Cell movement is guided by the rigidity of the substrate

Directional cell locomotion is critical in many physiological processes, including morphogenesis, the immune response, and wound healing. It is well known that in these processes cell movements can be guided by gradients of various chemical signals. In this study, we demonstrate that cell movement can also be guided by purely physical interactions at the cell-substrate interface. We cultured National Institutes of Health 3T3 fibroblasts on flexible polyacrylamide sheets coated with type I collagen. A transition in rigidity was introduced in the central region of the sheet by a discontinuity in the concentration of the bis-acrylamide cross-linker. Cells approaching the transition region from the soft side could easily migrate across the boundary, with a concurrent increase in spreading area and traction forces. In contrast, cells migrating from the stiff side turned around or retracted as they reached the boundary. We call this apparent preference for a stiff substrate "durotaxis." In addition to substrate rigidity, we discovered that cell movement could also be guided by manipulating the flexible substrate to produce mechanical strains in the front or rear of a polarized cell. We conclude that changes in tissue rigidity and strain could play an important controlling role in a number of normal and pathological processes involving cell locomotion.

Zebrafish hox clusters and vertebrate genome evolution

HOX genes specify cell fate in the anterior-posterior axis of animal embryos. Invertebrate chordates have one HOX cluster, but mammals have four, suggesting that cluster duplication facilitated the evolution of vertebrate body plans. This report shows that zebrafish have seven hox clusters. Phylogenetic analysis and genetic mapping suggest a chromosome doubling event, probably by whole genome duplication, after the divergence of ray-finned and lobe-finned fishes but before the teleost radiation. Thus, teleosts, the most species-rich group of vertebrates, appear to have more copies of these developmental regulatory genes than do mammals, despite less complexity in the anterior-posterior axis.

Stresses at the cell-to-substrate interface during locomotion of fibroblasts

Recent technological improvements in the elastic substrate method make it possible to produce spatially resolved measurements of the tractions exerted by single motile cells. In this study we have applied these developments to produce maps of the tractions exerted by 3T3 fibroblasts during steady locomotion. The resulting images have a spatial resolution of approximately 5 micrometers and a maximum intensity of approximately 10(2) kdyn/cm2 (10(4) pN/micrometers2). We find that the propulsive thrust for fibroblast locomotion, approximately 0.2 dyn, is imparted to the substratum within 15 micrometers of the leading edge. These observations demonstrate that the lamellipodium of the fibroblast is able to generate intense traction stress. The cell body and posterior seem to be mechanically passive structures pulled forward entirely by this action.

Exchange of actin subunits at the leading edge of living fibroblasts: possible role of treadmilling

Previous observations indicated that the lamellipodium ("leading edge") of fibroblasts contains a dense meshwork, as well as numerous bundles (microspikes) of actin filaments. Most, if not all, of the filaments have a uniform polarity, with the "barbed" end associated with the membrane. I investigated whether and how actin subunits exchange in this region by microinjecting living gerbil fibroma cells (IMR-33) with actin that had been labeled with iodoacetamidotetramethylrhodamine. After incorporation of the labeled actin into the lamellipodium, I used a laser microbeam to photobleach a 3-4-micron region at and surrounding a microspike, without disrupting the integrity of the structure. I then recorded the pattern of fluorescence recovery and analyzed it using a combination of TV image intensification and digital image processing techniques. Fluorescence recovery was first detected near the edge of the cell and then moved toward the cell's center at a constant rate of 0.79 +/- 0.31 micron/min. When only part of the lamellipodium near the edge of the cell was photobleached, the bleached spot also moved toward the cell's center and through an area unbleached by the laser beam. These results indicated that steady state incorporation of actin subunits occurred predominantly at the membrane-associated end of actin filaments, and that actin subunits in the lamellipodium underwent a constant movement toward the center of the cell. I suggest that treadmilling, possibly in combination with other molecular interactions, may provide an effective mechanism for the movement of actin subunits and the protrusion of cytoplasm in the lamellipodium of fibroblasts.

