Shared structural features of Miro binding control mitochondrial homeostasis.

Miro proteins are universally conserved mitochondrial calcium-binding GTPases that regulate a multitude of mitochondrial processes, including transport, clearance, and lipid trafficking. The exact role of Miro in these functions is unclear but involves binding to a variety of client proteins. How this binding is operated at the molecular level and whether and how it is important for mitochondrial health, however, remains unknown. Here, we show that known Miro interactors-namely, CENPF, Trak, and MYO19-all use a similar short motif to bind the same structural element: a highly conserved hydrophobic pocket in the first calcium-binding domain of Miro. Using these Miro-binding motifs, we identified direct interactors de novo, including MTFR1/2/1L, the lipid transporters Mdm34 and VPS13D, and the ubiquitin E3-ligase Parkin. Given the shared binding mechanism of these functionally diverse clients and its conservation across eukaryotes, we propose that Miro is a universal mitochondrial adaptor coordinating mitochondrial health.

Mitochondrial regulation of local supply of energy in neurons.

Brain computation is metabolically expensive and requires the supply of significant amounts of energy. Mitochondria are highly specialized organelles whose main function is to generate cellular energy. Due to their complex morphologies, neurons are especially dependent on a set of tools necessary to regulate mitochondrial function locally in order to match energy provision with local demands. By regulating mitochondrial transport, neurons control the local availability of mitochondrial mass in response to changes in synaptic activity. Neurons also modulate mitochondrial dynamics locally to adjust metabolic efficiency with energetic demand. Additionally, neurons remove inefficient mitochondria through mitophagy. Neurons coordinate these processes through signalling pathways that couple energetic expenditure with energy availability. When these mechanisms fail, neurons can no longer support brain function giving rise to neuropathological states like metabolic syndromes or neurodegeneration.

Mitochondrial Ca2+ uniporter haploinsufficiency enhances long-term potentiation at hippocampal mossy fibre synapses.

Long-term changes in synaptic strength form the basis of learning and memory. These changes rely upon energy-demanding mechanisms, which are regulated by local Ca2+ signalling. Mitochondria are optimised for providing energy and buffering Ca2+. However, our understanding of the role of mitochondria in regulating synaptic plasticity is incomplete. Here, we have used optical and electrophysiological techniques in cultured hippocampal neurons and ex vivo hippocampal slices from mice with haploinsufficiency of the mitochondrial Ca2+ uniporter (MCU+/-) to address whether reducing mitochondrial Ca2+ uptake alters synaptic transmission and plasticity. We found that cultured MCU+/- hippocampal neurons have impaired Ca2+ clearance, and consequently enhanced synaptic vesicle fusion at presynapses occupied by mitochondria. Furthermore, long-term potentiation (LTP) at mossy fibre (MF) synapses, a process which is dependent on presynaptic Ca2+ accumulation, is enhanced in MCU+/- slices. Our results reveal a previously unrecognised role for mitochondria in regulating presynaptic plasticity of a major excitatory pathway involved in learning and memory.

Loss of neuronal Miro1 disrupts mitophagy and induces hyperactivation of the integrated stress response.

Clearance of mitochondria following damage is critical for neuronal homeostasis. Here, we investigate the role of Miro proteins in mitochondrial turnover by the PINK1/Parkin mitochondrial quality control system in vitro and in vivo. We find that upon mitochondrial damage, Miro is promiscuously ubiquitinated on multiple lysine residues. Genetic deletion of Miro or block of Miro1 ubiquitination and subsequent degradation lead to delayed translocation of the E3 ubiquitin ligase Parkin onto damaged mitochondria and reduced mitochondrial clearance in both fibroblasts and cultured neurons. Disrupted mitophagy in vivo, upon post-natal knockout of Miro1 in hippocampus and cortex, leads to a dramatic increase in mitofusin levels, the appearance of enlarged and hyperfused mitochondria and hyperactivation of the integrated stress response (ISR). Altogether, our results provide new insights into the central role of Miro1 in the regulation of mitochondrial homeostasis and further implicate Miro1 dysfunction in the pathogenesis of human neurodegenerative disease.

