INTRODUCTION: Reissner fiber is a rope-like structure composed of microfilaments that originate from the midline floor plate and the anterior roof plate-derived subcommissural organ (SCO). Reissner fiber extends through the brain ventricular system (BVS) and the central canal to the end of the spinal cord. Reissner fiber is found in vertebrates with horizontal body posture. Reissner fiber consists of the giant matricellular protein Scospondin and several auxiliary polypeptides.

AIM(S): We aimed to understand the developmental mechanisms regulating the formation of Reissner fiber.

METHOD(S): We used a combination of the in vitro and vivo immunohistochemistry, light-sheet microscopy, and genetic and toxicological analyses to study Reissner fiber development in zebrafish.

RESULTS: The Reissner fiber develops through two mechanisms. First, the posterior Reissner fiber forms due to secretion from the apical surface of the floor plate. This fiber remains attached to the anterior floor plate (flexural organ); though in the hindbrain it begins to detach from the floor plate soon after formation. Second, the anterior Reissner fiber forms in the cerebral aqueduct cavity through secretion from the SCO. It then attaches to the flexural organ, where it fuses with the posterior fiber. Later, the SCO elongates posterior ward along the fiber trajectory through mechanoelastic stretching. Analysis of mutants affecting the two subunits of the voltage-gated potassium channel Kv2.1 revealed that Kv2.1 regulates BVS and Reissner fiber development. Toxicological analyses demonstrated that Reissner fiber formation depends on Hedgehog and Wnt/β-catenin signaling pathways, as well as cholesterol acting upstream of Kv2.1. Maintaining the embryo in abnormal orientation results in Reissner fiber and SCO deformation.

CONCLUSIONS: The development of the Reissner fiber takes place in two stages by the floor plate and SCO. It depends on multiple internal (developmental) and external (gravity) inputs. Reissner fiber shapes SCO and flexural organ.

FINANCIAL SUPPORT: NCN, OPUS 2020/39/B/NZ3/02729