Heart tissue possesses complex structural organization on multiple scales, from macro- to nano-, but nanoscale control of cardiac function has not been extensively analyzed. Inspired by ultrastructural analysis of the native tissue, we constructed a scalable, nanotopographically controlled model of myocardium mimicking the in vivo ventricular organization. Guided by nanoscale mechanical cues provided by the underlying hydrogel, the tissue constructs displayed anisotropic action potential propagation and contractility characteristic of the native tissue. Surprisingly, cell geometry, action potential conduction velocity, and the expression of a cell–cell coupling protein were exquisitely sensitive to differences in the substratum nanoscale features of the surrounding extracellular matrix. We propose that controlling cell–material interactions on the nanoscale can stipulate structure and function on the tissue level and yield novel insights into in vivo tissue physiology, while providing materials for tissue repair.

Three pancreatic beta-cell lines have been established from insulinomas derived from transgenic mice carrying a hybrid insulin-promoted simian virus 40 tumor antigen gene. The beta tumor cell (J3TC) lines maintain the features of differentiated beta cells for about 50 passages in culture. The cells produce both proinsulin I and II and efficiently process each into mature insulin, in a manner comparable to normal beta cells in isolated islets. Electron microscopy reveals typical beta-cell type secretory granules, in which insulin is stored. Insulin secretion is inducible up to 30-fold by glucose, although with a lower threshold for maximal stimulation than that for normal beta cells. (3TC lines can be repeatedly derived from primary beta-cell tumors that heritably arise in the transgenic mice. Thus, targeted expression of an oncogene with a cell-specific regulatory element can be used both to immortalize a rare cell type and to provide a selection for the maintenance of its differentiated phenotype.Pancreatic beta cells synthesize and secrete insulin, a hormone involved in regulation of glucose homeostasis. In rodents there are two nonallelic insulin genes (I and II), which differ in the number of introns as well as in chromosomal location. Both genes are expressed in beta cells (1). An adult murine pancreas contains about 106 beta cells, clustered in the islets of Langerhans, which are dispersed throughout the exocrine tissue. As a consequence, molecular analyses of beta-cell function has in large part depended on in vitro cultures. Cells from isolated islets do not grow well in culture, although they maintain viability for a few weeks (2). In recent years, several lines of transformed beta cells have been generated (3-6). Two of these, RIN-m SF, derived from an x-ray-induced rat insulinoma, and HIT, from hamster islets transformed by simian virus 40, have been used extensively for characterization of insulin gene expression (4,5,7,8).However, it is unclear to what extent they represent normal beta cells, given that the levels of insulin secreted are considerably lower than those of beta cells in vivo.The ability to target expression of oncogenes to particular cells in transgenic mice, by using cell-specific regulatory elements, presents a method for immortalization of rare cell types. We have reported that transgenic mice harboring insulin-simian virus 40 tumor (T) antigen (RIP-Tag) hybrid genes heritably develop beta-cell tumors (9-11). Here we describe the characterization of several beta tumor cell (J3TC) lines obtained from transgenic mouse tumors and propagated in culture for over 60 passages. These cells provide a useful tool for studies of beta-cell regulation and gene expression. METHODSCell Cultures. Pancreatic insulinomas were excised from transgenic mice and disrupted in Dulbecco's modified Eagle's medium (DMEM). To minimize contamination by fibroblasts and other nontransformed cells, the tumors were not trypsinized. Rather, the tumor capsule was gently removed, and the tumor cells were mech...

Abstract. In the yeast Saccharomyces cerevisiae, mitochondria are elongated organelles which form a reticulum around the cell periphery. To determine the mechanism by which mitochondrial shape is established and maintained, we screened yeast mutants for those defective in mitochondrial morphology. One of these mutants, mmml, is temperature-sensitive for the external shape of its mitochondria. At the restrictive temperature, elongated mitochondria appear to quickly collapse into large, spherical organelles. Upon return to the permissive temperature, wild-type mitochondrial structure is restored. The morphology of other cellular organelles is not affected in mmml mutants, and mmml does not disrupt normal actin or tubulin organization. Cells disrupted in the MMM1 gene are inviable when grown on nonfermentable carbon sources and show abnormal mitochondrial morphology at all temperatures. The lethality of mmml mutants appears to result from the inability to segregate the aberrant-shaped mitochondria into daughter cells. Mitochondrial structure is therefore important for normal cell function. Mmmlp is located in the mitochondrial outer membrane, with a large carboxyl-terminal domain facing the cytosol. We propose that Mmmlp maintains mitochondria in an elongated shape by attaching the mitochondrion to an external framework, such as the cytoskeleton.

These observations suggest that myosin-II along with actin crosslinkers establish local cortical tension and elasticity, allowing for contractility independent of a circumferential cytoskeletal array. Furthermore, myosin-II and actin crosslinkers may influence each other as they modulate the dynamics and mechanics of cell-shape change.