Absence of Meissner corpuscles in the digital pads of mice lacking functional TrkB

The Lamellar Cells of Vertebrate Meissner and Pacinian Corpuscles: Development, Characterization, and Functions

Sensory corpuscles, or cutaneous end-organ complexes, are complex structures localized at the periphery of Aβ-axon terminals from primary sensory neurons that primarily work as low-threshold mechanoreceptors. Structurally, they consist, in addition to the axons, of non-myelinating Schwann-like cells (terminal glial cells) and endoneurial- and perineurial-related cells. The terminal glial cells are the so-called lamellar cells in Meissner and Pacinian corpuscles. Lamellar cells are variably arranged in sensory corpuscles as a “coin stack” in the Meissner corpuscles or as an “onion bulb” in the Pacinian ones. Nevertheless, the origin and protein profile of the lamellar cells in both morphotypes of sensory corpuscles is quite similar, although it differs in the expression of mechano-gated ion channels as well as in the composition of the extracellular matrix between the cells. The lamellar cells have been regarded as supportive cells playing a passive role in the process of genesis of the action potential, i.e., the mechanotransduction process. However, they express ion channels related to the mechano–electric transduction and show a synapse-like mechanism that suggest neurotransmission at the genesis of the electrical action potential. This review updates the current knowledge about the embryonic origin, development modifications, spatial arrangement, ultrastructural characteristics, and protein profile of the lamellar cells of cutaneous end-organ complexes focusing on Meissner and Pacinian morphotypes.

The cellular and molecular basis of somatosensory neuron development

Primary somatosensory neurons convey salient information about our external environment and internal state to the CNS, allowing us to detect, perceive, and react to a wide range of innocuous and noxious stimuli. Pseudo-unipolar in shape, and among the largest (longest) cells of most mammals, dorsal root ganglia (DRG) somatosensory neurons have peripheral axons that extend into skin, muscle, viscera, or bone and central axons that innervate the spinal cord and brainstem, where they synaptically engage the central somatosensory circuitry. Here, we review the diversity of mammalian DRG neuron subtypes and the intrinsic and extrinsic mechanisms that control their development. We describe classical and contemporary advances that frame our understanding of DRG neurogenesis, transcriptional specification of DRG neurons, and the establishment of morphological, physiological, and synaptic diversification across somatosensory neuron subtypes.

Anatomy of the sense of touch in sea otters: Cutaneous mechanoreceptors and structural features of glabrous skin

Sea otters (Enhydra lutris) demonstrate rapid, accurate tactile abilities using their paws and facial vibrissae. Anatomical investigations of neural organization in the vibrissal bed and somatosensory cortex coincide with measured sensitivity, but no studies describe sensory receptors in the paws or other regions of glabrous (i.e., hairless) skin. In this study, we use histology to assess the presence, density, and distribution of mechanoreceptors in the glabrous skin of sea otters: paws, rhinarium, lips, and flipper digits, and we use scanning electron microscopy to describe skin-surface texture and its potential effect on the transduction of mechanical stimuli. Our results confirm the presence of Merkel cells and Pacinian corpuscles, but not Meissner corpuscles, in all sea otter glabrous skin. The paws showed the highest density of Merkel cells and Pacinian corpuscles. Within the paw, relative densities of mechanoreceptor types were highest in the distal metacarpal pad and digits, which suggests that the distal paw is a tactile fovea for sea otters. In addition to the highest receptor density, the paw displayed the thickest epidermis. Rete ridges (epidermal projections into the dermis) and dermal papillae (dermal projections into the epidermis) were developed across all glabrous skin. These quantitative and qualitative descriptions of neural organization and physical features, combined with previous behavioral results, contribute to our understanding of how structure relates to function in the tactile modality. Our findings coincide with behavioral observations of sea otters, which use touch to maintain thermoregulatory integrity of their fur, explore objects, and capture visually cryptic prey.

Meissner corpuscles and their spatially intermingled afferents underlie gentle touch perception

Meissner corpuscles are mechanosensory end organs that densely occupy mammalian glabrous skin. We generated mice that selectively lacked Meissner corpuscles and found them to be deficient in both perceiving the gentlest detectable forces acting on glabrous skin and fine sensorimotor control. We found that Meissner corpuscles are innervated by two mechanoreceptor subtypes that exhibit distinct responses to tactile stimuli. The anatomical receptive fields of these two mechanoreceptor subtypes homotypically tile glabrous skin in a manner that is offset with respect to one another. Electron microscopic analysis of the two Meissner afferents within the corpuscle supports a model in which the extent of lamellar cell wrappings of mechanoreceptor endings determines their force sensitivity thresholds and kinetic properties.