Substrate flexibility regulates growth and apoptosis of normal but not transformed cells

One of the hallmarks of oncogenic transformation is anchorage-independent growth (27). Here we demonstrate that responses to substrate rigidity play a major role in distinguishing the growth behavior of normal cells from that of transformed cells. We cultured normal or H-ras-transformed NIH 3T3 cells on flexible collagen-coated polyacrylamide substrates with similar chemical properties but different rigidity. Compared with cells cultured on stiff substrates, nontransformed cells on flexible substrates showed a decrease in the rate of DNA synthesis and an increase in the rate of apoptosis. These responses on flexible substrates are coupled to decreases in cell spreading area and traction forces. In contrast, transformed cells maintained their growth and apoptotic characteristics regardless of substrate flexibility. The responses in cell spreading area and traction forces to substrate flexibility were similarly diminished. Our results suggest that normal cells are capable of probing substrate rigidity and that proper mechanical feedback is required for regulating cell shape, cell growth, and survival. The loss of this response can explain the unregulated growth of transformed cells.

Intragenic deletion in the gene encoding ubiquitin carboxy-terminal hydrolase in gad mice

The gracile axonal dystrophy (gad) mouse is an autosomal recessive mutant that shows sensory ataxia at an early stage, followed by motor ataxia at a later stage. Pathologically, the mutant is characterized by 'dying-back' type axonal degeneration and formation of spheroid bodies in nerve terminals. Recent pathological observations have associated brain ageing and neurodegenerative diseases with progressive accumulation of ubiquitinated protein conjugates. In gad mice, accumulation of amyloid beta-protein and ubiquitin-positive deposits occur retrogradely along the sensory and motor nervous systems. We previously reported that the gad mutation was transmitted by a gene on chromosome 5 (refs 10,11). Here we find that the gad mutation is caused by an in-frame deletion including exons 7 and 8 of Uchl1, encoding the ubiquitin carboxy-terminal hydrolase (UCH) isozyme (Uch-l1) selectively expressed in the nervous system and testis. The gad allele encodes a truncated Uch-l1 lacking a segment of 42 amino acids containing a catalytic residue. As Uch-l1 is thought to stimulate protein degradation by generating free monomeric ubiquitin, the gad mutation appears to affect protein turnover. Our data suggest that altered function of the ubiquitin system directly causes neurodegeneration. The gad mouse provides a useful model for investigating human neurodegenerative disorders.

Risk factors for surgical site infection after elective resection of the colon and rectum: a single-center prospective study of 2,809 consecutive patients

OBJECTIVE

To identify the risk factors for surgical site infection (SSI) in patients undergoing elective resection of the colon and rectum.

SUMMARY BACKGROUND DATA

SSI causes a substantial number of deaths and complications. Determining risk factors for SSI may provide information on reducing complications and improving outcome.

METHODS

The authors performed a prospective study of 2,809 consecutive patients undergoing elective colorectal resection via laparotomy between February 1995 and December 1998 at a single institution. The outcome of interest was SSI, which was classified as being incisional or organ/space with or without clinical leakage. A likelihood ratio forward regression model was used to assess the independent association of variables with SSIs.

RESULTS

The overall SSI, incisional SSI, and organ/space SSI with and without clinical anastomotic leakage rates were 4.7%, 3%, 2%, and 0.8%, respectively. Risk factors for overall SSI were American Society of Anesthesiology (ASA) score 2 or 3 (odd ratio [OR] = 1.7), male gender (OR = 1.5), surgeons (OR = 1.3-3.3), types of operation (OR = 0.3-2.1), creation of ostomy (OR = 2.1), contaminated wound (OR = 2.9), use of drainage (OR = 1.6), and intra- or postoperative blood transfusion (1-3 units, OR = 5.3; >/=4 units, OR = 6.2). However, SSIs at specific sites differed from each other with respect to the risk factors. Among a variety of risk factors, only blood transfusion was consistently associated with a risk of SSI at any specific site.

CONCLUSIONS

In addition to ASA score and surgical wound class, blood transfusion, creation of ostomy, types of operation, use of drainage, sex, and surgeons were important in predicting SSIs after elective colorectal resection.

Parallel formation and synergism of hydrolytic enzymes and peptaibol antibiotics, molecular mechanisms involved in the antagonistic action of Trichoderma harzianum against phytopathogenic fungi

