Hair cycle-dependent plasticity of skin and hair follicle innervation in normal murine skin

Regulation of Hair Follicle Growth and Development by Different Alternative Spliceosomes of FGF5 in Rabbits

This study investigated the regulatory effect of alternative spliceosomes of the fibroblast growth factor 5 (FGF5) gene on hair follicle (HF) growth and development in rabbits. The FGF5 alternative spliceosomes (called FGF5-X1, FGF5-X2, FGF5-X3) were cloned. The overexpression vector and siRNA of spliceosomes were transfected into dermal papilla cells (DPCs) to analyze the regulatory effect on DPCs. The results revealed that FGF5-X2 and FGF5-X3 overexpression significantly decreased LEF1 mRNA expression (p < 0.01). FGF5-X1 overexpression significantly reduced CCND1 expression (p < 0.01). FGF5-X1 and FGF5-X2 possibly downregulated the expression level of FGF2 mRNA (p < 0.05), and FGF5-X3 significantly downregulated the expression level of FGF2 mRNA (p < 0.01). The FGF5 alternative spliceosomes significantly downregulated the BCL2 mRNA expression level in both cases (p < 0.01). FGF5-X1 and FGF5-X2 significantly increased TGFβ mRNA expression (p < 0.01). All three FGF5 alternative spliceosomes inhibited DPC proliferation. In conclusion, the expression profile of HF growth and development-related genes can be regulated by FGF5 alternative spliceosomes, inhibiting the proliferation of DPCs and has an influence on the regulation of HF growth in rabbits. This study provides insights to further investigate the mechanism of HF development in rabbits via FGF5 regulation.

Mechanical stimulation of human hair follicle outer root sheath cultures activates adjacent sensory neurons

Mechanical stimuli, such as stroking or pressing on the skin, activate mechanoreceptors transmitting information to the sensory nervous system and brain. It is well accepted that deflection of the hair fiber that occurs with a light breeze or touch directly activates the sensory neurons surrounding the hair follicle, facilitating transmission of mechanical information. Here, we hypothesized that hair follicle outer root sheath cells act as transducers of mechanical stimuli to sensory neurons surrounding the hair follicle. Using electrochemical analysis on human hair follicle preparations in vitro, we were able to show that outer root sheath cells release ATP and the neurotransmitters serotonin and histamine in response to mechanical stimulation. Using calcium imaging combined with pharmacology in a coculture of outer root sheath cells with sensory neurons, we found that the release of these three molecules from hair follicle cells leads to activation of sensory neurons.

Microfluidic-based skin-on-chip systems for safety assessment of nanomaterials

The skin is the body's largest organ, continuously exposed to and affected by natural and anthropogenic nanomaterials (materials with external and internal dimensions in the nanoscale range). This broad spectrum of insults gives rise to irreversible health effects (from skin corrosion to cancer). Organ-on-chip systems can recapitulate skin physiology with high fidelity and potentially revolutionize the safety assessment of nanomaterials. Here, we review current advances in skin-on-chip models and their potential to elucidate biological mechanisms. Further, strategies are discussed to recapitulate skin physiology on-chip, improving control over nanomaterials exposure and transport across cells. Finally, we highlight future opportunities and challenges from design and fabrication to acceptance by regulatory bodies and industry.

Alk1 acts in non-endothelial VE-cadherin+ perineurial cells to maintain nerve branching during hair homeostasis

Vascular endothelial (VE)-cadherin is a well-recognized endothelial cell marker. One of its interacting partners, the TGF-β receptor Alk1, is essential in endothelial cells for adult skin vasculature remodeling during hair homeostasis. Using single-cell transcriptomics, lineage tracing and gene targeting in mice, we characterize the cellular and molecular dynamics of skin VE-cadherin+ cells during hair homeostasis. We describe dynamic changes of VE-cadherin+ endothelial cells specific to blood and lymphatic vessels and uncover an atypical VE-cadherin+ cell population. The latter is not a predicted adult endovascular progenitor, but rather a non-endothelial mesenchymal perineurial cell type, which forms nerve encapsulating tubular structures that undergo remodeling during hair homeostasis. Alk1 acts in the VE-cadherin+ perineurial cells to maintain proper homeostatic nerve branching by enforcing basement membrane and extracellular matrix molecular signatures. Our work implicates the VE-cadherin/Alk1 duo, classically known as endothelial-vascular specific, in perineurial-nerve homeostasis. This has broad implications in vascular and nerve disease.

Spatiotemporal dynamics of sensory neuron and Merkel-cell remodeling are decoupled during epidermal homeostasis

As the juncture between the body and environment, epithelia are both protective barriers and sensory interfaces that continually renew. To determine whether sensory neurons remodel to maintain homeostasis, we used in vivo two-photon imaging of somatosensory axons innervating Merkel cells in adult mouse skin. These touch receptors were highly plastic: 63% of Merkel cells and 89% of branches appeared, disappeared, grew, regressed and/or relocated over a month. Interestingly, Merkel-cell plasticity was synchronized across arbors during rapid epithelial turnover. When Merkel cells remodeled, the degree of plasticity between Merkel-cell clusters and their axons was well correlated. Moreover, branches were stabilized by Merkel-cell contacts. These findings highlight the role of epithelial-neural crosstalk in homeostatic remodeling. Conversely, axons were also dynamic when Merkel cells were stable, indicating that intrinsic neural mechanisms drive branch plasticity. Two terminal morphologies innervated Merkel cells: transient swellings called boutons, and stable cups termed kylikes. In Atoh1 knockout mice that lack Merkel cells, axons showed higher complexity than control mice, with exuberant branching and no kylikes. Thus, Merkel cells limit axonal branching and promote branch maturation. Together, these results reveal a previously unsuspected high degree of plasticity in somatosensory axons that is biased, but not solely dictated, by plasticity of target epithelial cells. This system provides a platform to identify intrinsic and extrinsic mechanisms that govern axonal patterning in epithelial homeostasis.

