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Mitochondria are multifunctional organelles that actively participatein the immune-inflammatory response in various pathologies. This volume updates readers onknowledge about mitochondria function. The editors have compiled six chaptersabout inflammation in its broadest sense, with contributions from active groupsof cell biologists, infectologists and pathologists. The chapters in this volume focus on research related to five notablediseases:(1) two diseases (one bacterial and one viral) in which theexacerbation of the inflammatory response can lead to neuropathies: leprosy(one of the oldest diseases in the world) and Zika fever (a disease relativelynew in Brazil) (2) three diseases (two bacterial and one viral) in which theexacerbation of the inflammatory response can lead to irreversible lung damagethat can cause rapid death: tuberculosis, pneumonia and the most recent globaldisease, COVID- 19. New information about mitochondrial biology is presented, such as theeffect of aerobic physical exercise as a stimulator for mitochondriamultiplication, and the role of mitochondrial damage in inducing immune-inflammatoryresponses to pathogens. The contents shed light on mitochondrial biochemicalpathways that could serve as potential therapeutic targets. This is an important reference for scholars (cell biologists,microbiologists) in universities, hospitals and scientific research centersworking on biological and biomedical problems, and for health professionalsinvolved in infection control.

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Table of Contents
BENTHAM SCIENCE PUBLISHERS LTD.
End User License Agreement (for non-institutional, personal use)
Usage Rules:
Disclaimer:
Limitation of Liability:
General:
FOREWORD
PREFACE
DEDICATION
List of Contributors
An Auspicious Bacterium: How Mitochondria can be Beneficial to the Innate Immunity through Aerobic Exercises
Abstract
1. Mitochondrial Biogenesis
2. Mitochondria – a multifunctional organelle: from the correlation of DNA mitochondrial with inflammatory diseases and innate immune response
3. Mitochondria can be involved in the pathogenesis of some viral diseases
4. Mitochondria and aerobic exercise: new frontiers in health promotion
5. The Tight Link between PGC -1, PPARγ, Mitochondrial DNA and the Immuno-inflammatory Response
6. FINAL CONSIDERATIONS AND PERSPECTIVES
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
ACKNOWLEDGEMENTS
REFERENCES
Mitochondrial Dysfunction in Leprosy: Shedding Light on the Neurodegenerative Consequences
Abstract
1. Leprosy-an overview
2. Clinical forms of leprosy
3. Treatment for leprosy
3.1. Treatment for Reactions in Leprosy
4. Outline of some inflammatory cytokines in leprosy reactional episodes
5. TNF-α and leprosy neuritis
6. Mitochondria, mitochondrial DNA and leprosy
7. Leprosy - special remarks
8. Final considerations and perspectives
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
ACKNOWLEDGEMENTS
REFERENCES
The Multifaceted Interface Between the Host Immune Cell and Mycobacterium Tuberculosis - Mitochondria at the Crux of the Matter
Abstract
1. Tuberculosis-an overview
2. Status of the TB epidemic
3. Tuberculosis-Disease, Diagnosis, Vaccine and Treatment
4. Latent tuberculosis
5. The relevance of macrophages on TB pathogenesis
6. Host cell death during M. tuberculosis infection - the active role of mitochondria
7. Final considerations and perspectives
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
ACKNOWLEDGEMENTS
REFERENCES
Mitochondrial DNA and Streptococcus pneumoniae Infection – Induction of Immuno-inflammatory Response
Abstract
1. INTRODUCTION
2. Pneumococcal pathogenesis: virulence factors, inflammation and mitochondrial DNA leakage
3. Mitochondrial DNA and S. pneumoniae
4. Final considerations and perspectives for therapeutic interventions
CONCLUSION
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
ACKNOWLEDGEMENT
REFERENCES
Mitochondrial DNA Role in Zika Virus Infection
Abstract
1. Introducing Zika Virus (ZIKV)
2. Brief overview of the innate immune response to zika virus
3. Mitochondria Fragmentation
4. Mitochondrial DNA and the immuno-inflammatory response in ZIKV
5. Final considerations and perspectives
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
ACKNOWLEDGEMENT
REFERENCES
Mitochondrial Dysfunction and the Immuno-inflammatory Response Induced by SARS-CoV-2 Infection: the Role of Mitochondrial DNA
Abstract
1. Introducing Coronavirus Infectious Disease 2019 (COVID-19)
2. The immunopathogenesis of COVID-19
3. Mechanisms involved in mitochondrial-induced cellular stress in SARS-CoV-2 Infection
4. Mitochondrial dysfunction and the immuno-inflammatory response in COVID-19: The role of mtDNA
4.1. mtDNA
4.2. mtROS
4.3. MAVS
5. Final considerations and perspectives
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
ACKNOWLEDGEMENT
REFERENCES
Frontiers in Inflammation
(Volume 3)
Mitochondrial DNA and the Immuno-inflammatory Response: New Frontiers to Control Specific Microbial Diseases
Edited by
Dilvani Oliveira Santos (Editor)
Postgraduate Program in Applied Microbiology and Parasitology of the University
and Postgraduate Program in Science and Biotechnology University Federal Fluminense (UFF)
Niterói, RJ
Brazil
&
Paulo Renato Zuquim Antas (Co-Editor)
Clinical Immunology Laboratory/ Postgraduate Program in Tropical Medicine/ Oswaldo
Cruz Foundation
Manguinhos, RJ
Brazil

