Pacinian corpuscle development involves multiple Trk signaling pathways

Coding of self and environment by Pacinian neurons in freely moving animals

Pacinian corpuscle neurons are specialized low-threshold mechanoreceptors (LTMRs) that are tuned to detect high-frequency vibration (~40-2000 Hz), however it is unclear how Pacinians and other LTMRs encode mechanical forces encountered during naturalistic behavior. Here, we developed methods to record LTMRs in awake, freely moving mice. We find that Pacinians, but not other LTMRs, encode subtle vibrations of surfaces encountered by the animal, including low-amplitude vibrations initiated over two meters away. Strikingly, Pacinians are also highly active during a wide variety of natural behaviors, including walking, grooming, digging, and climbing. Pacinians in the hindlimb are sensitive enough to be activated by forelimb- or upper-body-dominant behaviors. Finally, we find that Pacinian LTMRs have diverse tuning and sensitivity. Our findings suggest a Pacinian population code for the representation of vibro-tactile features generated by self-initiated movements and low-amplitude environmental vibrations emanating from distant locations.

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 Anatomical Distribution of Mechanoreceptors in Mouse Hind Paw Skin and the Influence of Integrin α1β1 on Meissner-Like Corpuscle Density in the Footpads

Afferent neurons and their mechanoreceptors provide critical sensory feedback for gait. The anatomical distribution and density of afferents and mechanoreceptors influence sensory feedback, as does mechanoreceptor function. Electrophysiological studies of hind paw skin reveal the different types of afferent responses and their receptive fields, however, the anatomical distribution of mechanoreceptor endings is unknown. Also, the role of integrin α1β1 in mechanoreceptor function is unclear, though it is expressed by keratinocytes in the stratum basale where it is likely involved in a variety of mechanotransduction pathways and ion channel functionalities. For example, it has been shown that integrin α1β1 is necessary for the function of TRPV4 that is highly expressed by afferent units. The purpose of this study, therefore, was to determine and compare the distribution of mechanoreceptors across the hind paw skin and the footfall patterns of itga1-null and wild type mice. The itga1-null mouse is lacking the integrin α1 subunit, which binds exclusively to the β1 subunit, thus rendering integrin α1β1 nonfunctional while leaving the numerous other pairings of the β1 subunit undisturbed. Intact hind paws were processed, serially sectioned, and stained to visualize mechanoreceptors. Footfall patterns were analyzed as a first step in correlating mechanoreceptor distribution and functionality. Merkel cells and Meissner-like corpuscles were present, however, Ruffini endings and Pacinian corpuscles were not observed. Meissner-like corpuscles were located exclusively in the glabrous skin of the footpads and digit tips, however, Merkel cells were found throughout hairy and glabrous skin. The increased density of Merkel cells and Meissner-like corpuscles in footpads 1 and 3 and Meissner-like corpuscles in footpad 4 suggests their role in anteroposterior balance, while Meissner-like corpuscle concentrations in digits 2 and 5 support their role in mediolateral balance. Finally, a larger density of Meissner-like corpuscles in footpads 3 and 4 in male itga1-null mice compared to wild type controls paves the way for future site-specific single fiber in vivo recordings to provide insight into the role of integrin α1β1 in tactile mechanotransduction.

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.

A RET-ER81-NRG1 Signaling Pathway Drives the Development of Pacinian Corpuscles

Axon-Schwann cell interactions are crucial for the development, function, and repair of the peripheral nervous system, but mechanisms underlying communication between axons and nonmyelinating Schwann cells are unclear. Here, we show that ER81 is functionally required in a subset of mouse RET⁺ mechanosensory neurons for formation of Pacinian corpuscles, which are composed of a single myelinated axon and multiple layers of nonmyelinating Schwann cells, and Ret is required for the maintenance of Er81 expression. Interestingly, Er81 mutants have normal myelination but exhibit deficient interactions between axons and corpuscle-forming nonmyelinating Schwann cells. Finally, ablating Neuregulin-1 (Nrg1) in mechanosensory neurons results in no Pacinian corpuscles, and an Nrg1 isoform not required for communication with myelinating Schwann cells is specifically decreased in Er81-null somatosensory neurons. Collectively, our results suggest that a RET-ER81-NRG1 signaling pathway promotes axon communication with nonmyelinating Schwann cells, and that neurons use distinct mechanisms to interact with different types of Schwann cells.

