Cellular complexity of the peripheral nervous system: Insights from single-cell resolution

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TL;DR

This review summarizes how single-cell RNA sequencing has revealed the cellular diversity of the peripheral nervous system (PNS), identifying multiple subtypes of neurons and glial cells in both naïve and injured states. Key findings include the classification of sensory neurons into neurofilament-containing, non-peptidergic nociceptor, and peptidergic nociceptor subtypes, as well as the identification of injury-specific cell states such as repair Schwann cells and novel neuronal clusters marked by Atf3. The clinical significance lies in providing a cellular and molecular basis for developing targeted therapies for peripheral nerve injury and neurological disorders.

Full summary

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**Background:** The peripheral nervous system (PNS) transmits sensory and motor information between the central nervous system and the body. Traditional classification of PNS cells relied on anatomical location, size, or functional response. The advent of single-cell RNA sequencing (scRNA-seq) has enabled molecular-level classification of cellular diversity in the PNS. This review compiles findings from scRNA-seq studies to describe the heterogeneity of neurons and glial cells in the PNS under normal conditions and after nerve injury.

**Methods:** The authors reviewed published studies employing scRNA-seq technologies—including Drop-seq, 10x Genomics, Smart-seq2, and single-nucleus RNA sequencing—applied to PNS tissues from mice, rats, and humans. Key datasets discussed include: Usoskin et al. (2015) sequencing 799 single cells from mouse lumbar DRG with 3,574 ± 2,010 genes per cell; Renthal et al. (2020) analyzing 141,093 DRG nuclei; and Zeisel et al. (2018) surveying nearly half a million single cells from mouse nerve tissues. Studies of development (embryonic day 11.5 to postnatal day 60), injury models (sciatic nerve transection, crush, spinal nerve transection), and cross-species comparisons (mouse, rat, human) were included.

**Key Results:** - **Neuronal diversity:** Peripheral neurons are classified into sensory, sympathetic, and enteric subdivisions. Sensory neurons in DRG are divided into 11 principal types (Usoskin et al., 2015) or 9 subtypes in naïve state (Renthal et al., 2020), including neurofilament-containing (NF), non-peptidergic nociceptors (NP), peptidergic nociceptors (PEP), and TH neurons. Human DRG neurons comprise 15 subtypes, with some human-specific clusters (H4, H9, H12). Trigeminal ganglia neurons show 12–13 distinct subtypes, largely conserved with DRG. - **Glial diversity:** Schwann cells (SCs) are divided into 6 subtypes: proliferating SCs (Mki67), immature SCs (Ngfr), pro-myelinating SCs (Ncmap), myelinating SCs (Mpz), transition SCs (Ncam1), and mature non-myelinating SCs (Ncam1). Satellite glial cells (SGCs) account for ~30% of DRG cells, with markers Kcnj10, Cdh19, Fabp7, and include 4 subtypes in adult DRG. Enteric glial cells (EGCs) include 3–7 subtypes; Gfap⁺ EGCs are essential for intestinal stem cell activity and regeneration. - **Injury responses:** After peripheral nerve injury, neurons upregulate Atf3, Gal, and Sox11, entering a shared injured-neuron transcriptional state. Novel injury-induced neuronal clusters (Atf3/Gfra3/Gal, Atf3/Mrgprd, Atf3/S100b/Gal) appear. Repair Schwann cells (Shh⁺) increase after spinal nerve transection. Macrophage infiltration (Lyz2⁺) increases 7 days post-injury. SGC numbers show little change but upregulate lipid metabolism and immune genes. Endoneurial Pdgfra⁺ nerve mesenchymal cells increase after injury and secrete paracrine factors (Angpt1, Ccl11, Vegfc) promoting axon growth. - **Cross-species conservation:** Mouse, rat, and human SGCs have similar transcriptional profiles, though human SGCs express more ion channels and receptors. Human and mouse DRG neurons show highly conserved genetic signatures, with some human-specific subtypes.

**Clinical Implications:** The detailed cellular atlas of the PNS provides a foundation for understanding peripheral neuropathies, nerve injury, and pain conditions. Identification of injury-specific cell states and regeneration-associated genes (e.g., Atf3) offers potential therapeutic targets for promoting nerve regeneration. The conservation between rodent and human PNS cell types supports the use of rodent models for preclinical studies. Single-cell multi-omics approaches, currently focused on the CNS, should be extended to the PNS to uncover regulatory mechanisms underlying development, regeneration, and disease.