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FOREWORD

Microorganisms are the most abundant and diverse beings on Earth and are capable of occupying various ecological niches. Among them, there are pathogenic microorganisms that have the ability to cause infections or diseases when interacting with a host, who they need to thrive and survive. Once the pathogen sets itself up in a host, it manages to avoid the host’s immune response and uses its resources to replicate before spreading to new ones.

Infectious diseases are among the main cause of morbidity and mortality worldwide and are a major challenge for the biomedical sciences. Recently, much progress has been made towards unraveling the mechanisms of microbial pathogenesis, including the immuno-inflammatory response elicited by the parasite-host relationship. It is worth mentioning that the mitochondrial DNA stands out, known for its role in oxidative phosphorylation and maternally inherited mitochondrial diseases. The release of mitochondrial DNA into the cellular cytoplasm and out to the extracellular milieu activates different pattern recognition receptors and innate immune responses leading to robust actions.

Mitochondrial DNA and the Immuno-inflammatory response: new frontiers to control specific microbial diseases aims to present state-of-the-art coverage on topics central to the understanding of the interactions between pathogenic microorganism (bacteria and virus) and hosts. The book is divided into six chapters written by professionals with expertise in the field of cell biology and immuno-inflammatory response. The chapters cover the complexity of mitochondrial metabolism; the mitochondrial dysfunction in leprosy; mitochondria and the host immune cell against Mycobacterium tuberculosis; disturbance of mitochondrial function in Streptococcus pneumoniae infection; inflammatory response in Zika virus infection and mitochondrial dysfunction; and the potential role of mtDNA as an important marker of hyper inflammation in the progress of COVID-19. This book represents a comprehensive and an indispensable tool for researchers in immunology and microbiology wishing to keep abreast with the latest developments in cellular immunology and mitochondrial DNA. In addition, it provides a reliable reference for undergraduate and graduate students in their pursuit of becoming competent future immunologists/microbiologists, as well as for health professionals in general.

Cintia de Moraes Borba Laboratory of Taxonomy, Biochemistry and Bioprospecting of Fungi Oswaldo Cruz Institute, Oswaldo Cruz Foundation FIOCRUZ, Rio de Janeiro-RJ Brazil

PREFACE

The first cases of the infection caused by the virus SARS-CoV-2 were reported in December, 2019, in China, hence the name of the disease: COVID-19 (Corona Virus Disease, year 2019). The world saw the emergence of a global pandemic in 2020, which still poses a threat to global health in 2021, despite the recent mass vaccination. In some countries like the USA, Brazil and India, the number of deaths is still worrisome. The most important lesson learnt from this disease is its destructive potential of triggering a sudden and uncontrolled inflammatory response to the virus, which can rapidly decimate populations worldwide. That was our biggest motivation for choosing the topic Mitochondrial DNA and the Immuno-inflammatory response: new frontiers to control specific microbial diseases as the second volume in the book series "Frontiers in Inflammation".

