Spectrin is a mechanoresponsive protein shaping fusogenic synapse architecture during myoblast fusion

Abstract

Spectrin is a membrane skeletal protein best known for its structural role in maintaining cell shape and protecting cells from mechanical damage. Here, we report that α/β-spectrin (β is also called karst) dynamically accumulates and dissolves at the fusogenic synapse between fusing Drosophila muscle cells, where an attacking fusion partner invades its receiving partner with actin-propelled protrusions to promote cell fusion. Using genetics, cell biology, biophysics and mathematical modelling, we demonstrate that spectrin exhibits a mechanosensitive accumulation in response to shear deformation, which is highly elevated at the fusogenic synapse. The transiently accumulated spectrin network functions as a cellular fence to restrict the diffusion of cell-adhesion molecules and a cellular sieve to constrict the invasive protrusions, thereby increasing the mechanical tension of the fusogenic synapse to promote cell membrane fusion. Our study reveals a function of spectrin as a mechanoresponsive protein and has general implications for understanding spectrin function in dynamic cellular processes.

Mesoscale Dynamics of Spectrin and Acto-Myosin shape Membrane Territories during Mechanoresponse

“…This result confirmed that changes in spectrin density based on fluorescence intensity could be observed upon drug perturbations by our approach (Figure 4 A-B). Since microtubules (MT) have also been proposed to control βII-spectrin and βV-spectrin dynamics at cell-cell junctions (Jenkins, He and Bennett, 2015; Duan et al ., 2018), we investigated their role in βII-spectrin recruitment to the cell cortex. Upon MT depolymerization, even at early time points (5 minutes), spectrin density increased by almost 20% (Figure 4 A-B).…”

“…Based on different mechanical perturbations of the PM, we propose that spectrin is maintained globally at a constant ratio with the lipid bilayer. Locally, it reacts instead to intrinsic cues, such as PM collapse (Figure 7 I, zone VII), or to external perturbations, such as cell-cell fusion (Duan et al ., 2018). These reactions correspond to a meshwork condensation rather than de-novo recruitment, as a self- and cell-protective mechanism.…”

“…βII-spectrin has also been involved in the maintenance of epithelial cell-cell contact through microtubule-dependent processes, and its dynamics was shown to inversely correlate with endocytic capacities (Jenkins, He and Bennett, 2015). A mechanoresponsive role during myoblasts fusion in muscle development has recently been proposed for the αII/βV-spectrin dimer (Duan et al ., 2018). This developmental process is conserved among different species (e.g.…”

Radial contractility of Actomyosin-II rings facilitates cargo trafficking and maintains axonal structural stability following cargo-induced transient axonal expansion

“…In mammalian cells, radial contractility of subcortical actin (Duan et al, 2018) or actin rings (Gormal et al, 2015) is regulated by mechanosensory NM-II, which has the ability to convert ATP into mechanical force. Recently, the activated form of NM-II light chain (p-MRLC) was shown to distribute with a similar periodicity and largely overlap with the actin MPS at the AIS (Berger et al, 2018).…”

The engulfment receptor Draper organizes the postsynaptic spectrin cytoskeleton into corrals containing synaptic proteins and promotes synaptic renewal

“…This is supported by our FRAP result showing that Dlg can flow out of the SSR even in wild-type body walls. A recent study of the role of the spectrin cytoskeleton in Drosophila myoblast fusion demonstrates another function for spectrin corralling in muscle development: the corralling of the cellular adhesion molecules Dumbfounded and Antisocial in embryonic myoblasts (Duan et al, 2018). In wild-type embryos Dumbfounded and Antisocial are tightly clustered but become dispersed in a/bH-spectrin double mutants.…”