Joshua M. Shulman

Parkinson's Disease: Genetics and Pathogenesis

Recent investigation into the mechanisms of Parkinsons disease (PD) has generated remarkable insight while simultaneously challenging traditional conceptual frameworks. Although the disease remains defined clinically by its cardinal motor manifestations and pathologically by midbrain dopaminergic cell loss in association with Lewy bodies, it is now recognized that PD has substantially more widespread impact, causing a host of nonmotor symptoms and associated pathology in multiple regions throughout the nervous system. Further, the discovery and validation of PD-susceptibility genes contradict the historical view that environmental factors predominate, and blur distinctions between familial and sporadic disease. Genetic advances have also promoted the development of improved animal models, highlighted responsible molecular pathways, and revealed mechanistic overlap with other neurodegenerative disorders. In this review, we synthesize emerging lessons on PD pathogenesis from clinical, pathological, and genetic studies toward a unified concept of the disorder that may accelerate the design and testing of the next generation of PD therapies.

Prickle Mediates Feedback Amplification to Generate Asymmetric Planar Cell Polarity Signaling

Planar cell polarity signaling in Drosophila requires the receptor Frizzled and the cytoplasmic proteins Dishevelled and Prickle. From initial, symmetric subcellular distributions in pupal wing cells, Frizzled and Dishevelled become highly enriched at the distal portion of the cell cortex. We describe a Prickle-dependent intercellular feedback loop that generates asymmetric Frizzled and Dishevelled localization. In the absence of Prickle, Frizzled and Dishevelled remain symmetrically distributed. Prickle localizes to the proximal side of pupal wing cells and binds the Dishevelled DEP domain, inhibiting Dishevelled membrane localization and antagonizing Frizzled accumulation. This activity is linked to Frizzled activity on the adjacent cell surface. Prickle therefore functions in a feedback loop that amplifies differences between Frizzled levels on adjacent cell surfaces.

The Drosophila homolog of C. elegans PAR-1 organizes the oocyte cytoskeleton and directs oskar mRNA localization to the posterior pole.

In C. elegans, the PAR-1 kinase is localized to the posterior of the zygote and is required for anterior-posterior axis formation. Here, we report that a Drosophila PAR-1 homolog localizes to the posterior of the oocyte with oskar mRNA. Furthermore, par-1 mutants show a novel polarity phenotype in which bicoid mRNA accumulates normally at the anterior, but oskar mRNA is redirected to the center of the oocyte, resulting in embryonic patterning defects. These phenotypes arise from a disorganization of the oocyte microtubule cytoskeleton, consistent with reports that mammalian PAR-1 homologs regulate microtubule dynamics. Thus, Drosophila PAR-1 may remodel the oocyte microtubule network to define the posterior as the site for oskar localization. These results identify a molecular parallel between anterior-posterior polarization in Drosophila and C. elegans.

Functional Links Between Aβ Toxicity, Endocytic Trafficking, and Alzheimer’s Disease Risk Factors in Yeast

The use of yeast as a model organism reveals cellular factors involved in beta-amyloid toxicity. Aβ (beta-amyloid peptide) is an important contributor to Alzheimer’s disease (AD). We modeled Aβ toxicity in yeast by directing the peptide to the secretory pathway. A genome-wide screen for toxicity modifiers identified the yeast homolog of phosphatidylinositol binding clathrin assembly protein (PICALM) and other endocytic factors connected to AD whose relationship to Aβ was previously unknown. The factors identified in yeast modified Aβ toxicity in glutamatergic neurons of Caenorhabditis elegans and in primary rat cortical neurons. In yeast, Aβ impaired the endocytic trafficking of a plasma membrane receptor, which was ameliorated by endocytic pathway factors identified in the yeast screen. Thus, links between Aβ, endocytosis, and human AD risk factors can be ascertained with yeast as a model system.

TOR-mediated cell-cycle activation causes neurodegeneration in a Drosophila tauopathy model.

BACKGROUND Previous studies have demonstrated reexpression of cell-cycle markers within postmitotic neurons in neurodegenerative tauopathies, including Alzheimers disease (AD). However, the critical questions of whether cell-cycle activation is causal or epiphenomenal to tau-induced neurodegeneration and which signaling pathways mediate cell-cycle activation in tauopathy remain unresolved. RESULTS Cell-cycle activation accompanies wild-type and mutant tau-induced neurodegeneration in Drosophila, and genetically interfering with cell-cycle progression substantially reduces neurodegeneration. Our data support a role for cell-cycle activation downstream of tau phosphorylation, directly preceding apoptosis. We accordingly show that ectopic cell-cycle activation leads to apoptosis of postmitotic neurons in vivo. As in AD, TOR (target of rapamycin kinase) activity is increased in our model and is required for neurodegeneration. TOR activation enhances tau-induced neurodegeneration in a cell cycle-dependent manner and, when ectopically activated, drives cell-cycle activation and apoptosis in postmitotic neurons. CONCLUSIONS TOR-mediated cell-cycle activation causes neurodegeneration in a Drosophila tauopathy model, identifying TOR and the cell cycle as potential therapeutic targets in tauopathies and AD.

