PubMed Health⌕ Search

Biomedical subjects

S J Singer

Publications and source records attributed to S J Singer.

At least 19 recordsLinked to original sources
PubMed1992

Intercellular communication and cell-cell adhesion.

In developmental biology, binary cell-cell interactions often determine the fate of one or both cell partners. The two cells must adhere to one another to allow chemical signals to be transmitted in one or both directions across the regions of cell-cell contact. The molecular mechanisms of cell-cell adhesion and intercellular communication, even if they are mediated by different cell surface components, may be functionally integrated in several different ways. Studies of helper T cells with antigen-presenting B cells in culture have illuminated such binary interactions. The possible application of similar mechanisms to other binary developmental systems is briefly explored.

Animals↗
PubMed1992

Cek5, a membrane receptor-type tyrosine kinase, is in neurons of the embryonic and postnatal avian brain.

Cek5 is a recently identified receptor-type tyrosine kinase of the Eph subclass that is nearly ubiquitously expressed during embryonic development (Pasquale, 1991). Cek5 is predominantly expressed in the avian CNS throughout development, and high levels remain apparent in adult neurons. By means of immunofluorescence microscopy and high-resolution immunoelectron microscopy, Cek5 was found to be expressed in many regions of the chicken brain at various developmental stages, most notably in the hippocampus and cerebellum. The highest concentration of Cek5 was observed in the molecular layer of the cerebellum, associated within the axons of mature granule cells (parallel fibers) and with the cell bodies of immature granule cells. In the axons of parallel fibers, Cek5 was concentrated in the fasciculated nonsynaptic portions. This localization, together with the "adhesion" motifs present in the Cek5 extracellular region suggest that Cek5 may interact with other cell surface-associated molecules and be involved in the growth, guidance, and/or bundling of certain unmyelinated axonal processes. Alternatively (or in addition), Cek5 may represent the receptor for a neurotrophic substance, similar to several other neuronal transmembrane tyrosine kinases.

Animals↗
PubMed1990

Sequence and domain structure of talin.

Talin is a high-molecular-weight cytoskeletal protein concentrated at regions of cell-substratum contact and, in lymphocytes, at cell-cell contacts. Integrin receptors are involved in the attachment of adherent cells to extracellular matrices and of lymphocytes to other cells. In these situations, talin codistributes with concentrations of integrins in the cell surface membrane. Furthermore, in vitro binding studies suggest that integrins bind to talin, although with low affinity. Talin also binds with high affinity to vinculin, another cytoskeletal protein concentrated at points of cell adhesion. Finally, talin is a substrate for the Ca2(+)-activated protease, calpain II, which is also concentrated at points of cell-substratum contact. To learn more about the structure of talin and its involvement in transmembrane connections between extracellular adhesions and the cytoskeleton, we have cloned and sequenced murine talin. We describe a model for the structure of talin based on this sequence and other data. Homologies between talin and other proteins define a novel family of submembranous cytoskeleton-associated proteins all apparently involved in connections to the plasma membrane.

Amino Acid Sequence↗
PubMed1990

Embedded or not? Hydrophobic sequences and membranes.

The same translocation machinery appears to be responsible for both the translocation of soluble proteins across membranes and the insertion of integral membrane proteins into the bilayer. A single mechanism is proposed to accommodate these two functions. This model is also extended to explain the paradoxical translocation of mitochondrial and chloroplastic membrane proteins across one or more membranes before they are finally inserted into their target membranes.

Amino Acid Sequence↗
PubMed1990

Transfer of secretory proteins from the endoplasmic reticulum to the Golgi apparatus: discrimination between homologous and heterologous transfer in intact heterokaryons.

To examine aspects of the transfer of secretory proteins from the endoplasmic reticulum to the Golgi apparatus in situ, heterokaryons were formed between Hep G2 human hepatoma cells and WI-38 human fibroblasts. The cells were appropriately treated with cycloheximide before fusion, which emptied them of their respective secretory proteins, serum albumin for the Hep G2 cells and procollagen I for the WI-38 cells. After fusion was complete, the cycloheximide was washed out, protein synthesis was resumed, and the rates of reappearance of serum albumin and procollagen I in the two separated Golgi apparatuses within each heterokaryon were followed by immunofluorescence microscopy. Serum albumin was found to always reappear first in the Golgi apparatus contributed by the Hep G2 half of the heterokaryon, and procollagen I in the Golgi apparatus of the WI-38 half. These results suggest that the endoplasmic reticulum-to-Golgi apparatus transfer in situ is not simply a stochastic process but is either spatially restricted or exhibits cell-type specificity or both.

