Increased Plp1 gene expression leads to massive microglial cell activation and inflammation throughout the brain

PMD (Pelizaeus–Merzbacher disease) is a rare neurodegenerative disorder that impairs motor and cognitive functions and is associated with a shortened lifespan. The cause of PMD is mutations of the PLP1 [proteolipid protein 1 gene (human)] gene. Transgenic mice with increased Plp1 [proteolipid protein 1 gene (non-human)] copy number model most aspects of PMD patients with duplications. Hypomyelination and demyelination are believed to cause the neurological abnormalities in mammals with PLP1 duplications. We show, for the first time, intense microglial reactivity throughout the grey and white matter of a transgenic mouse line with increased copy number of the native Plp1 gene. Activated microglia in the white and grey matter of transgenic mice are found as early as postnatal day 7, before myelin commences in normal cerebra. This finding indicates that degeneration of myelin does not cause the microglial response. Microglial numbers are doubled due to in situ proliferation. Compared with the jp (jimpy) mouse, which has much more oligodendrocyte death and hardly any myelin, microglia in the overexpressors show a more dramatic microglial reactivity than jp, especially in the grey matter. Predictably, many classical markers of an inflammatory response, including TNF-α (tumour necrosis factor-α) and IL-6, are significantly up-regulated manyfold. Because inflammation is believed to contribute to axonal degeneration in multiple sclerosis and other neurodegenerative diseases, inflammation in mammals with increased Plp1 gene dosage may also contribute to axonal degeneration described in patients and rodents with PLP1 increased gene dosage.     

Drosophila ref(2)P is required for the parkin-mediated suppression of mitochondrial dysfunction in pink1 mutants

Autophagy is a critical regulator of organellar homeostasis, particularly of mitochondria. Upon the loss of membrane potential, dysfunctional mitochondria are selectively removed by autophagy through recruitment of the E3 ligase Parkin by the PTEN-induced kinase 1 (PINK1) and subsequent ubiquitination of mitochondrial membrane proteins. Mammalian sequestrome-1 (p62/SQSTM1) is an autophagy adaptor, which has been proposed to shuttle ubiquitinated cargo for autophagic degradation downstream of Parkin. Here, we show that loss of ref(2)P, the Drosophila orthologue of mammalian P62, results in abnormalities, including mitochondrial defects and an accumulation of mitochondrial DNA with heteroplasmic mutations, correlated with locomotor defects. Furthermore, we show that expression of Ref(2)P is able to ameliorate the defects caused by loss of Pink1 and that this depends on the presence of functional Parkin. Finally, we show that both the PB1 and UBA domains of Ref(2)P are crucial for mitochondrial clustering. We conclude that Ref(2)P is a crucial downstream effector of a pathway involving Pink1 and Parkin and is responsible for the maintenance of a viable pool of cellular mitochondria by promoting their aggregation and autophagic clearance.     

Progesterone Antagonist Therapy in a Pelizaeus-Merzbacher Mouse Model

Pelizaeus-Merzbacher disease (PMD) is a severe hypomyelinating disease, characterized by ataxia, intellectual disability, epilepsy, and premature death. In the majority of cases, PMD is caused by duplication of PLP1 that is expressed in myelinating oligodendrocytes. Despite detailed knowledge of PLP1, there is presently no curative therapy for PMD. We used a Plp1 transgenic PMD mouse model to test the therapeutic effect of Lonaprisan, an antagonist of the nuclear progesterone receptor, in lowering Plp1 mRNA overexpression. We applied placebo-controlled Lonaprisan therapy to PMD mice for 10 weeks and performed the grid slip analysis to assess the clinical phenotype. Additionally, mRNA expression and protein accumulation as well as histological analysis of the central nervous system were performed. Although Plp1 mRNA levels are increased 1.8-fold in PMD mice compared to wild-type controls, daily Lonaprisan treatment reduced overexpression at the RNA level to about 1.5-fold, which was sufficient to significantly improve the poor motor phenotype. Electron microscopy confirmed a 25% increase in the number of myelinated axons in the corticospinal tract when compared to untreated PMD mice. Microarray analysis revealed the upregulation of proapoptotic genes in PMD mice that could be partially rescued by Lonaprisan treatment, which also reduced microgliosis, astrogliosis, and lymphocyte infiltration.     