Ageing of the somatosensory system at the periphery: age-related changes in cutaneous mechanoreceptors

Decline of tactile sensation associated with ageing depends on modifications in skin and both central and peripheral nervous systems. At present, age-related changes in the periphery of the somatosensory system, particularly concerning the effects on mechanoreceptors, remain unknown. Here we used immunohistochemistry to analyse the age-dependent changes in Meissner's and Pacinian corpuscles as well as in Merkel cell-neurite complexes. Moreover, variations in the neurotrophic TrkB-BDNF system and the mechanoprotein Piezo2 (involved in maintenance of cutaneous mechanoreceptors and light touch, respectively) were evaluated. The number of Meissner's corpuscles and Merkel cells decreased progressively with ageing. Meissner's corpuscles were smaller, rounded in morphology and located deeper in the dermis, and signs of corpuscular denervation were found in the oldest subjects. Pacinian corpuscles generally showed no relevant age-related alterations. Reduced expression of Piezo2 in the axon of Meissner's corpuscles and in Merkel cells was observed in old subjects, as well was a decline in the BDNF-TrkB neurotrophic system. This study demonstrates that cutaneous Meissner's corpuscles and Merkel cell-neurite complexes (and less evidently Pacinian corpuscles) undergo morphological and size changes during the ageing process, as well as a reduction in terms of density. Furthermore, the mechanoprotein Piezo2 and the neurotrophic TrkB-BDNF system are reduced in aged corpuscles. Taken together, these alterations might explain part of the impairment of the somatosensory system associated with ageing.

The development of human digital Meissner's and Pacinian corpuscles

Meissner's and Pacinian corpuscles are cutaneous mechanoreceptors responsible for different modalities of touch. The development of these sensory formations in humans is poorly known, especially regarding the acquisition of the typical immunohistochemical profile related to their full functional maturity. Here we used a panel of antibodies (to specifically label the main corpuscular components: axon, Schwann-related cells and endoneurial-perineurial-related cells) to investigate the development of digital Meissner's and Pacinian corpuscles in a representative sample covering from 11 weeks of estimated gestational age (wega) to adulthood. Development of Pacinian corpuscles starts at 13 wega, and it is completed at 4 months of life, although their basic structure and immunohistochemical characteristics are reached at 36 wega. During development, around the axon, a complex network of S100 positive Schwann-related processes is progressively compacted to form the inner core, while the surrounding mesenchyme is organized and forms the outer core and the capsule. Meissner's corpuscles start to develop at 22 wega and complete their typical morphology and immunohistochemical profile at 8 months of life. In developing Meissner's corpuscles, the axons establish complex relationships with the epidermis and are progressively covered by Schwann-like cells until they complete the mature arrangement late in postnatal life. The present results demonstrate an asynchronous development of the Meissner's and Pacini's corpuscles and show that there is not a total correlation between morphological and immunohistochemical maturation. The correlation of the present results with touch-induced cortical activity in developing humans is discussed.

Molecular basis of tactile specialization in the duck bill

Tactile-foraging ducks are specialist birds known for their touch-dependent feeding behavior. They use dabbling, straining, and filtering to find edible matter in murky water, relying on the sense of touch in their bill. Here, we present the molecular characterization of embryonic duck bill, which we show contains a high density of mechanosensory corpuscles innervated by functional rapidly adapting trigeminal afferents. In contrast to chicken, a visually foraging bird, the majority of duck trigeminal neurons are mechanoreceptors that express the Piezo2 ion channel and produce slowly inactivating mechano-current before hatching. Furthermore, duck neurons have a significantly reduced mechano-activation threshold and elevated mechano-current amplitude. Cloning and electrophysiological characterization of duck Piezo2 in a heterologous expression system shows that duck Piezo2 is functionally similar to the mouse ortholog but with prolonged inactivation kinetics, particularly at positive potentials. Knockdown of Piezo2 in duck trigeminal neurons attenuates mechano current with intermediate and slow inactivation kinetics. This suggests that Piezo2 is capable of contributing to a larger range of mechano-activated currents in duck trigeminal ganglia than in mouse trigeminal ganglia. Our results provide insights into the molecular basis of mechanotransduction in a tactile-specialist vertebrate.

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.

The specification and wiring of mammalian cutaneous low-threshold mechanoreceptors

The mammalian cutaneous low-threshold mechanoreceptors (LTMRs) are a diverse set of primary somatosensory neurons that function to sense external mechanical force. Generally, LTMRs are composed of Aβ-LTMRs, Aδ-LTMRs, and C-LTMRs, which have distinct molecular, physiological, anatomical, and functional features. The specification and wiring of each type of mammalian cutaneous LTMRs is established during development by the interplay of transcription factors with trophic factor signalling. In this review, we summarize the cohort of extrinsic and intrinsic factors generating the complex mammalian cutaneous LTMR circuits that mediate our tactile sensations and behaviors. For further resources related to this article, please visit the WIREs website.

