Matthew J. LaVoie

PINK1 and Parkin Target Miro for Phosphorylation and Degradation to Arrest Mitochondrial Motility

Cells keep their energy balance and avoid oxidative stress by regulating mitochondrial movement, distribution, and clearance. We report here that two Parkinsons disease proteins, the Ser/Thr kinase PINK1 and ubiquitin ligase Parkin, participate in this regulation by arresting mitochondrial movement. PINK1 phosphorylates Miro, a component of the primary motor/adaptor complex that anchors kinesin to the mitochondrial surface. The phosphorylation of Miro activates proteasomal degradation of Miro in a Parkin-dependent manner. Removal of Miro from the mitochondrion also detaches kinesin from its surface. By preventing mitochondrial movement, the PINK1/Parkin pathway may quarantine damaged mitochondria prior to their clearance. PINK1 has been shown to act upstream of Parkin, but the mechanism corresponding to this relationship has not been known. We propose that PINK1 phosphorylation of substrates triggers the subsequent action of Parkin and the proteasome.

γ-Secretase is a membrane protein complex comprised of presenilin, nicastrin, aph-1, and pen-2

γ-Secretase catalyzes the intramembrane proteolysis of Notch, β-amyloid precursor protein, and other substrates as part of a new signaling paradigm and as a key step in the pathogenesis of Alzheimers disease. This unusual protease has eluded identification, though evidence suggests that the presenilin heterodimer comprises the catalytic site and that a highly glycosylated form of nicastrin associates with it. The formation of presenilin heterodimers from the holoprotein is tightly gated by unknown limiting cellular factors. Here we show that Aph-1 and Pen-2, two recently identified membrane proteins genetically linked to γ-secretase, associate directly with presenilin and nicastrin in the active protease complex. Coexpression of all four proteins leads to marked increases in presenilin heterodimers, full glycosylation of nicastrin, and enhanced γ-secretase activity. These findings suggest that the four membrane proteins comprise the limiting components of γ-secretase and coassemble to form the active enzyme in mammalian cells.

Dopamine covalently modifies and functionally inactivates parkin

Inherited mutations in PARK2, the gene encoding parkin, cause selective degeneration of catecholaminergic neurons in the substantia nigra and locus coeruleus of the brainstem, resulting in early-onset parkinsonism. But the role of parkin in common, sporadic forms of Parkinson disease remains unclear. Here we report that the neurotransmitter dopamine covalently modifies parkin in living dopaminergic cells, a process that increases parkin insolubility and inactivates its E3 ubiquitin ligase function. In the brains of individuals with sporadic Parkinson disease, we observed decreases in parkin solubility consistent with its functional inactivation. Using a new biochemical method, we detected catechol-modified parkin in the substantia nigra but not other regions of normal human brain. These findings show a vulnerability of parkin to modification by dopamine, the principal transmitter lost in Parkinson disease, suggesting a mechanism for the progressive loss of parkin function in dopaminergic neurons during aging and sporadic Parkinson disease.

Dopamine Quinone Formation and Protein Modification Associated with the Striatal Neurotoxicity of Methamphetamine: Evidence against a Role for Extracellular Dopamine

Methamphetamine-induced toxicity has been shown to require striatal dopamine and to involve mechanisms associated with oxidative stress. Dopamine is a reactive molecule that can oxidize to form free radicals and reactive quinones. Although this has been suggested to contribute to the mechanism of toxicity, the oxidation of dopamine has never been directly measured after methamphetamine exposure. In this study we sought to determine whether methamphetamine-induced toxicity is associated with the oxidation of dopamine by measuring the binding of dopamine quinones to cysteinyl residues on protein. We observed that administration of neurotoxic doses of methamphetamine to rats resulted in a two- to threefold increase in protein cysteinyl-dopamine in the striatum 2, 4, and 8 hr after treatment. When methamphetamine was administered at an ambient temperature of 5°C, no increase in dopamine oxidation products was observed, and toxicity was prevented. Furthermore, as shown by striatal microdialysis, animals treated with methamphetamine at 5°C showed DA release identical to that of animals treated at room temperature. These data suggest that the toxicity of methamphetamine and the associated increase in dopamine oxidation are not exclusively the result of increases in extracellular dopamine. Because dopamine-induced modifications of protein structure and function may result in cellular toxicity, it is likely that dopamine oxidation contributes to methamphetamine-induced toxicity to dopamine terminals, adding support to the role of dopamine and the evidence of oxidative stress in this lesion model.