Intra-mitochondrial Methylation Deficiency Due to Mutations in SLC25A26

S-adenosylmethionine (SAM) is the predominant methyl group donor and has a large spectrum of target substrates. As such, it is essential for nearly all biological methylation reactions. SAM is synthesized by methionine adenosyltransferase from methionine and ATP in the cytoplasm and subsequently distributed throughout the different cellular compartments, including mitochondria, where methylation is mostly required for nucleic-acid modifications and respiratory-chain function. We report a syndrome in three families affected by reduced intra-mitochondrial methylation caused by recessive mutations in the gene encoding the only known mitochondrial SAM transporter, SLC25A26. Clinical findings ranged from neonatal mortality resulting from respiratory insufficiency and hydrops to childhood acute episodes of cardiopulmonary failure and slowly progressive muscle weakness. We show that SLC25A26 mutations cause various mitochondrial defects, including those affecting RNA stability, protein modification, mitochondrial translation, and the biosynthesis of CoQ10 and lipoic acid.     

Detection of a large duplication mutation in the myosin-binding protein C3 gene in a case of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a cardiovascular disease with autosomal dominant inheritance caused by mutations in genes coding for sarcomeric and/or regulatory proteins expressed in cardiomyocytes. In a small cohort of HCM patients (n = 8), we searched for mutations in the two most common genes responsible for HCM and found four missense mutations in the MYH7 gene encoding cardiac β-myosin heavy chain (R204H, M493V, R719W, and R870H) and three mutations in the myosin-binding protein C3 gene (MYBPC3) including one missense (A848V) and two frameshift mutations (c.3713delTG and c.702ins26bp). The c.702ins26bp insertion resulted from the duplication of a 26-bp fragment in a 54-year-old female HCM patient presenting with clinical signs of heart failure due to diastolic dysfunction. Although such large duplications (> 10 bp) in the MYBPC3 gene are very rare and have been identified only in 4 families reported so far, the identical duplication mutation was found earlier in a Dutch patient, demonstrating that it may constitute a hitherto unknown founder mutation in central European populations. This observation underscores the significance of insertions into the coding sequence of the MYBPC3 gene for the development and pathogenesis of HCM.     

Therapy of Pelizaeus-Merzbacher disease in mice by feeding a cholesterol-enriched diet

Duplication of PLP1 (proteolipid protein gene 1) and the subsequent overexpression of the myelin protein PLP (also known as DM20) in oligodendrocytes is the most frequent cause of Pelizaeus-Merzbacher disease (PMD), a fatal leukodystrophy without therapeutic options. PLP binds cholesterol and is contained within membrane lipid raft microdomains. Cholesterol availability is the rate-limiting factor of central nervous system myelin synthesis. Transgenic mice with extra copies of the Plp1 gene are accurate models of PMD. Dysmyelination followed by demyelination, secondary inflammation and axon damage contribute to the severe motor impairment in these mice. The finding that in Plp1-transgenic oligodendrocytes, PLP and cholesterol accumulate in late endosomes and lysosomes (endo/lysosomes), prompted us to further investigate the role of cholesterol in PMD. Here we show that cholesterol itself promotes normal PLP trafficking and that dietary cholesterol influences PMD pathology. In a preclinical trial, PMD mice were fed a cholesterol-enriched diet. This restored oligodendrocyte numbers and ameliorated intracellular PLP accumulation. Moreover, myelin content increased, inflammation and gliosis were reduced and motor defects improved. Even after onset of clinical symptoms, cholesterol treatment prevented disease progression. Dietary cholesterol did not reduce Plp1 overexpression but facilitated incorporation of PLP into myelin membranes. These findings may have implications for therapeutic interventions in patients with PMD.     

Yeast Phosducin-Like Protein 2 Acts as a Stimulatory Co-Factor for the Folding of Actin by the Chaperonin CCT via a Ternary Complex

