O-Acetylated
sialic acids have been reported in many
sialoglycoproteins where they mediate a variety of immune and other
biological events. We have previously demonstrated that the protective mucus barrier on the surface of the canine eye contains
sialoglycoproteins. We have also investigated the occurrence of O-acetylated
sialic acids in these ocular
mucins. Mucus aspirated from the surface of normal dog eyes and those with
keratoconjunctivitis sicca (KCS) was fractionated into three pools by density gradient centrifugation.
Sialic acids comprised 0.6-0.9% of the dry weight of the
mucins isolated. The
sialic acid profile in these pools was examined using HPLC. O-Acetylated
sialic acids, mainly Neu5,9Ac2, were detected in normal animals and made up 10-30% of the total
sialic acids detected. A doubling of the
sialic acid content was found in KCS
mucins, but the level of 9-O-acetylated
sialic acid was reduced below 4% of total. Histological analysis of conjunctival tissue from normal and KCS dogs showed the presence of
sialic acids, detected with the alpha(2-6)
sialic acid-specific
lectin Sambucus nigra, in the goblet cells and corresponding to the staining pattern for MUC5AC, the major ocular-secreted
mucin gene product. In KCS animals a disruption of the normal pattern of conjunctival goblet cells was seen with preservation of the pattern of
lectin binding observed in normal animals. Thus the data demonstrate the presence of mono-O-Acetylated
sialic acids in normal canine ocular
mucins and a loss of this population of
sialic acids in
dry eye disease in spite of a significant increase in total
sialic acids in KCS
mucin.