Figure 2: Ca²⁺ regulated checkpoints in canonical NF-κB signaling in lymphocytes.

A schematic representation of known mechanisms by which Ca²⁺ regulates the activation of NF-κB dependent gene expression. In resting lymphocytes, homo- and heterodimers composed of NF-κB p50, p65, and c-Rel proteins are sequestered in the cytoplasm by IκB proteins including IκBα. BCR or TCR engagement activates a series of protein tyrosine kinases (PTKs) that subsequently activate PLCγ isoforms, cleaving PtdIns(4,5)P2 to generate InsP3 (IP3) and DAG. IP3 binds to IP3 receptors (IP3R) channels allowing release of Ca²⁺ from the endoplasmic reticulum (ER). The resulting decrease in ER [Ca²⁺] causes oligomerization of STIM1 proteins and activation of plasma membrane Orai1 channels facilitating extracellular Ca²⁺ entry and a significant or sustained elevation in cytoplasmic Ca²⁺. In B cells, Ca²⁺ and DAG activation of PKCβ and in T cells, DAG activation of PKCθ controls assembly and activity of the CARMA-BCL10-MALT1 (CBM) complex, which is required for antigen receptor induced activation of the IKK complex (NEMO-IKKα-IKKβ). Furthermore, PKCα, the CaM dependent phosphatase Calcineurin (CaN), and calmodulin (CaM)-dependent kinase II (CaMKII) each also regulate CBM complex formation by controlling the phosphorylation levels of these proteins. Ca²⁺ also plays a negative regulatory role via CaM activation that can directly bind to BCL10 to disrupt the CBM complex, activate CaMKII that phosphorylates BCL10 on S138 or binds to NF-κB to block nuclear translocation (all depicted by red lines). NEMO recruitment to the CBM complex facilitates full activation of the IKK complex, which phosphorylates IκB proteins, marking them for ubiquitylation and proteasomal degradation. Both p65 and c-Rel can undergo Ca²⁺ dependent (e.g. Ser536 phosphorylation, CaM binding) and independent post-translational modifications that regulate their nuclear localization, transactivation, and target gene binding potential. Thus, Ca²⁺ dependent mechanisms that regulate the IκB protein degradation and expression and those that modify the post-translational landscape of NF-κB proteins cooperate to regulate gene expression.