Diving into the zebrafish locomotor network : A study on dI6 interneurons

Abstract: The generation and coordination of locomotion comprise of inhibitory and excitatory interneurons that govern motor neuron output to muscles. This neuronal network is known as the central pattern generator (CPG). Spinal inhibitory interneurons that originate from the dI6 population play a crucial role in the coordination of locomotor output. This thesis provides new insights into the role of two subsets of interneurons within the dI6 population, marked by the expression of Doublesex and mab-3 related transcription factor 3 (dmrt3a) or Wilms tumor 1 (wt1a). We showed that dmrt3a neurons play an important role during zebrafish locomotion by using a transgenic line lacking dmrt3a expression. The absence of Dmrt3a generated acceleration and coordination problems in larvae and reduced the maximum speed in juveniles. Analysis of the transcriptome of the dmrt3a subpopulation in both mice and zebrafish established their characteristics as well as revealed unique markers for novel dI6 subpopulations. The knock-down of Wt1a protein resulted in impaired left-right alternation and a reduction of fast swims. On a cellular level, we found alterations in dmrt3a-, wt1a- and evx2- interneuron composition, indicating a link between the two dI6 populations and changes in their fate assignments affecting cell type composition within the locomotor network. Moreover, we linked the absence of specific subtypes of dI6 interneurons with the observed locomotor phenotype in knock-down animals.While searching for unique markers genes within dmrt3a-expressing interneurons, we observed calb2b (encoding for calretinin) expression in one specific subpopulation. Following the generation of a calb2b transgenic line, we found it expressed in interneurons and motor neurons. Knock-down of calretinin generated a development disorder of dmrt3a interneurons and motor neurons, leading to a disruption of the left-right alternation and escape responses performance. Together, the work presented here provides new cellular, molecular, and behavioral information related to the dI6 population, helping us to better understand their role within the CPG of zebrafish specifically and vertebrates as a whole.