Myosin I phosphorylation is increased by chemotactic stimulation

Directed cell migration occurs in response to extracellular cues. Following stimulation of a cell with chemoattractant, a significant rearrangement of the actin cytoskeleton is mediated by intracellular signaling pathways and results in polarization of the cell and movement via pseudopod extension. Amoeboid myosin Is play a critical role in regulating pseudopod formation in Dictyostelium, and their activity is activated by heavy chain phosphorylation. The effect of chemotactic stimulation on the in vivo phosphorylation level of a Dictyostelium myosin I, myoB, was tested. The myoB heavy chain is phosphorylated in vivo on serine 322 (the myosin TEDS rule phosphorylation site) in chemotactically competent cells. The level of myoB phosphorylation increases following stimulation of starving cells with the chemoattractant cAMP. A 3-fold peak increase in the level of phosphorylation is observed at 60 s following stimulation, a time at which the Dictyostelium cell actively extends pseudopodia. These findings suggest that chemotactic stimulation results in increased myoB activity via heavy chain phosphorylation and contributes to the global extension of pseudopodia that occurs prior to polarization and directed motility.

Hepatocyte growth factor/scatter factor is a neurotrophic survival factor for lumbar but not for other somatic motoneurons in the chick embryo

Hepatocyte growth factor/scatter factor (HGF/SF) is expressed in the developing limb muscles of the chick embryo during the period of spinal motoneuron (MN) programmed cell death, and its receptor c-met is expressed in lumbar MNs during this same period. Although cultured motoneurons from brachial, thoracic, and lumbar segments are all rescued from cell death by chick embryo muscle extract (CMX) as well as by other specific trophic agents, HGF/SF only promotes the survival of lumbar MNs. Similarly, treatment of embryos in ovo with exogenous HGF/SF rescues lumbar but not other somatic MNs from cell death. Blocking antibodies to HGF/SF (anti-HGF) reduce the effects of CMX on MN survival in vitro and decrease the number of lumbar MNs in vivo. The expression of c-met on MNs in vivo is regulated by a limb-derived trophic signal distinct from HGF/SF. HGF/SF is a potent, select, and physiologically relevant survival factor for a subpopulation of developing spinal MNs in the lumbar segments of the chick embryo.

The myosin I SH3 domain and TEDS rule phosphorylation site are required for in vivo function

The class I myosins play important roles in controlling many different types of actin-based cell movements. Dictyostelium cells either lacking or overexpressing amoeboid myosin Is have significant defects in cortical activities such as pseudopod extension, cell migration, and macropinocytosis. The existence of Dictyostelium null mutants with strong phenotypic defects permits complementation analysis as a means of exploring important functional features of the myosin I heavy chain. Mutant Dictyostelium cells lacking two myosin Is exhibit profound defects in growth, endocytosis, and rearrangement of F-actin. Expression of the full-length myoB heavy chain in these cells fully rescues the double mutant defects. However, mutant forms of the myoB heavy chain in which a serine at the consensus phosphorylation site has been altered to an alanine or in which the C-terminal SH3 domain has been removed fail to complement the null phenotype. The wild-type and mutant forms of the myoB heavy chain appeared to be properly localized when they were expressed in the myosin I null mutants. These results suggest that the amoeboid myosin I consensus phosphorylation site and SH3 domains do not play a role in the localization of myosin I, but are absolutely required for in vivo function.

Changes in computerized videokeratography induced by artificial tears

PURPOSE

To analyze the effect of several standard artificial tear preparations on computerized videokeratographic measurements.

SETTING

Cullen Eye Insitute, Baylor College of Medicine, Department of Ophthalmology, Houston, Texas, USA.

METHODS

We evaluated one eye each in 18 normal volunteers. Using the EyeSys Corneal Analysis System (EyeSys Technologies), we obtained corneal topographic measurements at baseline and 0.5, 1, 2, 3, 4, 5, 6, 8, and 10 minutes after instillation of the following preparations: balanced salt solution, Tears Naturale II, Tears Naturale Free, Cellufresh, Celluvisc, HypoTears, and HypoTears PF. We analyzed changes in curvature of the keratographic rings at radii 1 to 5 mm and changes in keratometric-equivalent astigmatic power and meridian.

RESULTS

All preparations except HypoTears and Tears Naturale II induced statistically significant, time-dependent changes in mean corneal power in the central 5 mm corneal zone compared with baseline measurements (P < .05). The relationship between change in dioptric power over time varied with preparation type and was nonlinear in nature. In all cases, the mean induced change was < or = 0.5 diopter. Except for Celluvisc, tear administration produced minimal changes in the values of corneal astigmatic power or meridian.

CONCLUSION

When performing serial measurements of mean corneal power, the greatest consistency was achieved with no tears or with instillation of HypoTears or Tears Naturale II.

