Dynamic aspects of adhesion receptor function — integrins both twist and shout

The recognition of extracellular molecules by cell surface receptors is the principal mechanism used by cells to sense their environment. Consequently, signals transduced as a result of these interactions make a major contribution to the regulation of cellular phenotype. Historically, particular emphasis has been placed on elucidating the intracellular consequences of growth factor and cytokine binding to cells. In addition to these interactions, however, cells are usually in intimate contact with a further source of complex structural and functional information, namely immobilised extracellular matrix and/or cell surface adhesion proteins. A key question in recent years has been whether cells use the myriad of adhesion protein-receptor interactions purely for structural and migratory function, or whether these interactions also make a more varied contribution to cell phenotype. Here we review dynamic aspects of the function of one major class of adhesion receptor, the integrins. In particular, we focus on the evidence for shape changes in integrin molecules, the mechanisms responsible for regulating ligand binding, and the signals transduced following integrin occupancy.     

Granzyme B activates procaspase-3 which signals a mitochondrial amplification loop for maximal apoptosis

Granzyme B (GrB), acting similar to an apical caspase, efficiently activates a proteolytic cascade after intracellular delivery by perforin. Studies here were designed to learn whether the physiologic effector, GrB-serglycin, initiates apoptosis primarily through caspase-3 or through BH3-only proteins with subsequent mitochondrial permeabilization and apoptosis. Using four separate cell lines that were either genetically lacking the zymogen or rendered deficient in active caspase-3, we measured apoptotic indices within whole cells (active caspase-3, mitochondrial depolarization [DeltaPsim] and TUNEL). Adhering to these conditions, the following were observed in targets after GrB delivery: (a) procaspase-3-deficient cells fail to display a reduced DeltaPsim and DNA fragmentation; (b) Bax/Bak is required for optimal DeltaPsim reduction, caspase-3 activation, and DNA fragmentation, whereas BID cleavage is undetected by immunoblot; (c) Bcl-2 inhibits GrB-mediated apoptosis (reduced DeltaPsim and TUNEL reactivity) by blocking oligomerization of caspase-3; and (d) in procaspase-3-deficient cells a mitochondrial-independent pathway was identified which involved procaspase-7 activation, PARP cleavage, and nuclear condensation. The data therefore support the existence of a fully implemented apoptotic pathway initiated by GrB, propagated by caspase-3, and perpetuated by a mitochondrial amplification loop but also emphasize the presence of an ancillary caspase-dependent, mitochondria-independent pathway.     

Barbara McClintock — a personal view

While this special issue of TIG was in preparation, the news came that Barbara McClintock bad died at the age of 90 (2 September 1992). Her pioneering work on transposable elements, which was fundamental to the development of research on DNA rearrangements, makes it appropriate that we pay tribute to her in this issue. Regine Kahmann, now a Professor at the University of Munich, worked in the Cold Spring Harbor Laboratory from 1974 until 1980, and came to know Barbara McClintock well. In this ‘personal view’, she remembers Barbara McClintock as an inspiration to an aspiring young scientist.     

Quasi Competition — a New Aspect

The model of quasi competition put forward in 1967 is reinvestigated under the aspect that only large (N →∞) populations are considered. Under this new angle the conclusion that myomas develop from single cells seems better justified than the original discussion indicated.     

The cardiolipin-cytochrome c interaction and the mitochondrial regulation of apoptosis

While many studies have focused on cytochrome c release from mitochondria, little attention has been given to the specific interaction between cardiolipin (CL) and cytochrome c, the breaching of which likely represents a critical event in the initiation of mitochondrially mediated apoptosis. Mounting evidence suggests that a decrease in the level of CL affects cytochrome c binding to the inner membrane, thus leading to higher levels of soluble cytochrome c in the mitochondrial intermembrane space. Among the factors known to affect CL levels are thyroid status, plasma concentrations of free fatty acids, Ca²⁺ dysregulation, and reactive oxygen species (ROS). These factors, especially Ca²⁺ and ROS, have long been recognized as triggers of cell death and, more recently, as modulators of mitochondrially mediated apoptosis. In this review, we discuss the significance of the disruption of the CL-cytochrome c interaction for cytochrome c release and apoptosis.     

t-Tests for Single Means of Autocorrelated Data — A Simulation Study —

A simulation study is reported in which the effect of positive and negative autocorrelation on the quantiles of Student's t-test variable is investigated. It is shown that negative autocorrelations lead to smaller quantiles. Positive autocorrelations lead to larger quantiles.     

Differential regulation of nuclear and mitochondrial Bcl-2 in T cell apoptosis

Activated T cells require anti-apoptotic cytokines for their survival. The anti-apoptotic effects of these factors are mediated by their influence on the balance of expression and localisation of pro- and anti-apoptotic members of the Bcl-2 family. Among the anti-apoptotic Bcl-2 family members, the expression level of Bcl-2 itself and its interaction with the pro-apoptotic protein Bim are now regarded as crucial for the regulation of survival in activated T cells. We studied the changes in Bcl-2 levels and its subcellular distribution in relation to mitochondrial depolarisation and caspase activation in survival factor deprived T cells. Intriguingly, the total Bcl-2 level appeared to remain stable, even after caspase 3 activation indicated entry into the execution phase of apoptosis. However, cell fractionation experiments showed that while the dominant nuclear pool of Bcl-2 remained stable during apoptosis, the level of the smaller mitochondrial pool was rapidly downregulated. Signals induced by anti-apoptotic cytokines continuously replenish the mitochondrial pool, but nuclear Bcl-2 is independent of such signals. Mitochondrial Bcl-2 is lost rapidly by a caspase independent mechanism in the absence of survival factors, in contrast only a small proportion of the nuclear pool of Bcl-2 is lost during the execution phase and this loss is a caspase dependent process. We conclude that these two intracellular pools of Bcl-2 are regulated through different mechanisms and only the cytokine-mediated regulation of the mitochondrial pool is relevant to the control of the initiation of apoptosis. D. Scheel-Toellner and K. Raza have contributed equally to this study.