Transcranial focused ultrasound to human rIFG improves response inhibition through modulation of the P300 onset latency

Response inhibition in humans is important to avoid undesirable behavioral action consequences. Neuroimaging and lesion studies point to a locus of inhibitory control in the right inferior frontal gyrus (rIFG). Electrophysiology studies have implicated a downstream event-related potential from rIFG, the fronto-central P300, as a putative neural marker of the success and timing of inhibition over behavioral responses. However, it remains to be established whether rIFG effectively drives inhibition and which aspect of P300 activity uniquely indexes inhibitory control-ERP timing or amplitude. Here, we dissect the connection between rIFG and P300 for inhibition by using transcranial-focused ultrasound (tFUS) to target rIFG of human subjects while they performed a Stop-Signal task. By applying tFUS simultaneously with different task events, we found behavioral inhibition was improved, but only when applied to rIFG simultaneously with a 'stop' signal. Improved inhibition through tFUS to rIFG was indexed by faster stopping times that aligned with significantly shorter N200/P300 onset latencies. In contrast, P300 amplitude was modulated during tFUS across all groups without a paired change in behavior. Using tFUS, we provide evidence for a causal connection between anatomy, behavior, and electrophysiology underlying response inhibition.

Collagenolytic/gelatinolytic enzymes in corneal wound healing

We have documented changes in expression of collagenolytic/gelatinolytic enzymes of the matrix metalloproteinase family (MMP) in healing or ulcerating corneal wounds of rat or rabbit. Correlation of our findings with specific changes in the extracellular matrix of the repair tissue suggests two different roles for the enzymes, MMP-2 and MMP-9. MMP-2 is expressed in undamaged corneal stroma where it may degrade the occasional collagen molecule that becomes damaged. After corneal wounding, expression of this enzyme is increased and much of it appears in the active form. These changes persist for at least seven months, suggesting that MMP-2 is involved in the prolonged process of collagen remodelling in the stromal repair tissue. MMP-9 is expressed in the epithelial layer of repair tissue with a timing suggesting it might participate in controlling resynthesis of the basement membrane. MMP-9 also appears to be involved in degradation of the epithelial basement membrane that precedes corneal ulceration.

Effects of matrix metalloproteinase-9 gene knock-out on the proteolysis of blood-brain barrier and white matter components after cerebral ischemia

Deleterious processes of extracellular proteolysis may contribute to the progression of tissue damage after acute brain injury. We recently showed that matrix metalloproteinase-9 (MMP-9) knock-out mice were protected against ischemic and traumatic brain injury. In this study, we examined the mechanisms involved by focusing on relevant MMP-9 substrates in blood-brain barrier, matrix, and white matter. MMP-9 knock-out and wild-type mice were subjected to transient focal ischemia. MMP-9 levels increased after ischemia in wild-type brain, with expression primarily present in vascular endothelium. Western blots showed that the blood-brain barrier-associated protein and MMP-9 substrate zonae occludens-1 was degraded after ischemia, but this was reduced in knock-out mice. There were no detectable changes in another blood-brain barrier-associated protein, occludin. Correspondingly, blood-brain barrier disruption assessed via Evans Blue leakage was significantly attenuated in MMP-9 knock-out mice compared with wild types. In white matter, ischemic degradation of the MMP-9 substrate myelin basic protein was significantly reduced in knock-out mice compared with wild types, whereas there was no degradation of other myelin proteins that are not MMP substrates (proteolipid protein and DM20). There were no detectable changes in the ubiquitous structural protein actin or the extracellular matrix protein laminin. Finally, 24 hr lesion volumes were significantly reduced in knock-out mice compared with wild types. These data demonstrate that the protective effects of MMP-9 gene knock-out after transient focal ischemia may be mediated by reduced proteolytic degradation of critical blood-brain barrier and white matter components.

Matrix metalloproteinase 2 gene knockout has no effect on acute brain injury after focal ischemia

Matrix metalloproteinases (MMPs) may contribute to tissue damage after cerebral ischemia. In this study, wildtype and MMP-2 knockout mice were subjected to permanent and transient (2 h) occlusions of the middle cerebral artery. Gelatin zymography showed that MMP-9 levels were increased in all brains after ischemia. MMP-2 levels did not show a significant increase in wildtype mice, and were not detectable in knockout mice. Laser doppler flowmetry demonstrated equivalent ischemic reductions in perfusion in wildtype and knockout mice. In both permanent and transient occlusion paradigms, there were no statistically significant differences between wildtype and knockout mice in terms of 24 h ischemic lesion volumes. These data suggest that MMP-2 does not contribute to acute tissue damage in this model of focal ischemia.

Induction of the ubiquitin-proteasome pathway during the keratocyte transition to the repair fibroblast phenotype

PURPOSE

To examine dynamics and function of the ubiquitin (Ub)-proteasome pathway (UPP) during corneal stromal cell acquisition of the repair fibroblast phenotype.

METHODS

An established cell culture model was used in which freshly isolated rabbit corneal stromal cells acquire a repair fibroblast phenotype, thereby mimicking injury-induced stromal cell activation.

RESULTS

Transition to the repair fibroblast phenotype during the 72 hours after initial plating was coincident with progressive UPP induction. Levels of Ub, Ub-conjugated proteins, ubiquitinylating enzymes E1 and E2-25K, and 26 S proteasome increased two- to fivefold in activated stromal cells. These increases were associated with enhanced (>10-fold) capacity for Ub-dependent proteolysis of (125)I-labeled H2A and with progressive (>6-fold) increases in the UPP substrate, inhibitor of kappaBalpha (IkappaBalpha). Because IkappaBalpha expression is induced by nuclear factor (NF)-kappaB, this finding suggests that rates of constitutive NF-kappaB activation, and thus IkappaBalpha degradation, are elevated in activated stromal cells. Both freshly isolated and activated stromal cells degraded IkappaBalpha in response to IL-1alpha; yet, only activated stromal cells maintained autocrine IL-1alpha expression after 24 hours. UPP induction was coincident with a more than 90% loss of tissue transketolase (TKT) and aldehyde dehydrogenase (ALDH) class 1. TKT was stabilized during the repair phenotype transition by proteasome inhibition and was degraded (>30%/h) by the UPP in cell-free assays.

CONCLUSIONS

Coordinate induction of the UPP during stromal cell activation alters levels of IkappaBalpha and TKT, two UPP substrates that are implicated in the loss of tissue stasis and corneal clarity after injury.

Trefoil peptides promote restitution of wounded corneal epithelial cells

The ocular surface shares many characteristics with mucosal surfaces. In both, healing is regulated by peptide growth factors, cytokines, and extracellular matrix proteins. However, these factors are not sufficient to ensure most rapid healing. Trefoil peptides are abundantly expressed epithelial cell products which exert protective effects and are key regulators of gastrointestinal epithelial restitution, the critical early phase of cell migration after mucosal injury. To assess the role of trefoil peptides in corneal epithelial wound healing, the effects of intestinal trefoil factor (ITF/TFF3) and spasmolytic polypeptide (SP/TFF2) on migration and proliferation of corneal epithelial cells were analyzed. Both ITF and SP enhanced restitution of primary rabbit corneal epithelial cells in vitro. While the restitution-enhancing effects of TGF-alpha and TGF-beta were both inhibited by neutralizing anti-TGF-beta-antibodies, trefoil peptide stimulation of restitution was not. Neither trefoil peptide significantly affected proliferation of primary corneal epithelial cells. ITF but not SP or pS2 mRNA was present in rabbit corneal and conjunctival tissues. In summary, the data indicate an unanticipated role of trefoil peptides in healing of ocular surface and demand rating their functional actions beyond the gastrointestinal tract.

