shigeru yanagi

The proximal domains of mitochondria and the endoplasmic reticulum (ER) are linked by tethering factors on each membrane, allowing the efficient transport of substances, including lipids and calcium, between them. However, little is known about the regulation and function of mitochondria-ER contacts (MERCs) dynamics under mitochondrial damage. In this study, we apply NanoBiT technology to develop the MERBiT system, which enables the measurement of reversible MERCs formation in living cells. Analysis using this system suggests that induction of mitochondrial ROS increases MERCs formation via RMDN3 (also known as PTPIP51)-VAPB tethering driven by RMDN3 phosphorylation. Disruption of this tethering caused lipid radical accumulation in mitochondria, leading to cell death. The lipid radical transfer activity of the TPR domain in RMDN3, as revealed by an in vitro liposome assay, suggests that RMDN3 transfers lipid radicals from mitochondria to the ER. Our findings suggest a potential role for MERCs in cell survival strategy by facilitating the removal of mitochondrial lipid radicals under mitochondrial damage.

During gestation, the choroid plexus (ChP) produces protein-rich cerebrospinal fluid and matures prior to brain development. It is assumed that ChP dysfunction has a profound effect on developmental neuropsychiatric disorders, such as autism spectrum disorder (ASD). However, the mechanisms linking immature ChP to the onset of ASD remain unclear. Here, we find that ChP-specific CAMDI-knockout mice develop an immature ChP alongside decreased multiciliogenesis and expression of differentiation marker genes following disruption of the cerebrospinal fluid barrier. These mice exhibit ASD-like behaviors, including anxiety and impaired socialization. Additionally, the administration of metformin, an FDA-approved drug, before the social critical period achieves ChP maturation and restores social behaviors. Furthermore, both the ASD model mice and organoids derived from patients with ASD developed an immature ChP. These results propose the involvement of an immature ChP in the pathogenesis of ASD and suggest the targeting of functional maturation of the ChP as a therapeutic strategy for ASD.

Brown adipocytes are characterized by a high abundance of mitochondria, allowing them to consume fatty acids for heat production. Increasing the number of brown adipocytes is considered a promising strategy for combating obesity. However, the molecular mechanisms underlying their differentiation remain poorly understood. In this study, we demonstrate that etomoxir, an inhibitor of Carnitine Palmitoyltransferase 1 (CPT1), inhibits their differentiation through mechanisms independent of β-oxidation inhibition. In the presence of etomoxir during brown adipocyte differentiation, reduced expression of the thermogenic gene UCP1 and decreased lipid droplets formation were observed. Furthermore, a transient reduction in the expression of PPARγ2, a critical factor in adipocyte differentiation, was also observed in the presence of etomoxir. These findings suggest the presence of a regulatory mechanism that specifically enhances PPARγ2 expression during brown adipocyte differentiation, thereby modulating thermogenic gene expression.

Abstract Hematopoietic stem cell (HSC) divisional fate and function are determined by cellular metabolism, yet the contribution of specific cellular organelles and metabolic pathways to blood maintenance and stress-induced responses in the bone marrow remains poorly understood. The outer mitochondrial membrane-localized E3 ubiquitin ligase MITOL/MARCHF5 (encoded by the Mitol gene) is known to regulate mitochondrial and endoplasmic reticulum (ER) interaction and to promote cell survival. Here, we investigated the functional involvement of MITOL in HSC maintenance by generating MX1-cre inducible Mitol knockout mice. MITOL deletion in the bone marrow resulted in HSC exhaustion and impairment of bone marrow reconstitution capability in vivo. Interestingly, MITOL loss did not induce major mitochondrial dysfunction in hematopoietic stem and progenitor cells. In contrast, MITOL deletion induced prolonged ER stress in HSCs, which triggered cellular apoptosis regulated by IRE1α. In line, dampening of ER stress signaling by IRE1α inihibitor KIRA6 partially rescued apoptosis of long-term-reconstituting HSC. In summary, our observations indicate that MITOL is a principal regulator of hematopoietic homeostasis and protects blood stem cells from cell death through its function in ER stress signaling.