The eukaryotic chaperonin-containing TCP-1 (CCT) folds the cytoskeletal protein actin. The folding mechanism of this 16-subunit, 1-MDa machine is poorly characterised due to the absence of quantitative in vitro assays. We identified phosducin-like protein 2, Plp2p (=PLP2), as an ATP-elutable binding partner of yeast CCT while establishing the CCT interactome. In a novel in vitro CCT-ACT1 folding assay that is functional under physiological conditions, PLP2 is a stimulatory co-factor. In a single ATP-driven cycle, PLP2-CCT-ACT1 complexes yield 30-fold more native actin than CCT-ACT1 complexes. PLP2 interacts directly with ACT1 through the C-terminus of its thioredoxin fold and the CCT-binding subdomain 4 of actin. The in vitro CCT-ACT1-PLP2 folding cycle of the preassembled complex takes 90 s at 30 °C, several times slower than the canonical chaperonin GroEL. The specific interactions between PLP2, CCT and ACT1 in the yeast-component in vitro system and the pronounced stimulatory effect of PLP2 on actin folding are consistent with in vivo genetic approaches demonstrating an essential and positive role for PLP2 in cellular processes involving actin in Saccharomyces cerevisiae. In mammalian systems, however, several members of the PLP family, including human PDCL3, the orthologue of PLP2, have been shown to be inhibitory toward CCT-mediated folding of actin in vivo and in vitro. Here, using a rabbit-reticulocyte-derived in vitro translation system, we found that inhibition of β-actin folding by PDCL3 can be relieved by exchanging its acidic C-terminal extension for that of PLP2. It seems that additional levels of regulatory control of CCT activity by this PLP have emerged in higher eukaryotes.     

HtrA2/Omi influences the stability of LON protease 1 and prohibitin,proteins involved in mitochondrial homeostasis

High temperature requirement A2 (HtrA2)/Omi is a serine protease localized in mitochondria. In response to apoptotic stimuli, HtrA2 is released to the cytoplasm and cleaves many proteins, including XIAP, Apollon/BRUCE, WT1, and Ped/Pea-15, to promote apoptosis. However, the function of HtrA2 in mitochondria under normal conditions remains unclear. Here, we show that the mitochondrial proteins, LON protease 1 (LONP1) and prohibitin (PHB), are overexpressed in HtrA2^−/− mouse embryonic fibroblast (MEF) cells and HtrA2 knock-down HEK293T cells. We also confirm the effect of the HtrA2 protease on the stability of the above mitochondrial quality control proteins in motor neuron degeneration 2 (mnd2) mice, which have a greatly reduced protease activity as a result of a Ser276Cys missense mutation of the HtrA2 gene. In addition, PHB interacts with and is directly cleaved by HtrA2. Luminescence assays demonstrate that the intracellular ATP level is decreased in HtrA2^−/− cells compared to HtrA2^+/+ cells. HtrA2 deficiency causes a decrease in the mitochondrial membrane potential, and reactive oxygen species (ROS) generation is greater in HtrA2^−/− cells than in HtrA2^+/+ cells. Our results implicate that HtrA2 might be an upstream regulator of mitochondrial homeostasis.     

综述与专论: 少突胶质细胞病——佩梅病的临床特点及致病机制

佩梅病（Pelizaeus-Merzbacher disease,PMD）是髓鞘形成低下性脑白质营养不良疾病中最常见的一种,其临床特点主要表现为发育落后尤其是大运动落后、眼震、肌张力低下等。其致病机制主要为脑白质髓鞘形成细胞-少突胶质细胞发生病理性改变从而导致髓鞘形成不良,相应理论基础包括以往研究中PLP1点突变通过影响PLP1/DM20寡聚体的形成,进而影响少突胶质细胞的存活,髓鞘分子结构的形成等;而PLP1重复突变则使少突胶质细胞及髓鞘脂的发育停止。近年来对细胞器互作网络（organelle interaction network,OIN）的研究进一步揭示了PLP1突变的致病机制：PLP1点突变通过影响PLP1蛋白上膜进而影响少突胶质细胞髓鞘化。PLP1重复突变则改变内质网线粒体间的连接,继而影响线粒体的形态功能等产生致病作用。目前已有相关研究表明,一些小分子化合物或药物例如胆固醇、吡拉西坦等以及基因疗法在动物体内对PMD临床症状有改善作用,其在PMD 患者体内的疗效有待进一步证实。     

Proteolipid/DM-20 proteins bearing the paralytic tremor mutation in peripheral nerves and transfected Cos-7 cells

