Examining the function of Piccolino at the photoreceptor ribbon synapse

• Applicant: Professor Dr. Johann Helmut Brandstätter, since 8/2018
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2016 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 290790462

Piccolo is the largest known cytomatrix protein at active zones of chemical synapses, and a growing number of studies on conventional chemical synapses assign Piccolo a role in the recruitment and integration of molecules relevant for both exo- and endocytosis of synaptic vesicles, the dynamic assembly of presynaptic F-actin, as well as the proteostasis of presynaptic proteins. Sensory ribbon synapses of the eye and ear are specialized for sustained high vesicle release rates, and here, the function of Piccolo remains largely unknown. We recently identified Piccolino, a ribbon synapse specific Piccolo splice variant with a large C-terminal truncation. The resulting lack of interaction with active zone proteins present in conventional chemical synapses and its localization at the synaptic ribbon compartment of the photoreceptor active zone suggests a function of Piccolino at sensory ribbon synapses different from Piccolo function at conventional chemical synapses. Interestingly, the in vivo knockdown of Piccolino via an adeno-associated virus-based RNA interference approach revealed a striking photoreceptor synaptic phenotype with morphological changes of synaptic ribbon ultrastructure. Moreover, we observed a reduced number of clustered vesicles at the photoreceptor ribbon synaptic site in Piccolino knockdown photoreceptor terminals, and impaired photoreceptor synaptic transmission in a Piccolo/Piccolino knockout rat. However, we still do not know Piccolino's exact contribution to ribbon maturation, maintenance, and function. Based on our data we postulate that the C-terminal truncation of Piccolino represents an adaptation to the functional needs of sensory ribbon synapses. By examining a Piccolo/Piccolino deficient rat model, we want to address two working hypotheses concerning Piccolinos contribution to photoreceptor ribbon synaptic structure and function: (1) Piccolino is an essential scaffold for the structural maturation, maintenance and function of the photoreceptor synaptic ribbon, and (2) Piccolino is important for the recruitment of synaptic vesicles to the photoreceptor synaptic ribbon, and thus for continuous and sufficient vesicle supply. This analysis will provide novel insight into the structural organization and function of sensory ribbon synapses.

• DFG Programme: Research Grants
• Ehemalige Antragstellerin: Privatdozentin Dr. Hanna Regus-Leidig, until 8/2018