Project Summary/Abstract Fatigue is one of the most common and disruptive symptoms experienced by patients. It is present in about one out of five individuals in the general population, and its prevalence increases dramatically, up to and above 50%, in several medical conditions such as cancer and viral infections. The specific mechanisms responsible for fatigue remain largely unknown. Consequently, there are no mechanism-guided therapies for this condition. Although inflammation is often presented as a primary cause of fatigue, it is not always present in many instances of chronic fatigue. An alternative mechanism is mitochondrial dysfunction, as attested by the pivotal role of mitochondria in energy metabolism and the sensitivity of these organelles to cellular stress caused by toxins and pathogens. While it is not surprising that fatigue is the hallmark of mitochondrial diseases, what is still missing is how the deficit in energy metabolism caused by mitochondrial stress in metabolically active peripheral organs is transmitted to the brain to give rise to the feeling of fatigue and its behavioral expression. Mitokines such as GDF15 and FGF21 are candidates for such a communication as they are produced as both paracrine and hormonal factors to maintain energy homeostasis in conditions of mitochondrial stress. To test our hypothesis that mitokines induce fatigue by acting in the brain we will assess the ability of exogenously administered GDF15 and FGF21 to induce fatigue and study their receptor mechanisms (Aim 1) before determining whether their release as endogenous factors in response to mitochondrial stress mediates the fatigue that develops in this condition (Aim 2). In both aims, we will measure fatigue and its motivational component (the “can” and “will” aspects of fatigue) by decreased voluntary activity and reduced ability to engage in effortful behavior to obtain a reward. The tools used to study the role of GDF15 include a long-acting form of GDF15 and a neutralizing monoclonal antibody targeting its receptor, GFRAL. In addition, we will use chemogenetic approaches targeting CCK or TH contained in GFRAL expressing neurons in the hindbrain to identify its central site of action. The tools used to study the role of FGF21 include an analog of FGF21, LY2405319 administered via subcutaneously implanted osmotic minipumps, and a peptide antagonist of -klotho, the obligatory co-receptor of FGF21 receptors, that will be administered peripherally or centrally. Mitochondrial stress will be induced by administration of cisplatin, a cytotoxic agent used in chemotherapy, tunicamycin, a toxic compound that induces the unfolded protein response, and lipopolysaccharide. They all induce the release of both GDF15 and FGF21. The importance of mitochondrial stress will be checked by comparing wild type mice to Tfam+/- mice that have mitochondrial instability and should be more sensitive to the fatigue inducing effects of cisplatin. This preclinical proj...