miro1

MIRO1 (RHOT1)

Mitochondrial Rho GTPase 1 — an atypical GTPase anchored to the mitochondrial outer membrane (OMM) that physically couples mitochondria to microtubule-based motors (kinesin-1, dynein) for anterograde and retrograde trafficking. MIRO1’s two EF-hand calcium-sensing domains provide a direct link between local [Ca²⁺] and the arrest of mitochondrial movement. In the pink1-parkin-pathway, MIRO1 is among the first OMM substrates phosphorylated by PINK1 and subsequently ubiquitinated by parkin following mitochondrial depolarization, committing the organelle to immobility and packaging for mitophagy. Loss-of-function or impaired MIRO1 regulation is a convergence point for neurodegeneration and age-associated mitochondrial trafficking defects.

Identity

• UniProt: Q8IXI2 (MIRO1_HUMAN)
• NCBI Gene: 55288
• HGNC: 21168 (symbol: RHOT1)
• Mouse ortholog: Rhot1 (Miro1)
• Length: 618 amino acids (canonical isoform; single annotated isoform in Swiss-Prot)
• Topology: single-pass type IV transmembrane protein (C-terminal anchor in OMM, cytoplasmic N-terminus); all functional domains face the cytoplasm

Domain architecture

Domain	Residues (approx.)	Function
Miro1 GTPase domain (GTPase 1)	2–168	Atypical Rho-family GTPase; nucleotide binding and hydrolysis
EF-hand 1	184–219	Ca²⁺ sensor; binds Ca²⁺ at micromolar concentrations
EF-hand 2	304–339	Ca²⁺ sensor; cooperative with EF-hand 1
Miro2 GTPase domain (GTPase 2)	416–579	Second atypical GTPase domain; proposed structural/scaffolding role
Transmembrane helix	593–615	C-terminal OMM anchor

MIRO1 is classified as an “atypical” Rho GTPase because its GTPase domains diverge substantially from canonical Rho family members ¹. The N-terminal GTPase domain has been reported to have GTPase activity; the C-terminal GTPase domain additionally displays NTPase activity (hydrolyzing ATP and UTP) — but this biochemical characterization derives from later structural studies, not Fransson 2003 unsourced — primary citation for NTPase activity of the C-terminal GTPase domain needed. The two central EF-hands are the key regulatory feature distinguishing MIRO proteins from all other GTPases.

Function

Mitochondrial trafficking

MIRO1 (and its paralog miro2/RHOT2) serves as the OMM receptor that links mitochondria to the microtubule motor machinery. MIRO1 interacts with the adaptor protein complex TRAK1/TRAK2 (also called Milton/OIP106), which in turn binds kinesin-1 (KIF5B) for anterograde (plus-end) transport and dynein–dynactin for retrograde (minus-end) transport ². This three-part MIRO–TRAK–motor assembly is essential for the directional distribution of mitochondria throughout neurons and other polarized cells.

Germline-null Miro1 mice die perinatally, indicating that MIRO1-dependent mitochondrial trafficking is essential for postnatal survival ². needs-human-replication — the cellular lethality mechanism in Miro1-null mice has not been confirmed in human tissues; tissue-specific conditional knockouts (cardiac, neuronal) are the tractable model system for adult-tissue phenotypes.

Calcium-dependent arrest of movement

The two EF-hand domains confer Ca²⁺ sensitivity ³. At resting cytoplasmic Ca²⁺ (~100 nM), mitochondria are motile. At elevated local [Ca²⁺] (~1–5 μM, as occurs near active synapses or following plasma-membrane Ca²⁺ influx), Ca²⁺ occupancy of the EF-hands triggers dissociation of the MIRO–TRAK–kinesin complex from the motor, arresting mitochondrial movement in place ⁴. This mechanism is proposed to retain mitochondria at high-energy demand sites (active synaptic boutons, growth cones) and to redirect them away from Ca²⁺-loaded regions facing excitotoxic risk.