Our objective is to present to the reader a book on the topic of "inflammation" in its broadest sense, including relatively recent scientific discoveries concerning the active participation of a cell organelle—mitochondria—and its respective constituents, mainly mitochondrial DNA (mtDNA), in the immuno-inflammatory responses. It is worth mentioning that, coincidently, this organelle was studied by James P. Allison, PhD, and Tasuku Honjo, PhD, who were awarded the Nobel Prize in Physiology or Medicine for their discovery regarding cancer therapy by inhibition of negative immune regulation. Thus, the mechanism for oxygen sensing (mitochondria) has fundamental importance in Physiology and Pathology, in areas such as the metabolism, immune response and ability to adapt to exercise. All in all, the role of mitochondria goes far beyond their contribution to cellular energy metabolism. Mitochondria are multifuncional organelles that actively participate in the immuno-inflammatory response in several pathologies. To develop this subject, we chose some pathologies which have already been studied under the light of this specific area. Therefore, this book will address: (1) two diseases (one bacterial and the other one viral) in which the exacerbation of the inflammatory response can lead to neuropathies— leprosy (one of the oldest diseases in the world) and Zika fever (a relatively new disease in Brazil)—and (2) three diseases (two bacterial and one viral) in which the exacerbation of the inflammatory response can lead to irreversible lung damage that can cause rapid death—tuberculosis, pneumonia and the most recent global pathology, COVID-19. In addition, the introductory chapter of this book deals with updates on mitochondria as multifunctional organelles, enabling Cell Biology to better interface with Physiology, Pathology and Immunology.

Our goal is to provide up-to-date content on the chosen topic, aiming at broadening horizons and awakening readers, especially infectologists and pathologists, about the importance of investigation and research on the subject of inflammation, a very fascinating and promising topic for new discoveries of therapeutic targets.

We hope that this content may be useful in universities, hospitals and scientific research centers, as well as for health professionals in general. It is worth mentioning that each author and co-author presents their experience in their area of expertise in Cell Biology and Infectious Diseases.

Brazil is one of the countries listed with a high incidence of leprosy, Zika fever, tuberculosis, bacterial pneumonia and COVID-19. We would like to express our sincere thanks to all authors who have contributed chapters to this book. We would also like to thank Bentham Science Publishers for the publication opportunity and for their support in disseminating knowledge.

Dilvani Oliveira Santos (Editor) Postgraduate Program in Applied Microbiology and Parasitology, Biomedical Institute, University Federal Fluminense (UFF) & Postgraduate Program in Science and Biotechnology / Institute of Biology, University Federal Fluminense (UFF) Niterói, RJ Brazil & Paulo Renato Zuquim Antas (Co-Editor) Clinical Immunology Laboratory/Postgraduate Program in Tropical Medicine/Oswaldo Cruz Foundation Manguinhos, RJ Brazil

DEDICATION

This book is dedicated to healthcare professionals and all individuals who have dealt or are dealing with one or more of the illnesses covered in this book.