Chitinase, beta-1,3-glucanase, and protease activities were formed when Trichoderma harzianum mycelia, grown on glucose as the sole carbon source, were transferred to fresh medium containing cell walls of Botrytis cinerea. Chitobiohydrolase, endochitinase, and beta-1,3-glucanase activities were immunologically detected in culture supernatants by Western blotting (immunoblotting), and the first two were quantified by enzyme-linked immunosorbent assay. Under the same conditions, exogenously added [U-14C]valine was incorporated in acetone-soluble compounds with an apparent M(r) of < 2,000. These compounds comigrated with the peptaibols trichorzianines A1 and B1 in thin-layer chromatography and released [U-14C]valine after incubation in 6N HCl. Incorporation of radioactive valine into this material was stimulated by the exogenous supply of alpha-aminoisobutyric acid, a rare amino acid which is a major constituent of peptaibols. The obtained culture supernatants inhibited spore germination as well as hyphal elongation of B. cinerea. Culture supernatants from mycelia placed in fresh medium without cell walls of B. cinerea did not show hydrolase activities, incorporation of [U-14C]valine into peptaibol-like compounds, and inhibition of fungal growth. Purified trichorzianines A1 and B1 as well as purified chitobiohydrolase, endochitinase, or beta-1,3-glucanase inhibited spore germination and hyphal elongation, but at concentrations higher than those observed in the culture supernatants. However, when the enzymes and the peptaibols were tested together, an antifungal synergistic interaction was observed and the 50% effective dose values obtained were in the range of those determined in the culture supernatants. Therefore, the parallel formation and synergism of hydrolytic enzymes and antibiotics may have an important role in the antagonistic action of T. harzianum against fungal phytopathogens.

Mammalian spindle orientation and position respond to changes in cell shape in a dynein-dependent fashion

In animal cells, positioning of the mitotic spindle is crucial for defining the plane of cytokinesis and the size ratio of daughter cells. We have characterized this phenomenon in a rat epithelial cell line using microscopy, micromanipulation, and microinjection. Unmanipulated cells position the mitotic spindle near their geometric center, with the spindle axis lying roughly parallel to the long axis of the cell. Spindles that were initially misoriented underwent directed rotation and caused a delay in anaphase onset. To gain further insight into this process, we gently deformed cells with a blunted glass needle to change the spatial relationship between the cortex and spindle. This manipulation induced spindle movement or rotation in metaphase and/or anaphase, until the spindle reached a proper position relative to the deformed shape. Spindle positioning was inhibited by either treatment with low doses of nocodazole or microinjection of antibodies against dynein, apparently due to the disruption of the organization of dynein and/or astral microtubules. Our results suggest that mitotic cells continuously monitor and maintain the position of the spindle relative to the cortex. This process is likely driven by interactions among astral microtubules, the motor protein dynein, and the cell cortex and may constitute part of a mitotic checkpoint mechanism.

Fatal bilateral chylothorax in mice lacking the integrin alpha9beta1

Members of the integrin family of adhesion receptors mediate both cell-cell and cell-matrix interactions and have been shown to play vital roles in embryonic development, wound healing, metastasis, and other biological processes. The integrin alpha9beta1 is a receptor for the extracellular matrix proteins osteopontin and tenacsin C and the cell surface immunoglobulin vascular cell adhesion molecule-1. This receptor is widely expressed in smooth muscle, hepatocytes, and some epithelia. To examine the in vivo function of alpha9beta1, we have generated mice lacking expression of the alpha9 subunit. Mice homozygous for a null mutation in the alpha9 subunit gene appear normal at birth but develop respiratory failure and die between 6 and 12 days of age. The respiratory failure is caused by an accumulation of large volumes of pleural fluid which is rich in triglyceride, cholesterol, and lymphocytes. alpha9(-/-) mice also develop edema and lymphocytic infiltration in the chest wall that appears to originate around lymphatics. alpha9 protein is transiently expressed in the developing thoracic duct at embryonic day 14, but expression is rapidly lost during later stages of development. Our results suggest that the alpha9 integrin is required for the normal development of the lymphatic system, including the thoracic duct, and that alpha9 deficiency could be one cause of congenital chylothorax.

Aqueous humor dynamics in the aging human eye

PURPOSE

Healthy subjects were recruited to identify normal, age-associated changes in intraocular pressure and aqueous humor dynamics.

METHODS

Normal healthy subjects from two age groups were enrolled in the study: (1) those from 20 to 30 years of age (n = 51) and (2) those 60 years of age and older (n = 53). Intraocular pressure was measured by pneumatonometry, tonographic outflow facility by pneumatonography, and episcleral venous pressure by venomanometry. Aqueous flow and outflow facility were determined by a fluorophotometric technique. Uveoscleral outflow and anterior chamber volume were calculated. Results from the older group were compared with those from the younger group by means of unpaired, two-tailed t tests.

