"BM180": a novel basement membrane protein with a role in stimulus-secretion coupling by lacrimal acinar cells

The Lacritin-Syndecan-1-Heparanase Axis in Dry Eye Disease

Homeostasis and visual acuity of the surface of the eye are dependent on tears, a thin film comprising at least 1800 different extracellular proteins and numerous species of lipids through which 80% of entering light is refracted at the air interface. Loss of homeostasis in dry eye disease affects 5-7% of the world's population, yet little is known about key molecular players. Our story began as an unbiased screen for regulators of tearing that led to the discovery of homeostasis-restorative 'lacritin', a tear protein whose active form is selectively deficient in dry eye. Heparanase acts as a novel 'on-switch' for lacritin ligation of syndecan-1 necessary to trigger basal tearing, as well as pertussis toxin-sensitive and FOXO-dependent signaling pathways for healing of inflammation-damaged epithelia and restoring epithelial oxidative phosphorylation by mitochondrial fusion downstream of transiently accelerated autophagy. A phase 2 clinical trial has tested the applicability of this mechanism to the resolution of dry eye disease. Results are not yet available. With lacritin proteoforms detected in cerebral spinal fluid, plasma, and urine, the capacity of the lacritin-syndecan-1-heparanase axis to restore homeostasis might have wide systemic relevance to other organs.

Toll-like receptor 3 stimulation promotes Ro52/TRIM21 synthesis and nuclear redistribution in salivary gland epithelial cells, partially via type I interferon pathway

Up-regulated expression of Ro52/tripartite motif-containing protein 21 (TRIM21), Ro60/TROVE domain family, member 2 (TROVE2) and lupus LA protein/Sjögren's syndrome antigen B (La/SSB) autoantigens has been described in the salivary gland epithelial cells (SGEC) of patients with Sjögren's syndrome (SS). SGECs, the key regulators of autoimmune SS responses, express high levels of surface functional Toll-like receptor (TLR)-3, whereas Ro52/TRIM21 negatively regulates TLR-3-mediated inflammation. Herein, we investigated the effect of TLR-3-signalling on the expression of Ro52/TRIM21, as well as Ro60/TROVE2 and La/SSB autoantigens, by SGECs. The effect of TLR-3 or TLR-4 stimulation on autoantigen expression was evaluated by polyI:C or lipopolysaccharide (LPS) treatment, respectively, of SGEC lines (10 from SS patients, 12 from non-SS controls) or HeLa cells, followed by analysis of mRNA and protein expression. PolyI:C, but not LPS, resulted in a two-step induction of Ro52/TRIM21 mRNA expression by SGECs, a 12-fold increment at 6 h followed by a 2.5-fold increment at 24-48 h, whereas it induced a late two-fold up-regulation of Ro60/TROVE2 and La/SSB mRNAs at 48 h. Although protein expression levels were not affected significantly, the late up-regulation of Ro52/TRIM21 mRNA was accompanied by protein redistribution, from nucleolar-like pattern to multiple coarse dots spanning throughout the nucleus. These late phenomena were mediated significantly by interferon (IFN)-β production, as attested by cognate secretion and specific inhibition experiments and associated with IFN regulatory factor (IRF)3 degradation. TLR-3-signalling had similar effects on SGECs obtained from SS patients and controls, whereas it did not affect the expression of these autoantigens in HeLa cells. TLR-3 signalling regulates the expression of autoantigens by SGECs, implicating innate immunity pathways in their over-expression in inflamed tissues and possibly in their exposure to the immune system.

Lymphocytic infiltration leads to degradation of lacrimal gland extracellular matrix structures in NOD mice exhibiting a Sjögren's syndrome-like exocrinopathy

We previously reported that lacrimal glands (LGs) of male non-obese diabetic (NOD) mice, an established mouse model of autoimmune inflammatory LG disease that displays many features of human LGs in patients afflicted with Sjögren's syndrome (SjS), exhibit significant degradation of extracellular matrix (ECM) structures as well as increased expression of matrix metalloproteinases (MMPs). The purpose of the current study was to expand the spectrum of proteases identified, to clarify their probable origin as well as to identify the contribution of these changes to disease pathogenesis. We explored in depth the changes in ECM structures and ECM protease expression at the onset of disease (6 weeks) versus late stage disease (18 weeks) in male NOD mouse LGs, relative to LGs of age-matched male NODscid, a severely immunocompromised congenic strain, and healthy BALB/c mice. LG tissues were examined using routine histological, immunohistochemical, Western Blot and gene expression analyses novel multiphoton imaging technologies. We further characterized the profile of infiltrating immune cells under each condition using flow cytometry. Our results show that the initial infiltrating cells at 6 weeks of age are responsible for increased MMP and cathepsin H expression and therefore initiate the LG ECM degradation in NOD mice. More importantly, NODscid mice exhibited normal LG ECM structures, indicating the lymphocytes seen in the LGs of NOD mice are responsible for the degradation of the LG ECM. The disease-related remodeling of LG ECM structures may play a crucial role in altering the acinar signaling environment, disrupting the signaling scaffolds within the cells, which are required to mobilize the exocytotic trafficking machinery, ultimately leading to a loss of LG function in patients afflicted with SjS.

