Neurocalcin-immunopositive nerve terminals in the muscle spindle, Golgi tendon organ and motor endplate

Long-Term SMN- and Ncald-ASO Combinatorial Therapy in SMA Mice and NCALD-ASO Treatment in hiPSC-Derived Motor Neurons Show Protective Effects

For SMA patients with only two SMN2 copies, available therapies might be insufficient to counteract lifelong motor neuron (MN) dysfunction. Therefore, additional SMN-independent compounds, supporting SMN-dependent therapies, might be beneficial. Neurocalcin delta (NCALD) reduction, an SMA protective genetic modifier, ameliorates SMA across species. In a low-dose SMN-ASO-treated severe SMA mouse model, presymptomatic intracerebroventricular (i.c.v.) injection of Ncald-ASO at postnatal day 2 (PND2) significantly ameliorates histological and electrophysiological SMA hallmarks at PND21. However, contrary to SMN-ASOs, Ncald-ASOs show a shorter duration of action limiting a long-term benefit. Here, we investigated the longer-term effect of Ncald-ASOs by additional i.c.v. bolus injection at PND28. Two weeks after injection of 500 µg Ncald-ASO in wild-type mice, NCALD was significantly reduced in the brain and spinal cord and well tolerated. Next, we performed a double-blinded preclinical study combining low-dose SMN-ASO (PND1) with 2× i.c.v. Ncald-ASO or CTRL-ASO (100 µg at PND2, 500 µg at PND28). Ncald-ASO re-injection significantly ameliorated electrophysiological defects and NMJ denervation at 2 months. Moreover, we developed and identified a non-toxic and highly efficient human NCALD-ASO that significantly reduced NCALD in hiPSC-derived MNs. This improved both neuronal activity and growth cone maturation of SMA MNs, emphasizing the additional protective effect of NCALD-ASO treatment.

Neurocalcin Delta Suppression Protects against Spinal Muscular Atrophy in Humans and across Species by Restoring Impaired Endocytosis

Homozygous SMN1 loss causes spinal muscular atrophy (SMA), the most common lethal genetic childhood motor neuron disease. SMN1 encodes SMN, a ubiquitous housekeeping protein, which makes the primarily motor neuron-specific phenotype rather unexpected. SMA-affected individuals harbor low SMN expression from one to six SMN2 copies, which is insufficient to functionally compensate for SMN1 loss. However, rarely individuals with homozygous absence of SMN1 and only three to four SMN2 copies are fully asymptomatic, suggesting protection through genetic modifier(s). Previously, we identified plastin 3 (PLS3) overexpression as an SMA protective modifier in humans and showed that SMN deficit impairs endocytosis, which is rescued by elevated PLS3 levels. Here, we identify reduction of the neuronal calcium sensor Neurocalcin delta (NCALD) as a protective SMA modifier in five asymptomatic SMN1-deleted individuals carrying only four SMN2 copies. We demonstrate that NCALD is a Ca²⁺-dependent negative regulator of endocytosis, as NCALD knockdown improves endocytosis in SMA models and ameliorates pharmacologically induced endocytosis defects in zebrafish. Importantly, NCALD knockdown effectively ameliorates SMA-associated pathological defects across species, including worm, zebrafish, and mouse. In conclusion, our study identifies a previously unknown protective SMA modifier in humans, demonstrates modifier impact in three different SMA animal models, and suggests a potential combinatorial therapeutic strategy to efficiently treat SMA. Since both protective modifiers restore endocytosis, our results confirm that endocytosis is a major cellular mechanism perturbed in SMA and emphasize the power of protective modifiers for understanding disease mechanism and developing therapies.

Synaptic-like vesicles and candidate transduction channels in mechanosensory terminals