Miro1-dependent mitochondrial dynamics in parvalbumin interneurons.

The spatiotemporal distribution of mitochondria is crucial for precise ATP provision and calcium buffering required to support neuronal signaling. Fast-spiking GABAergic interneurons expressing parvalbumin (PV+) have a high mitochondrial content reflecting their large energy utilization. The importance for correct trafficking and precise mitochondrial positioning remains poorly elucidated in inhibitory neurons. Miro1 is a Ca²+-sensing adaptor protein that links mitochondria to the trafficking apparatus, for their microtubule-dependent transport along axons and dendrites, in order to meet the metabolic and Ca2+-buffering requirements of the cell. Here, we explore the role of Miro1 in PV+ interneurons and how changes in mitochondrial trafficking could alter network activity in the mouse brain. By employing live and fixed imaging, we found that the impairments in Miro1-directed trafficking in PV+ interneurons altered their mitochondrial distribution and axonal arborization, while PV+ interneuron-mediated inhibition remained intact. These changes were accompanied by an increase in the ex vivo hippocampal γ-oscillation (30-80 Hz) frequency and promoted anxiolysis. Our findings show that precise regulation of mitochondrial dynamics in PV+ interneurons is crucial for proper neuronal signaling and network synchronization.

Peroxisomal fission is modulated by the mitochondrial Rho-GTPases, Miro1 and Miro2.

Peroxisomes are essential for a number of cellular functions, including reactive oxygen species metabolism, fatty acid ß-oxidation and lipid synthesis. To ensure optimal functionality, peroxisomal size, shape and number must be dynamically maintained; however, many aspects of how this is regulated remain poorly characterised. Here, we show that the localisation of Miro1 and Miro2-outer mitochondrial membrane proteins essential for mitochondrial trafficking-to peroxisomes is not required for basal peroxisomal distribution and long-range trafficking, but rather for the maintenance of peroxisomal size and morphology through peroxisomal fission. Mechanistically, this is achieved by Miro negatively regulating Drp1-dependent fission, a function that is shared with the mitochondria. We further find that the peroxisomal localisation of Miro is regulated by its first GTPase domain and is mediated by an interaction through its transmembrane domain with the peroxisomal-membrane protein chaperone, Pex19. Our work highlights a shared regulatory role of Miro in maintaining the morphology of both peroxisomes and mitochondria, supporting a crosstalk between peroxisomal and mitochondrial biology.

Axonal autophagosome maturation defect through failure of ATG9A sorting underpins pathology in AP-4 deficiency syndrome.

Adaptor protein (AP) complexes mediate key sorting decisions in the cell through selective incorporation of transmembrane proteins into vesicles. Little is known of the roles of AP-4, despite its loss of function leading to a severe early onset neurological disorder, AP-4 deficiency syndrome. Here we demonstrate an AP-4 epsilon subunit knockout mouse model that recapitulates characteristic neuroanatomical phenotypes of AP-4 deficiency patients. We show that ATG9A, critical for autophagosome biogenesis, is an AP-4 cargo, which is retained within the trans-Golgi network (TGN) in vivo and in culture when AP-4 function is lost. TGN retention results in depletion of axonal ATG9A, leading to defective autophagosome generation and aberrant expansions of the distal axon. The reduction in the capacity to generate axonal autophagosomes leads to defective axonal extension and de novo generation of distal axonal swellings containing accumulated ER, underlying the impaired axonal integrity in AP-4 deficiency syndrome.Abbreviations: AP: adaptor protein; AP4B1: adaptor-related protein complex AP-4, beta 1; AP4E1: adaptor-related protein complex AP-4, epsilon 1; ATG: autophagy-related; EBSS: Earle's balanced salt solution; ER: endoplasmic reticulum; GFAP: glial fibrillary acidic protein; GOLGA1/Golgin-97/GOLG97: golgi autoantigen, golgin subfamily a, 1; GOLGA2/GM130: golgi autoantigen, golgin subfamily a, 2; HSP: hereditary spastic paraplegia; LC3/MAP1LC3B: microtubule-associated protein 1 light chain 3 beta; MAP2: microtubule-associated protein 2; MAPK8IP1/JIP1: mitogen-acitvated protein kinase 8 interacting protein 1; NEFH/NF200: neurofilament, heavy polypeptide; RBFOX3/NeuN (RNA binding protein, fox-1 homolog [C. elegans] 3); SQSTM1/p62: sequestosome 1; TGN: trans-Golgi network; WIPI2: WD repeat domain, phosphoinositide interacting protein 2.

