Biomarkers for Progenitor and Differentiated Epithelial Cells in the Human Meibomian Gland

The effect of airborne particulate matter 2.5 (PM2.5) on meibomian gland

Exposure to particulate matters in air pollution of 2.5 μm or less (PM2.5) was associated with loss of meibomian glands. The aim of this study was to verify that PM2.5 could directly impact meibomian gland epithelial cells and damage their function. To investigate the impact of PM2.5 on meibomian gland, immortalized human meibomian gland epithelial cells were treated with various concentrations of PM2.5in vitro. Meibomian gland cell microstructure, cell viability, expression of proliferating cell nuclear antigen and IL-1β, and intracellular accumulation of acidic vesicles were measured by transmission electron microscopy, cell counting, Western blot and LysoTracker staining, respectively. To further study the effect of PM2.5in vivo, male C57BL/6J mice were treated with 5 mg/ml PM2.5 or vehicle for 3 months. Corneal fluorescein staining and ocular examinations were done before and after the treatment. Eyelids tissues were processed for morphological studies, immunostaining and Oil Red O staining. Our data suggest that exposure to PM2.5 caused significant meibomian gland dropout, clogged gland orifice and increased corneal fluorescein staining that were consistent with the clinical presentations of meibomian gland dysfunction. Prominent changes in the morphology and ultrastructure of meibomian glands was observed with PM2.5 treatment. PM2.5 promoted ductal keratinization, inhibited cell proliferation, induced cell apoptosis and increased Interleukin-1β production in meibomian gland epithelial cells. This study may explain the association between PM2.5 exposure and meibomian gland dropout observed in clinic. PM2.5 resuspension instillation could be used to induce a meibomian gland dysfunction animal model.

Adult stem cells in the eye: Identification, characterisation, and therapeutic application in ocular regeneration - A review

Adult stem cells, present in various parts of the human body, are undifferentiated cells that can proliferate and differentiate to replace dying cells within tissues. Stem cells have specifically been identified in the cornea, trabecular meshwork, crystalline lens, iris, ciliary body, retina, choroid, sclera, conjunctiva, eyelid, lacrimal gland, and orbital fat. The identification of ocular stem cells broadens the potential therapeutic strategies for untreatable eye diseases. Currently, stem cell transplantation for corneal and conjunctival diseases remains the most common stem cell-based therapy in ocular clinical management. Lens epithelial stem cells have been applied in the treatment of paediatric cataracts. Several early-phase clinical trials for corneal and retinal regeneration using ocular stem cells are also underway. Extensive preclinical studies using ocular stem cells have been conducted, showing encouraging outcomes. Ocular stem cells currently demonstrate great promise in potential treatments of eye diseases. In this review, we focus on the identification, characterisation, and therapeutic application of adult stem cells in the eye.

Meibomian Gland Probing Stimulates a Proliferative Epithelial Response Resulting in Duct Regeneration

Purpose

To demonstrate that the meibomian gland ductal basement membrane and basal epithelial cell layer are in continuity with and may derive from lid margin orifice-associated rete ridge epithelial/basement membrane structures (OARREBS) and to characterize changes in the distal duct microanatomy after meibomian gland probing (MGP) using in vivo confocal microscopy (IVCM).

Patients and Methods

Pre/post-MGP IVCM examinations were performed on upper lids. Thirty-six identical glands from 20 lids of 16 patients (49.24 ±17.11 y/o with 13:3 F:M) were identified, analyzed, and compared to control cases. Statistical analyses were performed using ImageJ software and IBM SPSS version 27. All MGPs were performed within 12 weeks of the initial examination. Post-MGP follow-up exams occurred at 5.03 ±4.48 months.

