Mitochondrial pyruvate carrier as a key regulator of fever and neuroinflammation
Introduction
Over the last years, neuroinflammation has been linked to changes in cellular metabolism (Ghosh et al., 2016, Mills et al., 2017, Nair et al., 2019, Peruzzotti-Jametti and Pluchino, 2018). Under inflammatory conditions, neuronal and non-neuronal cell types have an increased need in energy to fulfill their protective functions. Thus, changes in the bioenergetic pathways are required to supply the increased demand for energy (Lynch, 2020).
Fever is a brain-mediated increase in body core temperature, a nonspecific host defense response to infectious or inflammatory insults. The increase and maintenance of body temperature associated with fever is due to behavioral changes and autonomic responses aimed at increasing heat production and decreasing heat loss (Blomqvist and Engblom, 2018, Roth and Blatteis, 2014). The body metabolic rate is estimated to increase by about 10% to 12.5% for every 1 °C rise in body temperature (Evans et al., 2015, Kluger, 1986). Nevertheless, little is known about the metabolic changes that occur in the hypothalamus and the preoptic hypothalamic area (POA), the major thermoregulatory region in the central nervous system (Roth and Blatteis, 2014). Indeed, metabolomic profiling of the hypothalamus in yeast-induced fever in rats revealed an increase in the metabolism of purines, lipids, amino acids, and energy metabolism in the febrile group (Liu et al., 2015). Additionally, we recently performed label-free quantitative proteomics of the hypothalamus in a rat model of fever induced by lipopolysaccharide (LPS). Pathway analysis showed changes in several proteins related to cellular metabolic processes, including glycolysis, citric acid cycle and oxidative phosphorylation (Firmino et al., 2018).
Pyruvate is a key metabolite linking cytoplasmic and mitochondrial metabolism. It is generated in the cytoplasm mainly through glycolysis and is subsequently transported into the mitochondria by the mitochondrial pyruvate carrier (MPC), a protein complex in the inner mitochondrial membrane that is composed of two essential components, MPC1 and MPC2 (Olson et al., 2016). The activity of this transporter, its kinetics and specificity for substrates and inhibitors was described decades ago (Halestrap, 1975), but its molecular identity was only recently determined (Bricker et al., 2012, Herzig et al., 2012). Constitutive knockout of MPC1 (Bowman et al., 2016, Vanderperre et al.,2016) or MPC2 (Vigueira et al., 2014) in mice leads to embryonic lethality, whereas partial deletion of MPC1 results in low body weight, decreased movement, low body shell temperature, and less adipose tissue accumulation in mice (Zou et al., 2018).
Interestingly, in the previously mentioned proteomic analysis study, we also found that MPC1 is upregulated in the hypothalamus of febrile rats (Firmino et al., 2018). The functional cause of MPC1 upregulation remains to be investigated, but given the central metabolic position of the MPC complex, changes in its activity, either through post-translational modifications or protein abundance, may regulate overall cellular metabolism (Gray et al., 2016). Since changes in cellular metabolism also influence neuroinflammation and redox signaling (Aguilera et al., 2018), the reduction in the inflammatory response and the promotion of neuroprotection as a consequence of blocking pyruvate transport by MPC inhibition have been documented in experimental models of diabetes, aging, and neurodegenerative diseases (Divakaruni et al., 2017, Quansah et al., 2018, Ghosh et al., 2016).
In this study, we evaluated the functional capacity of hypothalamic mitochondria in a model of fever induced by LPS. We found a critical modulation of hypothalamic mitochondrial function during the peak of fever that was related to increased MPC activity. We also investigated the link between MPC inhibition and the reduction of fever, mitochondrial H2O2 production and neuroinflammation.
Section snippets
Animals
Fever experiments were performed in female and male Wistar rats (180–200 g body weight). The animals were obtained from the Animal House of the University of Brasília and housed at 24 ± 1 °C, under a 12:12-hour light–dark cycle (lights on at 7 a.m.), with access to food and water ad libitum. Animal handling and experiments were approved by the Animal Research Ethics Committee of the University of Brasília (Protocol no. 99/2017). This study was carried out according to the Guide for the Care and
Inhibition of mitochondrial pyruvate carrier attenuates fever induced by LPS
Administration of LPS (5 µg/kg, intravenously) induced an increase in body temperature, which started approximately one hour after injection and produced a biphasic fever with the first peak occurring around 2.5 and the second peak around 5 h [FLPS (1, 25) = 32.01, p < 0.0001; Ftime (26, 650) = 16.91, p < 0.0001; Ftime x LPS (26, 650) = 7.807, p < 0.0001, Fig. 1A]. Pretreatment of the animals with a selective MPC inhibitor, UK 5099 (1 µg, i.c.v.), decreased the febrile response mainly during
Discussion
This study revealed an important association between hypothalamic mitochondrial metabolism and the modulation of fever and neuroinflammation. We recently demonstrated the upregulation of MPC1 in the hypothalamus of febrile rats (Firmino et al., 2018), which was associated with a higher metabolic demand. Since the pharmacological inhibition of hypothalamic MPC attenuated fever and neuroinflammation induced by LPS, we postulated that pyruvate metabolism has a key role under these conditions.
Funding
This work was supported by the Fundação de Apoio à Pesquisa do Distrito Federal (FAPDF grants 193.001.730/2017, 00193-00000106/2019-75, 00193–00001324/2019-27); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq grant 424809-2018-4); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) – Finance Code 001; and Instituto Nacional de Ciência e Tecnologia e Neuro-ImunoModulação (INCT-NIM grant 485489/2014-1). N.C.G is a recipient of a scholarship from CNPq
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge Prof. Dr. Connie McManus (Institute of Biological Sciences, University of Brasília, Brazil) for the valid discussions about the statistical analysis, which contributed to the quality of our article and we thank Priscila Batista da Rosa for excellent graphical design.
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