Immunohistochemical detection of ENaCbeta in the terminal Schwann cells associated with the periodontal Ruffini endings of the rat incisor

Periarticular Proprioception: Analyzing the Three-Dimensional Structure of Corpuscular Mechanosensors in the Dorsal Part of the Scapholunate Ligament

INTRODUCTION

Sensory nerve endings transmit mechanical stimuli into afferent neural signals and form the basis of proprioception, giving rise to the self-perception of dynamic stability of joints. We aimed to analyze the three-dimensional structure of periarticular corpuscular sensory nerve endings in a carpal ligament to enhance our understanding of their microstructure.

METHODS

Two dorsal parts of the scapholunate ligament were excised from two human cadaveric wrist specimens. Consecutive cryosections were stained with immunofluorescence markers protein S100B, neurotrophin receptor p75, protein gene product 9.5 (PGP 9.5), and 4',6-diamidino-2-phenylindole. Three-dimensional images of sensory nerve endings were obtained using confocal laser scanning microscopy, and subsequent analysis was performed using Imaris software.

RESULTS

Ruffini endings were characterized by a PGP 9.5-positive central axon, with a median diameter of 4.63 μm and a median of 25 cells. The p75-positive capsule had a range in thickness of 0.94 μm and 15.5 μm, consisting of single to three layers of lamellar cells. Ruffini endings were significantly smaller in volume than Pacini corpuscles or Golgi-like endings. The latter contained a median of three intracorpuscular structures. Ruffini endings and Golgi-like endings presented a similar structural composition of their capsule and subscapular space. The central axon of Pacini corpuscles was surrounded by S100-positive cells forming the inner core which was significantly smaller than the outer core, which was immunoreactive for p75 and PGP 9.5.

CONCLUSION

This study reports new data regarding the intricate outer and intracorpuscular three-dimensional morphology of periarticular sensory nerve endings, including the volume, number of cells, and structural composition. These results may form a basis to differ between normal and pathological morphological changes in periarticular sensory nerve endings in future studies.

Wrist proprioception-An update on scientific insights and clinical implications in rehabilitation of the wrist

The field of wrist proprioception, as it relates to rehabilitation and surgery, has gone through a period of intense growth in the past decade. From being primarily focused on the function of the joint and ligaments in patients with wrist trauma or after wrist surgery, the understanding is now that of a greater complexity in treating not just the wrist but the hand and arm as a whole. Proprioception is derived from the Latin words "proprius" - belonging to (oneself) and "-ception" to sense. In other words, how to sense ourselves. To have a complete sense of self, multiple sensory afferents originating from joints, ligaments, muscles, tendons, nerves, skin, vision, and hearing work together to orchestrate a balanced integration of sensorimotor functions, with the true goal to perceive and adapt to the physical world around us. In this update on wrist proprioception, we review current developments in the understanding of proprioception, with an implication for our everyday work as hand therapists and hand surgeons. Each contributing sense-joint, ligaments, muscles, skin, and brain-will be reviewed, and the clinical relevance will be discussed. An updated wrist rehabilitation protocol is proposed where the therapist is guided to rehabilitate a patient after wrist trauma and/or surgery in 4 stages: (1) basic hand and wrist rehabilitation with a focus on reducing edema, pain, and scar formation; (2) proprioception awareness to improve the sense of joint motion and position; (3) conscious neuromuscular rehabilitation where isometric exercises of muscles that are beneficial for a particular injury are promoted, whereas others that are potentially harmful are avoided; and (4) unconscious neuromuscular rehabilitation with training of the reflex and joint protective senses.

A helping hand: roles for accessory cells in the sense of touch across species

During touch, mechanical forces are converted into electrochemical signals by tactile organs made of neurons, accessory cells, and their shared extracellular spaces. Accessory cells, including Merkel cells, keratinocytes, lamellar cells, and glia, play an important role in the sensation of touch. In some cases, these cells are intrinsically mechanosensitive; however, other roles include the release of chemical messengers, the chemical modification of spaces that are shared with neurons, and the tuning of neural sensitivity by direct physical contact. Despite great progress in the last decade, the precise roles of these cells in the sense of touch remains unclear. Here we review the known and hypothesized contributions of several accessory cells to touch by incorporating research from multiple organisms including C. elegans, D. melanogaster, mammals, avian models, and plants. Several broad parallels are identified including the regulation of extracellular ions and the release of neuromodulators by accessory cells, as well as the emerging potential physical contact between accessory cells and sensory neurons via tethers. Our broader perspective incorporates the importance of accessory cells to the understanding of human touch and pain, as well as to animal touch and its molecular underpinnings, which are underrepresented among the animal welfare literature. A greater understanding of touch, which must include a role for accessory cells, is also relevant to emergent technical applications including prosthetics, virtual reality, and robotics.

