Defects in adhesion and migration, but not in proliferation and differentiation, of embryonic stem cells upon replacement of integrin subunit beta1A by beta1D

Integrins and integrin-associated proteins in the cardiac myocyte

Integrins are heterodimeric, transmembrane receptors that are expressed in all cells, including those in the heart. They participate in multiple critical cellular processes including adhesion, extracellular matrix organization, signaling, survival, and proliferation. Particularly relevant for a contracting muscle cell, integrins are mechanotransducers, translating mechanical to biochemical information. Although it is likely that cardiovascular clinicians and scientists have the highest recognition of integrins in the cardiovascular system from drugs used to inhibit platelet aggregation, the focus of this article will be on the role of integrins specifically in the cardiac myocyte. After a general introduction to integrin biology, the article will discuss important work on integrin signaling, mechanotransduction, and lessons learned about integrin function from a range of model organisms. Then we will detail work on integrin-related proteins in the myocyte, how integrins may interact with ion channels and mediate viral uptake into cells, and also play a role in stem cell biology. Finally, we will discuss directions for future study.

Integrins contribute to initial morphological development and process outgrowth in rat adult hippocampal progenitor cells

Adult rat hippocampal progenitor cells (AHPCs) are self-renewing, multipotent neural progenitor cells (NPCs) that can differentiate into neurons, oligodendrocytes, and astrocytes. AHPCs contact a variety of molecular cues within their surrounding microenvironment via integrins. We hypothesize that integrin receptors are important for NPCs. In this study, we have examined the distribution of integrins in neuronal-like, oligodendrocyte-like, and astrocyte-like AHPCs when grown on substrates that support integrin-mediated adhesion (laminin, fibronectin), and those that do not (poly-L: -ornithine, PLO) using immunocytochemistry as well as characterized the phenotypic differentiation of AHPCs plated on laminin and fibronectin. Focal adhesions were prominent in AHPCs plated on purified substrates, but were also found in AHPCs plated on PLO. The focal adhesions observed in AHPCs plated on PLO substrates may be formed by self-adhesion to the endogenously produced laminin or fibronectin. We have demonstrated that integrins contribute to the initial morphological differentiation of AHPCs, as inhibition of fibronectin binding with the competitive inhibitor echistatin significantly decreased the number of processes and microspikes present in treated cells, and also decreased overall cell area. Finally, we have characterized the genetic profile of a subset of integrins and integrin-related genes in the AHPCs using reverse transcriptase polymerase chain reaction. These results demonstrate an important role of integrins, in vitro, for the initial morphological differentiation of AHPCs.

Subunit 3 of the COP9 signalosome is poised to facilitate communication between the extracellular matrix and the nucleus through the muscle-specific beta1D integrin

Yeast two-hybrid analysis (Fields and Song, 1989, Nature, 340:245-246) was used to screen a human heart library to isolate proteins interacting with the adult muscle-specific beta1D integrin but not with beta1A integrin. In addition to previously identified interactions (RACK 1(Liliental and Chang, 1998, Journal of Biological Chemistry, 273:2379-2383) and alpha-actinin (Otey et al., 1990, Journal of Cell Biology, 111:721-729), the authors isolated several novel candidates. These include subunit 3 (CSN3/Sgn3) of the COP9 signalosome complex, cyclins D1, D2, and D3, RanBPM, and a recently identified protein COG8/DOR1. These protein interactions were specific for beta1D integrin, as no binding to beta1A integrin cytoplasmic domain was measurable by two-hybrid analysis. This paper presents the initial characterization of the interaction of CSN3 with beta1D integrin, the localization of CSN3 and the other COP9 signalosome subunits in embryonic and adult cardiac myocytes and their response to muscle cell differentiation.

Effect of cyclic stretch on β1D-integrin expression and activation of FAK and RhoA

Integrins play a pivotal role in proliferation, differentiation, and survival in skeletal and cardiac myocytes. The β1D-isoform of the β1-integrin is specifically expressed in striated skeletal muscle. However, little is known about the role and the mechanisms by which the splice variant β1D-integrin regulates myogenesis and mechanotransduction. We observed that cyclic mechanical stretch increases β1D-integrin protein levels and activates the downstream cytoskeletal signaling proteins focal adhesion kinase (FAK) and RhoA. Elimination of native β1D-integrin expression by RNA interference in immature developing myoblasts abolished stretch-induced increases in FAK phosphorylation and further downregulated RhoA activity. Blocking of β1D-integrin expression prevented myocellular fusion to form multinucleated mature myotubes. Restoration of human β1D-integrin expression in β1D-integrin-deficient cells partially restored myotube formation. The onset of myofusion also requires the generation of nitric oxide (NO). The release of NO affects cytoskeletal proteins by mediating RhoA activity and protein degradation. Our previous study demonstrated that stretch-induced NO positively modulates mechanical properties of differentiating skeletal myocytes. We found a significant decrease in NO production and apparent elastic modulus in β1D-integrin-deficient cells, suggesting signaling interactions between β1D-integrin and neuronal NO synthase to mediate mechanotransduction and myogenesis in skeletal myocytes. These results suggest that, in addition to regulating differentiation, the β1D-integrin isoform plays a critical role in the response of skeletal myoblasts to cyclic stretch by activating the downstream components of FAK and RhoA activity and affecting NO release.