RESULTS

Compared with the younger group, the older group showed significant differences as follows: smaller anterior chamber volume (160+/-39 vs. 247+/-39 microl; mean +/- SD; P< .00001), reduced aqueous flow (2.4+/-0.6 vs. 2.8+/-0.8 microl/minute; P = .002), and reduced uveoscleral outflow (1.10+/-0.81 vs. 1.52+/-0.81 microl/minute; P = .009).

CONCLUSIONS

In the healthy aging eye, there is a reduction in the production of aqueous humor and a reduction in its drainage through the uveoscleral outflow pathway.

Mechanism of the formation of contractile ring in dividing cultured animal cells. II. Cortical movement of microinjected actin filaments

The contractile ring in dividing animal cells is formed primarily through the reorganization of existing actin filaments (Cao, L.-G., and Y.-L. Wang. 1990. J. Cell Biol. 110:1089-1096), but it is not clear whether the process involves a random recruitment of diffusible actin filaments from the cytoplasm, or a directional movement of cortically associated filaments toward the equator. We have studied this question by observing the distribution of actin filaments that have been labeled with fluorescent phalloidin and microinjected into dividing normal rat kidney (NRK) cells. The labeled filaments are present primarily in the cytoplasm during prometaphase and early metaphase, but become associated extensively with the cell cortex 10-15 min before the onset of anaphase. This process is manifested both as an increase in cortical fluorescence intensity and as movements of discrete aggregates of actin filaments toward the cortex. The concentration of actin fluorescence in the equatorial region, accompanied by a decrease of fluorescence in polar regions, is detected 2-3 min after the onset of anaphase. By directly tracing the distribution of aggregates of labeled actin filaments, we are able to detect, during anaphase and telophase, movements of cortical actin filaments toward the equator at an average rate of 1.0 micron/min. Our results, combined with previous observations, suggest that the organization of actin filaments during cytokinesis probably involves an association of cytoplasmic filaments with the cortex, a movement of cortical filaments toward the cleavage furrow, and a dissociation of filaments from the equatorial cortex.

New horizons for cytokinesis

The mechanism of cytokines is an old problem in cell biology that has received fresh attention recently with a large variety of powerful approaches and experimental systems. Significant advances have been made on the structure of the cortical cytoskeleton, the identification of proteins and genes involved, and the regulatory mechanism. Many surprises have surfaced within the past two years, leading us toward a major revision in our understanding of this important process.

Signals from the spindle midzone are required for the stimulation of cytokinesis in cultured epithelial cells

The interaction between the mitotic spindle and the cellular cortex is thought to play a critical role in stimulating cell cleavage. However, little is understood about the nature of such interactions, particularly in tissue culture cells. We have investigated the role of the spindle midzone in signaling cytokinesis by creating a barrier in cultured epithelial cells with a blunted needle, to block signals that may emanate from this region. When the barrier was created during metaphase or early anaphase, cleavage took place only on the sides of the cortex facing the mitotic spindle. Microtubules on the cleaving side showed organization typical of that in normal dividing cells. On the noncleaving side, most microtubules passed from one side of the equator into the other without any apparent organization, and actin filaments failed to organize in the equatorial region. When the barrier was created after the first minute of anaphase, cells showed successful cytokinesis, with normal organization of microtubules and actin filaments on both sides of the barrier. Our study suggests that transient signals from the midzone of early anaphase spindles are required for equatorial contraction in cultured cells and that such signaling may involve the organization of microtubules near the equator.

Mechanical stretch-induced endoplasmic reticulum stress, apoptosis and inflammation contribute to thoracic aortic aneurysm and dissection

Thoracic aortic aneurysm/dissection (TAAD) is characterized by excessive smooth muscle cell (SMC) loss, extracellular matrix (ECM) degradation and inflammation. In response to certain stimuli, endoplasmic reticulum (ER) stress is activated and regulates apoptosis and inflammation. Excessive apoptosis promotes aortic inflammation and degeneration, leading to TAAD. Therefore, we studied the role of ER stress in TAAD formation. A lysyl oxidase inhibitor, 3-aminopropionitrile fumarate (BAPN), was administrated to induce TAAD formation in mice, which showed significant SMC loss (α-SMA level). Excessive apoptosis (TUNEL staining) and ER stress (ATF4 and CHOP), along with inflammation, were present in TAAD samples from both mouse and human. Transcriptional profiling of SMCs after mechanical stress demonstrated the expression of genes for ER stress and inflammation. To explore the causal role of ER stress in initiating degenerative signalling events and TAAD, we treated wild-type (CHOP(+/+)) or CHOP(-/-) mice with BAPN and found that CHOP deficiency protected against TAAD formation and rupture, as well as reduction in α-SMA level. Both SMC apoptosis and inflammation were significantly reduced in CHOP(-/-) mice. Moreover, SMCs isolated from CHOP(-/-) mice were resistant to mechanical stress-induced apoptosis. Taken together, our results demonstrated that mechanical stress-induced ER stress promotes SMCs apoptosis, inflammation and degeneration, providing insight into TAAD formation and progression.

