Some cells in our bodies need to increase their surface area significantly in order to perform their functions. This is particularly true of neurons, whose cell membrane surface area increases by 20% every day during development. When neurons reach maturity, their membrane surface area is 250,000 µm², totalling 25000 m² when all the neurons in the brain are considered. Starting at only ~1256 μm², the total surface area of the ensemble of brain neurons thus increases 200 times, eventually covering an area equivalent to 4 football fields.
In a study published in Nature Cell Biology, Thierry Galli, director of research at Inserm and his colleagues reveal a new mechanism involved in cell membrane growth.
We already knew that secretory vesicles add membrane to the existing cell membrane, making it grow. This occurs through fusion of the vesicles with the cell membrane and involves special proteins known as SNARE proteins whose discovery earned Thomas Südhof, James E Rothman and Randy Schekman the Nobel Prize in Physiology or Medicine in 2013.
The Inserm research team looked at SNARE proteins in mice that were especially abundant at the ends of growing neurons or axons. The researchers identified a specific protein, ‘Sec 22’, first discovered in yeast by Randy Schekman.
“It emerged that the protein ‘Sec22’ plays an active role in cell growth by forming bridges within cells which appear to transport the material needed for their membranes to grow” explains Thierry Galli Inserm Director of Research.
These SNARE bridges between the endoplasmatic reticulum and the plasma membrane, which do not mediate membrane fusion, appear instead to help synthesised lipids pass directly from the endoplasmatic reticulum to the cell surface.
“It is entirely possible that this new mechanism for cell growth is prevalent in rapidly-dividing cells such as cancer cells.” he concludes.
Special types of vesicular transport in rat hippocampal neurons after 3 days’ growth (SNARE proteins in red).
© Inserm/Burgo, Andrea
 Inserm Research Team Unit 950 “Normal and pathological membrane traffic” (Inserm/CNRS/Paris Diderot University) in collaboration with the “Membrane dynamics and intracellular traffic” team (CNRS/Paris Diderot University; Cathy Jackson and Jean-Marc Verbavatz), the “Cellular biology of the synapse” team (Inserm/CNRS/ENS; Antoine Triller) and Xavier Darzacq, Institut de Biologie de l’Ecole Normale Supérieure (IBENS), “Imaging of transcriptional machinery” team