Funktionen und Interaktionen von Homöodomänenproteinen während der Entwicklung des Rückenmarks

Abstract

The dorsal spinal cord consists of several types of interneurons which process and relay sensory information from the periphery. These dorsal interneurons are generated during two distinct temporal waves. Six populations of early-born dorsal interneurons (dI1-dI6) are born during the first wave, whereas two late-born populations (dILA/B) are generated during the second wave of neurogenesis. This project investigates the role of two homeodomain proteins, Gsh1 and Gsh2, in the specification of interneurons in the dorsal half of the spinal cord. Gsh1 and Gsh2 are expressed in three different progenitor populations of early-born dorsal interneurons (dI3-dI5), as well as in the progenitor population of the two late-born dorsal interneurons. During early neurogenesis, Gsh1 and Gsh2 are necessary for the generation of excitatory dI3 and dI5 neurons. These factors function by maintaining expression of the bHLH protein Mash1, a necessary determinant of excitatory dI3 and dI5 cell fate, through repression of the bHLH transcription factor Ngn1, which is expressed in wild type embryos in dorsally and ventrally adjacent domains to Gsh1/2. In the spinal cord of Gsh1/2 double knockout mice embryos Ngn1 expands, leading to a reduction in Mash1 protein, which results in a loss of dI3 and dI5 neurons. Gsh1 and Gsh2 are also necessary for the specification of the excitatory dILB neurons evidenced by the loss of these neurons in the spinal cord of Gsh1/2-deficient mice embryos. Interestingly, Mash1 function changes during the second wave of neurogenesis in the dorsal spinal cord. During this period, Mash1 opposes Gsh1/2 function and is now necessary for the specification of inhibitory dILA neurons by inducing Ptf1a expression in dILA progenitors. This bHLH transcription factor antagonizes Gsh1/2 function, thereby promoting an inhibitory dILA cell fate from bipotential Gsh1/2+ progenitors. Taken together, this work identifies the homeodomain proteins Gsh1/2 and the bHLH transcription factor Mash1 as key players in the establishment of inhibitory/excitatory interneuron fates in the developing dorsal spinal cord.