Miro clusters regulate ER-mitochondria contact sites and link cristae organization to the mitochondrial transport machinery.

Mitochondrial Rho (Miro) GTPases localize to the outer mitochondrial membrane and are essential machinery for the regulated trafficking of mitochondria to defined subcellular locations. However, their sub-mitochondrial localization and relationship with other critical mitochondrial complexes remains poorly understood. Here, using super-resolution fluorescence microscopy, we report that Miro proteins form nanometer-sized clusters along the mitochondrial outer membrane in association with the Mitochondrial Contact Site and Cristae Organizing System (MICOS). Using knockout mouse embryonic fibroblasts we show that Miro1 and Miro2 are required for normal mitochondrial cristae architecture and Endoplasmic Reticulum-Mitochondria Contacts Sites (ERMCS). Further, we show that Miro couples MICOS to TRAK motor protein adaptors to ensure the concerted transport of the two mitochondrial membranes and the correct distribution of cristae on the mitochondrial membrane. The Miro nanoscale organization, association with MICOS complex and regulation of ERMCS reveal new levels of control of the Miro GTPases on mitochondrial functionality.

Autism and Schizophrenia-Associated CYFIP1 Regulates the Balance of Synaptic Excitation and Inhibition.

Altered excitatory/inhibitory (E/I) balance is implicated in neuropsychiatric and neurodevelopmental disorders, but the underlying genetic etiology remains poorly understood. Copy number variations in CYFIP1 are associated with autism, schizophrenia, and intellectual disability, but its role in regulating synaptic inhibition or E/I balance remains unclear. We show that CYFIP1, and the paralog CYFIP2, are enriched at inhibitory postsynaptic sites. While CYFIP1 or CYFIP2 upregulation increases excitatory synapse number and the frequency of miniature excitatory postsynaptic currents (mEPSCs), it has the opposite effect at inhibitory synapses, decreasing their size and the amplitude of miniature inhibitory postsynaptic currents (mIPSCs). Contrary to CYFIP1 upregulation, its loss in vivo, upon conditional knockout in neocortical principal cells, increases expression of postsynaptic GABAA receptor ß2/3-subunits and neuroligin 3, enhancing synaptic inhibition. Thus, CYFIP1 dosage can bi-directionally impact inhibitory synaptic structure and function, potentially leading to altered E/I balance and circuit dysfunction in CYFIP1-associated neurological disorders.

Miro proteins coordinate microtubule- and actin-dependent mitochondrial transport and distribution.

In the current model of mitochondrial trafficking, Miro1 and Miro2 Rho-GTPases regulate mitochondrial transport along microtubules by linking mitochondria to kinesin and dynein motors. By generating Miro1/2 double-knockout mouse embryos and single- and double-knockout embryonic fibroblasts, we demonstrate the essential and non-redundant roles of Miro proteins for embryonic development and subcellular mitochondrial distribution. Unexpectedly, the TRAK1 and TRAK2 motor protein adaptors can still localise to the outer mitochondrial membrane to drive anterograde mitochondrial motility in Miro1/2 double-knockout cells. In contrast, we show that TRAK2-mediated retrograde mitochondrial transport is Miro1-dependent. Interestingly, we find that Miro is critical for recruiting and stabilising the mitochondrial myosin Myo19 on the mitochondria for coupling mitochondria to the actin cytoskeleton. Moreover, Miro depletion during PINK1/Parkin-dependent mitophagy can also drive a loss of mitochondrial Myo19 upon mitochondrial damage. Finally, aberrant positioning of mitochondria in Miro1/2 double-knockout cells leads to disruption of correct mitochondrial segregation during mitosis. Thus, Miro proteins can fine-tune actin- and tubulin-dependent mitochondrial motility and positioning, to regulate key cellular functions such as cell proliferation.

