Fibroblast Growth Factor Receptor 2 (FGFR2) Is Required for Meibomian Gland Homeostasis in the Adult Mouse

Models for Meibomian gland dysfunction: In vivo and in vitro

Meibomian gland dysfunction (MGD) is a chronic abnormality of the Meibomian glands (MGs) that is recognized as the leading cause of evaporative dry eye worldwide. Despite its prevalence, however, the pathophysiology of MGD remains elusive, and effective disease management continues to be a challenge. In the past 50 years, different models have been developed to illustrate the pathophysiological nature of MGD and the underlying disease mechanisms. An understanding of these models is crucial if researchers are to select an appropriate model to address specific questions related to MGD and to develop new treatments. Here, we summarize the various models of MGD, discuss their applications and limitations, and provide perspectives for future studies in the field.

Eyes on the Lid: The Impact of Fibroblast Growth Factor Receptor Targeting Drug on Human Meibomian Gland

OBJECTIVE

To determine whether fibroblast growth factor receptor (FGFR)-targeting drug could impact human meibomian gland.

METHODS

We followed up with three patients who were using pemigatinib for 4 to 10 weeks. The patients were evaluated for their ocular surface disease index, best-corrected visual acuity, Schirmer test, cornea staining, meibum expressibility score, tear meniscus height, noninvasive tear film breakup time, and meibomian gland area. The distribution of the FGFR family, FGF7, and FGF10 were evaluated by immunofluorescence staining and Western blot in fresh tarsal tissues from deidentified patients who underwent lid plastic surgeries.

RESULTS

All patients developed apparent meibomian gland atrophy, shortening and narrowing of ducts, and significantly increased meibum expressibility and decreased noninvasive tear film breakup time within 5 to 8 weeks. Laboratory evaluations confirmed that human meibomian gland expresses abundant fibroblast growth factor receptors.

CONCLUSIONS

These findings indicate that meibomian gland is a target tissue of FGFR inhibitors, and patients who use these drugs may develop meibomian gland dysfunction.

Role of Fibroblast Growth Factor Receptor 2 (FGFR2) in Corneal Stromal Thinning

Purpose

To determine the role of fibroblast growth factor receptor 2 (FGFR2)-mediated signaling in keratocytes during corneal development, a keratocyte-specific FGFR2-knockout (named FGFR2cKO) mouse model was generated, and its phenotypic characteristics were determined.

Methods

A FGFR2cKO mouse model was generated by the following method: FGFR2 flox mice were crossed with the inducible keratocyte specific-Cre mice (Kera-rtTA/tet-O-Cre). Both male and female FGFR2cKO- and control mice (1 to 3-months-old) were analyzed for changes in corneal topography and pachymetry maps using the optical coherence tomography (OCT) method. The comparative TUNEL assay and immunohistochemical analyses were performed using corneas of FGFR2cKO and control mice to determine apoptotic cells, and expression of collagen-1 and fibronectin. Transmission electron microscopic analysis was conducted to determine collagen structures and their diameters in corneas of FGFR2cKO and control mice.

Results

OCT-analyses of corneas of FGFR2cKO mice (n = 24) showed localized central thinning and an increased corneal steepness compared to control mice (n = 23). FGFR2cKO mice further showed a decreased expression in collagen-1, decreased collagen diameters, acute corneal hydrops, an increased fibronectin expression, and an increased number of TUNEL-positive cells suggesting altered collagen structures and keratocytes' apoptosis in the corneas of FGFR2cKO mice compared to control mice.

Conclusions

The FGFR2cKO mice showed several corneal phenotypes (as described above in the results) that are also exhibited by the human keratoconus corneas. The results suggested that the FGFR2cKO mouse model serves to elucidate not only the yet unknown role of FGFR2-mediated signaling in corneal physiology but also serves as a model to determine molecular mechanism of human keratoconus development.

