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H Holtzer

Publications and source records attributed to H Holtzer.

At least 55 records · Page 3Linked to original sources
PubMed1983

Taxol induces microtubule-rough endoplasmic reticulum complexes and microtubule-bundles in cultured chondroblasts.

Taxol induces a vast increase in the number of microtubules (MTs) in functional chondroblasts. The drug also induces a marked change in MT distribution. In control cultures, anti-tubulin stains long, fine, sinuous filaments radiating from a perinuclear center. In taxol-treated cells, anti-tubulin stains stubby, straight, chevron-like structures that assume a striking antipodal distribution. Such MT-bundles are relatively stable: they persist for over 48 h after removal of taxol, and even for 16-24 h in Colcemid. Many of these supernumerary MTs bind to, and align on, the cytoplasmic face of the rough endoplasmic reticulum (RER). In binding, the MTs displace the numerous ribosomes that normally stud the surface of the cisternae of the RER. The bound MTs form a remarkably uniform layer with center-to-center spacings of 40 nm. The attached parallel arrays of MTs achieve lengths of over 10 microns. These bound MTs not only dislodge ribosomes from the RER surface, but they also zip together adjacent ER complexes, forming tiers of two to eight cisternae. Numerous cytoplasmic bundles of hexagonally-ordered MTs are also induced. When closely aligned, the MTs assume a crystalline configuration with a six-fold symmetry, a central MT being surrounded by six equidistant MTs. A single cell can have over 100 MT-bundles and the number of MTs per bundles varies from 2-30. The forces aggregating cytoplasmic MT-bundles probably differ from those that bind MTs to the RER. Taxol also fragments the prominent Golgi complex that characterizes actively secreting chondroblasts. No obvious morphologic relationship has yet been detected between these induced MTs and other organelles such as intermediate-sized filaments, microfilaments, mitochondria, Golgi cisternae, or secretory vesicles.

Alkaloids↗
PubMed1983

Altered cell spreading in cytochalasin B: a possible role for intermediate filaments.

Trypsinized chicken embryo dermal fibroblasts plated in the presence of cytochalasin B (CB) quickly attached to the substrate and within 24 h obtained an arborized morphology. This morphology is the result of the pushing out of pseudopodial processes along the substrate from the round central cell body. There were no microfilament bundles in the processes of these cells plated in the presence of CB; however, the processes were packed with highly oriented, parallel-aligned intermediate filaments. Only a few scattered microtubules were seen in these processes. These results demonstrated that in CB, cells are capable of a form of movement, i.e., the extension of pseudopodial processes, without the presence of the microfilament structures usually associated with extensions of the cytoplasm and pseudopodial movements. We also found that arborization did not depend on fibronectin since cells plated in CB did not have fibronectin fibers associated with the processes. Chicken fibroblasts transformed with tsLA24A, a Rous sarcoma virus which is temperature sensitive for pp60src, formed arborized cells with properties similar to those of uninfected fibroblasts when plated in the presence of CB at the nonpermissive temperature (41 degrees C). At the permissive temperature for transformation (36 degrees C), the cells attached to the substrate but remained round. These round cells were not only deficient in microfilament bundles but also lacked the highly organized intermediate filaments found in the processes of the arborized cells at 41 degrees C. Although both microfilament bundles and the fibronectin matrix were decreased after transformation with Rous sarcoma virus, neither was involved in the formation of processes in normal cells plated in CB. Therefore, the inability of the transformed cells to form or maintain processes in CB must be the result of another structural alteration in the transformed cells, such as that of the intermediate filaments.

Actins↗
PubMed1983

Infection of chick limb bud presumptive chondroblasts by a temperature-sensitive mutant of Rous sarcoma virus and the reversible inhibition of their terminal differentiation in culture.

Stage 21 to 22 chicken embryo limb bud cells were infected with a temperature-sensitive mutant of Rous sarcoma virus and were grown in culture. Although control, uninfected cells yielded definitive chondroblasts (by day 4) which initiated the synthesis of the cartilage-characteristic proteoglycan, the transformed cells grown at the permissive temperature failed to do so. These effects were fully reversible after a shift to the nonpermissive temperature. In addition, infected cells at the nonpermissive temperature expressed traits of terminal chondrogenic maturation 2 to 3 days earlier than parallel, uninfected cells. Thus, Rous sarcoma virus-induced transformation reversibly blocks terminal limb bud cell chondrogenesis in culture, at the nonpermissive temperature, viral infection may also induce intracellular or extracellular conditions which favor or accelerate the process of chondrogenic cell maturation.