Abstract Mitochondria are involved in various cellular processes, such as energy production, inflammatory responses, and cell death. Mitochondrial dysfunction is associated with many age-related diseases, including neurological disorders and heart failure. Mitochondrial quality is strictly maintained by mitochondrial dynamics linked to an adequate supply of phospholipids and other substances from the endoplasmic reticulum (ER). The outer mitochondrial membrane-localized E3 ubiquitin ligase MITOL/MARCHF5 is responsible for mitochondrial quality control through the regulation of mitochondrial dynamics, formation of mitochondria-ER contacts, and mitophagy. MITOL deficiency has been shown to impair mitochondrial function, cause an excessive inflammatory response, and increase vulnerability to stress, resulting in the exacerbation of the disease. In this study, we overview the ubiquitin-mediated regulation of mitochondrial function by MITOL and the relationship between MITOL and diseases.

Abnormal mitochondrial fragmentation by dynamin-related protein1 (Drp1) is associated with the progression of aging-associated heart diseases, including heart failure and myocardial infarction (MI). Here, we report a protective role of outer mitochondrial membrane (OMM)-localized E3 ubiquitin ligase MITOL/MARCH5 against cardiac senescence and MI, partly through Drp1 clearance by OMM-associated degradation (OMMAD). Persistent Drp1 accumulation in cardiomyocyte-specific MITOL conditional-knockout mice induced mitochondrial fragmentation and dysfunction, including reduced ATP production and increased ROS generation, ultimately leading to myocardial senescence and chronic heart failure. Furthermore, ischemic stress-induced acute downregulation of MITOL, which permitted mitochondrial accumulation of Drp1, resulted in mitochondrial fragmentation. Adeno-associated virus-mediated delivery of the MITOL gene to cardiomyocytes ameliorated cardiac dysfunction induced by MI. Our findings suggest that OMMAD activation by MITOL can be a therapeutic target for aging-associated heart diseases, including heart failure and MI.

Most oncolytic virotherapy has thus far employed viruses deficient in genes essential for replication in normal cells but not in cancer cells. Intra-tumoral injection of such viruses has resulted in clinically significant anti-tumor effects on the lesions in the vicinity of the injection sites but not on distant visceral metastases. To overcome this limitation, we have developed a receptor-retargeted oncolytic herpes simplex virus employing a single-chain antibody for targeting tumor-associated antigens (RR-oHSV) and its modified version with additional mutations conferring syncytium formation (RRsyn-oHSV). We previously showed that RRsyn-oHSV exhibits preserved antigen specificity and an ∼20-fold higher tumoricidal potency in vitro relative to RR-oHSV. Here, we investigated the in vivo anti-tumor effects of RRsyn-oHSV using human cancer xenografts in immunodeficient mice. With only a single intra-tumoral injection of RRsyn-oHSV at very low doses, all treated tumors regressed completely. Furthermore, intra-venous administration of RRsyn-oHSV resulted in robust anti-tumor effects even against large tumors. We found that these potent anti-tumor effects of RRsyn-oHSV may be associated with the formation of long-lasting tumor cell syncytia not containing non-cancerous cells that appear to trigger death of the syncytia. These results strongly suggest that cancer patients with distant metastases could be effectively treated with our RRsyn-oHSV.

Ineffective hematopoiesis is a fundamental process leading to the pathogenesis of myelodysplastic syndromes (MDS). However, the pathobiological mediators of ineffective hematopoiesis in MDS remain unclear. Here, we demonstrated that overwhelming mitochondrial fragmentation in mutant hematopoietic stem cells and progenitors (HSC/Ps) triggers ineffective hematopoiesis in MDS. Mouse modeling of CBL exon-deletion with RUNX1 mutants, previously unreported co-mutations in MDS patients, recapitulated not only clinically relevant MDS phenotypes but also a distinct MDS-related gene signature. Mechanistically, dynamin-related protein 1 (DRP1)-dependent excessive mitochondrial fragmentation in HSC/Ps led to excessive ROS production, induced inflammatory signaling activation, and promoted subsequent dysplasia formation and impairment of granulopoiesis. Mitochondrial fragmentation was generally observed in patients with MDS. Pharmacological inhibition of DRP1 attenuated mitochondrial fragmentation and rescued ineffective hematopoiesis phenotypes in MDS mice. These findings provide mechanistic insights into ineffective hematopoiesis and indicate that dysregulated mitochondrial dynamics could be a therapeutic target for bone marrow failure in MDS.

