Unexpected expression of thrombospondin 1 by corneal and iris fibroblasts in the pseudoexfoliation syndrome

Thrombospondin-1 Signaling Through the Calreticulin/LDL Receptor Related Protein 1 Axis: Functions and Possible Roles in Glaucoma

Thrombospondin-1 (TSP-1) is a matricellular extracellular matrix protein. Matricellular proteins are components of the extracellular matrix (ECM) that regulate key cellular functions and impact ECM organization, but which lack direct primary structural roles in the ECM. TSP-1 expression is upregulated in response to injury, hypoxia, growth factor stimulation, inflammation, glucose, and by reactive oxygen species. Relevant to glaucoma, TSP-1 is also a mechanosensitive molecule upregulated by mechanical stretch. TSP-1 expression is increased in ocular remodeling in glaucoma in both the trabecular meshwork and in the optic nerve head. The exact roles of TSP-1 in glaucoma remain to be defined, however. It plays important roles in cell behavior and in ECM remodeling during wound healing, fibrosis, angiogenesis, and in tumorigenesis and metastasis. At the cellular level, TSP-1 can modulate cell adhesion and migration, protease activity, growth factor activity, anoikis resistance, apoptosis, and collagen secretion and matrix assembly and cross-linking. These multiple functions and macromolecular and receptor interactions have been ascribed to specific domains of the TSP-1 molecule. In this review, we will focus on the cell regulatory activities of the TSP-1 N-terminal domain (NTD) sequence that binds to cell surface calreticulin (Calr) and which regulates cell functions via signaling through Calr complexed with LDL receptor related protein 1 (LRP1). We will describe TSP-1 actions mediated through the Calr/LRP1 complex in regulating focal adhesion disassembly and cytoskeletal reorganization, cell motility, anoikis resistance, and induction of collagen secretion and matrix deposition. Finally, we will consider the relevance of these TSP-1 functions to the pathologic remodeling of the ECM in glaucoma.

Cell-Matrix Interactions in the Eye: From Cornea to Choroid

The extracellular matrix (ECM) plays a crucial role in all parts of the eye, from maintaining clarity and hydration of the cornea and vitreous to regulating angiogenesis, intraocular pressure maintenance, and vascular signaling. This review focuses on the interactions of the ECM for homeostasis of normal physiologic functions of the cornea, vitreous, retina, retinal pigment epithelium, Bruch's membrane, and choroid as well as trabecular meshwork, optic nerve, conjunctiva and tenon's layer as it relates to glaucoma. A variety of pathways and key factors related to ECM in the eye are discussed, including but not limited to those related to transforming growth factor-β, vascular endothelial growth factor, basic-fibroblastic growth factor, connective tissue growth factor, matrix metalloproteinases (including MMP-2 and MMP-9, and MMP-14), collagen IV, fibronectin, elastin, canonical signaling, integrins, and endothelial morphogenesis consistent of cellular activation-tubulogenesis and cellular differentiation-stabilization. Alterations contributing to disease states such as wound healing, diabetes-related complications, Fuchs endothelial corneal dystrophy, angiogenesis, fibrosis, age-related macular degeneration, retinal detachment, and posteriorly inserted vitreous base are also reviewed.

Matricellular Proteins Play a Potential Role in Acute Primary Angle Closure

PURPOSE

To quantify levels of matricellular proteins in aqueous humor samples from acute primary angle closure (APAC) and non-glaucomatous cataract eyes and investigate their correlation with intraocular pressure (IOP) fluctuation.

MATERIALS AND METHODS

Aqueous humor samples were collected from 63 eyes including 29 current APAC eyes, 12 previous APAC eyes, and 22 cataract eyes. Concentrations of four main matricellular proteins (SPARC, tenascin-C, thrombospondin-2, and osteopontin) were measured using multiplexed immunoassay kits. Correlations between matricellular proteins and age, sex, and IOP were then detected using Spearman's rank correlation coefficient.

