Cell Motility through Blebbing

A bleb is a rounded membrane protrusion formed when the cortical actin network destabilizes and ruptures, causing localized detachment of the membrane from the cortex. Internal hydrostatic pressure then pushes the detached membrane outward, rapidly filling it with cytoplasm. Blebs are observed in fibroblasts, immune cells, and organisms like amoebas, serving functions in cell locomotion and apoptosis.

Inside the bleb, Rnd3 prevents RhoA activation and inhibits actin cortex reassembly, allowing bleb expansion. As the bleb expands, Rnd3 concentration decreases, triggering RhoA activation. Activated RhoA promotes reassembly of actin and ezrin, re-establishing the cortex. Myosin-II then contracts the actin bundles, retracting the bleb and propelling the cell forward.

The actomyosin cortex is an internal network of actin and myosin proteins that lines the cell and links to the membrane via linker proteins like ezrin. When this cortex destabilizes and ruptures in response to signals, it allows membrane detachment and bleb formation. Myosin-II later reassociates with actin bundles to generate contractile force for bleb retraction and cell movement.

Blebs allow migrating cells to squeeze through narrow gaps in the extracellular matrix by pushing the flexible plasma membrane into small spaces. Bleb initiation and expansion create pathways through the matrix, while subsequent bleb retraction pulls the cell forward through the three-dimensional environment. This mechanism complements actin-driven membrane protrusions used for surface crawling.

Blebbing is a characteristic feature of apoptosis, or programmed cell death. Apoptotic cells initiate multiple blebs on their surface that pinch off to form tiny apoptotic blebs containing cytoplasm and proteins. Larger blebs may contain fragments of disrupted organelles like mitochondria and nucleus. Immune cells such as neutrophils and macrophages phagocytose these apoptotic blebs to recycle cellular components.

Ezrin is a key linker protein that connects the actomyosin cortex to the cell membrane. During bleb formation, ezrin detaches from the membrane when the cortex ruptures. As RhoA becomes activated during bleb expansion, ezrin reassembles along with actin to re-establish the cortex and stabilize the membrane, preparing the bleb for retraction.

Internal hydrostatic pressure of the cytoplasm is the driving force behind bleb formation. When the cortical actin network ruptures and the membrane detaches from the cortex, this pressure causes the detached membrane to swell outward and rapidly fill with cytoplasm, creating the characteristic rounded protrusion. The pressure continues to expand the bleb until RhoA activation triggers cortex reassembly and myosin-mediated retraction.