The ζ Isoform of Diacylglycerol Kinase Plays a Predominant Role in Regulatory T Cell Development and TCR-Mediated Ras Signaling

Diacylglycerol (DAG) is a critical second messenger that mediates T cell receptor (TCR)-stimulated signaling. The abundance of DAG is reduced by the diacylglycerol kinases (DGKs), which catalyze the conversion of DAG to phosphatidic acid (PA) and thus inhibit DAG-mediated signaling. In T cells, the predominant DGK isoforms are DGK alpha and DGK zeta, and deletion of the genes encoding either isoform enhances DAG-mediated signaling. We found that DGK zeta, but not DGK alpha, suppressed the development of natural regulatory T (T-reg) cells and predominantly mediated Ras and Akt signaling downstream of the TCR. The differential functions of DGK alpha and DGK zeta were not attributable to differences in protein abundance in T cells or in their localization to the contact sites between T cells and antigen-presenting cells. RasGRP1, a key DAG-mediated activator of Ras signaling, associated to a greater extent with DGK zeta than with DGK alpha; however, in silico modeling of TCR-stimulated Ras activation suggested that a difference in RasGRP1 binding affinity was not sufficient to cause differences in the functions of each DGK isoform. Rather, the model suggested that a greater catalytic rate for DGK zeta than for DGK alpha might lead to DGK zeta exhibiting increased suppression of Ras-mediated signals compared to DGK alpha. Consistent with this notion, experimental studies demonstrated that DGK zeta was more effective than DGK alpha at catalyzing the metabolism of DAG to PA after TCR stimulation. The enhanced effective enzymatic production of PA by DGK zeta is therefore one possible mechanism underlying the dominant functions of DGK zeta in modulating T-reg cell development.