Single-cell RNAseq analysis of spinal locomotor circuitry in larval zebrafish

Zebrafish as a model to understand extraocular motor neuron diversity

Motor neurons have highly diverse anatomical, functional and molecular features, and differ significantly in their susceptibility in disease. Extraocular motor neurons, residing in the oculomotor, trochlear and abducens cranial nuclei (nIII, nIV and nVI), control eye movements. Recent work has begun to clarify the developmental mechanisms by which functional diversity among extraocular motor neurons arises. However, we know little about the role and consequences of extraocular motor neuron diversity in eye movement control. Here, we highlight recent work investigating the anatomical, functional and molecular features of extraocular motor neurons. Further, we frame hypotheses where studying ocular motor circuits in the larval zebrafish is poised to illuminate the consequences of motor neuron diversity for behavior.

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 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.