Calbindins

Two types of calbindins have been described, calbindin D28k and calbindin Dpk- The latter (now renamed as S100G) belongs to the SI00 protein family. 

In the CNS, calbindin D28k is widely expressed in a subpopulation of neurons distinct from those immunoreactive for parvalbumin or calretinin. 

Calretinin is homologous to calbindin D28k- It is abundantly expressed in the central and peripheral nervous system and other organs. The protein contains four EF-hand domains homologous to the first four of calbindin D28k-... 

Calbindins are hormonally controlled by vitamin D and are expressed in the kidney, intestine, and brain. 

S100G S100 calcium binding protein G Calbindin 3 (CALB3), CaBP9K, CABP1 Xp22 NM 004057... 

In addition to its role in regulating calcium homeostasis, vitamin D is required for the intestinal absorption of calcium. Synthesis of the intracellular calciumbinding protein, calbindin, required for calcium absorption, is induced by vitamin D, which also affects the permeability of the mucosal cells to calcium, an effect that is rapid and independent of protein synthesis. 

Alonso J., Arevalo R., Garciaojeda E., Porteros A., et al. (1995). NADPH-diaphorase active and calbindin d-28k-immunoreactive neurons and fibers in the olfactory-bulb of the hedgehog (Erinaceus europaeus). J Comp Neurol 351, 307-327. 

Johnson E.W., Eller P.M., Jafek B.W. and Norman A.W. (1992). Calbindin-like immunoreactivity in two peripheral chemosensory tissues of the rat taste buds and the vomeronasal organ. Brain Res 572, 319-324. 

Calbindin Provides storage and buffering capacity for the regulation of Ca2+ concentrations (357)... 

The dynamics of Ca2+-bound calbindin have been investigated by 13C NMR relaxation... 

As both fast and slow adaptation mechanisms are regulated by Ca2+, the stereocilia mechanisms that control the free concentration of this ion also play central roles in transduction. Entering Ca2+ is thought to be buffered very rapidly by the mobile buffers parvalbumin 3, calbindin, and calretinin [22,23]. Even before bound Ca2+ can diffuse out of stereocilia, it is pumped back out into the endo-lymph by isoform 2a of the plasma-membrane Ca2+-ATPase (PMCA2) [24,25] (see also Ca2+ transport in Ch. 5). 

One of the crucial questions is why certain motoneuron populations are particularly prone to death in ALS. Although there is no clear-cut answer, one of the possibilities is weak Ca " " buffering capacity connected with lowered levels of cytoplasmic proteins responsible for such buffering, such as parvalbumin and calbindin (Krieger et al. 1994 Roy et al. 1998). 

Parvalbumins, which are also found in other vertebrates, are high-affinity Ca2+-buffers." Additional calcium buffers with EF-hand structures are the vitamin D-induced calbindins. One 9-kDa calbindin is found in mammalian intestinal tissue and in skin. It has two helix-loop-helix Ca2+-binding sites of differing affinity101102 that presumably function in the absorption of calcium. A 28-kDa vitamin D-dependent protein from chicken intestine contains six similar Ca2 +-binding loops.97 103... 

The hormonally active la,25-dihydroxyvitamin D is distributed through the bloodstream to all parts of the body. It is taken up rapidly by nuclei of cells of the intestinal lining where it binds to a 55-kDa receptor protein. In response, the cells synthesize calbindins (Chapter 6), Ca2+-binding proteins which facilitate the uptake of calcium ions by the body.1 v (see also Fig. 6-7). 

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