Jayanth Chandran

The Chaperone Activity of Heat Shock Protein 90 Is Critical for Maintaining the Stability of Leucine-Rich Repeat Kinase 2

Parkinsons disease (PD), a progressive neurodegenerative disease characterized by bradykinesia, rigidity, and resting tremor, is the most common neurodegenerative movement disorder. Although the majority of PD cases are sporadic, some are inherited, including those caused by leucine-rich repeat kinase 2 (LRRK2) mutations. The substitution of serine for glycine at position 2019 (G2019S) in the kinase domain of LRRK2 represents the most prevalent genetic mutation in both familial and apparently sporadic cases of PD. Because mutations in LRRK2 are likely associated with a toxic gain of function, destabilization of LRRK2 may be a novel way to limit its detrimental effects. Here we show that LRRK2 forms a complex with heat shock protein 90 (Hsp90) in vivo and that inhibition of Hsp90 disrupts the association of Hsp90 with LRRK2 and leads to proteasomal degradation of LRRK2. Hsp90 inhibitors may therefore limit the mutant LRRK2-elicited toxicity to neurons. As a proof of principle, we show that Hsp90 inhibitors rescue the axon growth retardation caused by overexpression of the LRRK2 G2019S mutation in neurons. Therefore, inhibition of LRRK2 kinase activity can be achieved by blocking Hsp90-mediated chaperone activity and Hsp90 inhibitors may serve as potential anti-PD drugs.

RNA binding activity of the recessive parkinsonism protein DJ-1 supports involvement in multiple cellular pathways

Parkinsons disease (PD) is a major neurodegenerative condition with several rare Mendelian forms. Oxidative stress and mitochondrial function have been implicated in the pathogenesis of PD but the molecular mechanisms involved in the degeneration of neurons remain unclear. DJ-1 mutations are one cause of recessive parkinsonism, but this gene is also reported to be involved in cancer by promoting Ras signaling and suppressing PTEN-induced apoptosis. The specific function of DJ-1 is unknown, although it is responsive to oxidative stress and may play a role in the maintenance of mitochondria. Here, we show, using four independent methods, that DJ-1 associates with RNA targets in cells and the brain, including mitochondrial genes, genes involved in glutathione metabolism, and members of the PTEN/PI3K cascade. Pathogenic recessive mutants are deficient in this activity. We show that DJ-1 is sufficient for RNA binding at nanomolar concentrations. Further, we show that DJ-1 binds RNA but dissociates after oxidative stress. These data implicate a single mechanism for the pleiotropic effects of DJ-1 in different model systems, namely that the protein binds multiple RNA targets in an oxidation-dependent manner.

The G59S Mutation in p150glued Causes Dysfunction of Dynactin in Mice

The G59S missense mutation at the conserved microtubule-binding domain of p150glued, a major component of dynein/dynactin complex, has been linked to an autosomal dominant form of motor neuron disease (MND). To study how this mutation affects the function of the dynein/dynactin complex and contributes to motor neuron degeneration, we generated p150glued G59S knock-in mice. We found that the G59S mutation destabilizes p150glued and disrupts the function of dynein/dynactin complex, resulting in early embryonic lethality of homozygous knock-in mice. Heterozygous knock-in mice, which developed normally, displayed MND-like phenotypes after 10 months of age, including excessive accumulation of cytoskeletal and synaptic vesicle proteins at neuromuscular junctions, loss of spinal motor neurons, increase of reactive astrogliosis, and shortening of gait compared with wild-type littermates and age-matched p150glued heterozygous knock-out mice. Our findings indicate that the G59S mutation in p150glued abrogates the normal function of p150glued and accelerates motor neuron degeneration.

Amyotrophic Lateral Sclerosis 2-Deficiency Leads to Neuronal Degeneration in Amyotrophic Lateral Sclerosis through Altered AMPA Receptor Trafficking

