COQ4 is required for the oxidative decarboxylation of the C1 carbon of coenzyme Q in eukaryotic cells.

Coenzyme Q (CoQ) is a redox lipid that fulfills critical functions in cellular bioenergetics and homeostasis. CoQ is synthesized by a multi-step pathway that involves several COQ proteins. Two steps of the eukaryotic pathway, the decarboxylation and hydroxylation of position C1, have remained uncharacterized. Here, we provide evidence that these two reactions occur in a single oxidative decarboxylation step catalyzed by COQ4. We demonstrate that COQ4 complements an Escherichia coli strain deficient for C1 decarboxylation and hydroxylation and that COQ4 displays oxidative decarboxylation activity in the non-CoQ producer Corynebacterium glutamicum. Overall, our results substantiate that COQ4 contributes to CoQ biosynthesis, not only via its previously proposed structural role but also via the oxidative decarboxylation of CoQ precursors. These findings fill a major gap in the knowledge of eukaryotic CoQ biosynthesis and shed light on the pathophysiology of human primary CoQ deficiency due to COQ4 mutations.

Spatial heterogeneity for APRIL production by eosinophils in the small intestine.

Eosinophils may reside in the lower intestine to play several homeostatic functions. Regulation of IgA+ plasma-cell (PC) homeostasis is one of these functions. Here, we assessed regulation of expression for a proliferation-inducing ligand (APRIL), a key factor from the TNF superfamily for PC homeostasis, in eosinophils from the lower intestine. We observed a strong heterogeneity, since duodenum eosinophils did not produce APRIL at all, whereas a large majority of eosinophils from the ileum and right colon produced it. This was evidenced both in the human and mouse adult systems. At these places, the human data showed that eosinophils were the only cellular sources of APRIL. The number of IgA+ PCs did not vary along the lower intestine, but ileum and right colon IgA+ PC steady-state numbers significantly diminished in APRIL-deficient mice. Use of blood cells from healthy donors demonstrated that APRIL expression in eosinophils is inducible by bacterial products. Use of germ-free and antibiotics-treated mice confirmed the dependency on bacteria for APRIL production by eosinophils from the lower intestine. Taken together, our study shows that APRIL expression by eosinophils is spatially regulated in the lower intestine with a consequence on the APRIL dependency for IgA+ PC homeostasis.

COQ4 is required for the oxidative decarboxylation of the C1 carbon of Coenzyme Q in eukaryotic cells.

Coenzyme Q (CoQ) is a redox lipid that fulfills critical functions in cellular bioenergetics and homeostasis. CoQ is synthesized by a multi-step pathway that involves several COQ proteins. Two steps of the eukaryotic pathway, the decarboxylation and hydroxylation of position C1, have remained uncharacterized. Here, we provide evidence that these two reactions occur in a single oxidative decarboxylation step catalyzed by COQ4. We demonstrate that COQ4 complements an Escherichia coli strain deficient for C1 decarboxylation and hydroxylation and that COQ4 displays oxidative decarboxylation activity in the non-CoQ producer Corynebacterium glutamicum. Overall, our results substantiate that COQ4 contributes to CoQ biosynthesis, not only via its previously proposed structural role, but also via oxidative decarboxylation of CoQ precursors. These findings fill a major gap in the knowledge of eukaryotic CoQ biosynthesis, and shed new light on the pathophysiology of human primary CoQ deficiency due to COQ4 mutations.