Reply to: "Is NAFLD a key driver of brain dysfunction?"

Is NAFLD a key driver of brain dysfunction?Journal of HepatologyVol. 78Issue 4PreviewWe read with great interest the spectacular work of Hadjihambi and colleagues, published in August 2022 online ahead of print in the Journal.1 The authors aimed to establish the role of non-alcoholic fatty liver disease (NAFLD) in the development of brain dysfunction. To establish this role, the authors used a mouse model that they generated more than 10 years ago,2 monocarboxylate transporter-1 haploinsufficient (Mct1+/−) mice. MCT1 or SLC16A1 is a carrier of short-chain fatty acids, ketone bodies, and lactate in several tissues, including the liver, brain and adipose tissue, playing an important role in energy homeostasis in health and disease, including obesity, type 2 diabetes and cancer. Full-Text PDF Partial MCT1 invalidation protects against diet-induced non-alcoholic fatty liver disease and the associated brain dysfunctionJournal of HepatologyVol. 78Issue 1PreviewNon-alcoholic fatty liver disease (NAFLD) has been associated with mild cerebral dysfunction and cognitive decline, although the exact pathophysiological mechanism remains ambiguous. Using a diet-induced model of NAFLD and monocarboxylate transporter-1 (Mct1+/−) haploinsufficient mice, which resist high-fat diet-induced hepatic steatosis, we investigated the hypothesis that NAFLD leads to an encephalopathy by altering cognition, behaviour, and cerebral physiology. We also proposed that global MCT1 downregulation offers cerebral protection. Full-Text PDF Open Access We thank Sandforth and colleagues for their interest in our study.1Hadjihambi A. Konstantinou C. Klohs J. Monsorno K. Le Guennec A. Donnelly C. et al.Partial MCT1 invalidation protects against diet-induced non-alcoholic fatty liver disease and the associated brain dysfunction.J Hepatol. 2022; 78: 180-190Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar, 2Sandforth L El-Agroudy NN Birkenfeld AL Is NAFLD a key driver of brain dysfunction?.J Hepatol. 2023; 78Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar In their letter to the editor, Sandforth et al., refer to “body weight data missing in the report”. However, they do not indicate what would be the added value of reporting such data. Body weight was determined weekly, but these data were not presented as considered not of interest in the present context. More relevant to the focus of the article are the differences in fat mass. We would like to kindly point to Fig. 2A-B in our original paper, which depicts % fat mass and lean mass assessed by EchoMRI at the end of the 16-week feeding protocol, the time point at which all experiments were performed. Indeed, we found a significant difference in fat mass between Mct1+/+ and Mct1+/− mice under this diet, although substantial fat accumulation (essentially in adipose tissue) still occurs in Mct1+/− mice. In contrast, Mct1+/− mice are totally protected from fat accumulation in the liver as illustrated in Fig. 2D-E. Based on the body fat mass measurements by EchoMRI, we did not deem it necessary to repeat assessment of different fat depots or size of adipocytes in subcutaneous adipose tissue,[3]Lengacher S. Nehiri-Sitayeb T. Steiner N. Carneiro L. Favrod C. Preitner F. et al.Resistance to diet-induced obesity and associated metabolic perturbations in haploinsufficient monocarboxylate transporter 1 mice.PLoS One. 2013; 8e82505Crossref PubMed Scopus (58) Google Scholar as our study was focused on investigating the impact of NAFLD on the brain rather than elucidating the mechanism underlying the protective phenotype of Mct1+/− mice. While body fat content is responsible for low-grade subclinical inflammation in adipose tissue, a direct link with brain dysfunction remains to be established. However, we acknowledge that body fat distribution in different adipose tissues might be a contributing factor and would deserve to be further investigated as discussed in our manuscript. The study by Lengacher et al.[3]Lengacher S. Nehiri-Sitayeb T. Steiner N. Carneiro L. Favrod C. Preitner F. et al.Resistance to diet-induced obesity and associated metabolic perturbations in haploinsufficient monocarboxylate transporter 1 mice.PLoS One. 2013; 8e82505Crossref PubMed Scopus (58) Google Scholar aimed to characterise the peripheral changes which occur in Mct1+/− mice due to high-fat diet (HFD) feeding. In our study, the same solid diet composition was used, but a 16-week rather than 12-week feeding protocol was selected. In addition, liquid calories in the form of fructose and glucose in the water (HF/HG) were also introduced as they are known to lead to overconsumption of energy, and ultimately to weight gain.