The gene responsible for Friedreich's ataxia, a disease characterized by neurodegeneration and cardiomyopathy, has recently been cloned and its product designated frataxin. A gene in Saccharomyces cerevisiae was characterized whose predicted protein product has high sequence similarity to the human frataxin protein. The yeast gene (yeast frataxin homolog, YFH1) encodes a mitochondrial protein involved in iron homeostasis and respiratory function. Human frataxin also was shown to be a mitochondrial protein. Characterizing the mechanism by which YFH1 regulates iron homeostasis in yeast may help to define the pathologic process leading to cell damage in Friedreich's ataxia.

Mutations in the nuclear gene encoding the mitochondrial protein frataxin are responsible for the neurological disorder Friedreich ataxia (FA). Yeast strains with a deletion in the frataxin homologue YFH1 accumulate excess iron in mitochondria and demonstrate mitochondrial damage. We show that in the absence of YFH1, mitochondrial damage is proportional to the concentration and duration of exposure to extracellular iron, establishing mitochondrial iron accumulation as causal to mitochondrial damage. Reintroduction of YFH1 results in the rapid export of accumulated mitochondrial iron into the cytosol as free, non-heme bound iron, demonstrating that mitochondrial iron in the yeast FA model can be made bioavailable. These results demonstrate a mitochondrial iron cycle in which Yfh1p regulates mitochondrial iron efflux.Friedreich ataxia (FA) 1 is a neurodegenerative disease transmitted as an autosomal recessive trait with a prevalence of 1 in 50,000 individuals (1). The FA gene was identified by positional cloning and found to encode a 210-amino acid mitochondrial protein designated frataxin (2). Most cases of FA are because of the expansion of a polymorphic GAA trinucleotide repeat located in the first intron of the frataxin gene, resulting in reduced frataxin mRNA levels (3, 4). The defect responsible for FA also affects non-neuronal organs, and patients usually succumb to a cardiomyopathy in the fourth decade. Gait ataxia is the most common presenting symptom, and most patients eventually manifest dysarthria, areflexia, pyramidal weakness of the legs, extensor planar responses, and distal loss of joint position and vibration sense (1). The frataxin protein is localized to the mitochondria (5), but its function has not been determined. The frataxin protein is highly expressed in neuronal and heart tissue (2), both of which are postmitotic and highly dependent upon mitochondrial respiration (1). Iron deposits have been found in the myocardium of FA patients, and myocardial mitochondrial respiration has been found to be defective (6).Yeast disrupted for YFH1 (yeast frataxin homologue) accumulate iron in mitochondria (7). Mitochondrial DNA (mtDNA) is damaged and mitochondrial respiratory activity is impaired (8,9). Mitochondrial iron accumulation in yeast with YFH1 deletions is associated with subnormal cytosolic iron concentrations (7). Lowered cytosolic iron concentrations induce transcription of FET3, a component of the plasma membrane high affinity iron uptake system (10). The increased rate of iron uptake results in a doubling of cellular iron content relative to wild-type cells, but the excess iron is abnormally localized to mitochondria.Mitochondrial defects in patients with FA and in the yeast model could be a direct result of mitochondrial iron accumulation. Alternatively, a deficiency of frataxin protein could result in mitochondrial damage, and iron overload may be one manifestation of the mitochondrial damage. This phenomenon has been observed in two patients with acquired idiopathic sideroblastic anemi...

Hamiltonian-based quantum computation is a class of quantum algorithms in which the problem is encoded in a Hamiltonian and the evolution is performed by a continuous transformation of the Hamiltonian. Universal adiabatic quantum computing, quantum simulation, and quantum annealing are examples of such algorithms. Up to now, all implementations of this approach have been limited to qubits coupled via a single degree of freedom. This gives rise to a stoquastic Hamiltonian that has no sign problem in quantum Monte Carlo simulations. In this paper, we report implementation and measurements of two superconducting flux qubits coupled via two canonically conjugate degrees of freedom-charge and flux-to achieve a nonstoquastic Hamiltonian. We perform microwave spectroscopy to extract circuit parameters and show that the charge coupling manifests itself as a σ y σ y interaction in the computational basis. We observe destructive interference in quantum coherent oscillations between the computational basis states of the two-qubit system. Finally, we show that the extracted Hamiltonian is nonstoquastic over a wide range of parameters.