Carcinoma, Hepatocellular↗
PubMed1990

The early expression of myofibrillar proteins in round postmitotic myoblasts of embryonic skeletal muscle.

Previous reports on skeletal muscle myogenesis have shown that postmitotic spindle-shaped myoblasts express muscle-specific proteins, some of which are organized into nascent myofibrils. However, we show that, in skeletal muscle cultures derived from 12-day chick embryos, by 6 h after plating the predominant mononucleated cell type that expresses muscle-specific proteins is a round cell. These round myoblasts appear to precede spindle-shaped myoblasts in development, since the latter are more abundant in later cultures and contain larger amounts of muscle proteins and more highly organized myofibrils. By double immunofluorescence microscopy using antibodies specific for the muscle proteins titin, myosin heavy chain (MHC) and zeugmatin we find that 18 h after plating approximately 20% of the round myoblasts that are titin-positive are negative for myofibrillar MHC and zeugmatin. On the other hand, all spindle-shaped myocytes that are positive for titin are also positive for myofibrillar MHC and zeugmatin. These results suggest that titin expression precedes that of myofibrillar MHC and zeugmatin in the non-synchronized round myoblasts, and is consistent with earlier suggestions that titin may function as an initial organizer of myofibrillar proteins during myogenesis. Immunofluorescence data indicate that the earliest localization of the myofibrillar proteins titin, MHC, zeugmatin and alpha-actinin in the round myoblasts is surrounding the nucleus with no immunofluorescent labeling of the cytoplasm or near the plasma membrane. Furthermore, pairwise double immunofluorescence experiments show that these four myofibrillar proteins are all co-localized, at the light-microscopic level of resolution, in irregular patterns that may appear in either a punctate or a basket-like distribution. These labeling patterns around the nucleus are resistant to extraction with Triton X-100, suggesting that the proteins are associated in a stable array. These Triton X-100-resistant assemblies in round myoblasts appear to be composed solely of structural myofibrillar proteins, since the non-structural myofibrillar protein creatine kinase (CK) does not colocalize with the other myofibrillar proteins. These results indicate that in early myoblasts myofibrillar proteins form stable pre-myofibrillar assemblies surrounding the nucleus, and raise the possibility that these initial assemblies may play an organizing role during subsequent early stages of myofibrillogenesis.

Animals↗
PubMed1990

The PMA-induced specific association of LFA-1 and talin in intact cloned T helper cells.

Previous experiments with individual cell couples formed between cloned T helper (Th) cells and antigen-presenting cells have led us to suggest that the cytoskeletal protein talin may be associated with the cell surface protein LFA-1 in the Th cell. In order to examine this suggestion, we induced the surface capping of LFA-1 with suitable specific antibody reagents on the intact Th cells, and then determined by double immunofluorescence microscopic experiments, whether talin was co-clustered with the LFA-1 caps. With untreated Th cells, capping of LFA-1 did not result in any redistribution of intracellular talin. However, if the intact Th cells were treated with the phorbol ester PMA, the capping of LFA-1 resulted in a co-clustering of talin with the LFA-1 caps, but not a alpha-actinin. The capping of TCR or CD4 on the Th cells with or without pretreatment with PMA did not lead to any such co-clustering of talin with these caps. PMA treatment of the Th cells therefore induces a direct or indirect association of talin with LFA-1 underneath the Th cell surface. PMA treatment of the Th cells also increased their polarized spreading and adherence to substrata, as had been observed before. We found, furthermore, that this increased adherence upon PMA-treatment was inhibited by the presence of antibodies to LFA-1. The association of talin, and very likely also F-actin microfilaments, with LFA-1 appears to mediate a generalized increased adhesivity of the Th cells. The relevance of these findings with isolated Th cells to the interaction of Th cells with specific antigen-presenting cells is discussed.

Animals↗
PubMed1979

Mechanochemical proteins, cell motility and cell-cell contacts: the localization of mechanochemical proteins inside cultured cells at the edge of an in vitro "wound".

We have examined the distribution of several mechanochemical proteins inside rat A10 cells in monolayer culture, both in sparse cultures and at the edges of in vitro "wounds" in confluent cultures. The proteins examined were actin, myosin, tropomyosin, alpha-actinin, filamin, and tubulin. In each experiment, a pair of these proteins (one of which was usually actin) were examined simultaneously by double fluorescence staining methods. Actin was specificially stained by double fluorescence staining methods. Actin was specifically stained by a method based on heavy meromyosin binding, while the other proteins were specifically stained by indirect immunofluorescence procedures. The most important of the various results described was obtained with cells moving out from the edge of an in vitro wound. Within the flat leading lamella of such a cell, there was an extended region in which myosin was severely depleted or absent compared to the proximal regions of the same cells. By contrast, the other proteins were abundantly present throughout the leading lamella, except for tropomyosin, which was somewhat depleted but not as extensively as myosin. In Nomarski optics, there was no detectable morphological differentiation between the region depleted of myosin and the more proximal portion of the same lamella. While the depletion of myosin from the motile regions of cells does not rule out the involvement of some form of an actomyosin sliding filament mechanism, it suggests that other molecular mechanisms for generating motility be seriously considered.

