Cell biology reviews : CBR最新文献

Our understanding of the function of the corneal endothelium in corneal thickness regulation, and the role of ion transport mechanisms in endothelial physiology, has expanded greatly over the past 25 years. The basic events occurring across the apical and basolateral membranes of the cells are far better understood today, although gaps still exist in the area of the relationship of the cellular and paracellular pathways and their relative contribution to the overall behavior of the endothelium. Little is known about the movement of ions or fluid between the cells or in what proportion this may occur compared to the cellular events. Furthermore, although our knowledge of the ionic movement processes has been enhanced, the link between fluid transfer across the endothelium and ion movements remains an enigma. Important questions also remain concerning the link between electrical characteristics and either ion movement or fluid transport. Improved storage solutions are needed that will preserve endothelial function after transplantation through the provision of a significant improvement in long-term cell survival. The limit to preservation time at present is about 14 days, and the use of other variables in the storage solution may extend this time. In reality, however, extension of preservation time is now of secondary importance relative to the need to enhance cell survival and reduce cell loss following surgery. Whether such improvement can be made with manipulation of the solution alone, or whether refinements are needed in the surgical technique awaits further study. Our comprehension of the biochemical linkage between energy supply and ion movement also remains uncertain in view of the particular intracellular localization of the anionic ATPases to mitochondrial loci. Despite numerous attempts there have been only a few chemicals identified that stimulate the fluid pump, but the level of stimulation has been relatively small and short-lived. No sustained effects have been found that would be of clinical benefit in reducing corneal thickness. A considerable variety of chemicals has been tested on the endothelium and it is unlikely that any new compounds will be identified that will cause enhancement of the fluid pump that would be of clinical benefit in dystrophic, or otherwise swollen, corneas. Of all the toxic responses of the endothelium the majority have been identified because of a malfunction of corneal thickness regulation, with the resultant corneal swelling, or by morphological examination. Only in a few instances has the permeability to non-electrolytes (carboxyfluorescein, inulin/dextran) been measured, and even more rarely have ion fluxes, or pump activity (3H-ouabain binding), been measured.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinoids (retinoic acid and its biofunctional analogs) are widely involved in the control of cell proliferation, cell differentiation, and embryogenic development. A series of novel synthetic retinoids (called retinobenzoic acids), which include retinoid antagonists, have been developed and have been shown to be useful tools to investigate retinoidal action molecular mechanisms. Retinoids elicit their biological effects by binding to specific nuclear receptors (RARs) belonging to a steroid/thyroid nuclear receptor superfamily. RARs act as retinoid-dependent transcription factors which bind to a specific gene site and control the gene's expression. The diversity of retinoidal actions can possibly be interpreted by considering the following characteristics, all of which are quite diversified: the structure and spatial/temporal distribution of RARs, the base sequences which interact with RARs, the cell type specifically determined hierarchy of gene expression, and the nuclear coregulators which interact with RARs. Abnormality of an RAR gene which might cause acute promyelocytic leukemia is also discussed.

In in vitro cultures, isolated microspores and pollen grains can undergo two fundamentally different types of development. In conditions that simulate the tapetum they continue normal gametophytic development to produce functional pollen. Under stress conditions, they are induced to form haploid embryos and plants (sporophytes). In vitro matured pollen is a "minimal" pollen that lacks substances to enhance its reproductive success. In vitro pollen maturation is presented as an experimental system to study pollen development and its interaction with the tapetum. For young binucleate pollen grains, a starvation treatment is sufficient to induce sporophytic development, for microspores a heat shock treatment is required. During the stress treatments, transcription of specific mRNAs is activated. An important aspect of embryogenic induction of binucleate pollen is the derepression of the cell cycle in the G1-arrested vegetative cell.

