Here, we document changes in cell motility and organization of the contractile apparatus of human umbilical cord Wharton's jelly mesenchymal stem cells (MSCWJ-1) in the process of replicative senescence. Colocalization dynamics of F-actin and actin-binding proteins (myosin-9, α-actinin-4, RhoA) were examined in the MSCWJ-1 cell line. The results show that nuclear-cytoplasmic redistribution of RhoA occurs during replicative senescence, with maximal RhoA/nucleus colocalization evident at passage 15. At that time point, decreases in colocalization, namely myosin-9/F-actin and α-actinin-4/F-actin, were seen and myosin-9 was found in cytosolic extracts in the assembly-incompetent form. Using an automated intravital confocal cytometry system and quantitative analysis of MSCWJ-1 movements, we found that changes in cytoskeletal organization correlate with cell motility characteristics over a time period from passages 9 to 38. The factors examined (cytoskeleton structure, cell motility) indicate that the process by which cells transition to replicative senescence is best represented as three stages. The first stage lasts from cell culture isolation to passage 15 and is characterized by: accumulation of actin-binding proteins in assembly-incompetent forms; nuclear RhoA accumulation; and an increase in movement tortuosity. The second stage extends from passages 15 to 28 and is characterized by: an increase in the structural integrity of the actin cytoskeleton; exit of RhoA and alpha-actinin-4 from the nucleus; and a decrease in path tortuosity. The third stage extends from passage 28 to 38 and is marked by: a plateau in actin cytoskeleton structural integrity; significant decreases in nuclear RhoA levels; and decreases in cell speed.
Keywords: Actin cytoskeleton; Cell motility; Cellular senescence; Mesenchymal stem cells; RhoA.