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

Atrophic meibomian gland dysfunction induced by eyelid margin cryotherapy with liquid nitrogen

To develop an atrophic Meibomian Gland Dysfunction (MGD) animal model via liquid nitrogen cryotherapy, the eyelid edges of C57 mice exposure to liquid nitrogen for 30 s. Morphology of MG and ocular surface were assessed using stereomicroscopy and a slit lamp microscope at multiple time points post-injury. Acinar loss and atrophy were observed from day 7, with increased inflammation and apoptosis, and decreased proliferation in acinar cells. Corneal epithelial defects appeared after day 14. Liquid nitrogen induced selective damage to meibomian acinar cells, simulating MGD pathology effectively, with peak effects at day 21, providing a relevant model for atrophic MGD research.

A modified calculation formula for meibomian gland grading

This study aimed to establish a modified calculation formula for the grading of meibomian glands. Meibography images from 102 participants by different examiners on separate machines on two consecutive days were analyzed, quantified and compared side-by-side. Measure and calculate the ratio of the MGs area to the whole eyelid area and the ratio of the MGs to the corneal base area. Our findings demonstrate that there were significant differences in the ratio of the meibomian gland area to the whole eyelid area between two measurements, but not in the ratio of the meibomian gland area to the corneal base area. The measurement of the eyelid area showed bigger variations and poorer repeatability than the meibomian gland area and the corneal base area. As such, the ratio of the meibomian gland area to the corneal base area is a more stable indicator for the grading of meibomian glands over multiple measurement.

Deletion of Fgfr2 in Ductal Basal Epithelium With Tamoxifen Induces Obstructive Meibomian Gland Dysfunction

Purpose

Fibroblast growth factor receptor 2 (Fgfr2) is crucial for the homeostasis of meibomian gland (MG). However, the role of Fgfr2 in MG ductal epithelial progenitors remains to be delineated. Herein, we created a new transgenic mouse model with conditional deletion of Fgfr2 from MG ductal progenitors and investigated the cell-specific role in the pathogenesis of obstructive meibomian gland dysfunction.

Methods

Peritoneal injection of tamoxifen (TAM) at 50 µg/gm for three consecutive days was performed to induce conditional deletion of Fgfr2 in two-month-old Krt5Fgfr2CKO or Krt5Fgfr2CKO-mTmG mice. Phenotypes of MG after Fgfr2 deletion were monitored by meibography, lipid staining, and immunostaining against keratin-6a in MG whole mounts. Lineage tracing of the Krt5+ progenitors of MG and biomarkers for ductal differentiation and proliferation were also examined by immunostainings.

Results

The Krt5Fgfr2CKO mice developed extensive ductal occlusion and acinar atrophy at day 10 after TAM administration. Robust thickening of ductal epithelium with abnormal differentiation and proliferation of ductal basal meibocytes were observed in the MGs of Krt5Fgfr2CKO mice. In Krt5Fgfr2CKO-mTmG mice, the Krt5+ progenitors and its progeny were labeled by EGFP after Fgfr2 depletion by TAM with evident expansion of the suprabasal and superficial layers of MG ductal epithelium when compared with the controls.

Conclusions

Our results substantiated the crucial role of Fgfr2 in homeostasis of the MG ductal epithelium. Deletion of Fgfr2 affects the MG ductal basal progenitors by impacting the differentiation of ductal meibocytes and the maintenance of acinar meibocytes, which are likely the underlying pathogenesis of obstructive MGD.

Safety and Efficacy of Dry Eye Intelligent Therapeutic Device in the Treatment of Meibomian Gland Dysfunction in Rabbits

PURPOSE

To assess the safety and efficacy of the dry eye intelligent therapeutic device in rabbits with meibomian gland dysfunction.

METHODS

The meibomian gland dysfunction-afflicted rabbits were subjected to treatment using the dry eye intelligent therapeutic device. Various parameters, including eyelid margin, meibomian gland opening, redness, meibomian gland area, keratoconjunctival fluorescence staining, and intraocular pressure, were examined and analyzed using an ocular surface comprehensive examination instrument, slit lamp, and tonometer at corresponding times points. Hematoxylin and eosin staining was performed to examine the mucosal epithelium and meibomian gland.

RESULTS

In this study, eyelid margin congestion and meibomian gland opening obstruction were significantly improved after 3 weeks and 4 weeks of treatment, respectively (p < .01, p < .05). The treatment group showed a significant increase in tear meniscus height after 2 weeks, 3 weeks and 4 weeks of treatment (p < .001, p < .01, p < .05). No significant changes were noted in meibomian gland area, redness, intraocular pressure, and keratoconjunctival fluorescence staining of rabbits before and after treatment. Hematoxylin and eosin staining revealed a complete structure of mucosal epithelium and meibomian gland in the treatment group and that the expansion of the blocked meibomian gland duct was reduced.

CONCLUSION

The utilization of the dry eye intelligent therapeutic device in treating meibomian gland dysfunction-afflicted rabbits exhibits potential promising safety, efficacy, and overall benefits, thereby offering a novel alternative for managing meibomian gland dysfunction patients in clinical settings.

