The structure and organization of lanceolate mechanosensory complexes at mouse hair follicles

Schwann cell plasticity-roles in tissue homeostasis, regeneration, and disease

How tissues are maintained over a lifetime and repaired following injury are fundamental questions in biology with a disruption to these processes underlying pathologies such as cancer and degenerative disorders. It is becoming increasingly clear that each tissue has a distinct mechanism to maintain homeostasis and respond to injury utilizing different types of stem/progenitor cell populations depending on the insult and/or with a contribution from more differentiated cells that are able to dedifferentiate to aid tissue regeneration. Peripheral nerves are highly quiescent yet show remarkable regenerative capabilities. Remarkably, there is no evidence for a classical stem cell population, rather all cell-types within the nerve are able to proliferate to produce new nerve tissue. Co-ordinating the regeneration of this tissue are Schwann cells (SCs), the main glial cells of the peripheral nervous system. SCs exist in architecturally stable structures that can persist for the lifetime of an animal, however, they are not postmitotic, in that following injury they are reprogrammed at high efficiency to a progenitor-like state, with these cells acting to orchestrate the nerve regeneration process. During nerve regeneration, SCs show little plasticity, maintaining their identity in the repaired tissue. However, once free of the nerve environment they appear to exhibit increased plasticity with reported roles in the repair of other tissues. In this review, we will discuss the mechanisms underlying the homeostasis and regeneration of peripheral nerves and how reprogrammed progenitor-like SCs have broader roles in the repair of other tissues with implications for pathologies such as cancer.

Functional Anatomy of the Sensory Nervous System: Updates From the Neuroscience Bench

The simple tripartite classification of sensory neurons as A-beta, A-delta, and C fibers fails to convey the complexity of the neurons that encode stimuli as diverse as the texture of a surface, the location of a pinprick, or the direction of hair movement as a breeze moves across the skin. It has also proven to be inadequate when investigating the molecular mechanisms underlying pain, which can encompass any combination of chemical, tactile, and thermal modalities. Beginning with a brief overview of visceral and sensory neuroanatomy, this review expands upon sensory innervation of the skin as a prime example of the heterogeneity and complexity of the somatosensory nervous system. Neuroscientists have characterized defining features of over 15 subtypes of sensory neurons that innervate the skin of the mouse. This has enabled the study of cell-specific mechanisms of pain, which suggests that diverse sensory neuron subtypes may have distinct susceptibilities to toxic injury and different roles in pathologic mechanisms underlying altered sensation. Leveraging this growing body of knowledge for preclinical trials and models of neurotoxicity can vastly improve our understanding of peripheral nervous system dysfunction, advancing the fields of toxicologic pathology and neuropathology alike.

Somatosensory neurons integrate the geometry of skin deformation and mechanotransduction channels to shape touch sensing

Touch sensation hinges on force transfer across the skin and activation of mechanosensitive ion channels along the somatosensory neurons that invade the skin. This skin-nerve sensory system demands a quantitative model that spans the application of mechanical loads to channel activation. Unlike prior models of the dynamic responses of touch receptor neurons in Caenorhabditis elegans (Eastwood et al., 2015), which substituted a single effective channel for the ensemble along the TRNs, this study integrates body mechanics and the spatial recruitment of the various channels. We demonstrate that this model captures mechanical properties of the worm’s body and accurately reproduces neural responses to simple stimuli. It also captures responses to complex stimuli featuring non-trivial spatial patterns, like extended or multiple contacts that could not be addressed otherwise. We illustrate the importance of these effects with new experiments revealing that skin-neuron composites respond to pre-indentation with increased currents rather than adapting to persistent stimulation.