An Essential Role for the Tetraspanin LHFPL4 in the Cell-Type-Specific Targeting and Clustering of Synaptic GABAA Receptors.

Inhibitory synaptic transmission requires the targeting and stabilization of GABAA receptors (GABAARs) at synapses. The mechanisms responsible remain poorly understood, and roles for transmembrane accessory proteins have not been established. Using molecular, imaging, and electrophysiological approaches, we identify the tetraspanin LHFPL4 as a critical regulator of postsynaptic GABAAR clustering in hippocampal pyramidal neurons. LHFPL4 interacts tightly with GABAAR subunits and is selectively enriched at inhibitory synapses. In LHFPL4 knockout mice, there is a dramatic cell-type-specific reduction in GABAAR and gephyrin clusters and an accumulation of large intracellular gephyrin aggregates in vivo. While GABAARs are still trafficked to the neuronal surface in pyramidal neurons, they are no longer localized at synapses, resulting in a profound loss of fast inhibitory postsynaptic currents. Hippocampal interneuron currents remain unaffected. Our results establish LHFPL4 as a synapse-specific tetraspanin essential for inhibitory synapse function and provide fresh insights into the molecular make-up of inhibitory synapses.

Loss of Dendritic Complexity Precedes Neurodegeneration in a Mouse Model with Disrupted Mitochondrial Distribution in Mature Dendrites.

Correct mitochondrial distribution is critical for satisfying local energy demands and calcium buffering requirements and supporting key cellular processes. The mitochondrially targeted proteins Miro1 and Miro2 are important components of the mitochondrial transport machinery, but their specific roles in neuronal development, maintenance, and survival remain poorly understood. Using mouse knockout strategies, we demonstrate that Miro1, as opposed to Miro2, is the primary regulator of mitochondrial transport in both axons and dendrites. Miro1 deletion leads to depletion of mitochondria from distal dendrites but not axons, accompanied by a marked reduction in dendritic complexity. Disrupting postnatal mitochondrial distribution in vivo by deleting Miro1 in mature neurons causes a progressive loss of distal dendrites and compromises neuronal survival. Thus, the local availability of mitochondrial mass is critical for generating and sustaining dendritic arbors, and disruption of mitochondrial distribution in mature neurons is associated with neurodegeneration.

Miro1 Regulates Activity-Driven Positioning of Mitochondria within Astrocytic Processes Apposed to Synapses to Regulate Intracellular Calcium Signaling.

It is fast emerging that maintaining mitochondrial function is important for regulating astrocyte function, although the specific mechanisms that govern astrocyte mitochondrial trafficking and positioning remain poorly understood. The mitochondrial Rho-GTPase 1 protein (Miro1) regulates mitochondrial trafficking and detachment from the microtubule transport network to control activity-dependent mitochondrial positioning in neurons. However, whether Miro proteins are important for regulating signaling-dependent mitochondrial dynamics in astrocytic processes remains unclear. Using live-cell confocal microscopy of rat organotypic hippocampal slices, we find that enhancing neuronal activity induces transient mitochondrial remodeling in astrocytes, with a concomitant, transient reduction in mitochondrial trafficking, mediated by elevations in intracellular Ca(2+). Stimulating neuronal activity also induced mitochondrial confinement within astrocytic processes in close proximity to synapses. Furthermore, we show that the Ca(2+)-sensing EF-hand domains of Miro1 are important for regulating mitochondrial trafficking in astrocytes and required for activity-driven mitochondrial confinement near synapses. Additionally, activity-dependent mitochondrial positioning by Miro1 reciprocally regulates the levels of intracellular Ca(2+) in astrocytic processes. Thus, the regulation of intracellular Ca(2+) signaling, dependent on Miro1-mediated mitochondrial positioning, could have important consequences for astrocyte Ca(2+) wave propagation, gliotransmission, and ultimately neuronal function.

