Cellular adhesiveness, contractility, and traction: stick, grip, and slip control

Dissecting focal adhesions in cells differentially expressing calreticulin: a microscopy study

BACKGROUND INFORMATION

Our previous studies have shown that calreticulin, a Ca2+-binding chaperone located in the endoplasmic reticulum, affects cell-substratum adhesions via the induction of vinculin and N-cadherin. Cells overexpressing calreticulin contain more vinculin than low expressers and make abundant contacts with the substratum. However, cells that express low levels of calreticulin exhibit a weak adhesive phenotype and make few, if any, focal adhesions. To date, the identity of the types of focal adhesions made by calreticulin overexpressing and low expressing cells has not been dissected.

RESULTS

The results of the present study show that calreticulin affects fibronectin matrix assembly in L fibroblast cell lines that differentially express the protein, and that these cells also differ profoundly in focal adhesion formation. Although the calreticulin overexpressing cells generate numerous interference-reflection-microscopy-dark, vinculin- and paxillin-containing classical focal contacts, as well as some fibrillar adhesions, the cells expressing low levels of calreticulin generate only a few weak focal adhesions. The fibronectin receptor was found to be clustered in calreticulin overexpressing cells, but diffusely distributed over the cell surface in low expressing cells. Plating L fibroblasts on fibronectin-coated substrata induced extensive spreading in all cell lines tested. However, although calreticulin overexpressing cells were induced to form classical vinculin-rich focal contacts, the low calreticulin expressing cells overcame their weak adhesive phenotype by induction of many tensin-rich fibrillar adhesions, thus compensating for the low level of vinculin in these cells.

CONCLUSIONS

We propose that calreticulin affects fibronectin production and, thereby, assembly, and it indirectly influences the formation and/or stability of focal contacts and fibrillar adhesions, both of which are instrumental in matrix assembly and remodelling.

Quantification of substratum contact required for initiation of Colletotrichum graminicola appressoria

Colletotrichum graminicola, like many plant pathogenic fungi develop appressoria on germling apices, to facilitate penetration of their host. Induction of these structures occurs after contact with the host surface has been established by the germling. Surface contact and subsequent development of appressoria by germlings of C. graminicola was assessed using interference-reflection microscopy (IRM) and microfabricated pillared silicon substrata. Observations with IRM revealed that under low nutrient conditions, 90% of the germlings developed appressoria once they established 4.5 microm of continuous contact with the substratum. Substrata bearing pillars < or =5 microm in width supported < or =10% appressoria; however, as pillar width was increased the percentage of appressoria formed increased in a sigmoid fashion to a maximum of 80%. The percentage of appressoria produced experimentally on these surfaces was compared to data sets generated from a model designed to calculate the probability of appressorium development on similar pillar arrays at various germ tube contact lengths. These results indicate that germ tubes of C. graminicola require more than 4microm of continuous contact with a hydrophobic substratum for induction of appressoria.

The Differential Interfacial Tension Hypothesis (DITH): a comprehensive theory for the self-rearrangement of embryonic cells and tissues

A comprehensive theory, herein named the Differential Interfacial Tension Hypothesis for the self-rearrangement of embryonic cells and tissues is presented. These rearrangements include sorting, mixing and formation of checkerboard patterns in heterotypic aggregates of embryonic cells, and total or partial engulfment, separation and dissociation of tissues. This broadly-based theory accounts for the action of all currently known cytoskeletal components and cell adhesion mechanisms. The theory is used to derive conditions for the cell and tissue rearrangements named above. Finite element-based computer simulations involving two or more cell types confirm these conditions.

Involvement of the [uPAR:uPA:PAI-1:LRP] complex in human myogenic cell motility

The urokinase-type plasminogen activator system is a proteolytic system involved in tissue remodeling and cell migration. At the cell surface, receptor (uPAR)-bound urokinase (uPA) binds its inhibitor PAI-1, localized in the matrix, and the complex is internalized by endocytic receptors, such as the low-density lipoprotein receptor-related protein (LRP). We previously proposed a nonproteolytic role for the uPA system in human myogenic cell differentiation in vitro, i.e., cell fusion, and showed that myogenic cells can use PAI-1 as an adhesion matrix molecule. The aim of this study was to define the role of the uPA system in myogenic cell migration that is necessary for fusion. Using a two-dimensional motility assay and microcinematography, we showed that any interference with the [uPAR:uPA:PAI-1] complex formation, and interference with LRP binding to this complex, markedly decreased myogenic cell motility. This phenomenon was reversible and independent of plasmin activity. Inhibition of cell motility was associated with suppression of both filopodia and membrane ruffling activity. [uPAR:uPA:PAI-1:LRP] complex formation involves high-affinity molecular interactions and results in quick internalization of the complex. It is likely that this complex supports the membrane ruffling activity involved in the guidance of the migrating cell toward appropriate sites for attachment.

The mechanics of cell sorting and envelopment

Aggregates of embryonic cells undergo a variety of intriguing processes including sorting by histological type and envelopment of cell masses of one type by another. It has long been held that these processes were driven by differential adhesions, as embodied in the famous differential adhesion hypothesis (DAH). Here, we use analytical mechanics to investigate the forces that are generated by various sub-cellular structures including microfilaments, cell membranes and their associated proteins, and by sources of cell-cell adhesions. We consider how these forces cause the triple junctions between cells to move, and how these motions ultimately give rise to phenomena such as cell sorting and tissue envelopment. The analyses show that, contrary to the widely accepted DAH, differential adhesions alone are unable to drive sorting and envelopment. They show, instead, that these phenomena are driven by the combined effect of several force generators, as embodied in an equivalent surface or interfacial tension. These unconventional findings follow directly from the relevant surface physics and mechanics, and are consistent with well-known cell sorting and envelopment experiments, and with recent computer simulations.

Calreticulin affects focal contact-dependent but not close contact-dependent cell-substratum adhesion

We used two cell lines expressing fast (RPEfast) and slow (RPEslow) attachment kinetics to investigate mechanisms of cell-substratum adhesion. We show that the abundance of a cytoskeletal protein, vinculin, is dramatically decreased in RPEfast cells. This coincides with the diminished expression level of an endoplasmic reticulum chaperone, calreticulin. Both protein and mRNA levels for calreticulin and vinculin were decreased in RPEfast cells. After RPEfast cells were transfected with cDNA encoding calreticulin, both the expression of endoplasmic reticulum-resident calreticulin and cytoplasmic vinculin increased. The abundance of other adhesion-related proteins was not affected. RPEfast cells underexpressing calreticulin displayed a dramatic increase in the abundance of total cellular phosphotyrosine suggesting that the effects of calreticulin on cell adhesiveness may involve modulation of the activities of protein tyrosine kinases or phosphatases which may affect the stability of focal contacts. The calreticulin and vinculin underexpressing RPEfast cells lacked extensive focal contacts and adhered weakly but attached fast to the substratum. In contrast, the RPEslow cells that expressed calreticulin and vinculin abundantly developed numerous and prominent focal contacts slowly, but adhered strongly. Thus, while the calreticulin overexpressing RPEslow cells "grip" the substratum with focal contacts, calreticulin underexpressing RPEfast cells use close contacts to "stick" to it.