The cellular and molecular basis of somatosensory neuron development

Dendrite morphogenesis in Caenorhabditis elegans

Since the days of Ramón y Cajal, the vast diversity of neuronal and particularly dendrite morphology has been used to catalog neurons into different classes. Dendrite morphology varies greatly and reflects the different functions performed by different types of neurons. Significant progress has been made in our understanding of how dendrites form and the molecular factors and forces that shape these often elaborately sculpted structures. Here, we review work in the nematode Caenorhabditis elegans that has shed light on the developmental mechanisms that mediate dendrite morphogenesis with a focus on studies investigating ciliated sensory neurons and the highly elaborated dendritic trees of somatosensory neurons. These studies, which combine time-lapse imaging, genetics, and biochemistry, reveal an intricate network of factors that function both intrinsically in dendrites and extrinsically from surrounding tissues. Therefore, dendrite morphogenesis is the result of multiple tissue interactions, which ultimately determine the shape of dendritic arbors.

Dermal TRPV1 innervations engage a macrophage- and fibroblast-containing pathway to activate hair growth in mice

Pain, detected by nociceptors, is an integral part of injury, yet whether and how it can impact tissue physiology and recovery remain understudied. Here, we applied chemogenetics in mice to locally activate dermal TRPV1 innervations in naive skin and found that it triggered new regenerative cycling by dormant hair follicles (HFs). This was preceded by rapid apoptosis of dermal macrophages, mediated by the neuropeptide calcitonin gene-related peptide (CGRP). TRPV1 activation also triggered a macrophage-dependent induction of osteopontin (Spp1)-expressing dermal fibroblasts. The neuropeptide CGRP and the extracellular matrix protein Spp1 were required for the nociceptor-triggered hair growth. Finally, we showed that epidermal abrasion injury induced Spp1-expressing dermal fibroblasts and hair growth via a TRPV1 neuron and CGRP-dependent mechanism. Collectively, these data demonstrated a role for TRPV1 nociceptors in orchestrating a macrophage and fibroblast-supported mechanism to promote hair growth and enabling the efficient restoration of this mechano- and thermo-protective barrier after wounding.

Spinal Glycine Receptor Alpha 3 Cells Communicate Sensations of Chemical Itch in Hairy Skin

Glycinergic neurons regulate nociceptive and pruriceptive signaling in the spinal cord, but the identity and role of the glycine-regulated neurons are not fully known. Herein, we have characterized spinal glycine receptor alpha 3 (Glra3) subunit-expressing neurons in Glra3-Cre female and male mice. Glra3-Cre(+) neurons express Glra3, are located mainly in laminae III-VI, and respond to glycine. Chemogenetic activation of spinal Glra3-Cre(+) neurons induced biting/licking, stomping, and guarding behaviors, indicative of both a nociceptive and pruriceptive role for this population. Chemogenetic inhibition did not affect mechanical or thermal responses but reduced behaviors evoked by compound 48/80 and chloroquine, revealing a pruriceptive role for these neurons. Spinal cells activated by compound 48/80 or chloroquine express Glra3, further supporting the phenotype. Retrograde tracing revealed that spinal Glra3-Cre(+) neurons receive input from afferents associated with pain and itch, and dorsal root stimulation validated the monosynaptic input. In conclusion, these results show that spinal Glra3(+) neurons contribute to acute communication of compound 48/80- and chloroquine-induced itch in hairy skin.

The Mas-related G protein-coupled receptor d (Mrgprd) mediates pain hypersensitivity in painful diabetic neuropathy

ABSTRACT

Painful diabetic neuropathy (PDN) is one of the most common and intractable complications of diabetes. Painful diabetic neuropathy is characterized by neuropathic pain accompanied by dorsal root ganglion (DRG) nociceptor hyperexcitability, axonal degeneration, and changes in cutaneous innervation. However, the complete molecular profile underlying the hyperexcitable cellular phenotype of DRG nociceptors in PDN has not been elucidated. This gap in our knowledge is a critical barrier to developing effective, mechanism-based, and disease-modifying therapeutic approaches that are urgently needed to relieve the symptoms of PDN. Using single-cell RNA sequencing of DRGs, we demonstrated an increased expression of the Mas-related G protein-coupled receptor d (Mrgprd) in a subpopulation of DRG neurons in the well-established high-fat diet (HFD) mouse model of PDN. Importantly, limiting Mrgprd signaling reversed mechanical allodynia in the HFD mouse model of PDN. Furthermore, in vivo calcium imaging allowed us to demonstrate that activation of Mrgprd-positive cutaneous afferents that persist in diabetic mice skin resulted in an increased intracellular calcium influx into DRG nociceptors that we assess in vivo as a readout of nociceptors hyperexcitability. Taken together, our data highlight a key role of Mrgprd-mediated DRG neuron excitability in the generation and maintenance of neuropathic pain in a mouse model of PDN. Hence, we propose Mrgprd as a promising and accessible target for developing effective therapeutics currently unavailable for treating neuropathic pain in PDN.

Label-Free Visualization and Morphological Profiling of Neuronal Differentiation and Axonal Degeneration through Quantitative Phase Imaging

Understanding the intricate processes of neuronal growth, degeneration, and neurotoxicity is paramount for unraveling nervous system function and holds significant promise in improving patient outcomes, especially in the context of chemotherapy-induced peripheral neuropathy (CIPN). These processes are influenced by a broad range of entwined events facilitated by chemical, electrical, and mechanical signals. The progress of each process is inherently linked to phenotypic changes in cells. Currently, the primary means of demonstrating morphological changes rely on measurements of neurite outgrowth and axon length. However, conventional techniques for monitoring these processes often require extensive preparation to enable manual or semi-automated measurements. Here, a label-free and non-invasive approach is employed for monitoring neuronal differentiation and degeneration using quantitative phase imaging (QPI). Operating on unlabeled specimens and offering little to no phototoxicity and photobleaching, QPI delivers quantitative maps of optical path length delays that provide an objective measure of cellular morphology and dynamics. This approach enables the visualization and quantification of axon length and other physical properties of dorsal root ganglion (DRG) neuronal cells, allowing greater understanding of neuronal responses to stimuli simulating CIPN conditions. This research paves new avenues for the development of more effective strategies in the clinical management of neurotoxicity.