Amyotrophic lateral sclerosis (ALS), the most common adult-onset motor neuron disease is caused by a selective loss of motor neurons. One form of juvenile onset autosomal recessive ALS (ALS2) has been linked to the loss of function of the ALS2 gene. The pathogenic mechanism of ALS2-deficiency, however, remains unclear. To further understand the function of alsin that is encoded by the full-length ALS2 gene, we screened proteins interacting with alsin. Here, we report that alsin interacted with glutamate receptor interacting protein 1 (GRIP1) both in vitro and in vivo, and colocalized with GRIP1 in neurons. In support of the physiological interaction between alsin and GRIP1, the subcellular distribution of GRIP1 was altered in ALS2 −/− spinal motor neurons, which correlates with a significant reduction of AMPA-type glutamate receptor subunit 2 (GluR2) at the synaptic/cell surface of ALS2 −/− neurons. The decrease of calcium-impermeable GluR2-containing AMPA receptors at the cell/synaptic surface rendered ALS2 −/− neurons more susceptible to glutamate receptor-mediated neurotoxicity. Our findings reveal a novel function of alsin in AMPA receptor trafficking and provide a novel pathogenic link between ALS2-deficiency and motor neuron degeneration, suggesting a protective role of alsin in maintaining the survival of motor neurons.

ALS2/Alsin Knockout Mice and Motor Neuron Diseases

Autosomal recessive mutations in the ALS2 gene have been linked to juvenile-onset amyotrophic lateral sclerosis (ALS2), primary lateral sclerosis and juvenile-onset ascending hereditary spastic paraplegia. Except for two recently identified missense mutations, all other mutations in the ALS2 gene lead to a premature stop codon and likely abrogate all the potential functions of alsin, the protein encoded by the ALS2 gene. To study the pathologic mechanisms of ALS2 deficiency, four different lines of ALS2 knockout (ALS2–/–) mice have been generated by independent groups. The loss of ALS2/alsin does not have a drastic effect on the survival or function of motor neurons in mice. However, subtle deficits observed in the behavior and pathology of these mice have aided in our understanding of the relationship between alsin and motor neuron dysfunction. In this review, we summarize and reconcile major findings of ALS2–/– mice and attempt to place these results within the larger context of modeling recessive movement disorders in mice.

Regulation of endosomal motility and degradation by amyotrophic lateral sclerosis 2/alsin

Dysfunction of alsin, particularly its putative Rab5 guanine-nucleotide-exchange factor activity, has been linked to one form of juvenile onset recessive familial amyotrophic lateral sclerosis (ALS2). Multiple lines of alsin knockout (ALS2-/-) mice have been generated to model this disease. However, it remains elusive whether the Rab5-dependent endocytosis is altered in ALS2-/- neurons. To directly examine the Rab5-mediated endosomal trafficking in ALS2-/- neurons, we introduced green fluorescent protein (GFP)-tagged Rab5 into cultured hippocampal neurons to monitor the morphology and motility of Rab5-associated early endosomes. Here we report that Rab5-mediated endocytosis was severely altered in ALS2-/-neurons. Excessive accumulation of Rab5-positive vesicles was observed in ALS2-/- neurons, which correlated with a significant reduction in endosomal motility and augmentation in endosomal conversion to lysosomes. Consequently, a significant increase in endosome/lysosome-dependent degradation of internalized glutamate receptors was observed in ALS2-/- neurons. These phenotypes closely resembled the endosomal trafficking abnormalities induced by a constitutively active form of Rab5 in wild-type neurons. Therefore, our findings reveal a negatively regulatory mechanism of alsin in Rab5-mediated endosomal trafficking, suggesting that enhanced endosomal degradation in ALS2-/- neurons may underlie the pathogenesis of motor neuron degeneration in ALS2 and related motor neuron diseases.

DJ-1 is Essential for Long-Term Depression at Hippocampal CA1 Synapses

Mutations in DJ-1 cause inherited Parkinson’s disease (PD) in several families. The normal function of DJ-1 is unknown, but mice lacking DJ-1 exhibit a deficit in dopaminergic signaling in the striatum. Since the hippocampus contains relatively high levels of DJ-1, and PD patients are often cognitively impaired, we evaluated the effects of DJ-1 deficiency on the plasticity of hippocampal CA1 synapses. LTP was slightly impaired and LTD was abolished in DJ-1−/− mice, whereas DJ-1+/− mice exhibited no alterations in synaptic plasticity. The dopamine receptor D2/3 agonist quinpirole rescued LTD in DJ-1−/− mice, suggesting a role for impaired dopaminergic signaling in the hippocampal LTD deficit.

Mad Fly Disease

The prion disorders are a rare and intriguing group of neurodegenerative diseases characterized by spongiform degeneration of the brain, accumulation of a misfolded form of a host protein [the prion protein (PrP) or, in its misfolded form, PrPSc] and, uniquely, an etiology that can be inherited,