Paralytic tremor (Plp-pt) is a missense mutation of the myelin proteolipid gene (Plp) in rabbits. The myelin yield in the Plp-pt brain is reduced and the protein and lipid composition of central nervous system (CNS) myelin is abnormal. We studied the intracellular transport of the normal and Plp-pt mutant PLP and DM-20 in transiently transfected Cos-7 cells. While the mutant PLP accumulates in the rough endoplasmic reticulum and does not reach the plasma membrane, the spliced isoform of PLP, mutant DM-20, is normally transported to the cell surface and integrated into the membrane. Analysis of rabbit sciatic nerves revealed that concentration of peripheral nervous system (PNS) myelin proteins is normal in Plp-pt myelin. In the PNS like in the CNS, the level of Plp gene products is subnormal. But this does not affect myelination, in the PNS where PLP, present in low concentration, is not a structural component of compact myelin. The normal level of Plp gene expression in Schwann cells is low and these results suggest that, in the Plp-pt PNS, Schwann cell function is not affected by the deficiency in PLP and/or the impairment of intracellular PLP transport. Special issue dedicated to Dr Marion E. Smith.     

Mitochondrial proteolytic stress induced by loss of mortalin function is rescued by Parkin and PINK1

The mitochondrial chaperone mortalin was implicated in Parkinson''s disease (PD) because of its reduced levels in the brains of PD patients and disease-associated rare genetic variants that failed to rescue impaired mitochondrial integrity in cellular knockdown models. To uncover the molecular mechanisms underlying mortalin-related neurodegeneration, we dissected the cellular surveillance mechanisms related to mitochondrial quality control, defined the effects of reduced mortalin function at the molecular and cellular levels and investigated the functional interaction of mortalin with Parkin and PINK1, two PD-related proteins involved in mitochondrial homeostasis. We found that reduced mortalin function leads to: (1) activation of the mitochondrial unfolded protein response (UPR(mt)), (2) increased susceptibility towards intramitochondrial proteolytic stress, (3) increased autophagic degradation of fragmented mitochondria and (4) reduced mitochondrial mass in human cells in vitro and ex vivo. These alterations caused increased vulnerability toward apoptotic cell death. Proteotoxic perturbations induced by either partial loss of mortalin or chemical induction were rescued by complementation with native mortalin, but not disease-associated mortalin variants, and were independent of the integrity of autophagic pathways. However, Parkin and PINK1 rescued loss of mortalin phenotypes via increased lysosomal-mediated mitochondrial clearance and required intact autophagic machinery. Our results on loss of mortalin function reveal a direct link between impaired mitochondrial proteostasis, UPR(mt) and PD and show that effective removal of dysfunctional mitochondria via either genetic (PINK1 and Parkin overexpression) or pharmacological intervention (rapamycin) may compensate mitochondrial phenotypes.     

TMEM126A is a mitochondrial located mRNA (MLR) protein of the mitochondrial inner membrane

Background

Hereditary optic neuropathies (HONs) are a heterogeneous group of disorders that affect retinal ganglion cells (RGCs) and axons that form the optic nerve. Leber's Hereditary Optic Neuropathy and the autosomal dominant optic atrophy related to OPA1 mutations are the most common forms. Nonsyndromic autosomal recessive optic neuropathies are rare and their existence has been long debated. We recently identified the first gene responsible for these conditions, TMEM126A. This gene is highly expressed in retinal cellular compartments enriched in mitochondria and supposed to encode a mitochondrial transmembrane protein of unknown function.

Methods

A specific polyclonal antibody targeting the TMEM126A protein has been generated. Quantitative fluorescent in situ hybridization, cellular fractionation, mitochondrial membrane association study, mitochondrial sub compartmentalization analysis by both proteolysis assays and transmission electron microscopy, and expression analysis of truncated TMEM126A constructs by immunofluorescence confocal microscopy were carried out.

Results

TMEM126A mRNAs are strongly enriched in the vicinity of mitochondria and encode an inner mitochondrial membrane associated cristae protein. Moreover, the second transmembrane domain of TMEM126A is required for its mitochondrial localization.

Conclusions

TMEM126A is a mitochondrial located mRNA (MLR) that may be translated in the mitochondrial surface and the protein is subsequently imported to the inner membrane. These data constitute the first step toward a better understanding of the mechanism of action of TMEM126A in RGCs and support the importance of mitochondrial dysfunction in the pathogenesis of HON.

General significance

Local translation of nuclearly encoded mitochondrial mRNAs might be a mechanism for rapid onsite supply of mitochondrial membrane proteins.     