Uncovering the Cells and Circuits of Touch in Normal and Pathological Settings

The sense of touch is fundamental as it provides vital, moment-to-moment information about the nature of our physical environment. Primary sensory neurons provide the basis for this sensation in the periphery; however, recent work demonstrates that touch transduction mechanisms also occur upstream of the sensory neurons via non-neuronal cells such as Merkel cells and keratinocytes. Within the spinal cord, deep dorsal horn circuits transmit innocuous touch centrally and also transform touch into pain in the setting of injury. Here non-neuronal cells play a key role in the induction and maintenance of persistent mechanical pain. This review highlights recent advances in our understanding of mechanosensation, including a growing appreciation for the role of non-neuronal cells in both touch and pain.

Tiling and somatotopic alignment of mammalian low-threshold mechanoreceptors

Innocuous mechanical stimuli acting on the skin are detected by sensory neurons, known as low-threshold mechanoreceptors (LTMRs). LTMRs are classified based on their response properties, action potential conduction velocity, rate of adaptation to static indentation of the skin, and terminal anatomy. Here, we report organizational properties of the cutaneous and central axonal projections of the five principal hairy skin LTMR subtypes. We find that axons of neurons within a particular LTMR class are largely nonoverlapping with respect to their cutaneous end organs (e.g., hair follicles), with Aβ rapidly adapting-LTMRs being the sole exception. Individual neurons of each LTMR class are mostly nonoverlapping with respect to their associated hair follicles, with the notable exception of C-LTMRs, which exhibit multiple branches that redundantly innervate individual hair follicles. In the spinal cord, LTMR central projections exhibit rostrocaudal elongation and mediolateral compression, compared with their cutaneous innervation patterns, and these central projections also exhibit a fine degree of homotypic topographic adjacency. These findings thus reveal homotypic tiling of LTMR subtype axonal projections in hairy skin and a remarkable degree of spatial precision of spinal cord axonal termination patterns, suggesting a somatotopically precise tactile encoding capability of the mechanosensory dorsal horn.

The cellular basis of mechanosensory Merkel-cell innervation during development

Touch sensation is initiated by mechanosensory neurons that innervate distinct skin structures; however, little is known about how these neurons are patterned during mammalian skin development. We explored the cellular basis of touch-receptor patterning in mouse touch domes, which contain mechanosensory Merkel cell-neurite complexes and abut primary hair follicles. At embryonic stage 16.5 (E16.5), touch domes emerge as patches of Merkel cells and keratinocytes clustered with a previously unsuspected population of Bmp4-expressing dermal cells. Epidermal Noggin overexpression at E14.5 disrupted touch-dome formation but not hair-follicle specification, demonstrating a temporally distinct requirement for BMP signaling in placode-derived structures. Surprisingly, two neuronal populations preferentially targeted touch domes during development but only one persisted in mature touch domes. Finally, Keratin-17-expressing keratinocytes but not Merkel cells were necessary to establish innervation patterns during development. These findings identify key cell types and signaling pathways required for targeting Merkel-cell afferents to discrete mechanosensory compartments.

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.

Primary sensory neurons expressing tropomyosin receptor kinase A in the rat trigeminal ganglion

Tropomyosin receptor kinase A (trkA), a high affinity receptor for nerve growth factor (NGF), has been implicated in neuronal survival, neurite outgrowth and inflammatory pain. So far, the characterization of the primary sensory neurons that express trkA, and are thus potentially affected by NGF, has remained incomplete. The goal of this study was to investigate the trkA-expressing neurons and fibers in the rat trigeminal ganglion and its sensory root using light- and electron-microscopic immunohistochemistry and quantitative analysis. TrkA-immunopositive (+) trigeminal neurons varied from small to large. Double immunofluorescent staining showed that about 28%, 33% and 3% of the trkA(+) neurons coexpressed SP, CGRP and IB4, respectively. About 11% of the trkA(+) neurons also coexpressed parvalbumin. Electron microscopy revealed that trkA was expressed in all types of fibers: While the large majority of the trkA(+) fibers were unmyelinated (35.3%) and small myelinated (<20 μm² in cross-sectional area; 45.5%), a still considerable fraction (19.2%) was large myelinated. These findings indicate that all types of trigeminal neurons (ones with unmyelinated, small myelinated or large myelinated fibers) may be regulated by NGF/trkA signaling.

