The calcium binding loops of the cytosolic phospholipase A2 C2 domain specify targeting to Golgi and ER in live cells

Differential Compartmentalization of the Calpain/Calpastatin Network with the Endoplasmic Reticulum and Golgi Apparatus

Calpain, a calcium-activated cysteine protease, is involved in modulating a variety of cell activities such as shape change, mobility, and apoptosis. The two ubiquitous isoforms of this protease, calpain I and II, are considered to be cytosolic proteins that can translocate to various sites in the cell. The activity of calpain is modulated by two regulatory proteins, calpastatin, the specific endogenous inhibitor of calpain, and the 28-kDa regulatory subunit. Using velocity gradient centrifugation, the results of this study confirm and greatly expand upon our previous finding that the calpain/calpastatin network is associated with the endoplasmic reticulum and Golgi apparatus in cells. Moreover, confocal microscopy demonstrates that calpain II colocalizes with specific proteins found in these organelles. Additional experiments reveal that hydrophobic rather than electrostatic interactions are responsible for the association of the calpain/calpastatin network with these organelles. Treatment of the organelles with Na2CO3 or deoxycholate reveal that calpain I, 78-kDa calpain II, and the regulatory subunit are "embedded" within the organelle membranes similar to integral membrane proteins. Proteinase K treatment of the organelles shows that calpain I and II, calpastatin, and the regulatory subunit localize to the cytosolic surface of the organelle membranes, and a subset of calpain II and the regulatory subunit are also found within the lumen of these organelles. These results provide a new and novel explanation for how the calpain/calpastatin network is organized in the cell.

Regulation of the specific release of arachidonic acid by cytosolic phospholipase A2

Cytosolic phospholipase A(2) alpha (cPLA(2)alpha) is the only PLA(2) that exhibits specificity for sn-2 arachidonic acid consistent with its primary role in mediating the agonist-induced release of arachidonic acid for eicosanoid production. It is subject to complex mechanisms of regulation that ensure that levels of free arachidonic acid are tightly controlled. The calcium-induced translocation of cPLA(2)alpha from the cytosol to membrane regulates its interaction with phospholipid substrate. cPLA(2)alpha is additionally regulated by phosphorylation on sites in the catalytic domain. Because of its central position as the upstream regulatory enzyme for initiating production of several classes of bioactive lipid mediators (leukotrienes, prostaglandins and platelet-activating factor), it is a potentially important pharmacological target for the control of inflammatory diseases.

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.