List of Contributors

Alessandra D' Almeida FilardyDepartment of Immunology, Institute of Microbiology Paulo de Góes, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, BrazilAnalúcia Rampazzo XavierMultiuser Laboratory for Research Support in Nephrology and Medical Sciences, Faculty of Medicine, University Federal Fluminense, Niterói, Rio de Janeiro, RJ, Brazil Department of Pathology, Faculty of Medicine, University Federal Fluminense, Niterói, Rio de Janeiro, BrazilAndrea Alice SilvaMultiuser Laboratory for Research Support in Nephrology and Medical Sciences, Faculty of Medicine, University Federal Fluminense, Niterói, Rio de Janeiro, RJ, Brazil Department of Pathology, Faculty of Medicine, University Federal Fluminense, Niterói, Rio de Janeiro, BrazilAnna Fernandes Silva Chagas do NascimentoPostgraduate Program in Applied Microbiology and Parasitology of the University Federal Fluminense (UFF), Niterói, Rio de Janeiro, BrazilArthur Willkomm KazniakowskiFaculty of Pharmacy, University Federal Fluminense (UFF), Niterói, Rio de Janeiro, BrazilDébora Familiar-MacedoLaboratory of Viral Immunology, Oswaldo Cruz Foundation, Rio de Janeiro, BrazilDilvani Oliveira SantosPostgraduate Program in Applied Microbiology and Parasitology of the University Federal Fluminense (UFF), Niterói, Rio de Janeiro, Brazil Postgraduate Program in Science and Biotechnology (PPBI)/Institute of Biology, Federal Fluminense University (UFF), Niterói, Rio de Janeiro, BrazilFabiana Rabe CarvalhoMultiuser Laboratory for Research Support in Nephrology and Medical Sciences, Faculty of Medicine, University Federal Fluminense, Niterói, Rio de Janeiro, RJ, BrazilFelipe Piedade Gonçalves NevesDepartment of Microbiology and Parasitology, Biomedical Institute, University Federal Fluminense, Niterói, RJ, BrazilLaura Brandão MartinsBiomedical Institute, University Federal Fluminense (UFF), Niterói, Rio de Janeiro, BrazilMaria Elena SamarDepartment of Oral Biology, Faculty of Dentistry, National University of Cordoba, Cordoba, ArgentinaPaulo Renato Zuquim AntasClinical Immunology Laboratory, Oswaldo Cruz Foundation, Manguinhos, RJ, Brazil Postgraduate Program in Tropical Medicine, Oswaldo Cruz Foundation, Manguinhos, RJ, BrazilRodolfo AvilaDepartment of Cell Biology and Histology, Faculty of Medical Sciences, National University of Cordoba, Cordoba, ArgentinaSourou Credo Francisco Justus ZinsouBiomedical Institute, University Federal Fluminense (UFF), Niterói, Rio de Janeiro, BrazilTatiana de Castro Abreu PintoDepartment of Medical Microbiology, Institute of Microbiology Paulo de Góes, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, BrazilThalia MedeirosMultiuser Laboratory for Research Support in Nephrology and Medical Sciences, Faculty of Medicine, University Federal Fluminense, Niterói, Rio de Janeiro, RJ, Brazil

An Auspicious Bacterium: How Mitochondria can be Beneficial to the Innate Immunity through Aerobic Exercises

Dilvani Oliveira Santos1,2*,Arthur Willkomm Kazniakowski3,Anna Fernandes Silva Chagas do Nascimento1,Laura Brandão Martins4,Sourou Credo Francisco Justus Zinsou4,Rodolfo Avila5,Maria Elena Samar6
1 Postgraduate Program in Applied Microbiology and Parasitology of the University Federal Fluminense (UFF), Niterói, Rio de Janeiro, Brazil
2 Postgraduate Program in Sciences and Biotechnology of the University Federal Fluminense (UFF), Niterói, Rio de Janeiro, Brazil
3 Faculty of Pharmacy, University Federal Fluminense (UFF), Niterói, Rio de Janeiro, Brazil
4 Biomedical Institute, University Federal Fluminense (UFF), Niterói, Rio de Janeiro, Brazil
5 Department of Cell Biology and Histology, Faculty of Medical Sciences, National University of Cordoba, Cordoba, Argentina
6 Department of Oral Biology, Faculty of Dentistry, National University of Cordoba, Cordoba, Argentina

Abstract

Mitochondria are highly relevant organelles with regard to their unique function in generating energy and contributing to metabolism within the cell. Furthermore, recent studies suggest that they might have an influence on the innate immune and inflammatory responses, thus affecting antiviral immunity (as example: Zika virus (ZIKV), hepatitis C virus (HCV), dengue virus and SARS-CoV-2 virus) and antibacterial immunity as well (Streptococcus pneumoniae, Mycobacterium leprae and Mycobacterium tuberculosis). Therefore, this chapter aims at bringing a relevant debate about the role of mitochondria and their multifunctional capacity. We intend to discuss the complexity of mitochondrial metabolism, especially during aerobic physical exercises, which causes the modulation of the gene expression of proteins that lead to mitochondrial proliferation and, thus, promote health. In addition, considering the injuries caused by hypoxia, this chapter also stresses the enormous potential of mitochondria to enable the survival of eukaryotic cells by allowing them to turn to aerobic respiration, as shown in previous scientific studies. In conclusion, this chapter points out the importance of mitochondrial biogenesis (both natural and stimulated

biogenesis by aerobic exercise) and the benefits this organelle brings to the health, arguing that they go far beyond cellular respiration and oxidative phosphorylation.