Specialized mechanoreceptor systems in rodent glabrous skin

KEY POINTS

An ex vivo preparation was developed to record from single sensory fibres innervating the glabrous skin of the mouse forepaw. The density of mechanoreceptor innervation of the forepaw glabrous skin was found to be three times higher than that of hindpaw glabrous skin. Rapidly adapting mechanoreceptors that innervate Meissner's corpuscles were severalfold more responsive to slowly moving stimuli in the forepaw compared to those innervating hindpaw skin. We found a distinct group of small hairs in the centre of the mouse hindpaw glabrous skin that were exclusively innervated by directionally sensitive D-hair receptors. The directional sensitivity, but not the end-organ anatomy, were the opposite to D-hair receptors in the hairy skin. Glabrous skin hairs in the hindpaw are not ubiquitous in rodents, but occur in African and North American species that diverged more than 65 million years ago.

ABSTRACT

Rodents use their forepaws to actively interact with their tactile environment. Studies on the physiology and anatomy of glabrous skin that makes up the majority of the forepaw are almost non-existent in the mouse. Here we developed a preparation to record from single sensory fibres of the forepaw and compared anatomical and physiological receptor properties to those of the hindpaw glabrous and hairy skin. We found that the mouse forepaw skin is equipped with a very high density of mechanoreceptors; >3 times more than hindpaw glabrous skin. In addition, rapidly adapting mechanoreceptors that innervate Meissner's corpuscles of the forepaw were severalfold more sensitive to slowly moving mechanical stimuli compared to their counterparts in the hindpaw glabrous skin. All other mechanoreceptor types as well as myelinated nociceptors had physiological properties that were invariant regardless of which skin area they occupied. We discovered a novel D-hair receptor innervating a small group of hairs in the middle of the hindpaw glabrous skin in mice. These glabrous skin D-hair receptors were direction sensitive albeit with an orientation sensitivity opposite to that described for hairy skin D-hair receptors. Glabrous skin hairs do not occur in all rodents, but are present in North American and African rodent species that diverged more than 65 million years ago. The function of these specialized hairs is unknown, but they are nevertheless evolutionarily very ancient. Our study reveals novel physiological specializations of mechanoreceptors in the glabrous skin that likely evolved to facilitate tactile exploration.

The neurochemistry and morphology of functionally identified corneal polymodal nociceptors and cold thermoreceptors

It is generally believed that the unencapsulated sensory nerve terminals of modality specific C- and Aδ-neurons lack structural specialization. Here we determined the morphology of functionally defined polymodal receptors and cold thermoreceptors in the guinea pig corneal epithelium. Polymodal receptors and cold thermoreceptors were identified by extracellular recording at the surface of the corneal epithelium. After marking the recording sites, corneas were processed to reveal immunoreactivity for the transient receptor potential channels TRPV1 (transient receptor potential cation channel, subfamily V, member 1) or TPRM8 (transient receptor potential cation channel subfamily M member 8). Polymodal receptor nerve terminals (n = 6) were TRPV1-immunoreactive and derived from an axon that ascended from the sub-basal plexus to the squamous cell layer where it branched into fibers that ran parallel to the corneal surface and terminated with small bulbar endings (ramifying endings). Cold thermoreceptor nerve terminals were TRPM8-immunoreactive (n = 6) and originated from an axon that branched as it ascended through the wing cell and squamous cell layers and terminated with large bulbar endings (complex endings). These findings indicate that modality specific corneal sensory neurons with unencapsulated nerve endings have distinct nerve terminal morphologies that are likely to relate to their function.

