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Typus
KomplettPlagiat
Bearbeiter
Hindemith
Gesichtet
Yes.png
Untersuchte Arbeit:
Seite: 44, Zeilen: 15-47
Quelle: Hoenderop et al 2003
Seite(n): 1906, 1907, Zeilen: 1906: abstract, r.col: 10ff; 1907: l.col: 1ff
Transient receptor potential (TRP) cation channel subfamily V, members 5 and 6 (TRPV5 and TRPV6) have recently been postulated to be the molecular gatekeepers facilitating Ca2+ influx in these tissues and are members of the TRP family, which mediates diverse biological effects ranging from pain perception to male aggression. Genetic ablation of TRPV5 in the mouse allowed us to investigate the function of this novel Ca2+ channel in maintaining the Ca2+ balance. Here, we demonstrate that mice lacking TRPV5 display diminished active Ca2+ reabsorption despite enhanced vitamin D levels, causing severe hypercalciuria. In vivo micropuncture experiments demonstrated that Ca2+ reabsorption was malfunctioning within the early part of the distal convolution, exactly where TRPV5 is localized. In addition, compensatory hyperabsorption of dietary Ca2+was measured in TRPV5 knockout mice. Furthermore, the knockout mice exhibited significant disturbances in bone structure, including reduced trabecular and cortical bone thickness. These data demonstrate the key function of TRPV5 in active Ca2+ reabsorption and its essential role in the Ca2+ homeostasis.

In humans, the daily dietary Ca2+ intake is less than 1000 mg, of which only 30% is absorbed in the intestinal tract. This percentage is significantly enhanced during growth, pregnancy, and lactation by increased levels of circulating 1.25-(OH)2D3. Although there is continuous turnover of bone mass, there is no net gain or loss of Ca2+ from bone in a young and healthy individual. This indicates that healthy adults excrete a maximum of 300 mg Ca2+ in the urine to balance the intestinal Ca2+ uptake and that the remaining 98% of the Ca2+ filtered in the glomeruli is reabsorbed along the nephron. The molecular mechanism responsible for Ca2+ absorption in the small intestine and the kidney was elusive for a long time.

The cloning of transient receptor potential (TRP) cation channel subfamily V, member 5 (TRPV5; originally called ECaC1) from vitamin D–responsive rabbit renal epithelial cells and TRP cation channel subfamily V, member 6 (TRPV6; originally called CaT1) from rat duodenum has ignited research into transcellular Ca2+ (re)absorption at the molecular level. Mammals harbor at least 21 genes of the so-called TRP channels, whose functions remain mostly unknown. TRPV5 has been implicated as the Ca2+ influx channel in the process of vitamin D–responsive active Ca2+ reabsorption in the kidney. In comparison, the TRPV5 homolog TRPV6, which displays an amino acid sequence identity of about 75% to TRPV5, has been postulated to be the Ca2+ influx channel facilitating Ca2+ absorption in enterocytes. TRPV6 is ubiquitously expressed and has been implicated as part of the capacitative Ca2+ [entry mechanism and, therefore, intracellular Ca2+ signaling.]

Transient receptor potential (TRP) cation channel subfamily V, members 5 and 6 (TRPV5 and TRPV6) have recently been postulated to be the molecular gatekeepers facilitating Ca2+ influx in these tissues and are members of the TRP family, which mediates diverse biological effects ranging from pain perception to male aggression. Genetic ablation of TRPV5 in the mouse allowed us to investigate the function of this novel Ca2+ channel in maintaining the Ca2+ balance. Here, we demonstrate that mice lacking TRPV5 display diminished active Ca2+ reabsorption despite enhanced vitamin D levels, causing severe hypercalciuria. In vivo micropuncture experiments demonstrated that Ca2+ reabsorption was malfunctioning within the early part of the distal convolution, exactly where TRPV5 is localized. In addition, compensatory hyperabsorption of dietary Ca2+ was measured in TRPV5 knockout mice. Furthermore, the knockout mice exhibited significant disturbances in bone structure, including reduced trabecular and cortical bone thickness. These data demonstrate the key function of TRPV5 in active Ca2+ reabsorption and its essential role in the Ca2+ homeostasis.

[...]

[...] In humans, the daily dietary Ca2+ intake is less than 1,000 mg, of which only 30% is absorbed in the intestinal tract. This percentage is significantly enhanced during growth, pregnancy, and lactation by increased levels of circulating 1,25-(OH)2D3. Although there is continuous turnover of bone mass, there is no net gain or loss of Ca2+ from bone in a young and healthy individual. This indicates that healthy adults excrete a maximum of 300 mg Ca2+ in the urine to balance the intestinal Ca2+ uptake and that the remaining 98% of the Ca2+ filtered in the glomeruli is reabsorbed along the nephron. The molecular mechanism responsible for Ca2+ absorption in the small intestine and the kidney was elusive for a long time.

[page 1907]

The cloning of transient receptor potential (TRP) cation channel subfamily V, member 5 (TRPV5; originally called ECaC1) from vitamin D–responsive rabbit renal epithelial cells (1) and TRP cation channel subfamily V, member 6 (TRPV6; originally called CaT1) from rat duodenum (2) has ignited research into transcellular Ca2+ (re)absorption at the molecular level (1). Mammals harbor at least 21 genes of the so-called TRP channels, whose functions remain mostly unknown (3). TRPV5 has been implicated as the Ca2+ influx channel in the process of vitamin D–responsive active Ca2+ reabsorption in the kidney (1, 4). In comparison, the TRPV5 homolog TRPV6, which displays an amino acid sequence identity of about 75% to TRPV5, has been postulated to be the Ca2+ influx channel facilitating Ca2+ absorption in enterocytes (2). TRPV6 is ubiquitously expressed and has been implicated as part of the capacitative Ca2+ entry mechanism and, therefore, intracellular Ca2+ signaling (5).


1. Hoenderop, J.G., et al. 1999. Molecular identification of the apical Ca2+ channel in 1, 25-dihydroxyvitamin D3-responsive epithelia. J. Biol. Chem. 274:8375–8378.

2. Peng, J.B., et al. 1999. Molecular cloning and characterization of a channel- like transporter mediating intestinal calcium absorption. J. Biol. Chem. 274:22739–22746.

3. Montell, C., Birnbaumer, L., and Flockerzi, V. 2002. The TRP channels, a remarkably functional family. Cell. 108:595–598.

4. Hoenderop, J.G., Nilius, B., and Bindels, R.J. 2002. Molecular mechanisms of active Ca2+ reabsorption in the distal nephron. Ann. Rev. Physiol. 64:529–549.

5. Yue, L., Peng, J.B., Hediger, M.A., and Clapham, D.E. 2001. CaT1 manifests the pore properties of the calcium-release-activated calcium channel. Nature. 410:705–709.

Anmerkungen

The source is not mentioned.

Sichter
(Hindemith), WiseWoman

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