Relationships between ocular surface sphingomyelinases, Meibum and Tear Sphingolipids, and clinical parameters of meibomian gland dysfunction

Abstract

Purpose: Sphingolipids (SPL) are a class of lipid molecules that play important functional and structural roles in our body and are a component of meibum. Sphingomyelinases (SMases) are key enzymes in sphingolipid metabolism that hydrolyze sphingomyelin (SM) and generate ceramide (Cer). The purpose of this study was to examine relationships between ocular surface SMases, SPL composition, and parameters of Meibomian gland dysfunction (MGD).

Methods: Individuals were grouped by meibum quality (n = 25 with poor-quality, MGD, and n = 25 with good-quality, control). Meibum and tears were analyzed with LC-MS to quantify SPL classes: Cer, Hexosyl-Ceramide (Hex-Cer), SM, Sphingosine (Sph), and sphingosine 1-phosphate (S1P). SMase activity in tears were quantified using a commercially available 'SMase assay'. Statistical analysis included multiple linear regression analyses to assess the impact of SMase activity on lipid composition, as well as ocular surface symptoms and signs of MGD.

Results: Demographic characteristics were similar between the two groups. nSMase and aSMase levels were lower in the poor vs good quality group. aSMase activity in tears negatively correlated with SM in meibum and tears and positively with Sph in meibum and S1P in tears. Lower SMase activity were associated with signs of MGD, most notably Meibomian gland dropout.

Conclusion: This study suggests that individuals with MGD have reduced enzymatic activity of SMases in tears. Specifically, individuals with poor vs good meibum quality were noted to have alterations in SMase activity and SPL composition of meibum and tears which may reflect deviations from normal lipid metabolism in individuals with MGD.

Keywords: Ceramide; Meibomian gland dysfunction; Sphingolipid; Sphingomyelin; Sphingomyelinases; Sphingosine 1-phosphate; Tear film.

Fig. 1.. Overview of Bioactive Sphingolipid Metabolism from Sphingomyelins.

Sphingomyelins, hexosylceramides, cerebrosides and gangliosides are complex membrane sphingolipids synthesized from Ceramide, the key molecule of sphingolipid metabolism in a cell. The major catabolic pathway of ceramide generation in a cell is from the hydrolysis of Sphingomyelin by Sphingomyelinases (neutral: nSMase, acidic: aSMase). Ceramide kinase phosphorylates ceramide to Ceramide 1-phosphate, and Ceramidases hydrolyze ceramide to Sphingosine. Sphingosine is phosphorylated to Sphingosine 1-phosphate (S1P) by Sphingosine kinases. Ceramide and sphingosine are overall pro-apoptotic lipids, whereas S1P has overall antiapoptotic effects, and a balance of their activity is called sphingolipid rheostat, important for cellular homeostasis.