Results

Post-MGP images showed more superficially organized OARREBS with accelerated and more superficial basement membrane formation, and an average increase of 32.2%, 25.4%, 32.04%, 77.7%, and 81.3% in duct wall epithelial cell layers (DWECL) (p < 0.001, compared to control (CTC) p < 0.001), distal duct wall thickness (DWT) (p < 0.001, CTC p < 0.001), proximal DWT (p < 0.001, CTC p < 0.001), distal lumen area (p < 0.001, CTC p = 0.037), and proximal lumen area (p < 0.001, CTC p = 0.007), respectively. The increase in the distal DWT and lumen area correlated with the months of follow-up (p = 0.004 and p = 0.010, respectively). Immediate post-MGP imaging revealed the probe track confined to the ductal epithelial compartment.

Conclusion

MGP appears to stimulate a proliferative epithelial response characterized by an accelerated more superficial formation of ductal basement membrane with increased DWECL as well as DWT and lumen area at two separate duct foci. These findings suggest activation of lid margin meibomian gland precursor cells and confirm that MGP stimulates an epithelial regenerative phenomenon, not a fibrotic one.

Urban Particulate Matter Triggers Meibomian Gland Dysfunction

Purpose

The meibomian gland (MG), as the largest modified sebaceous gland, is potentially damaged by urban particulate matter (UPM) based on epidemiological evidence, but the specific experimental mechanisms remain unknown. This study investigated the effects of UPM on MG dysfunction (MGD) in rodent models.

Methods

Female C57BL/6J mice received eye drops containing UPM suspension or PBS for 14 days. The proliferative capacity and progenitor of MG were evaluated by immunofluorescence. Cell apoptosis was confirmed by TUNEL assay, along with the analysis of caspase family expression. Lipid accumulation was visualized by Oil Red O staining and LipidTox staining. Ductal hyperkeratinization, neutrophil infiltration, and pyroptosis activation were detected through immunostaining. The relative gene expression and signaling pathway activation were determined by Western blot analysis.

Results

Administration of UPM caused MGD-like clinical signs, manifested as distinct corneal epithelial erosion, increased MG orifice occlusion, and glandular dropout. UPM exposure significantly induced progenitor loss, cellular apoptosis, and lipogenic disorder in MG, by reducing P63/Lrig1 expression and increasing cleaved caspase-8, -9, and -3 and meibum lipogenic protein (HMGCR/SREBP-1) expression. UPM-treated mice exhibited ductal hyperkeratinization and neutrophil recruitment. Simultaneously, pyroptosis was motivated, as indicated by the heightened expression of NLRP3 and the cleavage of caspase-1 and -4 and gasdermin D, as well as the increase in IL-1β and IL-18 downstream. The underlying pathological mechanisms of UPM involve the phosphorylation of mitogen-activated protein kinase and nuclear factor-κB.

Conclusions

These results provided direct evidence for the toxicity of UPM in MG. UPM-induced activation of pyroptosis and mitogen-activated protein kinase/nuclear factor-κB signaling pathway might account for the inflammatory MGD.

Isolation and Culture of Human Meibomian Gland Ductal Cells

Purpose

Hyperkeratinization of meibomian gland (MG) ducts is currently recognized as the primary pathologic mechanism of meibomian gland dysfunction (MGD). This research figured out a method to isolate the MG ducts and established a novel system to culture the human meibomian gland ductal cells (HMGDCs) for investigating the process of MGD.

Methods

The MG ducts were obtained from the eyelids of recently deceased donors and subjected to enzymatic digestion. The acini were then removed to isolate independent ducts. These MG ducts were subsequently cultivated on Matrigel-coated wells and covered with a glass plate to obtain HMGDCs. The HMGDCs were further cultivated until passage 2, and when they reached 60% confluence, they were treated with IL-1β and rosiglitazone for a duration of 48 hours. Immunofluorescence staining and Western blot techniques were employed to identify ductal cells and analyze the effects of IL-1β on HMGDCs in an in vitro setting.

Results

Ophthalmic micro-forceps and insulin needles can be employed for the purpose of isolating ducts. Within this particular culture system, the rapid expansion of HMGDCs occurred in close proximity to the duct tissue. MG ducts specifically expressed keratin 6 (Krt6) and hardly synthesized lipids. Furthermore, the expression of Krt6 was significantly higher (P < 0.0001) in HMGDCs compared to human meibomian gland cells. Upon treatment with IL-1β, HMGDCs exhibited an overexpression of keratin 1, which was effectively blocked by the administration of rosiglitazone.