Potential Roles of Specific Subclasses of Premotor Interneurons in Spinal Cord Function Recovery after Traumatic Spinal Cord Injury in Adults

The differential expression of transcription factors during embryonic development has been selected as the main feature to define the specific subclasses of spinal interneurons. However, recent studies based on single-cell RNA sequencing and transcriptomic experiments suggest that this approach might not be appropriate in the adult spinal cord, where interneurons show overlapping expression profiles, especially in the ventral region. This constitutes a major challenge for the identification and direct targeting of specific populations that could be involved in locomotor recovery after a traumatic spinal cord injury in adults. Current experimental therapies, including electrical stimulation, training, pharmacological treatments, or cell implantation, that have resulted in improvements in locomotor behavior rely on the modulation of the activity and connectivity of interneurons located in the surroundings of the lesion core for the formation of detour circuits. However, very few publications clarify the specific identity of these cells. In this work, we review the studies where premotor interneurons were able to create new intraspinal circuits after different kinds of traumatic spinal cord injury, highlighting the difficulties encountered by researchers, to classify these populations.

In vivo imaging of the neuronal response to spinal cord injury: a narrative review

Deciphering the neuronal response to injury in the spinal cord is essential for exploring treatment strategies for spinal cord injury (SCI). However, this subject has been neglected in part because appropriate tools are lacking. Emerging in vivo imaging and labeling methods offer great potential for observing dynamic neural processes in the central nervous system in conditions of health and disease. This review first discusses in vivo imaging of the mouse spinal cord with a focus on the latest imaging techniques, and then analyzes the dynamic biological response of spinal cord sensory and motor neurons to SCI. We then summarize and compare the techniques behind these studies and clarify the advantages of in vivo imaging compared with traditional neuroscience examinations. Finally, we identify the challenges and possible solutions for spinal cord neuron imaging.

Mineralocorticoid Receptor Antagonism Reduces Inflammatory Pain Measures in Mice Independent of the Receptors on Sensory Neurons

Corticosteroids are commonly used in the treatment of inflammatory low back pain, and their nominal target is the glucocorticoid receptor (GR) to relieve inflammation. They can also have similar potency at the mineralocorticoid receptor (MR). The MR has been shown to be widespread in rodent and human dorsal root ganglia (DRG) neurons and non-neuronal cells, and when MR antagonists are administered during a variety of inflammatory pain models in rats, pain measures are reduced. In this study we selectively knockout (KO) the MR in sensory neurons to determine the role of MR in sensory neurons of the mouse DRG in pain measures as MR antagonism during the local inflammation of the DRG (LID) pain model. We found that MR antagonism using eplerenone reduced evoked mechanical hypersensitivity during LID, but MR KO in paw-innervating sensory neurons only did not. This could be a result of differences between prolonged (MR KO) versus acute (drug) MR block or an indicator that non-neuronal cells in the DRG are driving the effect of MR antagonists. MR KO unmyelinated C neurons are more excitable under normal and inflamed conditions, while MR KO does not affect excitability of myelinated A cells. MR KO in sensory neurons causes a reduction in overall GR mRNA but is protective against reduction of the anti-inflammatory GRα isoform during LID. These effects of MR KO in sensory neurons expanded our understanding of MR's functional role in different neuronal subtypes (A and C neurons), and its interactions with the GR.

Vagal sensory pathway for the gut-brain communication

The communication between the gut and brain is crucial for regulating various essential physiological functions, such as energy balance, fluid homeostasis, immune response, and emotion. The vagal sensory pathway plays an indispensable role in connecting the gut to the brain. Recently, our knowledge of the vagal gut-brain axis has significantly advanced through molecular genetic studies, revealing a diverse range of vagal sensory cell types with distinct peripheral innervations, response profiles, and physiological functions. Here, we review the current understanding of how vagal sensory neurons contribute to gut-brain communication. First, we highlight recent transcriptomic and genetic approaches that have characterized different vagal sensory cell types. Then, we focus on discussing how different subtypes encode numerous gut-derived signals and how their activities are translated into physiological and behavioral regulations. The emerging insights into the diverse cell types and functional properties of vagal sensory neurons have paved the way for exciting future directions, which may provide valuable insights into potential therapeutic targets for disorders involving gut-brain communication.

Human assembloid model of the ascending neural sensory pathway

The ascending somatosensory pathways convey crucial information about pain, touch, itch, and body part movement from peripheral organs to the central nervous system. Despite a significant need for effective therapeutics modulating pain and other somatosensory modalities, clinical translation remains challenging, which is likely related to species-specific features and the lack of in vitro models to directly probe and manipulate this polysynaptic pathway. Here, we established human ascending somatosensory assembloids (hASA)- a four-part assembloid completely generated from human pluripotent stem cells that integrates somatosensory, spinal, diencephalic, and cortical organoids to model the human ascending spinothalamic pathway. Transcriptomic profiling confirmed the presence of key cell types in this circuit. Rabies tracing and calcium imaging showed that sensory neurons connected with dorsal spinal cord projection neurons, which ascending axons further connected to thalamic neurons. Following noxious chemical stimulation, single neuron calcium imaging of intact hASA demonstrated coordinated response, while four-part concomitant extracellular recordings and calcium imaging revealed synchronized activity across the assembloid. Loss of the sodium channel SCN9A, which causes pain insensitivity in humans, disrupted synchrony across the four-part hASA. Taken together, these experiments demonstrate the ability to functionally assemble the essential components of the human sensory pathway. These findings could both accelerate our understanding of human sensory circuits and facilitate therapeutic development.

