Calcineurin (CaN) is a protein phosphatase that plays an important role in the activation of immune system T cells, but is also active in other calcium-mediated signaling pathways throughout the body. By dephosphorylating the NF-AT protein, this enzyme initiates a series of gene transcripts that are primarily responsible for the characteristic work of T lymphocytes. Thanks to this key position, calcineurin is the starting point of several therapeutic procedures for immunosuppression.
What is calcineurin?
Structured, the enzyme is composed of two subunits: Calcineurin A (approximately 60 kDa) provides catalytic function and carries one calmodulin-binding site, whereas calcineurin B (approximately 19 kDa) is regulatory active and has two calcium ion-binding sites. In the ground state, CaN is inactive because part of the protein blocks the active site – this is known as autoinhibition. Binding of calcium-activated calmodulin and calcium ions is required for full activation. As a phosphatase, calcineurin is assigned to EC number 3.1.3.16, which includes those enzymes that catalyze the hydrolytic dephosphorylation of serine and threonine residues of other proteins.
Function, action, and roles
The substrate-binding site of the enzyme is selective primarily for NF-ATc (nuclear factor of activated T-cells, cytosolic). This transcription factor is present in the cytoplasm of lymphocytes. In the ground state, NF-Atc is phosphorylated and thus inactive. The role of calcineurin in the immune response begins with the uptake of an antigen – e.g. a virus, a bacterium or components of degenerated cells – by a cell of the immune system (monocytes, macrophages, dendritic cells and B cells). This substance is then processed and presented on the surface of the cell. Upon contact of antigen-presenting cells with the T cell receptor of the T cells, a signaling cascade is initiated. These extracellular stimuli increase the calcium concentration in the cell. Binding of calcium ions to CaN B occurs, which by structural change of the protein releases the autoinhibitory domain of CaN A and mediates calmodulin binding to CaN A. Hereby, calcineurin becomes fully catalytically active and dephosphorylates the serine-rich region (SRR) in the amino terminus of NF-ATc. This results in a conformational change of NF-ATc, as a consequence of which the transcription factor is transported into the nucleus. There, it triggers the transcription of several genes that are responsible, among other things, for the production of interleukins such as IL-2. IL-2 also ensures the activation of T helper cells and the synthesis of cytokines, thus directing the work of cytotoxic T cells. While helper cells direct other lymphocytes in the immune response – such as by maturation of B cells into plasma cells or memory cells and activation of phagocytes – cytotoxic T cells are responsible for destruction of infected or degenerated somatic cells. Since this pathway cannot be followed without calcineurin, the enzyme plays a key role in the immune response. Other target proteins of the enzyme are the cAMP response element binding protein (CREB) with influence e.g. on the nervous system and the internal clock and myocyte enhancer factor 2 (MEF2) , which is partly responsible for cell differentiation during embryonic development and plays a role in the stress response of some tissues in adults.
Formation, occurrence, properties, and optimal levels
Different isoforms of the two subunits exist (CaN A: 3 isoforms, CaN B: 2 isoforms), some of which are expressed differently depending on the body region. CaN A γ in particular stands out, which is found exclusively in the testis and is involved in sperm maturation there. Despite its prominent role in the immune system and nerves, calcineurin can be assumed to be found in almost all tissues. The regulation is not so much via an increase or decrease of the synthesis but via the calcineurin inhibitor CAIN. This prevents, for example, the dephosphorylation of NF-AT. Negative feedback regulation by RCAN1 ensures that excessive cytosolic concentrations of CaN are not generated. In this process, activated (dephosphorylated) NF-AT binds to the gene promoter of RCAN1 in the nucleus, thereby triggering transcription. The resulting RCAN1 binds to CaN and inhibits its activity.
Diseases and disorders
Calcineurin is the active target of calcineurin inhibitors such as cyclosporine, pimecrolimus, and tacrolimus. Inhibition of the phosphatase action of CaN causes immunosuppression, which is desirable, for example, after organ transplantation to reduce the likelihood of rejection or in autoimmune diseases to combat inflammatory processes. Thus, CaN inhibitors are also used for the treatment of rheumatoid diseases. Other approaches currently being explored are the treatment of tuberculosis infection, schizophrenia and diabetes. The exclusive occurrence of CaN A γ in the testis implies a possible role in the development of contraceptives. In cases of cardiac hypertrophy involving the CaN-NA-FT pathway, the development of hypertrophy could be prevented by administration of CaN inhibitors. People with Down syndrome have three 21 chromosomes instead of the usual two, which encode a calcineurin inhibitory protein. This inhibitor prevents calcineurin from interacting with cells of blood vessels and triggering proliferation processes in them. This is particularly important in the case of tumors, since they ensure their blood supply via calcineurin, among other things. Intervening at this point can therefore effectively prevent the progression of cancer. For example, a much lower incidence of tumors is found in people with Down syndrome, and it is hoped that targeted inhibition of this process will provide advantages in the fight against cancer in the future. Recently, there has also been increasing evidence that age-related dysregulation of calcineurin may also play a role in the development of neuronal diseases such as Alzheimer’s disease. Research into the signaling pathways in which the enzyme is involved is revealing more and more white spots on the biochemical map. At the same time, it opens up the hope that a variety of different diseases can be better understood and treated in the future with the help of this key protein.
