Three members of the Rho family, Cdc42, Rac, and Rho are known to regulate the organization of actin-based cytoskeletal structures. In Bac1.2F5 macrophages, we have shown that Rho regulates cell contraction, whereas Rac and Cdc42 regulate the formation of lamellipodia and filopodia, respectively. We have now tested the roles of Cdc42, Rac, and Rho in colony stimulating factor-1 (CSF-1)–induced macrophage migration and chemotaxis using the Dunn chemotaxis chamber. Microinjection of constitutively activated RhoA, Rac1, or Cdc42 inhibited cell migration, presumably because the cells were unable to polarize significantly in response to CSF-1. Both Rho and Rac were required for CSF-1–induced migration, since migration speed was reduced to background levels in cells injected with C3 transferase, an inhibitor of Rho, or with the dominant-negative Rac mutant, N17Rac1. In contrast, cells injected with the dominant-negative Cdc42 mutant, N17Cdc42, were able to migrate but did not polarize in the direction of the gradient, and chemotaxis towards CSF-1 was abolished. We conclude that Rho and Rac are required for the process of cell migration, whereas Cdc42 is required for cells to respond to a gradient of CSF-1 but is not essential for cell locomotion.
Desmogleins are members of the cadherin superfamily which form the core of desmosomes. In vitro studies indicate that the cytoplasmic domain of desmogleins associates with plakoglobin; however, little is known about the role of this domain in desmosome recognition or assembly in vivo, or about the possible relation of desmoglein mutations to epidermal differentiation and disease. To address these questions we used transgenic mouse technology to produce an NH2-terminally truncated desmoglein (Pemphigus Vulgaris Antigen or Dsg3) in cells known to express its wild-type counterpart. Within 2 d, newborn transgenic animals displayed swelling of their paws, flakiness on their back, and blackening of the tail tip. When analyzed histologically and ultrastructurally, widening of intercellular spaces and disruption of desmosomes were especially striking in the paws and tail. Desmosomes were reduced dramatically in number and were smaller and often peculiar in structure. Immunofluorescence and immunoelectron microscopy revealed no major abnormalities in localization of hemidesmosomal components, but desmosomal components organized aberrantly, resulting in a loss of ultrastructure within the plaque. In regions where desmosome loss was prevalent but where some adhesive structures persisted, the epidermis was thickened, with a marked increase in spinous and stratum corneum layers, variability in granular layer thickness, and parakeratosis in some regions. Intriguingly, a dramatic increase in cell proliferation was also observed concomitant with biochemical changes, including alterations in integrin expression, known to be associated with hyperproliferation. An inflammatory response was also detected in some skin regions. Collectively, these findings demonstrate that a mutation in a desmoglein can perturb epidermal cell-cell adhesion, triggering a cascade of changes in the skin.
The accumulation of tropomyosin in cultures of differentiating muscle cells was quantitatively measured. Tropomyosin was isolated from cultured cells during and after myoblast fusion; both alpha- and beta-subunits were present in myotube cultures. During fusion small amounts of tropomyosin were detectable, but, as fusion approached a maximum, tropomyosin accumulation began to increase. The increased synthesis of tropomyosin after the initiation of muscle cell fusion is consistent with the increased synthesis of other proteins characteristic of muscle, including myosin.