Presence of alpha-and beta-integrin subunits in rabbit lacrimal gland acinar cells cultured on a laminin-rich matrix

FGF Gradient Controls Boundary Position Between Proliferating and Differentiating Cells and Regulates Lacrimal Gland Growth Dynamics

Fibroblast growth factor (FGF) signaling plays an important role in controlling cell proliferation, survival, and cell movements during branching morphogenesis of many organs. In mammals branching morphogenesis is primarily regulated by members of the FGF7-subfamily (FGF7 and FGF10), which are expressed in the mesenchyme, and signal to the epithelial cells through the “b” isoform of fibroblast growth factor receptor-2 (FGFR2). Our previous work demonstrated that FGF7 and FGF10 form different gradients in the extracellular matrix (ECM) and induce distinct cellular responses and gene expression profiles in the lacrimal and submandibular glands. The last finding was the most surprising since both FGF7 and FGF10 bind signal most strongly through the same fibroblast growth factor receptor-2b isoform (FGFR2b). Here we revisit this question to gain an explanation of how the different FGFs regulate gene expression. For this purpose, we employed our ex vivo epithelial explant migration assay in which isolated epithelial explants are grown near the FGF loaded beads. We demonstrate that the graded distribution of FGF induces activation of ERK1/2 MAP kinases that define the position of the boundary between proliferating “bud” and differentiating “stalk” cells of growing lacrimal gland epithelium. Moreover, we showed that gene expression profiles of the epithelial explants exposed to distinct FGFs strictly depend on the ratio between “bud” and “stalk” area. Our data also suggests that differentiation of “stalk” and “bud” regions within the epithelial explants is necessary for directional and persistent epithelial migration. Gaining a better understanding of FGF functions is important for development of new approaches to enhance tissue regeneration.

Novel fiber-dependent entry mechanism for adenovirus serotype 5 in lacrimal acini

The established mechanism for infection of most cells with adenovirus serotype 5 (Ad5) involves fiber capsid protein binding to coxsackievirus-adenovirus receptor (CAR) at the cell surface, followed by penton base capsid protein binding to alpha(v) integrins, which triggers clathrin-mediated endocytosis of the virus. Here we determined the identity of the capsid proteins responsible for mediating Ad5 entry into the acinar epithelial cells of the lacrimal gland. Ad5 transduction of primary rabbit lacrimal acinar cells was inhibited by excess Ad5 fiber or knob (terminal region of the fiber) but not excess penton base. Investigation of the interactions of recombinant Ad5 penton base, fiber, and knob with lacrimal acini revealed that the penton base capsid protein remained surface associated, while the knob domain of the fiber capsid protein was rapidly internalized. Introduction of rabbit CAR-specific small interfering RNA (siRNA) into lacrimal acini under conditions that reduced intracellular CAR mRNA significantly inhibited Ad5 transduction, in contrast to a control (nonspecific) siRNA. Preincubation of Ad5 with excess heparin or pretreatment of acini with a heparinase cocktail each inhibited Ad5 transduction by a separate and apparently additive mechanism. Functional and imaging studies revealed that Ad5, fiber, and knob, but not penton base, stimulated macropinocytosis in acini and that inhibition of macropinocytosis significantly reduced Ad5 transduction of acini. However, inhibition of macropinocytosis did not reduce Ad5 uptake. We propose that internalization of Ad5 into lacrimal acini is through a novel fiber-dependent mechanism that includes CAR and heparan sulfate glycosaminoglycans and that the subsequent intracellular trafficking of Ad5 is enhanced by fiber-induced macropinocytosis.

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.

ICAM-1 expression predisposes ocular tissues to immune-based inflammation in dry eye patients and Sjögrens syndrome-like MRL/lpr mice

PURPOSE

We previously reported that immune-based inflammation occurs on the ocular surface of humans as well as canines with keratoconjunctivitis sicca (KCS). Intercellular adhesion molecule-1 (ICAM-1) was found to be upregulated on lymphocytes and/or vascular endothelial cells resulting in lymphocytic diapedesis to the lacrimal and conjunctival tissues. The purpose of the current study was to demonstrate the role of ICAM-1 in (1) resident epithelial cell response during ocular inflammation, (2) local and/or peripheral lymphocyte activation or accumulation in the ocular tissues, and (3) whether anti-ICAM-1 is effective to attenuate immune-mediated ocular inflammation.