Keywords: Aerobic exercises, Immunoinflammatory response, Mitochondrial biogenesis.
*Corresponding author Dilvani Oliveira Santos: Postgraduate Program in Applied Microbiology and Parasitology of the University Federal Fluminense (UFF), Niterói, RJ, Brazil and Postgraduate Program in Science and Biotechnology (PPBI)/Institute of Biology; Federal Fluminense University (UFF), Niterói, Rio de Janeiro, Brazil; Tel: 55(21)2629-2552; E-mails: [email protected] and [email protected]

1. Mitochondrial Biogenesis

The way in which life emerged on Earth is the subject of study by many scientists throughout the history of science. Assumptions and theories have been elaborated to suggest how cellular metabolism developed [1]. Despite those scientific efforts, the origin of the eukaryotic cell remains unknown. Nevertheless, it is known that there are two main cell types: the prokaryote and the eukaryote cell. The prokaryote cell is a simpler example consisting of cytoplasm, genetic material, cell wall, plasma membrane, cilia, flagella, ribosome and plasmid. The eukaryotic cell has some components similar to the prokaryotic cell, while it has additional components such as mitochondria, Golgi apparatus, nucleus, lysosomes, secretion vesicles, endoplasmic reticulum and peroxisomes (which can be found in most eukaryotic cells). However, glyoxysomes, chloroplasts, chlorophylls and the cell wall are present only in the plant cell. Together, these peculiarities make the eukaryotic cell (Fig. 1) a very complex system. In order to understand the complexity of these cells, it is necessary to have a closer look at relevant events in tissues, organs, systems and organisms.

The energetic role of mitochondria is intimately linked to the origin of the eukaryotic cell and their development in complex organisms. Comprehension of the evolutionary origin of mitochondria is essential for understanding any biological structure or process specially involved in birth, aging-related diseases and cell death. In this context, it is relevant to begin this review by introducing the meaning of “Biogenesis”, mentioned by Attardi et al. [2] and Leaver et al. [3] to refer to the production of new mitochondria inside the cells. According to the review of Nisoli et al. [4], mitochondrial division occurs concurrently with the nuclear division. Despite the kinetics of mitochondrial division not coinciding with cell proliferation all the time, it is verified that, in muscle cells, mitochondria divide during both events: myogenesis and physical exercise. Furthermore, mitochondria can also duplicate after some special circumstances, such as under benzodiazepine treatment, inhibitors of oxidative phosphorylation, phorbol esters and calcium modulation [4].

Fig. (1)) Eukaryotic cell and its organelles. Note that mitochondria are the only intracellular organelle with DNA inside, besides the nucleus.

The scientist Lynn Margulis was the main proponent of the endosymbiotic theory of the mitochondria’s evolution. She thoroughly changed the understanding of the evolution of nuclear cells by proposing that it was the result of symbiotic fusions of bacteria. Throughout her career, Margulis's research has not received due credit in the scientific community and her article entitled “On the Origin of Mitosing Cells” appeared in 1967, after being rejected by about fifteen journals [5]. Margulis defended the theory that cellular organelles, such as mitochondria and chloroplasts, had been independent bacteria, and this knowledge was ignored for another decade, only being accepted after robust genetic evidence [6, 7]. Anderson et al. [8] reported that complete genomic sequences for many mitochondria, as well as for some bacteria, were a consistent demonstration to explain the origin of mitochondria. In addition, they argued that phylogenetic reconstructions with genes encoding proteins active in metabolism and energy translation were the confirmation of the simplest version of the endosymbiosis hypothesis. These same authors warned that the hypotheses of hydrogen and syntrophy for the origin of the mitochondria, however, were not yet completely clear, but that future research in this direction would probably show that the evolution of hydrogenosomes could be related to that of mitochondria.

