Signaling pathways and cell mechanics involved in wound closure by epithelial cell sheets.

BACKGROUND: Sheets of cells move together as a unit during wound healing and embryonic tissue movements, such as those occurring during gastrulation and neurulation. We have used epithelial wound closure as a model system for such movements and examined the mechanisms of closure and the importance of the Rho family of Ras-related small GTPases in this process. RESULTS: Wounds induced in Madin-Darby canine kidney (MDCK) epithelial cell monolayers close by Rac- and phosphoinositide-dependent cell crawling, with formation of lamellipodia at the wound margin, and not by contraction of a perimarginal actomyosin purse-string. Although Rho-dependent actin bundles usually form at the margin, neither Rho activity nor formation of these structures is required for wound closure to occur at a normal rate. Cdc42 activity is also not required for closure. Inhibition of Rho or Cdc42 results, however, in statistically significant decreases in the regularity of wound closure, as determined by the ratio of wound margin perimeter over the remaining denuded area at different times. The Rac-dependent force generation for closure is distributed over several rows of cells from the wound margin, as inhibition of motility in the first row of cells alone does not inhibit closure and can be compensated for by generation of motile force in cells behind the margin. Furthermore, we observed high levels of Rac-dependent actin assembly in the first few rows of cells from the wound margin. CONCLUSIONS: Wounds in MDCK cell sheets do not close by purse-string contraction but by a crawling behavior involving Rac, phosphoinositides and active movement of multiple rows of cells. This finding suggests a new distributed mode of signaling and movement that, nevertheless, resembles individual cell motility. Although Rho and Cdc42 activities are not required for closure, they have a role in determining the regularity of closure.

Lactacystin and clasto-lactacystin beta-lactone modify multiple proteasome beta-subunits and inhibit intracellular protein degradation and major histocompatibility complex class I antigen presentation.

The antibiotic lactacystin was reported to covalently modify beta-subunit X of the mammalian 20 S proteasome and inhibit several of its peptidase activities. However, we demonstrate that [3H]lactacystin treatment modifies all the proteasome's catalytic beta-subunits. Lactacystin and its more potent derivative beta-lactone irreversibly inhibit protein breakdown and the chymotryptic, tryptic, and peptidylglutamyl activities of purified 20 S and 26 S particles, although at different rates. Exposure to these agents for 1 to 2 h reduced the degradation of short- and long-lived proteins in four different mammalian cell lines. Unlike peptide aldehyde inhibitors, lactacystin and the beta-lactone do not inhibit lysosomal degradation of an endocytosed protein. These agents block class I antigen presentation of a model protein, ovalbumin (synthesized endogenously or loaded exogenously), but do not affect presentation of the peptide epitope SIINFEKL, which does not require proteolysis for presentation. Generation of most peptides required for formation of stable class I heterodimers is also inhibited. Because these agents inhibited protein breakdown and antigen presentation similarly in interferon-gamma-treated cells (where proteasomes contain LMP2 and LMP7 subunits in place of X and Y), all beta-subunits must be affected similarly. These findings confirm our prior conclusions that proteasomes catalyze the bulk of protein breakdown in mammalian cells and generate the majority of class I-bound epitopes for immune recognition.

Specific inhibition of the chymotrypsin-like activity of the proteasome induces a bipolar morphology in neuroblastoma cells.

BACKGROUND: Lactacystin inhibits cell proliferation and induces a distinctive, predominantly bipolar (two-neurite-bearing) morphology in Neuro 2A murine neuroblastoma cells. It binds with high specificity to the multicatalytic 20S proteasome and inhibits at least three of its peptidase activities (chymotrypsin-like, trypsin-like and peptidylglutamyl-peptide hydrolyzing), each at a different rate, without inhibiting other known proteases. The chymotrypsin-like and trypsin-like activities of the proteasome are inhibited most rapidly, and irreversibly. In an effort to determine which of the peptidase activities needs to be inhibited for neurite outgrowth to occur, we treated Neuro 2A cells with peptide aldehydes that selectively inhibit different proteasome activities. RESULTS: Treatment with peptide aldehydes ending in a hydrophobic residue, all of which inhibit the chymotrypsin-like activity, results in a bipolar morphology in Neuro 2A cells, whereas treatment with a peptide aldehyde inhibitor of the trypsin-like activity does not lead to a detectable change in morphology. One of the inhibitors that induces neurite outgrowth has been previously shown to inhibit the chymotrypsin-like activity of the proteasome without inhibiting the other apparently distinct peptidase activities that cleave after neutral residues, the so-called 'branched chain amino acid preferring' (BrAAP) and 'small neutral amino acid preferring' (SNAAP) activities, or the peptidylglutamyl-peptide hydrolyzing (PGPH) activity. CONCLUSIONS: The chymotrypsin-like activity appears to antagonize bipolar-type neurite outgrowth in Neuro 2A cells, while the trypsin-like, PGPH, BrAAP and SNAAP appear not to do so. Selective inhibition of a single peptidase activity, as opposed to general inhibition of the proteasome, appears sufficient to induce a specific cellular process. Selective inhibition might be useful in managing diseases where only one activity is involved without completely inhibiting the proteasome. It is also possible that endogenous regulators of the proteasome could affect cellular processes and that certain peptidase activities of the proteasome may have roles in specifying a given cell fate.