Myosin I overexpression impairs cell migration

Dictyostelium myoB, a member of the myosin I family of motor proteins, is important for controlling the formation and retraction of membrane projections by the cell's actin cortex (Novak, K.D., M.D. Peterson, M.C. Reedy, and M.A. Titus. 1995. J. Cell Biol. 131:1205-1221). Mutants that express a three- to sevenfold excess of myoB (myoB+ cells) were generated to further analyze the role of myosin I in these processes. The myoB+ cells move with an instantaneous velocity that is 35% of the wild-type rate and exhibit a 6-8-h delay in initiation of aggregation when placed under starvation conditions. The myoB+ cells complete the developmental cycle after an extended period of time, but they form fewer fruiting bodies that appear to be small and abnormal. The myoB+ cells are also deficient in their ability both to form distinct F-actin filled projections such as crowns and to become elongate and polarized. This defect can be attributed to the presence of at least threefold more myoB at the cortex of the myoB+ cells. In contrast, threefold overexpression of a truncated myoB that lacks the src homology 3 (SH3) domain (myoB/SH3- cells) or myoB in which the consensus heavy chain phosphorylation site was mutated to an alanine (S332A-myoB) does not disturb normal cellular function. However, there is an increased concentration of myoB in the cortex of the myoB/SH3- and S332A-myoB cells comparable to that found in the myoB+ cells. These results suggest that excess full-length cortical myoB prevents the formation of the actin-filled extensions required for locomotion by increasing the tension of the F-actin cytoskeleton and/or retracting projections before they can fully extend. They also demonstrate a role for the phosphorylation site and SH3 domain in mediating the in vivo activity of myosin I.

Examination of the endosomal and lysosomal pathways in Dictyostelium discoideum myosin I mutants

The role of myosin Is in endosomal trafficking and the lysosomal system was investigated in a Dictyostelium discoideum myosin I double mutant myoB-/C-, that has been previously shown to exhibit defects in fluid-phase endocytosis during growth in suspension culture (Novak et al., 1995). Various properties of the endosomal pathway in the myoB-/C- double mutant as well as in the myoB- and myoC- single mutants, including intravesicular pH, and intracellular retention time and exocytosis of a fluid phase marker, were found to be indistinguishable from wild-type parental cells. The intimate connection between the contractile vacuole complex and the endocytic pathway in Dictyostelium, and the localization of a myosin I to the contractile vacuole in Acanthamoeba, led us to also examine the structure and function of this organelle in the three myosin I mutants. No alteration in contractile vacuole structure or function was observed in the myoB-, myoC- or myoB-/C- cell lines. The transport, processing, and localization of a lysosomal enzyme, alpha-mannosidase, were also unaltered in all three mutants. However, the myoB- and myoB-/C- cell lines, but not the myoC- cell line, were found to oversecrete the lysosomal enzymes alpha-mannosidase and acid phosphatase, during growth and starvation. None of the mutants oversecreted proteins following the constitutive secretory pathway. Two additional myosin I mutants, myoA- and myoA-/B-, were also found to oversecrete the lysosomally localized enzymes alpha-mannosidase and acid phosphatase. Taken together, these results suggest that these myosins do not play a role in the intracellular movement of vesicles, but that they may participate in controlling events that occur at the actin-rich cortical region of the cell. While no direct evidence has been found for the association of myosin Is with lysosomes, we predict that the integrity of the lysosomal system is tied to the fidelity of the actin cortex, and changes in cortical organization could influence lysosomal-related membrane events such as internalization or transit of vesicles to the cell surface.

Dictyostelium myosin I double mutants exhibit conditional defects in pinocytosis

The functional relationship between three Dictyostelium myosin Is, myoA, myoB, and myoC, has been examined through the creation of double mutants. Two double mutants, myoA-/B- and myoB-/C-, exhibit similar conditional defects in fluid-phase pinocytosis. Double mutants grown in suspension culture are significantly impaired in their ability to take in nutrients from the medium, whereas they are almost indistinguishable from wild-type and single mutant strains when grown on a surface. The double mutants are also found to internalize gp126, a 116-kD membrane protein, at a slower rate than either the wild-type or single mutant cells. Ultrastructural analysis reveals that both double mutants possess numerous small vesicles, in contrast to the wild-type or myosin I single mutants that exhibit several large, clear vacuoles. The alterations in fluid and membrane internalization in the suspension-grown double mutants, coupled with the altered vesicular profile, suggest that these cells may be compromised during the early stages of pinocytosis, a process that has been proposed to occur via actin-based cytoskeletal rearrangements. Scanning electron microscopy and rhodamine-phalloidin staining indicates that the myosin I double mutants appear to extend a larger number of actin-filled structures, such as filopodia and crowns, than wild-type cells. Rhodamine-phalloidin staining of the F-actin cytoskeleton of these suspension-grown cells also reveals that the double mutant cells are delayed in the rearrangement of cortical actin-rich structures upon adhesion to a substrate. We propose that myoA, myoB, and myoC play roles in controlling F-actin filled membrane projections that are required for pinosome internalization in suspension.