Conclusions

The present study successfully isolated human MG ducts and cultured HMGDCs, providing a valuable in vitro model for investigating the mechanism of MGD. Additionally, the potential therapeutic efficacy of rosiglitazone in treating hyperkeratinization of ducts in patients with MGD was identified.

Age related changes in hyaluronan expression leads to Meibomian gland dysfunction

The prevalence of dry eye disease (DED) ranges from ∼5 to 50 % and its associated symptoms decrease productivity and reduce the quality of life. Approximately 85 % of all DED cases are caused by Meibomian gland dysfunction (MGD). As humans and mice age, their Meibomian glands (MGs) undergo age-related changes resulting in age related-MGD (ARMGD). The precise cause of ARMGD remains elusive, which makes developing therapies extremely challenging. We previously demonstrated that a hyaluronan (HA)-rich matrix exists surrounding the MG, regulating MG morphogenesis and homeostasis. Herein, we investigated whether changes to the HA matrix in the MG throughout life contributes towards ARMGD, and whether altering this HA matrix can prevent ARMGD. For such, HA synthase (Has) knockout mice were aged and compared to age matched wild type (wt) mice. MG morphology, lipid production, PPARγ expression, basal cell proliferation, stem cells, presence of atrophic glands and MG dropout were analyzed at 8 weeks, 6 months, 1 year and 2 years of age and correlated with the composition of the HA matrix. We found that as mice age, there is a loss of HA expression in and surrounding the MGs of wt mice, while, in contrast, Has1-/-Has3-/- mice present a significant increase in HA expression through Has2 upregulation. At 1 year, Has1-/-Has3-/- mice present significantly enlarged MGs, compared to age-matched wt mice and compared to all adult mice. Thus, Has1-/-Has3-/- mice continue to develop new glandular tissue as they age, instead of suffering MG atrophy. At 2 years, Has1-/-Has3-/- mice continue to present significantly larger MGs compared to age-matched wt mice. Has1-/-Has3-/- mice present increased lipid production, increased PPARγ expression and an increase in the number of proliferating cells when compared to wt mice at all-time points analyzed. Taken together, our data shows that a loss of the HA matrix surrounding the MG as mice age contributes towards ARMGD, and increasing Has2 expression, and consequently HA levels, prevents ARMGD in mice.

Recent advances in age-related meibomian gland dysfunction (ARMGD)

Meibomian glands (MGs), located within the tarsal plate of the eyelid, secrete meibum which is the lipid-rich secretion necessary for stabilizing the tear film and preventing tear evaporation. Changes in the quality and quantity of meibum produced causes MG dysfunction (MGD), the leading cause of evaporative dry eye disease (EDED). MGD is an underdiagnosed disease and it is estimated that, in the US, approximately 70 % of the population over 60 have MGD. Three forms of MGD occur based on their meibum secretion: hyposecretory, obstructive, and hypersecretory MGD. The pathophysiology of MGD remains poorly understood, however aging is the primary risk factor. With age, MGs undergo various age-related changes, including decreased acinar basal cell proliferation, hyperkeratinization, MG atrophy, and eventual MG drop-out, leading to age-related MGD (ARMGD). Additionally, studies have suggested that MGs can suffer inflammatory cell infiltration and changes innervation patterns with aging, which could also contribute towards ARMGD. This review focuses on how the aging process affects the MG, and more importantly, how age-related changes to the MG can lead to MG atrophy and MG drop-out, ultimately leading to ARMGD. This review also highlights the most recent developments in potential therapeutic interventions for ARMGD.