Sensory Neurons, Ion Channels, Inflammation and the Onset of Neuropathic Pain

Neuropathic pain often fails to respond to conventional pain management procedures. here we review the aetiology of neuropathic pain as would result from peripheral neuropathy or injury. We show that inflammatory mediators released from damaged nerves and tissue are responsible for triggering ectopic activity in primary afferents and that this, in turn, provokes increased spinal cord activity and the development of ‘central sensitization’. Although evidence is mounting to support the role of interleukin-1β, prostaglandins and other cytokines in the onset of neuropathic pain, the clinical efficacy of drugs which antagonize or prevent the actions of these mediators is yet to be determined. basic science findings do, however, support the use of pre-emptive analgesia during procedures which involve nerve manipulation and the use of anti-inflammatory steroids as soon as possible following traumatic nerve injury.

Brain-derived neurotrofic factor and its receptor TrkB are present, but segregated, within mature cutaneous Pacinian corpuscles of Macaca fascicularis

Some mechanoreceptors in mammals depend totally or in part on the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4), and their receptor TrkB, for development and maintenance. These actions are presumably exerced regulating the survival of discrete sensory neurons in the dorsal root ganglia which form mechanoreceptors at the periphery. In addition, the cells forming the mechanoreceptors also express both neurotrophins and their receptors although large differences have been described among species. Pacinian corpuscles are rapidly adapting low-threshold mechanoreceptors whose dependence from neurotrophins is not known. In the present study, we analyzed expression of TrkB and their ligands BDNF and NT-4 in the cutaneous Pacinian corpuscles of Macaca fascicularis using immunohistochemistry and fluorescent microscopy. TrkB immunoreactivity was found in Pacinian corpuscles where it co-localized with neuron-specific enolase, and occasionally with S100 protein, thus suggesting that TrkB expression is primarily into axons but also in the lamellar cells and even in the outer core. On the other hand, BDNF immunoreactivity was found the inner core cells where it co-localized with S100 protein but also in the innermost layers of the outer core; NT-4 immunostaining was not detected. These results describe for the first time the expression and distribution of a full neurotrophin system in the axon-inner core complex of mature Pacinian corpuscles. The data support previous findings demonstrating large differences in the expression of BDNF-TrkB in mammalian mechanoreceptors, and also suggest the existence of a retrograde trophic signaling mechanism to maintain morphological and functional integrity of sensory neurons supplying Pacinian corpuscles.

The anatomy, function, and development of mammalian Aβ low-threshold mechanoreceptors

Touch sensation is critical for our social and environmental interactions. In mammals, most discriminative light touch sensation is mediated by the Aβ low-threshold mechanoreceptors. Cell bodies of Aβ low-threshold mechanoreceptors are located in the dorsal root ganglia and trigeminal ganglia, which extend a central projection innervating the spinal cord and brain stem and a peripheral projection innervating the specialized mechanosensory end organs. These specialized mechanosensory end organs include Meissner's corpuscles, Pacinian corpuscles, lanceolate endings, Merkel cells, and Ruffini corpuscles. The morphologies and physiological properties of these mechanosensory end organs and their innervating neurons have been investigated for over a century. In addition, recent advances in mouse genetics have enabled the identification of molecular mechanisms underlying the development of Aβ low-threshold mechanoreceptors, which highlight the crucial roles of neurotrophic factor signaling and transcription factor activity in this process. Here, we will review the anatomy, physiological properties, and development of mammalian low-threshold Aβ mechanoreceptors.

Signals and switches in Mammalian neural crest cell differentiation

Neural crest cells (NCCs) comprise a multipotent, migratory cell population that generates a diverse array of cell and tissue types during vertebrate development. These include cartilage and bone, tendons, and connective tissue, as well as neurons, glia, melanocytes, and endocrine and adipose cells; this remarkable lineage potential persists into adult life. Taken together with a limited capacity for self-renewal, neural crest cells bear the hallmarks of stem and progenitor cells and are considered to be synonymous with vertebrate evolution. The neural crest has provided a system for exploring the mechanisms that govern developmental processes such as morphogenetic induction, cell migration, and fate determination. Today, much of the focus on neural crest cells revolves around their stem cell-like characteristics and potential for use in regenerative medicine. A thorough understanding of the signals and switches that govern mammalian neural crest patterning is central to potential therapeutic application of these cells and better appreciation of the role that neural crest cells play in vertebrate evolution, development, and disease.

