Expression patterns of conjunctival mucin 5AC and aquaporin 5 in response to acute dry eye stress

Structural and genetic diversity in the secreted mucins, MUC5AC and MUC5B

The secreted mucins MUC5AC and MUC5B play critical defensive roles in airway pathogen entrapment and mucociliary clearance by encoding large glycoproteins with variable number tandem repeats (VNTRs). These polymorphic and degenerate protein coding VNTRs make the loci difficult to investigate with short reads. We characterize the structural diversity of MUC5AC and MUC5B by long-read sequencing and assembly of 206 human and 20 nonhuman primate (NHP) haplotypes. We find that human MUC5B is largely invariant (5761-5762aa); however, seven haplotypes have expanded VNTRs (6291-7019aa). In contrast, 30 allelic variants of MUC5AC encode 16 distinct proteins (5249-6325aa) with cysteine-rich domain and VNTR copy number variation. We grouped MUC5AC alleles into three phylogenetic clades: H1 (46%, ~5654aa), H2 (33%, ~5742aa), and H3 (7%, ~6325aa). The two most common human MUC5AC variants are smaller than NHP gene models, suggesting a reduction in protein length during recent human evolution. Linkage disequilibrium (LD) and Tajima's D analyses reveal that East Asians carry exceptionally large MUC5AC LD blocks with an excess of rare variation (p<0.05). To validate this result, we used Locityper for genotyping MUC5AC haplogroups in 2,600 unrelated samples from the 1000 Genomes Project. We observed signatures of positive selection in H1 and H2 among East Asians and a depletion of the likely ancestral haplogroup (H3). In Africans and Europeans, H3 alleles show an excess of common variation and deviate from Hardy-Weinberg equilibrium, consistent with heterozygote advantage and balancing selection. This study provides a generalizable strategy to characterize complex protein coding VNTRs for improved disease associations.

Methods for studying mammalian aquaporin biology

Aquaporins (AQPs), transmembrane water-conducting channels, have earned a great deal of scrutiny for their critical physiological roles in healthy and disease cell states, especially in the biomedical field. Numerous methods have been implemented to elucidate the involvement of AQP-mediated water transport and downstream signaling activation in eliciting whole cell, tissue, and organ functional responses. To modulate these responses, other methods have been employed to investigate AQP druggability. This review discusses standard in vitro, in vivo, and in silico methods for studying AQPs, especially for biomedical and mammalian cell biology applications. We also propose some new techniques and approaches for future AQP research to address current gaps in methodology.

Aquaporin 5 in the eye: Expression, function, and roles in ocular diseases

As a water channel protein, aquaporin 5 (AQP5) is essential for the maintenance of the normal physiological functions of ocular tissues. This review provides an overview of the expression and function of AQP5 in the eye and discusses their role in related eye diseases. Although AQP5 plays a vital role in ocular functions, such as maintaining corneal and lens transparency, regulating water movement, and maintaining homeostasis, some of its functions in ocular tissues are still unclear. Based on the key role of AQP5 in eye function, this review suggests that in the future, eye diseases may be treated by regulating the expression of aquaporin.

Aquaporin-5 Dynamic Regulation

Aquaporin-5 (AQP5), belonging to the aquaporins (AQPs) family of transmembrane water channels, facilitates osmotically driven water flux across biological membranes and the movement of hydrogen peroxide and CO₂. Various mechanisms have been shown to dynamically regulate AQP5 expression, trafficking, and function. Besides fulfilling its primary water permeability function, AQP5 has been shown to regulate downstream effectors playing roles in various cellular processes. This review provides a comprehensive overview of the current knowledge of the upstream and downstream effectors of AQP5 to gain an in-depth understanding of the physiological and pathophysiological processes involving AQP5.

