Delamere Nicholas A, Shahidullah Mohammad
Department of Physiology, University of Arizona, Tucson, AZ, United States.
Front Physiol. 2022 Jan 31;12:834916. doi: 10.3389/fphys.2021.834916. eCollection 2021.
Aside from a monolayer of epithelium at the anterior surface, the lens is formed by tightly compressed multilayers of fiber cells, most of which are highly differentiated and have a limited capacity for ion transport. Only the anterior monolayer of epithelial cells has high Na, K-ATPase activity. Because the cells are extensively coupled, the lens resembles a syncytium and sodium-potassium homeostasis of the entire structure is largely dependent on ion transport by the epithelium. Here we describe recent studies that suggest TRPV4 and TRPV1 ion channels activate signaling pathways that play an important role in matching epithelial ion transport activity with needs of the lens cell mass. A TRPV4 feedback loop senses swelling in the fiber mass and increases Na, K-ATPase activity to compensate. TRPV4 channel activation in the epithelium triggers opening of connexin hemichannels, allowing the release of ATP that stimulates purinergic receptors in the epithelium and results in the activation of Src family tyrosine kinases (SFKs) and SFK-dependent increase of Na, K-ATPase activity. A separate TRPV1 feedback loop senses shrinkage in the fiber mass and increases NKCC1 activity to compensate. TRPV1 activation causes calcium-dependent activation of a signaling cascade in the lens epithelium that involves PI3 kinase, ERK, Akt and WNK. TRPV4 and TRPV1 channels are also evident in the ciliary body where Na, K-ATPase is localized on one side of a bilayer in which two different cell types, non-pigmented and pigmented ciliary epithelium, function in a coordinated manner to secrete aqueous humor. TRPV4 and TRPV1 may have a role in maintenance of cell volume homeostasis as ions and water move through the bilayer.
除了前表面的单层上皮细胞外,晶状体由紧密压缩的多层纤维细胞组成,其中大多数细胞高度分化,离子转运能力有限。只有前单层上皮细胞具有高钠钾ATP酶活性。由于细胞广泛连接,晶状体类似于一个合胞体,整个结构的钠钾稳态在很大程度上依赖于上皮细胞的离子转运。在这里,我们描述了最近的研究,这些研究表明TRPV4和TRPV1离子通道激活了信号通路,这些通路在使上皮离子转运活性与晶状体细胞团的需求相匹配方面发挥着重要作用。一个TRPV4反馈环感知纤维团的肿胀并增加钠钾ATP酶活性以进行补偿。上皮细胞中TRPV4通道的激活触发连接蛋白半通道的开放,允许ATP释放,ATP刺激上皮细胞中的嘌呤能受体,导致Src家族酪氨酸激酶(SFKs)的激活以及SFK依赖的钠钾ATP酶活性增加。另一个TRPV1反馈环感知纤维团的收缩并增加NKCC1活性以进行补偿。TRPV1的激活导致晶状体上皮细胞中涉及PI3激酶、ERK、Akt和WNK的信号级联的钙依赖性激活。TRPV4和TRPV1通道在睫状体中也很明显,在那里钠钾ATP酶位于双层的一侧,其中两种不同类型的细胞,非色素睫状上皮和色素睫状上皮,以协调的方式发挥作用以分泌房水。当离子和水通过双层时,TRPV4和TRPV1可能在维持细胞体积稳态中发挥作用。