Culture of primary human meibomian gland cells from surgically excised eyelid tissue

Meibomian gland dysfunction is one of the most common ocular diseases, with therapeutic treatment being primarily palliative due to our incomplete understanding of meibomian gland (MG) pathophysiology. To progress in vitro studies of human MG, this study describes a comprehensive protocol, with detailed troubleshooting, for the successful isolation, cultivation and cryopreservation of primary MG cells using biopsy-size segments of human eyelid tissue that would otherwise be discarded during surgery. MG acini were isolated and used to establish and propagate lipid-producing primary human MG cells. The primary cell viability during culture procedure was maintained through the application of Rho-associated coiled-coil containing protein kinase inhibitor (Y-27632, 10 μM) and collagen I from rat tails. Transcriptomic analysis of differentiated primary human MG cells confirmed cell origin and revealed high-level expression of many lipogenesis-related genes such as stearoyl-CoA desaturase (SCD), ELOVL Fatty Acid Elongase 1 (ELOVL1) and fatty acid synthase (FASN). Primary tarsal plate fibroblasts were also successfully isolated, cultured and cryopreserved. Established primary human MG cells and tarsal plate fibroblasts presented in this study have potential for applications in 3D models and bioengineered tissue that facilitate research in understanding of MG biology and pathophysiology.

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.

Meibomian gland development: Where, when and how?

The Meibomian gland (MG) is an indispensable adnexal structure of eye that produces meibum, an important defensive component for maintaining ocular homeostasis. Normal development and maintenance of the MGs is required for ocular health since atrophic MGs and disturbances in composition and/or secretion of meibum result in major ocular pathologies, collectively termed as Meibomian gland dysfunction (MGD). Currently available therapies for MGD merely provide symptomatic relief and do not treat the underlying deficiency of the MGs. Hence, a thorough understanding of the timeline of MG development, maturation and aging is required for regenerative purposes along with signaling molecules & pathways controlling proper differentiation of MG lineage in mammalian eye. Understanding the factors that contribute to the development of MGs, developmental abnormalities of MGs, and changes in the quality & quantity of meibum with developing phases of MGs are essential for developing potential treatments for MGD. In this review, we compiled a timeline of events and the factors involved in the structural and functional development of MGs and the associated developmental defects of MGs during development, maturation and aging.

Transitory alkali exposure on meibomian gland orifices induces meibomian gland dysfunction

PURPOSE

To determine pathological changes of meibomian glands (MGs) after transient exposure of the rat eyelid margin to alkali solution.

METHODS

Filter paper infiltrated with 1 N sodium hydroxide solution was applied to the eyelid margin of Sprague-Dawley rats for 30 s under general anesthesia, without touching the conjunctiva, after which the ocular surface and eyelid margin were examined by slit-lamp microscopy. In vivo confocal microscopy and stereomicroscopy were subsequently applied to observe MG morphology on day 5, day 10 and day 30 post alkali injury. Eyelid cross-sections were processed for H&E staining, Oil red O staining and immunofluorescent staining.

RESULTS

After alkali injury, there was marked plugging of MG orifices, telangiectasia and hypertrophy of the eyelid margin, while corneal epithelium was intact at post-injury days 5 and 10. However, 30 days after alkali injury, mild corneal epithelial damage was observed. Degeneration of MG acini was observed at days 5 and became aggravated at days 10 and 30, along with MG duct dilation and acini loss. Oil red O staining showed lipid accumulation in the dilated duct. Inflammatory cell infiltration and the presence of apoptotic cells was seen in the MG loci 5 days post injury, but diminished at days 10 and 30. Cytokeratin 10 expression was increased in dilated duct, while cytokeratin 14, PPAR-γ, Ki67 and LRIG1 expression were decreased in the acini of injured loci.

CONCLUSIONS

Transitory alkali exposure of the rat eyelid margin obstructs the MG orifice and induces pathological changes of MG dysfunction.

Ductal Hyperkeratinization and Acinar Renewal Abnormality: New Concepts on Pathogenesis of Meibomian Gland Dysfunction

Meibomian gland dysfunction (MGD) is a functional and morphological disorder of the meibomian glands which results in qualitative or quantitative alteration in meibum secretion and is the major cause of evaporative dry eye (EDE). EDE is often characterized by tear film instability, increased evaporation, hyperosmolarity, inflammation, and ocular surface disorder. The precise pathogenesis of MGD remains elusive. It has been widely considered that MGD develops as a result of ductal epithelial hyperkeratinization, which obstructs the meibomian orifice, halts meibum secretion, and causes secondary acinar atrophy and gland dropout. Abnormal self-renewal and differentiation of the acinar cells also play a significant role in MGD. This review summarizes the latest research findings regarding the possible pathogenesis of MGD and provides further treatment strategies for MGD-EDE patients.