Association of CR1, CLU and PICALM with Alzheimer's disease in a cohort of clinically characterized and neuropathologically verified individuals

In this study, we assess 34 of the most replicated genetic associations for Alzheimers disease (AD) using data generated on Affymetrix SNP 6.0 arrays and imputed at over 5.7 million markers from a unique cohort of over 1600 neuropathologically defined AD cases and controls (1019 cases and 591 controls). Testing the top genes from the AlzGene meta-analysis, we confirm the well-known association with APOE single nucleotide polymorphisms (SNPs), the CLU, PICALM and CR1 SNPs recently implicated in unusually large data sets, and previously implicated CST3 and ACE SNPs. In the cases of CLU, PICALM and CR1, as well as in APOE, the odds ratios we find are slightly larger than those previously reported in clinical samples, consistent with what we believe to be more accurate classification of disease in the clinically characterized and neuropathologically confirmed AD cases and controls.

GWAS of Cerebrospinal Fluid Tau Levels Identifies Risk Variants for Alzheimer’s Disease

Cerebrospinal fluid (CSF) tau, tau phosphorylated at threonine 181 (ptau), and Aβ₄₂ are established biomarkers for Alzheimers disease (AD) and have been used as quantitative traits for genetic analyses. We performed the largest genome-wide association study for cerebrospinal fluid (CSF) tau/ptau levels published to date (n = 1,269), identifying three genome-wide significant loci for CSF tau and ptau: rs9877502 (p = 4.89 × 10⁻⁹ for tau) located at 3q28 between GEMC1 and OSTN, rs514716 (p = 1.07 × 10⁻⁸ and p = 3.22 × 10⁻⁹ for tau and ptau, respectively), located at 9p24.2 within GLIS3 and rs6922617 (p = 3.58 × 10⁻⁸ for CSF ptau) at 6p21.1 within the TREM gene cluster, a region recently reported to harbor rare variants that increase AD risk. In independent data sets, rs9877502 showed a strong association with risk for AD, tangle pathology, and global cognitive decline (p = 2.67 × 10⁻⁴, 0.039, 4.86 × 10⁻⁵, respectively) illustrating how this endophenotype-based approach can be used to identify new AD risk loci.

Common variants at 12q14 and 12q24 are associated with hippocampal volume

Aging is associated with reductions in hippocampal volume that are accelerated by Alzheimers disease and vascular risk factors. Our genome-wide association study (GWAS) of dementia-free persons (n = 9,232) identified 46 SNPs at four loci with P values of <4.0 × 10−7. In two additional samples (n = 2,318), associations were replicated at 12q14 within MSRB3-WIF1 (discovery and replication; rs17178006; P = 5.3 × 10−11) and at 12q24 near HRK-FBXW8 (rs7294919; P = 2.9 × 10−11). Remaining associations included one SNP at 2q24 within DPP4 (rs6741949; P = 2.9 × 10−7) and nine SNPs at 9p33 within ASTN2 (rs7852872; P = 1.0 × 10−7); along with the chromosome 12 associations, these loci were also associated with hippocampal volume (P < 0.05) in a third younger, more heterogeneous sample (n = 7,794). The SNP in ASTN2 also showed suggestive association with decline in cognition in a largely independent sample (n = 1,563). These associations implicate genes related to apoptosis (HRK), development (WIF1), oxidative stress (MSR3B), ubiquitination (FBXW8) and neuronal migration (ASTN2), as well as enzymes targeted by new diabetes medications (DPP4), indicating new genetic influences on hippocampal size and possibly the risk of cognitive decline and dementia.

The role of PAR-1 in regulating the polarised microtubule cytoskeleton in the Drosophila follicular epithelium.

The PAR-1 kinase plays a conserved role in cell polarity in C. elegans, Drosophila and mammals. We have investigated the role of PAR-1 in epithelial polarity by generating null mutant clones in the Drosophila follicular epithelium. Large clones show defects in apicobasal membrane polarity, but small clones induced later in development usually have a normal membrane polarity. However, all cells that lack PAR-1 accumulate spectrin and F-actin laterally, and show a strong increase in the density of microtubules. This is consistent with the observation that the mammalian PAR-1 homologues, the MARKs, dramatically reduce the number of microtubules, when overexpressed in tissue culture cells. The MARKs have been proposed to destabilize microtubules by inhibiting the stabilizing activity of the Tau family of microtubule-associated proteins. This is not the case in Drosophila, however, as null mutations in the single tau family member in the genome have no effect on the microtubule organisation in the follicle cells. Furthermore, PAR-1 activity stabilises microtubules, as microtubules in mutant cells depolymerise much more rapidly after cold or colcemid treatments. Loss of PAR-1 also disrupts the basal localisation of the microtubule plus ends, which are mislocalised to the centre of mutant cells. Thus, Drosophila PAR-1 regulates the density, stability and apicobasal organisation of microtubules. Although the direct targets of PAR-1 are unknown, we suggest that it functions by regulating the plus ends, possibly by capping them at the basal cortex.

CR1 is associated with amyloid plaque burden and age-related cognitive decline

Recently, genome‐wide association studies have identified 3 new susceptibility loci for Alzheimers disease (AD), CLU, CR1, and PICALM. We leveraged available neuropsychological and autopsy data from 2 cohort studies to investigate whether these loci are associated with cognitive decline and AD neuropathology.