Charging Up the Periphery: Glial Ionic Regulation in Sensory Perception

The peripheral nervous system (PNS) receives diverse sensory stimuli from the environment and transmits this information to the central nervous system (CNS) for subsequent processing. Thus, proper functions of cells in peripheral sense organs are a critical gate-keeper to generating appropriate animal sensory behaviors, and indeed their dysfunction tracks sensory deficits, sensorineural disorders, and aging. Like the CNS, the PNS comprises two major cell types, neurons (or sensory cells) and glia (or glia-like supporting neuroepithelial cells). One classic function of PNS glia is to modulate the ionic concentration around associated sensory cells. Here, we review current knowledge of how non-myelinating support cell glia of the PNS regulate the ionic milieu around sensory cell endings across species and systems. Molecular studies reviewed here suggest that, rather than being a passive homeostatic response, glial ionic regulation may in fact actively modulate sensory perception, implying that PNS glia may be active contributors to sensorineural information processing. This is reminiscent of emerging studies suggesting analogous roles for CNS glia in modulating neural circuit processing. We therefore suggest that deeper molecular mechanistic investigations into critical PNS glial functions like ionic regulation are essential to comprehensively understand sensorineural health, disease, and aging.

From periodontal mechanoreceptors to chewing motor control: A systematic review

PURPOSE

This critical review summarizes the current knowledge of the structural and functional characteristics of periodontal mechanoreceptors, and understands their role in the signal pathways and functional motor control.

METHOD

A systematic review of the literature was conducted. Original articles were searched through Pubmed, Cochrane Central database and Embase until january 2016.

RESULT

1466 articles were identified through database searching and screened by reviewing the abstracts. 160 full-text were assessed for eligibility, and after 109 exclusion, 51 articles were included in the review process. Studies selected by the review process were mainly divided in studies on animal and studies on humans. Morphological, histological, molecular and electrophysiological studies investigating the periodontal mechanoreceptors in animals and in humans were included, evaluated and described.

CONCLUSION

Our knowledge of the periodontal mechanoreceptors, let us conclude that they are very refined neural receptors, deeply involved in the activation and coordination of the masticatory muscles during function. Strictly linked to the rigid structure of the teeth, they determine all the functional physiological and pathological processes of the stomatognathic system. The knowledge of their complex features is fundamental for all dental professionists. Further investigations are of utmost importance for guiding the technological advances in the respect of the neural control in the dental field.

Glial development and function in the nervous system of Caenorhabditis elegans

The nematode, Caenorhabditis elegans, has served as a fruitful setting for understanding conserved biological processes. The past decade has seen the rise of this model organism as an important tool for uncovering the mysteries of the glial cell, which partners with neurons to generate a functioning nervous system in all animals. C. elegans affords unparalleled single-cell resolution in vivo in examining glia-neuron interactions, and similarities between C. elegans and vertebrate glia suggest that lessons learned from this nematode are likely to have general implications. Here, I summarize what has been gleaned over the past decade since C. elegans glia research became a concerted area of focus. Studies have revealed that glia are essential elements of a functioning C. elegans nervous system and play key roles in its development. Importantly, glial influence on neuronal function appears to be dynamic. Key questions for the field to address in the near- and long-term have emerged, and these are discussed within.

