Mitochondrial Rho GTPase 1 (MIRO1) is an enzyme that in humans is encoded by the RHOT1 gene on chromosome 17. [5] [6] As a Miro protein isoform, the protein facilitates mitochondrial transport by attaching the mitochondria to the motor/adaptor complex. [7] Through its key role in mitochondrial transport, RHOT1 is involved in mitochondrial homeostasis and apoptosis, as well as Parkinson's disease (PD) and cancer. [7] [8] [9]
In mammals, RHOT1 is one of two Miro isoforms. Both isoforms share a structure consisting of two EF-hand motifs linking two GTP-binding domains and a C-terminal transmembrane domain that attaches the protein to the outer mitochondrial membrane (OMM). [7] [10] The EF-hand motifs serve as binding sites for the adaptor protein Milton and the kinesin heavy chain. [11] These domains can also bind calcium ions, and the binding results in a conformational change that dissociates the mitochondrial surface from kinesin. [7] [10]
RHOT1 is a member of the Rho GTPase family and one of two isoforms of the protein Miro: RHOT1 (Miro1) and RHOT2 (Miro2). [7] [11] Compared to the rest of the Rho GTPase family, the Miro isoforms are considered atypical due to their different regulation. [9] Moreover, the Miro isoforms are only expressed in the mitochondria. [12]
Miro associates with Milton (TRAK1/2) and the motor proteins kinesin and dynein to form the mitochondrial motor/adaptor complex. Miro functions to tether the complex to the mitochondrion while the complex transports the mitochondrion via microtubules within cells. [7] [8] Though Miro has been predominantly studied in neurons, the protein has also been observed to participate in the transport of mitochondria in lymphocytes toward inflamed endothelia. [11]
The motor/adaptor complex is regulated by calcium ion levels. At high concentrations, calcium ions arrest mitochondrial transport by binding Miro, causing the complex to detach from the organelle. Considering that physiological factors such as activation of glutamate receptors in dendrites, action potentials in axons, and neuromodulators may elevate calcium ion levels, this regulatory mechanism likely serves to keep mitochondria in such areas to provide calcium ion buffering and active export and, thus, maintain homeostasis. [7]
In addition, Miro regulates mitochondrial fusion and mitophagy in conjunction with mitofusin. According to one model, damaged mitochondria are sequestered from healthy mitochondria by the degradation of Miro and mitofusin. Miro degradation halts their movement while mitofusin degradation prevents them from fusing with healthy mitochondria, thus facilitating their clearance by autophagosomes. [7]
Though the exact mechanisms remain to be elucidated, RHOT1 has been implicated in promoting caspase-dependent apoptosis. [5]
Studies indicate that Miro may be involved in PD. [8] In neurons, Miro interacts with two key proteins involved in PD, PINK1 and Parkin. [7] Following depolarization of the mitochondria, PINK1 phosphorylates Miro at multiple sites, including S156, and Parkin ubiquitinates Miro, targeting it for proteasomal degradation. [7] [8] Degradation of Miro then halts mitochondrial transport. [7]
Though the Rho GTPase family is closely associated with cancer progression, there are few studies demonstrating such association with the atypical Miro proteins. Nonetheless, RHOT1 has been implicated in pancreatic cancer as a tumor suppressor through its regulation of mitochondrial homeostasis and apoptosis. Thus, this protein could serve as a therapeutic target for cancer treatment. [9]
RHOT1 has been shown to interact with:
Parkin is a 465-amino acid residue E3 ubiquitin ligase, a protein that in humans and mice is encoded by the PARK2 gene. Parkin plays a critical role in ubiquitination – the process whereby molecules are covalently labelled with ubiquitin (Ub) and directed towards degradation in proteasomes or lysosomes. Ubiquitination involves the sequential action of three enzymes. First, an E1 ubiquitin-activating enzyme binds to inactive Ub in eukaryotic cells via a thioester bond and mobilises it in an ATP-dependent process. Ub is then transferred to an E2 ubiquitin-conjugating enzyme before being conjugated to the target protein via an E3 ubiquitin ligase. There exists a multitude of E3 ligases, which differ in structure and substrate specificity to allow selective targeting of proteins to intracellular degradation.
Mitofusin-2 is a protein that in humans is encoded by the MFN2 gene. Mitofusins are GTPases embedded in the outer membrane of the mitochondria. In mammals MFN1 and MFN2 are essential for mitochondrial fusion. In addition to the mitofusins, OPA1 regulates inner mitochondrial membrane fusion, and DRP1 is responsible for mitochondrial fission.
PTEN-induced kinase 1 (PINK1) is a mitochondrial serine/threonine-protein kinase encoded by the PINK1 gene.
DnaJ homolog subfamily A member 3, mitochondrial, also known as Tumorous imaginal disc 1 (TID1), is a protein that in humans is encoded by the DNAJA3 gene on chromosome 16. This protein belongs to the DNAJ/Hsp40 protein family, which is known for binding and activating Hsp70 chaperone proteins to perform protein folding, degradation, and complex assembly. As a mitochondrial protein, it is involved in maintaining membrane potential and mitochondrial DNA (mtDNA) integrity, as well as cellular processes such as cell movement, growth, and death. Furthermore, it is associated with a broad range of diseases, including neurodegenerative diseases, inflammatory diseases, and cancers.