The impact of perceived stress on the hair follicle: Towards solving a psychoneuroendocrine and neuroimmunological puzzle

While popular belief harbors little doubt that perceived stress can cause hair loss and premature graying, the scientific evidence for this is arguably much thinner. Here, we investigate whether these phenomena are real, and show that the cyclic growth and pigmentation of the hair follicle (HF) provides a tractable model system for dissecting how perceived stress modulates aspects of human physiology. Local production of stress-associated neurohormones and neurotrophins coalesces with neurotransmitters and neuropeptides released from HF-associated sensory and autonomic nerve endings, forming a complex local stress-response system that regulates perifollicular neurogenic inflammation, interacts with the HF microbiome and controls mitochondrial function. This local system integrates into the central stress response systems, allowing the study of systemic stress responses affecting organ function by quantifying stress mediator content of hair. Focusing on selected mediators in this "brain-HF axis" under stress conditions, we distill general principles of HF dysfunction induced by perceived stress.

Hair Graying Regulators Beyond Hair Follicle

Hair graying is an interesting physiological alteration associated with aging and certain diseases. The occurrence is due to depigmentation of the hair caused by depletion and dysfunction of melanocyte stem cells (MeSCs). However, what causes the depletion and dysfunction of MeSCs remains unclear. MeSCs reside in the hair follicle bulge which provides the appropriate niche for the homeostasis of various stem cells within hair follicle including MeSCs. In addition to local signaling from the cells composed of hair follicle, emerging evidences have shown that nerves, adipocytes and immune cells outside of hair follicle per se also play important roles in the regulation of MeSCs. Here, we review the recent studies on different cells in the MeSCs microenvironment beyond the hair follicle per se, discuss their function in regulating hair graying and potentially novel treatments of hair graying.

Recent Progress in the Understanding of the Effect of Sympathetic Nerves on Hair Follicle Growth

Clinical observation and experimental studies have long suggested that the perifollicular nerves have nutritional and regulatory effects on the growth, development, and physiological cycle of hair follicles (HFs), even though the concrete mechanism remains obscure. Recently, with the progress of immunohistochemistry and molecular biology techniques, more innovation has been made in the study of the follicular sympathetic nerves and its nerve-effect factor norepinephrine affecting hair follicle stem cells. This review highlights the progress in the regulation of the sympathetic nervous system toward the growth of HFs.

Lgr6 marks epidermal stem cells with a nerve-dependent role in wound re-epithelialization

Stem cells support lifelong maintenance of adult organs, but their specific roles during injury are poorly understood. Here we demonstrate that Lgr6 marks a regionally restricted population of epidermal stem cells that interact with nerves and specialize in wound re-epithelialization. Diphtheria toxin-mediated ablation of Lgr6 stem cells delays wound healing, and skin denervation phenocopies this effect. Using intravital imaging to capture stem cell dynamics after injury, we show that wound re-epithelialization by Lgr6 stem cells is diminished following loss of nerves. This induces recruitment of other stem cell populations, including hair follicle stem cells, which partially compensate to mediate wound closure. Single-cell lineage tracing and gene expression analysis reveal that the fate of Lgr6 stem cells is shifted toward differentiation following loss of their niche. We conclude that Lgr6 epidermal stem cells are primed for injury response and interact with nerves to regulate their fate.

Hair follicle stem cells as a skin-organizing signaling center during adult homeostasis

Stem cells are the essential source of building blocks for tissue homeostasis and regeneration. Their behavior is dictated by both cell-intrinsic cues and extrinsic cues from the microenvironment, known as the stem cell niche. Interestingly, recent work began to demonstrate that hair follicle stem cells (HFSCs) are not only passive recipients of signals from the surroundings, but also actively send out signals to modulate the organization and function of their own niches. Here, we discuss recent findings, and briefly refer to the old, on the interaction of HFSCs and their niches with the emphasis on the outwards signals from HFSCs toward their niches. We also highlight recent technology advancements that further promote our understanding of HFSC niches. Taken together, the HFSCs emerge as a skin-organizing center rich in signaling output for niche remodeling during various stages of adult skin homeostasis. The intricate crosstalk between HFSCs and their niches adds important insight to skin biology that will inform clinical and bioengineering fields aiming to build complete and functional 3D organotypic cultures for skin replacement therapies.

Lower proximal cup and outer root sheath cells regenerate hair bulbs during anagen hair follicle repair after chemotherapeutic injury

The cell dynamics and cell origin for anagen hair follicle (HF) repair following chemotherapeutic injury are unclear. We first mapped the HF response to cyclophosphamide (CYP) at natural anagen VI in mice. We found that 30-60 mg/kg of CYP leads to dose-dependent HF dystrophy that was spontaneously repaired with anagen resumption, while 120 mg/kg of CYP prematurely induced catagen/telogen entry. To explore how anagen HF repair is achieved in the dystrophic anagen pathway, we analysed the cell dynamics at 30 mg/kg of CYP. Hair bulbs first shrunk due to matrix cell apoptosis associated with DNA double-strand breaks. DNA damage was repaired, and ordered hair bulb structures were restored within 96 hours. Bulge stem cells did not undergo apoptosis nor proliferation. K5⁺ basal lower proximal cup cells and outer root sheath cells quickly replenished the cells in the germinative zone and regenerated the concentric layered structures of the lower HF segment. Therefore, anagen HFs are able to summon extra-bulge progenitor cells in close proximity to the damaged matrix for quick repair after CYP injury.