Activation of a tissue-specific stress response in the aqueous outflow pathway of the eye defines the glaucoma disease phenotype

The glaucomas are a group of optic neuropathies comprising the leading cause of irreversible blindness worldwide. Elevated intraocular pressure due to a reduction in normal aqueous outflow is a major causal risk factor. We found that endothelial leukocyte adhesion molecule-1 (ELAM-1), the earliest marker for the atherosclerotic plaque in the vasculature, was consistently present on trabecular meshwork (TM) cells in the outflow pathways of eyes with glaucomas of diverse etiology. We determined expression of ELAM-1 to be controlled by activation of an interleukin-1 (IL-1) autocrine feedback loop through transcription factor NF-kappaB, and activity of this signaling pathway was shown to protect TM cells against oxidative stress. These findings characterize a protective stress response specific to the eye's aqueous outflow pathways and provide the first known diagnostic indicator of glaucomatous TM cells. They further indicate that common mechanisms contribute to the pathophysiology of the glaucomas and vascular diseases.

Role for matrix metalloproteinase 9 after focal cerebral ischemia: effects of gene knockout and enzyme inhibition with BB-94

It has been shown recently that matrix metalloproteinases (MMPs) are elevated after cerebral ischemia. In the current study, we investigated the pathophysiologic role for MMP-9 (gelatinase B, EC.3.4.24.35) in a mouse model of permanent focal cerebral ischemia, using a combination of genetic and pharmacologic approaches. Zymography and Western blot analysis demonstrated that MMP-9 protein levels were rapidly up-regulated in brain after ischemic onset. Reverse transcription polymerase chain reaction showed increased transcription of MMP-9. There were no differences in systemic hemodynamic parameters and gross cerebrovascular anatomy between wild type mice and mutant mice with a targeted knockout of the MMP-9 gene. After induction of focal ischemia, similar reductions in cerebral blood flow were obtained. In the MMP-9 knockout mice, ischemic lesion volumes were significantly reduced compared with wild type littermates in male and female mice. In normal wild type mice, the broad spectrum MMP inhibitor BB-94 (batimastat) also significantly reduced ischemic lesion size. However, BB-94 had no detectable protective effect when administered to MMP-9 knockout mice subjected to focal cerebral ischemia. These data demonstrate that MMP-9 plays a deleterious role in the development of brain injury after focal ischemia.

Matrix metalloproteinase 9 promoter activity is induced coincident with invasion during tumor progression

Matrix metalloproteinase 9 (MMP-9, also known as gelatinase B or 92-kd Type IV collagenase) is overexpressed in many human and murine cancers. We induced carcinomas in mice carrying a transgene that links the MMP-9 promoter to the reporter ss-galactosidase so that activation of the MMP-9 promoter would be indicated by ss-galactosidase. Mammary carcinomas were induced by mating the MMP-9 promoter reporter transgenic mice with mice carrying a transgene for murine mammary tumor virus promoter linked to polyoma middle T antigen, a transgene that leads to rapid development of mammary tumors in female mice. None of the hyperplastic mammary glands and none of the carcinomas in situ expressed ss-galactosidase. However, all invasive tumors had evidence of ss-galactosidase expression. In addition to the breast carcinomas, a malignant teratoma in a female and a papillary adenocarcinoma in the pelvic region of a male arose and were also ss-galactosidase positive. We also induced skin tumors in the mice with the MMP-9 reporter transgene with 7, 12-dimethylbenz[a]anthracene (DMBA) treatment followed by phorbol 12 myristate 13-acetate (TPA). None of the papillomas or in situ carcinomas showed any ss-galactosidase expression, but expression was seen in invasive carcinoma. Although normal skin epithelial cells did not express ss-galactosidase, we did find staining in a few cells at the duct of the sebaceous gland at the base of the hair follicles. The MMP-9 reporter transgene did not lead to expression in the alveolar macrophages, confirming that additional upstream sequences are required for expression in macrophages. These experiments have revealed that MMP-9 promoter activity is induced coincident with invasion during tumor progression. Furthermore, this indicates that the more proximal upstream elements of the promoter are sufficient for MMP-9 transcription during tumor progression.

Effects of matrix metalloproteinase-9 gene knock-out on morphological and motor outcomes after traumatic brain injury

Matrix metalloproteinases (MMPs) belong to a class of extracellular proteinases responsible for maintaining and remodeling the extracellular matrix. In addition to multiple functions in normal physiology, abnormal MMP expression and activity may also participate in the pathophysiology of cerebral disease. Here, we show that MMP-9 (gelatinase B; EC.3.4.24.35) contributes to the pathophysiology of traumatic brain injury. After controlled cortical impact in mice, MMP-9 was increased in traumatized brain. Total MMP-9 levels at 24 hr were significantly increased as measured by a substrate cleavage assay. Zymograms showed that MMP-9 was elevated as early as 3 hr after traumatic brain injury, reaching a maximum at approximately 24 hr. Increased MMP-9 levels persisted for up to 1 week. Western blot analysis indicated increased profiles of MMP-9 expression that corresponded with the zymographic data. Knock-out mice deficient in MMP-9 gene expression were compared with wild-type littermates in terms of morphological and motor outcomes after trauma. Motor outcomes were measured at 1, 2, and 7 d after traumatic brain injury by the use of a rotarod device. MMP-9 knock-out mice had less motor deficits than wild-type mice. At 7 d, traumatic brain lesion volumes on Nissl-stained histological sections were significantly smaller in MMP-9 knock-out mice. These data demonstrate that MMP-9 contributes to the pathophysiology of traumatic brain injury and suggest that interruption of the MMP proteolytic cascade may be a possible therapeutic approach for preventing the secondary progression of damage after brain trauma.

Perspectives on eye development

The lens of the vertebrate eye was the classic model used to demonstrate the concepts of inductive interactions controlling development. However, it is in the Drosophila model that the greatest progress in understanding molecular mechanisms of eye development have most recently been mode. This progress can be attributed to the power of molecular genetics, an approach that was once confined to simpler systems like worms and flies, but is now becoming possible in vertebrates. Thus, the use of transgenic and knock-out gene technology, coupled with the availability of new positional cloning methods, has recently initiated a surge of progress in the mouse genetic model and has also led to the identification of genes involved in human inherited disorders. In addition, gene transfer techniques have opened up opportunities for progress using chick, Xenopus, and other classic developmental systems. Finally, a new vertebrate genetic model, zebrafish, appears very promising for molecular studies. As a result of the opportunities presented by these new approaches, eye development has come into the limelight, hence the timeliness of this focus issue of Developmental Genetics. In this introductory review, we discuss three areas of current work arising through the use of these newer genetic approaches, and pertinent to research articles presented herein. We also touch on related studies reported at the first Keystone Meeting on Ocular Cell and Molecular Biology, recently held in Tamarron Springs, Colorado, January 7-12, 1997.

Pax-6 expression and activity are induced in the reepithelializing cornea and control activity of the transcriptional promoter for matrix metalloproteinase gelatinase B

Recent evidence supports the idea that matrix metalloproteinases (MMPs) act as morphogenetic regulators in embryonic and adult events of tissue remodeling. MMP activity is controlled primarily at the level of gene expression. In a recent study we characterized the transcriptional promoter of the MMP gene, gelatinase B (gelB), in transgenic mice, demonstrating the requirement for DNA sequences between -522 and +19 for appropriate activity. In this study we investigated factors required for gelB promoter activity in the developing eye and reepithelializing adult cornea. Pax-6 is a homeobox and paired domain transcription factor that acts at the top of the hierarchy of genes controlling eye development. Pax-6 is also expressed in the adult eye. We show here that the tissue expression pattern of Pax-6 overlaps extensively with gelB promoter activity in the developing and adult eye. In addition Pax-6 is observed to be upregulated in repairing corneal epithelium, as is gelB promoter activity. In cell culture transfection experiments, we identified two promoter regions which mediate positive response to Pax-6. By electrophoretic mobility shift assay, we further pinpoint two Pax-6 binding sites within these response regions and demonstrate direct interaction of the Pax-6 paired domain with one of these sites. These data suggest a mechanism by which Pax-6 may direct gelB expression in an eye-specific manner.