Developing a sense of touch

The sensation of touch is mediated by mechanosensory neurons that are embedded in skin and relay signals from the periphery to the central nervous system. During embryogenesis, axons elongate from these neurons to make contact with the developing skin. Concurrently, the epithelium of skin transforms from a homogeneous tissue into a heterogeneous organ that is made up of distinct layers and microdomains. Throughout this process, each neuronal terminal must form connections with an appropriate skin region to serve its function. This Review presents current knowledge of the development of the sensory microdomains in mammalian skin and the mechanosensory neurons that innervate them.

Opposing expression gradients of calcitonin-related polypeptide alpha (Calca/Cgrpα) and tyrosine hydroxylase (Th) in type II afferent neurons of the mouse cochlea

Type II spiral ganglion neurons (SGNs) are small caliber, unmyelinated afferents that extend dendritic arbors hundreds of microns along the cochlear spiral, contacting many outer hair cells (OHCs). Despite these many contacts, type II afferents are insensitive to sound and only weakly depolarized by glutamate release from OHCs. Recent studies suggest that type II afferents may be cochlear nociceptors, and can be excited by ATP released during tissue damage, by analogy to somatic pain-sensing C-fibers. The present work compares the expression patterns among cochlear type II afferents of two genes found in C-fibers: calcitonin-related polypeptide alpha (Calca/Cgrpα), specific to pain-sensing C-fibers, and tyrosine hydroxylase (Th), specific to low-threshold mechanoreceptive C-fibers, which was shown previously to be a selective biomarker of type II versus type I cochlear afferents (Vyas et al., ). Whole-mount cochlear preparations from 3-week- to 2-month-old CGRPα-EGFP (GENSAT) mice showed expression of Cgrpα in a subset of SGNs with type II-like peripheral dendrites extending beneath OHCs. Double labeling with other molecular markers confirmed that the labeled SGNs were neither type I SGNs nor olivocochlear efferents. Cgrpα starts to express in type II SGNs before hearing onset, but the expression level declines in the adult. The expression patterns of Cgrpα and Th formed opposing gradients, with Th being preferentially expressed in apical and Cgrpα in basal type II afferent neurons, indicating heterogeneity among type II afferent neurons. The expression of Th and Cgrpα was not mutually exclusive and co-expression could be observed, most abundantly in the middle cochlear turn.

An αII Spectrin-Based Cytoskeleton Protects Large-Diameter Myelinated Axons from Degeneration

Axons must withstand mechanical forces, including tension, torsion, and compression. Spectrins and actin form a periodic cytoskeleton proposed to protect axons against these forces. However, because spectrins also participate in assembly of axon initial segments (AISs) and nodes of Ranvier, it is difficult to uncouple their roles in maintaining axon integrity from their functions at AIS and nodes. To overcome this problem and to determine the importance of spectrin cytoskeletons for axon integrity, we generated mice with αII spectrin-deficient peripheral sensory neurons. The axons of these neurons are very long and exposed to the mechanical forces associated with limb movement; most lack an AIS, and some are unmyelinated and have no nodes. We analyzed αII spectrin-deficient mice of both sexes and found that, in myelinated axons, αII spectrin forms a periodic cytoskeleton with βIV and βII spectrin at nodes of Ranvier and paranodes, respectively, but that loss of αII spectrin disrupts this organization. Avil-cre;Sptan1^f/f mice have reduced numbers of nodes, disrupted paranodal junctions, and mislocalized Kv1 K⁺ channels. We show that the density of nodal βIV spectrin is constant among axons, but the density of nodal αII spectrin increases with axon diameter. Remarkably, Avil-cre;Sptan1^f/f mice have intact nociception and small-diameter axons, but severe ataxia due to preferential degeneration of large-diameter myelinated axons. Our results suggest that nodal αII spectrin helps resist the mechanical forces experienced by large-diameter axons, and that αII spectrin-dependent cytoskeletons are also required for assembly of nodes of Ranvier.SIGNIFICANCE STATEMENT A periodic axonal cytoskeleton consisting of actin and spectrin has been proposed to help axons resist the mechanical forces to which they are exposed (e.g., compression, torsion, and stretch). However, until now, no vertebrate animal model has tested the requirement of the spectrin cytoskeleton in maintenance of axon integrity. We demonstrate the role of the periodic spectrin-dependent cytoskeleton in axons and show that loss of αII spectrin from PNS axons causes preferential degeneration of large-diameter myelinated axons. We show that nodal αII spectrin is found at greater densities in large-diameter myelinated axons, suggesting that nodes are particularly vulnerable domains requiring a specialized cytoskeleton to protect against axon degeneration.