Transcriptomic Analysis of the Rat Dorsal Root Ganglion After Fracture

In fractures, pain signals are transmitted from the dorsal root ganglion (DRG) to the brain, and the DRG generates efferent signals to the injured bone to participate in the injury response. However, little is known about how this process occurs. We analyzed DRG transcriptome at 3, 7, 14, and 28 days after fracture. We identified the key pathways through KEGG and GO enrichment analysis. We then used IPA analysis to obtain upstream regulators and disease pathways. Finally, we compared the sequencing results with those of nerve injury to identify the unique transcriptome changes in DRG after fracture. We found that the first 14 days after fracture were the main repair response period, the 3rd day was the peak of repair activity, the 14th day was dominated by the stimulus response, and on the 28th day, the repair response had reached a plateau. ECM-receptor interaction, protein digestion and absorption, and the PI3K-Akt signaling pathway were most significantly enriched, which may be involved in repair regeneration, injury response, and pain transmission. Compared with the nerve injury model, DRG after fracture produced specific alterations related to bone repair, and the bone density function was the most widely activated bone-related function. Our results obtained some important genes and pathways in DRG after fracture, and we also summarized the main features of transcriptome function at each time point through functional annotation clustering of GO pathway, which gave us a deeper understanding of the role played by DRG in fracture.

Docking protein 6 (DOK6) selectively docks the neurotrophic signaling transduction to restrain peripheral neuropathy

The appropriate and specific response of nerve cells to various external cues is essential for the establishment and maintenance of neural circuits, and this process requires the proper recruitment of adaptor molecules to selectively activate downstream pathways. Here, we identified that DOK6, a member of the Dok (downstream of tyrosine kinases) family, is required for the maintenance of peripheral axons, and that loss of Dok6 can cause typical peripheral neuropathy symptoms in mice, manifested as impaired sensory, abnormal posture, paw deformities, blocked nerve conduction, and dysmyelination. Furthermore, Dok6 is highly expressed in peripheral neurons but not in Schwann cells, and genetic deletion of Dok6 in peripheral neurons led to typical peripheral myelin outfolding, axon destruction, and hindered retrograde axonal transport. Specifically, DOK6 acts as an adaptor protein for selectivity-mediated neurotrophic signal transduction and retrograde transport for TrkC and Ret but not for TrkA and TrkB. DOK6 interacts with certain proteins in the trafficking machinery and controls their phosphorylation, including MAP1B, Tau and Dynein for axonal transport, and specifically activates the downstream ERK1/2 kinase pathway to maintain axonal survival and homeostasis. This finding provides new clues to potential insights into the pathogenesis and treatment of hereditary peripheral neuropathies and other degenerative diseases.

Loss of G9a does not phenocopy the requirement for Prdm12 in the development of the nociceptive neuron lineage

Prdm12 is an epigenetic regulator expressed in developing and mature nociceptive neurons, playing a key role in their specification during neurogenesis and modulating pain sensation at adulthood. In vitro studies suggested that Prdm12 recruits the methyltransferase G9a through its zinc finger domains to regulate target gene expression, but how Prdm12 interacts with G9a and whether G9a plays a role in Prdm12's functional properties in sensory ganglia remain unknown. Here we report that Prdm12-G9a interaction is likely direct and that it involves the SET domain of G9a. We show that both proteins are largely co-expressed in dorsal root ganglia during early murine development, opening the possibility that G9a plays a role in DRG and may act as a mediator of Prdm12's function in the development of nociceptive sensory neurons. To test this hypothesis, we conditionally inactivated G9a in neural crest using a Wnt1-Cre transgenic mouse line. We found that the specific loss of G9a in the neural crest lineage does not lead to dorsal root ganglia hypoplasia due to the loss of somatic nociceptive neurons nor to the ectopic expression of the visceral determinant Phox2b as observed upon Prdm12 ablation. These findings suggest that Prdm12 function in the initiation of the nociceptive lineage does not critically involves its interaction with G9a.

Design of mechanosensory feedback during undulatory locomotion to enhance speed and stability

Undulatory locomotion relies on the propagation of a wave of excitation in the spinal cord leading to consequential activation of segmental skeletal muscles along the body. Although this process relies on self-generated oscillations of motor circuits in the spinal cord, mechanosensory feedback is crucial to entrain the underlying oscillatory activity and thereby, to enhance movement power and speed. This effect is achieved through directional projections of mechanosensory neurons either sensing stretching or compression of the trunk along the rostrocaudal axis. Different mechanosensory feedback pathways act in concert to shorten and fasten the excitatory wave propagating along the body. While inhibitory mechanosensory cells feedback inhibition on excitatory premotor interneurons and motor neurons, excitatory mechanosensory cells feedforward excitation to premotor excitatory interneurons. Together, diverse mechanosensory cells coordinate the activity of skeletal muscles controlling the head and tail to optimize speed and stabilize balance during fast locomotion.

Kinesin family member 2A gates nociception

Nociceptive axons undergo remodeling as they innervate their targets during development and in response to environmental insults and pathological conditions. How is nociceptive morphogenesis regulated? Here, we show that the microtubule destabilizer kinesin family member 2A (Kif2a) is a key regulator of nociceptive terminal structures and pain sensitivity. Ablation of Kif2a in sensory neurons causes hyperinnervation and hypersensitivity to noxious stimuli in young adult mice, whereas touch sensitivity and proprioception remain unaffected. Computational modeling predicts that structural remodeling is sufficient to explain the phenotypes. Furthermore, Kif2a deficiency triggers a transcriptional response comprising sustained upregulation of injury-related genes and homeostatic downregulation of highly specific channels and receptors at the late stage. The latter effect can be predicted to relieve the hyperexcitability of nociceptive neurons, despite persisting morphological aberrations, and indeed correlates with the resolution of pain hypersensitivity. Overall, we reveal a critical control node defining nociceptive terminal structure, which is regulating nociception.

