Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada.
Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada.
Biochim Biophys Acta Biomembr. 2017 Mar;1859(3):319-330. doi: 10.1016/j.bbamem.2016.12.010. Epub 2016 Dec 16.
The tear film lipid layer is formed on the anterior surface of the eye, functioning as a barrier to excess evaporation and foreign particles, while also providing stability to the tear film. The lipid layer is organized into a polar lipid layer consisting of phospholipids, ceramides, and free fatty acids that act as a surfactant to a non-polar multilayer of wax and cholesterol esters. Due to shear forces from eye movement and the compression and expansion of blinking, the tear lipids are under constant stress. However, tear film is able to resist immediate rupture and remains intact over multiple blinks. This work aimed to better understand the lateral organization of selected tear film polar lipids. The polar lipid biomimetic studied here consisted of dipalmitoyl phosphatidylcholine (DPPC), dipalmitoyl phosphatidylethanolamine (DPPE), palmitoyl glucosylceramide (PGC), and palmitoyl sphingomyelin (PSM). Surface pressure-area isocycles mimicked blinking and films were visualized by Brewster angle microscopy (BAM). All lipid systems formed relatively reversible films as indicated by limited hysteresis. However, pure DPPC and PSM films experienced greater changes in lipid packing upon compression and expansion compared to pure PGC and DPPE. This suggests that the driving force behind maintaining the lateral organization of the polar lipids from tear film may be the hydrogen bonding propensities of the head groups. Additionally, isocycles of films containing DPPC, DPPE, and PGC mixtures exhibited evidence for reversible multilayer formation or folding. This was supported by 3D analysis of structures that formed during compression but reintegrated back into the bulk lipid film during expansion near the in vitro tear film surface pressure of the open eye. Therefore, the polar lipids of tear film may be directly involved in preventing film rupture during a blink.
泪膜的脂质层位于眼球的前表面，起到阻止过多蒸发和外来颗粒的作用，同时为泪膜提供稳定性。脂质层组织成一个极性脂质层，由磷脂、神经酰胺和游离脂肪酸组成，它们作为表面活性剂作用于非极性的多层蜡质和胆固醇酯。由于眼球运动的剪切力以及眨眼时的压缩和扩张，泪液脂质处于持续的压力下。然而，泪膜能够抵抗立即破裂，并在多次眨眼后保持完整。本研究旨在更好地了解选定的泪膜极性脂质的侧向组织。这里研究的极性脂质仿生体系由二棕榈酰磷脂酰胆碱（DPPC）、二棕榈酰磷脂酰乙醇胺（DPPE）、棕榈酰葡萄糖神经酰胺（PGC）和棕榈酰鞘氨醇（PSM）组成。表面压力-面积等周环模拟眨眼，并用布鲁斯特角显微镜（BAM）观察薄膜。所有脂质体系都形成了相对可逆的薄膜，这表明滞后现象有限。然而，与纯 PGC 和 DPPE 相比，纯 DPPC 和 PSM 薄膜在压缩和扩张时脂质堆积的变化更大。这表明维持泪膜极性脂质侧向组织的驱动力可能是头基的氢键倾向。此外，含有 DPPC、DPPE 和 PGC 混合物的薄膜的等周环显示出可逆多层形成或折叠的证据。这得到了在体外泪膜表面压力接近睁眼时，压缩过程中形成的结构的 3D 分析的支持，这些结构在扩展过程中重新整合到大块脂质膜中。因此，泪膜的极性脂质可能直接参与防止眨眼时膜破裂。