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.

The Eyelid Meibomian Gland Deficiency in Fucosyltransferase 1 Knockout Mice

To investigate the effect of fucosyltransferase (FUT) 1-mediated fucosylation on meibomian glands (MG), we first confirmed that FUT1 and its fucosylated products were expressed in the eyelid, conjunctiva and skin in wild-type (WT) mice, whereas their mRNA and protein levels were downregulated in Fut1 knock-out (KO) mice. We then evaluated age-dependent changes in the total and acinar areas of MG, meibocyte differentiation, lipid synthesis, and eyelid inflammation and oxidative stress in Fut1 KO and WT mice. Results show that both the total and acinar areas of MG were smaller in Fut1 KO mice than in WT mice in all evaluated age groups. Meibocyte differentiation, lipid-producing capacities and the enzyme levels responsible for lipid synthesis were reduced in Fut1 KO mice, compared to WT controls. The levels of pro-inflammatory cytokines and oxidative-stress-related markers were elevated in the eyelids and MG of FUT1 KO mice. These findings demonstrate the physiologic function of FUT1-mediated fucosylation in MG development and function, and indicate its potential role in ocular surface homeostasis.

Candidate Molecular Compounds as Potential Indicators for Meibomian Gland Dysfunction

Meibomian gland dysfunction (MGD) is the leading cause of dry eye disease throughout the world. Studies have shown that several molecules in meibum, including but not limited to interleukins, amino acids, cadherins, eicosanoids, carbohydrates, and proteins, are altered in meibomian gland dysfunction compared with healthy normal controls. Some of these molecules such as antileukoproteinase, phospholipase A2, and lactoperoxidase also show differences in concentrations in tears between meibomian gland dysfunction and dry eye disease, further boosting hopes as candidate biomarkers. MGD is a complex condition, making it difficult to distinguish patients using single biomarkers. Therefore, multiple biomarkers forming a multiplex panel may be required. This review aims to describe molecules comprising lipids, proteins, and carbohydrates with the potential of serving various capacities as monitoring, predictive, diagnostic, and risk biomarkers for meibomian gland dysfunction.

Intracrine activity involving NAD-dependent circadian steroidogenic activity governs age-associated meibomian gland dysfunction

Canonically, hormones are produced in the endocrine organs and delivered to target tissues. However, for steroids, the concept of tissue intracrinology, whereby hormones are produced in the tissues where they exert their effect without release into circulation, has been proposed, but its role in physiology/disease remains unclear. The meibomian glands in the eyelids produce oil to prevent tear evaporation, which reduces with aging. Here, we demonstrate that (re)activation of local intracrine activity through nicotinamide adenine dinucleotide (NAD⁺)-dependent circadian 3β-hydroxyl-steroid dehydrogenase (3β-HSD) activity ameliorates age-associated meibomian gland dysfunction and accompanying evaporative dry eye disease. Genetic ablation of 3β-HSD nullified local steroidogenesis and led to atrophy of the meibomian gland. Conversely, reactivation of 3β-HSD activity by boosting its coenzyme NAD⁺ availability improved glandular cell proliferation and alleviated the dry eye disease phenotype. Both women and men express 3β-HSD in the meibomian gland. Enhancing local steroidogenesis may help combat age-associated meibomian gland dysfunction.

Basal Cells in the Epidermis and Epidermal Differentiation

A definite identification of epidermal stem cells is not known and the mechanism of epidermal differentiation is not fully understood. Toward both of these quests, considerable information is available from the research on lineage tracing and clonal growth analysis in the basal layer of the epidermis, on the hair follicle and the interfollicular epidermal stem cells, and on Wnt signaling along with its role in the developmental patterning and cell differentiation. In this paper, literature on the aforementioned research has been collated and analyzed. In addition, models of the basal layer cellular composition and the epidermal differentiation have been presented. Graphical Abstract.

