Mitochondrial connections with immune system in Zebrafish

Abstract

Mitochondria are organelles commonly associated with adenosine triphosphate (ATP) formation through the oxidative phosphorylation (OXPHOS) process. However, mitochondria are also responsible for functions such as calcium homeostasis, apoptosis, autophagy, and production of reactive oxygen species (ROS) that, in conjunction, can lead to different cell fate decisions. Mitochondrial morphology changes rely on nutrients' availability and the bioenergetics demands of the cells, in a process known as mitochondrial dynamics, which includes both fusion and fission. This organelle senses the microenvironment and can modify the cells to either a pro or anti-inflammatory profile. The zebrafish has been increasingly used to research mitochondrial dynamics and its connection with the immune system since the pathways and molecules involved in these processes are conserved on this fish. Several genetic tools and technologies are currently available to analyze the behavior of mitochondria in zebrafish. However, even though zebrafish presents several similar processes known in mammals, the effect of the mitochondria in the immune system has not been so broadly studied in this model. In this review, we summarize the current knowledge in zebrafish studies regarding mitochondrial function and immuno metabolism.

Keywords: Fish; Immuno metabolism; Immunology; Metabolism; Mitochondria; Mitochondrial functions.

Fig. 1

The metabolites once inside the mitochondrial matrix are converted to acetyl-CoA through the tricarboxylic acid (TCA) cycle. The oxidation of fatty acids and pyruvate to Acetyl-CoA in the TCA cycle happens through pyruvate dehydrogenase (PDH), followed by the reduction of NAD+ to NADH and production of H+ on the Complex I (I) . Complex II (II) oxidizes the succinate to fumarate and converts FAD to FADH₂. The oxidation process of metabolites allows the synthesis of ATP in the electron transport chain (ETC) as the formation of FADH₂ and NADH render electrons to Complex I and II . The electrons provided by the TCA cycle (red arrow) goes throughout ETC, composed by the Complex I, II, III and IV, this process allows the pumping of protons (H⁺) of the matrix to the inter membrane space, creating a proton-driving force. In Complex IV, the electron is used to build HO (2 H⁺+ 1/2O₂) . The high proton concentration on the inter membrane space is essential to the Complex V- ATP synthase complex- function. Once activated, the ATP synthase complex can join the phosphatase molecule (P) to the adenosine diphosphate (ADP) to generate adenosine triphosphate (ATP) [2,3].

Fig. 2

Microinjection method at the stage of one cell . A- Microinjection of morpholino oligonucleotides can reduce the expression of the selected gene or, by adding processed mRNA, enhance the expression of the select gene . B- CRISPR/Cas9 is a DNA editing technique that consists of the microinjection of the Cas9 protein/mRNA and a guide RNA that will induce a double-strand break on the DNA causing a mutation in the target gene. Novel versions are used to add genes to the genome using this technique . C- The Gal4/UAS system allows tissue-specific editing of the genome. The technique consists of a two-machinery program: the “driver fish” with a Tol2 transposition system: a specific mRNA synthesized of a Gal4 tissue-specific gene among two transposable elements- Tol2; and the “reporter fish” a transgenic fish carrying a reporter gene together with the UAS. Once crossed the “driver” and “reporter” fish it generates a linage with Gal4 as a transcription factor for UAS. [24, 29, 31, 32].