Curcuminoids inhibit the angiogenic response stimulated by fibroblast growth factor-2, including expression of matrix metalloproteinase gelatinase B

We have studied mechanisms controlling activation of the gelatinase B gene (matrix metalloproteinase-9) by fibroblast growth factor-2 (FGF-2) during angiogenesis, and the effects of the natural product curcuminoids on this process. Using a transgenic mouse (line 3445) harboring a gelatinase B promoter/lacZ fusion gene, we demonstrate FGF-2 stimulation of reporter gene expression in endothelial cells of invading neocapillaries in the corneal micropocket assay. Using cultured corneal cells, we show that FGF-2 stimulates DNA binding activity of transcription factor AP-1 but not NF-kappaB and that AP-1 stimulation is inhibited by curcuminoids. We further show that induction of gelatinase B transcriptional promoter activity in response to FGF-2 is dependent on AP-1 but not NF-kappaB response elements and that promoter activity is also inhibited by curcuminoids. In rabbit corneas, the angiogenic response induced by implantation of an FGF-2 pellet is inhibited by the co-implantation of a curcuminoid pellet, and this correlates with inhibition of endogenous gelatinase B expression induced by FGF-2. Angiostatic efficacy in the cornea is also observed when curcuminoids are provided to mice in the diet. Our findings provide evidence that curcuminoids target the FGF-2 angiogenic signaling pathway and inhibit expression of gelatinase B in the angiogenic process.

Failure to activate transcription factor NF-kappaB in corneal stromal cells (keratocytes)

PURPOSE

Freshly isolated cultures of corneal stromal cells (keratocytes) are incompetent to synthesize the tissue remodeling proteinase, collagenase, in response to agents such as cytochalasin B (CB) or phorbol myristate acetate (PMA), which are strong stimulators of collagenase expression in subcultured fibroblasts of all types, including those from corneal stroma. Incompetence is due to failure to activate an autocrine interleukin (IL)1alpha feedback loop required to mediate cell response. The goal of the present study was to investigate the mechanism for this failure.

METHODS

A cell culture model of freshly isolated corneal stromal cells and subcultured stromal fibroblasts from rabbits was used for these studies.

RESULTS

Competence to synthesize collagenase in response to CB was acquired as a differentiation property by corneal stromal cells placed in culture, and did not require subculture. Competence acquisition correlated with transition to a fibroblastic spindle shape, assembly of actin stress fibers, and the acquired capacity to collapse in response to CB. It was demonstrated that competence could be more precisely defined as the capacity to express IL-1alpha in response to IL-1, making possible activation of the feedback loop. Investigation into the signaling pathway for IL-1alpha expression in response to IL-1 revealed a requirement for reactive oxygen species and activity of the transcription factor nuclear factor (NF)kappaB. Importantly, freshly isolated stromal cells were found to be relatively incompetent to activate NF-kappaB in comparison to subcultured stromal fibroblasts.

CONCLUSIONS

Failure to activate NF-kappaB explains incompetence for expression of IL-1alpha in corneal stromal cells. Because NF-kappaB regulates many cell functions with potential to disturb corneal structure, including expression of inflammatory, stress, and degradative proteinase genes; protection against apoptosis; and cell replication; this seems likely to be an important mechanism protecting corneal stasis and preserving function.

Keratocyte and fibroblast phenotypes in the repairing cornea

In mammals, tissue damage is usually repaired by activation of a fibrotic response which saves the life of the organism, but which can never restore function to the damaged organ. In addition, fibrotic responses form the basis for diverse pathologies, including many that occur in the eye. It is intriguing, therefore, to observe the occasional circumstances in which repair in mammals appears to take on a regenerative character, such as during fetal wound healing or in certain types of corneal wounds. The thesis of this chapter is that the choice between regeneration or fibrosis lies in the control of fibroblast phenotype. The cornea of the eye has several features which make it a particularly useful model for the study of fibroblast phenotype. Studies discussed herein, identify failure to activate the transcription factor NF-kappaB as a control mechanism for inhibiting fibroblast activation in the cornea. Evidence is further presented for the view that transition in fibroblast phenotype in repair tissue is not simply a matter of differential gene expression, but is a developmental event which reflects changes in the hard wiring of signalling pathways by which the cell responds to environmental input.

Differential inhibition of collagenase and interleukin-1alpha gene expression in cultured corneal fibroblasts by TGF-beta, dexamethasone, and retinoic acid

PURPOSE

Expression of the genes for collagenase and interleukin-1alpha (IL-1alpha) are induced as stromal cells become activated to the repair fibroblast phenotype after injury to the cornea. This investigation examines the mechanisms whereby expression of these genes is inhibited by transforming growth factor-beta (TGF-beta), dexamethasone (DEX), or retinoic acid (RET A).

METHODS

A model of freshly isolated cultures of corneal stromal cells and early passage cultures of corneal fibroblasts was used in these studies. This model reproduces the events of stromal cell activation in the corneal wound.

RESULTS

In early passage cultures of corneal fibroblasts, expression of collagenase is under obligatory control by autocrine IL-1alpha. IL-1alpha controls its own expression through an autocrine feedback loop that is dependent on transcription factor NF-kappaB. TGF-beta, DEX, and RET A were each effective inhibitors of collagenase gene expression in these cells. Furthermore, these agents have the capacity to inhibit expression of IL-1alpha and this was correlated with their ability to affect DNA-binding activity of NF-kappaB. However, TGF-beta, DEX, and RET A were also effective inhibitors of the low level of collagenase expressed by freshly isolated corneal stromal cells that cannot express IL-1alpha.

CONCLUSIONS

In cells with an active IL-1alpha autocrine loop there are at least two distinct signaling pathways by which collagenase gene expression can be modulated. The results of this study demonstrate that TGF-beta, DEX, and RET A differentially inhibit collagenase and IL-1alpha gene expression. This information will be useful in the design of therapeutic modalities for fibrotic disease in the cornea and other parts of the eye.

Regulation of gelatinase B production in corneal cells is independent of autocrine IL-1alpha

PURPOSE

The matrix metalloproteinase gelatinase B is synthesized by cells at the leading edge of the corneal epithelium migrating to heal a wound. Recent data from the authors' laboratory suggest that excessive synthesis contributes to repair defects. The goal of the study reported here was to investigate mechanisms controlling gelatinase B production by corneal epithelial cells.

METHODS

Freshly isolated cultures of corneal epithelial cells and early passage stromal fibroblasts from rabbit were used for these studies.

RESULTS

In a previous study, it was found that the cytokine interleukin (IL)-1alpha is released into the culture medium of corneal epithelial cells more efficiently when they are plated at low density with limited cell-cell contact than when plated at high density. In this study, we show that production of gelatinase B by these cells is similarly affected by cell plating density. However, it is further demonstrated that these two events are not dependent on one another but occur in parallel: IL-1alpha does not regulate gelatinase B production (synthesis), nor was there evidence that any other secreted autocrine cytokine acts as mediator. Instead, our data suggest that gelatinase B production is downregulated directly by high cell density and indicate a connection to the level of protein kinase C activity. Nevertheless, the anticancer agent suramin, which blocks collagenase synthesis by interfering with autocrine cytokine-receptor interactions, still inhibits synthesis of gelatinase B.

CONCLUSIONS

Unlike collagenase synthesis by corneal stromal fibroblasts, production (synthesis) of gelatinase B does not appear to be controlled by secreted autocrine cytokines but can still be inhibited by suramin. Suramin may make an effective therapeutic agent for controlling pathologic overproduction of gelatinase B in corneal ulcers.

