Authors: Contreras N^1,2*, Macías-Camero A^1,2*, Delgado-Dolset MI^1,2.

Affiliations: ¹Centre for Metabolomics and Bioanalysis (CEMBIO), Department of Chemistry and Biochemistry, Facultad de Farmacia, Universidad San Pablo-CEU, CEU Universities, Urbanización Montepríncipe, 28660 Boadilla del Monte, Madrid, Spain.²Instituto de Medicina Molecular Aplicada Nemesio Díez (IMMA-NM), Departamento de Ciencias Médicas Básicas, Facultad de Medicina, Universidad San Pablo-CEU, CEU Universities, Urbanización Montepríncipe, 28660 Boadilla del Monte, Madrid, Spain.

*These authors contributed equally

Correspondence to: María Isabel Delgado-Dolset , Instituto de Medicina Molecular Aplicada Nemesio Díez (IMMA-NM), Departamento de Ciencias Médicas Básicas, Facultad de Medicina, Universidad San Pablo CEU, CEU Universities, Urbanización Montepríncipe, 28660 Boadilla del Monte, Madrid, Spain.

Campus Montepríncipe. Crtra. Boadilla del Monte km 5.3.

CP 28668 Boadilla del Monte. Madrid, Spain.

Tlf: +34 91 372 47 00 ext. 15068

E-mail: maria.delgadodolset@ceu.es

Conflict of interest: The authors have no conflicts of interest to declare.

Funding information: The authors received no specific funding for the elaboration of this article.

Authorship: All the authors approved the final version of the manuscript as submitted and agreed to be accountable for all aspects of the work.

Acknowledgments: The authors acknowledge the support by Instituto de Salud Carlos III (PI18/01467 and PI19/00044), co-funded by FEDER “Investing in your future” for the thematic network and co-operative research centres ARADyAL RD16/0006/0015 and RICORS Red de Enfermedades Inflamatorias (REI) RD21 0002 0008. Authors would also like to recognize the funding by the Ministry of Science and Innovation in Spain (PCI2018-092930), co-funded by the European program ERA HDHL—Nutrition and the Epigenome, project Dietary Intervention in Food Allergy: Microbiome, Epigenetic and Metabolomic interactions (DIFAMEM); and by Fundación Mutua Madrileña (AP177712021). N.C. and A.M.C. are supported by FPI-CEU predoctoral fellowships. The authors would like to thank Dr Domingo Barber, Dr María M Escribese and Dr Alma Villaseñor for their asserted comments.

List of abbreviations : 2-HG: 2-HidroxyGlutarate, α-KG: α-KetoGlutarate, CD4: Cluster of Differentiation 4, ETC: Electron Transport Chain, IL17A: InterLeukin 17A, LKB1: Liver-associated Kinase B1, NET: Neutrophil Extracellular Trap, OPA1: Optic Atrophy 1, PHGDH: PHosphoGlycerate DeHydrogenase, T_reg: Regulatory T Helper, T_H: T Helper

In the last 20 years, increasing evidence has arisen challenging the belief that mitochondria are mere ATP-synthesizing machines, shedding light on their role in cell signaling (1). Metabolites, energy mediators, and physical interactions involving membrane rearrangements are some of the mechanisms involved in mitochondria-driven cell regulation (1). In this sense, energetics plays a role in the development and function of immune cells, and immunometabolism is a flourishing field. Nonetheless, how mitochondria signaling networks, including membrane dynamics, affect T cell development and differentiation remains unclear (2).

In a recently published work, Baixauli et al (3) investigated how CD4⁺ T cell differentiation is influenced by mitochondrial membrane morphology. In vitro analysis showed that elongated mitochondria with tight cristae in T_H17 cells correlated with higher levels of the long isoform protein of OPA1 (L-OPA1) when compared to T_H1 and T_H2 cells. Moreover, they developed an OPA1 knockout mouse model (Opa1^Cd4-cre ) which showed that, besides controlling mitochondrial membrane dynamics, OPA1 also regulated IL17A production, suggesting its potential role in the regulation of T_H17 cells effector function.