EF-hand mutants (e.g., Miro1-EF1/2 double mutant) that cannot bind Ca²⁺ abolish this arrest response, resulting in constitutively motile mitochondria that fail to accumulate at active synapses ³. Conversely, supraphysiological Ca²⁺ promotes mitochondrial fragmentation and immobility ⁴.

Dimension	Status
Pathway conserved in humans?	partial — MIRO1 EF-hand Ca²⁺ arrest mechanism demonstrated in rat hippocampal neurons and H9c2 cells; human MIRO1 protein is highly conserved but direct human neuronal demonstration lacking
Phenotype conserved in humans?	partial — synaptic mitochondrial localization studied in rodent neurons; human in vivo confirmation lacking
Replicated in humans?	no — human in vivo trafficking measurements not available

Mitochondrial-ER contact sites

MIRO1 also participates in mitochondria–ER contact sites (MAMs), co-localizing at ER-mitochondria interface zones with VAMP-associated protein B (VAPB). This function is distinct from motor coupling and is proposed to facilitate local Ca²⁺ transfer and lipid exchange ². no-mechanism — the precise molecular bridge at ER-mito contacts involving MIRO1 remains poorly characterized; structural details are incomplete.

Role in the PINK1–Parkin pathway (critical)

MIRO1 is a key substrate at the initiation of pink1-parkin-pathway-dependent mitophagy ⁵. The sequence of events on a depolarized mitochondrion:

1. PINK1 stabilization — loss of ΔΨm prevents PINK1 import and proteolysis; PINK1 accumulates on the OMM and phosphorylates ubiquitin (Ser65) and Parkin (Ser65 in Ubl domain), activating parkin as an E3 ubiquitin ligase (detailed on pink1-parkin-pathway).
2. PINK1 phosphorylates MIRO1 — PINK1 directly phosphorylates MIRO1 at Ser156 (and possibly additional sites) on the depolarized mitochondrion ⁵. This is proposed to be an early “stop signal” that arrests the damaged organelle before ubiquitination of OMM proteins proceeds further.
3. Parkin ubiquitinates MIRO1 — activated Parkin polyubiquitinates MIRO1 (and its paralog MIRO2), targeting both proteins for proteasomal degradation ⁵.
4. Motor detachment — loss of MIRO1/2 from the OMM releases the MIRO–TRAK–kinesin linkage, physically isolating the depolarized mitochondrion from the microtubule network.
5. Mitophagy packaging — the immobilized mitochondrion is recognized by autophagy receptors (OPTN, NDP52, p62/p62) and enclosed in autophagosomes (LC3-II/lc3-mediated).

MIRO1/2 accumulate on depolarized mitochondria in PINK1-null or Parkin-null cells, confirming that their removal is PINK1/Parkin-dependent ⁵. Stabilized MIRO1 (phospho-dead mutant Ser156A) delays mitochondrial arrest and impairs downstream mitophagy, establishing MIRO1 removal as mechanistically required for efficient mitophagy rather than merely coincident with it.

MIRO2 paralog: RHOT2 (miro2 — verified R30) is ~60% identical to MIRO1 and is similarly regulated by the PINK1/Parkin axis; however, the K27-ubiquitin-chain / Parkin-Ser65-dependence detail (Birsa 2014) was demonstrated for MIRO1 only — MIRO2-specific chain-type characterization remains a gap. The two paralogs have partially non-redundant tissue distribution: MIRO1 dominant in neurons (long-range microtubule transport); MIRO2 broader, with a more prominent Myo19/actin-based perinuclear distribution role in non-neuronal cells (López-Domènech 2018). MIRO2 single-KO mice are viable and fertile; the Miro1+Miro2 double-KO is embryonic lethal at E10.5 ².