Mechanosensory neurons, cutaneous mechanoreceptors, and putative mechanoproteins

The mammalian skin has developed sensory structures (mechanoreceptors) that are responsible for different modalities of mechanosensitivity like touch, vibration, and pressure sensation. These specialized sensory organs are anatomically and functionally connected to a special subset of sensory neurons called mechanosensory neurons, which electrophysiologically correspond with Aβ fibers. Although mechanosensory neurons and cutaneous mechanoreceptors are rather well known, the biology of the sense of touch still remains poorly understood. Basically, the process of mechanosensitivity requires the conversion of a mechanical stimulus into an electrical signal through the activation of ion channels that gate in response to mechanical stimuli. These ion channels belong primarily to the family of the degenerin/epithelium sodium channels, especially the subfamily acid-sensing ion channels, and to the family of transient receptor potential channels. This review compiles the current knowledge on the occurrence of putative mechanoproteins in mechanosensory neurons and mechanoreceptors, as well as the involvement of these proteins on the biology of touch. Furthermore, we include a section about what the knock-out mice for mechanoproteins are teaching us. Finally, the possibilities for mechanotransduction in mechanoreceptors, and the common involvement of the ion channels, extracellular membrane, and cytoskeleton, are revisited.

Dependence on the transcription factor Shox2 for specification of sensory neurons conveying discriminative touch

Touch sensation is mediated by specific subtypes of sensory neurons which develop in a hierarchical process from common early progenitor neurons, but the molecular mechanism that underlies diversification of touch-sensitive mechanoreceptive neurons is not fully known. Here, we use genetically manipulated mice to examine whether the transcription factor short stature homeobox 2 (Shox2) participates in the acquisition of neuronal subtypes conveying touch sensation. We show that Shox2 encodes the development of category I low-threshold mechanoreceptive neurons in glabrous skin, i.e. discriminative touch-sensitive neurons which form innervations of epidermal Merkel cell and Meissner corpuscles. In contrast, other sensory fiber endings, including those innervating Pacinian corpuscles, are not dependent on Shox2. Shox2 is expressed in neurons of most or all classes of sensory neurons at early embryonic stages and is later confined to touch-sensitive neurons expressing Ret and/or TrkB. Conditional deletion of Shox2 and analysis of Runx3(-/-);Bax(-/-) mutant mice reveals that Runx3 is suppressing Shox2 while Shox2 is necessary for TrkB expression, and that these interactions are necessary for diversification of TrkB(+) and TrkC(+) mechanoreceptive neurons. In particular, development of TrkB(+)/Ret(+) and TrkB(+)/Ret(-) touch-sensitive neurons is critically dependent on Shox2. Consistently, Shox2 conditional mutant mice demonstrate a dramatic impairment of light touch responses. These results show that Shox2 is required for specification of a subclass of TrkB(+) sensory neurons which convey the sensation of discriminative touch arising from stimuli of the skin.

Brn3a/Pou4f1 regulates dorsal root ganglion sensory neuron specification and axonal projection into the spinal cord

The sensory neurons of the dorsal root ganglia (DRG) must project accurately to their central targets to convey proprioceptive, nociceptive and mechanoreceptive information to the spinal cord. How these different sensory modalities and central connectivities are specified and coordinated still remains unclear. Given the expression of the POU homeodomain transcription factors Brn3a/Pou4f1 and Brn3b/Pou4f2 in DRG and spinal cord sensory neurons, we determined the subtype specification of DRG and spinal cord sensory neurons as well as DRG central projections in Brn3a and Brn3b single and double mutant mice. Inactivation of either or both genes causes no gross abnormalities in early spinal cord neurogenesis; however, in Brn3a single and Brn3a;Brn3b double mutant mice, sensory afferent axons from the DRG fail to form normal trajectories in the spinal cord. The TrkA(+) afferents remain outside the dorsal horn and fail to extend into the spinal cord, while the projections of TrkC(+) proprioceptive afferents into the ventral horn are also impaired. Moreover, Brn3a mutant DRGs are defective in sensory neuron specification, as marked by the excessive generation of TrkB(+) and TrkC(+) neurons as well as TrkA(+)/TrkB(+) and TrkA(+)/TrkC(+) double positive cells at early embryonic stages. At later stages in the mutant, TrkB(+), TrkC(+) and parvalbumin(+) neurons diminish while there is a significant increase of CGRP(+) and c-ret(+) neurons. In addition, Brn3a mutant DRGs display a dramatic down-regulation of Runx1 expression, suggesting that the regulation of DRG sensory neuron specification by Brn3a is mediated in part by Runx1. Our results together demonstrate a critical role for Brn3a in generating DRG sensory neuron diversity and regulating sensory afferent projections to the central targets.