To address this matter, a multi-omic approach, including epigenomics, transcriptomics, proteomics, and metabolomics, was applied. They found several changes in the mitochondria due to the lack of OPA1 that could lead to the loss of Il17a expression. First, as a result of a disrupted inner mitochondrial membrane, electron transport chain (ETC) subunits uncouple, leading to an increase in the NADH/NAD⁺ ratio. Higher levels of NADH, together with an increase in the oxidation of glutamine, promote α-ketoglutarate (α-KG) conversion towards 2-hidroxyglutarate (2-HG) by phosphoglycerate dehydrogenase (PHGDH). 2-HG accumulation increases histone and DNA methylation that lastly alters chromatin accessibility in immune response genes interfering with Il17a expression.

Pathway analysis was performed to determine OPA1 intracellular biological mediators, raising LKB1 as its major upstream regulator. While LKB1 activity was increased inOpa1^Cd4-cre mice, authors demonstrated thatLkb1 deletion restored cell carbon metabolism and Il17aexpression by reducing the production of PHGDH and other serine biosynthesis enzymes.

This article provides a perspective of the OPA1-LKB1 axis and its role in immune regulation in T_H17 cells, which grants a deep understanding on how the different types of molecules are intertwined in the disease (4). As for possible limitations, this work was done using solely a mouse model, which, despite being as extraordinary as it is, does not necessarily match the conditions and metabolic changes that take place in human cells (1). It would have been interesting to see some of these experiments being done in T cells from human donors to corroborate these findings, which would be possible by using CRISPR/Cas technology to delete OPA1 and/or LKB1 .

Furthermore, it is still necessary to understand how, if at all, OPA1-LKB1 axis regulates other subsets of T cells, such as T_H1 or T_reg. It is known that LKB1 affects other immune cell types, even within the innate immune response. For example, LKB1 deficiency in mouse dendritic cells results in higher levels of T_reg in vivo that promote an immune-suppressed phenotype through mTOR signaling, impairing tumor growth control and protecting against allergic asthma development (5). Moreover, deletion of Lkb1 in mice alveolar macrophages leads to more severe asthma and higher susceptibility to S. aureusinfection through the AMPK pathway (6); and an increased number of neutrophils. Additionally, it has been described that OPA1-dependent ATP production is needed for neutrophil extracellular trap (NET) formation and effective antibacterial defense both in human and mouse neutrophils (7). However, these studies fail to analyze the OPA1-LKB1 relation, which, to our knowledge, has been described for the first time by Baixauli et al. All in all, these authors have uncover the great potential of the OPA-LKB1 axis in the immunometabolism research field.

REFERENCES

1. Picard M, Shirihai OS. Mitochondrial signal transduction. Cell Metab 2022;34 :1620–1653.

2. Shyer JA, Flavell RA, Bailis W. Metabolic signaling in T cells.Cell Res 2020;30 :649–659.

3. Baixauli F, Piletic K, Puleston DJ, Villa M, Field CS, Flachsmann LJ et al. An LKB1–mitochondria axis controls TH17 effector function.Nature 2022;610 :555–561.

4. Radzikowska U, Baerenfaller K, Cornejo-Garcia JA, Karaaslan C, Barletta E, Sarac BE et al. Omics technologies in allergy and asthma research: An EAACI position paper. Allergy2022;77 :2888–2908.

5. Pelgrom LR, Patente TA, Sergushichev A, Esaulova E, Otto F, Ozir-Fazalalikhan A et al. LKB1 expressed in dendritic cells governs the development and expansion of thymus-derived regulatory T cells.Cell Res 2019;29 :406–419.

6. Wang Q, Chen S, Li T, Yang Q, Liu J, Tao Y et al. Critical Role of Lkb1 in the Maintenance of Alveolar Macrophage Self-Renewal and Immune Homeostasis. Front Immunol 2021;12 :1–12.

7. Amini P, Stojkov D, Felser A, Jackson CB, Courage C, Schaller A et al. Neutrophil extracellular trap formation requires OPA1-dependent glycolytic ATP production. Nat Commun 2018;9 :2958.