Toward cell replacement therapy: promises and caveats

Studies in animal models have suggested a role for stem cells in repair and regeneration of the nervous system. Human equivalents of stem and precursor cells have been isolated and their efficacy is being evaluated in rodent and primate models. Difficulties exist in translating results of these preclinical models to therapy in humans. Evolutionary differences among rodents, primates, and humans; fundamental differences in the anatomy and physiology; differences in immune responses in xenotransplant models; the paucity of good transplant models of chronic disease; and allelic variability in the cells themselves make any study evaluating the efficacy of cells in transplant models difficult to interpret. As no better alternatives to testing in animals exist, we suggest that at this early stage a considered step-by-step approach to testing and comparison of different transplant strategies in isolation will prepare us better for clinical trials than simple evaluation of functional outcomes in various models of disease. We emphasize that we do not recommend delaying or abandoning clinical trials; rather, we suggest that one anticipate failures and design experiments and data collection such that we learn from these failures to ensure future success in as rapid a time frame as possible.

The beta1 cytoplasmic domain regulates the laminin-binding specificity of the alpha7X1 integrin

During muscle development, the laminin-specific alpha7 integrin is alternatively spliced in the putative ligand-binding domain to yield either the alpha7X1 or the alpha7X2 variant. The relative level of alpha7X1 and alpha7X2 is developmentally regulated. Similarly, the partner beta1 integrin cytoplasmic domain is converted from the beta1A to the beta1D splice variant. To determine whether beta1D modulates the activity of the alpha7 receptor, cells were transfected with alpha7X1 and beta1D cDNA. alpha7X1 coupled with beta1A failed to adhere to laminin-1, whereas cotransfectants expressing alpha7X1 and beta1D showed strong adhesion. Interestingly, alpha7X1 complexed with beta1A and beta1D displayed the same level of poor adhesion to laminin-2/4 or strong adhesion to laminin-10/11. These findings indicate that alpha7 function is regulated not only by X1/X2 in its extracellular domain but also by beta1 cytoplasmic splice variants. It is likely that expression of beta1D alters alpha7X1 binding to laminin isoforms by a process related to ligand affinity modulation. Functional regulation of alpha7beta1 by developmentally regulated splicing events may be important during myogenic differentiation and repair because the integrin mediates adhesion, motility, and cell survival.

Knock-in of integrin beta 1D affects primary but not secondary myogenesis in mice

Integrins are extracellular matrix receptors composed of alpha and beta subunits involved in cell adhesion, migration and signal transduction. The beta1 subunit has two isoforms, beta 1A ubiquitously expressed and beta 1D restricted to striated muscle. They are not functionally equivalent. Replacement of beta 1A by beta 1D (beta 1D knock-in) in the mouse leads to midgestation lethality on a 50% Ola/50% FVB background [Baudoin, C., Goumans, M. J., Mummery, C. and Sonnenberg, A. (1998). Genes Dev. 12, 1202-1216]. We crossed the beta 1D knock-in line into a less penetrant genetic background. This led to an attenuation of the midgestation lethality and revealed a second period of lethality around birth. Midgestation death was apparently not caused by failure in cell migration, but rather by abnormal placentation. The beta 1D knock-in embryos that survived midgestation developed until birth, but exhibited severely reduced skeletal muscle mass. Quantification of myotube numbers showed that substitution of beta 1A with beta 1D impairs primary myogenesis with no direct effect on secondary myogenesis. Furthermore, long-term primary myotube survival was affected in beta 1D knock-in embryos. Finally, overexpression of beta 1D in C2C12 cells impaired myotube formation while overexpression of beta 1A primarily affected myotube maturation. Together these results demonstrate for the first time distinct roles for beta1 integrins in primary versus secondary myogenesis and that the beta 1A and beta 1D variants are not functionally equivalent in this process.