Differences in vulnerability to desiccating stress between corneal and conjunctival epithelium in rabbit models of short-term ocular surface exposure

We evaluate the difference in vulnerability to desiccating stress (DS) between the corneal and conjunctival epithelia to understand different ocular surface staining patterns in dry eye patients. We generated a rabbit model of short-term exposure keratopathy. To induce DS in the ocular surface, rabbit right eyelids were opened for 30 min, with blinking once/minute. Corneal staining scores increased from 3-min post-DS exposure, while conjunctival staining increased from 20-min post-DS. At 20 min, the tear MUC5AC level doubled as compared to pre-DS (p = 0.007). In Western blot analysis, conjunctival AQP5, MUC5AC, and CFTR expression increased significantly in response to DS, compared to control (p = 0.039, 0.002, 0.039, respectively). Immunohistochemistry for CD31 and LYVE-1 were performed. CD31-positive cells and lymphatic space surrounded by LYVE-1-positive cells increased significantly in conjunctival tissue post-DS, compared to control (p = 0.0006, p < 0.0001, respectively). Surface damage was worse in the corneal than in the conjunctival epithelium after DS, by scanning electron microscopy. This study showed that the cornea and conjunctival epithelium show differences in vulnerability to DS. Increased blood vessels and dilated lymphatics, accompanied by increased conjunctival epithelial AQP5, MUC5AC, and CFTR expression, underlie the protective mechanism of the conjunctiva to desiccating stress.

Insight into the Mammalian Aquaporin Interactome

Aquaporins (AQPs) are a family of transmembrane water channels expressed in all living organisms. AQPs facilitate osmotically driven water flux across biological membranes and, in some cases, the movement of small molecules (such as glycerol, urea, CO₂, NH₃, H₂O₂). Protein-protein interactions play essential roles in protein regulation and function. This review provides a comprehensive overview of the current knowledge of the AQP interactomes and addresses the molecular basis and functional significance of these protein-protein interactions in health and diseases. Targeting AQP interactomes may offer new therapeutic avenues as targeting individual AQPs remains challenging despite intense efforts.

Ocular Surface Ion-Channels Are Closely Related to Dry Eye: Key Research Focus on Innovative Drugs for Dry Eye

Abundant ion-channels, including various perceptual receptors, chloride channels, purinergic receptor channels, and water channels that exist on the ocular surface, play an important role in the pathogenesis of dry eye. Channel-targeting activators or inhibitor compounds, which have shown positive effects in in vivo and in vitro experiments, have become the focus of the dry eye drug research and development, and individual compounds have been applied in clinical experimental treatment. This review summarized various types of ion-channels on the ocular surface related to dry eye, their basic functions, and spatial distribution, and discussed basic and clinical research results of various channel receptor regulatory compounds. Therefore, further elucidating the relationship between ion-channels and dry eye will warrant research of dry eye targeted drug therapy.

Aquaporin5-Targeted Treatment for Dry Eye Through Bioactive Compounds and Gut Microbiota

Dry eye and dry mouth are the principal sources of morbidity for patients with Sjögren's syndrome (SS). There are few effective treatments, particularly systemic ones. Targeting aquaprin-5 (AQP5)-mediated tear secretion has been tested as a novel ancillary strategy and has proved promising. Patients have a great interest in using complementary medicine, including nutraceuticals and bioactive compounds to alleviate their symptoms. Potential mechanisms by which phytocompounds and bioactive compounds may benefit SS ocular and mouth symptoms through modulation of AQP5 activity are presented within this review. Supplementation with prebiotics (such as polyphenols with high bioavailability) in SS patients with lower Firmicutes/Bacteroides (F/B) community ratio phenotype, through administration of butyrate-producing diets, is proposed as ancillary strategy for dry eye and mouth. The potential use of natural bioactive compounds to treat dry eye could also apply to dry mouth occurring in the context of aging and SS. This novel hypothesis could have implications with respect to planning a successful dietary regimen for achieving and maintaining a normal gut microbiota in SS patients. This regimen would include augmenting butyrate-producing foodstuffs and/or polyphenol-rich syrups, and high amounts of some specific probiotic-rich foodstuffs such as yogurt, soy yogurt, or as probiotic supplements. There are applications for pharmaceutical and nutraceutical products aiming to relieve dry eye and mouth.

A Novel Integrated Active Herbal Formulation Ameliorates Dry Eye Syndrome by Inhibiting Inflammation and Oxidative Stress and Enhancing Glycosylated Phosphoproteins in Rats