This article summarises progress to date over an exciting and very enjoyable first 15 years of collaboration with Bob Banks. Our collaboration began when I contacted him with (to me) an unexpected observation that a dye used to mark recycling synaptic vesicle membrane at efferent terminals also labelled muscle spindle afferent terminals. This observation led to the re-discovery of a system of small clear vesicles present in all vertebrate primary mechanosensory nerve terminals. These synaptic-like vesicles (SLVs) have been, and continue to be, the major focus of our work. This article describes our characterisation of the properties and functional significance of these SLVs, combining our complementary skills: Bob’s technical expertise and encyclopaedic knowledge of mechanosensation with my experience of synaptic vesicles and the development of the styryl pyridinium dyes, of which the most widely used is FM1-43. On the way we have found that SLVs seem to be part of a constitutive glutamate secretory system necessary to maintain the stretch-sensitivity of spindle endings. The glutamate activates a highly unusual glutamate receptor linked to phospholipase D activation, which we have termed the PLD-mGluR. It has a totally distinct pharmacology first described in the hippocampus nearly 20 years ago but, like the SLVs that were first described over 50 years ago, has since been little researched. Yet, our evidence and literature searches suggest this glutamate/SLV/PLD-mGluR system is a ubiquitous feature of mechanosensory endings and, at least for spindles, is essential for maintaining mechanosensory function. This article summarises how this system integrates with the classical model of mechanosensitive channels in spindles and other mechanosensory nerve terminals, including hair follicle afferents and baroreceptors controlling blood pressure. Finally, in this time when there is an imperative to show translational relevance, I describe how this fascinating system might actually be a useful therapeutic drug target for clinical conditions such as hypertension and muscle spasticity. This has been a fascinating 15-year journey in collaboration with Bob who, as well as having an astute scientific mind, is also a great enthusiast, motivator and friend. I hope this exciting and enjoyable journey will continue well into the future.

Effects of peripheral nerve injury on parvalbumin expression in adult rat dorsal root ganglion neurons

Background

Parvalbumin (PV) is a calcium binding protein that identifies a subpopulation of proprioceptive dorsal root ganglion (DRG) neurons. Calcitonin gene-related peptide (CGRP) is also expressed in a high proportion of muscle afferents but its relationship to PV is unclear. Little is known of the phenotypic responses of muscle afferents to nerve injury. Sciatic nerve axotomy or L5 spinal nerve ligation and section (SNL) lesions were used to explore these issues in adult rats using immunocytochemistry.

Results

In naive animals, the mean PV expression was 25 % of L4 or L5 dorsal root ganglion (DRG) neurons, and this was unchanged 2 weeks after sciatic nerve axotomy. Colocalization studies with the injury marker activating transcription factor 3 (ATF3) showed that approximately 24 % of PV neurons expressed ATF3 after sciatic nerve axotomy suggesting that PV may show a phenotypic switch from injured to uninjured neurons. This possibility was further assessed using the spinal nerve ligation (SNL) injury model where injured and uninjured neurons are located in different DRGs. Two weeks after L5 SNL there was no change in total PV staining and essentially all L5 PV neurons expressed ATF3. Additionally, there was no increase in PV-ir in the adjacent uninjured L4 DRG cells. Co-labelling of DRG neurons revealed that less than 2 % of PV neurons normally expressed CGRP and no colocalization was seen after injury.

Conclusion

These experiments clearly show that axotomy does not produce down regulation of PV protein in the DRG. Moreover, this lack of change is not due to a phenotypic switch in PV immunoreactive (ir) neurons, or de novo expression of PV-ir in uninjured neurons after nerve injury. These results further illustrate differences that occur when muscle afferents are injured as compared to cutaneous afferents.

Mechanotransduction in the muscle spindle

The focus of this review is on the principal sensory ending of the mammalian muscle spindle, known as the primary ending. The process of mechanosensory transduction in the primary ending is examined under five headings: (i) action potential responses to defined mechanical stimuli—representing the ending's input–output properties; (ii) the receptor potential—including the currents giving rise to it; (iii) sensory-terminal deformation—measurable changes in the shape of the primary-ending terminals correlated with intrafusal sarcomere length, and what may cause them; (iv) putative stretch-sensitive channels—pharmacological and immunocytochemical clues to their identity; and (v) synaptic-like vesicles—the physiology and pharmacology of an intrinsic glutamatergic system in the primary and other mechanosensory endings, with some thoughts on the possible role of the system. Thus, the review highlights spindle stretch-evoked output is the product of multi-ionic receptor currents plus complex and sophisticated regulatory gain controls, both positive and negative in nature, as befits its status as the most complex sensory organ after the special senses.