Schwann Cells in the Ventral Dermis Do Not Derive from Myf5-Expressing Precursors

The embryonic origin of lineage precursors of the trunk dermis is somewhat controversial. Precursor cells traced by Myf5 and Twist2 (Dermo1) promoter activation (i.e., cells of presumed dermomyotomal lineage) have been reported to generate Schwann cells. On the other hand, abundant data demonstrate that dermal Schwann cells derive from the neural crest. This is relevant because dermal precursors give rise to neural lineages, and multilineage differentiation potential qualifies them as adult stem cells. However, it is currently unclear whether neural lineages arise from dedifferentiated Schwann cells instead of mesodermally derived dermal precursor cells. To clarify these discrepancies, we traced SOX2⁺ adult dermal precursor cells by two independent Myf5 lineage tracing strains. We demonstrate that dermal Schwann cells do not belong to the Myf5⁺ cell lineage, indicating that previous tracing data reflected aberrant cre recombinase expression and that bona fide Myf5⁺ dermal precursors cannot transdifferentiate to neural lineages in physiological conditions.

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Adult Myf5-cre^Sor mice aberrantly trace dermal Schwann cells (dSCs)

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Dedifferentiated, SOX2⁺ dSCs are the neural-competent precursors in the dermis

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These findings cast doubt on the multipotency of adult skin-derived precursors

Evolutionary Specialization of Tactile Perception in Vertebrates

Evolution has endowed vertebrates with the remarkable tactile ability to explore the world through the perception of physical force. Yet the sense of touch remains one of the least well understood senses at the cellular and molecular level. Vertebrates specializing in tactile perception can highlight general principles of mechanotransduction. Here, we review cellular and molecular adaptations that underlie the sense of touch in typical and acutely mechanosensitive vertebrates.

Native Piezo2 Interactomics Identifies Pericentrin as a Novel Regulator of Piezo2 in Somatosensory Neurons

The ability of somatosensory neurons to perceive mechanical stimuli relies on specialized mechanotransducing proteins and their molecular environment. Only recently has the identity of a major transducer of mechanical forces in vertebrates been revealed by the discovery of Piezo2. Further work has established its pivotal role for innocuous touch in mice. Therefore, Piezo2 offers a unique platform for the molecular investigation of somatosensory mechanosensation. We performed a mass spectrometry-based interactomics screen on native Piezo2 in somatosensory neurons of mouse dorsal root ganglia (DRG). Stringent and quantitative data analysis yielded the identity of 36 novel binding partners of Piezo2. The biological significance of this data set is reflected by functional experiments demonstrating a role for Pericentrin in modulating Piezo2 activity and membrane expression in somatosensory neurons. Collectively, our findings provide a framework for understanding Piezo2 physiology and serve as a rich resource for the molecular dissection of mouse somatosensation.