SIGNIFICANCE STATEMENT

Communication between neurons and Schwann cells is critical for development, normal function, and regeneration of the peripheral nervous system. Despite many studies about axonal communication with myelinating Schwann cells, mostly via a specific isoform of Neuregulin1, the molecular nature of axonal communication with nonmyelinating Schwann cells is poorly understood. Here, we described a RET-ER81-Neuregulin1 signaling pathway in neurons innervating Pacinian corpuscle somatosensory end organs, which is essential for communication between the innervating axon and the end organ nonmyelinating Schwann cells. We also showed that this signaling pathway uses isoforms of Neuregulin1 that are not involved in myelination, providing evidence that neurons use different isoforms of Neuregulin1 to interact with different types of Schwann cells.

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.

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.

[Neurotrophins and pain]

Neurotrophins are a family of trophic factors well known for their effects on neuronal survival, growth and neuronal differentiation. During the last decade, a large literature has shown in humans and in animal models that nerve growth factor (NGF) is a peripheral mediator of pain, especially in the states of inflammatory pain. NGF synthesis is indeed increased in a wide variety of inflammatory diseases and NGF neutralizing molecules are effective analgesic agents in these models of persistent pain. Therapeutical strategies targeting the sequestering of NGF did yield very encouraging results in clinical trials (stages II and III) but have been on hold since 2010 due to potential harmful effects in combination with non-steroid anti-inflammatory drug. NGF regulates the expression of a second neurotrophin, Brain Derived Neurotrophic Factor (BDNF), in nociceptors. BDNF is released where nociceptors are activated, and it acts as a modulator of pain in the central nervous system and is involved in central sensitization.

Neurotrophins in the lower urinary tract: becoming of age

The lower urinary tract (LUT) comprises a storage unit, the urinary bladder, and an outlet, the urethra. The coordination between the two structures is tightly controlled by the nervous system and, therefore, LUT function is highly susceptible to injuries to the neuronal pathways involved in micturition control. These injuries may include lesions to the spinal cord or to nerve fibres and result in micturition dysfunction. A common trait of micturition pathologies, irrespective of its origin, is an upregulation in synthesis and secretion of neurotrophins, most notably Nerve Growth Factor (NGF) and Brain Derived Neurotrophic Factor (BDNF). These neurotrophins are produced by neuronal and non-neuronal cells and exert their effects upon binding to their high-affinity receptors abundantly expressed in the neuronal circuits regulating LUT function. In addition, NGF and BDNF are present in detectable amounts in the urine of patients suffering from various LUT pathologies, suggesting that analysis of urinary NGF and BDNF may serve as likely biomarkers to be studied in tandem with other factors when diagnosing patients. Studies with experimental models of bladder dysfunction using antagonists of NGF and BDNF receptors as well as scavenging agents suggest that those NTs may be key elements in the pathophysiology of bladder dysfunctions. In addition, available data indicates that NGF and BDNF might constitute future targets for designing new drugs for better treatment of bladder dysfunction.

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.

Combinatorial expression of Brn3 transcription factors in somatosensory neurons: genetic and morphologic analysis

The three members of the Brn3 family of POU-domain transcription factors (Brn3a/Pou4f1, Brn3b/Pou4f2, and Brn3c/Pou4f3) are expressed in overlapping subsets of visual, auditory/vestibular, and somatosensory neurons. Using unmarked Brn3-null alleles and Brn3 conditional alleles in which gene loss is coupled to expression of an alkaline phosphatase reporter, together with sparse Cre-mediated recombination, we describe the following: (1) the overlapping patterns of Brn3 gene expression in somatosensory neurons; (2) the manner in which these patterns correlate with molecular markers, peripheral afferent arbor morphologies, and dorsal horn projections; and (3) the consequences for these neurons of deleting individual Brn3 genes in the mouse. We observe broad expression of Brn3a among DRG neurons, but subtype-restricted expression of Brn3b and Brn3c. We also observe a nearly complete loss of hair follicle-associated sensory endings among Brn3a(-/-) neurons. Together with earlier analyses of Brn3 gene expression patterns in the retina and inner ear, these experiments suggest a deep functional similarity among primary somatosensory neurons, spiral and vestibular ganglion neurons, and retinal ganglion cells. This work also demonstrates the utility of sparse genetically directed labeling for visualizing individual somatosensory afferent arbors and for defining cell-autonomous mutant phenotypes.

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.