Pathophysiology of Sjögren's syndrome

The term Sjögren's syndrome refers to keratoconjunctivitis sicca and xerostomia due to lymphocytic infiltrates of lachrymal and salivary glands. The current used criteria for diagnosis of primary Sjögren's syndrome is the American-European consensus. Primary Sjögren's syndrome is an autoimmune disorder characterized by lymphocytic infiltrates and destruction of the salivary and lachrymal glands and systemic production of autoantibodies to the ribonucleoprotein particles SS-A/Ro and SS-B/La. The infiltrating cells (T- and B-cells, dendritic cells) interfere with glandular function at several points: destruction of glandular elements by cell-mediated mechanisms; secretion of cytokines that activate pathways bearing the signature of type 1 and 2 interferons; production of autoantibodies that interfere with muscarinic receptors; and secretion of metalloproteinases (MMPs) that interfere with the interaction of the glandular cell with its extracellular matrix, which is necessary for efficient glandular function. As the process progresses, the mucosal surfaces become sites of chronic inflammation and the start of a vicious circle. Despite extensive study of the underlying cause of Sjögren's syndrome, the pathogenesis remains obscure. In broad terms, pathogenesis is multifactorial; environmental factors are thought to trigger inflammation in individuals with a genetic predisposition to the disorder.

Integrin adhesion in regulation of lacrimal gland acinar cell secretion

The extracellular microenvironment regulates lacrimal gland acinar cell secretion. Culturing isolated rabbit lacrimal gland acinar cells on different extracellular matrix proteins revealed that laminin enhances carbachol-stimulated secretion to a greater extent than other extracellular matrix proteins investigated. Furthermore, immunofluorescence indicated that integrin subunits, potentially functioning as laminin receptors are present in acinar cells. Among these, the integrin alpha6 and beta1 subunit mRNA expression was also confirmed by RT-PCR and sequence analysis. Secretion assays, which measured beta-hexosaminidase activity released in the culture media, demonstrated that function-blocking integrin alpha6 and beta1 monoclonal antibodies (mAbs) induce a rapid, transient and dose-dependent secretory response in cultured cells. To determine the intracellular pathways by which integrin alpha6 and beta1 mAbs could induce secretion, selected second messenger molecules were inhibited. Although inhibitors of protein kinase C and IP(3)-induced Ca(2+) mobilization attenuated carbachol-stimulated secretion, no effect on integrin mAb-induced release was observed. In addition, protein tyrosine kinases do not appear to have a role in transducing signals arising from mAb interactions. Our data clearly demonstrate, though, that cell adhesion through integrins regulates secretion from lacrimal gland acinar cells. The fact that the integrin mAbs affect the cholinergic response differently and that the integrin beta1 mAb secretion, but not the alpha6, was attenuated by the phosphatase inhibitor, sodium orthovanadate, suggests that each subunit utilizes separate intracellular signaling pathways to induce exocytosis. The results also indicate that the secretory response triggered by the beta1 integrin mAb is generated through dephosphorylation events.

Molecular mechanisms of lacrimal acinar secretory vesicle exocytosis

The acinar epithelial cells of the lacrimal gland are responsible for the production, packaging and regulated exocytosis of tear proteins into ocular surface fluid. This review summarizes new findings on the mechanisms of exocytosis in these cells. Participating proteins are discussed within the context of different categories of trafficking effectors including targeting and specificity factors (rabs, SNAREs) and transport factors (microtubules, actin filaments and motor proteins). Recent information describing fundamental changes in basic exocytotic mechanisms in the NOD mouse, an animal model of Sjögren's syndrome, is presented.