Gelatinase B/lacZ transgenic mice, a model for mapping gelatinase B expression during developmental and injury-related tissue remodeling

Matrix metalloproteinases (MMPs) drive normal tissue remodeling and are implicated in a wide range of pathologies. Although MMP activity is controlled at multiple levels, the primary regulation of MMP activity is transcriptional. The transcriptional promoter elements required for MMP gene expression in cultured cells have been defined, but this has not been extended to the in vivo situation. In this paper, we show that the DNA sequences between -522 and +19 of the rabbit gelatinase B gene (MMP-9) (as characterized in the transgenic mouse line 3445) constitute a minimal promoter that drives appropriate developmental and injury-induced reporter gene expression in transgenic mice. We further show that the expression and activity of three transcription factors (NF-kappaB, AP-2, and Sp1) that control the activity of the gelatinase B promoter are selectively induced in the epithelium migrating to heal a wound. Although promoter activity parallels expression of the endogenous gene in cell cultures, we show by several criteria that cell cultures cannot model many aspects of promoter regulation in vivo. This study reveals that the transgenic mouse line 3445 might be a useful model for investigating the regulation of gelatinase B expression in vivo and for identifying and characterizing new drugs that can control gelatinase B gene transcription.

Proteolytic mechanisms in corneal ulceration and repair

Corneal stromal ulceration is a devastating disorder that can cause blindness. Stromal ulceration was once thought to be a physical dissolution process, which even now is described as "melting." However, a major paradigm change occurred about 25 years ago with the demonstration of extracellular matrix-degrading activity associated with tissues isolated from ulcerating corneas. Recent studies have identified the enzymes involved as specific members of the matrix metalloproteinase (MMP) family. These studies have further provided evidence that MMPs participate at all stages of the ulcerative process, from formation of the initiating epithelial defect to ulcer resolution and repair. Roles for MMPs in these processes are discussed in this review. Studies on corneal ulceration provide basic information about failure to heal, which is useful for understanding mechanisms common to other organ systems besides the cornea.

Collagen type IX and developmentally regulated swelling of the avian primary corneal stroma

A critical event in avian corneal development occurs when the acellular primary stroma swells and becomes populated by mesenchymal cells that migrate from the periphery. These cells then deposit the mature stromal matrix that exhibits the unique features necessary for corneal function. Our previous work correlated the disappearance of collagen type IX immunoreactivity at stage 27 (5 1/2-6 days) with matrix swelling and invasion. To investigate further the mechanism of this disappearance, we employed immunohistochemistry after tissue fixation with Histochoice, a non-crosslinking fixative, immunoblot analysis of protein extracts, and gel substrate chromatography (zymography) to detect endogenous proteolytic activity. We found that corneas fixed in Histochoice retain immunoreactivity for type IX collagen for 1-2 days after corneal swelling. This immunoreactivity, however, becomes extractable from tissue sections of unfixed corneas at the time of initiation of stromal swelling and mesenchymal cell invasion. Immunoblot analysis confirmed that, following swelling, immunoreactivity for collagen IX decreased substantially in corneas, but not in the vitreous body, which served as a comparison. Analysis of ammonium sulfate (AS) fractions of such extracts indicated that, at the time of swelling, much of the immunoreactivity for type IX collagen in cornea shifted from the AS precipitate (containing high molecular weight molecules) to the AS supernatant (containing smaller fragments). In contrast, collagen IX immunoreactivity from the vitreous was precipitated by ammonium sulfate throughout the period of study. Collagen type II, a major fibrillar collagen in both the corneal stroma and vitreous, remained in the high molecular weight fraction at all times examined. Zymography detected the presence of the latent (proenzyme) form of gelatinase A (MMP-2) before corneal swelling and invasion (4 days), and both the latent and active forms of the enzyme after corneal swelling. This suggests tissue-specific, developmentally regulated proteolysis of collagen IX as a trigger for corneal matrix swelling.

Role of serum amyloid A as an intermediate in the IL-1 and PMA-stimulated signaling pathways regulating expression of rabbit fibroblast collagenase

The matrix metalloproteinase collagenase is expressed by resident tissue cells only when needed for biological remodeling. Exogenous addition of inflammatory and growth-promoting cytokines stimulates collagenase expression in early passage fibroblast cultures. In addition, the signal for collagenase expression in response to phorbol-12 myristate-13 acetate (PMA) or to agents which alter cell shape in early passage fibroblast cultures is routed extracellularly to an autocrine cytokine intermediate, IL-1 alpha. Importantly, fibroblasts, when freshly isolated from the tissue, are not competent for IL-1 alpha gene expression and, therefore, cannot produce collagenase in response to shape change agents. However, they do make a small amount of collagenase in response to PMA via an IL-1-independent pathway that has not been further characterized. In this paper, we investigate the role of a second autocrine, serum amyloid A3 (SAA3), in IL-1-dependent and -independent collagenase gene expression. We demonstrate that SAA3 is required for effective stimulation of collagenase expression by either exogenous or endogenous IL-1. Furthermore, while freshly isolated fibroblasts cannot express IL-1 alpha they can express SAA3, and this autocrine mediator acts independently of IL-1 alpha to control the low level of collagenase expression that can be stimulated by PMA. These results provide further evidence for a newly emerging paradigm of collagenase regulation which emphasizes the requirement for extracellular routing of signals. They also suggest that SAA3 might be utilized independently of IL-1 alpha to control tissue remodeling in vivo.

Frog PNKT-4B cells express specific extracellular matrix-degrading enzymes and cytokines correlated with an invasive phenotype

A temperature-dependent metastatic phenotype reported for a frog cell line, PNKT-4B, provided a means for studying potential mediators of cell-matrix interaction involved in metastatic invasion. Zymography revealed that these cells secreted enzyme species with properties and characteristics of mammalian metalloproteinases: collagenase, stromelysin, gelatinase A, and gelatinase B. These enzymes were produced by PNKT-4B cultures maintained at both invasive-permissive (28 degrees C), and invasion-restrictive (20 degrees C) temperatures. However, under the invasive-permissive culture condition cells produced more of the putative gelatinase B and A enzymes. In addition, an activated form of gelatinase A was produced only in invasion-permissive cultures. DNA synthesis bioassays (Mv1Lu cell line and mouse thymocytes) to detect growth promoting and/or inhibitory cytokines, revealed that PNKT-4B cultures kept at 28 degrees C released significantly higher levels of stimulatory (interleukin-1-like) and latent inhibitory (transforming growth factor-beta-like) substances into the medium compared to 20 degrees C cultures. Pre-absorption of media samples with heparin-sepharose indicated a second stimulatory cytokine as well. A corneal fibroblast bioassay that tests for mediators of collagenase synthesis, detected a stimulatory substance whose activity was greatly reduced in the presence of interleukin-1 receptor antagonist protein. Collagenase stimulatory activity present in 28 degrees C culture medium was significantly higher than equal samples from 20 degrees C cultures. These studies provide a molecular correlation between release of cytokines with properties of the metastatic phenotype seen in vivo. They further provide some of the first characterizations of frog MMPs and cytokines, which are likely to be involved in other tissue remodeling events.

Repair phenotype in corneal fibroblasts is controlled by an interleukin-1 alpha autocrine feedback loop

PURPOSE

To explore the role of autocrine interleukin-1 alpha (IL-1 alpha) as a central regulator of the repair phenotype in corneal fibroblasts.

METHODS

Disruption of the actin cytoskeleton with cytochalasin B (CB), which mimics changes in shape that occur in repair tissues, was used to stimulate repair gene expression in early-passage fibroblasts. Changes in expression of IL-1 alpha, IL-8, collagenase, and ENA-78 were determined by Northern blot analysis, radioimmunoassay, and an enzyme-amplified sensitivity immunoassay (EASIA). Expression of repair genes was also examined in repair fibroblasts, isolated from healing, penetrating keratectomy wounds in rabbits.

RESULTS

Blocking IL-1 alpha activity prevented both constitutive and stimulated increases in synthesis of IL-8 and collagenase in early-passage cultures of corneal fibroblasts, demonstrating the role of IL-1 alpha as a necessary intermediate for expression of these genes. Evidence is also presented that the IL-1 alpha autocrine controls expression of an IL-8 related factor, ENA-78. Unlike early-passage fibroblasts, fibroblasts freshly isolated from the uninjured cornea did not express IL-1 alpha. However, fibroblasts freshly isolated from remodeling corneal repair tissue 3 weeks after injury were found to express substantial levels of IL-1 alpha, regulated through an autocrine feedback loop. Neutralization experiments demonstrated that the IL-1 alpha autocrine is largely responsible for controlling both collagenase and IL-8 synthesis in repair fibroblasts, as it is in early-passage fibroblasts.

