Search PubMed⌕ Search

Biomedical subjects

S J Singer

Publications and source records attributed to S J Singer.

At least 37 records · Page 2Linked to original sources
PubMed1995

Cell membrane formation during the cellularization of the syncytial blastoderm of Drosophila.

The early blastoderm of Drosophila is a syncytium in which about 6000 nuclei become localized in the peripheral cytoplasm. During cycle 14 interphase, a wave of membrane formation encircles each nucleus inside its own plasma membrane, thereby generating an intact epithelial layer. The details of this process of cellularization have been unclear. Using an improved method of fixation of the embryos for electron microscopy, we show by morphological observations that a large number of membrane-bounded, electron-transparent vesicles, of diameters ranging from 0.05 micron to 0.5 micron, are present in the periplasm and become redistributed during cellularization so as to provide the membrane mass required at each phase of the process. We recognize three phases. In the first two phases, the vesicles that were present in the apical periplasmic space at earlier stages become concentrated and aligned between the nuclei. The vesicles then undergo concerted but not precisely synchronous fusion to form double membranes, starting at furrows in the plasma membrane of the embryo and extending about 7 microns into the periplasmic space. Subsequently, in the third phase vesicles are recruited to the basal periplasmic space but do not become aligned between the nuclei as in the first phase. We presume that these vesicles fuse individually with the growing ends of the double membranes until encirclement of each nucleus is complete. We speculate that these vesicles are all derived from the Golgi apparatus and are moved about in the blastoderm by interactions with components of the cytoskeleton.

Animals↗
PubMed1994

A possible role for stable microtubules in intracellular transport from the endoplasmic reticulum to the Golgi apparatus.

The intracellular transport of secretory proteins involves at an early stage the formation of vesicles from transitional elements of the endoplasmic reticulum (ER) containing these proteins and the transfer of these vesicles to the cis-face of the Golgi apparatus. We propose that the latter transfer process does not occur by random diffusion, but is instead mediated by tracking along stable microtubules. To test this proposal, we have carried out double immunoelectron microscopic labeling experiments on frozen sections of HepG2 hepatoma cells secreting the protein human serum albumin (HSA). By a cycloheximide treatment protocol, the stage during which the transfer of newly synthesized HSA from the ER to the Golgi apparatus occurs in vivo was determined. Sections of the cells were then double immunolabeled using primary antibodies to HSA and to glu-tubulin, the latter specifically detecting stable microtubules. We observed a significantly high frequency of HSA-containing structures between the ER and the Golgi apparatus with which stable microtubules were closely associated. These results support the proposal that stable microtubules may play a critical role in directing the transfer process from the ER to the Golgi apparatus.

Biological Transport, Active↗
PubMed1994

A single-payer system in Jackson Hole clothing.

President Clinton's Health Security Act relies on government regulation, not market forces, to control costs. The act creates an entitlement to comprehensive benefits and places the federal budget at risk for total health care costs in order to achieve universal coverage; it creates a system of new state purchasing monopsonies; and it attempts to control costs with price controls on health plan premiums, set and administered by a National Health Board that would be part of the executive branch, not insulated from political considerations. We believe there is a better way.

Cost Control↗
PubMed1994

Choosing among health plans.

The health care reform system that is ultimately adopted, suggest the authors of this analysis, should rely on market forces, rather than government regulation, to reduce cost and improve quality in our health care delivery. A major portion of this paper compares the Health Security act (the Clinton proposal), the American Health Security Act (the McDermott bill), the Health Equity and Access Reform Today, HEART (the Chafee proposal), and the Managed Competition Act (the Cooper bill). The article focuses on the major areas of difference between these alternative proposals in the extent to which they would achieve true market-based reform, small group reform, universal coverage, and financing mechanisms.

Evaluation Studies as Topic↗
PubMed1993

A soluble secretory protein is first concentrated in the endoplasmic reticulum before transfer to the Golgi apparatus.

A soluble secretory protein is usually present at a much higher concentration in the Golgi apparatus than in the endoplasmic reticulum (ER) inside eukaryotic secretory cells in the steady state. We show by immunoelectron microscopic experiments with the soluble secretory protein serum albumin, inside Hep-G2 human hepatoma cells in culture, that the secretory protein is first concentrated at isolated sites within the ER before it is transferred to the cis face of the Golgi apparatus. This is contrary to expectations of the bulk-flow hypothesis of ER-to-Golgi transfer, and it suggests the involvement of concentration and transfer mechanisms within the ER that have not previously been recognized.

Carcinoma, Hepatocellular↗
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↗