Skin-type-dependent development of murine mechanosensory neurons

Mechanosensory neurons innervating the skin underlie our sense of touch. Fast-conducting, rapidly adapting mechanoreceptors innervating glabrous (non-hairy) skin form Meissner corpuscles, while in hairy skin, they associate with hair follicles, forming longitudinal lanceolate endings. How mechanoreceptors develop axonal endings appropriate for their skin targets is unknown. We report that mechanoreceptor morphologies across different skin regions are indistinguishable during early development but diverge post-natally, in parallel with skin maturation. Neurons terminating along the glabrous and hairy skin border exhibit hybrid morphologies, forming both Meissner corpuscles and lanceolate endings. Additionally, molecular profiles of neonatal glabrous and hairy skin-innervating neurons largely overlap. In mouse mutants with ectopic glabrous skin, mechanosensory neurons form end-organs appropriate for the altered skin type. Finally, BMP5 and BMP7 are enriched in glabrous skin, and signaling through type I bone morphogenetic protein (BMP) receptors in neurons is critical for Meissner corpuscle morphology. Thus, mechanoreceptor morphogenesis is flexibly instructed by target tissues.

Activity-dependent development of the body's touch receptors

We report a role for activity in the development of the primary sensory neurons that detect touch. Genetic deletion of Piezo2, the principal mechanosensitive ion channel in somatosensory neurons, caused profound changes in the formation of mechanosensory end organ structures and altered somatosensory neuron central targeting. Single cell RNA sequencing of Piezo2 conditional mutants revealed changes in gene expression in the sensory neurons activated by light mechanical forces, whereas other neuronal classes were less affected. To further test the role of activity in mechanosensory end organ development, we genetically deleted the voltage-gated sodium channel Nav1.6 (Scn8a) in somatosensory neurons throughout development and found that Scn8a mutants also have disrupted somatosensory neuron morphologies and altered electrophysiological responses to mechanical stimuli. Together, these findings indicate that mechanically evoked neuronal activity acts early in life to shape the maturation of the mechanosensory end organs that underlie our sense of gentle touch.

Epidural Injection of Harpagoside for the Recovery of Rats with Lumbar Spinal Stenosis

Epidural administration is the leading therapeutic option for the management of pain associated with lumbar spinal stenosis (LSS), which is characterized by compression of the nerve root due to narrowing of the spinal canal. Corticosteroids are effective in alleviating LSS-related pain but can lead to complications with long-term use. Recent studies have focused on identifying promising medications administered epidurally to affected spinal regions. In this study, we aimed to investigate the effectiveness of harpagoside (HAS) as an epidural medication in rats with LSS. HAS at various concentrations was effective for neuroprotection against ferrous sulfate damage and consequent promotion of axonal outgrowth in primary spinal cord neurons. When two concentrations of HAS (100 and 200 μg/kg) were administered to the rat LSS model via the epidural space once a day for 4 weeks, the inflammatory responses around the silicone block used for LSS were substantially reduced. Consistently, pain-related factors were significantly suppressed by the epidural administration of HAS. The motor functions of rats with LSS significantly improved. These findings suggest that targeted delivery of HAS directly to the affected area via epidural injection holds promise as a potential treatment option for the recovery of patients with LSS.

Intrinsic control of DRG sensory neuron diversification by Pten

Phosphatase and tensin homolog (PTEN) modulates intracellular survival and differentiation signaling pathways downstream of neurotrophin receptors in the developing peripheral nervous system (PNS). Although well-studied in the context of brain development, our understanding of the in vivo role of PTEN in the PNS is limited to models of neuropathic pain and nerve injury. Here, we assessed how alterations in PTEN signaling affects the development of peripheral somatosensory circuits. We found that sensory neurons within the dorsal root ganglia (DRG) in Pten heterozygous ( Pten Het ) mice exhibit defects in neuronal subtype diversification. Abnormal DRG differentiation in Pten Het mice arises early in development, with subsets of neurons expressing both progenitor and neuronal markers. DRGs in Pten Het mice show dysregulation of both mTOR and GSK-3β signaling pathways downstream of PTEN. Finally, we show that mice with an autism-associated mutation in Pten ( Pten Y68H/+ ) show abnormal DRG development. Thus, we have discovered a crucial role for PTEN signaling in the intrinsic diversification of primary sensory neuron populations in the DRG during development.

PTBP1 regulates injury responses and sensory pathways in adult peripheral neurons

Polypyrimidine tract binding protein 1 (PTBP1) is thought to be expressed only at embryonic stages in central neurons. Its down-regulation triggers neuronal differentiation in precursor and non-neuronal cells, an approach recently tested for generation of neurons de novo for amelioration of neurodegenerative disorders. Moreover, PTBP1 is replaced by its paralog PTBP2 in mature central neurons. Unexpectedly, we found that both proteins are coexpressed in adult sensory and motor neurons, with PTBP2 restricted mainly to the nucleus, while PTBP1 also shows axonal localization. Levels of axonal PTBP1 increased markedly after peripheral nerve injury, and it associates in axons with mRNAs involved in injury responses and nerve regeneration, including importin β1 (KPNB1) and RHOA. Perturbation of PTBP1 affects local translation in axons, nociceptor neuron regeneration and both thermal and mechanical sensation. Thus, PTBP1 has functional roles in adult axons. Hence, caution is required before considering targeting of PTBP1 for therapeutic purposes.