Our understanding of the function of the corneal endothelium in corneal thickness regulation, and the role of ion transport mechanisms in endothelial physiology, has expanded greatly over the past 25 years. The basic events occurring across the apical and basolateral membranes of the cells are far better understood today, although gaps still exist in the area of the relationship of the cellular and paracellular pathways and their relative contribution to the overall behavior of the endothelium. Little is known about the movement of ions or fluid between the cells or in what proportion this may occur compared to the cellular events. Furthermore, although our knowledge of the ionic movement processes has been enhanced, the link between fluid transfer across the endothelium and ion movements remains an enigma. Important questions also remain concerning the link between electrical characteristics and either ion movement or fluid transport. Improved storage solutions are needed that will preserve endothelial function after transplantation through the provision of a significant improvement in long-term cell survival. The limit to preservation time at present is about 14 days, and the use of other variables in the storage solution may extend this time. In reality, however, extension of preservation time is now of secondary importance relative to the need to enhance cell survival and reduce cell loss following surgery. Whether such improvement can be made with manipulation of the solution alone, or whether refinements are needed in the surgical technique awaits further study. Our comprehension of the biochemical linkage between energy supply and ion movement also remains uncertain in view of the particular intracellular localization of the anionic ATPases to mitochondrial loci. Despite numerous attempts there have been only a few chemicals identified that stimulate the fluid pump, but the level of stimulation has been relatively small and short-lived. No sustained effects have been found that would be of clinical benefit in reducing corneal thickness. A considerable variety of chemicals has been tested on the endothelium and it is unlikely that any new compounds will be identified that will cause enhancement of the fluid pump that would be of clinical benefit in dystrophic, or otherwise swollen, corneas. Of all the toxic responses of the endothelium the majority have been identified because of a malfunction of corneal thickness regulation, with the resultant corneal swelling, or by morphological examination. Only in a few instances has the permeability to non-electrolytes (carboxyfluorescein, inulin/dextran) been measured, and even more rarely have ion fluxes, or pump activity (3H-ouabain binding), been measured.(ABSTRACT TRUNCATED AT 400 WORDS)

The data on proteolytic control of protein topogenesis and of cell organelles assembly are critically reviewed. There are grounds to suggest that the slow conformational maturation of polypeptide chains, the defects in their modification (i. e., glycosylation) and the delay in some subcellular compartments on the exocytic pathway are responsible for the enhanced proteolytic degradation of proteins. The same holds true for the miscompartmentalized proteins. Another aspect of proteolytic control is the assembly of the multisubunit protein complexes in a cell. The cases of nonstoichiometric subunits synthesis, followed by the degradation of unassembled subunits, are summarized. Stoichiometric ratios of subunits in a cell are supposed to be established by this means. Such a mechanism is especially important when these subunits are synthesized asynchronously or when they are encoded by different genomes and are produced by different protein-synthesizing systems. Most probably, proteolytic control is operative at all stages of the respiratory competent mitochondria formation: (i) when protein precursors are imported from the cytosol, (ii) when the individual polypeptides are assembled into complexes, and (iii) when functioning multienzyme ensembles are formed. From this point of view, a pleiotropic character of mutations in individual proteins of mitochondria and chloroplasts is considered. Thus proteolytic systems of a cell play an important role in protein topogenesis and in the biogenes of cell organelles.

Retinoids (retinoic acid and its biofunctional analogs) are widely involved in the control of cell proliferation, cell differentiation, and embryogenic development. A series of novel synthetic retinoids (called retinobenzoic acids), which include retinoid antagonists, have been developed and have been shown to be useful tools to investigate retinoidal action molecular mechanisms. Retinoids elicit their biological effects by binding to specific nuclear receptors (RARs) belonging to a steroid/thyroid nuclear receptor superfamily. RARs act as retinoid-dependent transcription factors which bind to a specific gene site and control the gene's expression. The diversity of retinoidal actions can possibly be interpreted by considering the following characteristics, all of which are quite diversified: the structure and spatial/temporal distribution of RARs, the base sequences which interact with RARs, the cell type specifically determined hierarchy of gene expression, and the nuclear coregulators which interact with RARs. Abnormality of an RAR gene which might cause acute promyelocytic leukemia is also discussed.