Bioarchitecture最新文献

Eukaryotic cells display an asymmetric distribution of cellular compartments relying on their adhesion and the underlying anisotropy of the actin and microtubule cytoskeleton. Studies using a minimal cell culture system based on confined adhesion on micropatterns have illustrated that trafficking compartments are well organized at the single cell level in response to the geometry of cellular adhesion cues. Expanding our analysis on cellular uptake processes, we have found that cellular adhesion additionally defines the topology of endocytosis and signaling. During endocytosis, transferrin (Tfn) and epidermal growth factor (EGF) concentrate at distinct cellular sites in micropatterned cells. Tfn is enriched in adhesive sites during uptake, whereas EGF endocytosis is restricted to the dorsal cellular surface. This unexpected dorsal/ventral asymmetry is regulated by uptake mechanisms and actin dynamics. Interestingly, restricted EGF uptake leads to asymmetry of EGF receptor activation that is required to sustain downstream signaling. Based on our results, we propose that differential sorting begins at the plasma membrane leading to spatially distinct intracellular trafficking routes that are well defined in space. We speculate that the intracellular positioning of trafficking compartments sustains an important coupling between the endocytic and signaling systems that allows cells to sense their environment.

Plasma membrane organization is under the control of cytoskeletal networks and endocytic mechanisms, and a growing literature is showing how closely these influences are interconnected. Here, we review how plasma membranes are formed around individual nuclei of the syncytial Drosophila embryo. Specifically, we outline the pathways that promote and maintain the growth of pseudocleavage and cellularization furrows, as well as specific pathways that keep furrow growth in check. This system has become important for studies of actin regulators, such as Rho1, Diaphanous, non-muscle myosin II and Arp2/3, and endocytic regulators, such as a cytohesin Arf-GEF (Steppke), clathrin, Amphiphysin and dynamin. More generally, it provides a model for understanding how cytoskeletal-endocytic cross-talk regulates the assembly of a cell.

Microtubules play a central role in many essential cellular processes, including chromosome segregation, intracellular transport, and cell polarity. As these dynamic polymers are crucial components of eukaryotic cellular architecture, we were surprised by our recent discovery that a common human genetic difference leads to variation in microtubule stability in cells from different people. A single nucleotide polymorphism (SNP) near the TUBB6 gene, encoding class V β-tubulin, is associated with the expression level of this protein, which reduces microtubule stability at higher levels of expression. We discuss the novel cellular GWAS (genome-wide association study) platform that led to this discovery of natural, common variation in microtubule stability and the implications this finding may have for human health and disease, including cancer and neurological disorders. Furthermore, our generalizable approach provides a gateway for cell biologists to help interpret the functional consequences of human genetic variation.

Cell surface expansion is a necessary part of cell shape change. One long-standing hypothesis proposes that membrane for this expansion comes from the flattening out of cell surface projections such as microvilli and membrane folds. Correlative EM data of cells undergoing phagocytosis, cytokinesis, and morphogenesis has hinted at the existence of such an unfolding mechanism for decades; but unfolding has only recently been confirmed using live-cell imaging and biophysical approaches. Considering the wide range of cells in which plasma membrane unfolding has now been reported, it likely represents a fundamental mechanism of cell shape change.

Columnar epithelia (e.g., kidney, intestine) and hepatocytes embody the two major organizational phenotypes of non-stratified epithelial cells. Columnar epithelia establish their apical and basal domains at opposing poles and organize in monolayered cysts and tubules, in which their apical surfaces form a single continuous lumen whereas hepatocytes establish their apical domains in the midst of their basolateral domains and organize a highly branched capillary luminal network, the bile canaliculi, in which a single hepatocyte can engage in lumen formation with multiple neighbors. To maintain their distinct tissue architectures, columnar epithelial cells bisect their luminal domains during symmetric cell divisions, while the cleavage furrow in dividing hepatocytes avoids bisecting the bile canalicular domains. We discuss recently discovered molecular mechanisms that underlie the different cell division phenotypes in columnar and hepatocytic model cell lines. The serine/threonine kinase Par1b determines both the epithelial lumen polarity and cell division phenotype via cell adhesion signaling that converges on the small GTPase RhoA.

