SYK and ZAP70 kinases in lymphocyte selection

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ABSTRACT While B-cells are under intense selective pressure to eliminate autoreactive and pre-malignant clones, we identified SYK as central kinase to set the thresholds for negative selection. SYK and its highly homologous relative ZAP70 initiate B-cell receptor (BCR) and T-cell receptor (TCR) signaling in B- and T-lymphocytes, respectively. Even though the two kinases are almost identical and serve analogous functions, their expression in B- and T-cells is strictly segregated throughout evolution. Whereas the reason for separation of the two kinases is not known, aberrant coexpression of Syk and Zap70 was previously reported in B-cell chronic lymphocytic leukemia (CLL) and in peripheral T-cell lymphoma (PTCL). Our group recently demonstrated that aberrant ZAP70 expression is a common feature in pre- germinal center (GC) B-cell malignancies (B-ALL, CLL, MCL; Sadras et al., Mol Cell 2021). In genetic mouse models for B-ALL and B-CLL, inducible coexpression of Zap70 accelerated disease onset, while genetic deletion impaired malignant transformation. Likewise, inducible expression of Zap70 during early B-cell development subverted negative selection of autoreactive B-cells to promote pervasive autoantibody production. Mechanistically, ZAP70 competes with SYK and exerts a dominant-negative effect on SYK-dependent Ca2+-signaling. By occupying but not phosphorylating BLNK, BTK and PLC2 substrates upstream of calcium signaling, ZAP70 dramatically reduces the frequency of autonomous Ca2+-oscillations. Fast oscillations in the sole presence of SYK (4.5 mHz) are decoded by NFATC1 and result in anergy and cell death. In contrast, slow Ca2+-oscillations in the presence of ZAP70 (0.25 mHz) promote selective activation of NF-B and B-cell survival and proliferation. We test here the central hypothesis that B-cells sense pathological signaling downstream of an autoreactive BCR or a transforming oncogene through SYK-dependent high-frequency Ca2+- oscillations. At high frequencies, Ca2+-oscillations activate NFATC1 to initiate B-cell anergy and clonal deletion. Conversely, ZAP70 slows down Ca2+-oscillations to activate NF-B instead of NFATC1 and enables autoreactive and premalignant B-cell clones to persist and eventually give rise to autoimmune disease or pre-GC B-cell malignancies. We are proposing three Aims to (1) elucidate the mechanisms by which ZAP70 slows down SYK- dependent high-frequency Ca2+-oscillations, to (2) determine how coexpression of ZAP70 subverts negative selection of autoreactive B-cells in autoimmune disease and (3) to uncover how ZAP70 enables oncogenic signaling and overt malignant transformation of pre-malignant B-cells.