Hierarchy in the organization of the cell membrane

Novel biophysical techniques, including super-resolution nanoscopy and single molecule approaches, are providing indisputable proof that many, if not most, membrane proteins are clustered at the plasma membrane at different spatiotemporal scales. We discovered that pathogen recognition receptors organize in a highly hierarchical fashion on the membrane of immune cells, forming nanoclusters that further organize in meso-scale regions enriched with sites for endocytosis. This preferred organization increased the probability of internalizing virus pathogens for subsequent processing and degradation by dendritic cells. We advocate the view that nanoclustering is an important part of the hierarchical organization of proteins in the plasma membrane.

HFSP Program Grant holders Alessandra Cambi and Maria Garcia-Parajo and colleagues
authored on Tue, 27 January 2015

Organization by compartmentalization is a general property of natural systems that efficiently facilitates and orchestrates biological events in space and time.  Cells are primary examples of well-defined biological compartments within tissues. However, cells also exhibit a number of compartmentalization strategies that increases regulation efficiency. Interestingly, the plasma membrane with its extracellular matrix and the subjacent membrane-associated cytoskeleton is also highly organized. Once considered a relatively unstructured ‘sea’ of lipids and proteins potentially able to form aggregates, the plasma membrane is now widely accepted as being highly compartmentalized, thus allowing lipids and proteins to be organized in specific regions of varying size and composition. How and why this organization takes place are still questions that remain unsolved.

Figure: Artist impression of the hierarchical organization of the receptor DC-SIGN on the cell membrane. DC-SIGN (red) forms nanoclusters that are further confined in meso-scale regions of the membrane by extracellular glycans (gray patches). This confinement increases the probability of interactions with clathrin coated pits (green) favoring receptor internalization.

Recent work from the Cambi and Garcia-Parajo labs sponsored by HFSP has focussed on the spatiotemporal organization of the pathogen recognition receptor DC-SIGN expressed on the plasma membrane of immature dendritic cells (imDCs). The team had already discovered that DC-SIGN forms nanoclusters on the plasma membrane of imDCs that solely depended on the molecular structure of the receptor. Other molecules called glycans (sugars with different branching structures) which are ubiquitously present in the cell membrane, have the potential to interact with glycosylated proteins such as DC-SIGN, but until now their role on DC-SIGN was unknown.

Using advanced optical techniques that included optical nanoscopy and single particle tracking approaches, Cambi and Garcia-Parajo discovered that glycans confined DC-SIGN nanoclusters in regions of around 1 μm in size. In turn, this confinement corralled DC-SIGN nanoclusters into clathrin active regions (the primary internalization pathway in mammalian cells). Using a newly developed single molecule dynamic approach, the researchers saw increasing clathrin–receptor interactions and enhanced clathrin-mediated endocytosis of virus-like particles bound to DC-SIGN. The work thus demonstrated that efficient pathogen binding and uptake crucially depends on how receptors organize on the cell membrane. Importantly, the research provides a clear example on how Nature has devised strategies to optimize cellular function using concepts of compartmentalization and hierarchy.

In a separate commentary, and in collaboration with the groups of N. Thompson and K. Jacobson at the University of North Carolina, the HFSP research team recently discussed the state-of-art in the field, presenting an overview on results obtained from many different groups working on cell membrane biology. The emerging picture coming from all these results is that nanoclustering is an important part of the hierarchical organization of proteins in the plasma membrane. According to this emerging picture, nanoclusters can be organized on the meso-scale to form micro-domains that are capable of supporting cell adhesion, pathogen binding and immune cell-cell recognition, amongst other functions. Yet, a number of outstanding issues concerning nanoclusters remains open, including the details of their molecular composition, biogenesis, size, stability, function and regulation. The commentary put forth notions about nanocluster function and why this general feature of protein nanoclustering appears to be so prevalent.

References

Nanoclustering as a dominant feature of plasma membrane organization.M. F. Garcia-Parajo, A. Cambi, J. A. Torreno-Pina, N. Thompson, K. Jacobson. J. Cell Sci. 127, 4995-5005 (2014).

Enhanced receptor–clathrin interactions induced by N-glycan–mediated membrane micropatterning. J. A. Torreno-Pina, B. M. Castro, C. Manzo, S. I. Buschow, A. Cambi, M. F. Garcia-Parajo . P. Natl. Acad. Sci. USA 111, 11037-11042 (2014).

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