We cannot rule out that other organs or tissues are involved in the elevated circulating KYN levels in COPD as well, but these findings suggest that exercise training, currently a core component of pulmonary rehabilitation to treat muscle dysfunction in COPD [21], by restoring muscle PGC1α signaling and KAT4 expression, might also be beneficial in the treatment of mental comorbidities. Here, PPARGC1A is linked to chronic obstructive pulmonary disease.