The murine cone photoreceptor: a single cone type expresses both S and M opsins with retinal spatial patterning.

Mice express S and M opsins that form visual pigments for the detection of light and visual signaling in cones. Here, we show that S opsin transcription is higher than that of M opsin, which supports ultraviolet (UV) sensitivity greater than midwavelength sensitivity. Surprisingly, most cones coexpress both S and M opsins in a common cone cell type throughout the retina. All cones express M opsin, but the levels are graded from dorsal to ventral. The levels of S opsin are relatively constant. However, in the far dorsal retina, S opsin is repressed stochastically, such that some cones express M opsin only. These observations indicate that two different mechanisms control M and S opsin expression. We suggest that a common cone type is patterned across the retinal surface to produce phenotypic cone subtypes.

Cytoskeletal reorganizations responsible for the phorbol ester-induced formation of cytoplasmic processes: possible involvement of intermediate filaments.

The tumor promoter phorbol 12-myristate 13-acetate (PMA) induces characteristic reversible changes of cell shape in certain fibroblastic lines: motile lamellas are transformed into noncontractile narrow processes; simultaneously, the actin microfilament network of lamellas is locally disorganized. This reaction to PMA may be regarded as a prototype of reorganizations involving formation of stable cytoplasmic processes. Specific drugs, Taxol and Colcemid, were used to study the role of microtubules and vimentin-containing intermediate filaments (IF) in the development of PMA-induced reorganizations. PMA readily induced formation of noncontractile processes in Taxol-treated fibroblasts; these cells had a profoundly altered microtubular system but noncollapsed IF. A short (1 hr) exposure to PMA induced formation of processes in control cells but not in the Colcemid-treated cells, which had depolymerized microtubules and IF that collapsed around the nucleus. Longer (3-4 hr) exposure of the Colcemid-treated cells to PMA induced partial reversal of the IF collapse; those parts of the peripheral lamellas that contained IF were transformed into narrow noncontractile processes. It is suggested that the local interaction of IF with the actin system is an essential step in the formation of processes from lamellas. The microtubular system controls distribution of IF in the cytoplasm and thus plays an indirect role in the reorganization of the actin cortex.

Microtubule-dependent effect of phorbol ester on the contractility of cytoskeleton of cultured fibroblasts.

The effect of the tumor promoter phorbol 12-myristate 13-acetate (PMA) upon the contractility of permeabilized cell models (cytoskeletons) of mouse fibroblasts was examined. Contraction was induced by incubation of cell models in a solution containing ATP and was assessed quantitatively by measuring alterations of the area of cell model projection on the substrate. Immunofluorescence microscopy was used to assess alterations of cytoskeleton morphology in the course of permeabilization and contraction. It was found that contractility of cell models from PMA-treated fibroblasts was considerably diminished as compared with the models from control fibroblasts. ATP induced only local contraction of certain zones of actin cortex in models from PMA-treated fibroblasts; it did not induce general contraction, characteristic of control models. Normal high contractility was characteristic of the models from the cells preincubated with PMA in combination with Colcemid. PMA is a specific activator of protein kinase C, one of the key enzymes of the membrane signal-transduction pathway. It is suggested that protein kinase C regulates contractility of actin cortex and that the pathway of this regulation has a microtubule-dependent stage blocked by Colcemid.

Multinucleation-induced improvement of the spreading of transformed cells on the substratum.

Multinucleation of various cultured cells was produced by polyethylene glycol-induced fusion or by cytochalasin-induced block of mitosis. It was found that multinucleation induced by both methods considerably improved deficient spreading of all the tested transformed fibroblastic lines; average substratum area occupied by one cell and divided per number of nuclei was 2.0-2.5 times larger for multinucleated cells than for mononucleated ones. Improved spreading was accompanied by increased area of lamellar cytoplasm, increased number of focal contacts, and, in certain lines, by the appearance of actin bundles; numerous microtubules and intermediate filaments radiated from perinuclear zones into the lamellas of multinucleated cells. The number of cell-associated fibronectin fibrils was not increased by multinucleation. Cycloheximide did not prevent the improvement of spreading, suggesting that this effect was not due to any alterations of protein synthesis. Colcemid considerably decreased the effect of multinucleation but did not abolish it completely. It is suggested that increase of spreading is due to multinucleation-associated alterations of quantitative interrelationships between various cell components. One of these alterations is probably increased density of microtubules per unit length of outer cell edge.