Ann Saada

Control of pancreatic β cell regeneration by glucose metabolism.

Recent studies revealed a surprising regenerative capacity of insulin-producing β cells in mice, suggesting that regenerative therapy for human diabetes could in principle be achieved. Physiologic β cell regeneration under stressed conditions relies on accelerated proliferation of surviving β cells, but the factors that trigger and control this response remain unclear. Using islet transplantation experiments, we show that β cell mass is controlled systemically rather than by local factors such as tissue damage. Chronic changes in β cell glucose metabolism, rather than blood glucose levels per se, are the main positive regulator of basal and compensatory β cell proliferation in vivo. Intracellularly, genetic and pharmacologic manipulations reveal that glucose induces β cell replication via metabolism by glucokinase, the first step of glycolysis, followed by closure of K(ATP) channels and membrane depolarization. Our data provide a molecular mechanism for homeostatic control of β cell mass by metabolic demand.

Acute infantile liver failure due to mutations in the TRMU gene.

Acute liver failure in infancy accompanied by lactic acidemia was previously shown to result from mtDNA depletion. We report on 13 unrelated infants who presented with acute liver failure and lactic acidemia with normal mtDNA content. Four died during the acute episodes, and the survivors never had a recurrence. The longest follow-up period was 14 years. Using homozygosity mapping, we identified mutations in the TRMU gene, which encodes a mitochondria-specific tRNA-modifying enzyme, tRNA 5-methylaminomethyl-2-thiouridylate methyltransferase. Accordingly, the 2-thiouridylation levels of the mitochondrial tRNAs were markedly reduced. Given that sulfur is a TRMU substrate and its availability is limited during the neonatal period, we propose that there is a window of time whereby patients with TRMU mutations are at increased risk of developing liver failure.

Mutations in the Fatty Acid 2-Hydroxylase Gene Are Associated with Leukodystrophy with Spastic Paraparesis and Dystonia

Myelination is a complex, developmentally regulated process whereby myelin proteins and lipids are coordinately expressed by myelinating glial cells. Homozygosity mapping in nine patients with childhood onset spasticity, dystonia, cognitive dysfunction, and periventricular white matter disease revealed inactivating mutations in the FA2H gene. FA2H encodes the enzyme fatty acid 2-hydroxylase that catalyzes the 2-hydroxylation of myelin galactolipids, galactosylceramide, and its sulfated form, sulfatide. To our knowledge, this is the first identified deficiency of a lipid component of myelin and the clinical phenotype underscores the importance of the 2-hydroxylation of galactolipids for myelin maturation. In patients with autosomal-recessive unclassified leukodystrophy or complex spastic paraparesis, sequence analysis of the FA2H gene is warranted.

Mutations in NDUFAF3 (C3ORF60), Encoding an NDUFAF4 (C6ORF66)-Interacting Complex I Assembly Protein, Cause Fatal Neonatal Mitochondrial Disease

Mitochondrial complex I deficiency is the most prevalent and least understood disorder of the oxidative phosphorylation system. The genetic cause of many cases of isolated complex I deficiency is unknown because of insufficient understanding of the complex I assembly process and the factors involved. We performed homozygosity mapping and gene sequencing to identify the genetic defect in five complex I-deficient patients from three different families. All patients harbored mutations in the NDUFAF3 (C3ORF60) gene, of which the pathogenic nature was assessed by NDUFAF3-GFP baculovirus complementation in fibroblasts. We found that NDUFAF3 is a genuine mitochondrial complex I assembly protein that interacts with complex I subunits. Furthermore, we show that NDUFAF3 tightly interacts with NDUFAF4 (C6ORF66), a protein previously implicated in complex I deficiency. Additional gene conservation analysis links NDUFAF3 to bacterial-membrane-insertion gene cluster SecF/SecD/YajC and to C8ORF38, also implicated in complex I deficiency. These data not only show that NDUFAF3 mutations cause complex I deficiency but also relate different complex I disease genes by the close cooperation of their encoded proteins during the assembly process.

