Calbindin
Calbindin is a calcium-binding protein belonging to the S100 superfamily. More specifically, calbindin is a member of the EF-hand protein family, characterized by a helix-loop-helix structural motif involved in calcium ion (Ca2+) binding. There are two main isoforms of calbindin: calbindin-D28k (28 kDa) and calbindin-D9k (9 kDa).
Calbindin-D28k is primarily found in the brain, kidney, and intestine. In the brain, it is particularly abundant in certain neuronal populations, including cerebellar Purkinje cells and specific interneurons. Within the kidney, it is located in distal convoluted tubules. In the intestine, it plays a role in calcium absorption. Calbindin-D28k functions as an intracellular calcium buffer, modulating the concentration of free calcium ions within cells. This buffering action contributes to the regulation of neuronal excitability, calcium-dependent signaling pathways, and cellular protection against calcium overload (excitotoxicity).
Calbindin-D9k is predominantly expressed in the intestine and uterus, where it facilitates calcium transport across the intestinal epithelium during calcium absorption and across the uterine epithelium during pregnancy. It, too, acts as an intracellular calcium buffer, albeit with a tissue-specific role tailored to calcium transport.
The expression of calbindin can be influenced by factors such as vitamin D and hormonal signals. Alterations in calbindin levels have been implicated in various pathological conditions, including neurological disorders (e.g., Alzheimer's disease, Parkinson's disease), kidney disease, and cancer. Immunohistochemistry using antibodies against calbindin is a valuable tool in diagnostic pathology for identifying specific cell types and assessing the presence or absence of calbindin expression in tissue samples. This helps in characterizing tumors and understanding disease processes.