Parkinson's disease is a debilitating, age-associated movement disorder. A central aspect of the pathophysiology of Parkinson's disease is the progressive demise of midbrain dopamine neurons and ...their axonal projections, but the underlying causes of this loss are unclear. Advances in genetics and experimental model systems have illuminated an important role for defects in intracellular transport pathways to lysosomes. The accumulation of altered proteins and damaged mitochondria, particularly at axon terminals, ultimately might overwhelm the capacity of intracellular disposal mechanisms. Cell-extrinsic mechanisms, including inflammation and prion-like spreading, are proposed to have both protective and deleterious functions in Parkinson's disease.
Abstract
Human genetic studies implicate LRRK2 and RAB7L1 in susceptibility to Parkinson disease (PD). These two genes function in the same pathway, as knockout of Rab7L1 results in phenotypes ...similar to LRRK2 knockout, and studies in cells and model organisms demonstrate LRRK2 and Rab7L1 interact in the endolysosomal system. Recently, a subset of Rab proteins have been identified as LRRK2 kinase substrates. Herein, we find that Rab8, Rab10, and Rab7L1 must be membrane and GTP-bound for LRRK2 phosphorylation. LRRK2 mutations that cause PD including R1441C, Y1699C, and G2019S all increase LRRK2 phosphorylation of Rab7L1 four-fold over wild-type LRRK2 in cells, resulting in the phosphorylation of nearly one-third the available Rab7L1 protein in cells. In contrast, the most common pathogenic LRRK2 mutation, G2019S, does not upregulate LRRK2-mediated phosphorylation of Rab8 or Rab10. LRRK2 interaction with membrane and GTP-bound Rab7L1, but not Rab8 or Rab10, results in the activation of LRRK2 autophosphorylation at the serine 1292 position, required for LRRK2 toxicity. Further, Rab7L1 controls the proportion of LRRK2 that is membrane-associated, and LRRK2 mutations enhance Rab7L1-mediated recruitment of LRRK2 to the trans-Golgi network. Interaction studies with the Rab8 and Rab10 GTPase-activating protein TBC1D4/AS160 demonstrate that LRRK2 phosphorylation may block membrane and GTP-bound Rab protein interaction with effectors. These results suggest reciprocal regulation between LRRK2 and Rab protein substrates, where Rab7L1-mediated upregulation of LRRK2 kinase activity results in the stabilization of membrane and GTP-bound Rab proteins that may be unable to interact with Rab effector proteins.
Mutations in LRRK2 underlie an autosomal-dominant, inherited form of Parkinson's disease (PD) that mimics the clinical features of the common “sporadic” form of PD. The LRRK2 protein includes ...putative GTPase, protein kinase, WD40 repeat, and leucine-rich repeat (LRR) domains of unknown function. Here we show that PD-associated LRRK2 mutations display disinhibited kinase activity and induce a progressive reduction in neurite length and branching both in primary neuronal cultures and in the intact rodent CNS. In contrast, LRRK2 deficiency leads to increased neurite length and branching. Neurons that express PD-associated LRRK2 mutations additionally harbor prominent phospho-tau-positive inclusions with lysosomal characteristics and ultimately undergo apoptosis.
Somatic cells can be reprogrammed into induced pluripotent stem cells (iPSCs) by using the pluripotency factors Oct4, Sox2, Klf4 and c-Myc (together referred to as OSKM). iPSC reprogramming erases ...somatic epigenetic signatures—as typified by DNA methylation or histone modification at silent pluripotency loci—and establishes alternative epigenetic marks of embryonic stem cells (ESCs). Here we describe an early and essential stage of somatic cell reprogramming, preceding the induction of transcription at endogenous pluripotency loci such as Nanog and Esrrb. By day 4 after transduction with OSKM, two epigenetic modification factors necessary for iPSC generation, namely poly(ADP-ribose) polymerase-1 (Parp1) and ten-eleven translocation-2 (Tet2), are recruited to the Nanog and Esrrb loci. These epigenetic modification factors seem to have complementary roles in the establishment of early epigenetic marks during somatic cell reprogramming: Parp1 functions in the regulation of 5-methylcytosine (5mC) modification, whereas Tet2 is essential for the early generation of 5-hydroxymethylcytosine (5hmC) by the oxidation of 5mC (refs 3,4). Although 5hmC has been proposed to serve primarily as an intermediate in 5mC demethylation to cytosine in certain contexts, our data, and also studies of Tet2-mutant human tumour cells, argue in favour of a role for 5hmC as an epigenetic mark distinct from 5mC. Consistent with this, Parp1 and Tet2 are each needed for the early establishment of histone modifications that typify an activated chromatin state at pluripotency loci, whereas Parp1 induction further promotes accessibility to the Oct4 reprogramming factor. These findings suggest that Parp1 and Tet2 contribute to an epigenetic program that directs subsequent transcriptional induction at pluripotency loci during somatic cell reprogramming.
MicroRNAs (miRNAs) are evolutionarily conserved, 18- to 25-nucleotide, non-protein coding transcripts that posttranscriptionally regulate gene expression during development. miRNAs also occur in ...postmitotic cells, such as neurons in the mammalian central nervous system, but their function is less well characterized. We investigated the role of miRNAs in mammalian midbrain dopaminergic neurons (DNs). We identified a miRNA, miR-133b, that is specifically expressed in midbrain DNs and is deficient in midbrain tissue from patients with Parkinson's disease. miR-133b regulates the maturation and function of midbrain DNs within a negative feedback circuit that includes the paired-like homeodomain transcription factor Pitx3. We propose a role for this feedback circuit in the fine-tuning of dopaminergic behaviors such as locomotion.