Meibomian gland stem/progenitor cells: The hunt for gland renewal

Meibomian glands (MGs) secrete lipid (meibum) onto the ocular surface to form the outermost layer of the tear film. Proper meibum secretion is essential for stabilizing the tear film, reducing aqueous tear evaporation, and maintaining the homeostasis of the ocular surface. Atrophy of MG as occurs with aging, leads to reduction of meibum secretion, loss of ocular surface homeostasis and evaporative dry eye disease (EDED). Since MGs are holocrine glands, secretion of meibum requires continuous self-renewal of lipid-secreting acinar meibocytes by stem/progenitor cells, whose proliferative potential is dramatically reduced with age leading to MG atrophy and an age-related meibomian gland dysfunction (ARMGD). Understanding the cellular and molecular mechanisms regulating meibocyte stem/progenitor cell maintenance and renewal may provide novel approaches to regenerating MG and treating EDED. Towards that end, recent label retaining cell and lineage-tracing experiments as well as knock-out transgenic mouse studies have begun to identify the location and identities of meibocyte progenitor cells and potential growth and transcription factors that may regulate meibocyte renewal. In addition, recent reports have shown that ARMGD may be reversed by novel therapeutics in mice. Herein, we discuss our current understanding of meibocyte stem/progenitor cells and the hunt for gland renewal.

Role of FGF10/FGFR2b Signaling in Homeostasis and Regeneration of Adult Lacrimal Gland and Corneal Epithelium Proliferation

Purpose

Fibroblast growth factor 10 (FGF10) is involved in eye, meibomian, and lacrimal gland (LG) development, but its function in adult eye structures remains unknown. This study aimed to characterize the role of FGF10 in homeostasis and regeneration of adult LG and corneal epithelium proliferation.

Methods

Quantitative reverse transcription PCR was used for analysis of FGF10 expression in both early postnatal and adult mouse LG, and RNA sequencing was used to analyze gene expression during LG inflammation. FGF10 was injected into the LG of two mouse models of Sjögren's syndrome and healthy controls. Flow cytometry, BrdU cell proliferation assay, immunostaining, and hematoxylin and eosin staining were used to evaluate the effects of FGF10 injection on inflammation and cell proliferation in vivo. Mouse and human epithelial cell cultures were treated with FGF10 in vitro, and cell viability was assessed using WST-8 and adenosine triphosphate (ATP) quantification assays.

Results

The level of Fgf10 mRNA expression was lower in adult LG compared to early postnatal LG and was downregulated in chronic inflammation. FGF10 injection into diseased LGs significantly increased cell proliferation and decreased the number of B cells. Mouse and human corneal epithelial cell cultures treated with FGF10 showed significantly higher cell viability and greater cell proliferation.

Conclusions

FGF10 appears to promote regeneration in damaged adult LGs. These findings have therapeutic potential for developing new treatments for dry eye disease targeting the ability of the cornea and LG to regenerate.

Spontaneous acinar and ductal regrowth after meibomian gland atrophy induced by deletion of FGFR2 in a mouse model

PURPOSE

We have demonstrated that deletion of fibroblast growth factor receptor 2 gene (Fgfr2) leads to Meibomian gland (MG) atrophy in an inducible conditional knockout mouse model, referred as Fgfr2^CKO. Herein, we investigated whether MG spontaneously recovers after atrophy in this model.

METHODS

Two months old Fgfr2^CKO mice were injected peritoneally once or twice of doxycycline (Dox) at 80 μg/gm of body weight to induce MG atrophy of various severities via Fgfr2 deletion. Recovery of acinar and ductal tissues was monitored by meibography, lipid staining and immunofluorescence against keratin-6a in MG whole-mount. Biomarkers for acinar and ductal differentiation and proliferation were also examined by immunostaining.

RESULTS

Single Dox injection in Fgfr2^CKO mice caused severe acinar and moderate ductal atrophy. Severe ductal shortening or loss occurred after second Dox injection, presumably related to the reported slower cycling of the ductal epithelia. Spontaneous acinar regrowth after atrophy was observed over a period of 60 days in both injection regimens. However, less robust acinar recovery was associated with more disrupted ductal structures in twice injected Fgfr2^CKO mice.