RESULTS

The levels of SPARC, thrombospondin-2, and osteopontin were significantly elevated in the APAC group as compared to the cataract group (p < 0.001, p < 0.001, and p = 0.009, respectively). Further separation of the APAC group into current and previous APAC groups showed that only the differences of SPARC and thrombospondin-2 between the current APAC and cataract groups were significant (both p < 0.001). All four matricellular proteins were found to have a positive correlation with IOP in the current APAC group but no correlation was found in the previous APAC or cataract groups.

CONCLUSIONS

The levels of matricellular proteins were significantly elevated in the current APAC eyes and positively correlated to IOP. Further studies are necessary to investigate the molecular mechanisms and histological evidence of pathogenesis in matricellular proteins in APAC.

Is there an association between pseudoexfoliation syndrome and knee osteoarthritis?

AIM

The purpose of this study was to investigate whether there is an association between pseudoexfoliation (PEX) syndrome and knee osteoarthritis (OA), and compare to compare patients with PEX and OA to a control group without PEX.

DESIGN AND METHODS

This observational case-control study examined 254 subjects: 127 cases with PEX in at least one eye and 127 controls without PEX. A full ophthalmic examination including slit lamp biomicroscopy, gonioscopy, applanation tonometry, pupil dilation, and fundus examination was performed for the diagnosis of PEX. Knee pain was evaluated using a visual analogue scale (VAS). Knee OA was diagnosed according to American College of Rheumatology (ACR) criteria and graded according to the Kellgren and Lawrence (KL) grading scale on radiographic examination.

RESULTS

Of the 254 subjects, 171 (67.3%) had severe OA and 83 (32.7%) mild OA. Of the 171 patients with severe knee OA, 93 (54.4%) had ocular PEX and the remaining 78 (45.6%) had normal ophthalmologic examination results. Both the PEX and the control groups contained 127 patients; the number of patients with moderate-severe OA was significantly (p = 0.032) higher in the PEX group as compared to controls: 93 patients (73.2%) in the PEX group and 78 (61.5%) in the control group. VAS score (p = 0.037) and KL grade (p = 0.024) were significantly higher in the PEX group than in controls. As evidenced by the odds ratios (ORs) pertaining to the severity of OA, age (OR = 1.112; 95% confidence interval, CI: 1.054-1.173) and PEX (OR = 2.044; 95% CI: 1.164-3.584) had a significant influence, but gender did not.

CONCLUSION

This study suggests a probable relationship between OA and ocular PEX. Further studies are needed to analyze the molecular basis of this association. Patients with severe knee OA should be informed about the possibility of having PEX and may be referred for ophthalmologic examination.

The Function of Matricellular Proteins in the Lamina Cribrosa and Trabecular Meshwork in Glaucoma

PURPOSE

To review the current literature regarding the role of matricellular proteins in glaucoma, specifically in the lamina cribrosa (LC) region of the optic nerve head (ONH) and the trabecular meshwork (TM).

METHODS

A literature search was performed for published articles describing the expression and function of matricellular proteins such as thrombospondin (TSP), connective tissue growth factor (CTGF), secreted protein acidic and rich in cysteine (SPARC), and periostin in glaucoma.

RESULTS

In glaucoma, there are characteristic extracellular matrix (ECM) changes associated with optic disc cupping in the ONH and subsequent visual field defects. Matricellular proteins are a family of nonstructural secreted glycoproteins, which enable cells to communicate with their surrounding ECM, including CTGF, also known as CCN2, TSPs, SPARC, periostin, osteonectin, and tenascin-C and -X, and other ECM proteins. Such proteins appear to play a role in fibrosis and increased ECM deposition. Importantly, most are widely expressed in tissues particularly in the TM and ONH, and deficiency of TSP1 and SPARC has been shown to lower intraocular pressure in mouse models of glaucoma through enhanced outflow facility.

CONCLUSION

This article highlights the role of matricellular proteins in glaucoma pathology. The potential role of these proteins in glaucoma is emerging as some have an association with the pathophysiology of the TM and LC region and might therefore be potential targets for therapeutic intervention in glaucoma.