Mitochondrial trafficking in neurons and the role of the Miro family of GTPase proteins.

Correct mitochondrial dynamics are essential to neuronal function. These dynamics include mitochondrial trafficking and quality-control systems that maintain a precisely distributed and healthy mitochondrial network, so that local energy demands or Ca2+-buffering requirements within the intricate architecture of the neuron can be met. Mitochondria make use of molecular machinery that couples these organelles to microtubule-based transport via kinesin and dynein motors, facilitating the required long-range movements. These motors in turn are associated with a variety of adaptor proteins allowing additional regulation of the complex dynamics demonstrated by these organelles. Over recent years, a number of new motor and adaptor proteins have been added to a growing list of components implicated in mitochondrial trafficking and distribution. Yet, there are major questions that remain to be addressed about the regulation of mitochondrial transport complexes. One of the core components of this machinery, the mitochondrial Rho GTPases Miro1 (mitochondrial Rho 1) and Miro2 have received special attention due to their Ca2+-sensing and GTPase abilities, marking Miro an exceptional candidate for co-ordinating mitochondrial dynamics and intracellular signalling pathways. In the present paper, we discuss the wealth of literature regarding Miro-mediated mitochondrial transport in neurons and recently highlighted involvement of Miro proteins in mitochondrial turnover, emerging as a key process affected in neurodegeneration.

The non-canonical Wnt/PKC pathway regulates mitochondrial dynamics through degradation of the arm-like domain-containing protein Alex3.

The regulation of mitochondrial dynamics is vital in complex cell types, such as neurons, that transport and localize mitochondria in high energy-demanding cell domains. The Armcx3 gene encodes a mitochondrial-targeted protein (Alex3) that contains several arm-like domains. In a previous study we showed that Alex3 protein regulates mitochondrial aggregation and trafficking. Here we studied the contribution of Wnt proteins to the mitochondrial aggregation and dynamics regulated by Alex3. Overexpression of Alex3 in HEK293 cells caused a marked aggregation of mitochondria, which was attenuated by treatment with several Wnts. We also found that this decrease was caused by Alex3 degradation induced by Wnts. While the Wnt canonical pathway did not alter the pattern of mitochondrial aggregation induced by Alex3, we observed that the Wnt/PKC non-canonical pathway regulated both mitochondrial aggregation and Alex3 protein levels, thereby rendering a mitochondrial phenotype and distribution similar to control patterns. Our data suggest that the Wnt pathway regulates mitochondrial distribution and dynamics through Alex3 protein degradation.

The Eutherian Armcx genes regulate mitochondrial trafficking in neurons and interact with Miro and Trak2.

Brain function requires neuronal activity-dependent energy consumption. Neuronal energy supply is controlled by molecular mechanisms that regulate mitochondrial dynamics, including Kinesin motors and Mitofusins, Miro1-2 and Trak2 proteins. Here we show a new protein family that localizes to the mitochondria and controls mitochondrial dynamics. This family of proteins is encoded by an array of armadillo (Arm) repeat-containing genes located on the X chromosome. The Armcx cluster is unique to Eutherian mammals and evolved from a single ancestor gene (Armc10). We show that these genes are highly expressed in the developing and adult nervous system. Furthermore, we demonstrate that Armcx3 expression levels regulate mitochondrial dynamics and trafficking in neurons, and that Alex3 interacts with the Kinesin/Miro/Trak2 complex in a Ca(2+)-dependent manner. Our data provide evidence of a new Eutherian-specific family of mitochondrial proteins that controls mitochondrial dynamics and indicate that this key process is differentially regulated in the brain of higher vertebrates.

NP1 regulates neuronal activity-dependent accumulation of BAX in mitochondria and mitochondrial dynamics.