Sequestration of brain derived nerve factor by intravenous delivery of TrkB-Ig2 reduces bladder overactivity and noxious input in animals with chronic cystitis

Brain derived nerve factor (BDNF) is a trophic factor belonging to the neurotrophin family. It is upregulated in various inflammatory conditions, where it may contribute to altered pain states. In cystitis, little is known about the relevance of BDNF in bladder-generated noxious input and bladder overactivity, a matter we investigated in the present study. Female rats were intraperitoneally (i.p.) injected with cyclophosphamide (CYP; 200 mg/kg). They received saline or TrkB-Ig(2) via intravenously (i.v.) or intravesical administration. Three days after CYP-injection, animals were anaesthetized and cystometries performed. All animals were perfusion-fixed and the spinal cord segments L6 collected, post-fixed and processed for c-Fos and phosphoERK immunoreactivity. BDNF expression in the bladder, as well as bladder histology, was also assessed. Intravesical TrkB-Ig(2) did not change bladder reflex activity of CYP-injected rats. In CYP-animals treated with i.v. TrkB-Ig(2) a decrease in the frequency of bladder reflex contractions, in comparison with saline-treated animals, was observed. In spinal sections from the latter group of animals, the number of phosphoERK and c-Fos immunoreactive neurons was lower than in sections from saline-treated CYP-animals. BDNF immunoreactivity was higher during cystitis but was not changed by TrkB-Ig(2) i.v. treatment. Evaluation of the bladder histology showed similar inflammatory signs in the bladders of inflamed animals, irrespective of the treatment. Data show that i.v. but not intravesical administration of TrkB-Ig(2) reduced bladder hyperactivity in animals with cystitis to levels comparable to those observed in unirritated rats. Since i.v. TrkB-Ig(2) also reduced spinal extracellular signal-regulated kinase (ERK) activation, it is possible that BDNF contribution to inflammation-induced bladder hyperactivity is via spinal activation of the ERK pathway. Finally, the reduction in c-Fos expression indicates that TrkB-Ig(2) also reduced bladder-generated noxious input. Our results show that sequestration of BDNF may be considered a new therapeutic strategy to treat chronic cystitis.

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.

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.

Neurotrophin-4/5-depletion induces a delay in maturation of the periodontal Ruffini endings in mice

Neurotrophin-4/5 (NT-4/5) - a member of the neurotrophic factors - is a ligand for TrkB, which has been reported to be expressed in the mechanoreceptive Ruffini endings of the periodontal ligament. The present study examined developmental changes in the terminal morphology and neural density in homozygous mice with a targeted disruption of the nt-4/5 gene and wild-type mice by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker, and by quantitative analysis using an image analyzer. Postnatal development of terminal Schwann cells was also investigated by enzymatic histochemistry for non-specific cholinesterase activity (ChE). Furthermore, the immuno-expression of TrkB and low affinity nerve growth factor receptor (p75-NGFR) was surveyed in the periodontal Ruffini endings as well as trigeminal ganglion. At postnatal 1 week, the lingual periodontal ligament of both types of mice contained PGP 9.5-positive nerve fibers showing a tree-like ramification with axonal swellings in their course. In both types of mice at 2 weeks of age, comparatively thick nerve fibers with a smooth outline increased in number, and frequently ramified to form nerve terminals with dendritic profiles. However, no typical Ruffini endings with irregular outlines observed in the adult wild-type mice were found in the periodontal ligament at this stage. At postnatal 3 weeks, typical Ruffini endings with irregular outlines were discernable in the periodontal ligament of the wild-type mice while the dendritic endings showing smooth outlines were restricted to the homozygous mice. After postnatal 8 weeks, both types of mice showed an increase in the number of Ruffini endings, but the morphology differed between the wild-type and NT-4/5 homozygous mice. In the wild-type mice, a major population of the Ruffini endings expanded their axonal branches and developed many microprojections, resulting in a reduction of endings with smooth outlines. In contrast, we failed to find such typical Ruffini endings in the periodontal ligament of the homozygous mice: A majority of the periodontal Ruffini endings continued to show smooth outlines at postnatal 12 weeks. Quantitative analysis on neural density demonstrated a reduction in the homozygous mice with a significant difference by postnatal 8 weeks. Enzymatic histochemistry for non-specific ChE did not exhibit a distinct difference in the distribution and density of terminal Schwann cells between wild-type and homozygous mice. Furthermore, TrkB and p75-NGFR mRNA and proteins did not differ in the trigeminal ganglion between the two types. The periodontal Ruffini endings also displayed immunoreactivities for TrkB and p75- NGFR in both phenotypes. These findings suggest that the nt-4/5 gene depletion caused a delay in the formation and maturation of the periodontal Ruffini endings in the mice by inhibiting the growth of the periodontal nerves at an early stage, and indicate that multiple neurotrophins such as NT- 4/5 and BDNF might play roles in the development and/or maturation of the periodontal Ruffini endings.