TGF-β2-mediated ocular hypertension is attenuated in SPARC-null mice

PURPOSE

Transforming growth factor-β2 (TGF-β2) has been implicated in the pathogenesis of primary open-angle glaucoma through extracellular matrix (ECM) alteration among various mechanisms. Secreted protein acidic and rich in cysteine (SPARC) is a matricellular protein that regulates ECM within the trabecular meshwork (TM), and is highly upregulated by TGF-β2. We hypothesized that, in vivo, SPARC is a critical regulatory node in TGF-β2-mediated ocular hypertension.

METHODS

Empty (Ad.empty) or TGF-β2-containing adenovirus (Ad.TGF-β2) was injected intravitreally into C57BL6-SV129 WT and SPARC-null mice. An initial study was performed to identify a stable period for IOP measurement under isoflurane. The IOP was measured before injection and every other day for two weeks using rebound tonometry. Additional mice were euthanized at peak IOP for immunohistochemistry.

RESULTS

The IOP was stable under isoflurane during minutes 5 to 8. The IOP was significantly elevated in Ad.TGF-β2-injected (n = 8) versus Ad.empty-injected WT (n = 8) mice and contralateral uninjected eyes during days 4 to 11 (P < 0.03). The IOPs were not significantly elevated in Ad.TGF-β2-injected versus Ad.empty-injected SPARC-null mice. However, on day 8, the IOP of Ad.TGF-β2-injected SPARC-null eyes was elevated compared to that of contralateral uninjected eyes (P = 0.0385). Immunohistochemistry demonstrated that TGF-β2 stimulated increases in collagen IV, fibronectin, plasminogen activator inhibitor-1 (PAI-1), connective tissue growth factor (CTGF), and SPARC in WT mice, but only PAI-1 and CTGF in SPARC-null mice (P < 0.05).

CONCLUSIONS

SPARC is essential to the regulation of TGF-β2-mediated ocular hypertension. Deletion of SPARC significantly attenuates the effects of TGF-β2 by restricting collagen IV and fibronectin expression. These data provide further evidence that SPARC may have an important role in IOP regulation and possibly glaucoma pathogenesis.

Matricellular proteins in the trabecular meshwork: review and update

Abstract Primary open-angle glaucoma (POAG) is a leading cause of blindness worldwide, and intraocular pressure (IOP) is an important modifiable risk factor. IOP is a function of aqueous humor production and aqueous humor outflow, and it is thought that prolonged IOP elevation leads to optic nerve damage over time. Within the trabecular meshwork (TM), the eye's primary drainage system for aqueous humor, matricellular proteins generally allow cells to modulate their attachments with and alter the characteristics of their surrounding extracellular matrix (ECM). It is now well established that ECM turnover in the TM affects outflow facility, and matricellular proteins are emerging as significant players in IOP regulation. The formalized study of matricellular proteins in TM has gained increased attention. Secreted protein acidic and rich in cysteine (SPARC), myocilin, connective tissue growth factor (CTGF), and thrombospondin-1 and -2 (TSP-1 and -2) have been localized to the TM, and a growing body of evidence suggests that these matricellular proteins play an important role in IOP regulation and possibly the pathophysiology of POAG. As evidence continues to emerge, these proteins are now seen as potential therapeutic targets. Further study is warranted to assess their utility in treating glaucoma in humans.

The role of matricellular proteins in glaucoma

Glaucoma is an optic neuropathy affecting approximately 60million people worldwide and is the second most common cause of irreversible blindness. Elevated intraocular pressure (IOP) is the main risk factor for developing glaucoma and is caused by impaired aqueous humor drainage through the trabecular meshwork (TM) and Schlemm's canal (SC). In primary open angle glaucoma (POAG), this elevation in IOP in turn leads to deformation at the optic nerve head (ONH) specifically at the lamina cribrosa (LC) region where there is also a deposition of extracellular matrix (ECM) molecules such as collagen and fibronectin. Matricellular proteins are non-structural secreted glycoproteins that help cells communicate with their surrounding ECM. This family of proteins includes connective tissue growth factor (CTGF), also known as CCN2, thrombospondins (TSPs), secreted protein acidic and rich in cysteine (SPARC), periostin, osteonectin, and Tenascin-C and -X and other ECM proteins. All members appear to play a role in fibrosis and increased ECM deposition. Most are widely expressed in tissues particularly in the TM and ONH and deficiency of TSP1 and SPARC have been shown to lower IOP in mouse models of glaucoma through enhanced outflow facility. The role of these proteins in glaucoma is emerging as some have an association with the pathophysiology of the TM and LC regions and might therefore be potential targets for therapeutic intervention in glaucoma.