CONCLUSIONS

These findings provide evidence that activation of an autocrine IL-1 alpha feedback loop is an important mechanism by which fibroblasts adopt a repair phenotype during remodeling of the cornea.

Regulation of paracrine cytokine balance controlling collagenase synthesis by corneal cells

PURPOSE

Classic studies have demonstrated that corneal epithelial cell density in culture can alter the balance of stimulatory and inhibitory cytokines controlling the elaboration of collagenolytic activity by co-cultured stromal cells. The current study attempts to bring the understanding of this mechanism to a molecular level.

METHODS

A rabbit primary corneal cell culture model was used.

RESULTS

Using molecular probes that bind to and neutralize specific cytokines, a major stimulator for stromal cell collagenase synthesis released by corneal epithelial cells into culture medium was identified as interleukin-1 alpha (IL-1 alpha), and a secondary stimulator was characterized as a heparin-binding cytokine. An inverse relationship between net collagenase stimulatory activity and epithelial cell plating density was demonstrated. In contrast, the release of inhibitory activity for IL-1-stimulated collagenase synthesis was not subject to the cell density effect. Direct measurement of IL-1 alpha protein levels revealed that this cytokine was released much more efficiently on a per cell basis when cells were plated at low density than when they were plated at high density. The effect was not caused by greater cell lysis at low cell density and was mediated only partially by changes at the IL-1 alpha synthesis level.

CONCLUSIONS

These data provide evidence that epithelial cells release stimulatory cytokines for collagenase expression more efficiently when they have limited contact with their neighbors and that this has important consequences for the overall paracrine cytokine balance controlling collagenase synthesis. Alteration of the paracrine cytokine balance by changes in cell contact may be an important means for regulating epithelial-stromal interactions involved in corneal development and repair.

Role of matrix metalloproteinases in failure to re-epithelialize after corneal injury

Delayed re-epithelialization of the cornea after injury usually precedes stromal ulceration. Previous findings using a rat thermal injury model suggested that re-epithelialization is impeded by products of resident corneal cells, which destroy adhesive structures at the basement membrane zone. In this study, we provide additional evidence for this concept. Failure to re-epithelialize was found to correlate with an increase in the amounts of gelatinolytic matrix metalloproteinases present in the rat cornea. One of these gelatinases, gelatinase B, is synthesized by the resident corneal cells, and inhibitions of its synthesis correlated with inhibition of basement membrane dissolution. The matrix metalloproteinases collagenase and stromelysin are also synthesized by resident corneal cells in thermally injured corneas of rabbits, but the timing of bulk enzyme synthesis correlated more closely with deposition of repair tissue in the stroma than with failure to re-epithelialize. Nevertheless, in human corneas with repair defects, gelatinase B and collagenase are synthesized by cells in the basal layer of the epithelium directly adjacent to the basement membrane, suggesting that both could participate in dissolution of this structure. Importantly, treatment of thermally injured corneas with a synthetic inhibitor of matrix metalloproteinases significantly improved basement membrane integrity. These data support the concept that over-expression of matrix metalloproteinases by resident corneal cells impedes re-epithelialization after some types of corneal injury.

Regionalized growth patterns of young chicken corneas

PURPOSE

Extracellular matrix (ECM) accumulation, synthesis, and degradation were compared in central and peripheral corneal regions of 3-week-old chicks to identify regional differences within the cornea.

METHODS

For collagen accumulation experiments, corneas were isolated at the scleral junction, the epithelium was removed, and the central corneal region was isolated from the peripheral region with a 3 mm trephine. After corneal stromas were digested with proteinase K, aliquots were used to estimate total collagen from hydroxyproline and cell number from DNA measurements. For biosynthesis experiments, corneas were placed in organ culture containing either in 3H-thymidine, 3H-proline, or 35SO4. After radiolabeling, the epithelium was removed, central and peripheral regions were isolated, and each corneal sample was analyzed for incorporated radioactivity. Gelatinolytic species present in each corneal region were compared qualitatively by gelatin zymography.

RESULTS

Measurement of hydroxyproline content indicated that the central corneal stroma contained significantly more collagen per DNA than the peripheral corneal region (+40%, P = 0.013) and incorporated significantly higher levels of 3H-thymidine/ng DNA (+92%, P < 0.001), 3H-proline/ng DNA (+980%, P = 0.004), and 35SO4/ng DNA (+650%, P = 0.01) than the peripheral corneal region. Results of gelatin zymography indicated that the central cornea contained the proenzyme form of gelatinase A, whereas the peripheral cornea contained both the proenzyme form and the active form of gelatinase A.

CONCLUSIONS

Fibroblasts located in the central cornea have a significantly higher rate of proliferation and ECM production than those of the peripheral cornea, whereas the presence of active gelatinase only in the peripheral cornea suggests higher gelatinolytic activity and ECM turnover in the periphery.

Competence for collagenase gene expression by tissue fibroblasts requires activation of an interleukin 1 alpha autocrine loop

The enzyme collagenase (EC 3.4.24.7), a key mediator in biological remodeling, can be induced in early-passage fibroblasts by a wide variety of agents and conditions. In contrast, at least some primary tissue fibroblasts are incompetent to synthesize collagenase in response to many of these stimulators. In this study, we investigate mechanisms controlling response to two of the conditions in question: (i) trypsin or cytochalasin B, which disrupt actin stress fibers, or (ii) phorbol 12-myristate 13-acetate (PMA), which activates growth factor signaling pathways. We demonstrate that collagenase expression stimulated by trypsin or cytochalasin B is regulated entirely through an autocrine cytokine, interleukin 1 alpha (IL-1 alpha). The IL-1 alpha intermediate also constitutes the major mechanism by which PMA stimulates collagenase expression, although a second signaling pathway(s) contributes to a minor extent. Elevation of the IL-1 alpha level in response to stimulators is found to be sustained by means of an autocrine feedback loop in early-passage fibroblast cultures. In contrast, fibroblasts freshly isolated from the tissue are incompetent to activate and sustain the IL-1 alpha feedback loop, even though they synthesize collagenase in response to exogenous IL-1. We conclude that this is the reason why tissue fibroblasts are limited, in comparison with subcultured fibroblasts, in their capacity to synthesize collagenase. Activation of the IL-1 alpha feedback loop, therefore, seems likely to be an important mechanism by which resident tissue cells adopt the remodeling phenotype.

Unique regulation of the matrix metalloproteinase, gelatinase B

PURPOSE

The matrix metalloproteinase (MMP), gelatinase B, is expressed by both corneal cell types found at the epithelial-stromal tissue interface, the site of basement membrane repair in the healing cornea. This study investigates the relative regulation of gelatinase B compared to other MMPs in response to agents related to those found in the corneal repair environment or in corneal ulcers.

METHODS

A culture model of corneal cells isolated from rabbit was used.

RESULTS

Gelatinase B is the major MMP expressed by corneal epithelial cells, whereas stromal fibroblasts produce gelatinase B along with three other MMPs: collagenase, stromelysin, and gelatinase A. Phorbol-12-myristate 13-acetate (PMA) stimulates gelatinase B mRNA and protein synthesis by corneal cells, which is similar to its effect on the other MMPs. Stimulation occurs, at least partially, at the transcriptional level. PMA-stimulated MMP expression follows biphasic kinetics, with the major effect on collagenase, stromelysin, and gelatinase A occurring during the late component. In contrast, the major gelatinase B response occurs during the early component. Transforming growth factor-beta (TGF-beta) has no effect on constitutive expression of gelatinase B by fibroblasts; however, expression stimulated by PMA is enhanced. In contrast, constitutive expression of collagenase and stromelysin is inhibited by TGF-beta. However, in the presence of PMA, the initial inhibitory effect of TGF-beta is reversed after treatment.

CONCLUSION

Gelatinase B expression is regulated differently from other corneal MMPs. This provides a mechanism for control of basement membrane repair independent of repair processes in the stroma.