Blockade of ENaCs by amiloride induces c-Fos activation of the area postrema

Epithelial sodium channels (ENaCs) are strongly expressed in the circumventricular organs (CVOs), and these structures may play an important role in sensing plasma sodium levels. Here, the potent ENaC blocker amiloride was injected intraperitoneally in rats and 2h later, the c-Fos activation pattern in the CVOs was studied. Amiloride elicited dose-related activation in the area postrema (AP) but only ~10% of the rats showed c-Fos activity in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO). Tyrosine hydroxylase-immunoreactive (catecholamine) AP neurons were activated, but tryptophan hydroxylase-immunoreactive (serotonin) neurons were unaffected. The AP projects to FoxP2-expressing neurons in the dorsolateral pons which include the pre-locus coeruleus nucleus and external lateral part of the parabrachial nucleus; both cell groups were c-Fos activated following systemic injections of amiloride. In contrast, another AP projection target--the aldosterone-sensitive neurons of the nucleus tractus solitarius which express the enzyme 11-β-hydroxysteriod dehydrogenase type 2 (HSD2) were not activated. As shown here, plasma concentrations of amiloride used in these experiments were near or below the IC50 level for ENaCs. Amiloride did not induce changes in blood pressure, heart rate, or regional vascular resistance, so sensory feedback from the cardiovascular system was probably not a causal factor for the c-Fos activity seen in the CVOs. In summary, amiloride may have a dual effect on sodium homeostasis causing a loss of sodium via the kidney and inhibiting sodium appetite by activating the central satiety pathway arising from the AP.

Neurotoxic unc-8 mutants encode constitutively active DEG/ENaC channels that are blocked by divalent cations

Ion channels of the DEG/ENaC family can induce neurodegeneration under conditions in which they become hyperactivated. The Caenorhabditis elegans DEG/ENaC channel MEC-4(d) encodes a mutant channel with a substitution in the pore domain that causes swelling and death of the six touch neurons in which it is expressed. Dominant mutations in the C. elegans DEG/ENaC channel subunit UNC-8 result in uncoordinated movement. Here we show that this unc-8 movement defect is correlated with the selective death of cholinergic motor neurons in the ventral nerve cord. Experiments in Xenopus laevis ooctyes confirm that these mutant proteins, UNC-8(G387E) and UNC-8(A586T), encode hyperactivated channels that are strongly inhibited by extracellular calcium and magnesium. Reduction of extracellular divalent cations exacerbates UNC-8(G387E) toxicity in oocytes. We suggest that inhibition by extracellular divalent cations limits UNC-8 toxicity and may contribute to the selective death of neurons that express UNC-8 in vivo.

Two novel DEG/ENaC channel subunits expressed in glia are needed for nose-touch sensitivity in Caenorhabditis elegans

Neuronal DEG/ENaC (degenerin and epithelial Na(+) channel) Na(+) channels have been implicated in touch sensation. For example, MEC-4 is expressed in touch neurons in Caenorhabditis elegans and mediates gentle-touch response. Similarly, homologous mammalian ASIC2 and ASIC3 are expressed in sensory neurons and produce touch phenotypes when knocked out in mice. Here, we show that novel DEG/ENaC subunits DELM-1 and DELM-2 (degenerin-like channel mechanosensory linked-1 and degenerin-like channel mechanosensory linked-2) are expressed in glia associated with touch neurons in C. elegans and that their knock-out causes defects in mechanosensory behaviors related to nose touch and foraging, which are mediated by OLQ and IL1 sensory neurons. Cell-specific rescue supports that DELM-1 and DELM-2 are required cell-autonomously in glia to orchestrate mechanosensory behaviors. Electron microscopy reveals that in delm-1 knock-outs, OLQ and IL1 sensory neurons and associated glia are structurally normal. Furthermore, we show that knock-out of DELM-1 and DELM-2 does not disrupt the expression or cellular localization of TRPA-1, a TRP channel needed in OLQ and IL1 neurons for touch behaviors. Rather, rescue of the delm-1 nose-touch-insensitive phenotype by expression of a K(+) channel in socket glia and of a cationic channel in OLQ neurons suggests that DELM channels set basal neuronal excitability. Together, our data show that DELM-1 and DELM-2 are expressed in glia associated with touch neurons where they are not needed for neuronal structural integrity or cellular distribution of neuronal sensory channels, but rather for their function.

Immunohistochemical localization of acid-sensing ion channel 2 (ASIC2) in cutaneous Meissner and Pacinian corpuscles of Macaca fascicularis

Acid-sensing ion channel 2 (ASIC2) is a member of the degenerin/epithelial sodium channel superfamily, presumably involved mechanosensation. Expression of ASIC2 has been detected in mechanosensory neurons as well as in both axons and Schwann-like cells of cutaneous mechanoreceptors. In these studies we analysed expression of ASIC2 in the cutaneous sensory corpuscles of Macaca fascicularis using immunohistochemistry and laser confocal-scanner microscopy. ASIC2 immunoreactivity was detected in both Meissner and Pacinian corpuscles. It was found to co-localize with neuron-specific enolase and RT-97, but not with S100 protein, demonstrating that ASIC2 expression is restricted to axons supplying mechanoreceptors. These results demonstrate for the first time the presence of the protein ASIC2 in cutaneous rapidly adapting low-threshold mechanoreceptors of monkey, suggesting a role of this ion channel in touch sense.

