pH-Dependent channel activity of heterologously-expressed main intrinsic protein (MIP) from rat lens

Phylogenetic and specific sequence analysis of four paralogs in insect Aquaporins

Aquaporins (AQP) are proteins that form channels to facilitate the movement of water across cell membranes in plants, bacteria and animals. Insect AQPs are indispensable for cellular water management under stress, including dehydration and cold. To better understand the biological significance of molecular evolution of gene sequences, followed by structural and functional specialization, the present study used ClustalX2.1, MEGA7.0, Jalview and Mesquite software to build an insect AQP phylogenetic tree and visualize the evolutionary associations among insect AQPs. It was demonstrated that 45 AQPs were classified as four major paralogs with each amino acid sequence containing two conserved NPA (Asp‑Pro‑Ala) motifs located in the center and C‑terminal domains, and other residues conserved within the paralogous groups, however not among them. All these differences in amino acid content may affect the structure, function and classification of the AQPs. The findings provide a basis for further study to understand insect AQPs through sequence comparison, structure and predicted function.

Aquaporins: Multiple Roles in the Central Nervous System

Aquaporins (AQPs) represent a diverse family of membrane proteins found in prokaryotes and eukaryotes. The primary aquaporins expressed in the mammalian brain are AQP1, which is densely packed in choroid plexus cells lining the ventricles, and AQP4, which is abundant in astrocytes and concentrated especially in the end-feet structures that surround capillaries throughout the brain and are present in glia limitans structures, notably in osmosensory areas such the supraoptic nucleus. Water movement in brain tissues is carefully regulated from the micro- to macroscopic levels, with aquaporins serving key roles as multifunctional elements of complex signaling assemblies. Intriguing possibilities suggest links for AQP1 in Alzheimer's disease, AQP4 as a target for therapy in brain edema, and a possible contribution of AQP9 in Parkinson's disease. For all the aquaporins, new contributions to physiological functions are likely to continue to be discovered with ongoing work in this rapidly expanding field of research. NEUROSCIENTIST 13(5):470—485, 2007.

Structural function of MIP/aquaporin 0 in the eye lens; genetic defects lead to congenital inherited cataracts

Aquaporin 0 (AQP0) was originally characterized as a membrane intrinsic protein, specifically expressed in the lens fibers of the ocular lens and designated MIP, for major intrinsic protein of the lens. Once the gene was cloned, an internal repeat was identified, encoding for the amino acids Asp-Pro-Ala, the NPA repeat. Shortly, the MIP gene family was emerging, with members being characterized in mammals, insects, and plants. Once Peter Agre's laboratory developed a functional assay for water channels, the MIP family became the aquaporin family and MIP became known as aquaporin 0. Besides functioning as a water channel, aquaporin 0 also plays a structural role, being required for maintaining the transparency and optical accommodation of the ocular lens. Mutations in the AQP0 gene in human and mice result in genetic cataracts; deletion of the MIP/AQP0 gene in mice results in lack of suture formation required for maintenance of the lens fiber architecture, resulting in perturbed accommodation and focus properties of the ocular lens. Crystallography studies support the notion of the double function of aquaporin 0 as a water channel (open configuration) or adhesion molecule (closed configuration) in the ocular lens fibers. The functions of MIP/AQP0, both as a water channel and an adhesive molecule in the lens fibers, contribute to the narrow intercellular space of the lens fibers that is required for lens transparency and accommodation.

A fascinating tail: cGMP activation of aquaporin-1 ion channels

Aquaporin-1 (AQP1) is a member of the diverse major intrinsic protein family of water and solute channels. AQP1 is known as an osmotic water channel in kidney, brain, vascular system and other tissues, and recently has been demonstrated to function as a cation channel gated by cGMP. Electrophysiology and binding assays implicate direct cGMP binding in the AQP1 C-terminus and sequence similarities with cyclic-nucleotide-gated channels support the idea that the AQP1 C-terminus mediates ion channel activation. In this article, new data show that the AQP1 C-terminus also exhibits homology, at key residues, with the substrate-selectivity subdomain of cyclic nucleotide phosphodiesterases. Distinct pathways for fluxes of water and ions in the tetrameric AQP1 channel indicate an intriguing multifunctional capacity. The physiological role of AQP1 in transmembrane signaling remains to be elucidated for these channels expressed in native tissues.