Human meibomian gland epithelial cell culture models: Current progress, challenges, and future directions

The widely used immortalised human meibomian gland epithelia cell (iHMGEC) line has made possible extensive studies of the biology and pathophysiology of meibomian glands (MG). Tissue culture protocols for iHMGEC have been revised and modified to optimise the growth conditions for cell differentiation and lipid accumulation. iHMGEC proliferate in serum-free medium but require serum or other appropriate exogenous factors to differentiate. Several supplements can enhance differentiation and neutral lipid accumulation in iHMGEC grown in serum-containing medium. In serum-free medium, rosiglitazone, a peroxisome proliferator activator receptor-γ (PPARγ) agonist, is reported to induce iHMGEC differentiation, neutral lipid accumulation and expression of key biomarkers of differentiation. iHMGEC cultured in serum-containing medium under hypoxia or with azithromycin increases DNAse 2 activity, a biomarker of terminal differentiation in sebocytes. The production of lipids with composition similar to meibum has not been observed in vitro and this remains a major challenge for iHMGEC culture. Innovative methodologies such as 3D ex vivo culture of MG and generation of MG organoids from stem cells are important for further developing a model that more closely mimics the in vivo biology of human MG and to facilitate the next generation of studies of MG disease and dry eye.

Hedgehog Signaling Pathway Regulates the Proliferation and Differentiation of Rat Meibomian Gland Epithelial Cells

Purpose

Meibomian glands play a vital role in maintaining ocular surface stability. This study aimed to investigate whether Hedgehog signaling is involved in the regulation of meibomian gland epithelial cells.

Methods

Rat meibomian glands epithelial cells (RMGECs) were isolated from ducts and ductules, and then were cultivated to passage two on Matrigel coated wells in meibomian gland epithelial cells medium (MGECM). Cells were switched from MGECM to differentiation medium (DM) or DM added 10 µg/mL azithromycin (DM + AZM) when reached 50% to 60% confluence. The effects of the Smoothened (Smo) agonist (Smo agonist [SAG]) and antagonist (by cyclopamine) on RMGECs were analyzed using quantitative RT-PCR, cell proliferation analysis, immunofluorescence staining, and Nile red staining.

Results

The Hedgehog receptor, Smo, and its downstream molecules, Glis, were expressed both in vivo and in vitro. Smo and Gli1 both decreased with the increase of differentiation in vitro. Smo antagonist, cyclopamine, reduced cell numbers, and the expression of Ki67 in MGECM, and promoted the expression of SREBP1 and lipid production in DM + AZM. Smo agonist, SAG, inhibited the expression of SREBP1 and lipid accumulation in DM + AZM but showed no significant effects on raising cell numbers and the expression of Ki67 in MGECM.

Conclusions

The Hedgehog signaling pathway appears to play important roles in RMGECs proliferation and differentiation. This may provide a potential therapeutic way to treat meibomian gland dysfunction (MGD).

Pathophysiology of Meibomian Glands - An Overview

Purpose: The meibomian glands are located in the tarsal plate of the upper and lower eyelid and are responsible for the production of a lipid-rich secretion, the meibum, which forms the outer component of the tear film. Meibomian gland dysfunction results in excessive evaporation of the tear film and is the leading cause of dry eye disease (DED). Despite the high prevalence of DED, the etiology of meibomian gland dysfunction is only basically understood. In addition, the molecular mechanisms of meibomian gland maturation and physiological function are currently the focus of research.Methods: A systematic literature search was performed using the main scientific databases, including all relevant published articles up to September 2020.Results: This article provides an overview of the current state of knowledge about meibomian gland stem cells, cell surface marker expression and PPARγ signaling, as well as the pathological causes of meibomian gland dysfunction.Conclusion: Androgen deficiency, hyperkeratinization, PPARγ signaling and inflammatory reactions including neutrophil extracellular traps (NETs) seem to be key factors within the pathological processes of the meibomian gland.

