Developmental and Behavioral Analyses of clarin-2: A Novel Somatosensory Neuron Subtype-Enriched Gene

Abstract

The trigeminal ganglion (TG) is a somatosensory organ that relays stimuli in the head to the hindbrain and spinal cord, and it comprises multiple subtypes of sensory neurons that respond to different somatosensory stimuli and establish distinct neuronal circuits. The Trpa1b subtype of TG sensory neurons (TGSNs) are responsible for sensing noxious chemicals, but the molecular cues that specify the development of this neuronal subtype remain poorly understood. Zebrafish were previously established as a robust model for studying the development of TGSNs due to its small size, translucency, and robust somatosensory behaviors. A previous microarray study in zebrafish found a novel four transmembrane-domain protein, clarin-2, to be enriched in Trpa1b-expressing cells. Nothing is known about the function of clarin-2, but a close homolog, clarin-1, is one of the causative genes for Usher Syndrome Type 3, a disorder characterized by progressive hearing and vision loss. We hypothesize that clarin-2 may play a role in the development and sensory function of TGSNs. To test this hypothesis, we examined the expression of clarin-2 within the TG during development and used clarin-2 knockout (KO) fish to study the genesis and neurite outgrowth of Trpa1b TGSNs. We found that clarin-2 is indeed enriched in a subset of TGSNs but is not required for the morphogenesis of the TG or the specification of nociceptive sensory neurons. Furthermore, axon projections from Trpa1b neurons were normal in clarin-2 KO fish, compared to control siblings. To test whether clarin-2 is required for the function of TGSNs, we tested somatosensory behaviors in larval zebrafish, including chemo-, thermo-, and mechanosensation. Behavioral analyses showed that clarin-2 is not required for the ability of Trpa1b neurons to detect the chemical irritant mustard oil. Additionally, the detection of heat or vibration was not affected in clarin-2 KO fish. Together, these results suggest that although clarin-2 is enriched in a subset of TGSNs, it is not required for the general morphogenesis of TGSNs or for somatosensation.