Proteins in aqueous humor from cataract patients with and without pseudoexfoliation syndrome

The aim of this study was to investigate the protein content in aqueous humor in eyes with and without pseudoexfoliations (PEX) and to evaluate the quantitative proteomics method, isobaric tagging for relative and absolute protein quantification (iTRAQ), in combination with two separation methods followed by matrix-assisted Laser desorption/ionization (MALDI) mass spectrometry and tandem mass spectrometry (MS/MS). During cataract surgery, samples of aqueous humor were collected from 20 eyes with PEX and from 18 control eyes. The relative concentrations of proteins in the pooled samples of ten PEX eyes and eight controls were evaluated after trypsin digestion and Labeling of the peptides with (iTRAQ) reagent. Two separation methods, Liquid chromatography (LC) and capillary electrophoresis (CE) were used, followed by MALDI mass spectrometry and MS/MS. Furthermore, 1D gel electrophoresis was performed on the remaining ten pooled PEX samples and ten control samples. The gel material was separated by nano-liquid chromatography (nano-LC) followed by Linear-ion-trap quadrupole Fourier transformation ion cyclotron resonance (FT-ICR). Fifty four proteins were identified in the LC runs and 24 with CE. The relative concentrations of beta-crystallines B2 and S were raised and those of angiotensinogen and osteopontin Lowered in the PEX sample compared to the control. The trends regarding beta-crystallines B2, angiotensinogen and osteopontin were confirmed by the 1D gel electrophoresis.

Non-contact in vivo confocal scanning laser microscopy in exfoliation syndrome, exfoliation syndrome suspect and normal eyes

PURPOSE

This study aimed to evaluate the efficacy of non-contact confocal laser microscopy in detecting structural alterations of the cornea, iris and lens in fellow eyes of patients with clinically unilateral exfoliation syndrome (XFS) and XFS suspects.

METHODS

In a prospective observational case series, eyes of 12 patients with clinically unilateral XFS, six XFS suspects and six age-matched controls were divided into four groups. Group A included eyes with clinically visible exfoliation material (XFM) on the pupillary border or anterior lens capsule (n = 12); group B included fellow eyes of clinically unilateral XFS patients without visible XFM (n = 12); group C comprised eyes in XFS suspects with signs of pigment dispersion without visible XFM (n = 10), and group D consisted of control eyes with no evidence of XFM or pigment dispersion (n = 12). The cornea, iris and lens were imaged using a non-contact lens prototype for the Rostock Cornea Module (HRT II). Images were analysed by two observers masked to the clinical findings.

RESULTS

Visible XFM (group A) on the lens capsule was characterized by hyperreflectivity in the granular and central disc areas and hyporeflective spaces in the intermediate zones. Similar hyperreflectivity was noted in four of 12 and one of 10 eyes in groups B and C, respectively, but in none in group D. Corneal endothelial hyperreflective deposits were found in eight of 12, four of 12 and one of 10 eyes in groups A, B and C, respectively, and no eyes in group D.

CONCLUSIONS

This technology permits visualization of XFM and ⁄ or XFMrelated changes in the cornea and lens in the unaffected eyes of patients with clinically unilateral XFS. It may allow early detection of XFS and impact glaucoma screening and clinical surveillance decisions.

Matricellular proteins in the trabecular meshwork

The trabecular meshwork is one of the primary tissues of interest in the normal regulation and dysregulation of intraocular pressure (IOP) that is a causative risk factor for primary open-angle glaucoma. Matricellular proteins generally function to allow cells to modulate their attachments with and alter the characteristics of their surrounding extracellular matrix (ECM). In non-ocular tissues, matricellular proteins generally increase fibrosis. Since ECM turnover is very important to the outflow facility, matricellular proteins may have a significant role in the regulation of IOP. The formalized study of matricellular proteins in trabecular meshwork is in its infancy. SPARC, thrombospondins-1 and -2, and tenascins-C and -X, and osteopontin have been localized to varying areas within the trabecular meshwork. Preliminary evidence indicates that SPARC and thrombospondin-1 play a role in the regulation of IOP and possibly the pathophysiology of glaucoma. These data show promise that matricellular proteins are involved in IOP dysregulation and are potential therapeutic targets. Further study is needed to clarify these roles.