[4]de Graaf C. Why liquid energy results in overconsumption.Proc Nutr Soc. 2011; 70: 162-170Crossref PubMed Scopus (75) Google Scholar This was prominent also in Mct1+/− mice, which were no longer able to regulate their food intake and gained a significantly higher fat mass compared to mice fed a HFD alone.[3]Lengacher S. Nehiri-Sitayeb T. Steiner N. Carneiro L. Favrod C. Preitner F. et al.Resistance to diet-induced obesity and associated metabolic perturbations in haploinsufficient monocarboxylate transporter 1 mice.PLoS One. 2013; 8e82505Crossref PubMed Scopus (58) Google Scholar This feeding protocol modification was used to mimic a western diet-induced animal model of NAFLD. Our data emphasise that with this more challenging diet, the most resistant component in Mct1+/− mice is hepatic lipid accumulation. Since these mice are still protected from brain dysfunction, it makes it likely that prevention of hepatic steatosis must play an essential role. Mice were group caged to avoid inducing stress associated with long-term single caging. As a result, we were unable to monitor water intake individually. However, no significant differences were observed on average water intake per cage between groups. The possibility that lower caloric intake in Mct1+/− mice on HFDHF/HG could play a role in protecting against NAFLD and brain dysfunction, as proposed by Sandforth and colleagues, cannot be excluded. We did not collect data on energy absorption and expenditure in the present study, as this had been well characterised in a previously published study,[3]Lengacher S. Nehiri-Sitayeb T. Steiner N. Carneiro L. Favrod C. Preitner F. et al.Resistance to diet-induced obesity and associated metabolic perturbations in haploinsufficient monocarboxylate transporter 1 mice.PLoS One. 2013; 8e82505Crossref PubMed Scopus (58) Google Scholar and is established as part of the protective mechanism of Mct1+/− mice under HFD. Whether these parameters could be different under our new diet remains an open question that might be addressed subsequently. However, it is unlikely that it will substantially modify the conclusions of our study. Recently published studies in humans and experimental animals have proposed a very intriguing mechanism, suggesting that hepatic lipid accumulation is a driver of obesity-induced hyperinsulinemia and insulin resistance.[5]Geisler C.E. Ghimire S. Bruggink S.M. Miller K.E. Weninger S.N. Kronenfeld J.M. et al.A critical role of hepatic GABA in the metabolic dysfunction and hyperphagia of obesity.Cell Rep. 2021; 35109301Abstract Full Text Full Text PDF Scopus (9) Google Scholar,[6]Geisler C.E. Ghimire S. Hepler C. Miller K.E. Bruggink S.M. Kentch K.P. et al.Hepatocyte membrane potential regulates serum insulin and insulin sensitivity by altering hepatic GABA release.Cel Rep. 2021; 35109298Google Scholar These results support a key role for hepatocyte GABA as a neuro-hepatokine that is dysregulated in obesity and NAFLD, leading to dysfunctional glucoregulation and feeding behaviour,[6]Geisler C.E. Ghimire S. Hepler C. Miller K.E. Bruggink S.M. Kentch K.P. et al.Hepatocyte membrane potential regulates serum insulin and insulin sensitivity by altering hepatic GABA release.Cel Rep. 2021; 35109298Google Scholar and supporting our proposed hypothesis of NAFLD-induced dysfunction. However, we have clearly acknowledged the potential contribution of adipose tissues as well as other organs in any NAFLD-associated brain alterations, as discussed in our manuscript. Finally, Sandforth and colleagues proposed that reduced MCT1 expression in microglia might contribute to the observed brain protection. Indeed, it was something we have considered and addressed in our manuscript. We also agree with them that it is tempting to speculate that while the liver might have contributed to brain dysfunction, brain dysfunction could also have preceded and contributed to liver dysfunction. Notwithstanding, we hope that our study will bring attention to the need for early diagnosis and treatment of NAFLD, while having a direct impact on policies worldwide regarding the health risk associated with this disease, its prevention and treatment. The authors received no financial support to produce this manuscript. The authors declare no conflicts of interest that pertain to this work. Please refer to the accompanying ICMJE disclosure forms for further details. The manuscript was written by Anna Hadjihambi and Luc Pellerin, who both reviewed the final version critically. The following are the supplementary data to this article: Download .pdf (.21 MB) Help with pdf files Multimedia component 1