William Renthal

The striking diversity of mammalian body forms and surfaces, lifestyles, and habitats prompts the question of how each species’ somatosensory neuron repertoire accommodates such diversity. The visual and olfactory systems diversify by gaining and losing sensor/receptor genes and corresponding sensory cell types. Using multiomic single-cell analysis of dorsal root ganglia neurons from thirteen different mammalian species and cross-species functional studies enabled by cell-type-specific enhancer viruses, we show that mammals instead assemble species-specific, functionally distinct somatosensory neuron repertoires from a conserved set of neuron types (orthotypes). Repertoire diversification takes multiple forms: orthotype abundance scales with variable bodily traits like hair follicle density, facilitating relevant behaviors; sensor/receptor expression is shuffled across species, such that orthotypes can detect different stimuli in different species; and orthotypes split into functionally distinct, species-specific subtypes (paratypes). Thus, unlike other sensory systems, the mammalian somatosensory system diversifies through multifarious changes to conserved orthotypes, enabling adaptation to diverse habitats, lifestyles, and body forms.

Peripheral sensory neurons, long appreciated for their role in detecting environmental stimuli, also play an active role in local and systemic immune responses. In response to tissue damage, the coordinated activation of peripheral sensory neurons and immune cells facilitates tissue repair. When dysregulated, however, these neuroimmune interactions can become maladaptive and contribute to chronic disorders such as chronic pain or headache. Recent single-cell transcriptomic studies have uncovered multiple distinct sensory neuron subtypes, each with unique molecular profiles, anatomical niches, and immune functions. In this review, we examine how distinct sensory neuron subtypes coordinate immune responses in barrier and internal tissues, forming cell type– and context-specific neuroimmune circuits. We also discuss how disruption of these circuits contributes to neuroinflammatory diseases such as atopic dermatitis and neuropathic pain. Clarifying these subtype-specific neuroimmune mechanisms offers a path toward more precise and effective therapies that target maladaptive neuroimmune circuitry in multiple disorders.

Chronic pain accounts for nearly half of owner-reported canine euthanasia decisions, yet dogs remain underutilized as a large-animal model for studying pain and developing translational therapeutics. Here, we present a canine dorsal root ganglion (DRG) cell atlas generated from six donors, representing five breeds, both sexes, and three spinal segments. Our dataset comprises 3,026 neurons and 11,734 non-neuronal cells and resolves 15 neuronal subtypes that map cleanly onto A- and C-fiber classes. We further identify eight major non-neuronal subtypes, including glial, vascular, and immune populations and characterize neuronal and non-neuronal expression of physiologically relevant neuropeptides, receptors, and ion channels. We identify region-specific differences in subtype composition between lumbar and sacral DRGs, with transcriptional programs suggestive of enhanced tactile-associated signaling in lumbar DRGs and heightened nociception-associated signaling in sacral DRGs. Cross-species comparisons reveal that canine DRG subtypes are broadly conserved with human and mouse, while also exhibiting canine-specific and canine–human shared molecular features relevant for translation. Together, this atlas serves as a valuable resource for understanding canine sensory neurobiology, comparing DRG organization across mammals, and leveraging dogs as a translational model for pain research and therapeutic development.

Chronic pain conditions are often driven by hyperexcitability of nociceptors, the peripheral sensory neurons that detect noxious stimuli. Recent single-cell transcriptomic studies have begun to clarify the molecular identity of distinct peripheral sensory neuron subtypes, but tools that restrict transgene expression to nociceptors while sparing other dorsal root ganglion (DRG) subtypes remain limited. Here, we combined single-nucleus multi-omic profiling of human and mouse DRG with in vivo AAV enhancer screening to identify cis -regulatory elements that drive biased AAV expression in mouse DRG nociceptors and human iPSC-derived nociceptors. We then validated that an enhancer AAV designed to express K _ir 2.1 preferentially in nociceptors reduces DRG neuronal excitability. Leveraging these multi-omic datasets, we trained a sequence-based model to decode the cis -regulatory logic of nociceptors, enabling both the prioritization of native candidate elements and the design of synthetic enhancers with a range of nociceptor targeting properties. Together, these cross-species multi-omic resources define conserved DRG regulatory programs and provide a viral toolkit for pain research with potential translational applications for patients with refractory pain.

A key concern in early-stage analgesic discovery efforts is the extent to which mechanisms identified in rodents will translate to humans. To evaluate an alternative approach to the use of rodent dissociated DRG neurons for in vitro analyses of nociceptive signaling, we performed a transcriptomic analysis of the HD10.6 human dorsal root ganglion (DRG)-derived immortalized cell line. We conducted RNA-seq on proliferating and mature HD10.6 cells to characterize transcriptional changes associated with maturation. We then compared the transcriptomes of HD10.6 cells and several recently developed lines of human induced pluripotent stem cell-derived sensory neurons (iPSC-SN) to single-nucleus RNA-seq data from human DRGs. HD10.6 cells showed the highest correlation with 3 human sensory neuron subtypes associated with nociception and pruriception. Each of the iPSC-SN lines evaluated showed a distinct pattern of correlation with human sensory neuron subtypes. We identified G protein-coupled receptors (GPCRs) and ion channels that are expressed in both HD10.6 cells and human DRG neurons, as well as numerous genes that are expressed in human DRG but not in rodent, underscoring the need for human sensory neuron in vitro models. Proof-of-concept evaluations of protein kinase A, protein kinase C and Erk signaling provide examples of scalable assays using HD10.6 cells to investigate well-established GPCR signaling pathways. We conclude that HD10.6 cells provide a versatile model for exploring human neuronal signaling mechanisms.

