Comparison of target innervation by sympathetic axons in adult wild type and heterozygous mice for nerve growth factor or its receptor trkA

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.

Human TrkAR649W mutation impairs nociception, sweating and cognitive abilities: a mouse model of HSAN IV

A functional nerve growth factor NGF-Tropomyosin Receptor kinase A (TrkA) system is an essential requisite for the generation and maintenance of long-lasting thermal and mechanical hyperalgesia in adult mammals. Indeed, mutations in the gene encoding for TrkA are responsible for a rare condition, named Hereditary Sensory and Autonomic Neuropathy type IV (HSAN IV), characterized by the loss of response to noxious stimuli, anhidrosis and cognitive impairment. However, to date, there is no available mouse model to properly understand how the NGF-TrkA system can lead to pathological phenotypes that are distinctive of HSAN IV. Here, we report the generation of a knock-in mouse line carrying the HSAN IV TrkAR649W mutation. First, by in vitro biochemical and biophysical analyses, we show that the pathological R649W mutation leads to kinase-inactive TrkA also affecting its membrane dynamics and trafficking. In agreement with the HSAN IV human phenotype, TrkAR649W/m mice display a lower response to thermal and chemical noxious stimuli, correlating with reduced skin innervation, in addition to decreased sweating in comparison to TrkAh/m controls. Moreover, the R649W mutation decreases anxiety-like behavior and compromises cognitive abilities, by impairing spatial-working and social memory. Our results further uncover unexplored roles of TrkA in thermoregulation and sociability. In addition to accurately recapitulating the clinical manifestations of HSAN IV patients, our findings contribute to clarifying the involvement of the NGF-TrkA system in pain sensation.

Retrograde signaling via axonal transport through signaling endosomes

Neurons extend axons far from cell bodies, and retrograde communications from distal axons to cell bodies and/or dendrites play critical roles in the development and maintenance of neuronal circuits. In neurotrophin signaling, the retrograde axonal transport of endosomes containing active ligand-receptor complexes from distal axons to somatodendrite compartments mediates retrograde signaling. However, the generality and specificity of these endosome-based transportations called "signaling endosomes" remain to be elucidated. Here, I summarize the discovery of semaphorin3A signaling endosomes, the first example other than neurotrophins to regulate dendritic development via AMPA receptor GluA2 localization in dendrites. The molecular components of Sema3A and neurotrophin signaling endosomes are distinct, but partially overlap to regulate specific and common cellular events. Because receptors are transported back to the cell bodies, neurons must replenish receptors on the growth cone surface to ensure continued response to the target-derived ligands. Recent findings have demonstrated that retrograde signaling endosomes also induce anterograde delivery of nascent receptors in neurotrophin signaling. The coupling between anterograde and retrograde axonal transport via signaling endosomes therefore plays a critical role in regulating proper neuronal network formation.

Physiology, Pathology and Regeneration of Salivary Glands

Salivary glands are essential structures in the oral cavity. A variety of diseases, such as cancer, autoimmune diseases, infections and physical traumas, can alter the functionality of these glands, greatly impacting the quality of life of patients. To date, no definitive therapeutic approach can compensate the impairment of salivary glands, and treatment are purely symptomatic. Understanding the cellular and molecular control of salivary glands function is, therefore, highly relevant for therapeutic purposes. In this review, we provide a starting platform for future studies in basic biology and clinical research, reporting classical ideas on salivary gland physiology and recently developed technology to guide regeneration, reconstruction and substitution of the functional organs.

TrkA Bumps into Its Future Self

In neurons, correct targeting of receptors to the axon is critical for cell survival and circuit formation. In this issue of Developmental Cell, Yamashita et al. (2017) report that the ER-resident phosphatase PTP1B is required to prime TrkA for axonal transport.

Phospho-Regulation of Soma-to-Axon Transcytosis of Neurotrophin Receptors

Axonal targeting of signaling receptors is essential for neuronal responses to extracellular cues. Here, we report that retrograde signaling by target-derived nerve growth factor (NGF) is necessary for soma-to-axon transcytosis of TrkA receptors in sympathetic neurons, and we define the molecular underpinnings of this positive feedback regulation that enhances neuronal sensitivity to trophic factors. Activated TrkA receptors are retrogradely transported in signaling endosomes from distal axons to cell bodies, where they are inserted on soma surfaces and promote phosphorylation of resident naive receptors, resulting in their internalization. Endocytosed TrkA receptors are then dephosphorylated by PTP1B, an ER-resident protein tyrosine phosphatase, prior to axonal transport. PTP1B inactivation prevents TrkA exit from soma and causes receptor degradation, suggesting a "gatekeeper" mechanism that ensures targeting of inactive receptors to axons to engage with ligand. In mice, PTP1B deletion reduces axonal TrkA levels and attenuates neuron survival and target innervation under limiting NGF (NGF^+/-) conditions.