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.

Postnatal developmental changes in the expression of ErbB receptors in murine Pacinian cospucles

Neuregulins and their signaling ErbB receptors play a critical role during the development of the mammalian peripheral nervous system, including some kinds of mechanoreceptors such as the Pacinian corpuscles which become structurally and functionally mature postnatally. In this study, we investigated whether or not ErbBs in Pacinian corpuscles undergoes developmental changes, as well as if their expression depends on the innervation. Pacinian corpuscles from 7-day- and 3-month-old mice were assessed for the immunohistochemical detection of EGFR or ErbB1, ErbB2, ErbB3 and ErbB4. The effect of denervation on the expression of ErbBs in mature Pacinian corpuscles was also analyzed. Developing 7-day-old Pacinian corpuscles express ErbB2 and ErbB3 immunoreactivity in the inner-core (regarded as modified Schwann cells), whereas the mature 3-month-old Pacinian corpuscles exclusively displayed ErbB4 immunoreactivity in the outer core and the capsule (regarded as endoneurial and perineurial cells). Denervation was without effect on the ErbB expression. Present results demonstrate maturational related changes and cell segregation in the expression of ErbB receptors by murine Pacinian corpuscles, and that this expression is independent of the innervation.

Hypertrophy of Pacinian corpuscles in a young patient with neurofibromatosis

Hypertrophy of Pacinian corpuscles is a condition that has been reported very infrequently. It was thought not to be connected with neurofibromatosis. A 24-year-old female with a lifelong history of neurofibromatosis type 1, who presented with left ring finger pain and decreased sensation for over 1 year is reported. Histopathology showed hypertrophy of Pacinian corpuscles. Hypertrophy of Pacinian corpuscles should be considered in the differential diagnosis of any patient with persistent pain localized to the distal palm or digits. This is the first case reported in a patient with neurofibromatosis. The finding of this lesion in a patient with neurofibromatosis is somewhat intriguing, though the relationship of these two entities is unclear.

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.

Neurotrophins: mediators and modulators of pain

The neurotrophin family of neurotrophic factors are well-known for their effects on neuronal survival and growth. Over the past decade, considerable evidence has accumulated from both humans and animals that one neurotrophin, nerve growth factor (NGF), is a peripheral pain mediator, particularly in inflammatory pain states. NGF is upregulated in a wide variety of inflammatory conditions, and NGF-neutralizing molecules are effective analgesic agents in many models of persistent pain. Such molecules are now being evaluated in clinical trials. NGF regulates the expression of a second neurotrophin, brain-derived neurotrophic factor (BDNF), in nociceptors. BDNF is released when nociceptors are activated, and it acts as a central modulator of pain. The chapter reviews the evidence for these roles (and briefly the effects of other neurotrophins), the range of conditions under which they act, and their mechanism of action.

ETS transcription factor ER81 is required for the pacinian corpuscle development

ER81, a member of the ETS family of transcription factors, is involved in processes of specification of neuronal identity, control of sensory-motor connectivity, and differentiation of muscle spindles. Spindles either degenerate or are abnormal in mutant mice lacking ER81. We examined whether ER81 is required for the development of another class of mechanoreceptors, the Pacinian corpuscle. ER81 was expressed by the inner core cells of the corpuscles, as reflected by expression of the lacZ reporter gene in Er81(+/lacZ) mutants, thereby suggesting a role for ER81 in the corpuscle development. No Pacinian corpuscles or their afferent nerve fibers were present in the crus of Er81 null mice at birth. Legs of mutant embryos examined at E16.5 were also devoid of the corpuscles, but not of their afferents. Thus, Pacinian corpuscles do not form, and their afferents do not survive, in the absence of ER81. A deficiency of dorsal root ganglia neurons expressing calretinin, a marker for neurons subserving Pacinian corpuscles, correlated with the absence of corpuscles and their afferents in Er81 null mice. These observations indicate a requirement for ER81 in the assembly of Pacinian corpuscles and the survival of the sensory neurons that innervate them.

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.