Regulatory pathways in lacrimal gland epithelium

Tears are a complex fluid that continuously cover the exposed surface of the eye, namely the cornea and conjunctiva. Tears are secreted in response to the multitude of environmental stresses that can harm the ocular surface such as cold, mechanical stimulation, physical injury, noxious chemicals, as well as infections from various organisms. Tears also provide nutrients and remove waste from cells of the ocular surface. Because of the varied function of tears, tears are complex and are secreted by several different tissues. Tear secretion is under tight neural control allowing tears to respond rapidly to changing environmental conditions. The lacrimal gland is the main contributor to the aqueous portion of the tear film and the regulation of secretion from this gland has been well studied. Despite multiple redundencies in pathways to stimulate secretion from the lacrimal gland, defects can occur resulting in dry eye syndromes. These diseases can have deleterious effects on vision. In this review, we summarize the latest information regarding the regulatory pathways, which control secretion from the lacrimal gland, and their roles in the pathogenesis of dry eye syndromes.

cDNA and genomic cloning of lacritin, a novel secretion enhancing factor from the human lacrimal gland

Multiple extracellular factors are hypothesized to promote the differentiation of unstimulated and/or stimulated secretory pathways in exocrine secretory cells, but the identity of differentiation factors, particularly those organ-specific, remain largely unknown. Here, we report on the identification of a novel secreted glycoprotein, lacritin, that enhances exocrine secretion in overnight cultures of lacrimal acinar cells which otherwise display loss of secretory function. Lacritin mRNA and protein are highly expressed in human lacrimal gland, moderately in major and minor salivary glands and slightly in thyroid. No lacritin message or protein is detected elsewhere among more than 50 human tissues examined. Lacritin displays partial similarity to the glycosaminoglycan-binding region of brain-specific neuroglycan C (32 % identity over 102 amino acid residues) and to the possibly mucin-like amino globular region of fibulin-2 (30 % identity over 81 amino acid residues), and localizes primarily to secretory granules and secretory fluid. The lacritin gene consists of five exons, displays no alternative splicing and maps to 12q13. Recombinant lacritin augments unstimulated but not stimulated acinar cell secretion, promotes ductal cell proliferation, and stimulates signaling through tyrosine phosphorylation and release of calcium. It binds collagen IV, laminin-1, entactin/nidogen-1, fibronectin and vitronectin, but not collagen I, heparin or EGF. As an autocrine/paracrine enhancer of the lacrimal constitutive secretory pathway, ductal cell mitogen and stimulator of corneal epithelial cells, lacritin may play a key role in the function of the lacrimal gland-corneal axis.

Human genome search in celiac disease using gliadin cDNA as probe

Celiac disease is a wheat gliadin-promoted disorder that displays a complex genetic susceptibility associated with HLA-DQ2, and one or more unknown factor(s), possibly gliadin-like. The presence of mammalian proteins with partial gliadin similarity was suggested by transglutaminase-independent multi-tissue reactivity of gliadin-immunopurified antibodies from celiac patients. No non-plant sequence, however, was identified in gliadin peptide epitope searches of non-redundant and EST databanks via TBLASTN, BLASTP and FASTA, even at E values as high as 20. Therefore, an alpha-gliadin cDNA screen of human cDNA and genomic libraries was undertaken, an approach in keeping with positive human Northern and Southern analyses with the same probe. Four distinct cDNA clones were obtained, the most stringent of which (3.2 and 5.1 kb) were novel, and featured potential open reading frames with high gliadin domain II and domain IV homologies (BestFit quality scores >/=295 and 322, respectively, versus random value 126-127). Both were also homologous to ESTs. An additional 5' gliadin oligonucleotide screen identified the widely distributed cytoplasmic protein acyl coA hydrolase whose homology was restricted to the oligonucleotide probe (BestFit quality=215 versus 100 for random); and achaete-scute homologous protein, which displays particularly high gliadin domain II homology (BestFit quality 316 versus 111 for random). Genomic screening uncovered 16 positives, one of which was the ALR gene, whose similarity to three of gliadin's five domains (I, II and IV; BestFit quality 322-473 versus 121-154 for random) was remarkable. More extensive was novel genomic clone 2, with fragments hybridizing to cDNA probes approximating gliadin domains I, II+IV, V and the gliadin 5' untranslated region, and mapping by FISH to 19q13.11-13. 12. Two fragments were sequenced; one was exonic, as predicted by four different programs; and test oligonucleotides suggested widespread 4 and/or 2 kb mRNA expression, even at high stringency (t(m)-8.8 deg. C). Taken together, it is apparent that several genes with partial gliadin homology exist in the human genome. Many bear gliadin-like T-cell epitopes, are expressed in intestine and, like transglutaminase, are cytoplasmic. Glutamine to glutamic acid or other mutation within such epitopes followed by injury or infection-related release could explain enhanced disease susceptibility in affected families.

