Mammalian calcium pumps are mainly located in either the plasma membrane, or in the internal membranes of the sarcoplasmic or endoplasmic reticulum, and fall into three classes of enzymes: the plasma membrane calcium ATPases (PMCA), which extrude calcium from the cell, the sarcoplasmic/endoplasmic reticulum calcium ATPases (SERCA), which sequester calcium within intracellular organelles, and the secretory pathway calcium ATPases (SPCA), whose function is poorly understood. There are several genes encoding these different pumps, including four PMCA pumps (PMCA1-4; in humans ATP2B1-4), three SERCA pumps (SERCA1-3; in humans ATP2A1-3), and two putative SPCA pumps (SPCA1-2; in humans ATP2C1, ATP2C4). These genes can be alternatively spliced, creating a total of more than 40 different splice variants, each serving different biological and physiological functions.
The basic function of the plasma membrane calcium pumps are to maintain the 10,000-fold calcium gradient across the plasma membrane via the highly regulated active expulsion of calcium from the cell. In addition, they are involved in calcium signalling and the modulation of calcium spikes. PMCA isoforms can also have tissue-specific roles, such as the regulation of the rate of clot retraction in platelets. There are more than 30 splice variants formed from the four PMCA isoforms, each differing in its affinity for calcium and calmodulin, with some isoforms showing tissue-specific expression. For instance, in rat brain, PMCA isoforms are differentially expressed within different classes of neurons, suggesting that they play a complex role in calcium homeostasis. PMCA isoforms are differentially regulated by protein kinases (PKA, PKC), by proteases (calpain), by effector caspases, and by interaction with phospholipids (phosphotidylserine, phosphatidylinositol), which act to shape the time course of the calcium signals. PMCAs can be distinguished from SERCAs by the addition of an extended C-terminal tail that forms an auto-inhibitory domain, providing a mechanism for regulation of PMCA activity by cytosolic calcium concentration. The binding of calmodulin to this domain relieves the inhibition.
PMCA1 is ubiquitously distributed in tissues and cells, and is the most abundant isoform. c-Myb is known to repress PMCA1 expression in vascular smooth cells.
PMCA2 is mainly found in the central nervous system, where it is the major isoform in purkinje neurons, as well as serving organ-specific functions. The PMCA2a isoform is the only PMCA present in hair bundles, the sensory organelle of cochlear hair cells of the inner ear that affect hearing and balance. Mice carrying mutations in PMCA2 are profoundly deaf and have a balance defect due to the loss of PMCA2a in sensory hair cells.
PMCA3 is mainly found in the central nervous system, and was found at the highest levels in the cerebral cortex and cerebellar cortex.
PMCA4 is ubiquitously distributed in tissues and cells, and constitutes 80% of PMCA in erythrocytes. Calcineurin was found to mediate the repression of PMCA4 expression in neurons, thereby regulating cell cycle-associated calcium concentration. PMCA4b is the neuronal nitric oxide synthase (nNOS)-associating isoform that helps to regulate vascular tone by regulating intracellular calcium concentration.
The endoplasmic reticulum (ER) plays an important role in regulating cytosolic calcium levels through SERCA pumps, which accumulate calcium in the ER lumen. The mobilisation of calcium from intracellular organelles is highly specialised in cardiac and skeletal muscle, where SERCA makes up 90% of membrane proteins in the sarcoplasmic reticulum (SR) of skeletal muscle. In skeletal muscle, calcium ions are transported against a concentration gradient from the cytoplasm into the SR, which causes the relaxation of muscle cells following the excitatory effect of high cytosolic calcium. In cardiac muscle, the control of intracellular calcium is essential for the regulation of cardiac contractility, and relies upon SERCA and PMCA pumps. SERCA pumps display greater homology with sodium/potassium pumps, than with PMCA pumps, most differences occurring in the transmembrane domain. The three human SERCA genes encode up to 10 isoforms by alternative splicing.
The SERCA1 isoform is specific for the SR in fast-twitch skeletal muscle. There are at least two C-terminal variants, both of which mediate the uptake of calcium into skeletal muscle SR. Autosomal recessive (loss of function) mutations in SERCA1 are associated with Brody disease, a rare inherited muscle disorder characterised by exercise-induced impairment of skeletal muscle relaxation, resulting in stiffness and cramps.
SERCA2 transports calcium from the cytosol into the ER lumen to maintain low cytosolic calcium concentration. SERCA2 has been found in the SR of slow-twitch skeletal muscle, as well as in other tissues. The SERCA2a isoform is expressed in the SR of cardiac muscle, where it plays a key role in the contraction and relaxation of cardiac muscle through its control of cytosolic calcium levels. SERCA2a may be involved in the pathogenesis of cardiac hypertrophy and failure. SERCA2a activity is regulated by phospholamban and its homologue, sarcolipin, as well as by calcineurin.
SERCA2b is the major isoform expressed in smooth muscle and non-muscle tissues, such as the epidermis, where it plays an important role in maintaining epidermal integrity. SERCA2b differs from SERCA2a at the C-terminus, the extra sequences in SERCA2b containing an additional transmembrane domain that interacts with calreticulin. Autosomal dominant mutations in SERCA2b are associated with Darier disease, a skin disorder characterised by the loss of adhesion between epidermal cells and abnormal keratinisation, as well as being associated with a wide range of neuropsychiatric problems, such as epilepsy and depression. The presence of low lumenal calcium concentrations in Darier disease patients could cause defective processing of newly synthesised proteins required for normal adhesion between epithelial cells. SERCA2b also has a housekeeping function that is critical for most mammalian cell types.
SERCA3 has three splice variants that vary in their C-termini. SERCA3 is expressed in many tissues, and is often co-expressed with SERCA2b, yet unlike SERCA2b, SERCA3 does not have an essential housekeeping function. SERCA3 may be involved in the relaxation of vascular smooth muscle. SERCA3 may also play a role in regulating insulin secretion via glucose-activated beta cell calcium signalling. Mutations in SERCA3 are thought to be involved in non-insulin dependent type-II diabetes mellitus through a pancreatic beta cell defect. Abnormal calcium concentrations are a common defect in both type I and II diabetes, effecting beta cell function.
SPCA represents a third distinct type of calcium pump. The mammalian SPCA is closely related to a prokaryotic calcium pump, PACL, cloned from cyanobacterium. As such, SPCA may exhibit greater similarity to early eukaryotic calcium pumps than either PMCA or SERCA. Mutations in ATP2C1 (SPCA1) are associated with the autosomal recessive Hailey-Hailey disease, where abnormal calcium signalling in keratinocytes results in a blistering skin disorder. ATP2C1 is localised to the Golgi apparatus in keratinocytes, where it controls Golgi calcium stores. Mutations in ATP2C1 may effect protein sorting and maturation.