What's new in ocular and oral aspects of Sjögren's syndrome and do new treatments work?

Primary SS (pSS) is a systemic autoimmune disease characterized by lymphocytic infiltration of the exocrine glands leading to glandular dysfunction, resulting in dryness of the eyes, mouth and other mucosal surfaces. Systemic manifestations also occur in the majority of patients. There has been increasing interest in recent years, with a number of publications regarding the classification criteria, diagnostic tools, disease activity, damage, impact and potential treatments. This article reviews recent advances in the diagnosis and treatment of ocular and oral involvement in pSS. Recent stand-out developments include measurement of tear osmolarity as a marker in dry eye disease diagnosis, new devices measuring tear constituents and meibomian gland structure and treatment of its dysfunction. Lip biopsy is still valuable despite emerging evidence of non-invasive diagnostic techniques, notably salivary gland ultrasound.

Comparative influence of differentiation and proliferation on gene expression in human meibomian gland epithelial cells

We recently discovered that by changing environmental signals, differentiated immortalized human meibomian gland epithelial cells (IHMGECs) de-differentiate into proliferating cells. We also discovered that following exposure to appropriate stimuli, these proliferative cells re-differentiate into differentiated IHMGECs. We hypothesize that this plasticity of differentiated and proliferative IHMGECs is paralleled by very significant alterations in cellular gene expression. To begin to test this hypothesis, we compared the gene expression patterns of IHMGECs during differentiation and proliferation. IHMGECs were cultured for four days in either differentiating or proliferating media. After four days of culture, cells were processed for the analysis of gene expression by using Illumina BeadChips and bioinformatic software. Our study identified significant differences in the expression of more than 9200 genes in differentiated and proliferative IHMGECs. Differentiation was associated with significant increases in the expression of specific genes (e.g. S100 calcium binding protein P; 7,194,386-fold upregulation) and numerous ontologies (e.g. 83 biological process [bp] ontologies with ≥100 genes were upregulated), such as those related to development, transport and lysosomes. Proliferation also led to a significant rise in specific gene expressions (e.g. cathelicidin antimicrobial peptide; 859,100-fold upregulation) and many ontologies (115 biological process [bp] ontologies with ≥100 genes were upregulated), with most of the highly significant ontologies related to cell cycle (z scores > 13.9). Our findings demonstrate that gene expression in differentiated and proliferative IHMGECs is extremely different. These results may have significant implications for the regeneration of HMGECs and the reversal of MG dropout in MG dysfunction.

LRIG1, a regulator of stem cell quiescence and a pleiotropic feedback tumor suppressor

LRIG1, leucine-rich repeats and immunoglobulin-like domains protein 1, was discovered more than 20 years ago and has been shown to be downregulated or lost, and to function as a tumor suppressor in several cancers. Another well-reported biological function of LRIG1 is to regulate and help enforce the quiescence of adult stem cells (SCs). In both contexts, LRIG1 regulates SC quiescence and represses tumor growth via, primarily, antagonizing the expression and activities of ERBB and other receptor tyrosine kinases (RTKs). We have recently reported that in treatment-naïve human prostate cancer (PCa), LRIG1 is primarily regulated by androgen receptor (AR) and is prominently overexpressed. In castration-resistant PCa (CRPC), both LRIG1 and AR expression becomes heterogeneous and, frequently, discordant. Importantly, in both androgen-dependent PCa and CRPC models, LRIG1 exhibits tumor-suppressive functions. Moreover, LRIG1 induction inhibits the growth of pre-established AR⁺ and AR⁻ PCa. Here, upon a brief introduction of the LRIG1 and the LRIG family, we provide an updated overview on LRIG1 functions in regulating SC quiescence and repressing tumor development. We further highlight the expression, regulation and functions of LRIG1 in treatment-naïve PCa and CRPC. We conclude by offering the perspectives of identifying novel cancer-specific LRIG1-interacting signaling partners and developing LRIG1-based anti-cancer therapeutics and diagnostic/prognostic biomarkers.

