A general RNA-binding protein complex that includes the cytoskeleton-associated protein MAP 1A.

Association of mRNA with the cytoskeleton represents a fundamental aspect of RNA physiology likely involved in mRNA transport, anchoring, translation, and turnover. We report the initial characterization of a protein complex that binds RNA in a sequence-independent but size-dependent manner in vitro. The complex includes a approximately 160-kDa protein that is bound directly to mRNA and that appears to be either identical or highly related to a approximately 1600-kDa protein that binds directly to mRNA in vivo. In addition, the microtubule-associated protein, MAP 1A, a cytoskeletal associated protein is a component of this complex. We suggest that the general attachment of mRNA to the cytoskeleton may be mediated, in part, through the formation of this ribonucleoprotein complex.

Integrin binding and mechanical tension induce movement of mRNA and ribosomes to focal adhesions.

The extracellular matrix (ECM) activates signalling pathways that control cell behaviour by binding to cell-surface integrin receptors and inducing the formation of focal adhesion complexes (FACs). In addition to clustered integrins, FACs contain proteins that mechanically couple the integrins to the cytoskeleton and to immobilized signal-transducing molecules. Cell adhesion to the ECM also induces a rapid increase in the translation of preexisting messenger RNAs. Gene expression can be controlled locally by targeting mRNAs to specialized cytoskeletal domains. Here we investigate whether cell binding to the ECM promotes formation of a cytoskeletal microcompartment specialized for translational control at the site of integrin binding. High-resolution in situ hybridization revealed that mRNA and ribosomes rapidly and specifically localized to FACs that form when cells bind to ECM-coated microbeads. Relocation of these protein synthesis components to the FAC depended on the ability of integrins to mechanically couple the ECM to the contractile cytoskeleton and on associated tension-moulding of the actin lattice. Our results suggest a new type of gene regulation by integrins and by mechanical stress which may involve translation of mRNAs into proteins near the sites of signal reception.

Cellular control lies in the balance of forces.

Mechanical tension generated within the cytoskeleton of living cells is emerging as a critical regulator of biological function in diverse situations ranging from the control of chromosome movement to the morphogenesis of the vertebrate brain. In this article, we review recent advances that have been made in terms of understanding how cells generate, transmit and sense mechanical tension, as well as how they use these forces to control their shape and behavior. An integrated view of cell regulation that incorporates mechanics and structure as well as chemistry is beginning to emerge.

A possible role for plasmids in mediating the cell-cell proximity required for gene flux.

One of the major requirements for successful gene flux is a close proximity between participating organisms. In previous articles, we have proposed that plasmids act as powerful vehicles transporting genes collected by integration and transposition, mainly via the process of conjugation. However, in addition to conjugation, there are other processes, also mediated by plasmids, in which different cells come into very close contact with each other, such as symbiosis and the formation of multi-specific cellular communities. There is evidence that suggests that such intimate associations between cells may facilitate gene transfer events, even between distantly related organisms. Examples of symbiotic endosymbiotic, and parasitic associations provide evidence in support of the role of plasmids in bridging the genetic gap between species. In this purely theoretical article we attempt to conceptualize existing data on this subject, provide new insights and present testable predictions on how plasmids may facilitate gene flux by bringing cells together.

mRNA at the synapse: analysis of a synaptosomal preparation enriched in hippocampal dendritic spines.

Previous studies have demonstrated that the branched spines of the mossy fiber-CA3 hippocampal synapse contain a particularly large number of polyribosomes (Chicurel and Harris, 1989, 1992). We analyzed a preparation of synaptosomes isolated from this region and have found it to contain a restricted RNA population: certain mRNAs, presumably derived from the dendritic spines and the fine astrocytic processes surrounding the pre- and postsynaptic elements of the synapse, are enriched in the synaptosome preparation as compared to the total hippocampus; other mRNAs are less prevalent or altogether absent. In addition, neural BC1, a small noncoding RNA thought to be involved in pre- or posttranslational regulatory processes in dendrites, is a major RNA component of the dendritic spine. These results support the hypothesis that local translational regulation of gene expression may be important in establishing and modulating synaptic function.

Three-dimensional analysis of the structure and composition of CA3 branched dendritic spines and their synaptic relationships with mossy fiber boutons in the rat hippocampus.

This paper is the third in a series to quantify differences in the composition of subcellular organelles and three-dimensional structure of dendritic spines that could contribute to their specific biological properties. Proximal apical dendritic spines of the CA3 pyramidal cells receiving synaptic input from mossy fiber (MF) boutons in the adult rat hippocampus were evaluated in three sets of serial electron micrographs. These CA3 spines are unusual in that they have from 1 to 16 branches emerging from a single dendritic origin. The branched spines usually contain subcellular organelles that are rarely found in adult spines of other brain regions including ribosomes, multivesicular bodies (MVB), mitochondria, and microtubules. MVBs occur most often in the spine heads that also contain smooth endoplasmic reticulum, and ribosomes occur most often in spines that have spinules, which are small nonsynaptic protuberances emerging from the spine head. Most of the branched spines are surrounded by a single MF bouton, which establishes synapses with multiple spine heads. The postsynaptic densities (PSDs) occupy about 10-15% of the spine head membrane, a value that is consistent with spines from other brain regions, with spines of different geometries, and with immature spines. Individual MF boutons usually synapse with several different branched spines, all of which originate from the same parent dendrite. Larger branched spines and MF boutons are more likely to synapse with multiple MF boutons and spines, respectively, than smaller spines and boutons. Complete three-dimensional reconstructions of representative spines with 1, 6, or 12 heads were measured to obtain the volumes, total surface areas, and PSD surface areas. Overall, these dimensions were larger for the complete branched spines than for unbranched or branched spines in other brain regions. However, individual branches were of comparable size to the large mushroom spines in hippocampal area CA1 and in the visual cortex, though the CA3 branches were more irregular in shape. The diameters of each spine branch were measured along the cytoplasmic path from the PSD to the origin with the dendrite, and the lengths of branch segments over which the diameters remained approximately uniform were computed for subsequent use in biophysical models. No constrictions in the segments of the branched spines were thin enough to reduce charge transfer along their lengths.(ABSTRACT TRUNCATED AT 400 WORDS)