SY5Y cells were co-cultured with keratinocytes, they had no significant effect on axonal development. Substance P was also released after capsaicin treatment and reduced by TTBC under co-culture conditions. Moreover, the expression of inflammatory cytokines and neuropeptides was significantly increased in co-cultured keratinocytes compared to that under monoculture conditions. In addition, the stinging sensation was significantly induced after the application of capsaicin in vivo and was relieved after the application of the TRPV1 antagonist.

CONCLUSION

We demonstrated that the novel co-culture model is functionally valid through capsaicin and TRPV1 antagonist. We also confirmed that TTBC could be used for the treatment of sensitive skin through a co-culture model and in vivo tests. This co-culture model of keratinocytes and SH-SY5Y cells may be useful in vitro alternatives for studying the close communication between keratinocytes and neuronal cells and for screening therapeutic drugs for sensitive skin.

Towards a mechanistic understanding of axon transport and endocytic changes underlying paclitaxel-induced peripheral neuropathy

Paclitaxel is a common chemotherapeutic agent widely used to treat solid cancer. However, it frequently causes peripheral sensory neuropathy, resulting in sensory abnormalities and pain in patients receiving treatment for cancer. As one of the most widely used chemotherapeutics, many preclinical studies on paclitaxel-induced peripheral neuropathy (PIPN) have been performed. Yet, there remain no effective options for treatment or prevention. Due to paclitaxel's ability to bind to and stabilize microtubules, a change in microtubule dynamics and subsequent disruptions in axonal transport has been predicted as a major underlying cause of paclitaxel-induced toxicity. However, the systemic understanding of PIPN mechanisms is largely incomplete, and various phenotypes have not been directly attributed to microtubule-related effects. This review aims to provide an overview of the literature involving paclitaxel-induced alteration in microtubule dynamics, axonal transport, and endocytic changes. It also aims to provide insights into how the microtubule-mediated hypothesis may relate to various phenotypes reported in PIPN studies.

Elevated levels of cytochrome p450scc and 3β-hydroxysteroid dehydrogenase expression in skin of acne vulgaris patients

Background: Acne vulgaris belongs to androgen-dependent diseases where androgens cause sebaceous gland hypertrophy with a subsequent enlargement of sebum production. Aims: Determination of the CYP450scc 3-hydroxysteroid dehydrogenase expression profile in normal skin and skin of patients with acne. Material and methods: Immunohistochemical assay was carried out to evaluate cytochrome P450 side chain cleavage and 3-hydroxysteroid dehydrogenase expression in skin of patients with acne vulgaris and in normal skin. Results: Cytochrome P450 side chain cleavage expression was elevated in epidermal keratinocytes and sebocytes whereas 3-hydroxysteroid dehydrogenase levels were increased in mature sebocytes of acne vulgaris patients. Conclusions: The revealed alterations in expression of steroid-converting enzymes in acne skin may indicate not only the conversion of steroids produced by external steroidogenesis organs but elevated steroids local production as well. The fact may account both for sebaceous gland hyperplasia and inflammation sustenance in skin.

Chronic skin ultraviolet irradiation induces transcriptomic changes associated with microglial dysfunction in the hippocampus

Recent evidence indicates that ultraviolet (UV) exposure of the skin can affect brain functions such as learning and memory, addictive behavior, and hippocampal neurogenesis. These changes are closely associated with hippocampal function, which plays a pivotal role in learning and memory formation. However, the molecular mechanisms underlying these UV-induced skin-brain interactions remain unclear. To elucidate the molecular signature associated with UV-induced neurobehavioral changes, we analyzed the hippocampal transcriptome in a well-established mouse skin aging model, which showed thickened skin and impaired hippocampal memory. Transcriptome analysis revealed that significantly downregulated genes in UV-irradiated mice are enriched in neuroimmune-related signaling pathways. Furthermore, cell-type analysis showed that DEGs are also enriched in microglia. Consistently, immunofluorescence imaging showed an increased number of Iba1-positive microglia in the hippocampi of UV-irradiated mice. Collectively, our findings highlight that chronic UV irradiation of the skin causes significant changes in the neuroimmune system in the hippocampus, accompanied by microglial dysfunction and cognitive impairment.