Animals↗
PubMed1983

Effects of phorbol 12-myristate 13-acetate on the differentiation program of embryonic chick skeletal myoblasts.

The effects of phorbol 12-myristate 13-acetate (PMA) on three aspects of myogenesis have been analyzed: (a) fusion of mononucleated myogenic cells to form myotubes; (b) synthesis and accumulation of two muscle-specific proteins; and (c) DNA synthesis. Using autoradiography combined with immunofluorescent localization of muscle-specific light meromyosin and the muscle-specific intermediate filament protein desmin, we have found that embryonic chick myogenic cells cultured in the presence of PMA (50 nM) initiate the synthesis of both desmin and muscle-specific light meromyosin and, by these criteria, partially differentiate. These cells differ from normal definitive postmitotic myoblasts, however, since they (a) do not fuse; (b) do not assemble normal myofibrils; and (c) incorporate [3H]thymidine. PMA does not appear to induce DNA synthesis in postmitotic myoblasts, but it apparently permits cells to initiate expression of muscle-specific proteins while preventing complete withdrawal from the cell cycle. Inhibition of fusion by PMA has been reported, but continued incorporation of [3H]thymidine in nuclei of cells expressing muscle-specific proteins is a previously undescribed effect of PMA. This effect is not achieved by 4-alpha-phorbol-12, 13-didecanoate, a nonpromoting phorbol ester, and may be relevant to the action of PMA as a tumor promoter.

Animals↗
PubMed1983

Quantal and proliferative cell cycles: how lineages generate cell diversity and maintain fidelity.

There are no known differences between the mechanisms that generate diverse differentiation programs in a mosaic embryo such as Caenorhabdites elegans or in a regulative embryo such as a chick. Transit through an invariant sequence of compartments in a lineage is obligatory for a given precursor cell 1) to inherit its differentiation program from its mother, and 2) to transmit to its daughters, by way of a predetermined binary decision, a new differentiation program. The inheritability of a differentiation program must be encoded in a structural molecule. We postulate that during an S period of a quantal cell cycle, chromosomal structures are so altered that a network of genes that could not be transcribed in the mother becomes available for transcription in the daughters. We do not view as a likely possibility the traditional notion that cell-cell or cell-matrix interactions instruct or commit blank, naive cells to transform into cells with unique differentiation programs. From this perspective, we have initiated experiments to determine the minimal rounds of DNA synthesis, following fertilization, that are required to generate founder cells for several major lineages in the chick. Somewhere between the 15th and 18th generations after fertilization erythrogenic hematocytoblasts that are cytokeratin-positive and vimentin- and hemoglobin-negative undergo a quantal cell cycle. Their daughters are cytokeratin-negative and vimentin- and hemoglobin-positive. DNA synthesis, but not cytokinesis, is an obligatory requirement for this switch in differentiation programs. Essentially similar findings are presented for cells in the cardiogenic, neurogenic, melanogenic, and endothelial lineages. There is no evidence that cell-cell or cell-matrix interactions are required for this diversification. Such interactions, however, may be required for the large number of proliferative cell cycles within particular compartments of particular lineages that are characteristic of all growing or expanding systems. With respect to classical "CFU cells" it is of interest that definitive white blood cells have not yet been identified in these cultures. Lastly, the high ratio of primitive red blood cells to non-red blood cells in the first 40 hours of culture is consistent with the notion that the majority of all cells present in the blastodisc at these early stages are in fact already committed to a unipotent erythrogenic lineage [5, 18, 23, 44, 45]. The issue of changing ratios of cells within compartments of a lineage, as well as of cells in different lineages, is much neglected in consideration of (a) normal embryogenesis, (b) cell-renewal in mature organisms and, particularly,

Animals↗
PubMed1982

Regulation of the synthesis of extracellular matrix components in chondroblasts transformed by a temperature-sensitive mutant of Rous sarcoma virus.