Despite the classic formulations of the endosymbiotic theory, research by Martin et al. [9], in accordance with the previous study by Margulis [5], assumed that mitochondria were acquired by a host cell capable of ingesting food bacteria with the help of phagocytosis. The premise that the host was phagocytic never received experimental support. A decade later, the same authors [10] reported several theories about the origin of mitochondria that do not involve phagocytosis first, and, in 2017, they affirmed that all of those other theories are models based on anaerobic syntrophy that would be responsible for the common ancestry of mitochondria and hydrogenosomes [11].

Interestingly, Archibald [12] attests that the evolution of the simplest prokaryotic eukaryotic cell was probably the most important event in the history of life and that the greatest mystery in biology is the vast and ancient nature of the prokaryote-eukaryotic division. Archibald mentioned that, despite the efforts of the scientific community, several points are still beyond the understanding of scientists. He mentions that some scientists link the origin of the eukaryotic cell to the origin of the mitochondria and attribute this to a single event, which is that of energy conversion. Still, according to this author, other scientists believe that the nucleus, the cytoskeleton and other eukaryotic complexities evolved step by step before the mitochondria. Yet, according to Archibald (2014) [12], thanks to advances in DNA-based research, endosymbiosis played an important role in the evolution of cellular complexity (Fig. 2), first with the eukaryotic cell and then with plants and algae. This author points out that, without plants and animals of any kind, we would not be here to reflect on our existence.

2. Mitochondria – a multifunctional organelle: from the correlation of DNA mitochondrial with inflammatory diseases and innate immune response

To address mitochondrial multifunctionality, it is important to consider that there is a branch in Cell Biology that greatly enriched the knowledge of various mitochondrial functions – the so-called Bioenergetic [1]. This topic is complementary to the molecular and cellular studies, improving the information about the composition of this organelle (Fig. 3), including how mitochondrial DNA duplication occurs. Moreover, the studies on free radical production and coordinated expression of the mitochondrial (mtDNA) and nuclear (nDNA) genomes that encode proteins also provide relevant knowledge [1]. During their duplication / replication process, new mitochondrial proteins are recruited and added to pre-existing compartments or protein complexes. In some cases, there may be a change in the intermediate compounds, producing free radicals, which are harmful when in excessive quantity. Therefore, mitochondria have become an important tool for Pathology, Phisiology, Pharmacology, Biochemistry, Cellular and Molecular Biology, Immunology, Microbiology and Virology because they are able “to respond quickly” to external and internal cell signals. For example, the ability of animal cells to detect different oxygen concentrations and, as a result, to reconnect their gene expression patterns is essential for the survival of virtually all animals, and casting light on these issues has led researchers in these areas of study to receive the Nobel Prize in Physiology or Medicine [13]. Thus, in 2019, William Kaelin Jr., Peter Ratcliffe and Gregg Semenza [13] shared the award for their work, which played a critical role in understanding and developing treatments for diseases such as anemia and cancer. They identified molecular machinery that regulates gene activity in response to changing levels of oxygen. Then, these researchers helped reveal the mechanism through which cells in the body respond and adapt to the availability of oxygen. The oxygen-activated signalling pathways that are controlled by these pathways affect approximately 300 genes that are involved in a variety of regulatory mechanisms. These molecular pathways permeate numerous physiological processes, ranging from organ development and metabolic homeostasis to tissue regeneration and immunity, and have important positions in several diseases.

Fig. (2)) Mitochondrial biogenesis. The anaerobic eukaryotic cell endocytosed the aerobic bacteria but did not digest it. The aerobic bacteria became a cell organelle and, consequently, the eukaryotic cell became aerobic. Fig. (3)) Mitochondrial components.

Besides, mitochondrial toxicity, often induced by certain drugs, body nutritional status, or osmotic imbalance, is a reliable sensor for assessing the potential of chemical compounds “in vitro” and “in vivo”. It is relevant to highlight that modern studies suggest that drug screening, the analysis of the relationship between biopathogens and their hosts, as well as the mechanism of action of some biological agents in diseases or symptoms perceived in the human body could harm this organelle’s regulation. This urges us to pay more attention to the structure, composition and functionality of mitochondria, as well as highlight the magnitude of the scientific studies that can still be done on it. Aiming at comparing energy yield, metabolism in the diverse existing aerobic organisms and cellular interaction.