Cytarabine chemotherapy induces meibomian gland dysfunction

PURPOSE

Cytarabine (Ara-C) chemotherapy causes symptoms resembling meibomian gland dysfunction (MGD), suggesting potential associations between Ara-C and MGD. In this study, the pathological effects of Ara-C on MGD were investigated in a rodent model.

METHODS

Mice received Ara-C with or without rosiglitazone (PPARγ agonist) for 7 consecutive days. Slit-lamp biomicroscope was used for ocular examinations. Immunofluorescence detected acinar cell proliferation, differentiation, and ductal keratinization in the meibomian gland (MG). Lipid accumulation was evaluated by Oil Red O and LipidTox staining. Lipogenic status, FoxO1/FoxO3a cellular localization, and oxidative stress were visualized via immunohistochemistry. Western blotting assessed relative protein expression and AKT/FoxO1/FoxO3a pathway phosphorylation.

RESULTS

Ara-C (50 mg/kg) did not affect mouse survival but induced damage to ocular surface microenvironment, including corneal epithelial defects, MG orifice plugging and acinar dropout, and lacrimal gland (LG) dysfunction. Ara-C intervention inhibited proliferation and caused progenitor loss in the MG, as evidenced by reduced PCNA + labeling and P63+/Lrig1+ basal cell numbers. The MG ducts of Ara-C-treated mice exhibited marked dilatation, lipid deposition, and hyperkeratinization (K1/K10 overexpression). Ara-C disrupted MG lipid metabolism by downregulating PPARγ and its downstream lipogenic targets AWAT2/SOAT1/ELOVL4 and upregulating HMGCR. Dephosphorylation of AKT and the subsequent nuclear translocation of FoxO1/FoxO3a contributed to Ara-C-induced PPARγ downregulation. Ara-C triggered oxidative stress with increases in 4-HNE and 8-OHdG and Keap1/Nrf2/HO-1/SOD1 axis dysregulation. Rosiglitazone treatment ameliorated MGD-associated pathological manifestations, LG function, MG lipid metabolism, and oxidative stress in Ara-C-exposed mice.

CONCLUSIONS

Systemic Ara-C chemotherapy exerted topical cytotoxic effects on the ocular surface, and PPARγ restoration by rosiglitazone mitigated Ara-C-induced MGD alterations.

Identification of Meibomian gland stem cell populations and mechanisms of aging

Meibomian glands secrete lipid-rich meibum, which prevents tear evaporation. Aging-related Meibomian gland shrinkage may result in part from stem cell exhaustion and is associated with evaporative dry eye disease, a common condition lacking effective treatment. The identities and niche of Meibomian gland stem cells and the signals controlling their activity are poorly defined. Using snRNA-seq, in vivo lineage tracing, ex vivo live imaging, and genetic studies in mice, we identified markers for stem cell populations that maintain distinct regions of the gland and uncovered Hh signaling as a key regulator of stem cell proliferation. Consistent with this, human Meibomian gland carcinoma exhibited increased Hh signaling. Aged glands displayed decreased Hh and EGF signaling, deficient innervation, and loss of collagen I in niche fibroblasts, indicating that alterations in both glandular epithelial cells and their surrounding microenvironment contribute to age-related degeneration. These findings suggest new approaches to treat aging-associated Meibomian gland loss.

The Yin and Yang of non-immune and immune responses in meibomian gland dysfunction

Meibomian gland dysfunction (MGD) is a leading cause of dry eye disease and one of the most common ophthalmic conditions encountered in eye clinics worldwide. These holocrine glands are situated in the eyelid, where they produce specialized lipids, or meibum, needed to lubricate the eye surface and slow tear film evaporation - functions which are critical to preserving high-resolution vision. MGD results in tear instability, rapid tear evaporation, changes in local microflora, and dry eye disease, amongst other pathological entities. While studies identifying the mechanisms of MGD have generally focused on gland obstruction, we now know that age is a major risk factor for MGD that is associated with abnormal cell differentiation and renewal. It is also now appreciated that immune-inflammatory disorders, such as certain autoimmune diseases and atopy, may trigger MGD, as demonstrated through a T cell-driven neutrophil response. Here, we independently discuss the underlying roles of gland and immune related factors in MGD, as well as the integration of these two distinct mechanisms into a unified perspective that may aid future studies. From this unique standpoint, we propose a revised model in which glandular dysfunction and immunopathogenic pathways are not primary versus secondary contributors in MGD, but are fluid, interactive, and dynamic, which we likened to the Yin and Yang of MGD.

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.