The Notch Ligands, Jagged and Delta, Are Sequentially Processed by α-Secretase and Presenilin/γ-Secretase and Release Signaling Fragments

The cleavage of Notch by presenilin (PS)/γ-secretase is a salient example of regulated intramembrane proteolysis, an unusual mechanism of signal transduction. This cleavage is preceded by the binding of protein ligands to the Notch ectodomain, activating its shedding. We hypothesized that the Notch ligands, Delta and Jagged, themselves undergo PS-mediated regulated intramembrane proteolysis. Here, we show that the ectodomain of mammalian Jagged is cleaved by an A disintegrin and metalloprotease (ADAM) 17-like activity in cultured cells and in vivo, similar to the known cleavage of Drosophila Delta by Kuzbanian. The ectodomain shedding of ligand can be stimulated by Notch and yields membrane-tethered C-terminal fragments (CTFs) of Jagged and Delta that accumulate in cells expressing a dominant-negative form of PS or treated with γ-secretase inhibitors. PS forms stable complexes with Delta and Jagged and with their respective CTFs. PS/γ-secretase then mediates the cleavage of the latter to release the Delta and Jagged intracellular domains, a portion of which can enter the nucleus. The ligand CTFs compete with an activated form of Notch for cleavage by γ-secretase and can thus inhibit Notch signaling in vitro. The soluble Jagged intracellular domain can activate gene expression via the transcription factor AP1, and this effect is counteracted by the co-expression of the γ-secretase-cleaved product of Notch, Notch intracellular domain. We conclude that Delta and Jagged undergo ADAM-mediated ectodomain processing followed by PS-mediated intramembrane proteolysis to release signaling fragments. Thus, Notch and its cognate ligands are processed by the same molecular machinery and may antagonistically regulate each others signaling.

Wildfire responses to abrupt climate change in North America

It is widely accepted, based on data from the last few decades and on model simulations, that anthropogenic climate change will cause increased fire activity. However, less attention has been paid to the relationship between abrupt climate changes and heightened fire activity in the paleorecord. We use 35 charcoal and pollen records to assess how fire regimes in North America changed during the last glacial–interglacial transition (15 to 10 ka), a time of large and rapid climate changes. We also test the hypothesis that a comet impact initiated continental-scale wildfires at 12.9 ka; the data do not support this idea, nor are continent-wide fires indicated at any time during deglaciation. There are, however, clear links between large climate changes and fire activity. Biomass burning gradually increased from the glacial period to the beginning of the Younger Dryas. Although there are changes in biomass burning during the Younger Dryas, there is no systematic trend. There is a further increase in biomass burning after the Younger Dryas. Intervals of rapid climate change at 13.9, 13.2, and 11.7 ka are marked by large increases in fire activity. The timing of changes in fire is not coincident with changes in human population density or the timing of the extinction of the megafauna. Although these factors could have contributed to fire-regime changes at individual sites or at specific times, the charcoal data indicate an important role for climate, and particularly rapid climate change, in determining broad-scale levels of fire activity.