Roles of innervation in developing and regenerating orofacial tissues

The head is innervated by 12 cranial nerves (I-XII) that regulate its sensory and motor functions. Cranial nerves are composed of sensory, motor, or mixed neuronal populations. Sensory neurons perceive generally somatic sensations such as pressure, pain, and temperature. These neurons are also involved in smell, vision, taste, and hearing. Motor neurons ensure the motility of all muscles and glands. Innervation plays an essential role in the development of the various orofacial structures during embryogenesis. Hypoplastic cranial nerves often lead to abnormal development of their target organs and tissues. For example, Möbius syndrome is a congenital disease characterized by defective innervation (i.e., abducens (VI) and facial (VII) nerves), deafness, tooth anomalies, and cleft palate. Hence, it is obvious that the peripheral nervous system is needed for both development and function of orofacial structures. Nerves have a limited capacity to regenerate. However, neural stem cells, which could be used as sources for neural tissue maintenance and repair, have been found in adult neuronal tissues. Similarly, various adult stem cell populations have been isolated from almost all organs of the human body. Stem cells are tightly regulated by their microenvironment, the stem cell niche. Deregulation of adult stem cell behavior results in the development of pathologies such as tumor formation or early tissue senescence. It is thus essential to understand the factors that regulate the functions and maintenance of stem cells. Yet, the potential importance of innervation in the regulation of stem cells and/or their niches in most organs and tissues is largely unexplored. This review focuses on the potential role of innervation in the development and homeostasis of orofacial structures and discusses its possible association with stem cell populations during tissue repair.

The transcription factor Hmx1 and growth factor receptor activities control sympathetic neurons diversification

The sympathetic nervous system relies on distinct populations of neurons that use noradrenaline or acetylcholine as neurotransmitter. We show that fating of the sympathetic lineage at early stages results in hybrid precursors from which, genetic cell-lineage tracing reveals, all types progressively emerge by principal mechanisms of maintenance, repression and induction of phenotypes. The homeobox transcription factor HMX1 represses Tlx3 and Ret, induces TrkA and maintains tyrosine hydroxylase (Th) expression in precursors, thus driving segregation of the noradrenergic sympathetic fate. Cholinergic sympathetic neurons develop through cross-regulatory interactions between TRKC and RET in precursors, which lead to Hmx1 repression and sustained Tlx3 expression, thereby resulting in failure of TrkA induction and loss of maintenance of Th expression. Our results provide direct evidence for a model in which diversification of noradrenergic and cholinergic sympathetic neurons is based on a principle of cross-repressive functions in which the specific cell fates are directed by an active suppression of the expression of transcription factors and receptors that direct the alternative fate.

Toll-like receptor 4 activation reduces adrenal chromaffin cell excitability through a nuclear factor-κB-dependent pathway

The adrenal medulla contains fenestrated capillaries that allow catecholamines and neuropeptides secreted by adrenal chromaffin cells (ACCs) to readily access the circulation. These capillaries may also allow bacterial products to enter the adrenal medulla and interact with ACCs during infection. One potential mediator of this interaction is toll-like receptor 4 (TLR-4), a pattern-recognition receptor that detects lipopolysaccharide (LPS) from Gram-negative bacteria. Evidence suggests that excitable cells can express TLR-4 and that LPS can modulate important neuronal and endocrine functions. The present study was therefore performed to test the hypothesis that TLR-4 activation by LPS affects ACC excitability and secretory output. RT-PCR revealed that TLR-4, cluster of differentiation 14, myeloid differentiation protein-2, and myeloid-derived factor 88 are expressed within mouse adrenal medullae. TLR-4 immunoreactivity was observed within all tyrosine hydroxylase immunoreactive ACCs. Incubation of isolated ACCs in LPS dose dependently hyperpolarized the resting membrane potential and enhanced large conductance (BK) Ca(2+)-activated K(+) currents. LPS (10 μg/ml) also increased rheobase, decreased the number of action potentials fired at rheobase, and reduced the percentage of ACCs exhibiting spontaneous and anodal break action potentials. Although catecholamine release was unaltered, LPS significantly reduced high-K(+)-stimulated neuropeptide Y release from isolated ACCs. LPS did not alter the excitability of ACCs from TLR-4(-/-) mice. Inhibition of nuclear factor-κB signaling with SC-514 (20 μm) abolished the effects of LPS on ACC excitability. Our findings suggest that LPS acts at TLR-4 to reduce ACC excitability and neuropeptide Y release through an nuclear factor-κB-dependent pathway.