Oleic acid induces lipogenesis and NLRP3 inflammasome activation in organotypic mouse meibomian gland and human meibomian gland epithelial cells

The accumulation of oleic acid (OA) in the meibum from patients with meibomian gland dysfunction (MGD) suggests that it may contribute to meibomian gland (MG) functional disorder, as it is a potent stimulator of acne-related lipogenesis and inflammation in sebaceous gland. Therefore, we investigate whether OA induces lipogenesis and inflammasome activation in organotypic cultured mouse MG and human meibomian gland epithelial cells (HMGECs). Organotypic cultured mouse MG and HMGECs were exposed to OA or combinations with specific AMPK agonists 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR). Lipogenic status, ductal keratinization, squamous metaplasia, NLRP3/ASC/Caspase-1 inflammasome activation, proinflammatory cytokine IL-1β production, and AMPK pathway phosphorylation in MG were subsequently examined by lipid staining, immunofluorescence staining, immunohistochemical staining, ELISA assay, and Western blot analyses. We found that OA significantly induced lipid accumulation, ductal keratinization, and squamous metaplasia in organotypic cultured MG, as evidenced by increased lipids deposition within acini and duct, upregulated expression of lipogenic proteins (SREBP-1 and HMGCR), and elevation of K10/Sprr1b. Additionally, OA induced NLRP3/ASC/Caspase-1 inflammasome activation, cleavage of Caspase-1, and production of downstream proinflammatory cytokine IL-1β. The findings of lipogenesis and NLRP3-related proinflammatory response in OA-stimulated HMGECs were consistent with those in organotypic cultured MG. OA exposure downregulated phospho-AMPK in two models, while AICAR treatment alleviated lipogenesis by improving AMPK/ACC phosphorylation and SREBP-1/HMGCR expression. Furthermore, AMPK amelioration inhibited activation of the NLRP3/ASC/Caspase-1 axis and secretion of IL-1β, thereby relieving the OA-induced proinflammatory response. These results demonstrated that OA induced lipogenic disorder and NLRP3 inflammasome activation in organotypic cultured mouse MG and HMGECs by suppressing the AMPK signaling pathway, indicating OA may play an etiological role in MGD.

Expression of ATP-Binding Cassette Transporter A1 (ABCA1) in Eyelid Tissues and Meibomian Gland Epithelial Cells

Purpose

To validate the adenosine triphosphate (ATP)-binding cassette transporter A1 (ABCA1) expression and distribution in human eyelid tissues and meibomian gland epithelial cells.

Methods

Meibomian gland tissues from human eyelids were isolated by collagenase A digestion and cultured in defined keratinocyte serum-free medium (DKSFM). Infrared imaging was used to analyze the general morphology of meibomian glands. Hematoxylin and eosin (H&E) staining and Oil Red O staining were used to observe the morphological structure and lipid secretion in the human meibomian gland tissues. Quantitative real-time polymerase chain reaction, western blotting, and immunostaining were used to detect the mRNA and protein expression and cytolocalization of ABCA1 in the meibomian gland tissues and cultured cells.

Results

The degree of loss of human meibomian gland tissue was related to age. Meibomian gland lipid metabolism was also associated with age. Additionally, human meibomian gland tissues express ABCA1 mRNA and protein; glandular epithelial cells express more ABCA1 mRNA and protein than acinar cells, and their expression in acinar cells decreases with differentiation. Furthermore, the expression of ABCA1 was downregulated in abnormal meibomian gland tissues. ABCA1 was mainly localized on the cell membrane in primary human meibomian gland epithelial cells (pHMGECs), whereas it was localized in the cytoplasm of immortalized human meibomian gland epithelial cells (iHMGECs). The mRNA and protein levels of ABCA1 in pHMGECs were higher than those in iHMGECs.

Conclusions

Meibomian gland tissues of the human eyelid degenerate with age. ABCA1 expression in acinar cells decreases after differentiation and plays an important role in meibomian gland metabolism.

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.

Standardization of meibomian gland dysfunction in an Egyptian population sample using a non-contact meibography technique

AIM

To develop normative data for meibomian gland dysfunction (MGD) parameters, using non-contact meibography technique of Sirius Costruzione Strumenti Oftalmici (CSO) machine, in an Egyptian population sample.

METHODS

Observational, cross-sectional, analytic study, in which 104 Egyptian volunteers were included. Both upper lids were examined, using "Sirius CSO" machine. Each eyelid was given a degree of meibomian gland loss (MGL), which was calculated by the software of the machine.

RESULTS

Mean percentage MGL in right upper lid was of 30.9%±12.6%, and that of left upper lid was 32.6%±11.8%. Thirty-four volunteers (32.7%) had first-degree MGL in their right upper lid, and 67.3% had second-degree loss. One volunteer (1%) had zero-degree MGL in left upper lid, 28 (26.9%) had first-degree loss, and 75 (72.1%) had second-degree loss. Degree of MGL in right upper eyelid was not related to age, but degree of MGL in left upper eyelid increased with age. There was statistically significant difference between both genders for degree of MGL in right eye (P=0.036) and in left eye (P=0.027).

CONCLUSION

Noncontact meibography is a useful non-invasive tool for diagnosing MGL. MGL is diagnosed in 100% of apparently normal individuals; 26.9%-32.7% of which have first-degree MGL, and 67.3%-72.1% have second-degree MGL.

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.