T Cell Activation Triggers Reversible Inosine-5'-Monophosphate Dehydrogenase Assembly

Abstract

T cell-mediated adaptive immunity requires naïve, unstimulated T cells to transition from a quiescent metabolic state into a highly proliferative state upon T cell receptor engagement.This complex process depends on transcriptional changes mediated by Ca 2+ -dependent NFAT signaling, mTOR-mediated signaling and increased activity of the guanine nucleotide biosynthetic enzyme inosine-5'-monophosphate (IMP) dehydrogenase (IMPDH). Inhibitors of these pathways serve as potent immunosuppressants. Unexpectedly, we discovered that all three pathways converge to promote the assembly of IMPDH protein into micron-scale macromolecular filamentous structures in response to T cell activation. Assembly is posttranscriptionally controlled by mTOR and the Ca 2+ influx regulator STIM1. Furthermore, IMPDH assembly and catalytic activity were negatively regulated by guanine nucleotide levels, suggesting a negative feedback loop that limits biosynthesis of guanine nucleotides. Filamentous IMPDH may be more resistant to this inhibition, facilitating accumulation of the higher GTP levels required for T cell proliferation.

Ensemble cryo-EM structures demonstrate human IMPDH2 filament assembly tunes allosteric regulation

“…We and others previously reported that GTP can stabilize compressed IMPDH2 filaments or drive their disassembly, depending on what other ligands are present (Buey et al 2017; Anthony et al 2017; Duong-Ly et al 2018). To understand how ligand status of the active site tunes the response to GTP, we systematically explored the effects of different ligand combinations on filament assembly.…”

“…IMPDH activity is upregulated to increase guanine levels in proliferating tissues like tumors and regenerating liver (Tressler et al 1994; Yalowitz & Jayaram 2000; Huang et al 2018; He et al 2018; Nagai et al 1991). IMPDH plays a particularly important role in the immune response, where T-cell activation is dependant on increased production of purine nucleotides, and is associated with IMPDH filament assembly (Gu et al 2000; Zimmermann et al 1998; Duong-Ly et al 2018; Calise et al 2018). As a result, IMPDH is the target of several drugs used in immunosuppressive treatment of both autoimmune disease and organ transplant rejection, and is considered a promising target for antineoplastic agents (Shu & Nair 2008; Liao et al 2017; Bergan et al 2016).…”

“…Assembly and disassembly of IMPDH into filaments has been observed in healthy proliferative cells, and in cancer cells (Keppeke et al 2018; Wolfe et al 2019). IMPDH filaments reversibly assemble in stimulated T-cells as they transition to a proliferative state, in a mechanism dependent on multiple metabolic signaling pathways and on the levels of guanine nucleotides (Duong-Ly et al 2018; Calise et al 2018). Despite the importance of understanding human IMPDH regulation, until now the regulatory role of IMPDH filament assembly has not been explored at the structural level.…”

“…Production of IMP is upregulated in response to overall purine demand, as well as in response to proliferative signalling via the mechanistic target of rapamycin (mTOR) (Smith 1998; Ben-Sahra et al 2016). Inhibition of mTOR reverses IMPDH filament assembly in activated mouse T-cells, as well as proliferating mouse liver cells (Duong-Ly et al 2018; Chang et al 2015). Thus the dependence of filament assembly on IMP levels provides an avenue for regulation of assembly through established proliferative signalling pathways.…”

“…Additional stimuli that alter IMPDH catalytic flux result in assembly, including increased intracellular IMP, and treatment with IMPDH inhibitors or other anti-proliferative drugs (Keppeke et al 2018; Chang et al 2015; Ji et al 2006; Keppeke et al 2015). In vivo, IMPDH assembly is seen mainly in cases of high nucleotide demand, for example assembly is correlated with proliferation of mouse induced pluripotent stem cells and human T-cell activation (Keppeke et al 2018; Duong-Ly et al 2018; Calise et al 2018). However, the molecular mechanisms that underlie assembly-dependent regulation of IMPDH have not been described.…”