Control of Human PLP1 Expression Through Transcriptional Regulatory Elements and Alternatively Spliced Exons in Intron 1

Although the myelin proteolipid protein gene (PLP1) encodes the most abundant protein in central nervous system (CNS) myelin, not much is known about the mechanisms that govern expression of the human gene (hPLP1). Much more is known about the processes that regulate Plp1 gene expression in rodents. From studies with Plp1-lacZ transgenic mice, it was determined that the first intron of mouse Plp1 (mPlp1) is required to attain high levels of expression in brain, concurrent with the active myelination period. Other studies have suggested that within mPlp1 intron 1 (>8 kb) lie several regions with enhancer-like activity. To test whether these sequences (and possibly others) in hPLP1 intron 1 are functional, deletion-transfection analysis was performed with hPLP1-lacZ constructs that contain various portions of the intron, or lack it altogether. Results presented here demonstrate the importance of hPLP1 intron 1 in achieving maximal levels of expression in the immortalized oligodendroglial cell line, Oli-neu. Deletion analysis indicates that the intron contains multiple positive regulatory elements which are active in Oli-neu cells. Some of these elements appear to be functionally conserved between human and mouse, while others are not. Furthermore, our studies demonstrate that multiple splice variants can be formed due to inclusion of extra (supplementary) exons from what is classically thought of as hPLP1 intron 1. Thus, splicing of these novel exons (which are not recognized as such in mPlp1 due to lack of conserved splice sites) must utilize factors common to both human and mouse since Oli-neu cells are of mouse origin.     

A new vicious cycle involving glutamate excitotoxicity, oxidative stress and mitochondrial dynamics

Glutamate excitotoxicity leads to fragmented mitochondria in neurodegenerative diseases, mediated by nitric oxide and S-nitrosylation of dynamin-related protein 1, a mitochondrial outer membrane fission protein. Optic atrophy gene 1 (OPA1) is an inner membrane protein important for mitochondrial fusion. Autosomal dominant optic atrophy (ADOA), caused by mutations in OPA1, is a neurodegenerative disease affecting mainly retinal ganglion cells (RGCs). Here, we showed that OPA1 deficiency in an ADOA model influences N-methyl-D-aspartate (NMDA) receptor expression, which is involved in glutamate excitotoxicity and oxidative stress. Opa1^enu/+ mice show a slow progressive loss of RGCs, activation of astroglia and microglia, and pronounced mitochondrial fission in optic nerve heads as found by electron tomography. Expression of NMDA receptors (NR1, 2A, and 2B) in the retina of Opa1^enu/+ mice was significantly increased as determined by western blot and immunohistochemistry. Superoxide dismutase 2 (SOD2) expression was significantly decreased, the apoptotic pathway was activated as Bax was increased, and phosphorylated Bad and BcL-xL were decreased. Our results conclusively demonstrate that not only glutamate excitotoxicity and/or oxidative stress alters mitochondrial fission/fusion, but that an imbalance in mitochondrial fission/fusion in turn leads to NMDA receptor upregulation and oxidative stress. Therefore, we propose a new vicious cycle involved in neurodegeneration that includes glutamate excitotoxicity, oxidative stress, and mitochondrial dynamics.     

The Legs at odd angles (Loa) Mutation in Cytoplasmic Dynein Ameliorates Mitochondrial Function in SOD1G93A Mouse Model for Motor Neuron Disease

Amyotrophic lateral sclerosis (ALS) is a debilitating and fatal late-onset neurodegenerative disease. Familial cases of ALS (FALS) constitute ∼10% of all ALS cases, and mutant superoxide dismutase 1 (SOD1) is found in 15–20% of FALS. SOD1 mutations confer a toxic gain of unknown function to the protein that specifically targets the motor neurons in the cortex and the spinal cord. We have previously shown that the autosomal dominant Legs at odd angles (Loa) mutation in cytoplasmic dynein heavy chain (Dync1h1) delays disease onset and extends the life span of transgenic mice harboring human mutant SOD1^G93A. In this study we provide evidence that despite the lack of direct interactions between mutant SOD1 and either mutant or wild-type cytoplasmic dynein, the Loa mutation confers significant reductions in the amount of mutant SOD1 protein in the mitochondrial matrix. Moreover, we show that the Loa mutation ameliorates defects in mitochondrial respiration and membrane potential observed in SOD1^G93A motor neuron mitochondria. These data suggest that the Loa mutation reduces the vulnerability of mitochondria to the toxic effects of mutant SOD1, leading to improved mitochondrial function in SOD1^G93A motor neurons.