γ-Protocadherins control synapse formation and peripheral branching of touch sensory neurons

Light touch sensation begins with activation of low-threshold mechanoreceptor (LTMR) endings in the skin and propagation of their signals to the spinal cord and brainstem. We found that the clustered protocadherin gamma (Pcdhg) gene locus, which encodes 22 cell-surface homophilic binding proteins, is required in somatosensory neurons for normal behavioral reactivity to a range of tactile stimuli. Developmentally, distinct Pcdhg isoforms mediate LTMR synapse formation through neuron-neuron interactions and peripheral axonal branching through neuron-glia interactions. The Pcdhgc3 isoform mediates homophilic interactions between sensory axons and spinal cord neurons to promote synapse formation in vivo and is sufficient to induce postsynaptic specializations in vitro. Moreover, loss of Pcdhgs and somatosensory synaptic inputs to the dorsal horn leads to fewer corticospinal synapses on dorsal horn neurons. These findings reveal essential roles for Pcdhg isoform diversity in somatosensory neuron synapse formation, peripheral axonal branching, and stepwise assembly of central mechanosensory circuitry.

EGR4 is critical for cell-fate determination and phenotypic maintenance of geniculate ganglion neurons underlying sweet and umami taste

The sense of taste starts with activation of receptor cells in taste buds by chemical stimuli which then communicate this signal via innervating oral sensory neurons to the CNS. The cell bodies of oral sensory neurons reside in the geniculate ganglion (GG) and nodose/petrosal/jugular ganglion. The geniculate ganglion contains two main neuronal populations: BRN3A+ somatosensory neurons that innervate the pinna and PHOX2B+ sensory neurons that innervate the oral cavity. While much is known about the different taste bud cell subtypes, considerably less is known about the molecular identities of PHOX2B+ sensory subpopulations. In the GG, as many as 12 different subpopulations have been predicted from electrophysiological studies, while transcriptional identities exist for only 3 to 6. Importantly, the cell fate pathways that diversify PHOX2B+ oral sensory neurons into these subpopulations are unknown. The transcription factor EGR4 was identified as being highly expressed in GG neurons. EGR4 deletion causes GG oral sensory neurons to lose their expression of PHOX2B and other oral sensory genes and up-regulate BRN3A. This is followed by a loss of chemosensory innervation of taste buds, a loss of type II taste cells responsive to bitter, sweet, and umami stimuli, and a concomitant increase in type I glial-like taste bud cells. These deficits culminate in a loss of nerve responses to sweet and umami taste qualities. Taken together, we identify a critical role of EGR4 in cell fate specification and maintenance of subpopulations of GG neurons, which in turn maintain the appropriate sweet and umami taste receptor cells.

Pain sensitivity related to gamma oscillation of parvalbumin interneuron in primary somatosensory cortex in Dync1i1-/- mice

Cytoplasmic dynein is an important intracellular motor protein that plays an important role in neuronal growth, axonal polarity formation, dendritic differentiation, and dendritic spine development among others. The intermediate chain of dynein, encoded by Dync1i1, plays a vital role in the dynein complex. Therefore, we assessed the behavioral and related neuronal activities in mice with dync1i1 gene knockout. Neuronal activities in primary somatosensory cortex were recorded by in vivo electrophysiology and manipulated by optogenetic and chemogenetics. Nociception of mechanical, thermal, and cold pain in Dync1i1^-/- mice were impaired. The activities of parvalbumin (PV) interneurons and gamma oscillation in primary somatosensory were also impaired when exposed to mechanical nociceptive stimulation. This neuronal dysfunction was rescued by optogenetic activation of PV neurons in Dync1i1^-/- mice, and mimicked by suppressing PV neurons using chemogenetics in WT mice. Impaired pain sensations in Dync1i1^-/- mice were correlated with impaired gamma oscillations due to a loss of interneurons, especially the PV type. This genotype-driven approach revealed an association between impaired pain sensation and cytoplasmic dynein complex.

Cardiovascular Brain Circuits

The cardiovascular system is hardwired to the brain via multilayered afferent and efferent polysynaptic axonal connections. Two major anatomically and functionally distinct though closely interacting subcircuits within the cardiovascular system have recently been defined: The artery-brain circuit and the heart-brain circuit. However, how the nervous system impacts cardiovascular disease progression remains poorly understood. Here, we review recent findings on the anatomy, structures, and inner workings of the lesser-known artery-brain circuit and the better-established heart-brain circuit. We explore the evidence that signals from arteries or the heart form a systemic and finely tuned cardiovascular brain circuit: afferent inputs originating in the arterial tree or the heart are conveyed to distinct sensory neurons in the brain. There, primary integration centers act as hubs that receive and integrate artery-brain circuit-derived and heart-brain circuit-derived signals and process them together with axonal connections and humoral cues from distant brain regions. To conclude the cardiovascular brain circuit, integration centers transmit the constantly modified signals to efferent neurons which transfer them back to the cardiovascular system. Importantly, primary integration centers are wired to and receive information from secondary brain centers that control a wide variety of brain traits encoded in engrams including immune memory, stress-regulating hormone release, pain, reward, emotions, and even motivated types of behavior. Finally, we explore the important possibility that brain effector neurons in the cardiovascular brain circuit network connect efferent signals to other peripheral organs including the immune system, the gut, the liver, and adipose tissue. The enormous recent progress vis-à-vis the cardiovascular brain circuit allows us to propose a novel neurobiology-centered cardiovascular disease hypothesis that we term the neuroimmune cardiovascular circuit hypothesis.

Representation and control of pain and itch by distinct prefrontal neural ensembles

Pain and itch are two closely related but essentially distinct sensations that elicit different behavioral responses. However, it remains mysterious how pain and itch information is encoded in the brain to produce differential perceptions. Here, we report that nociceptive and pruriceptive signals are separately represented and processed by distinct neural ensembles in the prelimbic (PL) subdivision of the medial prefrontal cortex (mPFC) in mice. Pain- and itch-responsive cortical neural ensembles were found to significantly differ in electrophysiological properties, input-output connectivity profiles, and activity patterns to nociceptive or pruriceptive stimuli. Moreover, these two groups of cortical neural ensembles oppositely modulate pain- or itch-related sensory and emotional behaviors through their preferential projections to specific downstream regions such as the mediodorsal thalamus (MD) and basolateral amygdala (BLA). These findings uncover separate representations of pain and itch by distinct prefrontal neural ensembles and provide a new framework for understanding somatosensory information processing in the brain.

