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بسم الله الرحمن الرحيم

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بسم الله الرحمن الرحيم. Aquaporins. The membrane water channels of the biological world. Aquaporins (AQPs) are a family of proteins that form channels to facilitate the transport of water across cell membranes.

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Presentation Transcript
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Aquaporins

The membrane water channels

of the biological world

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Aquaporins (AQPs) are a family of proteins that form channels to facilitate the transport of water across cell membranes.
  • The classical aquaporins transport solute-free water across cell membranes; they are exclusive water channels and do not permeate membranes to ions or other small molecules.
  • Some aquaporins - known as aquaglyceroporins- transport water plus glycerol and a few other small molecules as CO2, ammonia and urea across the membrane, depending on the size of the pore.
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There are thirteen known types of aquaporins in mammals, and six of these are located in the kidney, but the existence of many more is suspected.
  • Single human aquaporin-1 channel facilitates water transport at a rate of roughly 3 billion water molecules per second. Such transport appears to be bidirectional, in accordance with the prevailing osmotic gradient.
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‘hour-glass model’, characterized by wide external openings to

the channel with a narrow central constriction where the

NPA motifs interact, forming the functional water pore.

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Aquaporin proteins are made up of six transmembraneα-helices arranged in a right-handed bundle, with the amino and the carboxyl termini located on the cytoplasmic surface of the membrane. The amino and carboxyl halves of the sequence show strong similarity to each other.
  • There are also five interhelical loop regions (A – E) that form the extracellular and cytoplasmic vestibules.
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Loops B and E are hydrophobic loops which contain the Aspargine-Proline-Alanine (NPA) motif, which overlap the middle of the lipid bilayer of the membrane forming a 3-D \'hourglass\' structure where the water flows through.
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Aquaporins form tetramers in the cell membrane, with each monomer acting as a water channel.
  • The different aquaporins contain differences in their peptide sequence which allows for the size of the pore in the protein to differ between aquaporins. The resultant size of the pore directly affects what molecules are able to pass through the pore.
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Aquaporins

and

skin

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Up to 6 different AQPs (AQP1, 3, 5, 7, 9, 10) may be selectively expressed in human skin cells.
  • Whereas AQP1 and AQP5 are strictly water channels,

AQP3, 7, 9, and 10 are permeable to both water and glycerol.

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AQP1 mRNA was detected in dermal endothelial cells, dermal fibroblasts, and melanocytes.
  • Basal keratinocytes and monocyte-derived dendritic cells (MDDCs) exhibit AQP3 mRNA.
  • Sweat gland cells express AQP5.
  • Differentiated preadipocytes expressed AQP7 mRNA.
  • AQP9 is expressed in diffrentiated keratinocytes, monocytes, (MDDCs) and preadipocytes.
  • AQP10 mRNA was detected in keratinocytes, monocytes.
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In the healthy epidermis AQP3 is mainly found in the stratum basale with decreasing expression towards the stratum granulosum. Thus, a gradient is formed.
  • No AQP3 is found in the SC.
  • The AQP3 gradient may lead to a corresponding water gradient in the epidermis, with a sharp decrease of water content in the SC. In this way, water loss is prevented at the same time as the living layers of the epidermis are optimally hydrated. This isolates SC from the living tissue lower in the epidermis and contributes to skin barrier function.
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The AQP3-induced water gradient may be strengthened by another gradient formed by pH that also decreases in the SC; as AQP3 is inhibited by low pH the water transporting capacity of AQP3 is decreased in upper layers of the epidermis.
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The expressions of AQP3 water channels are strongly affected by age and chronic sun exposure defective osmotic equilibrium in the epidermis skin dryness in older people and skin areas most exposed to sunlight.
  • Water transport through both aquaporins and aquaglyceroporins and glycerol transport through aquaglyceroporins alone are important to skin hydration.
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AQPs appear to be key protein targets to improve the resistance and quality of the skin surface through their roles in barrier formation and recovery.
  • AQP3 expression in human skin is increased in response to skin stress in diseases such as atopic eczema, to various agents such as retinoic acid, and in skin carcinomas.
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Aquaporins &

atopic dermatitis

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Dry skin is a major contributary factor to the pathogenesis of AE. Dryness impaired barrier function with increased antigen absorption and cutaneous hyper-reactivity.
  • Dry skin in atopic eczema depends on increased water loss. Increased water loss is enhanced by abnormal lipid composition, with both reduction in the amount of ceramides and altered distribution between the different ceramide types in the SC. Also the level of cholesterol is increased.

(Increased water transport to the SC which has reduced water-holding capacity in AE.)

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Increased expression of AQP3 was found in eczema compared with healthy skin.
  • In healthy skin, epidermal AQP3 is mainly found in the stratum basale. A gradient was formed with decreasing AQP3 staining in the lower layers of the stratum spinosum.
  • Increased expression and altered cellular distribution of AQP3 is found in eczema and this may contribute to water loss.
  • In atopic dermatitis AQP3 was found in both the stratum basale and the stratum spinosum. The staining was more pronounced in the stratum spinosum.
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Expression of AQP1 is regulated by glucocorticoids,

AQP3 by p73 and

AQP5 by tumour necrosis factor-alpha.

  • These findings indicate that AQP-mediated water transport may be an integral part of inflammatory processes.
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Aquaporins &

cancer biology

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AQPs are strongly expressed in tumor cells of different origins, particularly aggressive tumors.
  • AQP involvement in cell migration and proliferation suggest that AQPs play key roles in tumor biology.
  • AQP1 is expressed in tumor vascular endothelium, and AQP1-null mice show defective tumor angiogenesis due to impaired endothelial cell migration.
  • AQP-expressing cancer cells show enhanced migration in vitro and greater local tumor invasion, tumor cell metastases in vivo. AQP-dependent cell migration may involve AQP-facilitated water influx into lamellipodia at the front edge of migrating cells.
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The aquaglyceroporin AQP3, which is found in normal epidermis and becomes upregulated in basal cell carcinoma, facilitates cell proliferation in different cell types.
  • Remarkably, AQP3-null mice are resistant to skin tumorigenesis by a mechanism that may involve reduced tumor cell glycerol metabolism and ATP generation. Together, the data suggest that AQP expression in tumor cells and tumor vessels facilitates tumor growth and spread, suggesting AQP inhibition as a novel antitumor therapy.
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