Thrombospondin 1, thrombospondin 2 and the eye

Thrombospondin 1 and thrombospondin 2 (TSP1 and TSP2), which comprise the subgroup A thrombospondins, are matricellular proteins. As matricellular proteins, they modulate interactions between cells and the cellular environment, regulate cell adhesion and typically are expressed during tissue formative processes. In general, TSP1 and TSP2 counter angiogenesis (including tumour angiogenesis) and play important but contrasting roles during cutaneous repair. The two proteins are involved in development, including that of the eye, although evidence suggests that they have their greatest impact during tissue production in the adult. In the normal adult eye, they tend to be found at sites of ongoing matrix synthesis or cell-matrix interactions. At these sites, the two proteins possibly influence cellular differentiation and/or basement membrane deposition. TSP1 is also present in the intraocular fluids and drainage pathway, where it may function in maintaining the anti-angiogenic environment and in intraocular pressure control, respectively. TSP1 could also be involved in ocular immune privilege. Unlike in skin wounds, where TSP1 is derived from the blood and is present only in the early phases of repair, ocular tissue damage appears to lead to protacted TSP1 synthesis by local cells. This response might help suppress angiogenesis in the transparent tissues of the eye and so lessen visual axis opacification following injury. However, TSP2, which is also produced by damaged ophthalmic tissue and may be especially important in matrix organisation, seems to augment contraction in anomalous intraocular fibrosis. Elucidating the roles of TSP1 and TSP2 in ocular physiology and pathobiology may lead to improved therapies for neovascular, neoplastic, reparative and other ophthalmic diseases.

Recent Insights Into Angiogenesis, Apoptosis, Invasion, and Metastasis in Colorectal Carcinoma

The numerous studies profiling mechanisms in colorectal carcinoma have implicated multiple pathways in the malignant progression of a colorectal epithelial cell. Such pathways as aberrations in the cell cycle, deviation from apoptosis, neovascularization of tumors, and invasion and metastasis of malignant epithelial cells have been shown to occur in the progression of a normal epithelial cell to an adenoma and carcinoma. Today, we continue to search for communications or connections between these pathways as we try to get a more global picture of the events responsible for the adenoma-carcinoma sequence. This review focuses on the latest developments of three well-characterized pathways implicated in colorectal carcinoma: angiogenesis, apoptosis, and invasion and metastasis. We will attempt to highlight clinical correlates, when available, with some of the more interesting molecules.

Thrombospondins: multifunctional regulators of cell interactions

Thrombospondins are secreted, multidomain macromolecules that act as regulators of cell interactions in vertebrates. Gene knockout mice constructed for two members of this family demonstrate roles in the organization and homeostasis of multiple tissues, with particularly significant activities in the regulation of angiogenesis. This review discusses the functions of thrombospondins with regard to their cellular mechanisms of action and highlights recent advances in understanding how multifactorial molecular interactions, at the cell surface and within extracellular matrix, produce cell-type-specific effects on cell behavior and the organization of matrix and tissues.

Role of transforming growth factor-beta1 and its latent form binding protein in pseudoexfoliation syndrome

