Single-cell analysis of Rohon-Beard neurons implicates Fgf signaling in axon maintenance and cell survival

Peripheral sensory neurons are a critical part of the nervous system that transmit a multitude of sensory stimuli to the central nervous system. During larval and juvenile stages in zebrafish, this function is mediated by Rohon-Beard somatosensory neurons (RBs). RBs are optically accessible and amenable to experimental manipulation, making them a powerful system for mechanistic investigation of sensory neurons. Previous studies provided evidence that RBs fall into multiple subclasses; however, the number and molecular make up of these potential RB subtypes have not been well defined. Using a single-cell RNA sequencing (scRNA-seq) approach, we demonstrate that larval RBs in zebrafish of unknown sex fall into three, largely non-overlapping classes of neurons. We also show that RBs are molecularly distinct from trigeminal neurons in zebrafish. Cross-species transcriptional analysis indicates that one RB subclass is similar to a mammalian group of A-fiber sensory neurons. Another RB subclass is predicted to sense multiple modalities, including mechanical stimulation and chemical irritants. We leveraged our scRNA-seq data to determine that the fibroblast growth factor (Fgf) pathway is active in RBs. Pharmacological and genetic inhibition of this pathway led to defects in axon maintenance and RB cell death. Moreover, this can be phenocopied by treatment with dovitinib, an FDA-approved Fgf inhibitor with a common side effect of peripheral neuropathy. Importantly, dovitinib-mediated axon loss can be suppressed by loss of Sarm1, a positive regulator of neuronal cell death and axonal injury. This offers a molecular target for future clinical intervention to fight neurotoxic effects of this drug. SIGNIFICANCE STATEMENT Elucidating the signaling pathways and cellular mechanisms that regulate sensory neuron health and maintenance is critical for understanding neuropathy etiologies and exploring therapies. Rohon-Beard (RB) sensory neurons are an accessible in vivo model to explore neurobiological mechanisms of such neuropathies. However, the number and nature of Rohon-Beard subpopulations have not been fully explored. We demonstrate that there are three subpopulations of RBs with unique transcriptional profiles. Furthermore, we find Fgf signaling has a novel role in survival and axonal maintenance of RBs. Our work finds a new molecular pathway for the maintenance of sensory axons and provides the first transcriptionally-based classification of RB neurons. It also sets the stage to explore how Fgf signaling may be involved in peripheral neuropathies.