Topical anesthesia for phacoemulsification: initial 20-case series with one month follow-up

We report the results of our initial 20 phacoemulsification cases performed using topical anesthesia. The preferred topical regimen consisted of preservative-free 0.75% bupivacaine. Intravenous sedation was provided primarily with fentanyl and midazolam. Phacoemulsification was performed through a scleral tunnel incision, and a one-piece poly(methyl methacrylate) or three-piece silicone intraocular lens was implanted. There were no complications with the anesthetic technique. One day postoperatively, 69% of patients with a desired refractive error within 0.75 diopters of emmetropia had an uncorrected visual acuity of 20/40 or better. At one month, all patients had a best corrected acuity of 20/30 or better, and 60%, 20/20 or better. Eighteen patients reported complete intraoperative comfort, and 17 reported complete postoperative comfort. Seven of the 10 patients who had had previous peribulbar anesthesia preferred topical. All 10 "first eye" patients said they would choose topical anesthesia for future surgery. With appropriate case selection, topical anesthesia for phacoemulsification surgery can be used with excellent intraoperative and postoperative results.

Chlamydia trachomatis can be transmitted by a nonporous plastic surface in vitro

Chlamydial conjunctivitis is a disease associated with venereal transmission through direct sexual contact or autoinoculation with genital secretions. Appropriate therapy for patients and their sexual partners involves important questions regarding the source of infection and mode of transmission. This study explored the potential role of a fomite, i.e., an environmental surface, as a possible vector of transmission. We determined the in vitro recovery of Chlamydia trachomatis from a nonporous plastic surface under ambient and humid conditions using the standard shell vial technique and confirmation by direct monoclonal immunofluorescence. Under ambient conditions, the TP50 (time at which 50% of samples were positive for Chlamydia) was 5 min, with complete desiccation occurring at 45 min. Under humid conditions, the TP50 was 52.5 min and complete desiccation did not occur up to 3 h. Beyond 45 min, a significantly greater number of positive chlamydial samples were collected under humid conditions (11 of 30) than under ambient conditions (0 of 30) (p = 0.00016). We conclude that a fomite, such as a nonporous plastic surface, may serve as a potential vector for the transmission of chlamydial infection to the eye, especially under humid conditions. This new information may prove useful in counseling patients and their sexual partners.

Molecular genetic analysis of myoF, a new Dictyostelium myosin I gene

Several new members of the Dictyostelium myosin family have been identified by physical mapping techniques in combination with PCR. Here we describe the initial molecular genetic characterization of one of these, myoF. A 1-kb segment of the myoF gene was obtained by the PCR and used as a specific probe for Northern analysis and as a vehicle for gene-targeting studies. The myoF gene is expressed as a 3.7-kb message, a size consistent with it encoding a myosin I class unconventional myosin, bringing the total of myosin is present in Dictyostelium to six. Analysis of strains in which the myoF gene has been disrupted reveals that loss of the myoF protein does not result in obvious defects either in cellular translocation, or in other readily assayed actin-based processes. The results of our investigation indicate that the myosin I family is quite large in Dictyostelium, and that several members, including myoF, may either be functionally redundant or play roles in as yet undescribed actin-based processes.

Molecular genetic analysis of myoC, a Dictyostelium myosin I

The protozoan myosin Is are widely expressed actin-based motors, yet their in vivo roles remain poorly understood. Molecular genetic studies have been carried out to determine their in vivo function in the simple eukaryote Dictyostelium, an organism that contains a family of four myosin Is. Here we report the characterization of myoC, a gene that encodes a fifth member of this family. Analysis of the deduced amino acid sequence reveals that the myoC gene encodes a myosin that is homologous to the well-described Acanthamoeba myosin Is as well as to Dictyostelium myoB and -D. The expression pattern of the myoC mRNA is similar to that of myoB and myoD, with a peak of expression at times of maximal cell migration, around 6 hours development. Deletion of the myoB gene has been previously shown to result in mutant cells that are defective in pseudopod extension and phagocytosis. However, no obvious differences in cell growth, development, phagocytosis or motility were detected in cells in which the myoC gene had been disrupted by homologous recombination. F-actin localization and ultrastructural organization also appeared unperturbed in myoC- cells. This apparent ‘lack’ of phenotype in a myosin I single knockout cannot be simply explained by redundancy of function. Our results rather suggest that the present means of assessing myosin I function in vivo are insufficient to identify the unique roles of these actin-based motors.

Interaction between NF-kappa B- and serum response factor-binding elements activates an interleukin-2 receptor alpha-chain enhancer specifically in T lymphocytes

We find that a short enhancer element containing the NF-kappa B binding site from the interleukin-2 receptor alpha-chain gene (IL-2R alpha) is preferentially activated in T cells. The IL-2R alpha enhancer binds NF-kappa B poorly and is only weakly activated by the NF-kappa B site alone. Serum response factor (SRF) binds to a site adjacent to the NF-kappa B site in the IL-2R enhancer, and both sites together have strong transcriptional activity specifically in T cells. Surprisingly, the levels of SRF constitutively expressed in T cells are consistently higher than in other cell types. Overexpression of SRF in B cells causes the IL-2R enhancer to function as well as it does in T cells, suggesting that the high level of SRF binding in T cells is functionally important.