Tyrosine phosphorylation of phospholipase C␥2 (PLC␥2) is a crucial activation switch that initiates and maintains intracellular calcium mobilization in response to B cell antigen receptor (BCR) engagement. Although members from three distinct families of nonreceptor tyrosine kinases can phosphorylate PLC␥ in vitro, the specific kinase(s) controlling BCR-dependent PLC␥ activation in vivo remains unknown. Bruton's tyrosine kinase (Btk)-deficient human B cells exhibit diminished inositol 1,4,5-trisphosphate production and calcium signaling despite a normal inducible level of total PLC␥2 tyrosine phosphorylation. This suggested that Btk might modify a critical subset of residues essential for PLC␥2 activity. To evaluate this hypothesis, we generated site-specific phosphotyrosine antibodies recognizing four putative regulatory residues within PLC␥2. Whereas all four sites were rapidly modified in response to BCR engagement in normal B cells, Btkdeficient B cells exhibited a marked reduction in phosphorylation of the Src homology 2 (SH2)-SH3 linker region sites, Tyr 753 and Tyr 759 . Phosphorylation of both sites was restored by expression of Tec, but not Syk, family kinases. In contrast, phosphorylation of the PLC␥2 carboxyl-terminal sites, Tyr 1197 and Tyr 1217 , was unaffected by the absence of functional Btk. Together, these data support a model whereby Btk/Tec kinases control sustained calcium signaling via site-specific phosphorylation of key residues within the PLC␥2 SH2-SH3 linker.Signals generated by the pre-B and mature B cell receptors are essential for B cell development, activation, and maintenance of mature B cell populations (1). Engagement of the BCR 1 initiates the formation of a lipid-raft associated signaling complex, or "signalosome," containing tyrosine and serine/threonine kinases, adapter molecules, and lipid hydrolases including phospholipase C␥ isoforms. Together, these events promote a series of downstream signals including a sustained increase in intracellular calcium concentrations. PLC␥ is essential for antigen receptor-mediated calcium mobilization (2-4). Activated PLC␥ hydrolyzes its substrate, phosphatidylinositol 4,5-bisphosphate, generating the second messengers inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (5, 6). IP 3 acts to open intracellular calcium stores promoting an initial, transient rise in intracellular calcium. Depletion of intracellular calcium stores triggers the opening of plasma membrane store-operated calcium channels (7), resulting in an influx of extracellular calcium and a secondary, sustained calcium signal. The amplitude and duration of this sustained calcium signal is a key determinant of the specific transcription program initiated in response to antigen receptor engagement (8 -10).The most abundantly expressed PLC␥ isoform in B lineage cells is PLC␥2. Chicken B lymphoma cells lacking PLC␥2 are unable to generate IP 3 in response to BCR engagement, resulting in the abrogation of receptor-mediated calcium mobilization (11). Similarly, mice deficient in...

Kipp KR, Rezaei M, Lin L, Dewey EC, Weimbs T. A mild reduction of food intake slows disease progression in an orthologous mouse model of polycystic kidney disease.

Although both LPA and S1P induce signal transduction in all prostate cancer cell lines studied, a proliferation response is observed only when the Erk, Akt, and FAK pathways are activated. Other responses to the lipid mediators, such as PLD activation, likely contribute to other cellular outcomes.

IntroductionX-linked agammaglobulinemia (XLA) is the prototypic primary humoral immunodeficiency disorder, first described by Bruton who reported a boy with severe recurrent infections and absence of the gamma-globulin serum fraction. 1 This disorder has been of major interest for more than half a century, initiating an ongoing and fruitful search for the genetic basis of this and other primary immunodeficiency diseases.XLA is characterized by a lack of mature B cells and plasma cells and a profound deficiency of all immunoglobulin types. Most patients develop recurrent infections coincident with the loss of maternal antibodies. Pyogenic infection with encapsulated bacteria is the most common clinical manifestation. [2][3][4] Current therapy consists of regular immunoglobulin replacement and prompt attention to infection.Both XLA and a related murine immunodeficiency, X-linked immunodeficiency (Xid), result from deficiencies in Bruton tyrosine kinase (Btk). [5][6][7][8][9][10] Btk, a member of the Tec family of nonreceptor kinases, 11 is expressed throughout B-lineage development except in plasma cells. 12,13 Btk contains a Src homology 1 (SH1) catalytic domain, SH2 and SH3 protein interaction domains, and a unique amino-terminus with a pleckstrin homology (PH) domain. 14 Mutational studies indicate important functional roles for each of these domains. 15 The major developmental defect in XLA occurs at the pre-Bcell transition, resulting in an increase in pro-B cells and a marked reduction in cycling pre-B cells and all subsequent stages. [16][17][18] Few IgM ϩ B cells are detectable in the blood and the hypotrophic lymphoid tissues. 19,20 Clonal expansion at the pre-B stage is regulated by pre-B-cell receptor (pre-BCR) signaling and is essential for generating an adequate pool of immature B cells. 21 Pre-BCR engagement leads to the generation of a "signalosome" that includes activated Btk, 22 an event disrupted in XLA.Several murine models of Btk deficiency currently exist. Xid mice have a spontaneous missense mutation in the PH domain. 23 The mutant protein is inefficiently recruited to the plasma membrane and fails to enter the BCR signalosome. 24 The phenotype of both Xid mice and Btk null-knockout mice (Btk Ϫ/Ϫ ) is less severe than that of XLA. [25][26][27][28] These mice produce nearly normal numbers of peripheral B cells, but splenic B-cell development is significantly compromised. Btk-deficient transitional 2 (T2) immature B cells fail to generate the BCR-dependent pro-survival, proliferative, and differentiation signals required to produce mature B An Inside Blood analysis of this article appears in the front of this issue.Reprints: David J. Rawlings, Children's Hospital and Regional Medical Center, 307 Westlake Ave North, Suite 300, Seattle, WA 98109; e-mail: drawling@u.washington.edu.The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked ''advertisement'' in accordance with 18 U.S.C. section 173...