Sensory neurons within the dorsal root ganglion (DRG) are the primary trigger of pain, relaying activity about noxious stimuli from the periphery to the central nervous system; however, targeting DRG neurons for pain management has remained a clinical challenge. Here, we demonstrate the use of lipid nanoparticles (LNPs) for effective intrathecal delivery of small interfering RNA (siRNA) to DRG neurons, achieving potent silencing of the transient receptor potential vanilloid 1 (TRPV1) ion channel that is predominantly expressed in nociceptor sensory neurons. This leads to a reversible interruption of heat-, capsaicin-, and inflammation-induced nociceptive conduction, as observed by behavioral outputs. Our work provides a proof-of-concept for intrathecal siRNA therapy as a novel and selective analgesic modality.

BACKGROUND AND OBJECTIVES: Anatomic features of neuromas have been explored in imaging studies. However, there has been limited research into these features using resected, ex vivo human neuroma specimens. The aim of this study was to investigate the influence that time may have on neuroma growth and size, and the clinical significance of these parameters. METHODS: Patients who underwent neuroma excision between 2022 through 2023 were prospectively included in this study. Neuroma specimens were obtained after operative resection. Standardized neuroma size measurements, expressed as a neuroma-to-nerve ratio (NNR), were conducted with ImageJ software. Pain data (numeric rating scale, 0-10) were prospectively recorded during preoperative evaluation, and patient factors were collected from chart reviews. RESULTS: Fifty terminal neuroma specimens from 31 patients were included, with 94.0% of the neuromas obtained from individuals with amputations. Most neuromas were excised from the lower extremities (n = 44, 88.0%). The neuromas had a median NNR of 2.45, and the median injury to neuroma excision interval was 6.3 years. Larger NNRs were associated with a longer injury to neuroma excision interval and with a smaller native nerve diameter. In addition, sensory nerves were associated with a larger NNR compared with mixed nerves. NNR was not associated with preoperative pain or with anatomical nerve distribution. CONCLUSION: This study suggests that neuromas seem to continue to grow over time and that smaller nerves may form relatively larger neuromas. In addition, sensory nerves develop relatively larger neuromas compared with mixed nerves. Neuroma size does not appear to correlate with pain severity. These findings may stimulate future research efforts and contribute to a better understanding of symptomatic neuroma development.

Sensory neurons in the dorsal root ganglion (DRG) and trigeminal ganglion (TG) are specialized to detect and transduce diverse environmental stimuli to the central nervous system. Single-cell RNA sequencing has provided insights into the diversity of sensory ganglia cell types in rodents, nonhuman primates, and humans, but it remains difficult to compare cell types across studies and species. We thus constructed harmonized atlases of the DRG and TG that describe and facilitate comparison of 18 neuronal and 11 non-neuronal cell types across six species and 31 datasets. We then performed single-cell/nucleus RNA sequencing of DRG from both human and the highly regenerative axolotl and found that the harmonized atlas also improves cell type annotation, particularly of sparse neuronal subtypes. We observed that the transcriptomes of sensory neuron subtypes are broadly similar across vertebrates, but the expression of functionally important neuropeptides and channels can vary notably. The resources presented here can guide future studies in comparative transcriptomics, simplify cell-type nomenclature differences across studies, and help prioritize targets for future analgesic development.

Peripheral sensory neurons in the dorsal root ganglion (DRG) and trigeminal ganglion (TG) are specialized to detect and transduce diverse environmental stimuli including touch, temperature, and pain to the central nervous system. Recent advances in single-cell RNA-sequencing (scRNA-seq) have provided new insights into the diversity of sensory ganglia cell types in rodents, non-human primates, and humans, but it remains difficult to compare transcriptomically defined cell types across studies and species. Here, we built cross-species harmonized atlases of DRG and TG cell types that describe 18 neuronal and 11 non-neuronal cell types across 6 species and 19 studies. We then demonstrate the utility of this harmonized reference atlas by using it to annotate newly profiled DRG nuclei/cells from both human and the highly regenerative axolotl. We observe that the transcriptomic profiles of sensory neuron subtypes are broadly similar across vertebrates, but the expression of functionally important neuropeptides and channels can vary notably. The new resources and data presented here can guide future studies in comparative transcriptomics, simplify cell type nomenclature differences across studies, and help prioritize targets for future pain therapy development.