Transcription factor short stature homeobox 2 is required for proper development of tropomyosin-related kinase B-expressing mechanosensory neurons

Dorsal root ganglia (DRG) contain somatosensory neurons of diverse sensory modalities. Among these different types of sensory neurons, the molecular mechanisms that regulate the development and specification of touch neurons are the least well understood. We took a candidate approach and searched for transcription factors that are expressed in subsets of DRG neurons, and found that the transcription factor Shox2 (short stature homeobox 2) is expressed in subpopulations of TrkB (tropomyosin-related kinase B)- and Ret-expressing neurons at neonatal stages. Since TrkB is a known marker that is selectively expressed in touch sensory neurons, we decided to examine the function of Shox2 in specifying TrkB-positive DRG neurons. Conditional deletion of Shox2 in neural crest cells (which give rise to all DRG neurons) caused a 60 ∼ 65% reduction in the number of TrkB-expressing neurons. It also resulted in an increase in coexpression of TrkC in Ret-positive sensory neurons. Deletion of Shox2 in differentiating DRG neurons at later time points caused only a moderate reduction in TrkB expression. Overexpression of Shox2 in all neural crest cells resulted in a small increase in the number of TrkB-expressing neurons. Finally, Shox2 deletion also caused reduced touch sensory axonal innervation to layers III/IV of the spinal cord. Together, our findings identify Shox2 as an essential but not sufficient component of the transcription programs required in neural progenitor cells for the proper specification of subsets of TrkB-expressing touch/mechanosensory neurons.

Morphological development and expression of neurotrophin receptors in the laryngeal sensory corpuscles

Morphological development of sensory structures in the laryngeal mucosa of postnatal rats was observed by use of immunohistochemistry for protein gene-product 9.5 (PGP9.5). Moreover, expression changes of high affinity neurotrophin receptors, TrkA, TrkB and TrkC, and low affinity neurotrophin receptor p75(NTR) were examined to elucidate the relationship to morphogenesis. Intraepithelial nerve endings and parent axons of the laminar endings with immunoreactivity for PGP9.5 have already appeared in the rat on embryonic day 18 (E18) as well as solitary chemoreceptor cells in the glottic cleft. According to neurotrophin receptors, TrkA immunoreactivity were observed on and after postnatal week 3 (3W) in the nervous sensory structures, that is, free nerve endings, laminar endings and sub- and intragemmal plexuses of the taste buds. In the laminar endings, TrkC immunoreactivity was also observed on and after 3W. According to the laryngeal sensory cells, the solitary chemoreceptor cells were immunoreactive to TrkA, TrkB, and TrkC on and after postnatal day 3 (P3). In the taste buds in arytenoid region, taste cells were immunoreactive for TrkA, TrkB, and TrkC on and after 3W, P14, and 3W, respectively. Immunoreactivity for p75(NTR) was observed on the surface of taste cells on and after P9. The results of the present study suggest that sensory structures in the laryngeal mucosa were developed on perinatal days to involve respiratory reflex, and that neurotrophin receptors may take part in the regulation and maintenance of sensory structures.

Axonal projections of mechanoreceptive dorsal root ganglion neurons depend on Ret

Establishment of connectivity between peripheral and central organs is essential for sensory processing by dorsal root ganglion (DRG) neurons. Using Ret as a marker for mechanoreceptive DRG neurons, we show that both central and peripheral projections of mechanoreceptive neurons are severely impaired in the absence of Ret. Death of DRG neurons in Ret-deficient mice can be rescued by eliminating Bax, although their projections remain disrupted. Furthermore, ectopic expression of the Ret ligand neurturin, but not Gdnf, in the spinal cord induces aberrant projection of mechanoreceptive afferents. Our results demonstrate that Ret expression in DRG neurons is crucial for the neurturin-mediated formation of precise axonal projections in the central nervous system.

Development and neuronal dependence of cutaneous sensory nerve formations: Lessons from neurotrophins

Null mutations of genes from the NGF family of NTs and their receptors (NTRs) lead to loss/reduction of specific neurons in sensory ganglia; conversely, cutaneous overexpression of NTs results in skin hyperinnervation and increase or no changes in the number of sensory neurons innervating the skin. These neuronal changes are paralleled with loss of specific types of sensory nerve formations in the skin. Therefore, mice carrying mutations in NT or NTR genes represent an ideal model to identify the neuronal dependence of each type of cutaneous sensory nerve ending from a concrete subtype of sensory neuron, since the development, maintenance, and structural integrity of sensory nerve formations depend upon sensory neurons. Results obtained from these mouse strains suggest that TrkA positive neurons are connected to intraepithelial nerve fibers and other sensory nerve formations depending from C and Adelta nerve fibers; the neurons expressing TrkB and responding to BDNF and NT-4 innervate Meissner corpuscles, a subpopulation of Merkell cells, some mechanoreceptors of the piloneural complex, and the Ruffini's corpuscles; finally, a subpopulation of neurons, which are responsive to NT-3, support postnatal survival of some intraepithelial nerve fibers and Merkel cells in addition to the muscle mechanoreceptors. On the other hand, changes in NTs and NTRs affect the structure of non-nervous structures of the skin and are at the basis of several cutaneous pathologies. This review is an update about the role of NTs and NTRs in the maintenance of normal cutaneous innervation and maintenance of skin integrity.