Combined Recording of Mechanically Stimulated Afferent Output and Nerve Terminal Labelling in Mouse Hair Follicle Lanceolate Endings

A novel dissection and recording technique is described for monitoring afferent firing evoked by mechanical displacement of hairs in the mouse pinna. The technique is very cost-effective and easily undertaken with materials commonly found in most electrophysiology laboratories, or easily purchased. The dissection is simple and fast, with the mechanical displacement provided by a generic electroceramic wafer controlled by proprietary software. The same software also records and analyses the electroneurogram output. The recording of the evoked nerve activity is through a commercial differential amplifier connected to fire-polished standard glass microelectrodes. Helpful tips are given for improving the quality of the preparation, the stimulation and the recording conditions to optimize recording quality. The system is suitable for assaying the electrophysiological and optical properties of lanceolate terminals of palisade endings of hair follicles, as well as the outcomes from their pharmacological and/or genetic manipulation. An example of combining electrical recording with mechanical stimulation and labeling with a styryl pyridinium vital dye is given.

Genetic Identification of an Expansive Mechanoreceptor Sensitive to Skin Stroking

Touch perception begins with activation of low-threshold mechanoreceptors (LTMRs) in the periphery. LTMR terminals exhibit tremendous morphological heterogeneity that specifies their mechanical receptivity. In a survey of mammalian skin, we found a preponderance of neurofilament-heavy-chain(+) circumferential endings associated with hair follicles, prompting us to develop a genetic strategy to interrogate these neurons. Targeted in vivo recordings revealed them to be Aβ field-LTMRs, identified 50 years ago but largely elusive thereafter. Remarkably, while Aβ field-LTMRs are highly sensitive to gentle stroking of the skin, they are unresponsive to hair deflection, and they encode skin indentation in the noxious range across large, spotty receptive fields. Individual Aβ field-LTMRs form up to 180 circumferential endings, making them the most anatomically expansive LTMR identified to date. Thus, Aβ field-LTMRs are a major mammalian LTMR subtype that forms circumferential endings in hairy skin, and their sensitivity to gentle skin stroking arises through integration across many low-sensitivity circumferential endings.

Mechanically Activated Ion Channels

Mechanotransduction, the conversion of physical forces into biochemical signals, is essential for various physiological processes such as the conscious sensations of touch and hearing, and the unconscious sensation of blood flow. Mechanically activated (MA) ion channels have been proposed as sensors of physical force, but the identity of these channels and an understanding of how mechanical force is transduced has remained elusive. A number of recent studies on previously known ion channels along with the identification of novel MA ion channels have greatly transformed our understanding of touch and hearing in both vertebrates and invertebrates. Here, we present an updated review of eukaryotic ion channel families that have been implicated in mechanotransduction processes and evaluate the qualifications of the candidate genes according to specified criteria. We then discuss the proposed gating models for MA ion channels and highlight recent structural studies of mechanosensitive potassium channels.

Dynamic expression of transcription factor Brn3b during mouse cranial nerve development

During development, transcription factor combinatorial codes define a large variety of morphologically and physiologically distinct neurons. Such a combinatorial code has been proposed for the differentiation of projection neurons of the somatic and visceral components of cranial nerves. It is possible that individual neuronal cell types are not specified by unique transcription factors but rather emerge through the intersection of their expression domains. Brn3a, Brn3b, and Brn3c, in combination with each other and/or transcription factors of other families, can define subgroups of retinal ganglion cells (RGC), spiral and vestibular ganglia, inner ear and vestibular hair cell neurons in the vestibuloacoustic system, and groups of somatosensory neurons in the dorsal root ganglia. The present study investigates the expression and potential role of the Brn3b transcription factor in cranial nerves and associated nuclei of the brainstem. We report the dynamic expression of Brn3b in the somatosensory component of cranial nerves II, V, VII, and VIII and visceromotor nuclei of nerves VII, IX, and X as well as other brainstem nuclei during different stages of development into adult stage. We find that genetically identified Brn3b(KO) RGC axons show correct but delayed pathfinding during the early stages of embryonic development. However, loss of Brn3b does not affect the anatomy of the other cranial nerves normally expressing this transcription factor.

