Effects of osmolality on the expression of brain aquaporins in AQP11-null mice

Aquaporins Display a Diversity in their Substrates

Aquaporins constitute a family of transmembrane proteins that function to transport water and other small solutes across the cell membrane. Aquaporins family members are found in diverse life forms. Aquaporins share the common structural fold consisting of six transmembrane alpha helices with a central water-transporting channel. Four such monomers assemble together to form tetramers as their biological unit. Initially, aquaporins were discovered as water-transporting channels, but several studies supported their involvement in mediating the facilitated diffusion of different solutes. The so-called water channel is able to transport a variety of substrates ranging from a neutral molecule to a charged molecule or a small molecule to a bulky molecule or even a gas molecule. This article gives an overview of a diverse range of substrates conducted by aquaporin family members. Prime focus is on human aquaporins where aquaporins show a wide tissue distribution and substrate specificity leading to various physiological functions. This review also highlights the structural mechanisms leading to the transport of water and glycerol. More research is needed to understand how one common fold enables the aquaporins to transport an array of solutes.

Aquaporin 11 alleviates retinal Müller intracellular edema through water efflux in diabetic retinopathy

Retinal Müller glial dysfunction and intracellular edema are important mechanisms leading to diabetic macular edema (DME). Aquaporin 11 (AQP11) is primarily expressed in Müller glia with unclear functions. This study aims to explore the role of AQP11 in the pathogenesis of intracellular edema of Müller glia in diabetic retinopathy (DR). Here, we found that AQP11 expression, primarily located at the endfeet of Müller glia, was down-regulated with diabetes progression, accompanied by intracellular edema, which was alleviated by intravitreal injection of lentivirus-mediated AQP11 overexpression. Similarly, intracellular edema of hypoxia-treated rat Müller cell line (rMC-1) was aggravated by AQP11 inhibition, while attenuated by AQP11 overexpression, accompanied by enhanced function in glutamate metabolism and reduced cell death. The down-regulation of AQP11 was also verified in the Müller glia from the epiretinal membranes (ERMs) of proliferative DR (PDR) patients. Mechanistically, down-regulation of AQP11 in DR was mediated by the HIF-1α-dependent and independent miRNA-AQP11 axis. Overall, we deciphered the AQP11 down-regulation, mediated by miRNA-AQP11 axis, resulted in Müller drainage dysfunction and subsequent intracellular edema in DR, which was partially reversed by AQP11 overexpression. Our findings propose a novel mechanism for the pathogenesis of DME, thus targeting AQP11 regulation provides a new therapeutic strategy for DME.

Aquaporins in Nervous System

Aquaporins (AQPs) mediate water flux between the four distinct water compartments in the central nervous system (CNS). In the present chapter, we mainly focus on the expression and function of the nine AQPs expressed in the CNS, which include five members of aquaporin subfamily: AQP1, AQP4, AQP5, AQP6, and AQP8; three members of aquaglyceroporin subfamily: AQP3, AQP7, and AQP9; and one member of superaquaporin subfamily: AQP11. In addition, AQP1, AQP2, and AQP4 expressed in the peripheral nervous system are also reviewed. AQP4, the predominant water channel in the CNS, is involved both in the astrocyte swelling of cytotoxic edema and the resolution of vasogenic edema and is of pivotal importance in the pathology of brain disorders such as neuromyelitis optica, brain tumors, and neurodegenerative disorders. Moreover, AQP4 has been demonstrated as a functional regulator of recently discovered glymphatic system that is a main contributor to clearance of toxic macromolecule from the brain. Other AQPs are also involved in a variety of important physiological and pathological process in the brain. It has been suggested that AQPs could represent an important target in treatment of brain disorders like cerebral edema. Future investigations are necessary to elucidate the pathological significance of AQPs in the CNS.