Regenerative plasticity of intact human skin axons

The evaluation of human epidermal innervation and its impact by disease has largely focused on rigorous immunohistochemical counts of PGP 9.5 labelled axons. In this brief and preliminary report, we expand the repertoire of epidermal axon markers to include those with an influence on their regenerative plasticity. We studied human lower limb punch skin samples with tandem analyses of their mRNA content using qRT-PCR. Normal human subjects (n = 11) and two patients with newly diagnosed CIDP were sampled with the latter undergoing serial tandem biopsies before and after 3 months of immunotherapy. Controls expressed regeneration proteins within dermal and epidermal axons: GAP43 (growth associated protein 43), Shh (sonic hedgehog) and SCG (superior cervical ganglion-10; stathmin 2). Moreover, this expression accompanied intraepidermal nerve fiber density (IENF) within normal established values. CIDP patients had lower IENF but also expressed GAP43, Shh, and SCG. Tandem qRT-PCR identified confirmed the presence not only of these plasticity markers but of additional regeneration related mRNAs. CIDP patients had marked elevation of several mRNAs, with improvement following treatment. The findings support the concept of dynamic skin axon plasticity in humans is relevant toward consideration of newer therapeutic approaches.

Progress in studies of epidermal stem cells and their application in skin tissue engineering

The epidermis, which is the outermost layer of mammalian skin, provides an essential barrier that is essential for maintenance of life. The epidermis is a stratified epithelium, which is maintained by the proliferation of epidermal stem cells (EPSCs) at the basal layer of the epidermis. As a unique cell population characterized by self-renewal and differentiation capabilities, EPSCs ensure the maintenance of adult skin homeostasis and participate in repair of the epidermis after injury. Recently, the utilization of EPSCs for wound healing and tissue regeneration has been attracting increased attention from researchers. In addition, the advances in tissue engineering have increased the interest in applying EPSCs in tissue-engineered scaffolds to further reconstitute injured tissues. In this review, we introduce research developments related to EPSCs, including methods recently used in the culture and enrichment of EPSCs, as well as advanced tools to study EPSCs. The function and mechanism of the EPSC-dermal units in the development and homeostasis of the skin are also summarized. Finally, the potential applications of EPSCs in skin tissue engineering are discussed.

Riveting hammer vibration damages mechanosensory nerve endings

Hand-arm vibration syndrome (HAVS) is an irreversible neurodegenerative, vasospastic, and musculoskeletal occupational disease of workers who use powered hand tools. The etiology is poorly understood. Neurological symptoms include numbness, tingling, and pain. This study examines impact hammer vibration-induced injury and recoverability of hair mechanosensory innervation. Rat tails were vibrated 12 min/d for 5 weeks followed by 5 week recovery with synchronous non-vibrated controls. Nerve fibers were PGP9.5 immunostained. Lanceolate complex innervation was compared quantitatively in vibrated vs sham. Vibration peak acceleration magnitudes were characterized by frequency power spectral analysis. Average magnitude (2515 m/s² , root mean squared) in kHz frequencies was 109 times that (23 m/s² ) in low Hz. Percentage of hairs innervated by lanceolate complexes was 69.1% in 5-week sham and 53.4% in 5-week vibration generating a denervation difference of 15.7% higher in vibration. Hair innervation was 76.9% in 5-weeks recovery sham and 62.0% in 5-week recovery vibration producing a denervation difference 14.9% higher in recovery vibration. Lanceolate number per complex (18.4 ± 0.2) after vibration remained near sham (19.3 ± 0.3), but 44.9% of lanceolate complexes were abnormal in 5 weeks vibrated compared to 18.8% in sham. The largest vibration energies are peak kHz accelerations (approximately 100 000 m/s² ) from shock waves. The existing ISO 5349-1 standard excludes kHz vibrations, seriously underestimating vibration injury risk. The present study validates the rat tail, impact hammer vibration as a model for investigating irreversible nerve damage. Persistence of higher denervation difference after 5-week recovery suggests repeated vibration injury destroys the capability of lanceolate nerve endings to regenerate.

Functional complexity of hair follicle stem cell niche and therapeutic targeting of niche dysfunction for hair regeneration

Stem cell activity is subject to non-cell-autonomous regulation from the local microenvironment, or niche. In adaption to varying physiological conditions and the ever-changing external environment, the stem cell niche has evolved with multifunctionality that enables stem cells to detect these changes and to communicate with remote cells/tissues to tailor their activity for organismal needs. The cyclic growth of hair follicles is powered by hair follicle stem cells (HFSCs). Using HFSCs as a model, we categorize niche cells into 3 functional modules, including signaling, sensing and message-relaying. Signaling modules, such as dermal papilla cells, immune cells and adipocytes, regulate HFSC activity through short-range cell-cell contact or paracrine effects. Macrophages capacitate the HFSC niche to sense tissue injury and mechanical cues and adipocytes seem to modulate HFSC activity in response to systemic nutritional states. Sympathetic nerves implement the message-relaying function by transmitting external light signals through an ipRGC-SCN-sympathetic circuit to facilitate hair regeneration. Hair growth can be disrupted by niche pathology, e.g. dysfunction of dermal papilla cells in androgenetic alopecia and influx of auto-reacting T cells in alopecia areata and lichen planopilaris. Understanding the functions and pathological changes of the HFSC niche can provide new insight for the treatment of hair loss.

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.

Alcohol extract from Vernonia anthelmintica willd (L.) seed counteracts stress-induced murine hair follicle growth inhibition

BACKGROUND

Vernonia anthelmintica (L.) willd is a traditional urgur herb in China for a long history. Its alcohol extract (AVE) has been proved to promote hair follicle growth in C57BL/6 mice. We conducted this study to investigate the hair-growth effects of AVE in stressed mice and its possible mechanism of action.