Tissue engineering the mechanosensory circuit of the stretch reflex arc: sensory neuron innervation of intrafusal muscle fibers

The sensory circuit of the stretch reflex arc, composed of specialized intrafusal muscle fibers and type Ia proprioceptive sensory neurons, converts mechanical information regarding muscle length and stretch to electrical action potentials and relays them to the central nervous system. Utilizing a non-biological substrate, surface patterning photolithography and a serum-free medium formulation a co-culture system was developed that facilitated functional interactions between intrafusal muscle fibers and sensory neurons. The presence of annulospiral wrappings (ASWs) and flower-spray endings (FSEs), both physiologically relevant morphologies in sensory neuron-intrafusal fiber interactions, were demonstrated and quantified using immunocytochemistry. Furthermore, two proposed components of the mammalian mechanosensory transduction system, BNaC1 and PICK1, were both identified at the ASWs and FSEs. To verify functionality of the mechanoreceptor elements the system was integrated with a MEMS cantilever device, and Ca(2+) currents were imaged along the length of an axon innervating an intrafusal fiber when stretched by cantilever deflection. This system provides a platform for examining the role of this mechanosensory complex in the pathology of myotonic and muscular dystrophies, peripheral neuropathy, and spasticity inducing diseases like Parkinson's. These studies will also assist in engineering fine motor control for prosthetic devices by improving our understanding of mechanosensitive feedback.

Autogenic modulation of mechanoreceptor excitability by glutamate release from synaptic-like vesicles: evidence from the rat muscle spindle primary sensory ending

Fifty-nanometre diameter, clear, synaptic-like vesicles (SLVs) are found in primary mechanosensory nerve terminals of vertebrate and invertebrate animals. We have investigated their role in mechanosensory function using the muscle spindle primary endings of rat Ia afferents as a model. Uptake and release of the synaptic vesicle marker FM1-43 indicated that SLVs recycle like synaptic vesicles and do so in a Ca(2+)-sensitive manner. Mechanical stimulation increased SLV recycling, increasing both dye uptake and release. Immunogold/electronmicroscopy showed that, like the central synaptic endings, Ia peripheral endings are enriched with glutamate. Moreover, exogenous glutamate enhanced stretch-induced Ia excitability. Enhanced excitability persisted in the presence of antagonists to the commonest ionotropic and metabotropic glutamate receptors (kynurenate, MCPG, CPPG and MAP4). However, excitation by glutamate was abolished by (R,S)-3,5-dihydroxyphenylglycine (DHPG), and rather more effectively by (2R,1'-S,2'-R,3'-S)-2-(2'-carboxy-3'-phenylcyclopropyl) glycine (PCCG-13). PCCG-13 also significantly reduced stretch-activated excitability in the absence of exogenous glutamate. These data indicate that SLVs recycle at rest, releasing glutamate, and that mechanical activity increases this process. The blockade with DHPG and PCCG-13 suggests that endogenous glutamate release acts, at least in part, through the recently described phospholipase D-linked metabotropic Glu receptor to maintain the excitability of the sensory endings.

Neurocalcin-immunoreactive neurons in the petrosal ganglion innervate the taste bud

The distribution and origin of neurocalcin-immunoreactive (NC-ir) nerve fibers in the taste bud and carotid body were examined by an immunofluorescence method. In the circumvallate papilla of the tongue, NC-ir nerve fibers made subepithelial nerve plexuses and occasionally penetrated the taste bud. However, the carotid body was devoid of ir nerve fibers. In the petrosal ganglion, 32% of neurons were immunoreactive for NC. Such neurons were mostly medium-sized to large, and scattered throughout the ganglion. In the superior cervical and intralingual ganglia, numerous ir varicose fibers surrounded postsynaptic neurons. However, NC-ir could not be detected in cell bodies of these neurons. The retrograde tracing method indicated that NC-ir petrosal neurons innervated taste buds in the circumvallate papilla. NC-ir neurons may have a gustatory function in the petrosal ganglion.

Motor, sensory and autonomic nerve terminals containing NAP-22 immunoreactivity in the rat muscle

Neuron-enriched acidic protein having a molecular mass of 22 kDa, NAP-22, is a Ca(2+)-dependent calmodulin-binding protein and is phosphorylated with protein kinase C (PKC). This protein is localized to the biological membrane via myristoylation and found in the membrane fraction of the brain and in the synaptic vesicle fraction. Recent studies showed that NAP-22 is localized in the membrane raft domain in a cholesterol-dependent manner and suggest a role for NAP-22 in maturation and/or maintenance of nerve terminals by controlling cholesterol-dependent membrane dynamics. The present study revealed the immunohistochemical distribution of NAP-22 in the peripheral nerves in rat muscles. In all examined muscles, nerve terminals in the motor endplates showed NAP-22 immunoreactivity associated with the membranes of synaptic vesicles and nerve terminals. In the muscle spindles, annulospiral endings, which made spirals around the intrafusal muscles, showed intense NAP-22 immunoreactivity. Autonomic nerve fibers around the intramuscular blood vessels also showed the immunoreactivity for NAP-22. NAP-22 immunoreactivity in these peripheral nerves was observed from birth to adulthood (100 days after birth). Though growth-associated protein-43 (GAP-43) immunoreactivity in these nerves was observed from birth, this immunoreactivity decreased from 20 days after birth. These findings suggest that NAP-22 is distributed and regulates functions in the motor, sensory and autonomic nerve terminals in the peripheral nervous system.