MITOL/MARCH5 is an E3 ubiquitin ligase that plays a crucial role in the control of mitochondrial quality and function. However, the significance of MITOL in cardiomyocytes under physiological and pathological conditions remains unclear. First, to determine the significance of MITOL in unstressed hearts, we assessed the cellular changes with the reduction of MITOL expression by siRNA in neonatal rat primary ventricular cardiomyocytes (NRVMs). MITOL knockdown in NRVMs induced cell death via ferroptosis, a newly defined non-apoptotic programmed cell death, even under no stress conditions. This phenomenon was observed only in NRVMs, not in other cell types. MITOL knockdown markedly reduced mitochondria-localized GPX4, a key enzyme associated with ferroptosis, promoting accumulation of lipid peroxides in mitochondria. In contrast, the activation of GPX4 in MITOL knockdown cells suppressed lipid peroxidation and cell death. MITOL knockdown reduced the glutathione/oxidized glutathione (GSH/GSSG) ratio that regulated GPX4 expression. Indeed, the administration of GSH or N-acetylcysteine improved the expression of GPX4 and viability in MITOL-knockdown NRVMs. MITOL-knockdown increased the expression of the glutathione-degrading enzyme, ChaC glutathione-specific γ-glutamylcyclotransferase 1 (Chac1). The knockdown of Chac1 restored the GSH/GSSG ratio, GPX4 expression, and viability in MITOL-knockdown NRVMs. Further, in cultured cardiomyocytes stressed with DOX, both MITOL and GPX4 were reduced, whereas forced-expression of MITOL suppressed DOX-induced ferroptosis by maintaining GPX4 content. Additionally, MITOL knockdown worsened vulnerability to DOX, which was almost completely rescued by treatment with ferrostatin-1, a ferroptosis inhibitor. In vivo, cardiac-specific depletion of MITOL did not produce obvious abnormality, but enhanced susceptibility to DOX toxicity. Finally, administration of ferrostatin-1 suppressed exacerbation of DOX-induced myocardial damage in MITOL-knockout hearts. The present study demonstrates that MITOL determines the cell fate of cardiomyocytes via the ferroptosis process and plays a key role in regulating vulnerability to DOX treatment. (288/300).

Radial migration during cortical development is required for formation of the six-layered structure of the mammalian cortex. Defective migration of neurons is linked to several developmental disorders such as autism and schizophrenia. A unique swollen structure called the dilation is formed in migrating neurons and is required for movement of the centrosome and nucleus. However, the detailed molecular mechanism by which this dilation forms is unclear. We report that CAMDI, a gene whose deletion is associated with psychiatric-behavior, is degraded by Cdc20-APC/C cell-cycle machinery after centrosome migration into the dilation in mouse brain development. We also show that CAMDI is restabilized in the dilation until the centrosome enters the dilation, at which point it is once again immediately destabilized. CAMDI degradation is carried out by binding to Cdc20-APC/C via the destruction box (D-box) degron of CAMDI. CAMDI D-box mutant overexpression inhibits dilation formation and neuronal cell migration via maintaining the stabilized state of CAMDI. These results indicate that CAMDI is a substrate of the Cdc20-APC/C system and that the oscillatory regulation of CAMDI protein correlates with dilation formation for proper cortical migration.