METHODS

ICAM-1 levels in various ocular tissues of human with KCS and/or MRL/lpr mouse were evaluated by immunohistochemistry and in situ hybridization for protein and messenger RNA (mRNA) expression, respectively. Soluble ICAM-1 concentrations in MRL/lpr mouse plasma over the course of disease development were measured by ELISA. Cell proliferation within ocular tissues was assessed by bromodeoxyuridine (BrdU) incorporation and immunohistochemical detection. The level of T cell activation was determined by IL-2 receptor (CD25, a marker of T cell activation and proliferation) and CD69 (a marker of T cell activation) immunoreactivity using FACS analysis. To examine the effectiveness of anti-ICAM-1/LFA-1 in elimination of lacrimal gland inflammation, MRL/lpr mice were injected intraperitoneally (i.p.) with or without monoclonal antibodies against ICAM-1 and LFA-1 at three or eight weeks of age.

RESULTS

Increased endogenous ICAM-1 expression at the level of protein and mRNA was detected in the epithelial cells present in the conjunctival and accessory lacrimal tissues in dry eye patients. In MRL/lpr mice, ICAM-1 expression by lacrimal acinar epithelial cells and conjunctival epithelial cells were detected in addition to inflammatory infiltrates and vascular endothelial cells at 16 weeks of age. Soluble ICAM-1 levels were markedly increased concomitantly with disease progression over time as compared with the controls. No significant lymphocytic proliferation (a lack of BrdU and CD25 immunoreactivities) was detected within lacrimal glands of MRL/lpr mice at the disease onset. However, a population of the infiltrated T cells were CD69 positive, indicating the activation stage of a T cell subset. Treatment using monoclonal antibodies against murine ICAM-1 and LFA-1 resulted in a decrease in the number of inflammatory infiltrates in MRL/lpr mice.

CONCLUSIONS

Our findings suggest that ICAM-1 upregulation locally and systemically promote lymphocyte activation and migration to the ocular surface (OS). Ocular resident epithelium is an active component of ocular surface and is capable of interacting with invasive lymphocytes by ICAM-1 production in response to immune activation and inflammation. ICAM-1 synthesized by epithelial cells may serve as a signaling molecule for predisposition of ocular surface inflammation and facilitate potential antigen presentation by epithelial cells. Lymphocytic infiltrates in the lacrimal gland of the MRL/lpr mouse appeared to be the result of the accumulation, but not proliferation of circulating lymphocytes diapodesed from the vasculature that had migrated into the local ocular tissues. The potential use of anti-ICAM-1 therapy in treating immune-based inflammatory diseases such as dry eye deserves further investigation.

The role of the lacrimal functional unit in the pathophysiology of dry eye

The majority of dry eye symptoms are due to a chronic inflammation of the lacrimal functional unit resulting in a loss of tear film integrity and normal function. This leads to a reduction in the ability of the ocular surface to respond to environmental challenges. The underlying cause of tear film dysfunction is the alteration of tear aqueous, mucin, and lipid components. This may result from a systemic autoimmune disease or a local autoimmune event. A lack of systemic androgen support to the lacrimal gland has been shown to be a facilitative factor in the initiation of this type of pathophysiology. Tear secretion is controlled by the lacrimal functional unit consisting of the ocular surface (cornea, conjunctiva, accessory lacrimal glands, and meibomian glands), the main lacrimal gland and the interconnecting innervation. If any portion of this functional unit is compromised, lacrimal gland support to the ocular surface is impeded. Factors such as neurogenic inflammation and T cell involvement in the disease pathogenesis as well as newly developed animal models of ocular surface inflammation are discussed.