High-fidelity encoding of mechanostimuli by tactile food-sensing neurons requires an ensemble of ion channels

The nematode C. elegans uses mechanosensitive neurons to detect bacteria, which are food for worms. These neurons release dopamine to suppress foraging and promote dwelling. Through a screen of genes highly expressed in dopaminergic food-sensing neurons, we identify a K2P-family potassium channel-TWK-2-that damps their activity. Strikingly, loss of TWK-2 restores mechanosensation to neurons lacking the NOMPC-like channel transient receptor potential 4 (TRP-4), which was thought to be the primary mechanoreceptor for tactile food sensing. The alternate mechanoreceptor mechanism uncovered by TWK-2 mutation requires three Deg/ENaC channel subunits: ASIC-1, DEL-3, and UNC-8. Analysis of cell-physiological responses to mechanostimuli indicates that TRP and Deg/ENaC channels work together to set the range of analog encoding of stimulus intensity and to improve signal-to-noise characteristics and temporal fidelity of food-sensing neurons. We conclude that a specialized mechanosensory modality-tactile food sensing-emerges from coordination of distinct force-sensing mechanisms housed in one type of sensory neuron.

E26 transformation-specific transcription variant 5 in development and cancer: modification, regulation and function

E26 transformation-specific (ETS) transcription variant 5 (ETV5), also known as ETS-related molecule (ERM), exerts versatile functions in normal physiological processes, including branching morphogenesis, neural system development, fertility, embryonic development, immune regulation, and cell metabolism. In addition, ETV5 is repeatedly found to be overexpressed in multiple malignant tumors, where it is involved in cancer progression as an oncogenic transcription factor. Its roles in cancer metastasis, proliferation, oxidative stress response and drug resistance indicate that it is a potential prognostic biomarker, as well as a therapeutic target for cancer treatment. Post-translational modifications, gene fusion events, sophisticated cellular signaling crosstalk and non-coding RNAs contribute to the dysregulation and abnormal activities of ETV5. However, few studies to date systematically summarized the role and molecular mechanisms of ETV5 in benign diseases and in oncogenic progression. In this review, we specify the molecular structure and post-translational modifications of ETV5. In addition, its critical roles in benign and malignant diseases are summarized to draw a panorama for specialists and clinicians. The updated molecular mechanisms of ETV5 in cancer biology and tumor progression are delineated. Finally, we prospect the further direction of ETV5 research in oncology and its potential translational applications in the clinic.

High-resolution mapping of sensory fibers at the healthy and post-myocardial infarct whole transgenic hearts

The sensory nervous system is critical to maintain cardiac function. As opposed to efferent innervation, less is known about cardiac afferents. For this, we mapped the VGLUT2-expressing cardiac afferent fibers of spinal and vagal origin by using the VGLUT2::tdTomato double transgenic mouse as an approach to visualize the whole hearts both at the dorsal and ventral sides. For comparison, we colabeled mixed-sex transgenic hearts with either TUJ1 protein for global cardiac innervation or tyrosine hydroxylase for the sympathetic network at the healthy state or following ischemic injury. Interestingly, the nerve density for global and VGLUT2-expressing afferents was found significantly higher on the dorsal side compared to the ventral side. From the global nerve innervation detected by TUJ1 immunoreactivity, VGLUT2 afferent innervation was detected to be 15-25% of the total network. The detailed characterization of both the atria and the ventricles revealed a remarkable diversity of spinal afferent nerve ending morphologies of flower sprays, intramuscular endings, and end-net branches that innervate distinct anatomical parts of the heart. Using this integrative approach in a chronic myocardial infarct model, we showed a significant increase in hyperinnervation in the form of axonal sprouts for cardiac afferents at the infarct border zone, as well as denervation at distal sites of the ischemic area. The functional and physiological consequences of the abnormal sensory innervation remodeling post-ischemic injury should be further evaluated in future studies regarding their potential contribution to cardiac dysfunction.

Recent advances in understanding neuronal diversity and neural circuit complexity across different brain regions using single-cell sequencing

Neural circuits are characterized as interconnecting neuron networks connected by synapses. Some kinds of gene expression and/or functional changes of neurons and synaptic connections may result in aberrant neural circuits, which has been recognized as one crucial pathological mechanism for the onset of many neurological diseases. Gradual advances in single-cell sequencing approaches with strong technological advantages, as exemplified by high throughput and increased resolution for live cells, have enabled it to assist us in understanding neuronal diversity across diverse brain regions and further transformed our knowledge of cellular building blocks of neural circuits through revealing numerous molecular signatures. Currently published transcriptomic studies have elucidated various neuronal subpopulations as well as their distribution across prefrontal cortex, hippocampus, hypothalamus, and dorsal root ganglion, etc. Better characterization of brain region-specific circuits may shed light on new pathological mechanisms involved and assist in selecting potential targets for the prevention and treatment of specific neurological disorders based on their established roles. Given diverse neuronal populations across different brain regions, we aim to give a brief sketch of current progress in understanding neuronal diversity and neural circuit complexity according to their locations. With the special focus on the application of single-cell sequencing, we thereby summarize relevant region-specific findings. Considering the importance of spatial context and connectivity in neural circuits, we also discuss a few published results obtained by spatial transcriptomics. Taken together, these single-cell sequencing data may lay a mechanistic basis for functional identification of brain circuit components, which links their molecular signatures to anatomical regions, connectivity, morphology, and physiology. Furthermore, the comprehensive characterization of neuron subtypes, their distributions, and connectivity patterns via single-cell sequencing is critical for understanding neural circuit properties and how they generate region-dependent interactions in different context.

