By targeting different compartments of principal neurons, inhibitory interneurons integrate complex inputs into a coherent output. The mechanisms through which interneurons choose their subcellular targets is poorly understood.
In the cerebellum of mammals, two types of interneurons innervate Purkinje cells. Stellate cells target dendrites whereas basket cells synapse on the soma and axon initial segment (AIS). Stellate cells are ancient and can be found starting in fish. Fish, however, do not make basket cells. In contrast, turtles and other reptiles have both stellate cells and an intermediate cell type that targets the Purkinje cell soma but not the AIS. This suggests that the ancestral stellate cell diverged into a second cell type which first targeted the soma and later the soma and AIS of Purkinje cells. I hypothesise that there is a concomitant stepwise emergence of regulators of synaptic targeting.
I propose to develop the painted turtle into a tractable system to investigate this evolution of synaptic targeting in two ways. 1) Establish a viral approach to allow reliable genetic access to turtle neurons, 2) Employ novel cell-type specific drivers, single-cell morphology, and in situ hybridisation to determine the morphological and molecular make-up of turtle cerebellar interneurons. This study will allow me to assess the conservation of synaptic regulators and the emergence of novel gene expression patterns in concert with anatomical changes across species.