METHODS

The hair-follicle growth effects of AVE were examined by in vivo and in vitro study. We exposed C57BL/6 male mice to chronic restraint stress to induce murine hair follicle growth inhibition. The effects of AVE were examined by histological analysis, immunofluorescence for Ki67 and cytokeratin 19 immunoreactivity, western blot assay in tyrosinase and related proteins expressions and immunofluorescence for nerve fibers. In organ culture of mouse vibrissae follicles, we used substance P as a catagen-inducing factor of hair follicle growth, and measured the elongation of hair shafts and expression of neurokinin-1 receptor protein by application of AVE.

RESULTS

Our results showed that AVE counteract murine hair follicle growth inhibition caused by chronic restraint stress via inducing the conversion of telogen to anagen and inhibiting catagen premature, increasing bulb keratinocytes and bulge stem cells proliferation, promoting melanogenesis, and reducing the numbers of substance P and calcitonin gene-related peptide nerve fibers. Furthermore, AVE also counteracted murine hair follicle growth inhibition caused by substance P in organ culture.

CONCLUSION

These results suggest that AVE counteract stress-induced hair follicle growth inhibition in C57BL/6 mice in vivo and in vitro, and may be an effective new candidate for treatment of stress-induced hair loss.

A Contemporary View on Felix Pinkus' Concept of the Vitreous Membrane

Introduction

Felix Pinkus' concept of the vitreous membrane (VM) published in 1927 describes circular folds protruding into the outer root sheath (ORS), which, in his opinion, serve as interdigitations between the outer root sheath (ORS) and the VM. This concept currently seems to have fallen into oblivion.

Objective

To determine the origin and possible function of the VM in the proliferation and vascularization of the hair follicle (HF).

Methods

Serial investigation of healthy skin probes with histological (hematoxylin & eosin and periodic acid-Schiff) and immunohistochemical examination (Ki67, CD56, CD8, and collagen IV) were performed.

Results

Morphological variations of the VM in various HFs such as protrusions and folds, the latter unilateral, bilateral or circular, some acute-angled, were found. Similarly, protrusions of the VM into the ORS were observed, that consisted of capillary tissue together with perifollicular tissue and VM mimicking minimal variants of the dermal papilla.

Conclusions

Pinkus' concept of the VM is revisited, reproduced and possible functions are proposed. Since these structures are found in a HF region with a high metabolic dynamism, they may be involved in differentiation or nutrition, or else be formed as a result of pressure arising from outgrowing hair shafts.

Multi-tasking epidermal stem cells: Beyond epidermal maintenance

Over the past decade, multiple stem cell compartments have been identified within the epidermis. These stem cell pools have different transcriptional properties, proliferative modes and anatomical locations, and they maintain distinct epidermal compartments. The importance of this stem cell heterogeneity and compartmentalization has been understood as a key feature in epidermal homeostasis. However, recent studies have revealed that these heterogeneous stem cells themselves act as a niche for neighboring cells, thereby establishing spatially and temporally patterned epidermal-dermal functional units. These studies provide a new perspective for interpreting the biological significance of stem cell heterogeneity and compartmentalization beyond their role in epidermal maintenance.

Hair follicle epidermal stem cells define a niche for tactile sensation

The heterogeneity and compartmentalization of stem cells is a common principle in many epithelia, and is known to function in epithelial maintenance, but its other physiological roles remain elusive. Here we show transcriptional and anatomical contributions of compartmentalized epidermal stem cells in tactile sensory unit formation in the mouse hair follicle. Epidermal stem cells in the follicle upper-bulge, where mechanosensory lanceolate complexes innervate, express a unique set of extracellular matrix (ECM) and neurogenesis-related genes. These epidermal stem cells deposit an ECM protein called EGFL6 into the collar matrix, a novel ECM that tightly ensheathes lanceolate complexes. EGFL6 is required for the proper patterning, touch responses, and αv integrin-enrichment of lanceolate complexes. By maintaining a quiescent original epidermal stem cell niche, the old bulge, epidermal stem cells provide anatomically stable follicle-lanceolate complex interfaces, irrespective of the stage of follicle regeneration cycle. Thus, compartmentalized epidermal stem cells provide a niche linking the hair follicle and the nervous system throughout the hair cycle.

External light activates hair follicle stem cells through eyes via an ipRGC-SCN-sympathetic neural pathway

Changes in external light patterns can alter cell activities in peripheral tissues through slow entrainment of the central clock in suprachiasmatic nucleus (SCN). It remains unclear whether cells in otherwise photo-insensitive tissues can achieve rapid responses to changes in external light. Here we show that light stimulation of animals' eyes results in rapid activation of hair follicle stem cells with prominent hair regeneration. Mechanistically, light signals are interpreted by M1-type intrinsically photosensitive retinal ganglion cells (ipRGCs), which signal to the SCN via melanopsin. Subsequently, efferent sympathetic nerves are immediately activated. Increased norepinephrine release in skin promotes hedgehog signaling to activate hair follicle stem cells. Thus, external light can directly regulate tissue stem cells via an ipRGC-SCN autonomic nervous system circuit. Since activation of sympathetic nerves is not limited to skin, this circuit can also facilitate rapid adaptive responses to external light in other homeostatic tissues.

The Hair Follicle: An Underutilized Source of Cells and Materials for Regenerative Medicine

The hair follicle is one of only two structures within the adult body that selectively degenerates and regenerates, making it an intriguing organ to study and use for regenerative medicine. Hair follicles have been shown to influence wound healing, angiogenesis, neurogenesis, and harbor distinct populations of stem cells; this has led to cells from the follicle being used in clinical trials for tendinosis and chronic ulcers. In addition, keratin produced by the follicle in the form of a hair fiber provides an abundant source of biomaterials for regenerative medicine. In this review, we provide an overview of the structure of a hair follicle, explain the role of the follicle in regulating the microenvironment of skin and the impact on wound healing, explore individual cell types of interest for regenerative medicine, and cover several applications of keratin-based biomaterials.