The lysine methyltransferase SETDB1, an enzyme responsible for methylation of histone H3 at lysine 9, plays a key role in H3K9 tri-methylation-dependent silencing of endogenous retroviruses and developmental genes. Recent studies have shown that ubiquitination of human SETDB1 complements its catalytic activity and the silencing of endogenous retroviruses in human embryonic stem cells. However, it is not known whether SETDB1 ubiquitination is essential for its other major role in epigenetic silencing of developmental gene programs. We previously showed that SETDB1 contributes to the formation of H3K4/H3K9me3 bivalent chromatin domains that keep adipogenic Cebpa and Pparg genes in a poised state for activation and restricts the differentiation potential of pre-adipocytes. Here, we show that ubiquitin-resistant K885A mutant of SETDB1 represses adipogenic genes and inhibits pre-adipocyte differentiation similar to wild-type SETDB1. We show this was due to a compensation mechanism for H3K9me3 chromatin modifications on the Cebpa locus by other H3K9 methyltransferases Suv39H1 and Suv39H2. In contrast, the K885A mutant did not repress other SETDB1 target genes such as Tril and Gas6 suggesting SETDB1 represses its target genes by two mechanisms; one that requires its ubiquitination and another that still requires SETDB1 but not its enzyme activity.

The transfer of phospholipids from the endoplasmic reticulum to mitochondria via the mitochondria-endoplasmic reticulum (ER) contact site (MERCS) is essential for maintaining mitochondrial function and integrity. Here, we identified RMDN3/PTPIP51, possessing phosphatidic acid (PA)-transfer activity, as a neighboring protein of the mitochondrial E3 ubiquitin ligase MITOL/MARCH5 by proximity-dependent biotin labeling using APEX2. We found that MITOL interacts with and ubiquitinates RMDN3. Mutational analysis identified lysine residue 89 in RMDN3 as a site of ubiquitination by MITOL. Loss of MITOL or the substitution of lysine 89 to arginine in RMDN3 significantly reduced the PA-binding activity of RMDN3, suggesting that MITOL regulates the transport of PA to mitochondria by activating RMDN3. Our findings imply that ubiquitin signaling regulates phospholipid transport at the MERCS.

Amyloid-β (Aβ) plaques are strongly associated with the development of Alzheimer's disease (AD). However, it remains unclear how morphological differences in Aβ plaques determine the pathogenesis of Aβ. Here, we categorized Aβ plaques into four types based on the macroscopic features of the dense core, and found that the Aβ-plaque subtype containing a larger dense core showed the strongest association with neuritic dystrophy. Astrocytes dominantly accumulated toward these expanded/dense-core-containing Aβ plaques. Previously, we indicated that deletion of the mitochondrial ubiquitin ligase MITOL/MARCH5 triggers mitochondrial impairments and exacerbates cognitive decline in a mouse model with AD-related Aβ pathology. In this study, MITOL deficiency accelerated the formation of expanded/dense-core-containing Aβ plaques, which showed reduced contacts with astrocytes, but not microglia. Our findings suggest that expanded/dense-core-containing Aβ-plaque formation enhanced by the alteration of mitochondrial function robustly contributes to the exacerbation of Aβ neuropathology, at least in part, through the reduced contacts between Aβ plaques and astrocytes.

Mouse models of various neuropsychiatric disorders, such as schizophrenia, often display an immature dentate gyrus, characterized by increased numbers of immature neurons and neuronal progenitors and a dearth of mature neurons. We previously demonstrated that the CRMP5-associated GTPase (CRAG), a short splice variant of Centaurin-γ3/AGAP3, is highly expressed in the dentate gyrus. CRAG promotes cell survival and antioxidant defense by inducing the activation of serum response factors at promyelocytic leukemia protein bodies, which are nuclear stress-responsive domains, during neuronal development. However, the physiological role of CRAG in neuronal development remains unknown. Here, we analyzed the role of CRAG using dorsal forebrain-specific CRAG/Centaurin-γ3 knockout mice. The mice revealed maturational abnormality of the hippocampal granule cells, including increased doublecortin-positive immature neurons and decreased calbindin-positive mature neurons, a typical phenotype of immature dentate gyri. Furthermore, the mice displayed hyperactivity in the open-field test, a common measure of exploratory behavior, suggesting that these mice may serve as a novel model for neuropsychiatric disorder associated with hyperactivity. Thus, we conclude that CRAG is required for the maturation of neurons in the dentate gyrus, raising the possibility that its deficiency might promote the development of psychiatric disorders in humans.

