| Health / Medical Topics |
BCR Signaling Pathway
Significant progress has been made towards delineation of the intrinsic molecular processes that regulate B lymphocyte immune function. Recent observations have provided a clearer picture of the interactive signaling pathways that emanate from the mature B cell antigen receptor (BCR) complex and the different precursor complexes that are expressed during development. Studies have also revealed that the net functional response to a given antigenic challenge is affected by the combined action of BCR-dependent signaling pathways, as well as those originating from various coreceptors expressed by B cells (e.g. CD19, CD22, FcgRIIb and PIR-B).It is now well established that reversible tyrosine phosphorylation plays an important role in regulating B cell biology. In particular, binding of antigen to the BCR promotes the activation of several protein tyrosine kinases (PTK) that, in conjunction with protein tyrosine phosphatases (PTP), alter the homeostasis of reversible tyrosine phosphorylation in the resting B cell. The net effect is a transient increase in protein tyrosine phosphorylation that facilitates the phosphotyrosine dependent formation of effector protein complexes, promotes targeting of effector proteins to specific microenvironments within the B cell and initiates the catalytic activation of downstream effector proteins. Studies have demonstrated that Src family PTKs are activated initially and serve to phosphorylate CD79a and CD79b thereby creating phosphotyrosine motifs that recruit downstream signaling proteins. In particular, phosphorylation of the BCR complex leads to the recruitment and activation of the PTK Syk, which in turn promotes phosphorylation of PLCg, Shc and Vav. Additionally, the Tec family member Btk is recruited to the plasma membrane where it is involved in activation of PLCg. Initiation of B lymphocyte activation is dependent on the tyrosine phosphorylation-dependent formation of multi-molecular effector protein complexes that activate downstream signaling pathways. The formation of such complexes was initially hypothesized to occur primarily via effector protein binding to the BCR complex itself. However, recent studies have demonstrated that productive signaling via the BCR is in fact dependent on tyrosine phosphorylation of one or more adapter proteins that play a crucial role in recruitment and organization of effector proteins at the plasma membrane. The SLP-65/BLNK adapter protein has recently been shown to play a crucial role in recruitment and activation of key signal transducing effector proteins in the B cell. After the BCR has been engaged by antigen and the activation response has been initiated, numerous second messengers and intermediate signal transducing proteins are activated. These include the production of lipid second messengers by phosphatidylinositol 3-kinase, and the PLC-dependent hydrolysis of phosphatidylinositol 4,5-bisphosphate to yield diacylglycerol and 1,4,5-inositoltrisphosphate (IP3). DAG is important for activation of PKC whereas IP3 promote release of calcium from the endoplasmic reticulum and the subsequent influx Ca2+ from the extracellular space. Numerous intermediate signaling proteins are also activated including the Ras and Rap1, which are small molecular weight GTPases and these ultimately lead to the activation of MAP kinases including Erk, JNK and p38. The net effect of second messenger production and activation of intermediate signaling proteins is the concerted regulation of several transcription factors that mediate gene transcription in the B cell. (NCI Thesaurus/BIOCARTA)
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