Immunohistochemical profile of human pancreatic pacinian corpuscles

OBJECTIVES

To analyze the immunohistochemical profile of the human pancreatic pacinian corpuscles in comparison with that of the cutaneous pacinian corpuscles. In addition, we studied a Pacinilike corpuscle found in the adventitia of a pancreatic artery.

METHODS

We used immunohistochemistry to detect specific antigens for corpuscular constituents, specific antibodies for the identification of Adelta- and C-sensory fibers and for the detection of several growth factor receptors, and some members of the degenerin/epithelial Na channel superfamily of proteins.

RESULTS

Approximately 62% of pancreatic pacinian corpuscles have 2 to 10 axonic profiles each enclosed by its own inner core: 1 or 2 of these axonic profiles displayed RT-97 immunoreactivity (specific marker of mechanical axons). The cutaneous pacinian corpuscles showed not more than 2 axonic profiles with identical immunohistochemical characteristics. The expression of glial fibrillary acidic protein, epithelial membrane antigen, and tyrosine receptor kinase B was different between pancreatic and cutaneous pacinian corpuscles; the pattern of distribution of degenerin/epithelial Na channel proteins was identical in both cases. The arterial Pacinilike corpuscles displayed a specific immunohistochemical profile.

CONCLUSIONS

Pancreatic pacinian corpuscles slightly differ from the cutaneous ones, and these differences could be related to topography, growth factor requirements, or function of pacinian corpuscles in the pancreas.

BDNF is essentially required for the early postnatal survival of nociceptors

Neurotrophins promote the survival of specific types of neurons during development and ensure proper maintenance and function of mature responsive neurons. Significant effects of BDNF (Brain-Derived Neurotrophic Factor) on pain physiology have been reported but the contribution of this neurotrophin to the development of nociceptors has not been investigated. We present evidence that BDNF is required for the survival of a significant fraction of peptidergic and non-peptidergic nociceptors in dorsal root ganglia (DRG) postnatally. Bdnf homozygous mutant mice lose approximately half of all nociceptive neurons during the first 2 weeks of life and adult heterozygotes exhibit hypoalgesia and a loss of 25% of all nociceptive neurons. Our in vitro analyses indicate that BDNF-dependent nociceptive neurons also respond to NGF and GDNF. Expression analyses at perinatal times indicate that BDNF is predominantly produced within sensory ganglia and is more abundant than skin-derived NGF or GDNF. Function-blocking studies with BDNF specific antibodies in vitro or cultures of BDNF-deficient sensory neurons suggest that BDNF acts in an autocrine/paracrine way to promote the early postnatal survival of nociceptors that are also responsive to NGF and GDNF. Altogether, the data demonstrate an essential requirement for BDNF in the early postnatal survival of nociceptive neurons.

Molecular identification of rapidly adapting mechanoreceptors and their developmental dependence on ret signaling

In mammals, the first step in the perception of form and texture is the activation of trigeminal or dorsal root ganglion (DRG) mechanosensory neurons, which are classified as either rapidly (RA) or slowly adapting (SA) according to their rates of adaptation to sustained stimuli. The molecular identities and mechanisms of development of RA and SA mechanoreceptors are largely unknown. We found that the "early Ret(+)" DRG neurons are RA mechanoreceptors, which form Meissner corpuscles, Pacinian corpuscles, and longitudinal lanceolate endings. The central projections of these RA mechanoreceptors innervate layers III through V of the spinal cord and terminate within discrete subdomains of the dorsal column nuclei. Moreover, mice lacking Ret signaling components are devoid of Pacinian corpuscles and exhibit a dramatic disruption of RA mechanoreceptor projections to both the spinal cord and medulla. Thus, the early Ret(+) neurons are RA mechanoreceptors and Ret signaling is required for the assembly of neural circuits underlying touch perception.

The lamellar cells in human Meissner corpuscles express TrkB

Cutaneous Meissner corpuscles depend for development and survival exclusively on the NT system TrkB/BDNF/NT-4 unlike other types of sensory corpuscles and nerve endings, which have very complex neuronal and growth factor dependence. However, the pattern of expression of TrkB in human Meissner corpuscles is not known. The experiments in these studies were designed to pursue further findings that suggest that BDNF and NT-4 have critical roles in the development and maintenance of Meissner corpuscles by analyzing the pattern of expression of TrkB, their high-affinity receptor, in human glabrous skin. These experiments showed that TrkB is expressed in different patterns by the lamellar cells of Meissner corpuscles and not by the axon. The studies also show that while the percentage of Meissner corpuscles that express TrkB remains constant from birth till 50-year old cases, it decreases approximately 3-fold in subjects older than 50 years. These results are important since the study of Meissner corpuscles from cutaneous biopsies to diagnose some neurological diseases has rapidly become of high interest and therefore the proteins expressed in these corpuscles are potential diagnostic tools.

