Conjunctival goblet cells secrete gel-forming mucins that capture and clear allergens, pathogens, and debris from the eye, according to research from Japan.
The researchers, from Juntendo University, revealed that the protective function of these mucins is attributed to the presence of sialic acid in their structure, and that the sialylation status of ocular mucins may predict ocular sensitivity to allergens. Their findings may help find a new therapy for allergic conjunctivitis.
Their experiment was published in Volume 14 of Nature Communications on 17 March 2023.
Goblet Cells and Mucus
The gene responsible for sialylation of mucus is mutated in Balb/c mice. However, reintroducing the same gene, intact, restores the production of sialylated mucus, which effectively protects the mice from allergic conjunctivitis.
The goblet cell is a specialised cell that plays a vital role in protecting wet surfaces of the body, such as the eye. By secreting mucus, they lubricate surfaces and entrap foreign particles (such as pollen or debris), which protects the surface from damage.
Mucins – a major component of mucus – are high molecular weight, heavily glycosylated glycoproteins. It is these mucins that lend mucus its properties and enable its function. They contain repetitive sequences rich in serine, threonine residues that are heavily modified by O-glycosylation, and cysteine-rich domains. The latter allow them to multimerise through intermolecular disulfide bonding and result in mucus’ gel-like consistency.
A deficiency in an intestinal gel-forming mucin – Muc2 – has staggering effects. It results in spontaneous colitis and an increased risk of colon cancer. Contrastingly, a complete deficiency in the mucins of the eye – such as Muc5ac/Muc5b – has minimal effects on the occurrence of dry eyes. Conjunctival goblet cells also do not play a role in protecting the eye against bacterial infection.
The sialylation status of ocular mucins might predict ocular sensitivity to allergens. These findings may also help find a new therapy for allergic conjunctivitis
The Role of Single-Gel-Forming Ocular Mucins
The researchers set out to determine the specific role of single-gel-forming ocular mucins, and to work out whether they protect the eye against allergies.
Led by Dr Tomoaki Ando, Dr Jiro Kitaura, and Dr Nobuyuki Ebihara, they assessed the influence of sialylation (addition of sialic acid to glycoproteins like mucins) on the protective role of mucins against allergens.
The team initially used two strains of mice – C57BL/6J (B6J) and Balb/c – for their experiment. Balb/c mice carry a point mutation in their St6galnac1 gene, which codes for sialyltransferase and is responsible for the sialylation of mucins. As a result of this mutation, they show aberrant St6galnac1 transcription, and dysfunctional proteins. Moreover, when the conjunctiva of B6J mice, Balb/c mice, and humans was stained with Alcian Blue (AB, which detects acidic residues such as sialic acid), only the conjunctival goblet cells of humans and B6J mice were positive for the same. They said this showed the goblet cells of Balb/c mice are not sialylated in the steady state.
“Genetic studies,” says Dr Ando, “revealed that St6galnac1 was, indeed, the gene responsible for this phenomenon in Balb/c mice.”
Furthermore, the human conjunctival samples, which had been exposed to chronic irritation, were found to express an increased amount of ST6GALNAC1 and its enzymatic product, sialyl-Tn, thereby elevating their defense against allergens.
Discussing the results, Dr Ando said: “Humans and B6J mice had sialylated mucins that confer protection against allergens, while Balb/c mice lacked protection against allergens due to a lack of sialylated mucins”.
To confirm their findings, the team engineered a third strain of mice – Ao mice – by backcrossing B6J mice to Balb/c mice. This reinstated their intact St6galnac1 gene in the mice, restoring their ability to exhibit sialylated mucins.
The Difference of Sialylation Status
The researchers then set out to determine exactly how the ocular mucins make a difference.
By conducting a microscopic examination of the pollen aggregates the mice were exposed to, they found that mice with sialylated mucins encapsulated the pollen in an impenetrable gel-like layer. This enabled them to capture a large amount of pollen shells, which could then be removed from the eye. This was not the case with the Balb/c mice, who did not have sialylated mucins.
The team also conducted in vivo experiments with mice. These showed that Ao mice with sialylated ocular mucins had a reduced antigen response, retained lesser amount of pollen, and were better protected against allergic conjunctivitis, compared to Balb/c mice.
Overall, these findings elucidate the role of sialylated goblet cell mucins in protecting the ocular surface from allergens and highlight the importance of sialylation in regulating mucin functions in the conjunctiva. Dr. Kitaura sums up their findings, “The sialylation status of ocular mucins might predict ocular sensitivity to allergens. These findings may also help find a new therapy for allergic conjunctivitis.”
Reference
Matsuzawa M., Ando T., Fukase S., et al., The protective role of conjunctival goblet cell mucin sialylation. Nature Communications. DOI: https://doi.org/10.1038/s41467-023-37101-y