Macroautophagy is an evolutionarily conserved mechanism for bulk intracellular degradation of proteins and organelles. Pathological studies have implicated macroautophagy defects in human ...neurodegenerative disorders of aging including Alzheimer's disease and tauopathies. Neuronal deficiency of macroautophagy throughout mouse embryonic development results in neurodevelopmental defects and early postnatal mortality. However, the role of macroautophagy in mature CNS neurons, and the relationship with human disease neuropathology, remains unclear. Here we describe mice deficient in an essential macroautophagy component, Atg7, specifically within postnatal CNS neurons.
Postnatal forebrain-specific Atg7 conditional knockout (cKO) mice displayed age-dependent neurodegeneration and ubiquitin- and p62-positive inclusions. Phosphorylated tau was significantly accumulated in Atg7 cKO brains, but neurofibrillary tangles that typify end-stage human tauopathy were not apparent. A major tau kinase, glycogen synthase kinase 3β (GSK3β), was also accumulated in Atg7 cKO brains. Chronic pharmacological inhibition of tau phosphorylation, or genetic deletion of tau, significantly rescued Atg7-deficiency-mediated neurodegeneration, but did not suppress inclusion formation.
These data elucidate a role for macroautophagy in the long-term survival and physiological function of adult CNS neurons. Neurodegeneration in the context of macroautophagy deficiency is mediated through a phospho-tau pathway.
Leucine-rich repeat kinase 2 (LRRK2) has been linked to several clinical disorders including Parkinson's disease (PD), Crohn's disease, and leprosy. Furthermore in rodents, LRRK2 deficiency or ...inhibition leads to lysosomal pathology in kidney and lung. Here we provide evidence that LRRK2 functions together with a second PD-associated gene, RAB7L1, within an evolutionarily conserved genetic module in diverse cellular contexts. In C. elegans neurons, orthologues of LRRK2 and RAB7L1 act coordinately in an ordered genetic pathway to regulate axonal elongation. Further genetic studies implicated the AP-3 complex, which is a known regulator of axonal morphology as well as of intracellular protein trafficking to the lysosome compartment, as a physiological downstream effector of LRRK2 and RAB7L1. Additional cell-based studies implicated LRRK2 in the AP-3 complex-related intracellular trafficking of lysosomal membrane proteins. In mice, deficiency of either RAB7L1 or LRRK2 leads to prominent age-associated lysosomal defects in kidney proximal tubule cells, in the absence of frank CNS pathology. We hypothesize that defects in this evolutionarily conserved genetic pathway underlie the diverse pathologies associated with LRRK2 in humans and in animal models.
Parkinson's disease (PD) pathology is characterized by the degeneration of midbrain dopamine neurons (DNs) ultimately leading to a progressive movement disorder in patients. The etiology of DN loss ...in sporadic PD is unknown, although it is hypothesized that aberrant protein aggregation and cellular oxidative stress may promote DN degeneration. Homozygous mutations in DJ-1 were recently described in two families with autosomal recessive inherited PD (Bonifati et al. 2003). In a companion article (Martinat et al. 2004), we show that mutations in DJ-1 alter the cellular response to oxidative stress and proteasomal inhibition. Here we show that DJ-1 functions as a redox-sensitive molecular chaperone that is activated in an oxidative cytoplasmic environment. We further demonstrate that DJ-1 chaperone activity in vivo extends to alpha-synuclein, a protein implicated in PD pathogenesis.
Nuclear transplantation, cell fusion, and induced pluripotent stem cell studies have revealed a surprising degree of plasticity in mature mammalian cell fates. Somatic cell reprogramming also has ...been achieved more recently by the directed conversion of nonneuronal somatic cells, such as skin fibroblasts, to neuronal phenotypes. This approach appears particularly applicable to the in vitro modeling of human neurologic disorders. Central nervous system neurons are otherwise difficult to obtain from patients with neurologic disorders; however, nonhuman models may not reflect patient pathology. Somatic cell reprogramming may afford models of nonfamilial “sporadic” neurologic disorders, which are likely caused by multiple interacting genetic and nongenetic factors. Directed somatic cell reprogramming, which does not pass through typical in vivo developmental stages, toward many mature neuronal phenotypes has now been described. This article reviews the field and discusses the potential utilities of such models, such as for the development of personalized medicine strategies.
Parkinsonism genes: culprits and clues Abeliovich, Asa; Flint Beal, M.
Journal of neurochemistry,
November 2006, Letnik:
99, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Parkinson's disease (PD) is characterized by a unique clinical constellation that includes: slowness, rigidity, gait difficulty, and tremor at rest. Pathological studies have linked this presentation ...to the loss of midbrain dopamine neurons (Gelb et al. 1999) although other neuronal populations are also targeted in PD. Epidemiological data implicate both genetic and environmental factors in the etiology of the disease. The identification of a series of genes that underlie relatively rare, familial forms of Parkinsonism (a clinical term that encompasses ‘sporadic’ PD, familial Parkinson's‐like forms, as well as other related syndromes) has brought excitement to the field. Three of the mutated familial Parkinsonism (FP) genes: Parkin, DJ‐1, and PINK1, typically present with apparent autosomal recessive inheritance and are implicated in mitochondria and oxidative stress‐related survival pathways. Two other FP genes: α‐Synuclein (αSyn) and LRRK2, present in an autosomal dominant pattern and are associated with prominent intracellular protein inclusions. A series of recent publications suggest novel pathways that may link the FP genes.
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