Mitochondrial pathophysiology is implicated in the development of Alzheimer's disease (AD). An integrative database of gene dysregulation suggests that the mitochondrial ubiquitin ligase MITOL/MARCH5, a fine-tuner of mitochondrial dynamics and functions, is downregulated in patients with AD. Here, we report that the perturbation of mitochondrial dynamics by MITOL deletion triggers mitochondrial impairments and exacerbates cognitive decline in a mouse model with AD-related Aβ pathology. Notably, MITOL deletion in the brain enhanced the seeding effect of Aβ fibrils, but not the spontaneous formation of Aβ fibrils and plaques, leading to excessive secondary generation of toxic and dispersible Aβ oligomers. Consistent with this, MITOL-deficient mice with Aβ etiology exhibited worsening cognitive decline depending on Aβ oligomers rather than Aβ plaques themselves. Our findings suggest that alteration in mitochondrial morphology might be a key factor in AD due to directing the production of Aβ form, oligomers or plaques, responsible for disease development.

Parkin promotes cell survival by removing damaged mitochondria via mitophagy. However, although some studies have suggested that Parkin induces cell death, the regulatory mechanism underlying the dual role of Parkin remains unknown. Herein, we report that mitochondrial ubiquitin ligase (MITOL/MARCH5) regulates Parkin-mediated cell death through the FKBP38-dependent dynamic translocation from the mitochondria to the ER during mitophagy. Mechanistically, MITOL mediates ubiquitination of Parkin at lysine 220 residue, which promotes its proteasomal degradation, and thereby fine-tunes mitophagy by controlling the quantity of Parkin. Deletion of MITOL leads to accumulation of the phosphorylated active form of Parkin in the ER, resulting in FKBP38 degradation and enhanced cell death. Thus, we have shown that MITOL blocks Parkin-induced cell death, at least partially, by protecting FKBP38 from Parkin. Our findings unveil the regulation of the dual function of Parkin and provide a novel perspective on the pathogenesis of PD.

The molecular pathology of diseases seen from the mitochondrial axis has become more complex with the progression of research. A variety of factors, including the failure of mitochondrial dynamics and quality control, have made it extremely difficult to narrow down drug discovery targets. We have identified MITOL (mitochondrial ubiquitin ligase: also known as MARCH5) localized on the mitochondrial outer membrane and previously reported that it is an important regulator of mitochondrial dynamics and mitochondrial quality control. In this review, we describe the pathological aspects of MITOL revealed through functional analysis and its potential as a drug discovery target.

In mitochondrial disorders, short stature and growth failure are common symptoms, but their underlying mechanism remains unknown. In this study, we examined the cause of growth failure of mice induced by nestin promoter-driven knockout of the mitochondrial ubiquitin ligase MITOL (MARCH5), a key regulator of mitochondrial function. MITOL-knockout mice have congenital hypoplasia of the anterior pituitary caused by decreased expression of pituitary transcript factor 1 (Pit1). Consistently, both mRNA levels of growth hormone (GH) and prolactin levels were markedly decreased in the anterior pituitary of mutant mice. Growth failure of mutant mice was partly rescued by hypodermic injection of recombinant GH. To clarify whether this abnormality was induced by the primary effect of MITOL knockdown in the anterior pituitary or a secondary effect of other lesions, we performed lentiviral-mediated knockdown of MITOL on cultured rat pituitary GH3 cells, which secrete GH. GH production was severely compromised in MITOL-knockdown GH3 cells. In conclusion, MITOL plays a critical role in the development of the anterior pituitary; therefore, mice with MITOL dysfunction exhibited pituitary dwarfism caused by anterior pituitary hypoplasia. Our findings suggest that mitochondrial dysfunction is commonly involved in the unknown pathogenesis of pituitary dwarfism.

