William T. Dauer

Parkinson's disease: mechanisms and models.

Parkinsons disease (PD) results primarily from the death of dopaminergic neurons in the substantia nigra. Current PD medications treat symptoms; none halt or retard dopaminergic neuron degeneration. The main obstacle to developing neuroprotective therapies is a limited understanding of the key molecular events that provoke neurodegeneration. The discovery of PD genes has led to the hypothesis that misfolding of proteins and dysfunction of the ubiquitin-proteasome pathway are pivotal to PD pathogenesis. Previously implicated culprits in PD neurodegeneration, mitochondrial dysfunction and oxidative stress, may also act in part by causing the accumulation of misfolded proteins, in addition to producing other deleterious events in dopaminergic neurons. Neurotoxin-based models (particularly MPTP) have been important in elucidating the molecular cascade of cell death in dopaminergic neurons. PD models based on the manipulation of PD genes should prove valuable in elucidating important aspects of the disease, such as selective vulnerability of substantia nigra dopaminergic neurons to the degenerative process.

Dopamine-modified α-synuclein blocks chaperone-mediated autophagy

Altered degradation of alpha-synuclein (alpha-syn) has been implicated in the pathogenesis of Parkinson disease (PD). We have shown that alpha-syn can be degraded via chaperone-mediated autophagy (CMA), a selective lysosomal mechanism for degradation of cytosolic proteins. Pathogenic mutants of alpha-syn block lysosomal translocation, impairing their own degradation along with that of other CMA substrates. While pathogenic alpha-syn mutations are rare, alpha-syn undergoes posttranslational modifications, which may underlie its accumulation in cytosolic aggregates in most forms of PD. Using mouse ventral medial neuron cultures, SH-SY5Y cells in culture, and isolated mouse lysosomes, we have found that most of these posttranslational modifications of alpha-syn impair degradation of this protein by CMA but do not affect degradation of other substrates. Dopamine-modified alpha-syn, however, is not only poorly degraded by CMA but also blocks degradation of other substrates by this pathway. As blockage of CMA increases cellular vulnerability to stressors, we propose that dopamine-induced autophagic inhibition could explain the selective degeneration of PD dopaminergic neurons.

Interplay of LRRK2 with chaperone-mediated autophagy

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial Parkinsons disease. We found LRRK2 to be degraded in lysosomes by chaperone-mediated autophagy (CMA), whereas the most common pathogenic mutant form of LRRK2, G2019S, was poorly degraded by this pathway. In contrast to the behavior of typical CMA substrates, lysosomal binding of both wild-type and several pathogenic mutant LRRK2 proteins was enhanced in the presence of other CMA substrates, which interfered with the organization of the CMA translocation complex, resulting in defective CMA. Cells responded to such LRRK2-mediated CMA compromise by increasing levels of the CMA lysosomal receptor, as seen in neuronal cultures and brains of LRRK2 transgenic mice, induced pluripotent stem cell–derived dopaminergic neurons and brains of Parkinsons disease patients with LRRK2 mutations. This newly described LRRK2 self-perpetuating inhibitory effect on CMA could underlie toxicity in Parkinsons disease by compromising the degradation of α-synuclein, another Parkinsons disease–related protein degraded by this pathway.

The kinase domain of mitochondrial PINK1 faces the cytoplasm

Mutations in PTEN-induced putative kinase 1 (PINK1) are a cause of autosomal recessive familial Parkinsons disease (PD). Efforts in deducing the PINK1 signaling pathway have been hindered by controversy around its subcellular and submitochondrial localization and the authenticity of its reported substrates. We show here that this mitochondrial protein exhibits a topology in which the kinase domain faces the cytoplasm and the N-terminal tail is inside the mitochondria. Although deletion of the transmembrane domain disrupts this topology, common PD-linked PINK1 mutations do not. These results are critical in rectifying the location and orientation of PINK1 in mitochondria, and they should help decipher its normal physiological function and potential pathogenic role in PD.

The AAA+ protein torsinA interacts with a conserved domain present in LAP1 and a novel ER protein

A glutamic acid deletion (ΔE) in the AAA+ protein torsinA causes DYT1 dystonia. Although the majority of torsinA resides within the endoplasmic reticulum (ER), torsinA binds a substrate in the lumen of the nuclear envelope (NE), and the ΔE mutation enhances this interaction. Using a novel cell-based screen, we identify lamina-associated polypeptide 1 (LAP1) as a torsinA-interacting protein. LAP1 may be a torsinA substrate, as expression of the isolated lumenal domain of LAP1 inhibits the NE localization of “substrate trap” EQ-torsinA and EQ-torsinA coimmunoprecipitates with LAP1 to a greater extent than wild-type torsinA. Furthermore, we identify a novel transmembrane protein, lumenal domain like LAP1 (LULL1), which also appears to interact with torsinA. Interestingly, LULL1 resides in the main ER. Consequently, torsinA interacts directly or indirectly with a novel class of transmembrane proteins that are localized in different subdomains of the ER system, either or both of which may play a role in the pathogenesis of DYT1 dystonia.

α-Synuclein produces a long-lasting increase in neurotransmitter release

Wild‐type α‐synuclein, a protein of unknown function, has received much attention because of its involvement in a series of diseases that are known as synucleinopathies. We find that long‐lasting potentiation of synaptic transmission between cultured hippocampal neurons is accompanied by an increase in the number of α‐synuclein clusters. Conversely, suppression of α‐synuclein expression through antisense nucleotide and knockout techniques blocks the potentiation, as well as the glutamate‐induced increase in presynaptic functional bouton number. Consistent with these findings, α‐synuclein introduction into the presynaptic neuron of a pair of monosynaptically connected cells causes a rapid and long‐lasting enhancement of synaptic transmission, and rescues the block of potentiation in α‐synuclein null mouse cultures. Also, we report that the application of nitric oxide (NO) increases the number of α‐synuclein clusters, and inhibitors of NO‐synthase block this increase, supporting the hypothesis that NO is involved in the enhancement of the number of α‐synuclein clusters. Thus, α‐synuclein is involved in synaptic plasticity by augmenting transmitter release from the presynaptic terminal.