Coffee intake may promote sudomotor function activation via the contribution of caffeine

Objectives

To determine whether drinking coffee with caffeine accelerates the sympathetic response to acetylcholine (ACh).

Methods

Tests were performed twice at 1-week intervals following the intake of coffee. Subjects were randomly divided into two groups: Group A was administered 16 fluid oz of water (CON), while Group B was given 16 fluid oz of coffee (Coffee). After 1 week, Group A was administered 16 fluid oz of coffee (Coffee), while Group B was given 16 fluid oz of water (CON). The quantitative sudomotor axon reflex test (QSART) was performed after intake of coffee and water and a 40 min break. QSART with iontophoresis and 10% ACh was performed to determine axon reflex (AXR) mediated with and without iontophoresis [AXR (1) and AXR (2), respectively], and directly activated sweating (DIR).

Results

The sweat onset time of the AXR was shorter in the Coffee compared with the CON (p < 0.05). The sweat rates in AXR (1) AXR (2) and DIR were significantly higher in the Coffee than in the CON (p < 0.05, p < 0.05, p < 0.01, respectively). In addition, the Coffee showed significantly higher density of activated sweat glands and activated sweat gland output than the CON (p < 0.05, p < 0.01, respectively). The overall results of this study showed that coffee intake could stimulate higher activation in both AXR and DIR sweat responses.

Conclusion

Coffee intake can improve sweating sensitivity in both the AXR and DIR by the contribution of caffeine contained in coffee. This suggests that other compounds in coffee may not inhibit the sympathetic response to ACh. Therefore, coffee may be clinically worth considering as a supplement for the activation of the cholinergic and sudomotor function.

Enhancing Cutaneous Wound Healing Based on Human Induced Neural Stem Cell-derived Exosomes

Background

Wound healing of skin is a complicated process. Cutaneous innervation and neurotrophic factors could participate in multiple stages of wound healing. Neurotrophic factors are mainly produced and released by neurons and neural stem cells (NSCs) which could be obtained in large quantities from human-induced pluripotent stem cells (iPSCs) in vitro. However, the potential wound healing effects of NSC secretions, such as exosomes, are unexplored yet.

Methods

NSCs-derived exosomes (NSC-exo) and iPSCs-derived exosomes (iPSC-exo) were isolated from the cell culture supernatants by centrifugation, and then quantified and characterized. The effects of these exosomes on the migration of human dermal fibroblasts (HDF) cells and the tube formation of human umbilical vein endothelial cells (HUVECs) were investigated in vitro. And the in vivo wound healing effect of these exosomes were tested on the mouse skin trauma model. Therefore, a dipeptide/hyaluronic acid (Nap-FF/HA) composite hydrogel was used to encapsulate the exosomes as a sustained release carrier. Histological observations were performed to evaluate the wound healing effect of exosomes. Furthermore, the non-labeling proteomic analysis was performed to explore the possible mechanisms of NSC-exo on wound healing.

Results

We obtained extracellular vesicles in a bowl-like structure with membranes which meet the general standards of exosomes. NSC-exo showed promotion effect on the migration of HDF cells and the tube formation of HUVECs in vitro. In a mouse skin injury model, NSC-exo enhanced the wound healing and the Nap-FF/HA hydrogel that contained exosomes did so with less drug frequency by sustaining release of exosomes. Further proteomic analysis demonstrated that the carried neurotrophic factors and immunity-related proteins in NSC-exo may play a functional role in wound healing.

Conclusion

NSC-exo may enhance wound healing via neurotrophic factors and immunomodulation.