Mitochondria are highly dynamic organelles that constantly fuse, divide, and move, and their function is regulated and maintained by their morphologic changes. Mitochondrial disease (MD) comprises a group of disorders involving mitochondrial dysfunction. However, it is not clear whether changes in mitochondrial morphology are related to MD. In this study, we examined mitochondrial morphology in fibroblasts from patients with MD (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) and Leigh syndrome). We observed that MD fibroblasts exhibited significant mitochondrial fragmentation by upregulation of Drp1, which is responsible for mitochondrial fission. Interestingly, the inhibition of mitochondrial fragmentation by Drp1 knockdown enhanced cellular toxicity and led to cell death in MD fibroblasts. These results suggest that mitochondrial fission plays a critical role in the attenuation of mitochondrial damage in MD fibroblasts.

CRMP-5-associated GTPase (CRAG), a short splicing variant of centaurin-γ3/AGAP3, is predominantly expressed in the developing brain. We previously demonstrated that CRAG, but not centaurin-γ3, translocates to the nucleus and activates the serum response factor (SRF)-c-Fos pathway in cultured neuronal cells. However, the physiological relevance of CRAG in vivo is unknown. Here, we found that CRAG/centaurin-γ3-knockout mice showed intensively suppressed kainic acid-induced c-fos expression in the hippocampus. Analyses of molecular mechanisms underlying CRAG-mediated SRF activation revealed that CRAG has an essential role in GTPase activity, interacts with ELK1 (a co-activator of SRF), and activates SRF in an ELK1-dependent manner. Furthermore, CRAG and ELK1 interact with promyelocytic leukaemia bodies through SUMO-interacting motifs, which is required for SRF activation. These results suggest that CRAG plays a critical role in ELK1-dependent SRF-c-fos activation at promyelocytic leukaemia bodies in the developing brain.

Developing axons change responsiveness to guidance cues during the journey to synapse with target cells. Axon crossing at the ventral midline serves as a model for studying how axons accomplish such a switch in their response. Although primary neuron culture has been a versatile technique for elucidating various developmental mechanisms, many in vivo characteristics of neurons, such as long axon-extending abilities and axonal compartments, are not thoroughly preserved. In explant cultures, such properties of differentiated neurons and tissue architecture are maintained. To examine how the midline repellent Slit regulated the distribution of the Robo receptor in spinal cord commissural axons upon midline crossing and whether Robo trafficking machinery was a determinant of midline crossing, novel explant culture systems were developed. We have combined an "open-book" spinal cord explant method with that devised for flat-mount retinae. Here we present our protocol for explant culture of embryonic mouse spinal cords, which allows flexible manipulation of experimental conditions, immunostaining of extending axons and quantitative analysis of individual axons. In addition, we present a modified method that combines ex vivo electroporation and "closed-book" spinal cord explant culture. These culture systems provide new platforms for detailed analysis of axon guidance, by adapting gene knockdown, knockout and genome editing.

Mitochondrial abnormalities are associated with developmental disorders, although a causal relationship remains largely unknown. Here, we report that increased oxidative stress in neurons by deletion of mitochondrial ubiquitin ligase MITOL causes a potential neuroinflammation including aberrant astrogliosis and microglial activation, indicating that mitochondrial abnormalities might confer a risk for inflammatory diseases in brain such as psychiatric disorders. A role of MITOL in both mitochondrial dynamics and ER-mitochondria tethering prompted us to characterize three-dimensional structures of mitochondria in vivo. In MITOL-deficient neurons, we observed a significant reduction in the ER-mitochondria contact sites, which might lead to perturbation of phospholipids transfer, consequently reduce cardiolipin biogenesis. We also found that branched large mitochondria disappeared by deletion of MITOL. These morphological abnormalities of mitochondria resulted in enhanced oxidative stress in brain, which led to astrogliosis and microglial activation partly causing abnormal behavior. In conclusion, the reduced ER-mitochondria tethering and excessive mitochondrial fission may trigger neuroinflammation through oxidative stress.

