Characterization of the three zebrafish orthologs of the mitochondrial GTPase Miro/Rhot

Mitochondrial Miro GTPases coordinate mitochondrial and peroxisomal dynamics

Mitochondria and peroxisomes are highly dynamic, multifunctional organelles. Both perform key roles for cellular physiology and homoeostasis by mediating bioenergetics, biosynthesis, and/or signalling. To support cellular function, they must be properly distributed, of proper size, and be able to interact with other organelles. Accumulating evidence suggests that the small atypical GTPase Miro provides a central signalling node to coordinate mitochondrial as well as peroxisomal dynamics. In this review, I summarize our current understanding of Miro-dependent functions and molecular mechanisms underlying the proper distribution, size and function of mitochondria and peroxisomes.

Miro (Mitochondrial Rho GTPase), a key player of mitochondrial axonal transport and mitochondrial dynamics in neurodegenerative diseases

Miro (mitochondrial Rho GTPases) a mitochondrial outer membrane protein, plays a vital role in the microtubule-based mitochondrial axonal transport, mitochondrial dynamics (fusion and fission) and Mito-Ca²⁺ homeostasis. It forms a major protein complex with Milton (an adaptor protein), kinesin and dynein (motor proteins), and facilitates bidirectional mitochondrial axonal transport such as anterograde and retrograde transport. By forming this protein complex, Miro facilitates the mitochondrial axonal transport and fulfills the neuronal energy demand, maintain the mitochondrial homeostasis and neuronal survival. It has been demonstrated that altered mitochondrial biogenesis, improper mitochondrial axonal transport, and mitochondrial dynamics are the early pathologies associated with most of the neurodegenerative diseases (NDs). Being the sole mitochondrial outer membrane protein associated with mitochondrial axonal transport-related processes, Miro proteins can be one of the key players in various NDs such as Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic lateral sclerosis (ALS) and Huntington's disease (HD). Thus, in the current review, we have discussed the evolutionarily conserved Miro proteins and its role in the pathogenesis of the various NDs. From this, we indicated that Miro proteins may act as a potential target for a novel therapeutic intervention for the treatment of various NDs.

Mitochondria as targets for toxicity and metabolism research using zebrafish

BACKGROUND

The study of mitochondrial functions in zebrafish was initiated before the 1990s and has effectively supported many of the recent scientific advances in the functional studies of mitochondria.

SCOPE OF REVIEW

This work elaborates various peculiarities and general advances in the study of mitochondria using this animal model.

MAJOR CONCLUSIONS

The inclusion of zebrafish models in scientific research was initiated with structural studies of mitochondria. Then, toxicological studies involving chemical compounds were undertaken. Currently, there is a decisive tendency to use zebrafish to understand how chemicals impair mitochondrial bioenergetics. Zebrafish modeling has been fruitful for the analysis of ion homeostasis, especially for Ca²⁺ transport, since zebrafish and mammals have the same set of Ca²⁺ transporters and mitochondrial membrane microdomains. Based on zebrafish embryo studies, our understanding of ROS generation has also led to new insights.

GENERAL SIGNIFICANCE

For the study of mitochondria, a new era was begun with the inclusion of zebrafish in bioenergetics research.

Ectopic expression of Miro 1 ameliorates seizures and inhibits hippocampal neurodegeneration in a mouse model of pilocarpine epilepsy

Epilepsy is a common disease of the central nervous system. This study aimed to investigate the role of mitochondrial Rho (Miro) 1 in epilepsy, using a mouse model of pilocarpine-induced status epilepticus (SE). Intraperitoneal injection of pilocarpine induced epileptic seizures in mice and significantly decreased Miro 1 expression in the hippocampus. Moreover, pilocarpine treatment increased the serum levels of heat shock protein 70 (HSP70) and S100 calcium binding protein B (S100B) and led to hippocampal neuronal injury and apoptosis. The intrinsic apoptotic pathway was activated in the hippocampal neurons following pilocarpine-induced SE, as evidenced by increased levels of cleaved caspase-3 and Bax, downregulation of Bcl-2, and the release of cytochrome c from mitochondria to cytoplasm. By contrast, forced expression of Miro 1 by lateral ventricular administration of adenovirus mitigated pilocarpine-induced epileptic seizures, reduced the elevation of HSP70 and S100B, and inhibited hippocampal neuronal apoptosis by suppressing the intrinsic apoptotic pathway. In summary, our data demonstrates that ectopic expression of Miro 1 alleviated pilocarpine-induced SE and protected hippocampal neurons by inhibiting the intrinsic apoptotic pathway. These findings provide new insights into epileptic disorders and suggest a potential neuroprotective value of Miro 1 in the treatment of epilepsy.

MIRO GTPases in Mitochondrial Transport, Homeostasis and Pathology

The evolutionarily-conserved mitochondrial Rho (MIRO) small GTPase is a Ras superfamily member with three unique features. It has two GTPase domains instead of the one found in other small GTPases, and it also has two EF hand calcium binding domains, which allow Ca²⁺-dependent modulation of its activity and functions. Importantly, it is specifically associated with the mitochondria and via a hydrophobic transmembrane domain, rather than a lipid-based anchor more commonly found in other small GTPases. At the mitochondria, MIRO regulates mitochondrial homeostasis and turnover. In metazoans, MIRO regulates mitochondrial transport and organization at cellular extensions, such as axons, and, in some cases, intercellular transport of the organelle through tunneling nanotubes. Recent findings have revealed a myriad of molecules that are associated with MIRO, particularly the kinesin adaptor Milton/TRAK, mitofusin, PINK1 and Parkin, as well as the endoplasmic reticulum-mitochondria encounter structure (ERMES) complex. The mechanistic aspects of the roles of MIRO and its interactors in mitochondrial homeostasis and transport are gradually being revealed. On the other hand, MIRO is also increasingly associated with neurodegenerative diseases that have roots in mitochondrial dysfunction. In this review, I discuss what is currently known about the cellular physiology and pathophysiology of MIRO functions.