A corneal epithelial inhibitor of stromal cell collagenase synthesis identified as TGF-beta 2

PURPOSE

To determine the molecular mechanisms whereby substances released by corneal epithelial cells act to inhibit the elaboration of collagenolytic activity by corneal stromal cells and to determine whether inhibitory activity might be mediated by interleukin-1 receptor antagonist (IL-1ra) or transforming growth factor (TGF)-beta.

METHODS

Conditioned media were generated from primary cultures of rabbit corneal epithelial cells, from passaged cultures of rabbit corneal stromal fibroblasts, and from cells of the rabbit corneal epithelial cell line, SIRC. Pure populations of stromal cells were isolated from rabbit cornea and used directly for bioassay of the conditioned media to detect substances that inhibit collagenase synthesis. The mink lung epithelial cell line, Mv1Lu, was used for bioassay of TGF-beta-like activity. The addition of specific neutralizing antisera to bioassays allowed an assessment of the contribution of each isoform to the net regulatory activity. Reverse transcription-polymerase chain reaction and Northern blot analysis were employed to detect the presence of IL-1ra or TGF-beta mRNA species in cells from cultures used to generate conditioned media.

RESULTS

Both stimulatory and inhibitory substances that regulate the synthesis of stromal cell collagenase are released by corneal epithelial cells in primary culture. In contrast, only stimulatory activity is produced by corneal fibroblasts or SIRCs. MRNAs for a TGF-beta isoform, TGF-beta 2, and IL-1ra were identified in epithelial cells. Stromal fibroblasts also expressed TGF-beta 2 mRNA, but no evidence was found for expression of TGF-beta 3 mRNA in any of the three cell types. TGF-beta 2 is released by epithelial cells in both active and latent forms. This cytokine mediates the major portion of the net inhibitory activity against stromal cell collagenase synthesis produced by corneal epithelial cells.

CONCLUSIONS

This study demonstrates the expression of TGF-beta 2 and IL-1ra by corneal epithelial cells in culture. It is the TGF-beta 2 that acts as the major inhibitor of collagenase synthesis by corneal stromal cells in culture. However, IL-1ra and TGF-beta 2 are likely to play important roles in epithelial-mesenchymal interactions regulating corneal development, homeostatic maintenance, and repair.

The rabbit gene for 92-kDa matrix metalloproteinase. Role of AP1 and AP2 in cell type-specific transcription

We isolated the rabbit gene for the 92-kDa matrix metalloproteinase, gelatinase B, and sequenced 1802 contiguous bases covering the first three exons and 522 bases of DNA upstream of the start site for transcription. The DNA between bases -519 and +19 is sufficient to drive expression of a reporter gene in early passage cultures of corneal fibroblasts or primary cultures of corneal epithelial cells. Basal activity of the gelatinase B promoter in fibroblasts is lower than a collagenase promotor of 1800 base pairs, but activity of both promotors is similarly stimulated by treatment of transfected cells with phorbol 12-myristate 13-acetate, and stimulation is enhanced by co-treatment with transforming growth factor-beta. In contrast, basal activity of the gelatinase B promotor in epithelial cells is higher than the collagenase promotor. Deletion analysis demonstrated that sequences upstream of base -330 confer cell type-specific activity to the gelatinase B promotor. Site-directed mutagenesis revealed that an AP1-like element within this region is specifically utilized by fibroblasts. This region also contains elements that confer the capacity for activation by AP2, a transcription factor found to be expressed by corneal epithelial cells but not by corneal fibroblasts. In contrast, AP2 does not activate the collagenase promotor. These results provide a molecular basis for the unique cell type-specific expression pattern of gelatinase B as compared to other matrix metalloproteinases.

Interleukin 1 alpha mediates collagenase synthesis stimulated by phorbol 12-myristate 13-acetate

Stimulation of collagenase expression in cultures of normal diploid fibroblasts by the tumor promotor phorbol 12-myristate 13-acetate (PMA) occurs secondarily to synthesis of unknown intermediary proteins. We have investigated the hypothesis that a form of the cytokine interleukin 1 (IL-1) is one intermediate controlling PMA-stimulated collagenase expression. Treatment with an IL-1 receptor antagonist inhibits the constitutive synthesis of collagenase in early passage fibroblast cultures from rabbit. Radioimmunoassay demonstrates that, of the two known IL-1 forms, IL-1 alpha and IL-1 beta, only IL-1 alpha is synthesized and released into the medium of corneal fibroblast cultures. PMA treatment of cells increases the level of IL-1 alpha mRNA; this occurs prior to the increase in collagenase mRNA and corresponds with increased synthesis and release of IL-1 alpha protein. Neutralizing antiserum to IL-1 alpha inhibits constitutive collagenase synthesis. Reagents that inhibit the activity of IL-1 alpha (IL-1 receptor antagonist or neutralizing antibody) also inhibit the PMA-mediated stimulation of collagenase synthesis. These results indicate that constitutive and PMA-stimulated expression of collagenase is regulated through an IL-1 alpha intermediate. In vivo, regulation of the lytic phase of tissue remodeling through the IL-1 alpha intermediate may ensure the recruitment of cells adjacent to the one that received the initial stimulus.

MMPs and proteinase inhibitors in the human aqueous humor

PURPOSE

This study was performed to examine the gelatinolytic and caseinolytic activities and the levels of two proteinase inhibitors, alpha 1-proteinase inhibitor (alpha 1-antitrypsin) and alpha 2-macroglobulin, in the human aqueous humor.

METHODS

Aqueous humor samples were collected during elective surgery in patients with cataracts. Zymography with gelatin- and casein-containing gels was performed. The inhibitors were examined by Western blot analyses, enzyme-linked immunosorbent assay, and dot blot assays.

RESULTS

The aqueous humor contained a major band of gelatinolytic activity at a molecular weight of 66 kD and minor bands at 125, 95, and 62 kD. These gelatinases were inhibited by 10 mM ethylenediaminetetraacetic acid (EDTA) or 1,10-phenanthroline. After extended incubation (48 hours), zymography on casein-containing gels showed proteinase bands with molecular weights in the 80- to 84-kD range. Additional bands at 68 and 48 kD also were observed. All the caseinase activities were inhibited by 10 mM phenylmethylsulfonyl fluoride and 1 microgram/ml aprotinin. No inhibition was observed with 5 mM EDTA, 5 microM E-64, or 1 microM pepstatin. These results indicated that the caseinases are serine proteinases. Western blot analysis showed a 53-kD alpha 1-proteinase inhibitor band in the aqueous humor. The concentration was 32.2 +/- 9.9 micrograms/ml, constituting approximately 15% of the total protein. A 360-kD protein band immunoreactive to anti-alpha 2-macroglobulin also was detected. Its level in the aqueous humor was 3.2 +/- 1.3 micrograms/ml.

CONCLUSIONS

The gelatinases, serine-like proteinases, and proteinase inhibitors found in the aqueous humor may participate in the remodeling of extracellular matrices in the trabecular meshwork and other tissues bordering the anterior chamber.

Stromal fibroblasts synthesize collagenase and stromelysin during long-term tissue remodeling

The process of connective tissue remodeling is an important mechanism contributing to tissue morphogenesis in development and homeostasis. Although it has long been known that remodeling tissues actively mediate collagenolysis, little is understood about the molecular mechanisms controlling this cell-regulated process. In this study, we examined the biosynthesis of collagenase and the related metalloproteinase, stromelysin, during remodeling of repair tissue deposited after mechanical injury to the rabbit cornea. Neither enzyme was synthesized by uninjured corneas; however, synthesis and secretion was detectable within one day after injury. Collagenase accumulated in its latent form while stromelysin appeared to be partially activated. Enzymes were synthesized by cells having a fibroblast phenotype. These cells were found within the stroma. New synthesis was correlated with accumulation of enzyme-specific mRNA. Highest levels of enzyme synthesis were observed in the repair tissue. However, stromal cells outside of the repairing area also synthesized both enzymes. The level of synthesis decreased in a gradient radiating from the repair tissue. Total synthetic levels in a given area of cornea were dependent on both the number of cells expressing enzyme and the rate of enzyme synthesis. Synthesis of collagenase was detected in repair tissue as long as nine months after injury. Our findings provide direct support for the hypothesis that new collagenase synthesis by cells in repair tissue is the first step in collagen degradation during long-term tissue remodeling.

