Role of sedlin, a TRAPP complex subunit, in membrane trafficking and in the pathogenesis of Spondyleopyphiseal Dysplasia Tarda

Genetic defects occurring in the sedlin gene, a conserved component of TRAPP complex, cause Spondyloepiphyseal Dysplasia Tarda (SEDT), a rare progressive condition characterised by impaired chondrogenesis resulting in short stature, flattening of the vertebrae, and premature osteoarthritis. The role of sedlin in the pathogenesis of SEDT disease so far is still unknown. Prompted by the consideration that sedlin is ubiquitously expressed but that sedlin mutations cause cartilaginous-restricted dysfunctions, I hypothesized that sedlin might exert a role in membrane trafficking generally but in particular in the transport of chondrocyte- specific cargoes, such as type II procollagen (PCII). This hypothesis was reinforced by the fact that mutations in PCII give rise to autosomal dominant forms of spondyloepiphiseal dysplasia. I tested this hypothesis by analyzing the involvement of sedlin in the transport of different classes of secretory cargoes and found that sedlin is selectively required for PCII to exit the ER, while it is not essential for ER exit of small soluble and membrane-associated cargoes. I have also identified the molecular mechanism underlying this role of sedlin in its ability to bind the GTPase Sarl and to control the membrane-cytosol cycle of Sarl itself and of the COPIl coat complex at the level of the ER exit sites. Sedlin depletion and/or mutation in SEDT patients slows down the Sar1 cycle and prolongs the membrane association of Sar1-GTP at the ER exit sites, thus inducing constriction and premature fission of nascent carriers which fail to incorporate the large PC protofibrils but are still competent for smaller secretory cargoes. All together these findings provide new insights not only into understanding the role of sedlin but also shed new light on the molecular mechanisms underlying the onset of the SEDT disease.