Calcium-binding protein-immunoreactive innervation of the rat vibrissa

Immunohistochemistry detected calcium-binding proteins (CaBPs) in corpuscular and Merkel nerve endings of the rat vibrissa. CaBP-immunoreactive (ir) corpuscular endings were divided into two types: ramified and unramified endings. Ramified endings were subdivided into reticular and Ruffini endings. Unramified endings were identical to longitudinal lanceolate endings which have been described previously. Reticular and unramified endings as well as Merkel endings co-expressed neurocalcin (NC)- and parvalbumin (PV)-immunoreactivity (ir). However, such endings were devoid of peptide 19 (PEP19)-ir. PV-ir Ruffini endings were immunoreactive for PEP19 but not NC. The retrograde tracing method revealed that 34, 21 and 18% of trigeminal neurons which project to the infraorbital nerve exhibited NC-, PEP19- and PV-ir, respectively. In addition, 73 and 36% of the PV-ir neurons showed NC- and PEP19-ir, respectively. The content and co-expression of CaBPs in vibrissal low-threshold mechanoreceptors may depend on their terminal morphology.

Horse lumbrical muscle: possible structural and functional reorganization in regressive muscle

An anatomical study of horse lumbrical muscle (Lm) was carried out by light and electron microscopy in combination with immunochemical and cytochemical methods. Paraffin sections were subjected to haematoxylin and eosin (H & E) and Masson's trichrome staining for morphometric analysis. Paraffin sections were also used for immunostaining by anti-PGP 9.5 for reaction with nerve-protein associated-structures, anti-heat-shock protein 70 (hsp 70) for detection of gene expression changes, anti-fast myosin for the determination of muscle fibre types, and for detection of apoptotic gene expression of muscle fibres by the TUNEL method. The distribution of muscle fibre types on frozen sections was also examined by assaying ATPase (pH 4.2). We found that the proximal end of the tendon of the unipennate-shaped Lm binds to the deep digital flexor tendon, and the distal end of the Lm tendon connects to the medial surface of the palmar annular ligament. The Lm was not always present, but when found it varied in length greatly, up to 8 cm (muscle part alone), and weighed less than 1 g. The Lm was white, pale, or reddish in colour depending on the ratio of muscle fibre and connective tissue contents. The semi-tendinized regressive Lm was composed of rich vasculature, peripheral nerves, and nerve-like organs similar to the neuromuscular spindle (NMS). The extrafusal muscle fibres (e-lm) that surround the NMS were replaced with a thick outer capsule of connective tissues (CT) in the Lm nerve-like organ, which we named the neurotendinous capsule (NTC) organ. NTC organs exist alone or as multiple structures (up to eight) surrounded by a common outer capsule at the outermost CT ring. The NTC possesses several intrafusal muscle fibres (ifm) just as the NMS does. That the ifm was associated with nerve endings was confirmed by anti-PGP 9.5 and electron microscopic observation. Some muscle fibres in ifm and e-lm reacted with anti-fast twitch myosin and with anti-hsp 70. The e-lm exhibited at least two fibre types, determined by ATPase (pH 4.2) assay. The ifm exhibited mainly type I (slow twitch) fibres. No apoptotic gene expression was detected in either ifm or e-lm, suggesting the Lm is a vital organ. The degenerating fibres observed in ifm and e-lm indicate that the turnover rate of cytoplasmic components is accelerated. We attribute this phenomenon to the necessity for adaptation to new environmental demands. The surprising finding of tubular aggregates (TAs) in ifm of the NTC organ suggests that the Lm is continuously adapting. Some results related to variation in diameter of the collagen fibrils, isolation of the NTC organ and the myofibrillar protein constituents are also discussed. In conclusion, the so-called regressive Lm has rich vasculature, many peripheral nerves, and newly described NTC organs. The induction of heat-shock protein, lack of apoptotic gene expression in ifm and e-lm fibres, and TA formation in ifm suggest that horse Lm responds to environmental stress through reorganization and/or remodelling of cell constituents. We hypothesize that the horse Lm has lost its original role as a contractile element and changed to another function, likely as a vital nerve organ.