The expression of ENa(+)C and ASIC2 proteins in Pacinian corpuscles is differently regulated by TrkB and its ligands BDNF and NT-4

Pacinian corpuscles are innervated by large myelinated Aalpha-beta axons from the large- and intermediate-sized sensory neurons of dorsal root ganglia. These neurons express different members of the degenerin/epithelial Na(+) channel (DEG/ENa(+)C) superfamily of proteins with putative mechanosensory properties, whose expression is regulated by the TrkB-BDNF system. Thus, we hypothesized that BDNF and/or NT-4 signalling through activation of TrkB may regulate the expression of molecules supposed to be necessary for the mechanosensory function of Pacinian corpuscles. To test this hypothesis we analyzed the expression and distribution of ENa(+)C subunits and acid-sensing ion channel 2 (ASIC2) in Pacinian corpuscles from 25 days old mice deficient in TrkB, BDNF and NT-4. Pacinian corpuscles in these animals are normal in number, structure, and expression of several immunohistochemical markers. Using immunohistochemistry we observed that the beta-ENa(+)C and gamma-ENa(+)C subunits, but not the alpha-ENa(+)C subunit, were expressed in wild-type animals, and they were always found in the central axon. ASIC2 immunoreactivity was found in both the central axon and the inner core cells. The absence of TrkB or BDNF abolished expression of beta-ENa(+)C and ASIC2, whereas expression of gamma-ENa(+)C did not change. Expression of beta-ENa(+)C and gamma-ENa(+)C subunits in NT-4 deficient mice was found in the axons but also in the inner core cells whereas levels of expression of ASIC2 were increased in these animals. This study suggests that expression in Pacianian corpuscles of some potential mechanosensory proteins is regulated by BDNF, NT-4 and TrkB.

The Meissner and Pacinian sensory corpuscles revisited new data from the last decade

This article reviews the biochemical, physiological, and experimental data cumulated during the last decade on the Meissner and Pacinian corpuscles. It includes information about (i) the localization of molecules recently detected in sensory corpuscles; (ii) the unsolved problem of the accessory fibers in sensory corpuscles and the occurrence of myelin within them; (iii) the development of sensory corpuscles, especially their neuronal and growth factor dependency; (iv) the composition and functional significance of the extracellular matrix as an essential part of the mechanisms involved in the genesis of the stimuli generated in sensory corpuscles; (v) the molecular basis of mechanotransduction; (vi) a miscellaneous section containing sparse new data on the protein composition of sensory corpuscles, as well as in the proteins involved in live-death cell decisions; (vii) the changes in sensory corpuscles as a consequence of aging, the central, or peripheral nervous system injury; and finally, (viii) the special interest of Meissner corpuscles and Pacinian corpuscles for pathologists for the diagnosis of some peripheral neuropathies and neurodegenerative diseases.

Characterization of sensory deficits in TrkB knockout mice

The sensory deficit in TrkB deficient mice was evaluated by counting the neuronal loss in lumbar dorsal root ganglia (DRG), the absence of sensory receptors (cutaneous--associated to the hairy and glabrous skin - muscular and articular), and the percentage and size of the neurocalcin-positive DRG neurons (a calcium-binding protein which labels proprioceptive and mechanoceptive neurons). Mice lacking TrkB lost 32% of neurons, corresponding to the intermediate-sized and neurocalcin-positive ones. This neuronal lost was accomplished by the absence of Meissner corpuscles, and reduction of hair follicle-associated sensory nerve endings and Merkel cells. The mutation was without effect on Pacinian corpuscles, Golgi's organs and muscle spindles. Present results further characterize the sensory deficit of the TrkB-/- mice demonstrating that the intermediate-sized neurons in lumbar DRG, as well as the cutaneous rapidly and slowly adapting sensory receptors connected to them, are under the control of TrkB for survival and differentiation. This study might serve as a baseline for future studies in experimentally induced neuropathies affecting TrkB positive DRG neurons and their peripheral targets, and to use TrkB ligands in the treatment of neuropathies in which cutaneous mechanoreceptors are primarily involved.

Abnormal development of pacinian corpuscles in double trkB;trkC knockout mice

Pacinian corpuscles depend on either Aalpha or Abeta nerve fibers of the large- and intermediate-sized sensory neurons for the development and maintenance of the structural integrity. These neurons express TrkB and TrkC, two members of the family of signal transducing neurotrophin receptors, and mice lacking TrkB and TrkC lost specific neurons and the sensory corpuscles connected to them. The impact of single or double targeted mutations in trkB and trkC genes in the development of Pacinian corpuscles was investigated in 25-day-old mice using immunohistochemistry and ultrastructural techniques. Single mutations on trkB or trkC genes were without effect on the structure and S100 protein expression, and caused a slight reduction in the number of corpuscles. In mice carrying a double mutation on trkB;trkC genes most of the corpuscles were normal with a reduction of 17% in trkB-/-;trkC+/- mice, and 8% in trkB +/-;trkC -/- mice. Furthermore, a subset of the remaining Pacinian corpuscles (23% in trkB-/-;trkC+/- mice; 3% in trkB+/-;trkC-/- mice) were hypoplasic or atrophic. Present results strongly suggest that the development of a subset of murine Pacinian corpuscles is regulated by the Trk-neurotrophin system, especially TrkB, acting both at neuronal and/or peripheral level. The precise function of each member of this complex in the corpuscular morphogenesis remains to be elucidated, though.

