Ca++ controlled contraction-relaxation cycle in glycerinated amoeboid cells.
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Amoeba↗
Biomedical subjects
M Opas
Publications and source records attributed to M Opas.
Contractility of glycerinated Amoeba proteus and Chaos-chaos.
Immediate contact with large volumes of cold 50% (v/v) buffered glycerol preserved typical ameboid shape of Chaos chaos and Amoeba proteus with no visible distortions. These technics allowed determination of the contraction sites in these glycerinated models upon applications of ATP-Ca-Mg-solutions. The ectoplasmic tube was the main site of contraction. Preliminary EM investigations revealed thick and thin filaments, associated with the ectoplasmic tube near the plasma-lemma, which appeared to be the basis for the contractility of the ectoplasmic tube. There was no predominant contraction of the pseudopodial tips or the endoplasm in these models. The changes of volume were as much as 50%, and in some cases were not accompanied by any change in the length of the ameba; however, lengthwise contractions of the ectoplasmic tube in some amebae occurred to as much as 25%. The data substantiate a basic requirement of the ectoplasmic tube contraction theory of ameboid locomotion.
Adenosine Diphosphate↗
Ca2+ signaling and calcium binding chaperones of the endoplasmic reticulum.
The endoplasmic reticulum is a centrally located organelle which affects virtually every cellular function. Its unique luminal environment consists of Ca(2+) binding chaperones, which are involved in protein folding, post-translational modification, Ca(2+) storage and release, and lipid synthesis and metabolism. The environment within the lumen of the endoplasmic reticulum has profound effects on endoplasmic reticulum function and signaling, including apoptosis, stress responses, organogenesis, and transcriptional activity. Calreticulin, a major Ca(2+) binding (storage) chaperone in the endoplasmic reticulum, is a key component of the calreticulin/calnexin cycle which is responsible for the folding of newly synthesized proteins and glycoproteins and for quality control pathways in the endoplasmic reticulum. The function of calreticulin, calnexin and other endoplasmic reticulum proteins is affected by continuous fluctuations in the concentration of Ca(2+) in the endoplasmic reticulum. Thus, changes in Ca(2+) concentration may play a signaling role in the lumen of the endoplasmic reticulum as well as in the cytosol. Recent studies on calreticulin-deficient and transgenic mice have revealed that calreticulin and the endoplasmic reticulum may be upstream regulators in the Ca(2+)-dependent pathways that control cellular differentiation and/or organ development.
Animals↗
Calcium storage in nonmuscle tissues: is the retina special?
It is now well established that calreticulin is a high capacity Ca(2+)-binding protein which is a major Ca2+ storage protein of the lumen of endoplasmic reticulum membranes in a wide variety of tissues with the exception of skeletal and cardiac muscles. However, in nervous tissue, confusion exists regarding the nature of the intracellular Ca2+ stores, as the organelle responsible for Ca2+ storage has been identified as the endoplasmic reticulum by some investigators and as the specialized organelle, calciosome by others. Calreticulin, calsequestrin, and calsequestrin-like proteins have all been, on different occasions, reported to be present in calciosomes. Cerebral and cerebellar tissues, moreover, have been shown to contain somewhat different systems of Ca(2+)-buffering proteins. In the present paper we discuss evidence that the Ca2+ storage systems of the retina may prove to be more complex than those of other neuronal tissues. Biochemical and immunocytochemical evidence indicates the presence of either an isoform of calreticulin or another protein that is antigenically similar to calreticulin, but of slightly higher molecular weight, in the endoplasmic reticulum of both neurons and Müller glia from rabbit neural retina. However, as retinal neurons express Purkinje cell markers, one may expect to observe the presence of calsequestrin in these cells as well. Secondly, antibodies against the onchocercal RAL-1 antigen recognize a protein sharing 62-65% amino acid sequence identity with calreticulin. The anti-RAL-1 antibodies show specificity for the retina. Whether or not the RAL-1 antigen is an active part of the Ca2+ storage systems of the retina remains to be verified.(ABSTRACT TRUNCATED AT 250 WORDS)
Amino Acid Sequence↗
Regulation of adhesion-related protein tyrosine kinases during in vitro differentiation of retinal pigment epithelial cells: translocation of pp60c-src to the nucleus is accompanied by downregulation of pp125FAK.
In the present report we show that induction of expression of a differentiated phenotype in cultured retinal pigmented epithelium of chick embryo is accompanied by coordinate regulation of expression and distribution of two adhesion-related nonreceptor protein tyrosine kinases, pp60c-src and pp125FAK. pp60c-src translocates from the cell surface in flat undifferentiated cells to the nucleus in the packed differentiated cells. In contrast, pp125FAK, abundant in focal adhesions of flat undifferentiated cells, is downregulated in cells that have differentiated and packed into an epithelial sheet.
Animals↗
Cell shape, intracellular pH, and fibroblast growth factor responsiveness during transdifferentiation of retinal pigment epithelium into neuroepithelium in vitro.
In this report we show that some retinal pigment epithelial (RPE) cells, with no expression of neural cell adhesion molecule (N-CAM) (RPEN-CAM-), spontaneously lose pigment and start to express N-CAM in culture. Chick RPE cells normally do not express N-CAM, while the protein is present in chick neural retina. Thus some of the RPE cells in culture started to transdifferentiate into a neuroepithelium (NEN-CAM+). We have measured intracellular pH (pHi) in the RPE cultures and followed its changes in response to basic fibroblast growth factor (bFGF). The depigmented cells protrude above the RPE cell sheet and have a lower resting pHi (approximately 7.05) than the pigmented RPE cells (approximately 7.15). The majority of cells with low resting pHi express N-CAM. The difference in the resting pHi between RPEN-CAM- and NEN-CAM+ cells is not due to the N-CAM expression by NEN-CAM+ cells, as their pHi is the same as the pHi of freshly plated single "round" RPEN-CAM- cells that have not spread yet. NEN-CAM+ cells respond to bFGF with a quick and sustained pHi rise. In contrast, neither the cuboidal RPEN-CAM- cells in a colony centre nor single round RPEN-CAM- cells respond to bFGF with cytoplasmic alkalinization. RPE cells do not proliferate in response to bFGF, while NE cells respond to bFGF with a stimulation of growth. We conclude that bFGF acts not on the fully differentiated RPEN-CAM-, but only on those cells which have already started to transdifferentiate and changed their shape and (or) adhesive status.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Cellular adhesiveness, contractility, and traction: stick, grip, and slip control.
Translocation of cells over solid substrata depends on generation of motive force, in crawling tissue cells, brought about by regulated contractility of intracellular actomyosin. Intracellular contractile machinery has a direct, structural connection to the cell surface. Hence, regulated adhesiveness of the cell surface provides a mechanism whereby a cell can fine tune the extent of tractional forces that are necessary for effective translocation. Cells are able to control adhesiveness of surfaces (stick), contractility (grip), and the extent of traction exerted on the substratum (slip). Here, I discuss several aspects of local (subcellular) regulation of adhesiveness and contractility and speculate on how cells, given a choice of the substratum, decide on how and where to apply traction.
Biomechanical Phenomena↗