Nerve reinnervation and itch behavior in a rat burn wound model

In this study, we investigated whether postburn itch in rats, after a full thickness burn, is correlated to the nervous reinnervation of the burn wound area. For this purpose, we determined scratching duration (expressed as second/hour) at 24 hours, 2, 4, 8, and 12 weeks postburn and combined this with immunohistochemistry for protein gene product 9.5 (PGP9.5) to identify all nerve fibers, calcitonin gene related peptide (CGRP) to identify peptidergic fibers, tyrosine hydroxylase (TH) for sympathetic fibers, and growth-associated protein 43 (GAP-43) for regrowing fibers. We found a modest, but highly significant, increase in scratching duration of all burn wound rats from 3 to 12 weeks postburn (maximally 63 ± 9.5 second/hour compared to sham 3.1 ± 1.4 second/hour at 9 weeks). At 24 hours postburn, all nerve fibers had disappeared from the burn area. Around 4 weeks postburn PGP 9.5- and CGRP-immunoreactive nerve fibers returned to control levels. TH- and GAP-43-IR nerve fibers, which we found to be almost completely colocalized, did not regrow. No correlation was found between scratching duration and nervous reinnervation of the skin. The present results suggest that in rat, like in human, burn wound healing will induce increased scratching, which is not correlated to the appearance of nervous reinnervation.

Touch Receptors Undergo Rapid Remodeling in Healthy Skin

Sensory tissues exposed to the environment, such as skin, olfactory epithelia, and taste buds, continuously renew; therefore, peripheral neurons must have mechanisms to maintain appropriate innervation patterns. Although somatosensory neurons regenerate after injury, little is known about how these neurons cope with normal target organ changes. To elucidate neuronal plasticity in healthy skin, we analyzed the structure of Merkel-cell afferents, which are gentle touch receptors, during skin remodeling that accompanies mouse hair-follicle regeneration. The number of Merkel cells is reduced by 90% and axonal arbors are simplified during active hair growth. These structures rebound within just days. Computational modeling predicts that Merkel-cell changes are probabilistic, but myelinated branch stability depends on Merkel-cell inputs. Electrophysiology and behavior demonstrate that tactile responsiveness is less reliable during active growth than in resting skin. These results reveal that somatosensory neurons display structural plasticity at the cost of impairment in the reliability of encoding gentle touch.

Out of Many, One: Computational Reconstruction of Mouse Skin using Single-Cell Transcriptomics

Mammalian skin is a complex and heterogeneous tissue with several distinct compartments and stem cell populations. Joost et al. (2016) now use single-cell RNaseq to comprehensively reconstruct this complexity, revealing spatial and pseudotemporal differences between transcriptional programs in distinct compartments and a common basal program in skin stem cell populations.

The Adaptor Protein CD2AP Is a Coordinator of Neurotrophin Signaling-Mediated Axon Arbor Plasticity

Growth of intact axons of noninjured neurons, often termed collateral sprouting, contributes to both adaptive and pathological plasticity in the adult nervous system, but the intracellular factors controlling this growth are largely unknown. An automated functional assay of genes regulated in sensory neurons from the rat in vivo spared dermatome model of collateral sprouting identified the adaptor protein CD2-associated protein (CD2AP; human CMS) as a positive regulator of axon growth. In non-neuronal cells, CD2AP, like other adaptor proteins, functions to selectively control the spatial/temporal assembly of multiprotein complexes that transmit intracellular signals. Although CD2AP polymorphisms are associated with increased risk of late-onset Alzheimer's disease, its role in axon growth is unknown. Assessments of neurite arbor structure in vitro revealed CD2AP overexpression, and siRNA-mediated knockdown, modulated (1) neurite length, (2) neurite complexity, and (3) growth cone filopodia number, in accordance with CD2AP expression levels. We show, for the first time, that CD2AP forms a novel multiprotein complex with the NGF receptor TrkA and the PI3K regulatory subunit p85, with the degree of TrkA:p85 association positively regulated by CD2AP levels. CD2AP also regulates NGF signaling through AKT, but not ERK, and regulates long-range signaling though TrkA(+)/RAB5(+) signaling endosomes. CD2AP mRNA and protein levels were increased in neurons during collateral sprouting but decreased following injury, suggesting that, although typically considered together, these two adult axonal growth processes are fundamentally different. These data position CD2AP as a major intracellular signaling molecule coordinating NGF signaling to regulate collateral sprouting and structural plasticity of intact adult axons.

SIGNIFICANCE STATEMENT

Growth of noninjured axons in the adult nervous system contributes to adaptive and maladaptive plasticity, and dysfunction of this process may contribute to neurologic pathologies. Functional screening of genes regulated during growth of noninjured axons revealed CD2AP as a positive regulator of axon outgrowth. A novel association of CD2AP with TrkA and p85 suggests a distinct intracellular signaling pathway regulating growth of noninjured axons. This may also represent a novel mechanism of generating specificity in multifunctional NGF signaling. Divergent regulation of CD2AP in different axon growth conditions suggests that separate mechanisms exist for different modes of axon growth. CD2AP is the first signaling molecule associated with adult sensory axonal collateral sprouting, and this association may offer new insights for NGF/TrkA-related Alzheimer's disease mechanisms.

