Skin barrier fine-tuning through low-temperature lipid chain transition

The lipids in the mammalian stratum corneum (SC) adopt an unusually rigid arrangement to form a vital barrier preventing water loss and harmful environmental impacts. Just above the physiological temperature, a subset of barrier lipids undergoes a phase transition from a very tight orthorhombic to a looser hexagonal arrangement and vice versa. The purpose of this lipid transition in skin physiology is unknown. Permeability experiments on isolated human SC indicated that the transition affects the activation energy for a model compound that prefers lateral movement along lipid layers but not for water or a large polymer that would cross the SC via the pore pathway. The orthorhombic phase content of SC lipids, as determined by infrared spectroscopy, was also modulated by (de)hydration. Spontaneous rearrangement of human SC lipid monolayers into 10 nm higher multilamellar islets at 32 – 37°C, but not at room temperature, was revealed by atomic force microscopy. Our findings add to our knowledge of fundamental skin physiology suggesting a fine temperature- and hydration-controlled switch from fluid lipids required for lipid barrier assembly to rigid and tightly packed lipids in the mature SC necessary for the water and permeability barriers.

哺乳动物角质层（stratum corneum，SC）中的脂质会以异常刚性的排列方式形成关键屏障，防止水分流失并抵御外界有害因素的侵袭。略高于生理温度时，一部分屏障脂质会发生相变：从紧密的正交晶相（orthorhombic）转变为更为松散的六角晶相（hexagonal），该过程亦可逆向进行。目前学界尚未明确皮肤生理学中此类脂质相变的生理功能。针对分离得到的人角质层开展的渗透性实验表明，该相变会影响一种偏好沿脂质层侧向扩散的模型化合物的活化能，但对水或通过孔隙通路跨越角质层的大型聚合物则无此调控作用。通过红外光谱（infrared spectroscopy）检测发现，角质层脂质的正交晶相占比同样会受到水合与脱水过程的调控。原子力显微镜（atomic force microscopy）观测显示，人角质层脂质单分子层会在32~37℃下自发重排为高度达10 nm的多层膜微岛结构，但在室温下不会发生该现象。本研究结果深化了我们对皮肤基础生理学的认知，表明存在一种受温度与水合状态精密调控的相变开关：从脂质屏障组装所需的流动性脂质，切换为成熟角质层中维持水屏障与渗透屏障功能所必需的刚性致密排列脂质。