Actinin↗
PubMed1979

Molecular changes in the membranes of mouse erythroid cells accompanying differentiation.

The development of the mouse erythroblast to a mature erythrocyte is accompanied by changes in the composition and properties of the plasma membranes of these cells. Using double fluorescence techniques, we have simultaneously determined the distribution of lectin receptors and spectrin on the membranes of these cells. The lateral mobility of the lectin receptors in the membranes decreases as differentiation proceeds, and this is accompanied by an increasing concentration of spectrin associated with the membranes. The most significant concentration of spectrin occurs, however, during the enucleation of the late erythroblast, where we observe a complete segregation of the spectrin to the incipient reticulocyte, as well as a previously observed enrichment of receptors for concanavalin A into the plasma membrane surrounding the extruding nucleus. On the basis of these and other observations, we explore the possible molecular mechanisms involved in erythroblast enucleation and the role of spectrin in the regulation of protein mobility in erythroid cell membranes.

Actins↗
PubMed1979

The participation of alpha-actinin in the capping of cell membrane components.

By means of double fluorescence staining experiments, intracellular alpha-actinin was found to accumulate under caps and patches induced in several cells by a variety of ligands. This phenomenon was demonstrated in lymphocytes and lymphoma cells treated with anti-H-2 sera; spleen lymphocytes treated with concanavalin A or anti-immunoglobulin antibodies, and VSV-infected mouse fibroblast line MC57 treated with antiserum against viral antigens. It occurred during both rapid and slow capping processes, and could be obtained by either direct or indirect ligand-induced redistribution. These observations were carried out on whole cells. For other cytoskeletal proteins such as filamin, tropomyosin and myosin, a similar accumulation under caps was not readily apparent using whole cell mounts, although earlier experiments with frozen-sectioned cells had shown such an enrichment of myosin (as well as actin). The enrichment of alpha-actinin under the clustered surface molecules was already apparent in early stages (patching) of the capping process, with or without 10 mM sodium azide present. Prolonged incubation of the cells with the different ligands resulted in endocytosis of the ligand-receptor complex. alpha-Actinin was not associated with the inernalized complex, however, suggesting that it may dissociate from the patched or capped surface structures at some stage during endocytosis.

Actinin↗
PubMed1979

Evidence for a large internal pressure in biological membranes.

We have developed a method (hygroscopic desorption) for measuring the binding of small molecules to membranes. With this method, we have found that the binding of the amphipathic compounds chlorpromazine, 2,4-dinitrophenol, and 1-decanol to various cell membranes is remarkably low, with partition coefficients, Kp, no larger than about 0.1. On the other hand, with phospholipid vesicles of large or small diameters, Kp values for these compounds were much larger. The results suggest that there exists in membranes a large internal pressure that excludes the amphipaths from the membranes and that does not exist in phospholipid vesicles.

B-Lymphocytes↗
PubMed1979

Immunocytochemical localization of alpha-actinin in intestinal epithelial cells.

alpha-Actinin was localized in chicken intestinal epithelial cells by immunofluorescence and immunoferritin labeling of thin frozen sections. Most of the label of the brush border was confined to the terminal web area. The label there was concentrated mainly along the "roots" of the microvilli core microfilaments and in the vicinity of the zonula adherens. In the latter structure, the narrow electron-dense zones adjacent to the cell membranes, however, were not significantly labeled. This suggests that alpha-actinin does not mediate directly the association of the transverse terminal web microfilaments to the membrane at the zonula adherens. Sparse ferritin labeling was found near the tight junction, whereas the staining associated with the spot desmosome was negligible. The microvilli were not significantly labeled by either immunofluorescence or immunoferritin staining unless the sections were previously treated with detergent. Moreover, alpha-actinin (or a structurally related protein) was not detected in preparations of purified microvillar vesicles, suggesting the possibility that the alpha-actinin staining in the microvilli may be an artificial due to its translocation by the detergent from the terminal web onto the microvilli. The possible roles of alpha-actinin in the organization and function of the brush border are discussed.

Actinin↗