The immune system of mouse vibrissae follicles: cellular composition and indications of immune privilege

Although vibrissae hair follicles (VHFs) have long been a key research model in the life sciences, their immune system (IS) is essentially unknown. Therefore, we have characterized basic parameters of the VHF-IS of C57BL/6J mice by quantitative (immuno-)histomorphometry. Murine anagen VHF harbour few CD4+ and CD8+ T cells in the distal mesenchyme and sinuses but hardly any gamma-delta T cells in their distal epithelium. MHC class II+ Langerhans cells are seeded in the VHF infundibulum, which is also surrounded by MHC class II+ and CD11b+ cells (macrophages). The number of Langerhans cells then declines sharply in the VHF bulge, and the VHF bulb lacks MHC class II+ cells. Mast cells densely populate the VHF connective tissue sheath, where they strikingly cluster around the bulge. Both the bulge and the bulb of VHF display signs of immune privilege, that is, low MHC class I and MHC class II expression and local immunoinhibitor expression (CD200, TGFβ1). This immunophenotyping study fills an important gap in the immunobiology of murine skin and identifies differences between the IS of VHF, mouse pelage and human terminal HFs. This facilitates utilizing murine VHF as a versatile organ culture model for general immunology and immune privilege research in situ.

Tissue-engineered dermo-epidermal skin analogs exhibit de novo formation of a near natural neurovascular link 10 weeks after transplantation

PURPOSE

Human autologous tissue-engineered skin grafts are a promising way to cover skin defects. Clearly, it is mandatory to study essential biological dynamics after transplantation, including reinnervation. Previously, we have already shown that human tissue-engineered skin analogs are reinnervated by host nerve fibers as early as 8 weeks after transplantation. In this study, we tested the hypothesis that there is a de novo formation of a "classical" neurovascular link in tissue-engineered and then transplanted skin substitutes.

METHODS

Keratinocytes, melanocytes, and fibroblasts were isolated from human skin biopsies. After expansion in culture, keratinocytes and melanocytes were seeded on dermal fibroblast-containing collagen type I hydrogels. These human tissue-engineered dermo-epidermal skin analogs were transplanted onto full-thickness skin wounds on the back of immuno-incompetent rats. Grafts were analyzed after 3 and 10 weeks. Histological sections were examined with regard to the ingrowth pattern of myelinated and unmyelinated nerve fibers into the skin analogs using markers such as PGP9.5, NF-200, and NF-160. Blood vessels were identified with CD31, lymphatic vessels with Lyve1. In particular, we focused on alignment patterns between nerve fibers and either blood and/or lymphatic vessels with regard to neurovascular link formation.

RESULTS

3 weeks after transplantation, blood vessels, but no nerve fibers or lymphatic vessels could be observed. 10 weeks after transplantation, we could detect an ingrowth of myelinated and unmyelinated nerve fibers into the skin analogs. Nerve fibers were found in close proximity to CD31-positive blood vessels, but not alongside Lyve1-positive lymphatic vessels.

CONCLUSION

These data suggest that host-derived innervation of tissue-engineered dermo-epidermal skin analogs is initiated by and guided alongside blood vessels present early post-transplantation. This observation is consistent with the concept of a cross talk between neurovascular structures, known as the neurovascular link.

VGLUTs in Peripheral Neurons and the Spinal Cord: Time for a Review

Vesicular glutamate transporters (VGLUTs) are key molecules for the incorporation of glutamate in synaptic vesicles across the nervous system, and since their discovery in the early 1990s, research on these transporters has been intense and productive. This review will focus on several aspects of VGLUTs research on neurons in the periphery and the spinal cord. Firstly, it will begin with a historical account on the evolution of the morphological analysis of glutamatergic systems and the pivotal role played by the discovery of VGLUTs. Secondly, and in order to provide an appropriate framework, there will be a synthetic description of the neuroanatomy and neurochemistry of peripheral neurons and the spinal cord. This will be followed by a succinct description of the current knowledge on the expression of VGLUTs in peripheral sensory and autonomic neurons and neurons in the spinal cord. Finally, this review will address the modulation of VGLUTs expression after nerve and tissue insult, their physiological relevance in relation to sensation, pain, and neuroprotection, and their potential pharmacological usefulness.

Loss of innervation and axon plasticity accompanies impaired diabetic wound healing

Loss of cutaneous innervation from sensory neuropathy is included among mechanisms for impaired healing of diabetic skin wounds. The relationships between cutaneous axons and their local microenvironment during wound healing are challenged in diabetes. Here, we show that secondary wound closure of the hairy dorsal skin of mice is delayed by diabetes and is associated with not only a pre-existing loss of cutaneous axons but substantial retraction of axons around the wound. At 7d following a 3mm punch wound, a critical period of healing and reinnervation, both intact skin nearby the wound and skin directly at the wound margins had over 30-50% fewer axons and a larger deficit of ingrowing axons in diabetics. These findings contrasted with a pre-existing 10-15% deficit in axons. Moreover, new diabetic ingrowing axons had less evidence of plasticity. Unexpectedly, hair follicles adjacent to the wounds had a 70% reduction in their innervation associated with depleted expression of hair follicular stem cell markers. These impairments were associated with the local upregulation of two established axon regenerative ‘roadblocks’: PTEN and RHOA, potential but thus far unexplored mediators of these changes. The overall findings identify striking and unexpected superimposed cutaneous axon loss or retraction beyond that expected of diabetic neuropathy alone, associated with experimental diabetic skin wounding, a finding that prompts new considerations in diabetic wounds.

The characterisation of Pax3 expressant cells in adult peripheral nerve

Pax3 has numerous integral functions in embryonic tissue morphogenesis and knowledge of its complex function in cells of adult tissue continues to unfold. Across a variety of adult tissue lineages, the role of Pax3 is principally linked to maintenance of the tissue’s resident stem/progenitor cell population. In adult peripheral nerves, Pax3 is reported to be expressed in nonmyelinating Schwann cells, however, little is known about the purpose of this expression. Based on the evidence of the role of Pax3 in other adult tissue stem and progenitor cells, it was hypothesised that the cells in adult peripheral nerve that express Pax3 may be peripheral glioblasts. Here, methods have been developed for identification and visualisation of Pax3 expressant cells in normal 60 day old mouse peripheral nerve that allowed morphological and phenotypic distinctions to be made between Pax3 expressing cells and other nonmyelinating Schwann cells. The distinctions described provide compelling support for a resident glioblast population in adult mouse peripheral nerve.

