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Calcineurin is a protein enzyme sometimes known as protein phosphatase 3. This protein is a phosphatase, meaning that it functions by removing phosphate groups from other macromolecules. The effect of this action varies by substrate, but often includes turning other enzymes on or off.
This enzyme has several important purposes in the human body. Calcineurin can assist in the immune response by activating the immune cells known as T cells. It may also play a role in regulating working memory and attention due to its effects on certain brain receptors. In diabetics, it can cause pancreatic cells to function properly.
Calcineurin is part of a signaling pathway that allows it to exert its effects on T cells. When these T cells recognize invaders, they allow calcium into their cytoplasm. This increase in calcium activates calcineurin, via regulatory proteins.
Once activated, calcineurin removes a phosphate from the enzyme called "nuclear factor of activated T cell, cytoplasmic" (NFATc). NFATc is then activated, and moves from the cytoplasm to the nucleus, where it encourages the gene expression and production of interleukin-2 (IL-2). NFATc is rapidly expelled from the nucleus, so calcineurin must be continually activated for IL-2 production to continue.
IL-2, in turn, activates helper T cells, which produce signaling proteins called cytokines. These cytokines promote an immune response from several types of immune cells. Calcineurin is therefore an essential component of the immune system.
The same pathway involving calcinuerin and NFATc is used in neurons. When stimulated, neurons allow calcium into their cytoplasm, activating this enzyme. This process has the net effect of guiding axons to other neurons and strengthening connections. The hippocampus, a brain region involved in memory, is where this process has been seen at work, and accounts for the enzyme's integral role in learning and memory.
This enzyme seems to be essential to proper fetal development. Studies have shown that the calcineurin and NFATc system is involved in lung development and function. This same system has also been found to assist in the expression of genes involved in heart valve development. In the heart, calcineurin is only seen to be activated on the days that the heart valve forms.
In adults, this enzyme can still affect the heart. During periods of stress in development, heart cells grow larger in a condition known as hypertrophy. An overabundance of a deficient form of it causes this effect to carry over into adulthood. When this effect occurs in a person with hypertension, it can be quite dangerous.