Infantile globoid cell leukodystrophy (GLD, Krabbe disease) is a fatal demyelinating disorder caused by a deficiency in the lysosomal enzyme galactosylceramidase (GALC). GALC deficiency leads to the accumulation of the cytotoxic glycolipid, galactosylsphingosine (psychosine). Complementary evidence suggested that psychosine is synthesized via an anabolic pathway. Here, we show instead that psychosine is generated catabolically through the deacylation of galactosylceramide by acid ceramidase (ACDase). This reaction uncouples GALC deficiency from psychosine accumulation, allowing us to test the long-standing “psychosine hypothesis.” We demonstrate that genetic loss of ACDase activity (Farber disease) in the GALC-deficient mouse model of human GLD (twitcher) eliminates psychosine accumulation and cures GLD. These data suggest that ACDase could be a target for substrate reduction therapy (SRT) in Krabbe patients. We show that pharmacological inhibition of ACDase activity with carmofur significantly decreases psychosine accumulation in cells from a Krabbe patient and prolongs the life span of the twitcher (Twi) mouse. Previous SRT experiments in the Twi mouse utilized l-cycloserine, which inhibits an enzyme several steps upstream of psychosine synthesis, thus altering the balance of other important lipids. Drugs that directly inhibit ACDase may have a more acceptable safety profile due to their mechanistic proximity to psychosine biogenesis. In total, these data clarify our understanding of psychosine synthesis, confirm the long-held psychosine hypothesis, and provide the impetus to discover safe and effective inhibitors of ACDase to treat Krabbe disease.

The promise of quantum computing lies in harnessing programmable quantum devices for practical applications such as efficient simulation of quantum materials and condensed matter systems. One important task is the simulation of geometrically frustrated magnets in which topological phenomena can emerge from competition between quantum and thermal fluctuations. Here we report on experimental observations of equilibration in such simulations, measured on up to 1440 qubits with microsecond resolution. By initializing the system in a state with topological obstruction, we observe quantum annealing (QA) equilibration timescales in excess of one microsecond. Measurements indicate a dynamical advantage in the quantum simulation compared with spatially local update dynamics of path-integral Monte Carlo (PIMC). The advantage increases with both system size and inverse temperature, exceeding a million-fold speedup over an efficient CPU implementation. PIMC is a leading classical method for such simulations, and a scaling advantage of this type was recently shown to be impossible in certain restricted settings. This is therefore an important piece of experimental evidence that PIMC does not simulate QA dynamics even for sign-problem-free Hamiltonians, and that near-term quantum devices can be used to accelerate computational tasks of practical relevance.

The N-ethylmaleimide-sensitive factor (NSF) and soluble NSF attachment protein (SNAP) are cytosolic factors that promote vesicle fusion with a target membrane in both the constitutive and regulated secretory pathways. NSF and SNAP are thought to function by catalyzing the disassembly of a SNAP receptor (SNARE) complex consisting of membrane proteins of the secretory vesicle and target membrane. Although studies of NSF function have provided strong support for this model, the precise biochemical role of SNAP remains controversial. To further explore the function of SNAP, we have used mutational and transgenic approaches in Drosophila to investigate the effect of altered SNAP dosage on neurotransmitter release and SNARE complex metabolism. Our results indicate that reduced SNAP activity results in diminished neurotransmitter release and accumulation of a neural SNARE complex. Increased SNAP dosage results in defective synapse formation and a variety of tissue morphological defects without detectably altering the abundance of neural SNARE complexes. The SNAP overexpression phenotypes are enhanced by mutations in other secretory components and are at least partially overcome by co-overexpression of NSF, suggesting that these phenotypes derive from a specific perturbation of the secretory pathway. Our results indicate that SNAP promotes neurotransmitter release and SNARE complex disassembly but inhibits secretion when present at high abundance relative to NSF.

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Krabbe disease (KD) and metachromatic leukodystrophy (MLD) are caused by accumulation of the glycolipids galactosylceramide (GalCer) and sulfatide and their toxic metabolites psychosine and lysosulfatide, respectively. We discovered a potent and selective small molecule inhibitor (S202) of ceramide galactosyltransferase (CGT), the key enzyme for GalCer biosynthesis, and characterized its use as substrate reduction therapy (SRT). Treating a KD mouse model with S202 dose-dependently reduced GalCer and psychosine in the central (CNS) and peripheral (PNS) nervous systems and significantly increased lifespan. Similarly, treating an MLD mouse model decreased sulfatides and lysosulfatide levels. Interestingly, lower doses of S202 partially inhibited CGT and selectively reduced synthesis of non-hydroxylated forms of GalCer and sulfatide, which appear to be the primary source of psychosine and lysosulfatide. Higher doses of S202 more completely inhibited CGT and reduced the levels of both non-hydroxylated and hydroxylated forms of GalCer and sulfatide. Despite the significant benefits observed in murine models of KD and MLD, chronic CGT inhibition negatively impacted both the CNS and PNS of wild-type mice. Therefore, further studies are necessary to elucidate the full therapeutic potential of CGT inhibition.