Remodeling of the actin cytoskeleton in the contracting A7r5 smooth musclecell

It has been proposed that the reorganization of components of the actin cyt omatrix could contribute to force development and the low energy cost of su stained contraction in contractile cells which lack a structured sarcomere (A.S. Battistella-Patterson, S. Wang and G.L. Wright (1997) Can J Physiol P harmacol 75: 1287-1299). However, there has been no direct evidence of an a propos actin reorganization specifically linked to the contractile response in cells of this type. Remodeling of the alpha- and beta -actin domains wa s studied in A7r5 smooth muscle cells during phorbol 12,13 dibutyrate (PDB) -induced contraction using immunohistologic staining and beta -actin-green fluorescent protein (beta -actin-GFP) fusion protein expression. Cell stain ed with phalloidin as well as cells expressing beta -actin-GFP showed dense ly packed actin stress cables, arranged in parallel and extending across th e cell body. PDB caused approximately 85% of cells to contract with evidenc e of forcible detachment from peripheral adhesion sites seen in many cells. The contraction of the cell body was not uniform but occurred along a prin cipal axis parallel to the system of densely packed beta -actin cables. Dur ing the interval of contraction, the beta -actin cables shortened without e vidence of disassembly or new cable formation. The use of cytochalasin to i nhibit actin polymerization resulted in the dissolution of the actin cables at the central region of the cell and caused the elongation of precontract ed cells. In unstimulated cells, alpha -actin formed cables similar in arra ngement to the cell spanning beta -actin cables. Within a short interval af ter PDB addition; however, the majority of alpha -actin cables disassembled and reformed into intensely fluorescing column-like structures extending v ertically from the cell base at the center of clusters of alpha -actin fila ments. The alpha -actin columns of contracting cells showed strong colocali zation of alpha -actinin suggesting they could be structurally analogous to the dense bodies of highly differentiated smooth muscle cells. The results indicate that the alpha- and beta -actin domains of A7r5 cells undergo a h ighly structured reorganization during PDB-induced contraction. The extent and nature of this restructuring suggest that remodeling could play a role in contractile function.