Rho GTPase-activating protein 1 is an enzyme that in humans is encoded by the ARHGAP1 gene.
Dynamin-like 120 kDa protein, mitochondrial is a protein that in humans is encoded by the OPA1 gene. This protein regulates mitochondrial fusion and cristae structure in the inner mitochondrial membrane (IMM) and contributes to ATP synthesis and apoptosis, and small, round mitochondria. Mutations in this gene have been implicated in dominant optic atrophy (DOA), leading to loss in vision, hearing, muscle contraction, and related dysfunctions.
Partitioning defective 3 homolog is a protein that in humans is encoded by the PARD3 gene.
Dynamin-1-like protein is a GTPase that regulates mitochondrial fission. In humans, dynamin-1-like protein, which is typically referred to as dynamin-related protein 1 (Drp1), is encoded by the DNM1L gene and is part of the dynamin superfamily (DSP) family of proteins.
Trafficking kinesin-binding protein 1 is a protein that in humans is encoded by the TRAK1 gene.
Mitofusin-1 is a protein that in humans is encoded by the MFN1 gene.
Trafficking kinesin-binding protein 2 is a protein that in humans is encoded by the TRAK2 gene.
Mitochondrial Rho GTPase 2 is an enzyme that in humans is encoded by the RHOT2 gene. As a Miro protein isoform, the protein facilitates mitochondrial transport by attaching the mitochondria to the motor/adaptor complex. Through its key role in mitochondrial transport, RHOT2 is involved in mitochondrial homeostasis and apoptosis, as well as Parkinson's disease (PD).
Presenilins-associated rhomboid-like protein, mitochondrial (PSARL), also known as PINK1/PGAM5-associated rhomboid-like protease (PARL), is an inner mitochondrial membrane protein that in humans is encoded by the PARL gene on chromosome 3. It is a member of the rhomboid family of intramembrane serine proteases. This protein is involved in signal transduction and apoptosis, as well as neurodegenerative diseases and type 2 diabetes.
Voltage-dependent anion-selective channel protein 2 is a protein that in humans is encoded by the VDAC2 gene on chromosome 10. This protein is a voltage-dependent anion channel and shares high structural homology with the other VDAC isoforms. VDACs are generally involved in the regulation of cell metabolism, mitochondrial apoptosis, and spermatogenesis. Additionally, VDAC2 participates in cardiac contractions and pulmonary circulation, which implicate it in cardiopulmonary diseases. VDAC2 also mediates immune response to infectious bursal disease (IBD).
Voltage-dependent anion-selective channel protein 3 (VDAC3) is a protein that in humans is encoded by the VDAC3 gene on chromosome 8. The protein encoded by this gene is a voltage-dependent anion channel and shares high structural homology with the other VDAC isoforms. Nonetheless, VDAC3 demonstrates limited pore-forming ability and, instead, interacts with other proteins to perform its biological functions, including sperm flagella assembly and centriole assembly. Mutations in VDAC3 have been linked to male infertility, as well as Parkinson's disease.
Mitophagy is the selective degradation of mitochondria by autophagy. It often occurs to defective mitochondria following damage or stress. The process of mitophagy was first described over a hundred years ago by Margaret Reed Lewis and Warren Harmon Lewis. Ashford and Porter used electron microscopy to observe mitochondrial fragments in liver lysosomes by 1962, and a 1977 report suggested that "mitochondria develop functional alterations which would activate autophagy." The term "mitophagy" was in use by 1998.
Mitochondrial fission is the process where mitochondria divide or segregate into two separate mitochondrial organelles. Mitochondrial fission is counteracted by the process of mitochondrial fusion, whereby two separate mitochondria can fuse together to form a large one. Mitochondrial fusion in turn can result in elongated mitochondrial networks. Both mitochondrial fission and fusion are balanced in the cell, and mutations interfering with either processes are associated with a variety of diseases. Mitochondria can divide by prokaryotic binary fission and since they require mitochondrial DNA for their function, fission is coordinated with DNA replication. Some of the proteins that are involved in mitochondrial fission have been identified and some of them are associated with mitochondrial diseases. Mitochondrial fission has significant implications in stress response and apoptosis.
Mitochondrial E3 ubiquitin protein ligase 1 (MUL1) is an enzyme that in humans is encoded by the MUL1 gene on chromosome 1. This enzyme localizes to the outer mitochondrial membrane, where it regulates mitochondrial morphology and apoptosis through multiple pathways, including the Akt, JNK, and NF-κB. Its proapoptotic function thus implicates it in cancer and Parkinson's disease.
The pathophysiology of Parkinson's disease is death of dopaminergic neurons as a result of changes in biological activity in the brain with respect to Parkinson's disease (PD). There are several proposed mechanisms for neuronal death in PD; however, not all of them are well understood. Five proposed major mechanisms for neuronal death in Parkinson's Disease include protein aggregation in Lewy bodies, disruption of autophagy, changes in cell metabolism or mitochondrial function, neuroinflammation, and blood–brain barrier (BBB) breakdown resulting in vascular leakiness.
Mitochondria-associated membranes (MAM) represent a region of the endoplasmic reticulum (ER) which is reversibly tethered to mitochondria. These membranes are involved in import of certain lipids from the ER to mitochondria and in regulation of calcium homeostasis, mitochondrial function, autophagy and apoptosis. They also play a role in development of neurodegenerative diseases and glucose homeostasis.