The Parkinson Disease Protein Leucine-Rich Repeat Kinase 2 Transduces Death Signals via Fas-Associated Protein with Death Domain and Caspase-8 in a Cellular Model of Neurodegeneration

Neurodegenerative illnesses such as Parkinson and Alzheimer disease are an increasingly prevalent problem in aging societies, yet no therapies exist that retard or prevent neurodegeneration. Dominant missense mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson disease (PD), but the mechanisms by which mutant forms of LRRK2 disrupt neuronal function and cause cell death remain poorly understood. We report that LRRK2 interacts with the death adaptor Fas-associated protein with death domain (FADD), and that in primary neuronal culture LRRK2-mediated neurodegeneration is prevented by the functional inhibition of FADD or depletion of caspase-8, two key elements of the extrinsic cell death pathway. This pathway is activated by disease-triggering mutations, which enhance the LRRK2-FADD association and the consequent recruitment and activation of caspase-8. These results establish a direct molecular link between a mutant PD gene and the activation of programmed cell death signaling, and suggest that FADD/caspase-8 signaling contributes to LRRK2-induced neuronal death.

Primary dystonia: molecules and mechanisms

Primary dystonia is characterized by abnormal, involuntary twisting and turning movements that reflect impaired motor system function. The dystonic brain seems normal, in that it contains no overt lesions or evidence of neurodegeneration, but functional brain imaging has uncovered abnormalities involving the cortex, striatum and cerebellum, and diffusion tensor imaging suggests the presence of microstructural defects in white matter tracts of the cerebellothalamocortical circuit. Clinical electrophysiological studies show that the dystonic CNS exhibits aberrant plasticity—perhaps related to deficient inhibitory neurotransmission—in a range of brain structures, as well as the spinal cord. Dystonia is, therefore, best conceptualized as a motor circuit disorder, rather than an abnormality of a particular brain structure. None of the aforementioned abnormalities can be strictly causal, as they are not limited to regions of the CNS subserving clinically affected body parts, and are found in seemingly healthy patients with dystonia-related mutations. The study of dystonia-related genes will, hopefully, help researchers to unravel the chain of events from molecular to cellular to system abnormalities. DYT1 mutations, for example, cause abnormalities within the endoplasmic reticulum–nuclear envelope endomembrane system. Other dystonia-related gene products traffic through the endoplasmic reticulum, suggesting a potential cell biological theme underlying primary dystonia.

A molecular mechanism underlying the neural-specific defect in torsinA mutant mice

A striking but poorly understood feature of many diseases is the unique involvement of neural tissue. One example is the CNS-specific disorder DYT1 dystonia, caused by a 3-bp deletion (“ΔE”) in the widely expressed gene TOR1A. Disease mutant knockin mice (Tor1aΔE/ΔE) exhibit disrupted nuclear membranes selectively in neurons, mimicking the tissue specificity of the human disease and providing a model system in which to dissect the mechanisms underlying neural selectivity. Our in vivo studies demonstrate that lamina-associated polypeptide 1 (LAP1) and torsinB function with torsinA to maintain normal nuclear membrane morphology. Moreover, we show that nonneuronal cells express dramatically higher levels of torsinB and that RNAi-mediated depletion of torsinB (but not other torsin family members) causes nuclear membrane abnormalities in Tor1aΔE/ΔE nonneuronal cells. The Tor1aΔE/ΔE neural selective phenotype therefore arises because high levels of torsinB protect nonneuronal cells from the consequences of torsinA dysfunction, demonstrating how tissue specificity may result from differential susceptibility of cell types to insults that disrupt ubiquitous biological pathways.

Cerebellothalamocortical pathway abnormalities in torsinA DYT1 knock-in mice

The factors that determine symptom penetrance in inherited disease are poorly understood. Increasingly, magnetic resonance diffusion tensor imaging (DTI) and PET are used to separate alterations in brain structure and function that are linked to disease symptomatology from those linked to gene carrier status. One example is DYT1 dystonia, a dominantly inherited movement disorder characterized by sustained muscle contractions, postures, and/or involuntary movements. This form of dystonia is caused by a 3-bp deletion (i.e., ΔE) in the TOR1A gene that encodes torsinA. Carriers of the DYT1 dystonia mutation, even if clinically nonpenetrant, exhibit abnormalities in cerebellothalamocortical (CbTC) motor pathways. However, observations in human gene carriers may be confounded by variability in genetic background and age. To address this problem, we implemented a unique multimodal imaging strategy in a congenic line of DYT1 mutant mice that contain the ΔE mutation in the endogenous mouse torsinA allele (i.e., DYT1 knock-in). Heterozygous knock-in mice and littermate controls underwent microPET followed by ex vivo high-field DTI and tractographic analysis. Mutant mice, which do not display abnormal movements, exhibited significant CbTC tract changes as well as abnormalities in brainstem regions linking cerebellar and basal ganglia motor circuits highly similar to those identified in human nonmanifesting gene carriers. Moreover, metabolic activity in the sensorimotor cortex of these animals was closely correlated with individual measures of CbTC pathway integrity. These findings further link a selective brain circuit abnormality to gene carrier status and demonstrate that DYT1 mutant torsinA has similar effects in mice and humans.