Pax3( GFP ) , a new reporter for the melanocyte lineage, highlights novel aspects of PAX3 expression in the skin

The paired box gene 3 (Pax3) is expressed during pigment cell development. We tested whether the targeted allele Pax3(GFP) can be used as a reporter gene for pigment cells in the mouse. We found that enhanced green fluorescent protein (GFP) can be seen readily in every melanoblast and melanocyte in the epidermis and hair follicles of Pax3(GFP/+) heterozygotes. The GFP was detected at all differentiation stages, including melanocyte stem cells. In the dermis, Schwann cells and nestin-positive cells of the piloneural collars resembling the nestin-positive hair follicle multipotent stem cells exhibited a weaker GFP signal. Pigment cells could be purified by fluorescent activated cell sorting and grown in vitro without feeder cells, giving pure cultures of melanocytes. The Schwann cells and nestin-positive cells of the piloneural collars were FACS-isolated based on their weak expression of GFP. Thus Pax3(GFP) can discriminate distinct populations of cells in the skin.

How many hair follicles are innervated by one afferent axon? A confocal microscopic analysis of palisade endings in the auricular skin of thy1-YFP transgenic mouse

Hairs are known as a sensory apparatus for touch. Their follicles are innervated predominantly by palisade endings composed of longitudinal and circumferential lanceolate endings. However, little is known as to how their original primary neurons make up a part of the ending. In this study, innervation of the palisade endings was investigated in the auricular skin of thy1-YFP transgenic mouse. Major observations were 1) Only a small portion of PGP9.5-immunopositive axons showed YFP-positivity, 2) All of thy1-YFP-positive sensory axons were thick and myelinated, 3) Individual thy1-YFP-positive trunk axons innervated 4-54 hair follicles, 4) Most palisade endings had a gap of lanceolate ending arrangement, 5) PGP9.5-immunopositive 10-32 longitudinal lanceolate endings were closely arranged. Only a part of them were thy1-YFP-positive axons that originated from 1-3 afferents, and 6) Single nerve bundles of the dermal nerve network included both bidirectional afferents. Palisade endings innervated by multiple sensory neurons might be highly sensitive to hair movement.

Niche crosstalk: intercellular signals at the hair follicle

A recent series of papers, including Festa et al. (2011) in this issue, has revealed unexpected interdependent relationships among cell populations residing in and around the hair follicle. These interactions between different lineages of stem cells are crucial for hair follicle growth and cycling and point to a complex crosstalk in stem cell niches.

Nerve-derived sonic hedgehog defines a niche for hair follicle stem cells capable of becoming epidermal stem cells

In adult skin, stem cells in the hair follicle bulge cyclically regenerate the follicle, whereas a distinct stem cell population maintains the epidermis. The degree to which all bulge cells have equal regenerative potential is not known. We found that Sonic hedgehog (Shh) from neurons signals to a population of cells in the telogen bulge marked by the Hedgehog response gene Gli1. Gli1-expressing bulge cells function as multipotent stem cells in their native environment and repeatedly regenerate the anagen follicle. Shh-responding perineural bulge cells incorporate into healing skin wounds where, notably, they can change their lineage into epidermal stem cells. The perineural niche (including Shh) is dispensable for follicle contributions to acute wound healing and skin homeostasis, but is necessary to maintain bulge cells capable of becoming epidermal stem cells. Thus, nerves cultivate a microenvironment where Shh creates a molecularly and phenotypically distinct population of hair follicle stem cells.

Dynamic plasticity of axons within a cutaneous milieu

The skin is a repository of sensory axons immersed within the turnover of epidermal, follicular, and dermal cellular constituents. We show that epidermal and perifollicular axons within intact hairy skin of mice possess a remarkable dynamic plasticity linked to their microenvironment. For example, the majority of epidermal axons express the growth protein GAP43. Unexpectedly, we induced new cutaneous axogenesis by simple and noninvasive hair clipping, a response linked to a series of changes in their cutaneous neighbors. In thy-1 YFP transgenic mice with fluorescent axons, superficial epidermal and perifollicular cells newly acquired YFP, indicating diffuse activation by clipping despite the absence of skin injury. At 48 h after clipping, this activation was accompanied by a rise in the number of epidermal cells, transient rises in mRNA of Sox2, a marker of follicular stem cells, and a rise in mRNA of glial fibrillary acidic protein, a marker of glial cells. Axons responded with rises in their numbers in the epidermis and around dermal hair follicles. Linking these responses were early, large, and selective rises in hepatic growth factor (HGF) mRNA, with its protein identified in epidermal cells, perifollicular cells, and sensory axons. Moreover, these elements also expressed the HGF receptor c-Met, especially in small caliber sensory neurons. Finally, we identified concurrent rises in Rac1 activation, a downstream target of ligated c-Met. Together, these results confirm critical linkages between sensory axons and their cutaneous milieu. We believe that the plasticity is provoked by follicular-originating cutaneous activation with HGF and Rac1 signaling, allowing cross talk and axonal remodeling.