Mutations in LPIN1 Cause Recurrent Acute Myoglobinuria in Childhood

Recurrent episodes of life-threatening myoglobinuria in childhood are caused by inborn errors of glycogenolysis, mitochondrial fatty acid beta-oxidation, and oxidative phosphorylation. Nonetheless, approximately half of the patients do not suffer from a defect in any of these pathways. Using homozygosity mapping, we identified six deleterious mutations in the LPIN1 gene in patients who presented at 2-7 years of age with recurrent, massive rhabdomyolysis. The LPIN1 gene encodes the muscle-specific phosphatidic acid phosphatase, a key enzyme in triglyceride and membrane phospholipid biosynthesis. Of six individuals who developed statin-induced myopathy, one was a carrier for Glu769Gly, a pathogenic mutation in the LPIN1 gene. Analysis of phospholipid content disclosed accumulation of phosphatidic acid and lysophospholipids in muscle tissue of the more severe genotype. Mutations in the LPIN1 gene cause recurrent rhabdomyolysis in childhood, and a carrier state may predispose for statin-induced myopathy.

Antenatal mitochondrial disease caused by mitochondrial ribosomal protein (MRPS22) mutation.

Three patients born to the same set of consanguineous parents presented with antenatal skin oedema, hypotonia, cardiomyopathy and tubulopathy. The enzymatic activities of multiple mitochondrial respiratory chain complexes were reduced in muscle. Marked reduction of 12s rRNA, the core of the mitochondrial small ribosomal subunit, was found in fibroblasts. Homozygosity mapping led to the identification of a mutation in the MRPS22 gene, which encodes a mitochondrial ribosomal protein. Transfection of the patient cells with wild-type MRPS22 cDNA increased the 12s rRNA content and normalised the enzymatic activities. Quantification of mitochondrial transcripts is advisable in patients with multiple defects of the mitochondrial respiratory chain.

The Transgenic Overexpression of α-Synuclein and Not Its Related Pathology Associates with Complex I Inhibition

α-Synuclein (αS) is a protein involved in the cytopathology and genetics of Parkinson disease and is thought to affect mitochondrial complex I activity. Previous studies have shown that mitochondrial toxins and specifically inhibitors of complex I activity enhance αS pathogenesis. Here we show that αS overexpression specifically inhibits complex I activity in dopaminergic cells and in A53T αS transgenic mouse brains. Importantly, our results indicate that the inhibitory effect on complex I activity is not associated with αS-related pathology. Specifically, complex I activity measured in purified mitochondria from A53T αS transgenic mouse brains was not affected by mouse age; Parkinson disease-like symptoms; levels of αS soluble oligomers; levels of insoluble, lipid-associated αS; or αS intraneuronal depositions in vivo. Likewise, no correlation was found between complex I activity and polyunsaturated fatty acid-induced αS depositions in Lewy body-like inclusions in cultured dopaminergic cells. We further show that the effect of αS on complex I activity is not due to altered mitochondrial protein levels or affected complex I assembly. Based on the results herein, we suggest that αS expression negatively regulates complex I activity as part of its normal, physiological role.

The cytokine network of wallerian degeneration: IL-10 and GM-CSF.

