Urp1 and Urp2 act redundantly to maintain spine shape in zebrafish larvae.

Urp1 and Urp2 are two neuropeptides, members of the Urotensin 2 family, that have been recently involved in the control of body axis morphogenesis in zebrafish. They are produced by a population of sensory spinal neurons, called cerebrospinal fluid contacting neurons (CSF-cNs), under the control of signals relying on the Reissner fiber, an extracellular thread bathing in the CSF. Here, we have investigated further the function of Urp1 and Urp2 (Urp1/2) in body axis formation and maintenance. We showed that urp1;urp2 double mutants develop strong body axis defects during larval growth, revealing the redundancy between the two neuropeptides. These defects were similar to those previously reported in uts2r3 mutants. We observed that this phenotype is not associated with bone formation defects nor with increased inflammation status but, by using specific inhibitors, we found that the action of Urp1/2 depends on myosin II contraction. Finally, we provide evidence that while the Urp1/2 signaling is functioning during larval growth but is dispensable for embryonic development. Taken together, our results show that Urp1/2 signaling is required in larvae to promote correct vertebral body axis, most likely by regulating muscle tone.