Mitochondrial inner membrane protein (mitofilin) knockdown induces cell death by apoptosis via an AIF-PARP-dependent mechanism and cell cycle arrest.

Mitofilin is an inner membrane protein that has been defined as a mitochondria-shaping protein in controlling and maintaining mitochondrial cristae structure and remodeling. We determined the role of mitofilin in cell survival by investigating the mechanism underlying mitofilin knockdown-induced cell death by apoptosis. Cultured H9c2 myoblasts and HEK 293 cells were treated with mitofilin siRNA or scrambled siRNA for 24 h. Cell death (apoptosis), caspase 3 activity and cell cycle phases were assessed by flow cytometry, while cytochrome c release and intracellular ATP production were measured by ELISA. Mitofilin, apoptosis-inducing factor (AIF) and poly(ADP-ribose) polymerase (PARP) expression were measured by Western blot analysis and calpain activity was assessed using a calpain activity kit. Mitochondrial images were taken using electron microscopy. We found that mitofilin knockdown increases apoptosis mainly via activation of the AIF-PARP pathway leading to nuclear fragmentation that is correlated with S phase arrest of the cell cycle. Knockdown of mitofilin also led to mitochondrial swelling and damage of cristae that is associated with the increase in reactive oxygen species production and mitochondrial calpain activity, as well as a marked decrease in intracellular ATP production and mitochondrial membrane potential. Together, these results indicate that mitofilin knockdown by siRNA increases calpain activity that presumably leads to mitochondrial structural degradation resulting in a critical reduction of mitochondrial function that is responsible for the increase in cell death by apoptosis via an AIF-PARP mechanism and associated with nuclear fragmentation, and S phase arrest of the cell cycle.

Live imaging of targeted cell ablation in Xenopus: a new model to study demyelination and repair.

Live imaging studies of the processes of demyelination and remyelination have so far been technically limited in mammals. We have thus generated a Xenopus laevis transgenic line allowing live imaging and conditional ablation of myelinating oligodendrocytes throughout the CNS. In these transgenic pMBP-eGFP-NTR tadpoles the myelin basic protein (MBP) regulatory sequences, specific to mature oligodendrocytes, are used to drive expression of an eGFP (enhanced green fluorescent protein) reporter fused to the Escherichia coli nitroreductase (NTR) selection enzyme. This enzyme converts the innocuous prodrug metronidazole (MTZ) to a cytotoxin. Using two-photon imaging in vivo, we show that pMBP-eGFP-NTR tadpoles display a graded oligodendrocyte ablation in response to MTZ, which depends on the exposure time to MTZ. MTZ-induced cell death was restricted to oligodendrocytes, without detectable axonal damage. After cessation of MTZ treatment, remyelination proceeded spontaneously, but was strongly accelerated by retinoic acid. Altogether, these features establish the Xenopus pMBP-eGFP-NTR line as a novel in vivo model for the study of demyelination/remyelination processes and for large-scale screens of therapeutic agents promoting myelin repair.

Inner mitochondrial membrane protein MPV17 mutant mice display increased myocardial injury after ischemia/reperfusion.

MPV17 is an inner mitochondrial membrane protein whose mutation results in mitochondrial DNA (mtDNA) depletion diseases such as neurohepatopathy. MPV17 is expressed in several organs including the liver and kidneys. Here, we investigated its role and mechanism of action in cardiac ischemia/reperfusion (I/R) injury. Using isolated hearts from wild type and Mpv17 mutant (Mpv17mut) mice, we found that mtDNA levels and normal cardiac function were similar between the groups. Furthermore, reactive oxygen species (ROS) generation, mitochondrial morphology, and calcium levels required to trigger mitochondrial permeability transition pore (mPTP) opening were all similar in normal/non-ischemic animals. However, following I/R, we found that mutant mice had poorer cardiac functional recovery and exhibited more mitochondrial structural damage. We also found that after I/R, Mpv17mut heart mitochondria did not produce more ROS than wild type hearts but that calcium retention capacity was gravely compromised. Using immunoprecipitation and mass spectrometry, we identified ATP synthase, Cyclophilin D, MIC60 and GRP75 as proteins critical to mitochondrial cristae organization and calcium handling that interact with MPV17, and this interaction is reduced by I/R. Together our results suggest that MPV17 has a protective function in the heart and is necessary for recovery following insults to the heart.

Ascorbic acid inhibits PMP22 expression by reducing cAMP levels.