Mechanism of the formation of contractile ring in dividing cultured animal cells. I. Recruitment of preexisting actin filaments into the cleavage furrow

Cytokinesis of animal cells involves the formation of the circumferential actin filament bundle (contractile ring) along the equatorial plane. To analyze the assembly mechanism of the contractile ring, we microinjected a small amount of rhodamine-labeled phalloidin (rh-pha) or rhodamine-labeled actin (rh-actin) into dividing normal rat kidney cells. rh-pha was microinjected during prometaphase or metaphase to label actin filaments that were present at that stage. As mitosis proceeded into anaphase, the labeled filaments became associated with the cortex of the cell. During cytokinesis, rh-pha was depleted from polar regions and became highly concentrated into the equatorial region. The distribution of total actin filaments, as revealed by staining the whole cell with fluorescein phalloidin, showed a much less pronounced difference between the polar and the equatorial regions. The sites of de novo assembly of actin filaments during the formation of the contractile ring were determined by microinjecting rh-actin shortly before cytokinesis, and then extracting and fixing the cell during mid-cytokinesis. Injected rhodamine actin was only slightly concentrated in the contractile ring, as compared to the distribution of total actin filaments. Our results indicate that preexisting actin filaments, probably through movement and reorganization, are used preferentially for the formation of the contractile ring. De novo assembly of filaments, on the other hand, appears to take place preferentially outside the cleavage furrow.

Orientation and three-dimensional organization of actin filaments in dividing cultured cells

The current hypothesis of cytokinesis suggests that contractile forces in the cleavage furrow are generated by a circumferential band of actin filaments. However, relatively little is known about the global organization of actin filaments in dividing cells. To approach this problem we have used fluorescence-detected linear dichroism (FDLD) microscopy to measure filament orientation, and digital optical sectioning microscopy to perform three-dimensional reconstructions of dividing NRK cells stained with rhodamine-phalloidin. During metaphase, actin filaments in the equatorial region show a slight orientation along the spindle axis, while those in adjacent regions appear to be randomly distributed. Upon anaphase onset and through cytokinesis, the filaments become oriented along the equator in the furrow region, and along the spindle axis in adjacent regions. The degree of orientation appears to be dependent on cell-cell and cell-substrate adhesions. By performing digital optical sectioning microscopy on a highly spread NRK subclone, we show that actin filaments organize as a largely isotropic cortical meshwork in metaphase cells and convert into an anisotropic network shortly after anaphase onset, becoming more organized as cytokinesis proceeds. The conversion is most dramatic on the adhering ventral surface which shows little or no cleavage activity, and results in the formation of large bundles along the equator. On the dorsal surface, where cleavage occurs actively, actin filaments remain isotropic, showing only subtle alignment late in cytokinesis. In addition, stereo imaging has led to the discovery of a novel set of filaments that are associated with the cortex and traverse through the cytoplasm. Together, these studies provide important insights into the process of actin remodeling during cell division and point to possible additional mechanisms for force generation.

Molecular cytochemistry: incorporation of fluorescently labeled actin into living cells

Actin labeled with 5-iodoacetamidofluorescein has been incorporated into the functional pool of actin in Chaos carolinensis and Physarum polycephalum by direct microinjection. The functional activity of the labeled actin has been analyzed at three levels of organization as: (a) with the purified actin, (b) in motile extracts of cells, and (c) in living motile cells. The labeled actin exhibited normal polymerization and activated myosin ATPase to a similar extent as unlabeled controls. Labeled actin and endogenous actin were incorporated into contracted pellets to approximately the same extent in motile cell extracts. After labeled actin had been microinjected into single C. carolinensis cells, the fluorescent actin spread into both the endoplasm and etoplasm without forming distinct fibrils. In contrast, fluorescent bundles developed in the ectoplasm of P. polycephalum following microinjection of labeled actin. This experimental method in conjunction with fluorescence spectroscopic techniques could become a powerful tool for studying the intracellular distribution and structural changes of components in living cells.