Regulation of cartilage extracellular matrix synthesis has been examined in chondroblasts infected with a temperature-sensitive mutant of Rous sarcoma virus. Cells grown at the nonpermissive temperature synthesized large amounts of several chondroblast-specific polypeptides (type IV proteoglycan core protein, type II procollagen, a proteoglycan link protein(s) and a 60 kd protein) and very low levels of fibronectin. At the permissive temperature, synthesis of chondroblast-specific proteins was coordinately reduced, while fibronectin synthesis was greatly increased. These changes reflected comparable alterations in levels of translatable mRNAs encoding these proteins. This analysis also revealed the unexpected presence in the transformed cells of type I collagen mRNAs, which are not used in intact cells, indicating that a posttranscriptional control (or controls) may be acting in transformed chondroblasts.

Animals↗
PubMed1982

Effects of taxol and Colcemid on myofibrillogenesis.

To determine the relationship between thin filaments, Z-bands, microtubules, intermediate filaments (IFs), T-tubules, and sarcoplasmic reticulum (SR) during myofibrillogenesis, myotubes were selectively depleted of their myofibrils with 12-tetradecanoylphorbol 13-acetate (TPA) and then were allowed to regenerate in (i) normal medium, (ii) taxol, and (iii) Colcemid. Myofibrils assembled in normal medium formed typical A-, I-, Z-, M-, and H-bands and associated IFs, T-tubules, and SR. Myofibrils assembled in taxol formed "A-bands" of aligned thick filaments interdigitating with long microtubules and "I-bands" consisting only of microtubules. These unprecedented sarcomeres lacked thin filaments, Z-bands, and associated IFs and SR. "Solitary A-bands," consisting exclusively of laterally aligned bipolar thick filaments 1.6 microM in length without either thin filaments or microtubules, were observed. Myofibrils assembled in Colcemid formed all myofibrillar components in the absence of microtubules but these did not achieve rigorous lateral alignment. Colcemid and taxol induced the formation of patchy Z-bands that invariably served as insertion sites for thin filaments, irrespective of the presence or absence of adjacent thick filaments. Z-bands may function as actin-organizing centers for each sarcomere.

Actins↗
PubMed1982

The response of chicken embryo dermal fibroblasts to cytochalasin B is altered by Rous sarcoma virus-induced cell transformation.

The drug cytochalasin B (CB), which disrupts the cellular microfilament network, allows the identification of as yet unclassified structural differences between normal and Rous sarcoma virus-transformed chicken embryo fibroblasts. When exposed to CB, normal chick fibroblasts attain an arborized or dendritic morphology. This results as the cytoplasm collapses upon the remaining structural and adhesive components of the cell. Rous sarcoma virus-transformed cells did not form or maintain these dendritic-like processes in the presence of CB and, as a result, rounded up but still remained attached to the substrate. With a temperature-sensitive mutant of Rous sarcoma virus, LA24A, it was possible to show that these effects are completely reversible and dependent on the expression of pp60src. The cytoskeleton in these CB-treated cells was examined by both immunofluorescence and electron microscopy. After exposure to CB, the microfilaments were found to be disrupted similarly throughout both the transformed and the nontransformed cells. In the nontransformed cells arborized by exposure to CB, the extended processes were found to contain intermediate filaments in an unusually high concentration and degree of organization. The distribution of these filaments in the central body of the arborized cells was random. This lower concentration and random distribution was similar to that seen throughout the transformed cells rounded up by exposure to CB. The failure of these transformed cells to arborize in CB indicates that the structural component(s) which is necessary for the formation or maintenance or both of the arborized state is altered by the expression of pp60src.

Animals↗
PubMed1981

Different proteins associated with 10-nanometer filaments in cultured chick neurons and nonneuronal cells.

A protein of molecular size 180 kilodaltons is associated with 10-nanometer filaments in neurons and is immunologically distinct from smaller putative neurofilament subunits and from 10-nanometer filament proteins in nonneuronal cells, such as myotubes and fibroblasts. Neurons do not contain vimentin, the major filament protein in many other cells, including the nonneuronal cells in cultures of neural tissue.

Animals↗