Boundary Caps Give Rise to Neurogenic Stem Cells and Terminal Glia in the Skin

While neurogenic stem cells have been identified in rodent and human skin, their manipulation and further characterization are hampered by a lack of specific markers. Here, we perform genetic tracing of the progeny of boundary cap (BC) cells, a neural-crest-derived cell population localized at peripheral nerve entry/exit points. We show that BC derivatives migrate along peripheral nerves to reach the skin, where they give rise to terminal glia associated with dermal nerve endings. Dermal BC derivatives also include cells that self-renew in sphere culture and have broad in vitro differentiation potential. Upon transplantation into adult mouse dorsal root ganglia, skin BC derivatives efficiently differentiate into various types of mature sensory neurons. Together, this work establishes the embryonic origin, pathway of migration, and in vivo neurogenic potential of a major component of skin stem-like cells. It provides genetic tools to study and manipulate this population of high interest for medical applications.

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Boundary cap cells give rise to all types of sensory neurons in the developing DRG

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BC derivatives migrate along peripheral nerves to reach the trunk skin

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BC cell progeny include glia associated with nerve endings

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Dermal BC-derived stem cells possess powerful in vivo neurogenic potential

Mammalian touch catches up

An assortment of touch receptors innervate the skin and encode different tactile features of the environment. Compared with invertebrate touch and other sensory systems, our understanding of the molecular and cellular underpinnings of mammalian touch lags behind. Two recent breakthroughs have accelerated progress. First, an arsenal of cell-type-specific molecular markers allowed the functional and anatomical properties of sensory neurons to be matched, thereby unraveling a cellular code for touch. Such markers have also revealed key roles of non-neuronal cell types, such as Merkel cells and keratinocytes, in touch reception. Second, the discovery of Piezo genes as a new family of mechanically activated channels has fueled the discovery of molecular mechanisms that mediate and mechanotransduction in mammalian touch receptors.

Structure-function correlations of rat trigeminal primary neurons: Emphasis on club-like endings, a vibrissal mechanoreceptor

This study focuses on the structure and function of the primary sensory neurons that innervate vibrissal follicles in the rat. Both the peripheral and central terminations, as well as their firing properties were identified using intracellular labelling and recording in trigeminal ganglia in vivo. Fifty-one labelled neurons terminating peripherally, as club-like, Merkel, lanceolate, reticular or spiny endings were identified by their morphology. All neurons responded robustly to air puff stimulation applied to the vibrissal skin. Neurons with club-like endings responded with the highest firing rates; their peripheral processes rarely branched between the cell body and their terminal tips. The central branches of these neurons displayed abundant collaterals terminating within all trigeminal nuclei. Analyses of three-dimensional reconstructions reveal a palisade arrangement of club-like endings bound to the ringwulst by collagen fibers. Our morphological findings suggest that neurons with club-like endings sense mechanical aspects related to the movement of the ringwulst and convey this information to all trigeminal nuclei in the brainstem.

The gentle touch receptors of mammalian skin

The skin is our largest sensory organ, transmitting pain, temperature, itch, and touch information to the central nervous system. Touch sensations are conveyed by distinct combinations of mechanosensory end organs and the low-threshold mechanoreceptors (LTMRs) that innervate them. Here we explore the various structures underlying the diverse functions of cutaneous LTMR end organs. Beyond anchoring of LTMRs to the surrounding dermis and epidermis, recent evidence suggests that the non-neuronal components of end organs play an active role in signaling to LTMRs and may physically gate force-sensitive channels in these receptors. Combined with LTMR intrinsic properties, the balance of these factors comprises the response properties of mechanosensory neurons and, thus, the neural encoding of touch.