Assembly of the γ-Secretase Complex Involves Early Formation of an Intermediate Subcomplex of Aph-1 and Nicastrin

The γ-secretase complex is an unusual multimeric protease responsible for the intramembrane cleavage of a variety of type 1 transmembrane proteins, including the β-amyloid precursor protein and Notch. Genetic and biochemical data have revealed that this protease consists of the presenilin heterodimer, a highly glycosylated form of nicastrin, and the recently identified gene products, Aph-1 and Pen-2. Whereas current evidence supports the notion that presenilin comprises the active site of the protease and that the other three components are members of the active complex required for proteolytic activity, the individual roles of the three co-factors remain unclear. Here, we demonstrate that endogenous Aph-1 interacts with an immature species of nicastrin, forming a stable intermediate early in the assembly of the γ-secretase complex, prior to the addition of presenilin and Pen-2. Our data suggest 1) that Aph-1 is involved in the early stages of γ-secretase assembly through the stabilization and perhaps glycosylation of nicastrin and by scaffolding nicastrin to the immature γ-secretase complex, and 2) that presenilin, and later Pen-2, bind to this intermediate during the formation of the mature protease.

Mitophagy of damaged mitochondria occurs locally in distal neuronal axons and requires PINK1 and Parkin

The Parkinson’s disease–associated proteins PINK1 and Parkin mediate local mitophagy of dysfunctional mitochondria in distal neuronal axons, abrogating the need for retrograde organelle transport and ensuring rapid neuroprotection.

Microglial activation precedes dopamine terminal pathology in methamphetamine-induced neurotoxicity

Previous studies have demonstrated methamphetamine (METH)-induced toxicity to dopaminergic and serotonergic axons in rat striatum. Although several studies have identified the nature of reactive astrogliosis in this lesion model, the response of microglia has not been examined in detail. In this investigation, we characterized the temporal relationship of reactive microgliosis to neuropathological alterations of dopaminergic axons in striatum following exposure to methamphetamine. Adult male Sprague-Dawley rats were administered a neurotoxic regimen of methamphetamine and survived 12 h, or 1, 2, 4, and 6 days after treatment. Immunohistochemical methods were used to evaluate reactive changes in microglia throughout the brain of methamphetamine-treated rats, with a particular focus upon striatum. Pronounced morphological changes, indicative of reactive microgliosis, were evident in the brains of all methamphetamine-treated animals and were absent in saline-treated control animals. These included hyperplastic changes in cell morphology that substantially increased the size and staining intensity of reactive microglia. Quantitative analysis of reactive microglial changes in striatum demonstrated that these changes were most robust within the ventrolateral region and were maximal 2 days after methamphetamine administration. Analysis of tissue also revealed that microglial activation preceded the appearance of pathological changes in striatal dopamine fibers. Reactive microgliosis was also observed in extra-striatal regions (somatosensory and piriform cortices, and periaqueductal gray). These data demonstrate a consistent, robust, and selective activation of microglia in response to methamphetamine administration that, at least in striatum, precedes the appearance of morphological indicators of axon pathology. These observations raise the possibility that activated microglia may contribute to methamphetamine-induced neurotoxicity.

Leucine-rich repeat kinase 2 interacts with Parkin, DJ-1 and PINK-1 in a Drosophila melanogaster model of Parkinson's disease

Mutations in the LRRK2 gene are the most common genetic cause of familial Parkinsons disease (PD). However, its physiological and pathological functions are unknown. Therefore, we generated several independent Drosophila lines carrying WT or mutant human LRRK2 (mutations in kinase, COR or LRR domains, resp.). Ectopic expression of WT or mutant LRRK2 in dopaminergic neurons caused their significant loss accompanied by complex age-dependent changes in locomotor activity. Overall, the ubiquitous expression of LRRK2 increased lifespan and fertility of the flies. However, these flies were more sensitive to rotenone. LRRK2 expression in the eye exacerbated retinal degeneration. Importantly, in double transgenic flies, various indices of the eye and dopaminergic survival were modified in a complex fashion by a concomitant expression of PINK1, DJ-1 or Parkin. This evidence suggests a genetic interaction between these PD-relevant genes.