CONCLUSIONS

Our current findings further substantiate the role of FGFR2 in MG homeostasis, and suggest that FGFR2-signaling may provide a potential strategy for regenerating acini from age-related MG dysfunction in humans. Our data demonstrated that spontaneous MG recovery depends on the extent of ductal atrophy, suggesting that ductal epithelia may provide the progenitor cells for acinar regeneration. Nonetheless, the role of ductal tissue as the source of acinar progenitors awaits further investigation.

Induction of meibocyte differentiation by three-dimensional, matrigel culture of immortalized human meibomian gland epithelial cells to form acinar organoids

PURPOSE

Recent studies have shown that two-dimensional (2D) culture of primary rabbit and immortalized human meibomian gland epithelial cells (iHMGEC) do not recapitulate normal meibocyte differentiation and fail to express critical enzymes necessary for synthesis of meibum lipids. The purpose of this study was to test the hypothesis that 3D-spheroid culture of iHMGEC can facilitate meibocyte differentiation and induce the expression of acyl-CoA wax-alcohol acyltransferase 2 (AWAT2), shown to be required for synthesis of meibum wax esters.

METHODS

iHMGEC were suspended in matrigel/basement membrane matrix and grown in proliferation media to form distinct cell clusters or spheroids. Cells were then treated with serum-free, differentiation media (advanced DMEM/F12) with and without FGF10 and synthetic agonists for the nuclear lipid receptor, peroxisome proliferator activator receptor gamma (PPARγ). Cells were then evaluated for differentiation markers using western blotting, immunocytochemistry (ICC) and real-time PCR. Control cells were grown in standard 2D culture systems.

RESULTS

Under proliferative conditions, 3D culture induced the formation of KRT5+ spheroids that contained a Ki67+/P63+ undifferentiated, basal cell population. When spheroids were switched to differentiation media containing PPARγ agonists, two different organoid populations were detected, a KRT6low population that was AWAT2+/PPARγ+ and a KRT6high population that was AWAT2-/PPARγ-, suggesting that iHMGEC exhibit a dual differentiation potential toward either a ductal or meibocyte organoid phenotype.

CONCLUSION

The 3D culturing of iHMGEC can induce the formation of both meibocyte and ductal organoids and may thus serve as a better in vitro model system for studying the regulatory mechanisms controlling meibomian gland function.

Expression of extracellular matrix components in the meibomian gland

Purpose

Extracellular matrix (ECM) is a key component of the stem cell local microenvironment. Our study aims to explore the periglandular distribution of major components of ECM in the Meibomian gland (MG).

Methods

Human eyelids and mouse eyelids were collected and processed for immunofluorescence staining.

Results

Human MG tissues stained positive for collagen IV α1, collagen IV α2, collagen IV α5, and collagen IV α6 around the acini and duct, but negative for collagen IV α3 and collagen IV α4. The mouse MG were stained positive for the same collagen IV subunits as early as postnatal day 15. Laminin α2, laminin β1 and perlecan stained the regions surrounding the acini and the acinar/ductal junction in the human MG, but not the region around the duct. Tenascin-C was found specifically located at the junctions between the acini and the central ducts. Neither agrin nor endostatin was found in the human MG tissues.

Conclusion

The ECM expresses specific components in different regions around the MG, which may play a role in MG stem cell regulation, renewal, and regeneration.

Application of Animal Models in Interpreting Dry Eye Disease

Different pathophysiologic mechanisms are involved in the initiation, development, and outcome of dry eye disease (DED). Animal models have proven valuable and efficient in establishing ocular surface microenvironments that mimic humans, thus enabling better understanding of the pathogenesis. Several dry eye animal models, including lacrimal secretion insufficiency, evaporation, neuronal dysfunction, and environmental stress models, are related to different etiological factors. Other models may be categorized as having a multifactorial DED. In addition, there are variations in the methodological classification, including surgical lacrimal gland removal, drug-induced models, irradiation impairment, autoimmune antibody-induced models, and transgenic animals. The aforementioned models may manifest varying degrees of severity or specific pathophysiological mechanisms that contribute to the complexity of DED. This review aimed to summarize various dry eye animal models and evaluate their respective characteristics to improve our understanding of the underlying mechanism and identify therapeutic prospects for clinical purposes.