Inhibition of proteasome activities and subunit-specific amino-terminal threonine modification by lactacystin.

Lactacystin is a Streptomyces metabolite that inhibits cell cycle progression and induces neurite outgrowth in a murine neuroblastoma cell line. Tritium-labeled lactacystin was used to identify the 20S proteasome as its specific cellular target. Three distinct peptidase activities of this enzyme complex (trypsin-like, chymotrypsin-like, and peptidylglutamyl-peptide hydrolyzing activities) were inhibited by lactacystin, the first two irreversibly and all at different rates. None of five other proteases were inhibited, and the ability of lactacystin analogs to inhibit cell cycle progression and induce neurite outgrowth correlated with their ability to inhibit the proteasome. Lactacystin appears to modify covalently the highly conserved amino-terminal threonine of the mammalian proteasome subunit X (also called MB1), a close homolog of the LMP7 proteasome subunit encoded by the major histocompatibility complex. This threonine residue may therefore have a catalytic role, and subunit X/MB1 may be a core component of an amino-terminal-threonine protease activity of the proteasome.

A beta-lactone related to lactacystin induces neurite outgrowth in a neuroblastoma cell line and inhibits cell cycle progression in an osteosarcoma cell line.

Lactacystin, a microbial natural product, induces neurite outgrowth in Neuro 2A mouse neuroblastoma cells and inhibits progression of synchronized Neuro 2A cells and MG-63 human osteosarcoma cells beyond the G1 phase of the cell cycle. A related beta-lactone, clasto-lactacystin beta-lactone, formally the product of elimination of N-acetylcysteine from lactacystin, is also active, whereas the corresponding clastolactacystin dihydroxy acid is completely inactive. Structural analogs of lactacystin altered only in the N-acetylcysteine moiety are active, while structural or stereochemical modifications of the gamma-lactam ring or the hydroxyisobutyl group lead to partial or complete loss of activity. The inactive compounds do not antagonize the effects of lactacystin in either neurite outgrowth or cell cycle progression assays. The response to lactacystin involves induction of a predominantly bipolar morphology that is maximal 16-32 h after treatment and is distinct from the response to several other treatments that result in morphological differentiation. Neurite outgrowth in response to lactacystin appears to be dependent upon microtubule assembly, actin polymerization, and de novo protein synthesis. The observed structure-activity relationships suggest that lactacystin and its related beta-lactone may act via acylation of one or more relevant target molecule(s) in the cell.

Specific Inhibitors of Protein Synthesis Do Not Block RNA Synthesis or Settlement in Larvae of a Marine Gastropod Mollusk (Haliotis rufescens).

Antibiotic inhibitors of protein synthesis were tested for their effectiveness in larvae of the red abalone, Haliotis rufescens (gastropod mollusk). Emetine and anisomycin proved highly effective in this system, while cycloheximide, fusidic acid, puromycin, and tetracycline were less effective. Emetine and anisomycin specifically inhibited protein synthesis but not RNA synthesis. The contribution to protein synthesis by chloramphenicolsensitive prokaryotic contaminants was found to be undetectable, except following the onset of symptoms of toxicity resulting from prolonged exposure to emetine or anisomycin. The induction of larval settlement and plantigrade attachment by γ-aminobutyric acid (GABA), a functional analog of the natural inducer of settlement, occurred even under conditions in which most protein synthesis was inhibited, as expected for a chemosensory system response, whereas subsequent developmental metamorphosis was completely blocked. Because emetine and anisomycin block protein synthesis--including the synthesis of new transcription factors--but do not block early transcription, treatment of marine invertebrate embryos and larvae with these inhibitors can be used to obtain a selective enrichment in the mRNA population of "early gene" transcripts induced directly by GABA and other morphogenetic signals, without dilution by new mRNAs, the appearance of which is dependent on the synthesis of new protein transcription factors.