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Secondary (A), Tertiary (B), and Quaternary (C) depictions of an aquaporin channel

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AQUAPORIN 4[edit]

Aquaporin-4,also known as AQP4,is a water channel protein encoded by the AQP4 genein humans.[1]AQP4 belongs to the aquaporinfamily of integral membrane proteinsthat conduct water through the cell membrane. A limited number of aquaporins are found within the central nervous system(CNS): AQP1, 3, 4, 5, 8, 9, and 11, but more exclusive representation of AQP1, 4, and 9 are found in the brain and spinal cord.[2] AQP4 SHOWS THE LARGEST PRESENCE IN THE CEREBELLUM AND SPINAL CORD GREY MATTER.[3] In the CNS, AQP4 is the most prevalent aquaporin channel, specifically located at the perimicrovessel astrocytefoot processes, glia limitans, and ependyma.[4] IN ADDITION, THIS CHANNEL IS COMMONLY FOUND FACILITATING WATER MOVEMENT NEAR THE CSF AND VASCULATURE.[5]

Aquaporin-4 was first identified in 1986. It was the first evidence of the existence of water transport channels.[6] The method that was used to discover the existence of the transport channels was through knockout experiments. With this technique they were able to show the significant role of AQP4 in CNS injuries and brain water imbalances.[2] IN 1994 THE CHANNEL WAS SUCCESSFULLY CLONED AND INITIALLY NAMED MERCURY-INSENSITIVE WATER CHANNEL.[7]

Structure[edit]

The structure of AQP4 consists of six-transmembrane domains and five connecting loops to form the channel. Through x-ray crystallography, it was found that “each AQP4 monomer consists of six helical, membrane-spanning domains and two short helical segments surrounding a narrow aqueous pore.”[8] AT THE NARROWEST POINT THE AQUEOUS PORE MEASURES 2.8 Å, JUST LARGE ENOUGH FOR THE SINGLE-FILE PASSAGE OF WATER MOLECULES. WHILE EACH MONOMER IS INDIVIDUALLY CAPABLE OF WATER TRANSPORT, THE QUATERNARY STRUCTURE OF THE CHANNEL IS A TETRAMER.[3] THE ASSEMBLY OF AQP4 MONOMERS into tetramers is similar to other aquaporin channels.[9] In addition, AQP4 has two distinct structural isoforms located in the CNS: M1 and M23.[2]Both form homo- and hetero-tetramers that are permeable to water.[2]M23 isoforms are larger square arrays in the endfoot membranes of astrocytes compared to M1 isoforms, which are smaller and more unstable. The aquaporin-4 tetramers accumulate to transform into orthogonal arrays of particle (OAPs) in the cell plasma membrane.[8]

Tissue and cellular distribution[edit]

AQUAPORIN-4 IS THE MOST COMMON AQUAPORIN IN THE BRAIN, SPINAL CORD, AND OPTIC NERVE.[7] IT is highly expressed in the human body primarily at the end-feet of astrocytes.[8] Additionally, AQP4 can also be located in epithelial cells of many organs throughout the human body, such as the kidney, intestine, salivary glands, sensory organs, and skeletal muscles.[6]In these specific cases of epithelial cell expression, AQP4 is concentrated within the basolateral membrane layerof these locations.[9]

Furthermore, AQP4 also plays a role in the supportive cells of sensory organs, such as the retina, inner ear, and olfactory epithelium.[8]Within the retina, AQP4 is highly concentrated where the processes of Muller cellshave a basal laminaaround blood vessels and inner limiting membrane[6]and to a lesser degree in the inner and outer plexiform layers.[10]

AQP4 is also expressed in astrocytesand is upregulatedby direct insult to the central nervous system.[11]Specifically within the central nervous system (CNS), AQP4 can be found along the spinal cord and serves as the main water channel.[2]The AQP4 channels are highly concentrated in the blood-brain barrier(BBB), as well as in other cerebrospinal fluid barriers.[12]

In the kidneys, AQP4 is PRIMARILY FOUND IN THE INNER MEDULLA, AND SHOWS LITTLE TO NO PRESENCE IN THE OUTER MEDULLA AND CORTEX.[13] IT IS constitutively expressed in the basolateral cell membraneof principal collecting ductcells and provide a pathway for water to exit these cells.[14]

Function[edit]

Aquaporin-4’s overall function is to provide fast water transportation as well as maintain homeostatic balance within the central nervous system. THIS CHANNEL CAN TRANSPORT WATER UP TO SPEEDS OF 3E9 MOLECULES PER SECOND.[3] It is the primary water channel protein that reconciles the homeostasis of water in the CNS.[2]AQP4 may be involved in a variety of physiological processes such as waste removal (glymphatic system) and fine-tuning of potassium homeostasis.[12]Water flowing into and out of the brain or spinal cord is assisted by AQP4.[2]Here, AQP4 channels respond passively to osmotic gradients. In addition, they play a role in brain water transport, cell migration, brain edema, metabolism and cell homeostasis.[15]

Other systems are also regulated by AQP4. Within the inner ear, the main role is to provide osmotic balance in supporting epithelium cells within the organ of Cortiby recycling K+.[6]Another specific role AQP4 plays is to help odorant molecules bind to target receptors and binding proteins within olfactory epithelium.[6]Within the retina, the role of AQP-4 is to maintain homeostasis.[6]Aquaporin-4 is essential in the formation of memory as well as synaptic plasticity.[12]Other performances that aquaporin-4 is involved in are synaptic plasticity, astrocyte migration, regulation of extracellular space volume, and the homeostasis of potassium.[12]