Periplocin ameliorates mouse age-related meibomian gland dysfunction through up-regulation of Na/K-ATPase via SRC pathway

Age-related meibomian gland dysfunction (MGD) is the main cause of evaporative dry eye disease in an aging population. Decreased meibocyte cell renewal and lipid synthesis are associated with age-related MGD. Here, we found an obvious decline of Ki67, ΔNp63, and Na+/K+ ATPase expression in aged meibomian glands. Potential Na+/K+ ATPase agonist periplocin, a naturally occurring compound extracted from the traditional herbal medicine cortex periplocae, could promote the proliferation and stem cell activity of meibocyte cells in vitro. Moreover, we observed that periplocin treatment effectively increased the expression of Na+ /K+ ATPase, accompanied with the enhanced expression of Ki67 and ΔNp63 in aged meibomian glands, indicating that periplocin may accelerate meibocyte cell renewal in aged mice. LipidTox staining showed increased lipid accumulation after periplocin treatment in cultured meibomian gland cells and aged meibomian glands. Furthermore, we demonstrated that the SRC pathway was inhibited in aged meibomian glands; however, it was activated by periplocin. Accordingly, the inhibition of the SRC signaling pathway by saracatinib blocked periplocin-induced proliferation and lipid accumulation in meibomian gland cells. In sum, we suggest periplocin-ameliorated meibocyte cell renewal and lipid synthesis in aged meibomian glands via the SRC pathway, which could be a promising candidate for age-related MGD.

Jack of all trades, master of each: the diversity of fibroblast growth factor signalling in eye development

A central question in development biology is how a limited set of signalling pathways can instruct unlimited diversity of multicellular organisms. In this review, we use three ocular tissues as models of increasing complexity to present the astounding versatility of fibroblast growth factor (FGF) signalling. In the lacrimal gland, we highlight the specificity of FGF signalling in a one-dimensional model of budding morphogenesis. In the lens, we showcase the dynamics of FGF signalling in altering functional outcomes in a two-dimensional space. In the retina, we present the prolific utilization of FGF signalling from three-dimensional development to homeostasis. These examples not only shed light on the cellular basis for the perfection and complexity of ocular development, but also serve as paradigms for the diversity of FGF signalling.

Meibomian Gland Dysfunction: What Have Animal Models Taught Us?

Studies have estimated that currently 344 million people worldwide and 16.4 million adults in the US have some form of dry eye disease (DED). It is believed that approximately 70% of DED cases are due to some form of evaporative dry eye, for which Meibomian gland dysfunction (MGD) is the major cause. Unfortunately, currently there is no effective treatment for MGD, and solely palliative care is available. Given the importance of MGD in DED, there has been a growing interest in studying Meibomian gland development, homeostasis and pathology, and, also, in developing therapies for treating and/or preventing MGD. For such, animal models have shown to be a vital tool. Much of what is known today about the Meibomian gland and MGD was learnt from these important animal models. In particular, canine and rabbit models have been essential for studying the physiopathology and progression of DED, and the mouse model, which includes different knockout strains, has enabled the identification of specific pathways potentially involved in MGD. Herein, we provide a bibliographic review on the various animal models that have been used to study Meibomian gland development, Meibomian gland homeostasis and MGD, primarily focusing on publications between 2000 and 2020.

The Effect of Androgens on Proinflammatory Cytokine Secretion from Human Ocular Surface Epithelial Cells

