Sialic acids: the key to protection against eye allergies

The goblet cell is a specialized cell that plays a vital role in protecting wet surfaces of the body, such as the eye. Goblet cells secrete mucus, which lubricates these surfaces and also entraps any foreign particles (such as pollen or debris), protecting 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 multimerize 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. What, then, is the specific role of single-gel-forming ocular mucins? Do they protect the eye against allergies?

To find answers, a team of researchers from Juntendo University, Japan, led by Dr. Tomoaki Ando, Dr. Jiro Kitaura, and Dr. Nobuyuki Ebihara, assessed the influence of sialylation (addition of sialic acid to glycoproteins like mucins) on the protective role of mucins against allergens. A paper published in Volume 14 of Nature Communications on 17 March 2023 details their experiments. 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. This shows that 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.” What’s more, 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. Dr. Ando discusses their results: “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 an intact St6galnac1 gene in the mice, restoring their ability to exhibit sialylated mucins.

How exactly did the sialylation status of ocular mucins make a difference, though?
To answer this, the team conducted 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.”

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