Isoform-specific dephosphorylation of dynamin1 by calcineurin couples neurotrophin receptor endocytosis to axonal growth

Endocytic events are critical for neuronal survival in response to target-derived neurotrophic cues, but whether local axon growth is mediated by endocytosis-dependent signaling mechanisms remains unclear. Here, we report that Nerve Growth Factor (NGF) promotes endocytosis of its TrkA receptors and axon growth by calcineurin-mediated dephosphorylation of the endocytic GTPase dynamin1. Conditional deletion of calcineurin in sympathetic neurons disrupts NGF-dependent innervation of peripheral target tissues. Calcineurin signaling is required locally in sympathetic axons to support NGF-mediated growth in a manner independent of transcription. We show that calcineurin associates with dynamin1 via a PxIxIT interaction motif found only in specific dynamin1 splice variants. PxIxIT-containing dynamin1 isoforms colocalize with surface TrkA receptors, and their phosphoregulation is selectively required for NGF-dependent TrkA internalization and axon growth in sympathetic neurons. Thus, NGF-dependent phosphoregulation of dynamin1 is a critical event coordinating neurotrophin receptor endocytosis and axonal growth.

Regulation of NGF and NT-3 protein expression in peripheral targets by sympathetic input

Nerve growth factor (NGF) and neurotrophin-3 (NT-3) are target-derived proteins that regulate innervating sympathetic neurons. Here, we used western blot analysis to investigate changes in NGF and NT-3 protein in several peripheral tissues following loss of sympathetic input. Following removal of the superior cervical ganglion (SCG), large molecular weight (MW) NGF species, including proNGF-A, were increased in distal intracranial SCG targets, such as pineal gland and extracerebral blood vessels (bv). Mature NGF was a minor species in these tissues and unchanged following sympathectomy. Large MW NGF species also were increased when sympathectomy was followed by in vivo NGF administration. Mature NT-3, which was abundant in controls, was significantly decreased in these targets following sympathetic denervation. The decrease in mature NT-3 was enhanced following NGF administration. The trigeminal ganglion, which provides sensory input to these targets, showed increased NGF, but decreased NT-3, in these treatments, demonstrating that decreased NT-3 at the targets did not result from enhanced NT-3 uptake. Unlike pineal gland and extracerebral bv, the external carotid artery, an extracranial proximal SCG target, showed no change in NGF following denervation, and mature NT-3 was significantly increased. Following NGF administration, NT-3 was significantly decreased. We provide evidence for sympathetic regulation of NGF and NT-3 in peripheral targets and that elevated NGF can depress NT-3. The differential response in distal and proximal adult targets is consistent with the idea that neurons innervating proximal and distal targets may serve different roles in regulating neurotrophin protein. In addition, we conclude that previous ELISA results showing increased NGF protein following sympathetic denervation may have resulted from increases in large MW species, rather than an increase in mature NGF.

Regulation of salivary gland function by autonomic nerves

Oral homeostasis is dependent upon saliva and its content of proteins. Reflex salivary flow occurs at a low 'resting' rate and for short periods of the day more intense taste or chewing stimuli evoke up to ten fold increases in salivation. The secretion of salivary fluid and proteins is controlled by autonomic nerves. All salivary glands are supplied by cholinergic parasympathetic nerves which release acetylcholine that binds to M3 and (to a lesser extent) M1 muscarinic receptors, evoking the secretion of saliva by acinar cells in the endpieces of the salivary gland ductal tree. Most salivary glands also receive a variable innervation from sympathetic nerves which released noradrenaline from which tends to evoke greater release of stored proteins, mostly from acinar cells but also ductal cells. There is some 'cross-talk' between the calcium and cyclic AMP intracellular pathways coupling autonomic stimulation to secretion and salivary protein secretion is augmented during combined stimulation. Other non-adrenergic, non-cholinergic neuropeptides released from autonomic nerves evoke salivary gland secretion and parasympathetically derived vasointestinal peptide, acting through endothelial cell derived nitric oxide, plays a role in the reflex vasodilatation that accompanies secretion. Neuronal type, calcium-activated, soluble nitric oxide within salivary cells appears to play a role in mediating salivary protein secretion in response to autonomimetics. Fluid secretion by salivary glands involves aquaporin 5 and the extent to which the expression of aquaporin 5 on apical acinar cell membranes is upregulated by cholinomimetics remains uncertain. Extended periods of autonomic denervation, liquid diet feeding (reduced reflex stimulation) or duct ligation cause salivary gland atrophy. The latter two are reversible, demonstrating that glands can regenerate provided that the autonomic innervation remains intact. The mechanisms by which nerves integrate with salivary cells during regeneration or during salivary gland development remain to be elucidated.