Molecular identity of proprioceptor subtypes innervating different muscle groups in mice

The precise execution of coordinated movements depends on proprioception, the sense of body position in space. However, the molecular underpinnings of proprioceptive neuron subtype identities are not fully understood. Here we used a single-cell transcriptomic approach to define mouse proprioceptor subtypes according to the identity of the muscle they innervate. We identified and validated molecular signatures associated with proprioceptors innervating back (Tox, Epha3), abdominal (C1ql2), and hindlimb (Gabrg1, Efna5) muscles. We also found that proprioceptor muscle identity precedes acquisition of receptor character and comprise programs controlling wiring specificity. These findings indicate that muscle-type identity is a fundamental aspect of proprioceptor subtype differentiation that is acquired during early development and includes molecular programs involved in the control of muscle target specificity.

A role for axon-glial interactions and Netrin-G1 signaling in the formation of low-threshold mechanoreceptor end organs

Low-threshold mechanoreceptors (LTMRs) and their cutaneous end organs convert light mechanical forces acting on the skin into electrical signals that propagate to the central nervous system. In mouse hairy skin, hair follicle-associated longitudinal lanceolate complexes, which are end organs comprising LTMR axonal endings that intimately associate with terminal Schwann cell (TSC) processes, mediate LTMR responses to hair deflection and skin indentation. Here, we characterized developmental steps leading to the formation of Aβ rapidly adapting (RA)-LTMR and Aδ-LTMR lanceolate complexes. During early postnatal development, Aβ RA-LTMRs and Aδ-LTMRs extend and prune cutaneous axonal branches in close association with nascent TSC processes. Netrin-G1 is expressed in these developing Aβ RA-LTMR and Aδ-LTMR lanceolate endings, and Ntng1 ablation experiments indicate that Netrin-G1 functions in sensory neurons to promote lanceolate ending elaboration around hair follicles. The Netrin-G ligand (NGL-1), encoded by Lrrc4c, is expressed in TSCs, and ablation of Lrrc4c partially phenocopied the lanceolate complex deficits observed in Ntng1 mutants. Moreover, NGL-1-Netrin-G1 signaling is a general mediator of LTMR end organ formation across diverse tissue types demonstrated by the fact that Aβ RA-LTMR endings associated with Meissner corpuscles and Pacinian corpuscles are also compromised in the Ntng1 and Lrrc4c mutant mice. Thus, axon-glia interactions, mediated in part by NGL-1-Netrin-G1 signaling, promote LTMR end organ formation.

Neurosensory development of the four brainstem-projecting sensory systems and their integration in the telencephalon

Somatosensory, taste, vestibular, and auditory information is first processed in the brainstem. From the brainstem, the respective information is relayed to specific regions within the cortex, where these inputs are further processed and integrated with other sensory systems to provide a comprehensive sensory experience. We provide the organization, genetics, and various neuronal connections of four sensory systems: trigeminal, taste, vestibular, and auditory systems. The development of trigeminal fibers is comparable to many sensory systems, for they project mostly contralaterally from the brainstem or spinal cord to the telencephalon. Taste bud information is primarily projected ipsilaterally through the thalamus to reach the insula. The vestibular fibers develop bilateral connections that eventually reach multiple areas of the cortex to provide a complex map. The auditory fibers project in a tonotopic contour to the auditory cortex. The spatial and tonotopic organization of trigeminal and auditory neuron projections are distinct from the taste and vestibular systems. The individual sensory projections within the cortex provide multi-sensory integration in the telencephalon that depends on context-dependent tertiary connections to integrate other cortical sensory systems across the four modalities.

Etv4 regulates nociception by controlling peptidergic sensory neuron development and peripheral tissue innervation

The perception of noxious environmental stimuli by nociceptive sensory neurons is an essential mechanism for the prevention of tissue damage. Etv4 is a transcriptional factor expressed in most nociceptors in dorsal root ganglia (DRG) during the embryonic development. However, its physiological role remains unclear. Here, we show that Etv4 ablation results in defects in the development of the peripheral peptidergic projections in vivo, and in deficits in axonal elongation and growth cone morphology in cultured sensory neurons in response to NGF. From a mechanistic point of view, our findings reveal that NGF regulates Etv4-dependent gene expression of molecules involved in extracellular matrix (ECM) remodeling. Etv4-null mice were less sensitive to noxious heat stimuli and chemical pain, and this behavioral phenotype correlates with a significant reduction in the expression of the pain-transducing ion channel TRPV1 in mutant mice. Together, our data demonstrate that Etv4 is required for the correct innervation and function of peptidergic sensory neurons, regulating a transcriptional program that involves molecules associated with axonal growth and pain transduction.

Development of a polyacrylamide/chitosan composite hydrogel conduit containing synergistic cues of elasticity and topographies for promoting peripheral nerve regeneration

Substrate elasticity and topographical guidance are crucial factors for regulating tissue regeneration, but the synergistic effects of both cues on peripheral nerve regeneration are still unclear. In this paper, polyacrylamide/chitosan (PAM/CS) composite hydrogels with synergistic characteristics of elasticity and morphology were prepared using in situ free-radical polymerization and micro-molding. The physicochemical properties of hydrogels were characterized, and the effect on peripheral nerve regeneration was systematically evaluated via in vitro and in vivo experiments, respectively. The in vitro experiments showed that on a PAM/CS composite hydrogel with an elastic modulus of 5.822 kPa/8.41 kPa and a surface groove width of 30 μm, the dorsal root ganglion (DRG) neurite had a strong growth ability and better-oriented status. The samples were taken from each group at 2 and 12 weeks after bridging rabbit sciatic nerve defects with a PAM/CS composite hydrogel conduit. General observation of the rabbit body and transplanted nerve, nerve electro-physiological examination, muscle wet weight recovery rate detection and comparison, observation of sciatic nerve frozen section immunofluorescence staining and myelinated nerve fiber recovery rate comparison were used to evaluate the effect of nerve transplantation. The elastic modulus of 8.41 kPa and groove width of 30 μm were similar to those of the autograft group. At the same time, the signaling pathways, including the focal adhesion markers vinculin, p-FAK, and Rho A protein, referring to axon adhesion and extension, were initially revealed. In summary, our developed hydrogel implants containing synergistic cues of elasticity and topographies may provide a new and effective strategy for the treatment of peripheral nerve injury in the future.

