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Phospholipases C

Phosphatidylinositols are hydrolyzed by phospholipases to generate diacyl-glycerol and inositol phosphates including inositol 1, 4, 5-trisphosphate (IP3). The diacyl-glycerol generated activates protein kinase C (PKC) family members, while IP3 mediates the mobilization of Ca2+ from internal stores, resulting in a transient intracellular Ca2+ flux. Mammalian cells contain at least ten genes that encode phosphatidylinositol 4, 5-bisphosphate-specific phospholipase C isozymes.

A wide variety of cytokines, tyrosine kinase receptors, and immunoglobulin superfamily receptors (TcR, BcR and Fc receptors) induce the tyrosine phosphorylation of PLCγ 1 and/or PLCγ2. Both PLCγ1 and PLCγ 2 bind to a major site of tyrosine phosphorylation within the kinase domain of Jak2.

Disruption of the PLCγ1 gene resulted in embryonic lethality during early to midgestation, with death occurring at approximately embryonic day 9. Unlike PLCγ 1, disruption of the PLCγ2 gene does not lead to an embryonic lethality. However, the PLCγ 2-deficient mice have a number of defects in signalling through immunoglobulin receptor superfamily receptors. In B cells, the response to engagement of the IgM antigen receptor is defective, resulting in a phenotype that is very similar to that seen in Btk-deficient mice. The similarity to Btk- or Blnk-deficient mice demonstrates that PLCγ 2 is downstream in Btk/Blnk signalling In addition, collagen-induced platelet aggregation is defective, demonstrating a critical role for PLCγ 2 in signalling through a receptor requiring the FcRγ chain [1].

[1] Wang, D. et al. Immunity 13, 25-35 (2000).