In cultured cerebellar granule neurons, low neuronal activity triggers the intrinsic program of apoptosis, which requires protein synthesis-dependent BAX translocation to mitochondria, a process that may underlie neuronal damage in neurodegeneration. However, the mechanisms that link neuronal activity with the induction of the mitochondrial program of apoptosis remain unclear. Neuronal pentraxin 1 (NP1) is a pro-apoptotic protein induced by low neuronal activity that is increased in damaged neurites in Alzheimer's disease-affected brains. Here we report that NP1 facilitates the accumulation of BAX in mitochondria and regulates mitochondrial dynamics during apoptosis in rat and mouse cerebellar granule neurons in culture. Reduction of neuronal activity increases NP1 protein levels in mitochondria and contributes to mitochondrial fragmentation in a Bax-dependent manner. In addition, NP1 is involved in mitochondrial transport in healthy neurons. These results show that NP1 is targeted to mitochondria acting upstream of BAX and uncover a novel function for NP1 in the regulation of mitochondrial dynamics and trafficking during apoptotic neurodegeneration.

GSK3ß is involved in the relief of mitochondria pausing in a Tau-dependent manner.

Mitochondrial trafficking deficits have been implicated in the pathogenesis of several neurological diseases, including Alzheimer's disease (AD). The Ser/Thre kinase GSK3ß is believed to play a fundamental role in AD pathogenesis. Given that GSK3ß substrates include Tau protein, here we studied the impact of GSK3ß on mitochondrial trafficking and its dependence on Tau protein. Overexpression of GSK3ß in neurons resulted in an increase in motile mitochondria, whereas a decrease in the activity of this kinase produced an increase in mitochondria pausing. These effects were dependent on Tau proteins, as Tau (-/-) neurons did not respond to distinct GSK3ß levels. Furthermore, differences in GSK3ß expression did not affect other parameters like mitochondria velocity or mitochondria run length. We conclude that GSK3B activity regulates mitochondrial axonal trafficking largely in a Tau-dependent manner.

Podocalyxin is a novel polysialylated neural adhesion protein with multiple roles in neural development and synapse formation.

Neural development and plasticity are regulated by neural adhesion proteins, including the polysialylated form of NCAM (PSA-NCAM). Podocalyxin (PC) is a renal PSA-containing protein that has been reported to function as an anti-adhesin in kidney podocytes. Here we show that PC is widely expressed in neurons during neural development. Neural PC interacts with the ERM protein family, and with NHERF1/2 and RhoA/G. Experiments in vitro and phenotypic analyses of podxl-deficient mice indicate that PC is involved in neurite growth, branching and axonal fasciculation, and that PC loss-of-function reduces the number of synapses in the CNS and in the neuromuscular system. We also show that whereas some of the brain PC functions require PSA, others depend on PC per se. Our results show that PC, the second highly sialylated neural adhesion protein, plays multiple roles in neural development.

A collection of cDNAs enriched in upper cortical layers of the embryonic mouse brain.

In an attempt to elucidate the molecular basis of neuronal migration and corticogenesis, we performed subtractive hybridization of mRNAs from the upper cortical layers (layer I and upper cortical plate) against mRNAs from the remaining cerebral cortex at E15-E16. We obtained a collection of subtracted cDNA clones and analyzed their 3' UTR sequences, 47% of which correspond to EST sequences, and may represent novel products. Among the cloned sequences, we identified gene products that have not been reported in brain or in the cerebral cortex before. We examined the expression pattern of 39 subtracted clones, which was enriched in the upper layers of the cerebral cortex at embryonic stages. The expression of most clones is developmentally regulated, and especially high in embryonic and early postnatal stages. Four of the unknown clones were studied in more detail and identified as a new member of the tetraspanin superfamily, a putative RNA binding protein, a specific product of the adult dentate gyrus and a protein containing a beta-catenin repeat. We thus cloned a collection of subtracted cDNAs coding for protein products that may be involved in the development of the cerebral cortex.