The cellular and molecular basis of direction selectivity of Aδ-LTMRs

The perception of touch, including the direction of stimulus movement across the skin, begins with activation of low-threshold mechanosensory neurons (LTMRs) that innervate the skin. Here, we show that murine Aδ-LTMRs are preferentially tuned to deflection of body hairs in the caudal-to-rostral direction. This tuning property is explained by the finding that Aδ-LTMR lanceolate endings around hair follicles are polarized; they are concentrated on the caudal (downward) side of each hair follicle. The neurotrophic factor BDNF is synthesized in epithelial cells on the caudal, but not rostral, side of hair follicles, in close proximity to Aδ-LTMR lanceolate endings, which express TrkB. Moreover, ablation of BDNF in hair follicle epithelial cells disrupts polarization of Aδ-LTMR lanceolate endings and results in randomization of Aδ-LTMR responses to hair deflection. Thus, BDNF-TrkB signaling directs polarization of Aδ-LTMR lanceolate endings, which underlies direction-selective responsiveness of Aδ-LTMRs to hair deflection.

Different requirements for GFRα2-signaling in three populations of cutaneous sensory neurons

Many primary sensory neurons in mouse dorsal root ganglia (DRG) express one or several GFRα’s, the ligand-binding receptors of the GDNF family, and their common signaling receptor Ret. GFRα2, the principal receptor for neurturin, is expressed in most of the small nonpeptidergic DRG neurons, but also in some large DRG neurons that start to express Ret earlier. Previously, GFRα2 has been shown to be crucial for the soma size of small nonpeptidergic nociceptors and for their target innervation of glabrous epidermis. However, little is known about this receptor in other Ret-expressing DRG neuron populations. Here we have investigated two populations of Ret-positive low-threshold mechanoreceptors that innervate different types of hair follicles on mouse back skin: the small C-LTMRs and the large Aβ-LTMRs. Using GFRα2-KO mice and immunohistochemistry we found that, similar to the nonpeptidergic nociceptors, GFRα2 controls the cell size but not the survival of both C-LTMRs and Aβ-LTMRs. In contrast to the nonpeptidergic neurons, GFRα2 is not required for the target innervation of C-LTMRs and Aβ-LTMRs in the back skin. These results suggest that different factors drive target innervation in these three populations of neurons. In addition, the observation that the large Ret-positive DRG neurons lack GFRα2 immunoreactivity in mature animals suggests that these neurons switch their GFRα signaling pathways during postnatal development.

Sensory mechanotransduction at membrane-matrix interfaces

Sensory cells specialized to detect extremely small mechanical changes are common to the auditory and somatosensory systems. It is widely accepted that mechanosensitive channels form the core of the mechanoelectrical transduction in hair cells as well as the somatic sensory neurons that underlie the sense of touch and mechanical pain. Here, we will review how the activation of such channels can be measured in a meaningful physiological context. In particular, we will discuss the idea that mechanosensitive channels normally occur in transmembrane complexes that are anchored to extracellular matrix components (ECM) both in vitro and in vivo. One component of such complexes in sensory neurons is the integral membrane scaffold protein STOML3 which is a robust physiological regulator of native mechanosensitive currents. In order to better characterize such channels in transmembrane complexes, we developed a new electrophysiological method that enables the quantification of mechanosensitive current amplitude and kinetics when activated by a defined matrix movement in cultured cells. The results of such studies strongly support the idea that ion channels in transmembrane complexes are highly tuned to detect movement of the cell membrane in relation to the ECM.

Diversification and specialization of touch receptors in skin

Our skin is the furthest outpost of the nervous system and a primary sensor for harmful and innocuous external stimuli. As a multifunctional sensory organ, the skin manifests a diverse and highly specialized array of mechanosensitive neurons with complex terminals, or end organs, which are able to discriminate different sensory stimuli and encode this information for appropriate central processing. Historically, the basis for this diversity of sensory specializations has been poorly understood. In addition, the relationship between cutaneous mechanosensory afferents and resident skin cells, including keratinocytes, Merkel cells, and Schwann cells, during the development and function of tactile receptors has been poorly defined. In this article, we will discuss conserved tactile end organs in the epidermis and hair follicles, with a focus on recent advances in our understanding that have emerged from studies of mouse hairy skin.