The effect of water content on the ordered/disordered structures in starches

(13)C cross-polarization magic angle spinning NMR has been used to study the ordered and disordered structures of starches with different water contents. The amorphous regions of starch have been shown to produce NMR patterns only if they are in a glassy state, the widths, positions, and areas of the peaks to some extent being dependent on the temperature and the water content of the starch. In the amorphous region, the peaks were all Gaussian in shape, while the peaks in the ordered regions had Lorentz profiles. Water contents in the range 10-50% did not influence the proportion of double helices in the starch. Decreasing the water content to 1-3%, however, resulted in a significant decrease in the proportion of double helices, the effect being greater in B- than in A-type starches. It is suggested that short-range order structures in starches (double helices) are stabilized by becoming part of long-range order structures (crystallites).

Assembly and characterization of five-arm and six-arm DNA branched junctions

DNA branched junctions have been constructed that contain either five arms or six arms surrounding a branch point. These junctions are not as stable as junctions containing three or four arms; unlike the smaller junctions, they cannot be shown to migrate as a single band on native gels when each of their arms contains eight nucleotide pairs. However, they can be stabilized if their arms contain 16 nucleotide pairs. Ferguson analysis of these junctions in combination with three-arm and four-arm junctions indicates a linear increase in friction constant as the number of arms increases, with the four-arm junction migrating anomalously. The five-arm junction does not appear to have any unusual stacking structure, and all strands show similar responses to hydroxyl radical autofootprinting analysis. By contrast, one strand of the six-arm junction shows virtually no protection from hydroxyl radicals, suggesting that it is the helical strand of a preferred stacking domain. Both junctions are susceptible to digestion by T4 endonuclease VII, which resolves Holliday junctions. However, the putative helical strand of the six-arm junction shows markedly reduced cleavage, supporting the notion that its structure is largely found in a helical conformation. Branched DNA molecules can be assembled into structures whose helix axes form multiply connected objects and networks. The ability to construct five-arm and six-arm junctions vastly increases the number of structures and networks that can be built from branched DNA components. Icosahedral deltahedra and 11 networks with 432 symmetry, constructed from Platonic and Archimedean solids, are among the structures whose construction is feasible, now that these junctions can be made.

Distinct roles of frontal and rear cell-substrate adhesions in fibroblast migration

Cell migration involves complex physical and chemical interactions with the substrate. To probe the mechanical interactions under different regions of migrating 3T3 fibroblasts, we have disrupted cell-substrate adhesions by local application of the GRGDTP peptide, while imaging stress distribution on the substrate with traction force microscopy. Both spontaneous and GRGDTP-induced detachment of the trailing edge caused extensive cell shortening, without changing the overall level of traction forces or the direction of migration. In contrast, disruption of frontal adhesions caused dramatic, global loss of traction forces before any significant shortening of the cell. Although traction forces and cell migration recovered within 10-20 min of transient frontal treatment, persistent treatment with GRGDTP caused the cell to develop traction forces elsewhere and reorient toward a new direction. We conclude that contractile forces of a fibroblast are transmitted to the substrate through two distinct types of adhesions. Leading edge adhesions are unique in their ability to transmit active propulsive forces. Their functions cannot be transferred directly to existing adhesions upon detachment. Trailing end adhesions create passive resistance during cell migration and readily redistribute their loads upon detachment. Our results indicate the distinct nature of mechanical interactions at the leading versus trailing edges, which together generate the mechanical interactions for fibroblast migration.

Thymosin beta 4 (Fx peptide) is a potent regulator of actin polymerization in living cells

Thymosin beta 4 (beta 4) is a 5-kDa polypeptide originally identified in calf thymus. Although numerous activities have been attributed to beta 4, its physiological role remains elusive. Recently, beta 4 was found to bind actin in human platelet extracts and to inhibit actin polymerization in vitro, raising the possibility that it may be a physiological regulator of actin assembly. To examine this potential function, we have increased the cellular beta 4 concentration by microinjecting synthetic beta 4 into living epithelial cells and fibroblasts. The injection induced a diminution of stress fibers and a dose-dependent depolymerization of actin filaments as indicated by quantitative image analysis of phalloidin binding. Our results show that beta 4 is a potent regulator of actin assembly in living cells.