Aging context

Impaired mitochondrial trafficking in aged neurons

Neurons are the most stringent users of MIRO1-dependent trafficking because mitochondria must be delivered over long axonal distances (up to 1 m in motor neurons) to supply ATP at distal synaptic terminals. In aged neurons:

• Mitochondrial motility declines with age in rodent and human neuronal models, associated with reduced axonal mitochondrial density at terminals.
• Impaired MIRO1 function or expression would reduce ATP delivery to distal synapses and Ca²⁺ buffering capacity at active zones. unsourced — direct quantitative measurements of MIRO1 protein levels across aged human neuronal tissue are lacking.

Convergence with neurodegeneration

Loss of neuronal Miro1 (Rhot1) disrupts mitophagy flux and induces hyperactivation of the integrated stress response (ISR), with downstream translational reprogramming ⁶. This phenocopies aspects of early Parkinson’s disease pathology, consistent with the known roles of PINK1/Parkin (pink1-parkin-pathway) in AR-PD. MIRO1 has been proposed as a biomarker for Parkinson’s disease: fibroblasts from Parkinson’s patients (including idiopathic cases) show delayed MIRO1 degradation following mitochondrial depolarization, suggesting a functional impairment in the PINK1–Parkin–MIRO1 axis even in sporadic disease ⁷. needs-replication — fibroblast-based MIRO1 degradation assays have not yet been replicated across independent cohorts.

MIRO1 is also misregulated in other neurodegenerative contexts: elevated or stabilized MIRO1 is observed in amyotrophic lateral sclerosis (ALS) patient neurons, where it may contribute to pathological mitochondrial clustering. unsourced — ALS MIRO1 elevation requires primary citation.

Dimension	Status
Pathway conserved in humans?	yes — PINK1/Parkin/MIRO1 axis demonstrated in human cell lines and patient fibroblasts
Phenotype conserved in humans?	partial — fibroblast MIRO1 delay in PD patients; in vivo neuronal phenotype not directly measured
Replicated in humans?	no (preclinical + patient-cell data only)

Pathway membership

• mitophagy — MIRO1 removal is a required early step for motor detachment and mitophagy initiation on depolarized mitochondria
• pink1-parkin-pathway — MIRO1 is a direct PINK1 phosphosubstrate and Parkin ubiquitination target
• mitochondrial-dysfunction — impaired MIRO1 trafficking reduces mitochondrial quality control and ATP distribution

Key interactors

• parkin (PRKN) — E3 ubiquitin ligase; polyubiquitinates MIRO1 for proteasomal degradation
• pink1 — Ser/Thr kinase; phosphorylates MIRO1 Ser156 on depolarized OMM
• trak1 / trak2 — adaptor proteins bridging MIRO1 to kinesin-1 and dynein; required for mitochondrial motility
• miro2 (verified R30) — paralog (RHOT2, ~60% identity); partial functional redundancy; same PINK1/Parkin axis, but K27-chain Parkin-Ser65 dependence directly demonstrated only for MIRO1
• USP30 — deubiquitinase that counteracts Parkin-driven MIRO1 ubiquitination; USP30 inhibition promotes mitophagy

Limitations and gaps

• needs-human-replication — All mechanistic trafficking and PINK1/Parkin/MIRO1 substrate data derive from mouse models, rodent neurons, and human fibroblast/iPSC lines. In vivo human neuronal mitochondrial trafficking measurements are technically inaccessible.
• needs-replication — MIRO1 degradation delay in PD fibroblasts (Grossmann 2020 review context); large-cohort fibroblast studies are needed.
• unsourced — Quantitative MIRO1 protein level changes in aged human brain or motor neurons; direct comparison to young tissue.
• unsourced — ALS-associated MIRO1 elevation claim requires primary citation.
• no-mechanism — Molecular details of MIRO1’s role in ER–mitochondria contact sites; structural characterization of MIRO–TRAK–kinesin ternary complex is incomplete.
• needs-canonical-id — GenAge entry for RHOT1/MIRO1: not confirmed; RHOT1 does not appear in the GenAge human database (aging relevance is inferred from pathway membership, not direct longevity genetics). Tag if GenAge ID is later found.
• long-term-unknown — Consequences of partial MIRO1 reduction over the lifespan in conditional neuronal KO models have not been reported beyond early postnatal windows.

