Presynaptic Dystroglycan-pikachurin Complex Regulates The Proper Synaptic Connection Between Photoreceptor And Bipolar Cells

Abstract

Purpose: : Dystroglycan (DG) is a key component of the dystrophin-glycoprotein complex (DGC) at the neuromuscular junction postsynapse. In the mouse retina, the DGC is localized at the presynapse of photoreceptor cells. We previously reported that an extracellular matrix protein, Pikachurin, is essential for the proper formation of ribbon synaptic structures. Pikachurin KO mice showed a reduced amplitude and prolonged implicit time of the b-wave similar to other mutant mice with perturbed DGC complex formation. Pikachurin physically interacts with DG, and proper glycosylation of DG is required for its interaction with Pikachurin. It was reported that the loss of DG in Muller glial cells causes ERG abnormality. However, the functional role of presynaptic DG in photoreceptor cells remains unclear.

Methods: : To investigate DG function in the photoreceptor ribbon synapse, we ablated DG from photoreceptor cells by conditional gene targeting. To accomplish this, we mated the DGflox line with the Crx-Cre transgenic mouse line (DG CKO) in which Cre-mediated recombination occurs in both rod and cone photoreceptor precursors.

Results: : We found that the DG CKO retina showed a reduced amplitude and a prolonged implicit time of the ERG b-wave. EM analysis revealed that bipolar dendrite invagination into the photoreceptor terminus is perturbed in the DG CKO retina. Interestingly, in the Pikachurin KO retina, the DG signal at the ribbon synaptic terminus was severely reduced, suggesting that pikachurin is required for the presynaptic accumulation of DG at the photoreceptor synaptic terminus. Furthermore, we found that overexpression of pikachurin induces formation and clustering of a DG-pikachurin complex on the cell surface. The Laminin G repeats of pikachurin, which are critical for its oligomerization and interaction with DG, were essential for the clustering of the DG-pikachurin complex as well.

Conclusions: : These results suggest that oligomerization of pikachurin and its interaction with DG causes DG assembly on the synapse surface of the photoreceptor synaptic terminals. Our results reveal that the presynaptic interaction of pikachurin with DG at photoreceptor terminals is essential for both the formation of proper photoreceptor ribbon synaptic structures and normal retinal electrophysiology.

Keywords: synapse • photoreceptors • extracellular matrix