Purpose: The purpose of this study is to explore the effects of dihydrotestosterone (DHT) on lipopolysaccharide (LPS)-induced proinflammatory cytokine release in human ocular surface epithelial cells exposed to LPS and LPS-binding protein (LBP).Methods: Immortalized human corneal, conjunctival, and meibomian gland epithelial cells were cultured in keratinocyte-free medium. After confluency, they were exposed to a stratification medium Dulbecco's modified Eagle medium (DMEM)/F12 in the presence of fetal bovine serum and were exposed to vehicle, LPS + LBP, or DHT. Culture media were processed for multiplex-bead analysis of specific proinflammatory cytokines including interferon (IFN)-γ, tumor necrosis factor (TNF)-α, interleukin (IL)-2, IL-4, IL-8, IL-6, IL-10, IL-1β, vascular endothelial growth factor (VEGF)-A. Cytokine concentrations were compared by analysis of variance with Tukey post hoc testing. p < 0.05 was considered statistically significant.Results: The results are LPS + LBP-induced the secretion of IFN-γ, IL-6, IL-10, IL-1β, VEGF-A cytokines in corneal epithelial cells; TNF-α, IL-2, IL-8, IL-6, IL-1β, VEGF-A cytokines in conjunctival epithelial cells; and IL-8, IL-6, IL-1β, VEGF-A cytokines in meibomian gland epithelial cells. When these LPS + LBP-stimulated cells were exposed to DHT for 2 days, it was found that DHT suppressed the secretion of IL-6, IL-10, IL-1β, VEGF-A cytokines in corneal epithelial cells; TNF-α, IL-6, IL-1β, VEGF-A cytokines in conjunctival epithelial cells; and IL-6, IL-1β, VEGF-A cytokines in meibomian gland epithelial cells.Conclusion: LPS + LBP is shown to induce the secretion of certain proinflammatory cytokines from ocular surface and adnexal epithelial cells. DHT showed anti-inflammatory activity by suppressing some of those cytokines in these cell lines.

Histopathology and selective biomarker expression in human meibomian glands

Background/aims

Meibomian gland dysfunction (MGD) is the most common form of evaporative dry eye disease, but its pathogenesis is poorly understood. This study examined the histopathological features of meibomian gland (MG) tissue from cadaver donors to identify potential pathogenic processes that underlie MGD in humans.

Methods

Histological analyses was performed on the MGs in the tarsal plates dissected from four cadaver donors, two young and two old adults, including a 36-year-old female (36F) and three males aged 30, 63 and 64 years (30M, 63M and 64M).

Results

The MGs of 36F displayed normal anatomy and structure, whereas the MGs of 30M showed severe ductal obstruction with mild distortion. The obstruction was caused by increased cytokeratin levels in association with hyperproliferation, but not hyperkeratinisation. In two older males, moderate to severe MG atrophy was noted. Cell proliferation was significantly reduced in the MG acini of the two older donors as measured by Ki67 labelling index (6.0%±3.4% and 7.9%±2.8% in 63M and 64M, respectively) when compared with that of the two younger donors (23.2%±5.5% and 16.9%±4.8% in 30M and 36F, respectively) (p<0.001). The expression patterns of meibocyte differentiation biomarkers were similar in the older and younger donors.

Conclusion

Our histopathological study, based on a small sample size, suggests potentially distinct pathogenic mechanisms in MGD. In the young male adult, hyperproliferation and aberrant differentiation of the central ductal epithelia may lead to the obstruction by overproduced cytokeratins. In contrast, in older adults, decreased cell proliferation in acinar basal epithelia could be a contributing factor leading to MG glandular atrophy.

Ocular mucins in dry eye disease

Dry eye disease is a common and multifactorial disease with a high prevalence worldwide. Water loss, reduced expression of glycocalyx mucins, and loss of goblet cells secreting gel-forming mucins are hallmarks of dry eye disease. Mucins are large and complex heavily glycosylated proteins. Their organization in the tear film remains unclear, but they play a key role to protect and maintain integrity of the ocular surface. Mice have been extremely valuable mammalian models with which to study ocular physiology and disease, and to evaluate eye therapies. Genetically modified mice and spontaneously occurring mutants with eye defects have proven to be powerful tools for the pharmaceutical industry, clinicians, and basic researchers investigating dry eye disease. However, ocular mucins remain relatively under-studied and inadequately characterized. This review aims to summarize current knowledge about mucin production at the ocular surface in healthy individuals and in dry eye disease, and to compile an overview of mouse models available for the study of mucins in dry eye disease.