A switch in the response of commissural axons to the repellent Slit is crucial for ensuring that they cross the ventral midline only once. However, the underlying mechanisms remain to be elucidated. We have found that both endocytosis and recycling of Robo1 receptor are crucial for modulating Slit sensitivity in vertebrate commissural axons. Robo1 endocytosis and its recycling back to the cell surface maintained the stability of axonal Robo1 during Slit stimulation. We identified Arf6 guanosine triphosphatase and its activators, cytohesins, as previously unknown components in Slit-Robo1 signalling in vertebrate commissural neurons. Slit-Robo1 signalling activated Arf6. The Arf6-deficient mice exhibited marked defects in commissural axon midline crossing. Our data showed that a Robo1 endocytosis-triggered and Arf6-mediated positive-feedback strengthens the Slit response in commissural axons upon their midline crossing. Furthermore, the cytohesin-Arf6 pathways modulated this self-enhancement of the Slit response before and after midline crossing, resulting in a switch that reinforced robust regulation of axon midline crossing. Our study provides insights into endocytic trafficking-mediated mechanisms for spatiotemporally controlled axonal responses and uncovers new players in the midline switch in Slit responsiveness of commissural axons.

Little is known about the molecular mechanisms of cognitive deficits in psychiatric disorders. CAMDI is a psychiatric disorder-related factor, the deficiency of which in mice results in delayed neuronal migration and psychiatrically abnormal behaviors. Here, we found that CAMDI-deficient mice exhibited impaired recognition memory and spatial reference memory. Knockdown of CAMDI in hippocampal neurons increased the amount of internalized alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor (AMPAR) and attenuated the chemical long-term potentiation (LTP)-dependent cell surface expression of AMPAR. KIBRA was identified as a novel CAMDI-binding protein that retains AMPAR in the cytosol after internalization. KIBRA inhibited CAMDI-dependent Rab11 activation, thereby attenuating AMPAR cell surface expression. These results suggest that CAMDI regulates AMPAR cell surface expression during LTP. CAMDI dysfunction may partly explain the mechanism underlying cognitive deficits in psychiatric diseases.

In fed cells, syntaxin 17 (Stx17) is associated with microtubules at the endoplasmic reticulum-mitochondria interface and promotes mitochondrial fission by determining the localization and function of the mitochondrial fission factor Drp1. Upon starvation, Stx17 dissociates from microtubules and Drp1, and binds to Atg14L, a subunit of the phosphatidylinositol 3-kinase complex, to facilitate phosphatidylinositol 3-phosphate production and thereby autophagosome formation, but the mechanism underlying this phenomenon remains unknown. Here we identify MAP1B-LC1 (microtubule-associated protein 1B-light chain 1) as a critical regulator of Stx17 function. Depletion of MAP1B-LC1 causes Stx17-dependent autophagosome accumulation even under nutrient-rich conditions, whereas its overexpression blocks starvation-induced autophagosome formation. MAP1B-LC1 links microtubules and Stx17 in fed cells, and starvation causes the dephosphorylation of MAP1B-LC1 at Thr217, allowing Stx17 to dissociate from MAP1B-LC1 and bind to Atg14L. Our results reveal the mechanism by which Stx17 changes its binding partners in response to nutrient status.

DDHD2/KIAA0725p is a mammalian intracellular phospholipase A1 that exhibits phospholipase and lipase activities. Mutation of the DDHD2 gene causes hereditary spastic paraplegia (SPG54), an inherited neurological disorder characterized by lower limb spasticity and weakness. Although previous studies demonstrated lipid droplet accumulation in the brains of SPG54 patients and DDHD2 knockout mice, the cause of SPG54 remains elusive. Here, we show that ablation of DDHD2 in mice induces age-dependent apoptosis of motor neurons in the spinal cord. In vitro, motor neurons and embryonic fibroblasts from DDHD2 knockout mice fail to survive and are susceptible to apoptotic stimuli. Chemical and probe-based analysis revealed a substantial decrease in cardiolipin content and an increase in reactive oxygen species generation in DDHD2 knockout cells. Reactive oxygen species production in DDHD2 knockout cells was reversed by the expression of wild-type DDHD2, but not by an active-site DDHD2 mutant, DDHD2 mutants related to hereditary spastic paraplegia, or DDHD1, another member of the intracellular phospholipase A1 family whose mutation also causes spastic paraplegia (SPG28). Our results demonstrate the protective role of DDHD2 for mitochondrial integrity and provide a clue to the pathogenic mechanism of SPG54.