Molecular cloning, mapping and characterization of the human neurocalcin delta gene (NCALD)

We identified a new human gene that encodes a cognate of the bovine neurocalcin delta from a human fetal brain cDNA library; hence we named it human neurocalcin delta (NCALD) gene. The deduced polypeptide product of the cDNA is 22 kDa in size, and its amino acid sequence is 100% and 99% identical to that of the bovine and chicken neurocalcin, respectively. Northern blots showed that the NCALD gene is more abundantly expressed in brain, testis, ovary and small intestine. Tissue in situ hybridization confirmed the existence of the NCALD mRNA in the adult human testis. Radiation hybrid panel mapping localized the gene to chromosome 8 between molecular markers D8S270 and D8S257.

Neurocalcin-immunoreactive neurons in the mammalian dorsal root ganglia, including humans

Neurocalcin (NC) is a recently characterized EF-hand calcium-binding protein present in a discrete population of sensory neurons and their peripheral mechanoreceptors, but its presence in peripheral nervous system neurons other than in the rat is still unknown. The present study was designed to investigate the occurrence of NC in the dorsal root ganglia (DRG) of several mammalian species (horse, buffalo, cow, sheep, pig, dog, and rat), including humans. DRG were fixed, embedded in paraffin, and processed for immunohistochemistry using a polyclonal antibody against NC. The size of the immunoreactive neurons was measured. In all species examined, NC immunoreactivity (IR) was restricted to neurons but the percentage, as well as the size of the immunoreactive neurons, varied among different species. As a rule, small neurons (diameter <20 microm) lack NC IR. In some species (pig, dog, buffalo, cow), only the largest neurons showed IR, whereas in others (sheep, horse, rat, and humans) they covered the entire range of neuron sizes. The pattern of immunostaining was cytoplasmic, although in some species (cow and buffalo), it formed a peripheral "ring." The present results demonstrate that mammalian DRG contain a subpopulation of NC-positive neurons, which varies from one species to another. Based on the neuron size, the possible function of the NC-containing neurons is discussed.

Neurocalcin-immunoreactive primary sensory neurons in the trigeminal ganglion provide myelinated innervation to the tooth pulp and periodontal ligament

The distribution of neurocalcin-immunoreactive (NC-ir) primary sensory neurons was examined in the trigeminal ganglion (TG), mesencephalic trigeminal tract nucleus (Mes5) and intraoral structures. NC-ir primary sensory neurons were located in the TG but not the Mes5. The coexpression study demonstrated that virtually all NC-ir TG neurons exhibited S100-immunoreactivity (-ir). In the tooth pulp, NC-ir nerve fibers were observed in the subodontoblastic and odontoblastic layers. Immunoelectron microscopic and retrograde tracing methods revealed that myelinated pulpal axons derived from the TG mostly exhibited the ir. In the periodontal ligament, bush-like endings showed NC-ir. These endings were morphologically identical to Ruffini-like endings. The present study suggests that NC-ir trigeminal primary sensory neurons have their cell bodies in the TG. Their peripheral axons are probably myelinated. Such neurons include pulpal nociceptors and low-threshold mechanoreceptors.

Immunohistochemical localization of neurocalcin in human sensory neurons and mechanoreceptors

The localization of neurocalcin in the developing and adult human peripheral nervous system (dorsal root and sympathetic ganglia (DRG, SG), and enteric nervous system (ENS)) was investigated using immunohistochemistry. A subpopulation of large-sized neurons in DRG of 9 and 12 weeks old embryos showed immunoreactivity (IR), whereas the sympathetic ganglia or enteric neurons did not. In adults, neurocalcin IR was restricted to a subpopulation of large (13%) and intermediate (15%) sized neurons in DRG. The protein was also found in muscular (67%) and cutaneous (12%) nerve fibers, as well as in the axons supplying muscular (muscle spindles, Golgi's tendon organs, and perimysial Pacinian corpuscles) and cutaneous (Meissner's but not Pacinian corpuscles) mechanoreceptors, as well as motor end-plates. Present results demonstrate that neurocalcin in both developing and adult humans can be used as a specific marker for a subpopulation of sensory neurons coupled to proprioception and touch, and for axons of motoneurons forming motor end-plates.