Overexpression of neurotrophin 4 in skin enhances myelinated sensory endings but does not influence sensory neuron number

The growth factors neurotrophin 4 (NT4) and brain-derived neurotrophic factor (BDNF) are expressed in the developing skin, activate the trkB tyrosine kinase receptor, and influence the development and survival of specific types of sensory afferents. Whether each factor is capable of regulating the same or overlapping populations of cutaneous afferents during development is unknown. A previous study of mice overexpressing BDNF in the developing skin (BDNF-OE mice) revealed that these animals exhibited increased hair follicle innervation, Meissner corpuscle size, and Merkel cell number in glabrous skin, although no change in the total number of sensory neurons was observed. To determine if NT4 affects cutaneous innervation in a manner similar to BDNF, transgenic mice overexpressing NT4 in skin, under the control of the keratin 14 gene promoter, were examined. Similar to BDNF-OE mice, NT4-OE mice had increased innervation to the skin but no increase in sensory neuron number in either the dorsal root ganglion or trigeminal ganglion. NT4 overexpression also enhanced hair follicle innervation and the size and density of innervation to Meissner corpuscles. Unlike BDNF overexpression, NT4 overexpression did not alter the number of Merkel cells in the glabrous skin, but it did enhance the number of myelinated axons in nerves projecting to skin. Thus, the same pattern of BDNF and NT4 overexpression within the skin produces phenotypes that are both similar and distinctive.

Bcl-2 immunoreactivity in human cutaneous Meissner and Pacinian corpuscles

The occurrence and distribution of Bcl-2, a protein involved in the death-life cell pathways, was investigated in the peripheral sensory nervous system of healthy adult humans, including lumbar dorsal root ganglia, nerve trunks and glabrous skin (to analyze sensory corpuscles) using Western blot and immunohistochemistry. The antibody used labelled a protein of 26 kDa of estimated molecular weight corresponding with Bcl-2. Immunohistochemistry showed that only a neuronal population in dorsal root ganglia, some axons in peripheral nerves and the axon supplying Meissner and Pacinian corpuscles contained Bcl-2, whereas peripheral glial cells (i.e. satellite glial cells, Schwann cell, and lamellar cells of sensory corpuscles) did not. These results suggest that in normal conditions, Bcl-2 is only present in some neuronal, but not glial, elements of the sensory peripheral nervous system. The functional significance, if any, of these results remains to be determined.

Pacinian corpuscle development involves multiple Trk signaling pathways

The development of crural Pacinian corpuscles was explored in neonatal mutant mice lacking nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) or neurotrophin-4 (NT4), or their cognate Trk receptors. Deficits of the corpuscles and their afferents were greatest in NT3, less in BDNF, and least in NT4 null mice. Deletion of NGF or p75(NTR) genes had little or no impact. No Pacinian corpuscles were present in NT3;BDNF and NT3;NT4 double or NT3;BDNF;NT4 triple null mice. Deficits were larger in NT3 than TrkC mutants and were comparable to deficits observed in TrkB or TrkA mutants. Afferents of all corpuscles coexpressed TrkA and TrkB receptors, and some afferents coexpressed all three Trk receptors. Our results suggest that multiple neurotrophins, in particular NT3, regulate the density of crural Pacinian corpuscles, most likely by regulating the survival of sensory neurons. In addition, NT3/TrkB and/or NT3/TrkA signaling plays a greater role than NT3/TrkC signaling in afferents to developing Pacinian corpuscles.

BDNF, but not NT-4, is necessary for normal development of Meissner corpuscles

Meissner corpuscles are rapidly adapting cutaneous mechanoreceptors depending for development on TrkB expressing sensory neurons, but it remains to be established which of the known TrkB ligands, BDNF or NT-4, is responsible of this dependence. In this study we analyze Meissner corpuscles in the digital pads of mice with target mutations in the genes encoding for either BDNF or NT-4, using immunohistochemistry and transmission-electron microscopy, and they were identified based on their morphology and expression of S100 protein. All wild-type animals as well as NT-4(-/-) animals and BDNF and NT4 heterozygous animals have Meissner corpuscles that are normal in number and size. However, Meissner corpuscles are absent the BDNF(-/-) mice. These results suggest that BDNF is the only TrkB ligand involved in the development of Meissner corpuscles in murine glabrous skin, and it probably regulates the development of the sensory neurons that innervate Meissner corpuscles.