Three different genes each located on a different chromosome encode the heavy chains of nonmuscle myosin II in humans and mice. This review explores the functional consequences of the presence of three isoforms during embryonic development and beyond. The roles of the various isoforms in cell division, cell-cell adhesion, blood vessel formation and neuronal cell migration are addressed in animal models and at the cellular level. Particular emphasis is placed on the role of nonmuscle myosin II during cardiac and brain development, and during closure of the neural tube and body wall. Questions addressed include the consequences on organ development, of lowering or ablating a particular isoform as well as the effect of substituting one isoform for another, all in vivo. Finally the roles of the three isoforms in human diseases such as cancer as well as in syndromes affecting a variety of organs in humans are reviewed.

The semiflexible polymers filamentous actin (F-actin) and intermediate filaments (IF) both form complex networks within the cell, and together are key determinants of cellular stiffness. While the mechanics of F-actin networks together with stiff microtubules have been characterized, the interplay between F-actin and IF networks is largely unknown, necessitating the study of composite networks using mixtures of semiflexible biopolymers. We employ bulk rheology in a simplified in vitro system to uncover the fundamental mechanical interactions between networks of the 2 semiflexible polymers, F-actin and vimentin IF. Surprisingly, co-polymerization of actin and vimentin can produce composite networks either stronger or weaker than pure F-actin networks. We show that this effect occurs through steric constraints imposed by IF on F-actin during network formation and filament crosslinking, highlighting novel emergent behavior in composite semiflexible networks.

Force-regulation at cellular membranes relies on dynamic molecular platforms that integrate intra- and extracellular signals to control cell shape and function. To correctly respond to a continuously changing environment, activity of these platforms needs to be tightly controlled in space and time. Over the last few years, curvature-dependent mechano-chemical signal translation—a receptor-independent signaling mechanism where physical forces at the plasma membrane trigger nanoscale membrane deformations that are then translated into chemical signal transduction cascades—has emerged as a new signaling principle that cells use to regulate forces at the membrane. However, until recently, technical limitations have precluded studies of this force-induced curvature-dependent signaling at the physiological scale. Here, we comment on recent advancements that allow studying curvature-dependent signaling at membranes, and discuss processes where it may be involved in. Considering its general impact on cell function, a particular focus will be put on the curvature-dependence of feedback loops that control actin-based forces at cellular membranes.

The myosin holoenzyme is a multimeric protein complex consisting of heavy chains and light chains. Myosin light chains are calmodulin family members which are crucially involved in the mechanoenzymatic function of the myosin holoenzyme. This review examines the diversity of light chains within the myosin superfamily, discusses interactions between the light chain and the myosin heavy chain as well as regulatory and structural functions of the light chain as a subunit of the myosin holoenzyme. It covers aspects of the myosin light chain in the localization of the myosin holoenzyme, protein-protein interactions and light chain binding to non-myosin binding partners. Finally, this review challenges the dogma that myosin regulatory and essential light chain exclusively associate with conventional myosin heavy chains while unconventional myosin heavy chains usually associate with calmodulin.

Stabilization of axonal connections is an underappreciated, but critical, element in development and maintenance of neuronal functions. The ability to maintain the overall architecture of the brain for decades is essential for our ability to process sensory information efficiently, coordinate motor activity, and retain memories for a lifetime. While the importance of the neuronal cytoskeleton in this process is acknowledged, little has been known about specializations of the axonal cytoskeleton needed to stabilize neuronal architectures. A novel post-translational modification of tubulin that stabilizes normally dynamic microtubules in axons has now been identified. Polyamination appears to be enriched in axons and is developmentally regulated with a time course that correlates with increased microtubule stabilization. Identifying one of the molecular mechanisms for maintaining neuronal connections creates new research avenues for understanding the role of stabilizing neuronal architecture in neuronal function and in neuropathology.