Previous information copied from Aquaporin 4


[3][16][13][7]

Original References (1-11) My References (3,5,7,13,) 16 bad[edit]

  1. ^ Jung JS, Bhat RV, Preston GM, Guggino WB, Baraban JM, Agre P (December 1994). "Molecular characterization of an aquaporin cDNA from brain: candidate osmoreceptor and regulator of water balance". Proceedings of the National Academy of Sciences of the United States of America. 91 (26): 13052–6. Bibcode:1994PNAS...9113052J. doi:10.1073/pnas.91.26.13052. PMC 45579. PMID 7528931.
  2. ^ a b c d e f g Oklinski MK, Skowronski MT, Skowronska A, Rützler M, Nørgaard K, Nieland JD, et al. (December 2016). "Aquaporins in the Spinal Cord". International Journal of Molecular Sciences. 17 (12): 2050. doi:10.3390/ijms17122050. PMC 5187850. PMID 27941618.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  3. ^ a b c d Halsey, Andrea; Conner, Alex; Bill, Roslyn; Logan, Ann; Ahmed, Zubair (2018-10-18). "Aquaporins and Their Regulation after Spinal Cord Injury". Cells. 7 (10): 174. doi:10.3390/cells7100174. ISSN 2073-4409.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  4. ^ Saadoun S, Papadopoulos MC (July 2010). "Aquaporin-4 in brain and spinal cord oedema". Neuroscience. 168 (4): 1036–46. doi:10.1016/j.neuroscience.2009.08.019. PMID 19682555.
  5. ^ Hubbard, Jacqueline A.; Hsu, Mike S.; Seldin, Marcus M.; Binder, Devin K. (2015-10-15). "Expression of the Astrocyte Water Channel Aquaporin-4 in the Mouse Brain". ASN Neuro. 7 (5): 175909141560548. doi:10.1177/1759091415605486. ISSN 1759-0914.
  6. ^ a b c d e f Gleiser C, Wagner A, Fallier-Becker P, Wolburg H, Hirt B, Mack AF (August 2016). "Aquaporin-4 in Astroglial Cells in the CNS and Supporting Cells of Sensory Organs-A Comparative Perspective". International Journal of Molecular Sciences. 17 (9): 1411. doi:10.3390/ijms17091411. PMC 5037691. PMID 27571065.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  7. ^ a b c Mader, Simone; Brimberg, Lior (2019-01-27). "Aquaporin-4 Water Channel in the Brain and Its Implication for Health and Disease". Cells. 8 (2): 90. doi:10.3390/cells8020090. ISSN 2073-4409.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  8. ^ a b c d Verkman AS, Phuan PW, Asavapanumas N, Tradtrantip L (November 2013). "Biology of AQP4 and anti-AQP4 antibody: therapeutic implications for NMO". Brain Pathology. 23 (6): 684–95. doi:10.1111/bpa.12085. PMC 3890327. PMID 24118484.
  9. ^ a b Chu H, Huang C, Ding H, Dong J, Gao Z, Yang X, et al. (August 2016). "Aquaporin-4 and Cerebrovascular Diseases". International Journal of Molecular Sciences. 17 (8): 1249. doi:10.3390/ijms17081249. PMC 5000647. PMID 27529222.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  10. ^ Nagelhus EA, Veruki ML, Torp R, Haug FM, Laake JH, Nielsen S, et al. (April 1998). "Aquaporin-4 water channel protein in the rat retina and optic nerve: polarized expression in Müller cells and fibrous astrocytes". The Journal of Neuroscience. 18 (7): 2506–19. doi:10.1523/JNEUROSCI.18-07-02506.1998. PMID 9502811.
  11. ^ Nagelhus EA, Mathiisen TM, Ottersen OP (2004). "Aquaporin-4 in the central nervous system: cellular and subcellular distribution and coexpression with KIR4.1". Neuroscience. 129 (4): 905–13. doi:10.1016/j.neuroscience.2004.08.053. PMID 15561407.
  12. ^ a b c d Hubbard JA, Szu JI, Binder DK (January 2018). "The role of aquaporin-4 in synaptic plasticity, memory and disease". Brain Research Bulletin. 136: 118–129. doi:10.1016/j.brainresbull.2017.02.011. PMID 28274814.
  13. ^ a b Terris, J.; Ecelbarger, C. A.; Marples, D.; Knepper, M. A.; Nielsen, S. (1995-12-01). "Distribution of aquaporin-4 water channel expression within rat kidney". American Journal of Physiology-Renal Physiology. 269 (6): F775–F785. doi:10.1152/ajprenal.1995.269.6.f775. ISSN 1931-857X.
  14. ^ Agre P, Nielsen S (1996). "The aquaporin family of water channels in kidney". Nephrologie. 17 (7): 409–15. PMID 8987045.
  15. ^ Desai B, Hsu Y, Schneller B, Hobbs JG, Mehta AI, Linninger A (September 2016). "Hydrocephalus: the role of cerebral aquaporin-4 channels and computational modeling considerations of cerebrospinal fluid". Neurosurgical Focus. 41 (3): E8. doi:10.3171/2016.7.FOCUS16191. PMID 27581320.
  16. ^ Zeidel, Mark L.; Nielsen, Soren; Smith, Barbara L.; Ambudkar, Suresh V.; Maunsbach, Arvid B.; Agre, Peter (1994-02-15). "Ultrastructure, Pharmacologic Inhibition, and Transport Selectivity of Aquaporin CHIP in Proteoliposomes". Biochemistry. 33 (6): 1606–1615. doi:10.1021/bi00172a042. ISSN 0006-2960.
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