Dry eye syndrome (DES) is a chronic condition of the eye with insufficient production of tears leading to inadequate lubrication of eyes. Symptoms of DES are associated with discomfort and redness of the eye, blurred vision, and tear film instability which leads to the damaged ocular surface. Inflammation and oxidative stress play a significant role in the pathogenesis of the disease. In this study, the protective effect of different doses (100 or 200 mg/kg) of a novel multi-component oral formulation of lutein/zeaxanthin, curcumin, and vitamin D3 (LCD) was evaluated using a rat model with benzalkonium chloride (BAC)-induced dry eye syndrome. The formulation was administered orally to rats for 4 weeks. We observed a significant improvement in tear volume, tear breakup time, tear film integrity, and reduction in overall inflammation in rats fed with the LCD at dose 200 mg/kg performing better than 100 mg/kg. Furthermore, the formulation helped in lowering oxidative stress by increasing antioxidant levels and restored protective tear protein levels including MUC1, MUC4, and MUC5AC with 200 mg of LCD having the most significant effect. The results strongly suggest that the combination of lutein/zeaxanthin, curcumin, and vitamin-D3 is effective in alleviating the symptoms of dry eye condition with a multi-modal mechanism of action.

Altered mucins and aquaporins indicate dry eye outcome in patients undergoing Vitreo-retinal surgery

Vitreo-retinal (VR) surgeries induce conjunctival changes. However, there are no study reports regarding prevalence and severity of dry eye after these surgeries. This study evaluated dry eye outcome after VR surgery. Patients undergoing VR surgery classified as scleral buckle and microincision vitrectomy surgery (n = 44, mean age: 56.09±10.2 years) were recruited. Dry eye evaluation was done before and 8 weeks after surgery (2 weeks after omitting topical eye drops). Conjunctival imprint cytology for goblet cell count and tear Mucin 5AC (MUC5AC) protein estimation was done. Gene expressions of MUC5AC, MUC4, MUC16, Aquaporin 4 (AQP4) and AQP5 were analyzed in the conjunctival imprint cells by qPCR. None of the patients exhibited clinical signs of dry eye after VR surgery. But the conjunctival goblet cell density (GCD) was significantly lowered post-VR surgery (63% cases, **p = 0.012) with no alterations in the tear MUC5AC protein. Post-VR surgery, the conjunctival cell gene expression of MUC4, MUC16 and AQP4 were significantly increased (*p = 0.025, *p = 0.05 and *p = 0.02 respectively) and AQP5 was significantly lowered (*p = 0.037), with no change in MUC5AC expression. Tear cytokines were significantly increased post-VR surgery (anti-inflammatory: IL1RA, IL4, IL5, IL9, FGF; PDGFbb and pro-inflammatory: IL2, IL6, IL15, GMCSF and IFNg). Though clinical signs of dry eye were not observed after VR surgery, ocular surface changes in the form of reduced GCD, altered MUC5AC, MUC4, MUC16, AQP4, AQP5 and cytokines are suggestive of dry eye outcome at the molecular level especially inpatients aged above 51 years, especially female gender and those who are diabetic.

Clinical Implications of Goblet Cells in Dacryoadenosis and Normal Human Lacrimal Glands

PURPOSE

The purpose of this study was to investigate an enlarged dacryoadenotic lacrimal gland and normal lacrimal glands for the presence of goblet cells (mucocytes).

DESIGN

Retrospective clinicopathologic series.

METHODS

An enlarged lacrimal gland (dacryoadenosis) without obvious histopathologic alterations was extensively evaluated histochemically, immunohistochemically, and ultrastructurally to detect the presence of goblet cells and to compare the findings with those in five normal lacrimal glands.

RESULTS

Granular, zymogen-rich pyramidal acinar cells in normal glands predominated over a previously not reported subpopulation of nongranular, pale-staining cells in both dacryoadenotic and normal lacrimal glands. These cells histochemically stained positively with mucicarmine and Alcian blue. Immunohistochemical and electron microscopic evaluations established that there was a displacement or replacement of cytoplasmic gross cystic disease fluid protein-15 and CK 7-positive tonofilaments in the pale acinar cells by myriad mucus granules. The goblet cells constituted approximately 2% of the normal acinar cells and 5% of dacryoadenotic acinar cells. A depletion of myoepithelial cells and ectopic intra-acinar ductular cells were also observed in dacryoadenosis.

CONCLUSION

Dacryoadenosis is caused by an increase in the number of acini without individual acinar cell hyperplasia. A normal cytologic feature of the lacrimal gland is the presence of acinar goblet cells that had been long overlooked; they are increased in number in dacryoadenosis. Intra-acinar ductular cells and the scattered loss of myoepithelial cells are other abnormalities in dacryoadenosis. The presence of lacrimal gland goblet cells may have physiologic implications for the precorneal tear film and its derangements as well as for the histogenesis of mucus-producing carcinomas.