Collagenolytic/gelatinolytic metalloproteinases in normal and keratoconus corneas

Cells of keratoconus corneas have been reported to produce higher levels of collagenolytic/gelatinolytic enzymatic activities than do cells of normal corneas. The current study investigates the contribution of 1) specific enzyme gene products, and 2) the degree to which these proteins are present in the activated forms, to the increased enzymatic activities. We demonstrate that two neutral gelatinolytic enzymes, a 66/59 kD form and a 92 kD form, can be directly extracted from both normal and keratoconus corneas. These enzymes are identified as the pro- and activated forms of MMP-2 and as the pro-form of MMP-9, specific members of the matrix metalloproteinase family. Normal and keratoconus corneas show no significant differences in amounts or types of extractable neutral gelatinases, nor in the amounts or types that they synthesize in culture. Furthermore, in both the normal and keratoconus corneas, gelatinases are found primarily in the inactive form. These studies suggest the possible importance of changes in proteinase inhibitor levels to the characteristic biochemical features of keratoconus corneas.

Mechanism of basement membrane dissolution preceding corneal ulceration

In animal models for corneal ulceration, the degradation of extracellular matrix components of the stroma does not occur until after the basement membrane underlying the corneal epithelium has disappeared. Using a thermal-burn model, it was demonstrated that epithelial basement membrane is degraded actively by products of corneal cells in a process that does not require the participation of polymorphonuclear leukocytes. A new gelatinolytic metalloproteinase, preliminarily identified as the matrix metalloproteinase, MMP-9, is synthesized and secreted by corneal cells with a timing appropriate for a role in basement membrane degradation. During healing of ulcers, when new matrix is being deposited actively in the burned tissue, a second new gelatinase appears in the cornea, preliminarily identified as a stable activated form of the matrix metalloproteinase, MMP-2. The timing of expression suggests a role for this enzyme in appropriate deposition and remodeling of new matrix in the regenerating corneal tissue.

Differential roles for two gelatinolytic enzymes of the matrix metalloproteinase family in the remodelling cornea

We have documented changes in collagenolytic/gelatinolytic enzymes of the matrix metalloproteinase family (MMP) in remodelling rabbit cornea. MMP-2 (65 kDa gelatinase) in the proenzyme form is synthesized by the cells of the normal corneal stroma. After keratectomy the level of MMP-2 is increased in the stroma and enzyme appears in both pro- and activated forms. In addition, corneal cells synthesize MMP-9 (92 kDa gelatinase) in the proenzyme form after keratectomy; expression occurs in both the epithelial as well as stromal corneal layers. Changes in expression of both enzymes are precisely localized to the repairing portion of cornea, but demonstrate important differences in timing that correlate with the timing of specific events of matrix remodelling. Our data suggest that each of the gelatinases plays a different role in tissue remodelling after injury. We hypothesize that MMP-2 performs a surveillance function in normal cornea, catalyzing degradation of collagen molecules that occasionally become damaged. After wounding, this enzyme appears to participate in the prolonged process of collagen remodelling in the corneal stroma that eventually results in functional regeneration of the tissue. MMP-9 expression does not correlate with stromal remodelling, but we suggest that the enzyme might play a part in controlling resynthesis of the epithelial basement membrane.

An inhibitor of the matrix metalloproteinase synthesized by rabbit corneal epithelium

Normal and abnormal processes of cellular invasion often are initiated by degradation of basement membranes. The process of corneal ulceration might operate via similar mechanisms; degradation of the corneal stroma is not seen until after the basement membrane underlying the corneal epithelium in the preulcerative lesion is lost. Recent data implicate a member of the matrix metalloproteinase (MMP) family of enzymes, 92 kD gelatinase/type IV collagenase (MMP-9) in both cellular invasion processes and degradation of epithelial basement membrane before corneal ulceration. This suggests that use of nontoxic substances that block activity of MMP-9 might be useful in preventing or inhibiting pathologic invasion processes in vivo. An agent that fits these criteria is N-[D,L-2-isobutyl-3(N'-hydroxycarbonylamido)-propanoyl]-O- methyl-L-tyrosine methylamide, which previously has been characterized as an inhibitor of tumor cell collagenases. In this study, the authors show that the inhibitor can efficiently block activity of MMP-9 purified from cultures of rabbit corneal epithelial cells. Results suggest that the recently reported efficacy of a closely related inhibitor in blocking progression of alkali burns to ulceration might be attributable to its action against MMP-9.

Transforming growth factor-beta and interleukin-1 modulate metalloproteinase expression by corneal stromal cells

The enzyme collagenase participates in remodeling the extracellular matrix of corneal stroma during normal wound healing and mediates the degradation of extracellular matrix that occurs in many corneal pathologic states. Because this enzyme is synthesized and secreted by corneal cells, therapy of degradative disorders might be geared toward control of enzyme expression. The effects of two cytokines, transforming growth factor-beta (TGF-beta) and interleukin-1 (IL-1), on the expression of collagenase by cultured corneal stromal cells are reported. In addition, the concomitant effects of these cytokines on expression of three additional members of the matrix metalloproteinase (MMP) family--stromelysin, 72-kilodalton (kD) gelatinase, and 92-kD gelatinase--were investigated. When stromal cells are situated in the normal corneal stroma, they produce only a single MMP, 72-kD gelatinase. This pattern of expression was reproduced by stromal cells freshly plated in primary culture. However after passage in culture, the cells also began to secrete collagenase and stromelysin. Treatment of primary cultures with recombinant human IL-1 also induced collagenase and stromelysin expression. In addition, 92-kD gelatinase expression was induced and 72-kD gelatinase expression was increased further by IL-1 treatment. Treatment of passaged cultures or IL-1-treated primary cultures with recombinant human TGF-beta reverted the pattern of enzyme expression toward that exhibited by primary, untreated cultures, ie, expression of collagenase and stromelysin was repressed while expression of 72-kD gelatinase was increased. These results suggest that TGF-beta and IL-1 may be important agents for controlling MMP expression in healthy and diseased corneas.

The pattern of metalloproteinase expression by corneal fibroblasts is altered by passage in cell culture

We have examined the pattern of expression of four different matrix metalloproteinases (MMPs), collagenase, stromelysin, 92 kD gelatinase, and 72 kD gelatinase, by primary and passaged cultures of rabbit corneal fibroblasts. Primary cultures of this cell type have previously been shown to reproduce the normal tissue regulation of collagenase expression. We demonstrate qualitative and quantitative changes in the pattern of MMP expression as the cells are passaged in culture. Only a single MMP, 72 kD gelatinase, is constitutively expressed by primary fibroblast cultures. Phorbol myristate acetate (PMA) treatment upregulates expression of 72 kD gelatinase and turns on the expression of collagenase and stromelysin, as well as 92 kD gelatinase. However, the degree to which MMP expression is induced is minimal. Cells subcultured but a single time constitutively produce not only 72 kD gelatinase, but also collagenase and stromelysin. In addition, PMA treatment upregulates expression of collagenase, stromelysin and 92 kD gelatinase to high levels. In contrast, the expression of 72 kD gelatinase is repressed by treatment of passaged cell cultures with PMA. Our data indicate that the cell does not simply turn the MMP genes on or off, as a group, in response to various agents, but that it has the capacity for fine control over which MMPs are expressed and the degree to which each is expressed. Changes in MMP protein expression induced by PMA treatment are correlated with changes in specific mRNA levels in passaged cultures. The kinetics of mRNA accumulation suggest that the MMP genes can respond to multiple intracellular signals initiated in a temporal cascade by PMA. It is the combined effects of the individual signals on the accumulation of specific mRNAs that must determine the ultimate pattern of MMP protein expression. The distinct patterns of MMP expression produced by primary and passaged cell cultures may be analogous to patterns of expression that might occur under particular in vivo conditions.

