Final Report Summary - GALECTCOMPART (Endocytic Membrane Compartmentalization by Galectins)
Many proteins and lipids are decorated by sugars. These sugar modifications are critical for eukaryotic life. It mostly still remains unexplored how the enormous complexity of structural information that is contained in these sugars is translated into biological functions. It is on this that the ERC GalectCompart program has allowed us to come up with a new postulate, termed the glycolipid-lectin (GL-Lect) hypothesis. Sugar binding proteins, the lectins, play a key role here, as discussed in the following at the example of the cellular galectin-3 (Gal3). Extracellular Gal3 is a monomer, which oligomerizes at the cell surface when it binds to glycoproteins (e.g. CD44, alpha5beta1 integrin...). We have discovered that specifically the oligomeric form of Gal3 has the capacity to then also interact with glycosylated lipids, notably the glycosphingolipids (GSLs), in a way such as to induce the bending of the plasma membrane and the formation of tubular endocytic pits. When these scission from the plasma membrane, endocytic carriers are generated via which the Gal3-binding cargoes are internalized into cells.
The GL-Lect hypothesis that we have proposed based on the research of the GalectCompart ERC program has transforming potential in several domains. In the glycosciences, it provides a unifying mechanistic framework that can be applied to the control of the plasma membrane dynamics of many glycoproteins in different tissues. It could thereby explain the effects of glycosylation that are observed on areas of biology as diverse as immunity, cell signaling, and nutrient uptake. It will also allow to address the role of glycosylation in pathology, and notably cancer, from a fresh angle. In the field of endocytosis, the GL-Lect hypothesis provides an explanation for an almost 40 years old conundrum: How endocytic cargoes are recruited and membranes bent into endocytic pits for processes of endocytic uptake in which the canonical clathrin machinery is not involved. If further consolidated, the hypothesized GL-Lect mechanism has the potential to become an endocytic paradigm, complementary to the clathrin coat paradigm. The GalectCompart ERC program is expected to stimulate further research that spans from the structural examination of endocytic site construction according to the building plan that is suggested by the GL-Lect hypothesis to its regulation in the context of the most complex physiological functions.
Apart from these conceptual advances, we would like to highlight the following specific outcomes of the GalectCompart ERC program.
1. Different galectins (i.e. Gal3 versus Gal8) drive the endocytic uptake of different cargoes (i.e. CD44 and alpha5beta1 integrin versus CD166, respectively), which provides strong evidence for our initial postulate that the 15 members of the galectin family control the generation of different populations of endocytic carriers according to the hypothesized GL-Lect mechanism.
2. Discovery of a novel scission modality — termed friction-driven scission (FDS) — by which tubular endocytic pits that are generated by lectins according to the premises of the GL-Lect hypothesis are detached from the plasma membrane without the intervention of the conventional pinchase dynamin. FDS relies on the molecular motor-driven pulling on tubular endocytic pits that are scaffolded such that lipid flow is inhibited, which then leads to tube breakage and detachment.
3. Based on chemical biology and biophysics approaches, we have put forward a number of new ideas to explain how GSL-bound lectins are clustered (membrane fluctuation forces) and membranes then bent (evolutionarily conserved GSL binding site geometry on oligomeric lectins, asymmetrical lipid compression) to drive the formation of tubular endocytic pits according to the GL-Lect hypothesis.
4. Evidence was obtained in mice that transcytosis across intestinal enterocytes is achieved according to the GL-Lect hypothesis.
The GL-Lect hypothesis that we have proposed based on the research of the GalectCompart ERC program has transforming potential in several domains. In the glycosciences, it provides a unifying mechanistic framework that can be applied to the control of the plasma membrane dynamics of many glycoproteins in different tissues. It could thereby explain the effects of glycosylation that are observed on areas of biology as diverse as immunity, cell signaling, and nutrient uptake. It will also allow to address the role of glycosylation in pathology, and notably cancer, from a fresh angle. In the field of endocytosis, the GL-Lect hypothesis provides an explanation for an almost 40 years old conundrum: How endocytic cargoes are recruited and membranes bent into endocytic pits for processes of endocytic uptake in which the canonical clathrin machinery is not involved. If further consolidated, the hypothesized GL-Lect mechanism has the potential to become an endocytic paradigm, complementary to the clathrin coat paradigm. The GalectCompart ERC program is expected to stimulate further research that spans from the structural examination of endocytic site construction according to the building plan that is suggested by the GL-Lect hypothesis to its regulation in the context of the most complex physiological functions.
Apart from these conceptual advances, we would like to highlight the following specific outcomes of the GalectCompart ERC program.
1. Different galectins (i.e. Gal3 versus Gal8) drive the endocytic uptake of different cargoes (i.e. CD44 and alpha5beta1 integrin versus CD166, respectively), which provides strong evidence for our initial postulate that the 15 members of the galectin family control the generation of different populations of endocytic carriers according to the hypothesized GL-Lect mechanism.
2. Discovery of a novel scission modality — termed friction-driven scission (FDS) — by which tubular endocytic pits that are generated by lectins according to the premises of the GL-Lect hypothesis are detached from the plasma membrane without the intervention of the conventional pinchase dynamin. FDS relies on the molecular motor-driven pulling on tubular endocytic pits that are scaffolded such that lipid flow is inhibited, which then leads to tube breakage and detachment.
3. Based on chemical biology and biophysics approaches, we have put forward a number of new ideas to explain how GSL-bound lectins are clustered (membrane fluctuation forces) and membranes then bent (evolutionarily conserved GSL binding site geometry on oligomeric lectins, asymmetrical lipid compression) to drive the formation of tubular endocytic pits according to the GL-Lect hypothesis.
4. Evidence was obtained in mice that transcytosis across intestinal enterocytes is achieved according to the GL-Lect hypothesis.