Adult epidermal Notch activity induces dermal accumulation of T cells and neural crest derivatives through upregulation of jagged 1

Notch signalling regulates epidermal differentiation and tumour formation via non-cell autonomous mechanisms that are incompletely understood. This study shows that epidermal Notch activation via a 4-hydroxy-tamoxifen-inducible transgene caused epidermal thickening, focal detachment from the underlying dermis and hair clumping. In addition, there was dermal accumulation of T lymphocytes and stromal cells, some of which localised to the blisters at the epidermal-dermal boundary. The T cell infiltrate was responsible for hair clumping but not for other Notch phenotypes. Notch-induced stromal cells were heterogeneous, expressing markers of neural crest, melanocytes, smooth muscle and peripheral nerve. Although Slug1 expression was expanded in the epidermis, the stromal cells did not arise through epithelial-mesenchymal transition. Epidermal Notch activation resulted in upregulation of jagged 1 in both epidermis and dermis. When Notch was activated in the absence of epidermal jagged 1, jagged 1 was not upregulated in the dermis, and epidermal thickening, blister formation, accumulation of T cells and stromal cells were inhibited. Gene expression profiling revealed that epidermal Notch activation resulted in upregulation of several growth factors and cytokines, including TNFα, the expression of which was dependent on epidermal jagged 1. We conclude that jagged 1 is a key mediator of non-cell autonomous Notch signalling in skin.

Immunohistochemical comparison of whisker pad cutaneous innervation in Swiss Webster and hairless mice

To establish the mouse mutant, hairless (Hr), as a useful model for future analyses of target-ending interactions, we assessed the cutaneous innervation in the whisker pad after loss of primary hair targets. Postnatal (P) development of fur in Hr begins similarly to that of "normal" Swiss Webster (SW) mice. Around P10, hairs are shed and the follicles rendered permanently incompetent. Hair loss progresses rostrocaudally until the entire skin is denuded. Substantial alterations in the distribution and density of sensory and autonomic endings in the mystacial pad vibrissal and intervibrissal fur innervation were discovered. Pilo-neural complexes innervating fur hairs were dismantled in Hr. Epidermal innervation in SW was rich; only a few endings expressed growth-associated protein-43 kdal (GAP), suggesting limited changes in axonal elongation. Innervation in Hr formed a dense layer passing upward through the thickened epidermis, with substantial increases among all types of endings. Vibrissal follicle-sinus complexes were also hyperinnervated. Endings in Hr vibrissae and fur were strongly GAP-positive, suggesting reorganization of innervation. Dermal and vascular autonomic innervation in both strains co-localized tyrosine hydroxylase and neuropeptide Y, but only in Hr did neuropeptide Y co-localize calcitonin gene-related peptide (CGRP) and express GAP immunolabeling. Stereological quantitation of trigeminal ganglia revealed no differences in neuron number between Hr and SW, although there were small increases in cell volume in Hr trigeminal ganglion cells. These results suggested that a form of collateral sprouting was active in Hr mystacial pads, not in response to local injury, but as a result of loss of primary target tissues.

Prenatal hair development: implications for drug exposure determination

Neonatal hair is a clinically important toxicological matrix, as it allows determination of in utero drug exposure. This paper serves to review the physiological development of the hair follicle and hair production during fetal life. An understanding of the mechanisms and timing of hair development in the prenatal period is critical to effectively assessing the time window of exposure determination associated with toxicological analysis of neonatal hair.

The hair follicle as a dynamic miniorgan

Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. The hair shaft consists of terminally differentiated keratinocytes that are produced by the hair follicle. Hair follicle development takes place during fetal skin development and relies on tightly regulated ectodermal-mesodermal interactions. After birth, mature and actively growing hair follicles eventually become anchored in the subcutis, and periodically regenerate by spontaneously undergoing repetitive cycles of growth (anagen), apoptosis-driven regression (catagen), and relative quiescence (telogen). Our molecular understanding of hair follicle biology relies heavily on mouse mutants with abnormalities in hair structure, growth, and/or pigmentation. These mice have allowed novel insights into important general molecular and cellular processes beyond skin and hair biology, ranging from organ induction, morphogenesis and regeneration, to pigment and stem cell biology, cell proliferation, migration and apoptosis. In this review, we present basic concepts of hair follicle biology and summarize important recent advances in the field.

The EGFR is required for proper innervation to the skin

EGFR family members are essential for proper peripheral nervous system development. A role for EGFR itself in peripheral nervous system development in vivo, however, has not been reported. We investigated whether EGFR is required for cutaneous innervation using Egfr null and skin-targeted Egfr mutant mice. Neuronal markers; including PGP9.5, GAP-43, acetylated tubulin, and neurofilaments; revealed that Egfr null dorsal skin was hyperinnervated with a disorganized pattern of innervation. In addition, receptor subtypes such as lanceolate endings were disorganized and immature. To determine whether the hyperinnervation phenotype resulted from a target-derived effect of loss of EGFR, mice lacking EGFR expression in the cutaneous epithelium were examined. These mice retained other aspects of the cutaneous Egfr null phenotype but exhibited normal innervation. The sensory deficits in Egfr null dorsal skin were not associated with any abnormality in the morphology or density of dorsal root ganglion (DRG) neurons or Schwann cells. However, explant and dissociated cell cultures of DRG revealed more extensive branching in Egfr null cultures. These data demonstrate that EGFR is required for proper cutaneous innervation during development and suggest that it limits axonal outgrowth and branching in a DRG-autonomous manner.

Skin and hair follicle innervation in experimental models: a guide for the exact and reproducible evaluation of neuronal plasticity

The remodelling of skin innervation is an instructive example of neuronal plasticity in the peripheral nervous system. Cutaneous innervation displays dramatic plasticity during morphogenesis, adult remodelling, skin diseases and after skin nerve lesions. To recognize even subtle changes or abnormalities of cutaneous innervation under different experimental conditions, it is critically important to use a quantitative approach. Here, we introduce a simple, fast and reproducible quantitative method based on immunofluorescence histochemistry for the exact quantification of peripheral nerve fibres. Computer-generated schematic representations of cutaneous innervation in defined skin compartments are presented with the aim of standardizing reports on gene and protein expression patterns. This guide should become a useful tool when screening new mouse mutants, disease models affecting innervation or mice treated with pharmaceuticals for discrete morphologic abnormalities of skin innervation in a highly reproducible and quantifiable manner. Moreover, this method can be easily transferred to other densely innervated peripheral organs.