Charcot-Marie-Tooth [CMT] syndrome is the most common hereditary peripheral neuropathy. CMT1A, which accounts for 50% of all CMT cases, usually results from triploidy of the PMP22 gene. Preclinical trials using an animal model show that disabled mice force-fed with high doses of ascorbic acid partially recover muscular strength after a few months of treatment, and suggest that high doses of ascorbic acid repress PMP22 expression. In this study, we demonstrated that ascorbic acid represses PMP22 gene expression by acting on intracellular cAMP levels and adenylate cyclase activity. This action is dose dependent and specific to ascorbic acid, since repression is not observed after treatment with other antioxidants. The new properties of ascorbic acid are discussed, along with the implications of these findings for CMT disease treatment.

Ascorbic Acid and gene expression: another example of regulation of gene expression by small molecules?

Ascorbic acid (vitamin C, AA) has long been considered a food supplement necessary for life and for preventing scurvy. However, it has been reported that other small molecules such as retinoic acid (vitamin A) and different forms of calciferol (vitamin D) are directly involved in regulating the expression of numerous genes. These molecules bind to receptors that are differentially expressed in the embryo and are therefore crucial signalling molecules in vertebrate development. The question is: is ascorbic acid also a signalling molecule that regulates gene expression?We therefore present and discuss recent publications that demonstrate that AA regulates the expression of a battery of genes. We offer a clue to understanding the biochemical mechanism by which AA regulates gene expression. Finally we will discuss the question of a receptor for AA and its potential involvement in embryonic development and cell differentiation.

Antiproliferative effect of ascorbic acid is associated with the inhibition of genes necessary to cell cycle progression.

BACKGROUND: Ascorbic acid (AA), or Vitamin C, is most well known as a nutritional supplement with antioxidant properties. Recently, we demonstrated that high concentrations of AA act on PMP22 gene expression and partially correct the Charcot-Marie-Tooth disease phenotype in a mouse model. This is due to the capacity of AA, but not other antioxidants, to down-modulate cAMP intracellular concentration by a competitive inhibition of the adenylate cyclase enzymatic activity. Because of the critical role of cAMP in intracellular signalling, we decided to explore the possibility that ascorbic acid could modulate the expression of other genes. METHODS AND FINDINGS: Using human pangenomic microarrays, we found that AA inhibited the expression of two categories of genes necessary for cell cycle progression, tRNA synthetases and translation initiation factor subunits. In in vitro assays, we demonstrated that AA induced the S-phase arrest of proliferative normal and tumor cells. Highest concentrations of AA leaded to necrotic cell death. However, quiescent cells were not susceptible to AA toxicity, suggesting the blockage of protein synthesis was mainly detrimental in metabolically-active cells. Using animal models, we found that high concentrations of AA inhibited tumor progression in nude mice grafted with HT29 cells (derived from human colon carcinoma). Consistently, expression of tRNA synthetases and ieF2 appeared to be specifically decreased in tumors upon AA treatment. CONCLUSIONS: AA has an antiproliferative activity, at elevated concentration that could be obtained using IV injection. This activity has been observed in vitro as well in vivo and likely results from the inhibition of expression of genes involved in protein synthesis. Implications for a clinical use in anticancer therapies will be discussed.

Database of queryable gene expression patterns for Xenopus.

The precise localization of gene expression within the developing embryo, and how it changes over time, is one of the most important sources of information for elucidating gene function. As a searchable resource, this information has up until now been largely inaccessible to the Xenopus community. Here, we present a new database of Xenopus gene expression patterns, queryable by specific location or region in the embryo. Pattern matching can be driven either from an existing in situ image, or from a user-defined pattern based on development stage schematic diagrams. The data are derived from the work of a group of 21 Xenopus researchers over a period of 4 days. We used a novel, rapid manual annotation tool, XenMARK, which exploits the ability of the human brain to make the necessary distortions in transferring data from the in situ images to the standard schematic geometry. Developmental Dynamics 238:1379-1388, 2009. (c) 2009 Wiley-Liss, Inc.

Analysis of the benefits of vitamin cocktails in treating Charcot-Marie-Tooth disease type 1A.

We recently proposed that the use of high doses of ascorbic acid (AA) could constitute the first potential treatment for Charcot-Marie-Tooth disease type 1A (CMT1A).4 We investigated the potential benefits of using cocktails of vitamins for CMT1A therapy. We used transient transfection of Schwann cells with a construction placing the expression of a reporter gene under the control of the Schwann cell-specific promoter of PMP22. Transfected cells were cultured with or without addition of ascorbic acid, vitamin A, vitamin E, or a cocktail of these vitamins. Adding vitamin A or E counteracts the effect of ascorbic acid in inhibiting PMP22 expression. We thus recommend that vitamins A and E should not be included in combination with AA in clinical trials.