Discovery of the "Golgi-bypass" provides new insight into how disease-related proteins are transported to the cell surface.
In order to understand the underlying mechanisms behind certain specific diseases it is important to clarify how newly synthesised molecules are transported in the cell. Cystic fibrosis (CF) affords an example of such a disease.
Professor Jaakko Saraste and his research group at the Department of Biomedicine during 25 years of basic research in cell biology have studied transport phenomena occurring between the endoplasmic reticulum (ER) and Golgi apparatus in mammalian cells, with focus on the so-called “pre Golgi intermediate compartment” (IC). The traditional understanding of the secretory pathway in the cell is based on studies performed in the 1960´s. Since then one has considered that proteins synthesised in the ER are normally trafficked to the Golgi apparatus where they mature and are sorted prior to being transported to the plasma membrane (PM). Saraste´s group has recently published surprising findings (Marie et al, 2009) that challenge this well-established view.
The results have led to the identification of a hitherto unknown pericentrosomal membrane system (PCMS) that appears to be partially formed by the IC and this provides completely new information on how the secretory pathway is organised. This finding opens the possibility that many molecules, including lipids and disease-associated proteins, can be transported directly from the IC to the PM. This in turn means that these molecules do not use the classical secretory pathway, but instead use a Golgi- independent mechanism of transport i.e. “Golgi-bypass” routes.
In cystic fibrosis there occurs an accumulation of thick mucus in the air passages of the lungs, leading to frequent or even chronic pulmonary infection, a condition that today cannot be cured. The reason for the occurrence of CF is inherited mutations in the gene that codes for “cystic fibrosis transmembrane conductance regulator” (CFTR), a chloride channel that is expressed on the surface of all epithelial cells in the body. The most frequent mutation is at F508 that results in the protein being arrested in the ER/IC and thus does not appear at the PM. Despite the mutation the protein is still fully functional and earlier studies have suggested that transport to the cell surface is still feasible. An understanding of the transport pathway of CFTR from the ER to PM is therefore of importance when considering the development of a future strategy of treatment of CF. In the hunt to understand how proteins utilise Golgi-independent transport, CFTR is therefore an excellent candidate for study.
In the work on CFTR that we recently published we demonstrated for the first time that the protein arrives at the PM even when the Golgi apparatus is made non-functional. The molecule is transported to the cell surface via PCMS, in other words by using a form of “Golgi-bypass”. It is now important to unfold the mechanisms that govern CFTR during its “unconventional” mode of transport.
The work has been performed in close connection with the technology platform at the Molecular Imaging Center (MIC), housed in the Department of Biomedicine. We have visualised processes both in fixed and in living cells. It has been especially valuable to record the rapid dynamics in cells using a “so-called” spinning disc” confocal microscope. This has given us important information on membrane traffic in the secretory pathway which has hitherto not been documented. As far as possible this data has been combined with results from molecular biological and biochemical studies in order to throw light on the problem from several angles.