Simultaneous disruption of mouse ASIC1a, ASIC2 and ASIC3 genes enhances cutaneous mechanosensitivity

Three observations have suggested that acid-sensing ion channels (ASICs) might be mammalian cutaneous mechanoreceptors; they are structurally related to Caenorhabditis elegans mechanoreceptors, they are localized in specialized cutaneous mechanosensory structures, and mechanical displacement generates an ASIC-dependent depolarization in some neurons. However, previous studies of mice bearing a single disrupted ASIC gene showed only subtle or no alterations in cutaneous mechanosensitivity. Because functional redundancy of ASIC subunits might explain limited phenotypic alterations, we hypothesized that disrupting multiple ASIC genes would markedly impair cutaneous mechanosensation. We found the opposite. In behavioral studies, mice with simultaneous disruptions of ASIC1a, -2 and -3 genes (triple-knockouts, TKOs) showed increased paw withdrawal frequencies when mechanically stimulated with von Frey filaments. Moreover, in single-fiber nerve recordings of cutaneous afferents, mechanical stimulation generated enhanced activity in A-mechanonociceptors of ASIC TKOs compared to wild-type mice. Responses of all other fiber types did not differ between the two genotypes. These data indicate that ASIC subunits influence cutaneous mechanosensitivity. However, it is unlikely that ASICs directly transduce mechanical stimuli. We speculate that physical and/or functional association of ASICs with other components of the mechanosensory transduction apparatus contributes to normal cutaneous mechanosensation.

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.

Knockout of glial channel ACD-1 exacerbates sensory deficits in a C. elegans mutant by regulating calcium levels of sensory neurons

Degenerin/epithelial Na(+) channels (DEG/ENaCs) are voltage-independent Na(+) or Na(+)/Ca(2+) channels expressed in many tissues and are needed for a wide range of physiological functions, including sensory perception and transepithelial Na(+) transport. In the nervous system, DEG/ENaCs are expressed in both neurons and glia. However, the role of glial vs. neuronal DEG/ENaCs remains unclear. We recently reported the characterization of a novel DEG/ENaC in Caenorhabditis elegans that we named ACD-1. ACD-1 is expressed in glial amphid sheath cells. The glial ACD-1, together with the neuronal DEG/ENaC DEG-1, is necessary for acid avoidance and attraction to lysine. We report presently that knockout of acd-1 in glia exacerbates sensory deficits caused by another mutant: the hypomorphic allele of the cGMP-gated channel subunit tax-2. Furthermore, sensory deficits caused by mutations in G(i) protein odr-3 and guanylate cyclase daf-11, which regulate the activity of TAX-2/TAX-4 channels, are worsened by knockout of acd-1. We also show that sensory neurons of acd-1 tax-2(p694) double mutants fail to undergo changes in intracellular Ca(2+) when animals are exposed to low concentrations of attractant. Finally, we show that exogenous expression of TRPV1 in sensory neurons and exposure to capsaicin rescue sensory deficits of acd-1 tax-2(p694) mutants, suggesting that sensory deficits of these mutants are bypassed by increasing neuronal excitability. Our data suggest a role of glial DEG/ENaC channel ACD-1 in supporting neuronal activity.

The glia of Caenorhabditis elegans

Glia have been, in many ways, the proverbial elephant in the room. Although glia are as numerous as neurons in vertebrate nervous systems, technical and other concerns had left research on these cells languishing, whereas research on neurons marched on. Importantly, model systems to study glia had lagged considerably behind. A concerted effort in recent years to develop the canonical invertebrate model animals, Drosophila melanogaster and Caenorhabditis elegans, as settings to understand glial roles in nervous system development and function has begun to bear fruit. In this review, we summarize our current understanding of glia and their roles in the nervous system of the nematode C. elegans. The recent studies we describe highlight the similarities and differences between C. elegans and vertebrate glia, and focus on novel insights that are likely to have general relevance to all nervous systems.

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.

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.