Profile of dorsal root ganglion neurons: study of oxytocin expression

Although dorsal root ganglion (DRG) neurons have been so far classified according to the difference in their fibers (Aβ, Aδ, and C), this classification should be further subdivided according to gene expression patterns. We focused on oxytocin (OXT) and its related receptors, because OXT plays a local role in DRG neurons. We measured the mRNA levels of OXT, OXT receptor (OXTR), vasopressin V1a receptor (V1aR), transient receptor potential cation channel subfamily V member 1 (TRPV1), and piezo-type mechanosensitive ion channel component 2 (Piezo2) in single DRG neurons by using real-time PCR, and then performed a cluster analysis. According to the gene expression patterns, DRG neurons were classified into 4 clusters: Cluster 1 was characterized mainly by Piezo2, Cluster 2 by TRPV1, Cluster 4 by OXTR, and neurons in Cluster 3 did not express any of the target genes. The cell body diameter of OXT-expressing neurons was significantly larger in Cluster 1 than in Cluster 2. These results suggest that OXT-expressing DRG neurons with small cell bodies (Cluster 2) and large cell bodies (Cluster 1) probably correspond to C-fiber neurons and Aβ-fiber neurons, respectively. Furthermore, the OXT-expressing neurons contained not only TRPV1 but also Piezo2, suggesting that OXT may be released by mechanical stimulation regardless of nociception. Thus, mechanoreception and nociception themselves may induce the autocrine/paracrine function of OXT in the DRG, contributing to alleviation of pain.

Generation and characterization of hair-bearing skin organoids from human pluripotent stem cells

Human skin uses millions of hairs and glands distributed across the body surface to function as an external barrier, thermoregulator and stimuli sensor. The large-scale generation of human skin with these appendages would be beneficial, but is challenging. Here, we describe a detailed protocol for generating hair-bearing skin tissue entirely from a homogeneous population of human pluripotent stem cells in a three-dimensional in vitro culture system. Defined culture conditions are used over a 2-week period to induce differentiation of pluripotent stem cells to surface ectoderm and cranial neural crest cells, which give rise to the epidermis and dermis, respectively, in each organoid unit. After 60 d of incubation, the skin organoids produce hair follicles. By day ~130, the skin organoids reach full complexity and contain stratified skin layers, pigmented hair follicles, sebaceous glands, Merkel cells and sensory neurons, recapitulating the cell composition and architecture of fetal skin tissue at week 18 of gestation. Skin organoids can be maintained in culture using this protocol for up to 150 d, enabling the organoids to be used to investigate basic skin biology, model disease and, further, reconstruct or regenerate skin tissue.

The mechanical forces that shape our senses

Developing organs are shaped, in part, by physical interaction with their environment in the embryo. In recent years, technical advances in live-cell imaging and material science have greatly expanded our understanding of the mechanical forces driving organ formation. Here, we provide a broad overview of the types of forces generated during embryonic development and then focus on a subset of organs underlying our senses: the eyes, inner ears, nose and skin. The epithelia in these organs emerge from a common origin: the ectoderm germ layer; yet, they arrive at unique and complex forms over developmental time. We discuss exciting recent animal studies that show a crucial role for mechanical forces in, for example, the thickening of sensory placodes, the coiling of the cochlea and the lengthening of hair. Finally, we discuss how microfabricated organoid systems can now provide unprecedented insights into the physical principles of human development.

Nociceptors: Their Role in Body’s Defenses, Tissue Specific Variations and Anatomical Update

The human body is constantly under the influence of numerous pathological factors: both external and internal. These factors can be potentially harmful and are perceived as such with a specialized nervous system subunit: the nociceptive system. The functional unit of the nociceptive system is the nociceptor. Recent studies have shown that nociceptors play a crucial role in maintaining of defensive homeostasis (responsive, immune, behavioral). Nociceptors respond to potentially harmful stimuli within viscera, bones, muscles, skin and specialized sensory organs. They function as complex predictors of harm through formation of pain stimulus. Their function and structures vary within different tissues. This variability reflects the anatomical and pathological peculiarities of varying tissues. Nociceptors play a significant role in adaptive, protective and behavioral reactions. Their functional capabilities and vast spread throughout the body make them the main units of the body’s defense system, allowing us to interact with the inner and outer environments.

A functional subdivision within the somatosensory system and its implications for pain research

Somatosensory afferents are traditionally classified by soma size, myelination, and their response specificity to external and internal stimuli. Here, we propose the functional subdivision of the nociceptive somatosensory system into two branches. The exteroceptive branch detects external threats and drives reflexive-defensive reactions to prevent or limit injury. The interoceptive branch senses the disruption of body integrity, produces tonic pain with strong aversive emotional components, and drives self-caring responses toward to the injured region to reduce suffering. The central thesis behind this functional subdivision comes from a reflection on the dilemma faced by the pain research field, namely, the use of reflexive-defensive behaviors as surrogate assays for interoceptive tonic pain. The interpretation of these assays is now being challenged by the discovery of distinct but interwoven circuits that drive exteroceptive versus interoceptive types of behaviors, with the conflation of these two components contributing partially to the poor translation of therapies from preclinical studies.