Wallerian degeneration (WD) is the inflammatory response of peripheral nerves to injury. Evidence is provided that granulocyte macrophage colony stimulating factor (GM‐CSF) contributes to the initiation and progression of WD by activating macrophages and Schwann, whereas IL‐10 down‐regulates WD by inhibiting GM‐CSF production. A significant role of activated macrophages and Schwann for future regeneration is myelin removal by phagocytosis and degradation. We studied the timing and magnitude of GM‐CSF and IL‐10 production, macrophage and Schwann activation, and myelin degradation in C57BL/6NHSD and C57BL/6‐WLD/OLA/NHSD mice that display normal rapid‐WD and abnormal slow‐WD, respectively. We observed the following events in rapid‐WD. The onset of GM‐CSF production is within 5 h after injury. Production is steadily augmented during the first 3 days, but is attenuated thereafter. The onset of production of the macrophage and Schwann activation marker Galectin‐3/MAC‐2 succeeds that of GM‐CSF. Galectin‐3/MAC‐2 production is up‐regulated during the first 6 days, but is down‐regulated thereafter. The onset of myelin degradation succeeds that of Galectin‐3/MAC‐2, and is almost complete within 1 week. IL‐10 production displays two phases. An immediate low followed by a high that begins on the fourth day, reaching highest levels on the seventh. The timing and magnitude of GM‐CSF production thus enable the rapid activation of macrophages and Schwann that consequently phagocytose and degrade myelin. The timing and magnitude of IL‐10 production suggest a role in down‐regulating WD after myelin is removed. In contrast, slow‐WD nerves produce low inefficient levels of GM‐CSF and IL‐10 throughout. Therefore, deficient IL‐10 levels cannot account for inefficient GM‐CSF production, whereas deficient GM‐CSF levels may account, in part, for slow‐WD.

Ablation of ceramide synthase 2 causes chronic oxidative stress due to disruption of the mitochondrial respiratory chain.

Background: Ceramide synthase 2 null mice, which cannot synthesize very-long chain ceramides, display severe hepatopathy. Results: These mice have elevated sphinganine and altered N-acyl chain ceramides that disrupt mitochondrial function by modifying respiratory chain activity. Conclusion: Alteration of mitochondrial sphingolipids results in formation of reaction oxygen species in liver. Significance: Ceramides with defined acyl chains influence oxidative stress signaling pathways. Ceramide is a key intermediate in the pathway of sphingolipid biosynthesis and is an important intracellular messenger. We recently generated a ceramide synthase 2 (CerS2) null mouse that cannot synthesize very long acyl chain (C22-C24) ceramides. This mouse displays severe and progressive hepatopathy. Significant changes were observed in the sphingolipid profile of CerS2 null mouse liver, including elevated C16-ceramide and sphinganine levels in liver and in isolated mitochondrial fractions. Because ceramide may be involved in reactive oxygen species (ROS) formation, we examined whether ROS generation was affected in CerS2 null mice. Levels of a number of anti-oxidant enzymes were elevated, as were lipid peroxidation, protein nitrosylation, and ROS. ROS were generated from mitochondria due to impaired complex IV activity. C16-ceramide, sphingosine, and sphinganine directly inhibited complex IV activity in isolated mitochondria and in mitoplasts, whereas other ceramide species, sphingomyelin, and diacylglycerol were without effect. A fluorescent analog of sphinganine accumulated in mitochondria. Heart mitochondria did not display a substantial alteration in the sphingolipid profile or in complex IV activity. We suggest that C16-ceramide and/or sphinganine induce ROS formation through the modulation of mitochondrial complex IV activity, resulting in chronic oxidative stress. These results are of relevance for understanding modulation of ROS signaling by sphingolipids.

Type 2 diabetes and congenital hyperinsulinism cause DNA double-strand breaks and p53 activity in β cells.

β cell failure in type 2 diabetes (T2D) is associated with hyperglycemia, but the mechanisms are not fully understood. Congenital hyperinsulinism caused by glucokinase mutations (GCK-CHI) is associated with β cell replication and apoptosis. Here, we show that genetic activation of β cell glucokinase, initially triggering replication, causes apoptosis associated with DNA double-strand breaks and activation of the tumor suppressor p53. ATP-sensitive potassium channels (KATP channels) and calcineurin mediate this toxic effect. Toxicity of long-term glucokinase overactivity was confirmed by finding late-onset diabetes in older members of a GCK-CHI family. Glucagon-like peptide-1 (GLP-1) mimetic treatment or p53 deletion rescues β cells from glucokinase-induced death, but only GLP-1 analog rescues β cell function. DNA damage and p53 activity in T2D suggest shared mechanisms of β cell failure in hyperglycemia and CHI. Our results reveal membrane depolarization via KATP channels, calcineurin signaling, DNA breaks, and p53 as determinants of β cell glucotoxicity and suggest pharmacological approaches to enhance β cell survival in diabetes.