Measurement of Mitochondrial Oxygen Consumption in Permeabilized Fibers of Drosophila Using Minimal Amounts of Tissue

The fruit fly, Drosophila melanogaster, represents an emerging model for the study of metabolism. Indeed, drosophila have structures homologous to human ...

Université de Moncton

Cellular activity in the brain depends on the high energetic support provided by mitochondria, the cell organelles which use energy sources to generate ATP. Acute cannabinoid intoxication induces amnesia in humans and animals, and the activation of type-1 cannabinoid receptors present at brain mitochondria membranes (mtCB) can directly alter mitochondrial energetic activity. Although the pathological impact of chronic mitochondrial dysfunctions in the brain is well established, the involvement of acute modulation of mitochondrial activity in high brain functions, including learning and memory, is unknown. Here, we show that acute cannabinoid-induced memory impairment in mice requires activation of hippocampal mtCBreceptors. Genetic exclusion of CBreceptors from hippocampal mitochondria prevents cannabinoid-induced reduction of mitochondrial mobility, synaptic transmission and memory formation. mtCBreceptors signal through intra-mitochondrial Gαprotein activation and consequent inhibition of soluble-adenylyl cyclase (sAC). The resulting inhibition of protein kinase A (PKA)-dependent phosphorylation of specific subunits of the mitochondrial electron transport system eventually leads to decreased cellular respiration. Hippocampal inhibition of sAC activity or manipulation of intra-mitochondrial PKA signalling or phosphorylation of the Complex I subunit NDUFS2 inhibit bioenergetic and amnesic effects of cannabinoids. Thus, the G protein-coupled mtCBreceptors regulate memory processes via modulation of mitochondrial energy metabolism. By directly linking mitochondrial activity to memory formation, these data reveal that bioenergetic processes are primary acute regulators of cognitive functions.

A cannabinoid link between mitochondria and memory.

During starvation, intra-mitochondrial sirtuins, NADsensitive deacylating enzymes that modulate metabolic homeostasis and survival, directly adjust mitochondrial function to nutrient availability; concomitantly, mitochondria elongate to escape autophagic degradation. However, whether sirtuins also impinge on mitochondrial dynamics is still uncharacterized. Here we show that the mitochondrial Sirtuin 5 (Sirt5) is essential for starvation induced mitochondrial elongation. Deletion of Sirt5 in mouse embryonic fibroblasts increased levels of mitochondrial dynamics of 51kDa protein and mitochondrial fission protein 1, leading to mitochondrial accumulation of the pro-fission dynamin related protein 1 and to mitochondrial fragmentation. During starvation, Sirt5 deletion blunted mitochondrial elongation, resulting in increased mitophagy. Our results indicate that starvation induced mitochondrial elongation and evasion from autophagic degradation requires the energy sensor Sirt5.

Sirtuin 5 protects mitochondria from fragmentation and degradation during starvation.

Recent evidence indicates that, besides its canonical localization at cell plasma membranes, the type-1 cannabinoid receptor, CB1 is functionally present at brain and muscle mitochondrial membranes (mtCB1). Through mtCB1 receptors, cannabinoids can directly regulate intramitochondrial signaling and respiration. This new and surprising discovery paves the way to new potential fields of research, dealing with the direct impact of G protein-coupled receptors on bioenergetic processes and its functional implications. In this chapter, we summarize some key experimental approaches established in our laboratories to identify anatomical, biochemical, and functional features of mtCB1 receptors in the brain. In particular, we describe the procedures to obtain reliable and controlled detection of mtCB1 receptors by immunogold electromicroscopy and by immunoblotting methods. Then, we address the study of direct cannabinoid effects on the electron transport system and oxidative phosphorylation. Finally, we present a functional example of the impact of mtCB1 receptors on mitochondrial mobility in cultured neurons. Considering the youth of the field, these methodological approaches will very likely be improved and refined in the future, but this chapter aims at presenting the methods that are currently used and, in particular, at underlining the need of rigorous controls to obtain reliable results. We hope that this chapter might help scientists becoming interested in this new and exciting field of research.

Functional Analysis of Mitochondrial CB1 Cannabinoid Receptors (mtCB1) in the Brain.

[Mitochondria link between cannabinoid and memory].

CB1 receptors are functionally present within brain mitochondria (mtCB1), although they are usually considered specifically targeted to plasma membrane. Acute activation of mtCB1 alters mitochondrial ATP generation, synaptic transmission, and memory performance. However, the detailed mechanism linking disrupted mitochondrial metabolism and synaptic transmission is still uncharacterized. CB1 receptors are among the most abundant G protein-coupled receptors in the brain and impact on several processes, including fear coping, anxiety, stress, learning, and memory. Mitochondria perform several key physiological processes for neuronal homeostasis, including production of ATP and reactive oxygen species, calcium buffering, metabolism of neurotransmitters, and apoptosis. It is therefore possible that acute activation of mtCB1 impacts on these different mitochondrial functions to modulate synaptic transmission. In reviewing and integrating across the literature in this area, we describe the possible mechanisms involved in the regulation of brain physiology by mtCB1 receptors.

Linking Mitochondria and Synaptic Transmission: The CB1 Receptor.

The small heat shock protein (sHsp) Hsp22 from Drosophila melanogaster (DmHsp22) is part of the family of sHsps in this diptera. This sHsp is characterized by its presence in the mitochondrial matrix as well as by its preferential expression during ageing. Although DmHsp22 has been demonstrated to be an efficient in vitro chaperone, its function within mitochondria in vivo remains largely unknown. Thus, determining its protein-interaction network (interactome) in the mitochondrial matrix would help to shed light on its function(s). In the present study we combined immunoaffinity conjugation (IAC) with mass spectroscopy analysis of mitochondria from HeLa cells transfected with DmHsp22 in non-heat shock condition and after heat shock (HS). 60 common DmHsp22-binding mitochondrial partners were detected in two independent IACs. Immunoblotting was used to validate interaction between DmHsp22 and two members of the mitochondrial chaperone machinery; Hsp60 and Hsp70. Among the partners of DmHsp22, several ATP synthase subunits were found. Moreover, we showed that expression of DmHsp22 in transiently transfected HeLa cells increased maximal mitochondrial oxygen consumption capacity and ATP contents, providing a mechanistic link between DmHsp22 and mitochondrial functions.

Identification of proteins interacting with the mitochondrial small heat shock protein Hsp22 of Drosophila melanogaster: Implication in mitochondrial homeostasis.

Etienne Hebert-Chatelain

Département de Biologie

Université de Moncton

Measurement of Mitochondrial Oxygen Consumption in Permeabilized Fibers of Drosophila Using Minimal Amounts of Tissue

Etienne Hebert-Chatelain

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Département de Biologie

Université de Moncton

Measurement of Mitochondrial Oxygen Consumption in Permeabilized Fibers of Drosophila Using Minimal Amounts of Tissue

Département de Biologie