Quantitation of rat lacrimal secretion: a novel sandwich ELISA with high sensitivity

Modulation of lacrimal acinar cell tear secretion may involve multiple factors acting both in subtle and pronounced ways. Functional screens of recombinant protein products arising from gene array technologies, or protein fractions derived from lacrimal conditioned media or extracellular matrix, will require a highly sensitive assay capable of monitoring tear protein secretion by small replicate cultures. To improve significantly on current methods, a rat- and mouse-specific sandwich ELISA was developed. For this purpose, chickens and rabbits were immunized with serum-free secretion media from carbachol and VIP-stimulated rat lacrimal acinar cell cultures. Immune sera were characterized by ELISA, Western blotting and immunohistochemistry, and subsequently optimized for use in a sandwich ELISA. Both antisera detected a wide range of different rat tear proteins, and immunostained only the secretory granule-rich juxtalumenal region in sections of rat lacrimal gland. Chicken, but not rabbit, antiserum cross-reacted with rabbit and human tears. In sandwich ELISA, capture with purified chicken immunoglobulin fraction and detection with rabbit antiserum detected as little as 1 ng ml-1 tear protein in 10,000-fold diluted rat secretion medium--a level of sensitivity 8000 times greater than the rat tear peroxidase assay. Such specificity and sensitivity greatly reduce the quantity of media needed for assay, and makes feasible functional screens for scarce factors that may influence lacrimal secretory processes, and in turn possibly play a role in human lacrimal insufficiency syndromes.

Role of laminin-1, collagen IV, and an autocrine factor(s) in regulated secretion by lacrimal acinar cells

Adhesion to novel basement membrane component BM180 in the presence of laminin-1 promotes stimulus-secretion coupling in lacrimal acinar cells [G. W. Laurie, J. D. Glass, R. A. Ogle, C. M. Stone, J. R. Sluss, and L. Chen. Am. J. Physiol. 270 (Cell Physiol. 39): C1743-C1750, 1996]. The identity of the active laminin-1 site and the possibility that other promoters of coupling are present in the acinar cell microenvironment were probed by use of different substrates, media, neutralizing antibodies and cell numbers. Regulated peroxidase secretion was unaffected by basement membrane coat concentration and was detectable at reduced levels in serum-free medium. Anti-laminin-1 antibodies, particularly against sites in the beta1 and gamma1 chains, but not alpha1 chains, partially suppressed regulated secretion, as did an anti-collagen IV antibody. Without effect were RGD peptide and antibodies against entactin, the beta1-integrin subunit, and several growth factors. Increasing cell number in serum-free medium revealed an unknown, serum-maskable, secretion-enhancing activity with a remarkable specificity for regulated secretion. Stimulus-secretion coupling, therefore, appears to be modulated by several extracellular factors whose relative contributions remain to be determined.

Gene cloning of BM180, a lacrimal gland enriched basement membrane protein with a role in stimulated secretion

BM180 is a novel basement membrane component with a role in regulated tear secretion by lacrimal acinar cells. BM180 bears some sequence similarity to alpha-gliadin, a plant protein against which antibodies have been reported in patients with Sjögren's syndrome. A precedent for plantlike sequence in the mammalian genome is provided by selectins, which possess a plant lectinlike domain involved in inflammatory cell homing. Cloning the mouse and human BM180 gene will aid molecular investigation of lacrimal acinar cell-BM180 interactions and may lead to a new molecular understanding of mechanisms contributing to dry eye.

Primary rat lacrimal cells undergo acinar-like morphogenesis on reconstituted basement membrane and express secretory component under androgen stimulation

Single cells or small cell clusters, isolated from the rat lacrimal gland, were incubated on reconstituted basement membrane (matrigel) in a well-defined serum-free medium. During the first days of culture, cells reassociated and reorganized in structures resembling acini. These multicellular structures, maintained in culture for 2 weeks, consisted of well-polarized cuboidal cells surrounding a central lumen and exhibiting apically located microvilli. Myoepithelial cells were observed at the periphery of the acinar structures. Both in the native lacrimal and in the cultured aggregates, epithelial cells displayed strong immunoreactivity for cytokeratin 8, while myoepithelial cells were immunoreactive for vimentin and alpha-smooth muscle isoactin. These data indicate that the cultured aggregates closely mimic the in vivo architecture of lacrimal glands both by morphology and immunohistochemistry. We further demonstrated the presence of an intact androgen receptor and the ability of the cultured aggregates to respond to androgens with increased secretion of the secretory component. Comparable androgen responses were observed in lacrimal gland cultures of 5-week-old male and female rats. In conclusion, we report a morphologically and functionally differentiated culture system of primary rat lacrimal cells, in which androgen-regulated gene expression was observed. This culture model provides a unique experimental paradigm for studying the effects of hormones, cytokines, and growth factors on the morphogenesis, growth, and functional differentiation of lacrimal glands.