Organotypic Culture of Mouse Meibomian Gland: A Novel Model to Study Meibomian Gland Dysfunction In Vitro

Purpose

Meibomian glands are essential in maintaining the integrity and health of the ocular surface. Meibomian gland dysfunction (MGD), mainly induced by ductal occlusion, is considered as the major cause of dry eye disease. In this study, a novel in vitro model was established for investigating the role of inflammation in the process of MGD.

Methods

Mouse tarsal plates were removed from eyelids after dissection and explants were cultured during various time ranging from 24 to 120 hours. Meibomian gland epithelial cells were further enzymatically digested and dissociated from tarsal plates before culturing. Both explants and cells were incubated in different media with or without serum or azithromycin (AZM). Furthermore, explants were treated with IL-1β or vehicle for 48 hours. Analyses for tissue viability, histology, biomarker expression, and lipid accumulation were performed with hematoxylin and eosin (H&E) staining, immunofluorescence staining, and Western blot.

Results

Higher viability was preserved when explants were cultured on Matrigel with immediate addition of culture medium. The viability, morphology, biomarker expression, and function of meibomian glands were preserved in explants cultured for up to 72 hours. Lipid accumulation and peroxisome proliferator-activated receptor γ (PPARγ) expression increased in both explants and cells cultured in media containing serum or AZM. Treatment with IL-1β induced overexpression of Keratin (Krt) 1 in meibomian gland ducts.

Conclusions

Intervention with pro-inflammatory cytokine IL-1β induces hyperkeratinization in meibomian gland ducts in vitro. This novel organotypic culture model can be used for investigating the mechanism of MGD.

Neuroendocrinology and neurobiology of sebaceous glands

The nervous system communicates with peripheral tissues through nerve fibres and the systemic release of hypothalamic and pituitary neurohormones. Communication between the nervous system and the largest human organ, skin, has traditionally received little attention. In particular, the neuro-regulation of sebaceous glands (SGs), a major skin appendage, is rarely considered. Yet, it is clear that the SG is under stringent pituitary control, and forms a fascinating, clinically relevant peripheral target organ in which to study the neuroendocrine and neural regulation of epithelia. Sebum, the major secretory product of the SG, is composed of a complex mixture of lipids resulting from the holocrine secretion of specialised epithelial cells (sebocytes). It is indicative of a role of the neuroendocrine system in SG function that excess circulating levels of growth hormone, thyroxine or prolactin result in increased sebum production (seborrhoea). Conversely, growth hormone deficiency, hypothyroidism, and adrenal insufficiency result in reduced sebum production and dry skin. Furthermore, the androgen sensitivity of SGs appears to be under neuroendocrine control, as hypophysectomy (removal of the pituitary) renders SGs largely insensitive to stimulation by testosterone, which is crucial for maintaining SG homeostasis. However, several neurohormones, such as adrenocorticotropic hormone and α-melanocyte-stimulating hormone, can stimulate sebum production independently of either the testes or the adrenal glands, further underscoring the importance of neuroendocrine control in SG biology. Moreover, sebocytes synthesise several neurohormones and express their receptors, suggestive of the presence of neuro-autocrine mechanisms of sebocyte modulation. Aside from the neuroendocrine system, it is conceivable that secretion of neuropeptides and neurotransmitters from cutaneous nerve endings may also act on sebocytes or their progenitors, given that the skin is richly innervated. However, to date, the neural controls of SG development and function remain poorly investigated and incompletely understood. Botulinum toxin-mediated or facial paresis-associated reduction of human sebum secretion suggests that cutaneous nerve-derived substances modulate lipid and inflammatory cytokine synthesis by sebocytes, possibly implicating the nervous system in acne pathogenesis. Additionally, evidence suggests that cutaneous denervation in mice alters the expression of key regulators of SG homeostasis. In this review, we examine the current evidence regarding neuroendocrine and neurobiological regulation of human SG function in physiology and pathology. We further call attention to this line of research as an instructive model for probing and therapeutically manipulating the mechanistic links between the nervous system and mammalian skin.