Down-regulation of B Cell Receptor Signaling by Hematopoietic Progenitor Kinase 1 (HPK1)-mediated Phosphorylation and Ubiquitination of Activated B Cell Linker Protein (BLNK)

Abstract

Background: HPK1 is a hematopoiesis-specific Ste20-like serine/threonine kinase that suppresses immune responses and autoimmunity. Results: HPK1 knock-out B cells show loss of Thr-152 phosphorylation, 14-3-3 binding, and Lys-37/38/42-ubiquitination of BLNK. Conclusion: HPK1 attenuates BCR signaling via inducing phosphorylation and ubiquitination of BLNK.Significance: This is the first report of BLNK ubiquitination that is initiated by HPK1-induced phosphorylation.

Dual Phosphorylation of Btk by Akt/Protein Kinase B Provides Docking for 14-3-3ζ, Regulates Shuttling, and Attenuates both Tonic and Induced Signaling in B Cells

“…Interestingly, BLNK, also known as SLP-65, which in B cells has a function related to that of SLP-76 in T cells, is also regulated by 14-3-3. Thus, it was recently reported that hematopoietic progenitor kinase 1 (HPK1), by phosphorylating threonine 152 in BLNK, attenuates BCR-mediated activation, since 14-3-3 binding to T152 causes ubiquitination and degradation of BLNK (59). This also suggests that other components than Btk can contribute to the enhanced activation that we observed upon BV02 treatment.…”

Attenuation of T Cell Receptor Signaling by Serine Phosphorylation-mediated Lysine 30 Ubiquitination of SLP-76 Protein

“…Finally, other SLP-76 family adaptor proteins, including BLNK and CLNK, also interact with and activate HPK1. In fact, we note that HPK1 feedback induces BLNK phosphorylation and ubiquitination in B cells during B cell receptor signaling (30). A future study of the potential regulation of CLNK activation by HPK1-induced phosphorylation may lead to interesting findings.…”

Actin-binding protein 1 links B-cell antigen receptors to negative signaling pathways

“…BCR signaling is tightly regulated, and elevated or sustained BCR signaling has been shown to be associated with autoimmunity (4). Attenuation of BCR signaling is mediated by various phosphatases and kinases, including SH2-containing inositol 5-phosphatase (SHIP-1) (5) and hematopoietic progenitor kinase 1 (HPK1) (6). SHIP-1 inhibits activation of phospholipase-Cγ2 (PLCγ2), Bruton's tyrosine kinase, and Akt by eliminating their membrane docking sites, consequently blocking their downstream signaling (5).…”

“…SHIP deficiency causes hyperresponsiveness and impaired affinity maturation of B cells in germinal centers (GCs) (7). HPK1 inhibits BCR signaling by inducing phosphorylation and subsequent ubiquitination of B-cell linker protein (BLNK) (6), the key adaptor molecule of the BCR. HPK1 deficiency results in elevated levels of activated BLNK, MAP kinases, B-cell proliferation, and resultant susceptibility to induced autoimmunity (6).…”

“…HPK1 inhibits BCR signaling by inducing phosphorylation and subsequent ubiquitination of B-cell linker protein (BLNK) (6), the key adaptor molecule of the BCR. HPK1 deficiency results in elevated levels of activated BLNK, MAP kinases, B-cell proliferation, and resultant susceptibility to induced autoimmunity (6). BCR clustering is also involved in negative regulation, as we recently demonstrated that coalescence of BCR microclusters into a central cluster facilitates BCR signal attenuation.…”