Iron uptake and transport (Bos taurus)
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The transport of iron between cells is mediated by transferrin. However, iron can also enter and leave cells not only by itself, but also in the form of heme and siderophores. When entering the cell via the main path (by transferrin endocytosis), its goal is not the (still elusive) chelated iron pool in the cytosol nor the lysosomes but the mitochondria, where heme is synthesized and iron-sulfur clusters are assembled (Kurz et al,2008, Hower et al 2009, Richardson et al 2010).
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Genetic variations in SLC17A3 influence the variance in serum uric acid concentrations and define the serum uric acid concentration quantitative trait locus 4 (UAQTL4; MIM:612671). Excess serum urate (hyperuricemia) can lead to the development of gout, characterized by tissue deposition of monosodium urate crystals.
CLCNKA and B (Kieferle et al. 1994) are predominantly expressed in distal nephron segments of the kidney (Takeuchi et al. 1995) and the inner ear (Estevez et al. 2001, Schlingmann et al. 2004). They are tightly associated with their essential beta subunit barttin (BSND), requiring it to be fully functional channels (Fischer et al. 2010, Scholl et al. 2006). These channels bound to BSND are essential for renal Cl- reabsorption (Waldegger & Jentsch 2000) and K+ recycling in the inner ear (Estevez et al. 2001). Defects in CLCNKA and B cause Bartter syndrome type 4B (BS4B; MIM:613090) characterized by impaired salt reabsorption and sensorineural deafness (Schlingmann et al. 2004, Nozu et al. 2008). Defects in BSND cause Bartter syndrome type 4A (BS4A aka infantile Bartter syndrome with sensorineural deafness; MIM:602522) characterized by impaired salt reabsorption in the thick ascending loop of Henle and sensorineural deafness (Birkenhager et al. 2001, Nozu et al. 2008).
A member of the intracellular Cl- channel protein family, CLIC2, has also been determined to inhibit RYR-mediated Ca2+ transport (Board et al. 2004), potentially playing a role in the homeostasis of Ca2+ release from intracellular stores. Inhibition is thought to be via reducing activation of the channels by their primary endogenous cytoplasmic ligands, ATP and Ca2+ (Dulhunty et al. 2005).
Defects in CLCN7 cause osteopetrosis autosomal recessive types 2 and 4 (OPTB2, MIM:166600 and OPTB4, MIM:611490) (Frattini et al. 2003, Pangrazio et al. 2010). Defects in OSTM1 cause osteopetrosis autosomal recessive type 5 (OPTB5, MIM:259720) (Pangrazio et al. 2006).
The human gene SLC11A2 encodes the divalent cation transporter DCT1 (NRAMP2, Natural resistance-associated macrophage protein 2). NRAMP2 resides on the apical membrane of enterocytes and mediates the uptake of ferrous iron into these cells (Tandy S et al, 2000). DCT1 can also accept a broad range of transition metal ions.
The human gene SLC40A1 encodes a metal transporter protein MTP1 (also called ferroportin or IREG1). This protein resides on the basolateral membrane of enterocytes and mediates ferrous iron efflux into the portal vein (Schimanski LM et al, 2005). MTP1 colocalizes with hephaestin (HEPH) which stablizes MTP1 and is necessary for the efflux reaction to occur (Han O and Kim EY, 2007; Chen H et al, 2009). As well as the dudenum, MTP1 is also highly expressed on macrophages (where it mediates iron efflux from the breakdown of haem) and the placenta (where it may mediate the transport of iron from maternal to foetal circulation). It is also expressed in muscle and spleen.
The human gene SLC40A1 encodes a metal transporter protein MTP1 (also called ferroportin or IREG1) (Schimanski LM et al, 2005). MTP1 colocalizes with ceruloplasmin (CP) which stablizes MTP1 and is necessary for the efflux reaction to occur (Texel SJ et al, 2008). Ceruloplasmin also catalyzes the conversion of iron from ferrous (Fe²⁺) to ferric form (Fe³⁺), therefore assisting in its transport in the plasma in association with transferrin, which can only carry iron in the ferric state. As well as on macrophages, MTP1 is also highly expressed in the duodenum, placenta (where it may mediate the transport of iron from maternal to foetal circulation), in muscle and the spleen.
Four human genes encode the catalytic alpha subunits, ATP1A1-4 (Kawakami et al, 1986; Shull et al, 1989; Ovchinnikov et al, 1988; Keryanov and Gardner, 2002). Defects in ATP1A2 cause alternating hemiplegia of childhood (AHC) (Swoboda et al, 2004). Another defect in ATP1A2 causes familial hemiplegic migraine type 2 (FHM2) (Vanmolkot et al, 2003). Defects in ATP1A3 are the cause of dystonia type 12 (DYT12) (de Carvalho Aguiar et al, 2004).
Three human genes encode the non-catalytic beta subunits, ATP1B1-3. The beta subunits are thought to mediate the number of sodium pumps transported to the plasma membrane (Lane et al, 1989; Ruiz et al, 1996; Malik et al, 1996). FXYD proteins belong to a family of small membrane proteins that are auxiliary gamma subunits of Na-K-ATPase. At least six members of this family, FYD1-4, 6 and 7, have been shown to regulate Na-K-ATPase activity (Geering 2006, Choudhury et al. 2007). Defects in FXYD2 are the cause of hypomagnesemia type 2 (HOMG2) (Meij et al, 2000).