As reviewed by Kausar et al. [14], the assumption of the common evolutionary origin between mitochondria and prokaryote denotes that mitochondrial DNA shares several characteristics with bacterial DNA as it consists of an extraordinary number of unmethylated DNA. Also, this fact suggests that mitochondrial DNA can be recognized as a molecular pattern associated with pathogens by the innate immune system. In pathological conditions, mitochondrial membranes rupture and mitochondrial DNA is released into the cell's cytoplasm. Once in the cytosol, this mitochondrial DNA, now cytosolic, induces several innate immune signalling pathways involving several types of receptors that participate in triggering the production of effector molecules such as the activation of NLRP3, TLR9, and STING by mitochondrial DNA [15-18].

Hence, mitochondrial DNA is responsible for inflammatory diseases after stress and cell damage. In addition, it is also involved in innate antiviral and antibacterial immunity. Furthermore, mitochondria are multifunctional organelles that, besides being focused on cell metabolism and energy production through mitochondrial DNA, seem to be of great relevance to trigger an innate immune response against various antimicrobial agents as well. Kausar et al. [14] mentioned that the relationship between mitochondrial DNA and its biological role in innate immunity has not yet been fully understood. These authors mention a study that mitochondrial DNA releases are mediated by the transition from mitochondrial permeability based on sensitivity to cyclosporin A, which is plausible if the transition from mitochondrial permeability is followed by osmotic swelling and rupture of the mitochondrial membrane [19]. Also, according to Kausar et al. [14], a recent study suggested that the hydrolysis of the mitochondrial membrane by secreted phospholipase A2 IIA (sPLA2-IIA) produces inflammatory mediators that induce leukocyte activation, leading to different inflammatory responses [20]. It is worth mentioning that mitochondria have a Nitric Oxyde sintase – NOS isoform (mtNOS) – and it has been described as a constitutive protein of the mitochondrial inner membrane that generates NO in a Ca2+-dependent reaction [21], although its existence has been doubted by other studies [21].

Furthermore, some studies also demonstrate that several viral proteins are responsible for the release of mitochondrial DNA in the cytosol [21]. In addition, Kausar et al. [14] also mentioned that, during infection with Mycobacterium tuberculosis, due to mitochondrial damage, mitochondrial DNA is released and it is considered to be an important source of cGAS binding [22]. Moreover, M. tuberculosis DNA is also a primary source for the activation of this signalling cascade [23]. The contribution of mitochondrial DNA to bacterial autophagy is especially interesting, given the similarity of this signalling pathway to mitophagy and the fact that the mitochondria itself were once a bacterium [22, 24].

Growing evidence suggests that mitochondrial DNA also participates in the anti-viral immunity [14]. Mitochondrial DNA can be recognized as a cell-intrinsic elicit of anti-microbial (anti-viral) signalling, and mitochondrial DNA monitoring homeostasis collaborates with typical virus detecting mechanisms to entirely incite anti-viral innate immunity. It has been shown that the C-terminus of the dengue virus M protein targets the membrane of mitochondria, causing swelling of its matrix, permeabilization, and potential loss of mitochondrial membrane. There are multiple lines of evidence suggesting the involvement of mitochondria in anti-bacterial immunity by producing reactive oxygen species [14].

Thus, the liberation of mitochondrial DNA, for instance, in response to Streptococcus pneumoniae, strongly activates the innate immune system via STING (stimulator of interferon genes) signalling pathway for the clearance of bacterial pathogens [14]. Interestingly, under stress conditions, particularly during microbial infection, physiological changes occur in mitochondrial membranes leading to the leakage of mitochondrial DNA, which is an evolutionary beneficial biological mechanism in a host, amplifying anti-viral and anti-bacterial signalling in response to these pathogen invasions [14]. However, the abnormal accumulation of damaged mitochondria and the release of mitochondrial DNA into the cytosol may also be responsible for various inflammatory diseases [14]. Furthermore, another interesting question is to understand the mechanism by which DNA sensing molecules can potentiate various outcomes of microbial infection. For example, in the case of M. tuberculosis, cGAS, STING, and TBK1 are all needed for both autophagosomal targeting and type I IFN expression [14].

3. Mitochondria can be involved in the pathogenesis of some viral diseases