We have long been searching for chemicals that could

We have long been searching for chemicals that could prevent such cell death in vivo. For this purpose, we have hypothesized that such chemicals would maintain cellular ATP levels under these pathological conditions. In order to address this hypothesis, we developed KUSs as specific inhibitors of the ATPase activity of VCP, the most abundant soluble ATPase in essentially all types of cells. Indeed, KUSs could prevent ATP decreases provoked by several cell death-inducing insults, e.g. glucose or serum deprivation, mitochondrial respiratory chain inhibition, etc., leading to the suppression of ER stress and cell death in cultured cells (Ikeda et al., 2014; Nakano et al., 2016). Furthermore, KUS administration was able to inhibit in vivo retinal neuronal cell death and mitigate disease phenotypes in several mouse models of retinal diseases, e.g. retinitis pigmentosa, glaucoma, and ischemic retinal disease (Hasegawa et al., 2016b; Hata et al., 2017; Ikeda et al., 2014; Nakano et al., 2016). On the other hand, our previous results indicated that activation of nuclear receptors, specifically ERRs, would lead to increased ATP production in the targeted cells or organs (Kamei et al., 2003). We, thus, searched for synthetic agonistic ligands of ERRs and showed in this study that esculetin could function as an ERR agonist. To the best of our knowledge, this is the first small chemical that functions as an ERR agonist for all three ERRs. As expected, esculetin treatment dose-dependently induced an increase in glucose and oxygen consumption in neuronally differentiated PC12 cells, eventually leading to high ATP levels, up to 180% relative to the non-treated cells (Fig. 2b, c, e, Fig. S1). We thus call these chemicals, as a whole, “ATP regulators”. Pathological hallmarks of Parkinson\'s disease are the loss of dopaminergic neurons in the substantia nigra and the presence of Lewy bodies, protein resperidone manufacturer of α-synuclein, in the extant dopaminergic neurons. To date, 18 genetic loci have been linked to familial human Parkinson\'s disease, and are named PARK1 to PARK18 (Klein and Westenberger, 2012; Lin and Farrer, 2014). PARK1 is the α-synuclein gene itself. These causative genes have been overexpressed, in the case of dominant inheritance, or knocked out, in the case of recessive inheritance, to produce mouse models for Parkinson\'s disease. However, no such mice have yet successfully manifested either the loss of dopaminergic neurons or Parkinson\'s disease-like phenotypes. On the other hand, there are established chemically induced Parkinson\'s disease rodent models, using MPTP, rotenone, 6-hydroxydopamine, etc. (Dauer and Przedborski, 2003; Lindholm et al., 2016). As contrasted with genetically manipulated models, these models did manifest prominent loss of dopaminergic neurons. Since we are interested in the death of dopaminergic neurons and their protection, we used chemically induced Parkinson\'s disease models in this study. Beyond our expectations, it was evident that KUSs and esculetin behave very similarly in preventing ATP decrease, ER stress, and cell death, in cell culture as well as in a mouse model of Parkinson\'s disease brought on by administration of MPTP. In neuronally differentiated PC12 cells and in primary cultures of dopaminergic neurons, MPP+ treatment led to decreased ATP levels, induction of CHOP expression (ER stress), and eventually cell death; yet both KUSs and esculetin suppressed all of these (Figs. 3–5g–i, 7, Figs. S2–S5). Moreover, administration of KUS121 or esculetin concurrently with MPTP maintained ATP levels and prevented ER stress in dopaminergic neurons in the substantia nigra, and ultimately suppressed the manifestation of Parkinson\'s disease phenotypes, as seen with the rotarod test (Figs. 8, 10). We confirmed very similar efficacies of KUS121 and esculetin in protecting dopaminergic neurons in the substantia nigra of a rotenone-induced mouse Parkinson\'s disease model (Fig. S15). We also observed that administration of GSK4716, an ERRγ-specific agonist, could mitigate the Parkinson\'s disease phenotypes. On the other hand, with an ERRγ-specific inverse-agonist, GSK5182, Parkinson\'s disease features were either unchanged or exacerbated (Fig. 8).