One of the major focus areas of our lab is the control of reactive oxygen species (ROS) by glutathione (GSH) in immune cells and the physiological consequences of ROS accumulation. ROS are balanced by antioxidants, which generally include molecules that are stable enough to donate electrons and thus act as ROS scavengers. The most abundant intracellular antioxidant system is GSH, a tri-peptide of γ-glutamyl-L-cysteinyl-glycine. The enzyme γ-glutamyl cysteine ligase (GCL), which is composed of a regulatory subunit (Gclm) and a catalytic subunit (Gclc), ligates glutamate and cysteine to form a di-peptide in the rate-limiting step of the synthetic pathway. This di-peptide is then covalently linked to glycine by GSH synthase (GS) to generate GSH.
We investigate the function of GSH in immune cells by reversed genetic approaches in vivo and in vitro. Conditional deletion of Gclc, the rate limiting enzyme in GSH-synthesis, in immune cells impairs the production of GSH.
For example, our group has identified GSH as an important regulator of T cell metabolism. We could show that the absence of GSH in conventional T cell lead to the accumulating ROS, which shuts down Teff responses. This prevents the onset of autoimmunity, but at the same time increases the susceptibility to viral infections. High ROS levels impair metabolic reprogramming in T cells by hindering proper activation of mTOR, NFAT and Myc and the establishment of T effector functions (Mak and Grusdat et al., Immunity 2017) (Figure 2).
Figure 2. GSH is a metabolic regulator of conventional T cells.
Mak T and Grusdat M et al, Immunity 2017
Interestingly GSH exhibits subset specific functions, which we are currently investigating in detail. It seems that GSH acts as a rheostat to adjust cellular ROS concentrations to cell subset-specific threshold level that allows for productive signaling and metabolism.
Our group has recently shown that Tregs contain significantly higher cellular GSH concentrations, which renders them more resistant to increasing ROS. Conditional deletion of Gclc in murine Tregs impairs their production of GSH. These mutant Tregs consequently display increased levels of reactive oxygen species (ROS) but no apparent defects in Treg homeostasis, stability or differentiation. However, Gclc-deficient Tregs do show increased proliferation and activation and the mutant mice develop spontaneous autoimmunity. Importantly, Gclc-deficient Tregs show elevations in both their synthesis and uptake of serine. Accordingly, dietary intervention in the form of feeding the mutant mice on serine/glycine-deficient chow prevents the onset of autoimmunity. In line with this finding, dampening of serine metabolism reinstated the suppressive function of Tregs, firmly linking serine metabolism to Treg function (Kurniawan et al. Cell Metabolism 2020) (Figure 3).
Figure 3. GSH restrains serine metabolism to ensure Treg cell functionality.
Kurniawan et al. Cell Metabolism 2020.