Expression of collagenolytic/gelatinolytic metalloproteinases by normal cornea

Members of the gelatinase subclass of the matrix metalloproteinase family have the capacity to degrade denatured collagens of all types and native types IV, V, and VII collagens. The authors identified the metalloproteinase species of the gelatinase class produced by the cells of rabbit corneal tissue. Two different molecular forms of gelatinase, visualized as enzymatic activities, that undergo electrophoresis with different mobilities on gelatin zymograms are synthesized by corneal cells in serum-free organ culture. The enzyme species that has the slower mobility is biochemically and immunologically related to a gelatinase synthesized by macrophages and neutrophils which has been called both type IV and type V collagenase. The second gelatinase species is related to a second enzyme, the product of a different gene, which has also been called type IV collagenase. The electrophoretic mobilities of these enzymes on polyacrylamide gels indicate the inactive proenzyme forms. The authors refer to these enzymes as 92-kilodalton (kD) gelatinase and 72-kD gelatinase based on their electrophoretic mobilities under sulfhydryl-reducing conditions. In primary cell culture, corneal epithelial cells were found to synthesize predominantly the 92-kD gelatinase species whereas the 72-kD gelatinase is synthesized mostly by stromal fibroblasts. However, each cell type can produce small amounts of the other enzyme. The 72-kD gelatinase, mostly in the proenzyme form, can be extracted from the normal corneal stroma without culturing, but expression of 92-kD gelatinase can only be detected in cell or organ culture. The substrate specificities of these enzymes suggests that they may be of central importance in the degradation of the epithelial basement membrane and in formation of the epithelial defect that precedes corneal ulceration.

Hsr-omega, A Novel Gene Encoded by a Drosophila Heat Shock Puff

Although originally identified because of its abundant transcription in heat shock, the hsr-omega gene is active, at generally lower levels, in non-stressed cells. The locus produces an unusual set of three transcripts. Evidence from a variety of experiments suggests that one of these transcripts acts in the nucleus, possibly to regulate the activity of a nuclear protein. Another of the transcripts appears to act in the cytoplasm, possibly monitoring or regulating some aspect of translation. The two transcripts together could have a role in coordinating nuclear and cytoplasmic activity. A number of processes occur in eukaryotic cells in which nuclear and cytoplasmic activities need to be coordinated; we suggest that hsr-omega plays a role in such coordination.

Unusual behavior of the cytoplasmic transcript of hsr omega: an abundant, stress-inducible RNA that is translated but yields no detectable protein product

Although a major site of transcription in heat shock, the Drosophila hsr omega gene does not encode any known heat shock proteins. Instead, studies of the hsr omega transcripts suggest that the RNA molecules, rather than encoded proteins, are the active products of this gene. The cytoplasmic RNA, omega 3, is spliced and polyadenylated and yet has only very small open reading frames (ORFs), and these are poorly conserved in different Drosophila species. Surprisingly, the work reported here leads us to conclude that one of the tiny ORFs in this RNA is translated. This ORF, designated ORF-omega, is notable in being the only ORF that shows sequence conservation in the three Drosophila species examined. However, translation of this ORF does not lead to detectable accumulation of the protein product. We suggest that ORF-omega may be an example of an unusual type of translated ORF. The act of translation itself may be important rather than the generation of a functional protein product. This nonproductive translation may play a role in regulation of cellular activities.

Cloning of a complementary DNA for rabbit proactivator. A metalloproteinase that activates synovial cell collagenase, shares homology with stromelysin and transin, and is coordinately regulated with collagenase

Rabbit proactivator is a neutral metalloproteinase that activates another metalloproteinase, procollagenase, and degrades noncollagenous matrix. We describe the construction of an activator complementary DNA (cDNA) clone, which is 1.9 kb, that selects a 2.1-kb messenger RNA (mRNA) in Northern blot hybridizations. Nucleic acid sequence studies of the activator cDNA indicate 1) that it encodes protein Mr 53,881, 2) that this protein exhibits approximately 80% homology with rat transin, an oncogene-induced protein with a previously unknown function, and 3) that, in the first 172 residues, it is virtually identical to the rabbit metalloproteinase, stromelysin. Homology between rabbit activator and human skin collagenase is approximately 50%. Activator and collagenase mRNA are coordinately regulated; untreated cultures of rabbit synovial fibroblasts produce low levels of each protein, but addition of phorbol myristate acetate (10(-8)M) results in an increase in mRNA for both proteins by 2.5-5 hours. Adding all-trans-retinoic acid (10(-6)M) or dexamethasone (10(-7)M) to phorbol-stimulated cells coordinately suppresses both activator and collagenase mRNA. Our data suggest the existence of coordinately regulated metalloproteinases that are important in the modulation of connective tissue metabolism.

A gene for rabbit synovial cell collagenase: member of a family of metalloproteinases that degrade the connective tissue matrix

We have determined the nucleotide sequence of a collagenase mRNA from rabbit synovial cells from which the primary structure of the encoded protein was deduced. This proteinase is 51% homologous to the enzyme that activates it from the zymogen form, rabbit synovial cell activator/stromelysin. Rabbit collagenase and activator/stromelysin thus share comembership in a gene family that includes human skin collagenase; the human and rabbit metalloproteinase, activator/stromelysin; and an oncogene-induced proteinase from rat named transin. The mRNA sequence of collagenase enabled us to completely map the structure of its gene, which is 9.1 kilobases and is composed of 10 exons and 9 introns. This is the first report of the structure of a collagenase gene. We show that it has striking similarity to additional members of this metalloproteinase gene family, transin genes I and II of rat. We have further sequenced genomic DNA flanking the collagenase gene and have identified nucleic acid elements of possible importance in gene regulation.

Molecular cloning of human synovial cell collagenase and selection of a single gene from genomic DNA

We used a subclone of a rabbit genomic clone for collagenase that cross-hybridizes with human synovial cell messenger RNA (mRNA) to identify a human collagenase complementary DNA (cDNA) clone. The human cDNA clone is 2.1 kilobases (kb) and selects a mRNA transcript of approximately the same size from primary cultures of rheumatoid synovial cells that produce collagenase, but no mRNA is selected from control (nonproducing) synovial fibroblasts. Restriction enzyme analysis and DNA sequence data indicate that our cDNA clone is full length and that it is identical to that recently described for human skin fibroblast collagenase. The cDNA clone identified a single collagenase gene of approximately 17 kb from blots of human genomic DNA. The identity of human skin and synovial cell collagenase and the ubiquity of this enzyme and of its substrates, the interstitial collagens types I, II, and III, imply that common mechanisms controlling collagenolysis throughout the human body may be operative in both normal and disease states.

Characterization of rabbit genes for synovial cell collagenase

To provide tools for understanding collagenase gene expression in rheumatoid arthritis, we have isolated and characterized genomic clones for rabbit synovial cell collagenase. These clones represent 2 types of collagenase gene, at least 1 of which is transcribed in synovial fibroblasts. By examining the rabbit genome in situ, we provide evidence that there are only 2 different synovial cell collagenase genes found in a haploid genome. Amplification of these genes is not a mechanism for collagenase messenger RNA induction by phorbol esters.

Homology between exon-containing portions of rabbit genomic clones for synovial cell collagenase and human foreskin and synovial cell mRNAs

Portions of rabbit genomic clones containing exons cross-hybridize with human synovial cell and human foreskin fibroblast mRNAs. These cross-hybridizing genomic fragments have been subcloned into pBR328 and may be useful probes in isolating cDNA and genomic clones from human tissue, and perhaps from other species as well. In addition, DNA sequence data on a 530 bp cDNA clone for rabbit synovial-cell collagenase indicate that this clone is composed primarily of 3'-untranslated region. As such, it is probably not useful in cross-species hybridizations.