Myoblast fusion and inositol phospholipid breakdown: causal relationship or coincidence?

Mike Wakelam: an appreciation

This issue of Essays in Biochemistry explores lipid mediators - biologically active metabolites formed by enzymic and non-enzymic oxidation of polyunsaturated fatty acids. These can be exported across the cell membrane into the extracellular space, where they activate cell surface receptors to stimulate the cells of origin (autocrine) or nearby cells (paracrine). Lipid mediators are involved in many physiological processes, which may become dysregulated during ageing and in lipid-related diseases such as diabetes, atherosclerosis, arthritis, cancer, Alzheimer's disease and metabolic syndrome. Following the death in March 2020 of Professor Mike Wakelam, with the loss of his major input into the lipid signalling field, Portland Press and Guest Editors John Harwood and Emyr Lloyd-Evans decided to dedicate this issue to his memory. This Editorial briefly recalls his work and influence.

Assessment of Membrane Fluidity Fluctuations during Cellular Development Reveals Time and Cell Type Specificity

Cell membrane is made up of a complex structure of lipids and proteins that diffuse laterally giving rise to what we call membrane fluidity. During cellular development, such as differentiation cell membranes undergo dramatic fluidity changes induced by proteins such as ARC and Cofilin among others. In this study we used the generalized polarization (GP) property of fluorescent probe Laurdan using two-photon microscopy to determine membrane fluidity as a function of time and for various cell lines. A low GP value corresponds to a higher fluidity and a higher GP value is associated with a more rigid membrane. Four different cell lines were monitored such as hN2, NIH3T3, HEK293 and L6 cells. Membrane fluidity was measured at 12h, 72h and 92 h. Our results show significant changes in membrane fluidity among all cell types at different time points. GP values tend to increase significantly within 92 h in hN2 cells and 72 h in NIH3T3 cells and only at 92 h in HEK293 cells. L6 showed a marked decrease in membrane fluidity at 72 h and starts to increase at 92 h. As expected, NIH3T3 cells have more rigid membrane at earlier time points. On the other hand, neurons tend to have the highest membrane fluidity at early time points emphasizing its correlation with plasticity and the need for this malleability during differentiation. This study sheds light on the involvement of membrane fluidity during neuronal differentiation and development of other cell lines.

Genetic analysis of myoblast fusion: blown fuse is required for progression beyond the prefusion complex

The events of myoblast fusion in Drosophila are dissected here by combining genetic analysis with light and electron microscopy. We describe a new and essential intermediate step in the process, the formation of a prefusion complex consisting of "paired vesicles." These pairs of vesicles from different cells align with each other across apposed plasma membranes. This prefusion complex resolves into dense membrane plaques between apposed cells; these cells then establish cytoplasmic continuity by fusion of small areas of plasma membrane followed by vesiculation of apposed membranes. Different steps in this process are specifically blocked by mutations in four genes required for myoblast fusion. One of these genes, blown fuse, encodes a novel cytoplasmic protein expressed in unfused myoblasts that is essential for progression beyond the prefusion complex stage.