Early stages of retinal development depend on Sec13 function

ER-to-Golgi transport of proteins destined for the extracellular space or intracellular compartments depends on the COPII vesicle coat and is constitutive in all translationally active cells. Nevertheless, there is emerging evidence that this process is regulated on a cell- and tissue-specific basis, which means that components of the COPII coat will be of differential importance to certain cell types. The COPII coat consists of an inner layer, Sec23/24 and an outer shell, Sec13/31. We have shown previously that knock-down of Sec13 results in concomitant loss of Sec31. In zebrafish and cultured human cells this leads to impaired trafficking of large cargo, namely procollagens, and is causative for defects in craniofacial and gut development. It is now widely accepted that the outer COPII coat is key to the architecture and stability of ER export vesicles containing large, unusual cargo proteins. Here, we investigate zebrafish eye development following Sec13 depletion. We find that photoreceptors degenerate or fail to develop from the onset. Impaired collagen trafficking from the retinal pigment epithelium and defects in overall retinal lamination also seen in Sec13-depleted zebrafish might have been caused by increased apoptosis and reduced topical proliferation in the retina. Our data show that the outer layer of the COPII coat is also necessary for the transport of large amounts of cargo proteins, in this case rhodopsin, rather than just large cargo as previously thought.