Pseudoexfoliation (PEX) syndrome is a common and clinically important systemic condition characterized by the pathologic production and accumulation of an abnormal fibrillar extracellular material in many intra- and extraocular tissues. Recent evidence suggests that it is a type of elastosis associated with the excess synthesis of elastic microfibrillar components such as fibrillin-1. Since transforming growth factor (TGF)-beta is a major modulator of extracellular matrix formation, the potential involvement of TGF-beta and its latent form binding protein (LTBP) in this aberrant matrix process was investigated. The expression of various isoforms of TGF-beta and LTBP was investigated in the anterior segment tissues of PEX and control eyes on the protein and mRNA level by light and electron microscopic immunohistochemistry, in situ hybridization, and semiquantitative RT-PCR. TGF-beta1 and TGF-beta2 levels were measured in aqueous humor and serum of PEX and control patients by ELISA. Cultures of Tenon's capsule fibroblasts were established to study the effect of TGF-beta1 on fibrillin-1 mRNA expression by Northern blot analysis. Significantly increased concentrations of both total and active TGF-beta1 were measured in the aqueous humor of PEX eyes without and with glaucoma as compared to control eyes, whereas levels of TGF-beta2 were not significantly different. The expression of TGF-beta1, LTBP-1, and LTBP-2, but not TGF-beta2, was markedly increased in anterior segment tissues of PEX eyes, particularly in the non-pigmented epithelium of the ciliary body, on both the mRNA and the protein level. Latent TGF-beta1 staining was consistently associated with PEX material deposits and could be released by proteolytic processing. Double immunolabeling revealed clear co-localization of LTBP-1 and -2 with latent TGF-beta1 and with fibrillin-1 on PEX fibrils. The expression of mRNA coding for fibrillin-1 was up-regulated in vitro by TGF-beta1. This study provides evidence for a significant role of TGF-beta1 and the LTBPs 1 and 2 in PEX syndrome. The results suggest that increased levels of latent and active TGF-beta1 in the aqueous humor of PEX patients, derived from enhanced local synthesis and activation, promote the buildup of the abnormal extracellular elastic material characteristic of PEX syndrome. They further support a dual role for LTBPs, both as integral structural components of PEX fibers and as a means of matrix anchorage of latent TGF-beta1, representing one possible mechanism for the regulation of TGF-beta1 activity in PEX eyes. Future therapeutic strategies might focus on TGF-beta1 antagonistic approaches.

Exfoliation syndrome

Exfoliation syndrome (XFS) is an age-related disease in which abnormal fibrillar extracellular material is produced and accumulates in many ocular tissues. Its ocular manifestations involve all of the structures of the anterior segment, as well as conjunctiva and orbital structures. Glaucoma occurs more commonly in eyes with XFS than in those without it; in fact, XFS has recently been recognized as the most common identifiable cause of glaucoma. Patients with XFS are also predisposed to develop angle-closure glaucoma, and glaucoma in XFS has a more serious clinical course and worse prognosis than primary open-angle glaucoma. There is increasing evidence for an etiological association of XFS with cataract formation, and possibly with retinal vein occlusion. XFS is now suspected to be a systemic disorder and has been associated preliminarily with transient ischemic attacks, stroke, systemic hypertension, and myocardial infarction. Further ramifications await discovery. Deposits of white material on the anterior lens surface are the most consistent and important diagnostic feature of XFS. The classic pattern consists of three distinct zones that become visible when the pupil is fully dilated. Whereas the classic picture of manifest XFS has been often described, the early stages of beginning exfoliation have not been well defined. Next to the lens, exfoliation material is most prominent at the pupillary border. Pigment loss from the iris sphincter region and its deposition on anterior chamber structures is a hallmark of XFS. Despite extensive research, the exact chemical composition of exfoliation material (XFM) remains unknown. An overproduction and abnormal metabolism of glycosaminoglycans have been suggested as one of the key changes in XFS. The protein components of XFM include both noncollagenous basement membrane components and epitopes of the elastic fiber system such as fibrillium. Regardless of etiology, typical exfoliation fibers have been demonstrated electron microscopically in close association with the pre-equatorial lens epithelium, the nonpigmented ciliary epithelium, the iris pigment epithelium, the corneal endothelium, the trabecular endothelium, and with almost all cell types of the iris stroma, such as fibrocytes, melanocytes, vascular endothelial cells, pericytes, and smooth muscle cells. The presence of XFS should alert the physician to the increased risks of intraocular surgery, most commonly zonular dehiscence, capsular rupture, and vitreous loss during cataract extraction. Heightened awareness of this condition and its associated clinical signs are important in the detection and management of glaucoma, and preoperative determination of those patients at increased risk for surgical complications.