Footnotes

Footnotes

1. doi:10.1074/jbc.M208609200 · Fransson A et al. 2003 J Biol Chem · in-vitro (mammalian cell lines) · model: NIH3T3 fibroblasts + COS 7 cells; Myc-tagged MIRO1/MIRO2 overexpression (yeast mentioned only as evolutionary context, not used experimentally) · 399 citations; (OA hybrid) · identifies MIRO1/2 as atypical Rho GTPases (618 aa each) with roles in mitochondrial morphology; constitutively active MIRO1-Val13 induces mitochondrial network collapse and increased apoptosis ↩

2. doi:10.1016/j.bbrc.2006.03.163 · Fransson S et al. 2006 Biochem Biophys Res Commun · in-vivo (mouse) + in-vitro · model: Miro1-null perinatal lethal phenotype; MIRO–TRAK motor complex · 460 citations; archive not_oa (closed-access) no-fulltext-access · establishes essential role in mitochondrial trafficking + Miro1-null lethality ↩ ↩² ↩³ ↩⁴

3. doi:10.1016/j.neuron.2009.01.030 · Macaskill AF et al. 2009 Neuron · in-vitro (primary neurons + COS7 pull-down) · model: rat hippocampal neurons (dendrites, DIV 12–14), Miro1 ΔEF mutant (E208K + E328K) · 660 citations; (OA hybrid) · demonstrates MIRO1 as Ca²⁺-sensor for glutamate-receptor-dependent synaptic mitochondrial localization via KIF5 (kinesin-1); Ca²⁺ at IC₅₀ ~1 μM dissociates Miro1–KIF5 complex; EF-hand mutant abolishes arrest and synaptic accumulation ↩ ↩²

4. doi:10.1073/pnas.0808953105 · Saotome M et al. 2008 PNAS · in-vitro (cell lines + primary neurons) · model: H9c2 cardiomyocyte cell line + primary cortical neurons (neonatal rat) · 523 citations; (OA green) · bidirectional Ca²⁺-dependent control of mitochondrial dynamics; immobilization and fragmentation at high [Ca²⁺]; Miro GTPase and EF-hand domains both required for full Ca²⁺ sensitivity ↩ ↩²

5. doi:10.1016/j.cell.2011.10.018 · Wang X et al. 2011 Cell · in-vitro (Drosophila neurons + mammalian cell lines) · model: Drosophila motor neurons + rat/mouse hippocampal neurons (Parkin-/- mice) + HeLa + HEK293T cells; PINK1-null + Parkin-null comparison · 1,139 citations; (OA bronze) · definitive demonstration that PINK1 phosphorylates MIRO at Ser156 and Parkin ubiquitinates MIRO1/2 → proteasomal degradation → motor detachment → mitophagy initiation ↩ ↩² ↩³ ↩⁴

6. doi:10.15252/embj.2018100715 · López-Doménech G et al. 2021 EMBO J · in-vitro + in-vivo · model: Miro1/2-DKO MEFs; CaMKIIα-Cre conditional Rhot1-KO neurons (hippocampus + cortex); primary mouse cortical neurons · 65 citations; (OA hybrid) · Miro1 ubiquitination and degradation are sequential requirements for Parkin recruitment and mitophagy progression; conditional neuronal loss leads to megamitochondria, Mfn1/2 upregulation, and pS51-eIF2α ISR activation at 12 months ↩

7. doi:10.3389/fneur.2020.00587 · Grossmann D et al. 2020 Front Neurol · review · model: literature synthesis covering PD patient fibroblasts, iPSC-derived neurons, and in vivo models · 46 citations; (OA gold) · comprehensive review of RHOT1/MIRO1 in PD pathogenesis: impaired Miro1 degradation in patient-derived fibroblasts and iPSC neurons; RHOT1 heterozygous mutations (R272Q, T351A, R450C, T610A) identified in PD patients; MERCs role discussed ↩