Amyotrophic lateral sclerosis-related VAPB P56S mutation differentially affects the function and survival of corticospinal and spinal motor neurons.

The substitution of Proline with Serine at residue 56 (P56S) of vesicle-associated membrane protein-associated protein B (VAPB) has been linked to an atypical autosomal dominant form of familial amyotrophic lateral sclerosis 8 (ALS8). To investigate the pathogenic mechanism of P56S VAPB in ALS, we generated transgenic (Tg) mice that heterologously express human wild-type (WT) and P56S VAPB under the control of a pan-neuronal promoter Thy1.2. While WT VAPB Tg mice did not exhibit any overt motor behavioral phenotypes, P56S VAPB Tg mice developed progressive hyperactivities and other motor abnormalities. VAPB protein was accumulated as large punctate in the soma and proximal dendrites of both corticospinal motor neurons (CSMNs) and spinal motor neurons (SMNs) in P56S VAPB Tg mice. Concomitantly, a significant increase of endoplasmic reticulum stress and unfolded protein response and the resulting up-regulation of pro-apoptotic factor CCAAT/enhancer-binding protein homologous protein expression were observed in the CSMNs and SMNs of P56S VAPB Tg mice. However, only a progressive loss of CSMNs but not SMNs was found in P56S VAPB Tg mice. In SMNs, P56S VAPB promoted a rather selective translocation of VAPB protein onto the postsynaptic site of C-boutons that altered the morphology of C-boutons and impaired the spontaneous rhythmic discharges of SMNs. Therefore, these findings provide new pathophysiological mechanisms of P56S VAPB that differentially affect the function and survival of CSMNs and SMNs in ALS8.

The chaperone activity of heat shock protein 90 is critical for maintaining the stability of leucine-rich repeat kinase 2.

Parkinson's 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.

The G59S mutation in p150(glued) causes dysfunction of dynactin in mice.

The G59S missense mutation at the conserved microtubule-binding domain of p150(glued), 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 p150(glued) G59S knock-in mice. We found that the G59S mutation destabilizes p150(glued) 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 p150(glued) heterozygous knock-out mice. Our findings indicate that the G59S mutation in p150(glued) abrogates the normal function of p150(glued) and accelerates motor neuron degeneration.

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.

Leucine-rich repeat kinase 2 regulates the progression of neuropathology induced by Parkinson's-disease-related mutant alpha-synuclein.

Mutations in alpha-synuclein and Leucine-rich repeat kinase 2 (LRRK2) are linked to autosomal dominant forms of Parkinson's disease (PD). However, little is known about any potential pathophysiological interplay between these two PD-related genes. Here we show in transgenic mice that although overexpression of LRRK2 alone did not cause neurodegeneration, the presence of excess LRRK2 greatly accelerated the progression of neuropathological abnormalities developed in PD-related A53T alpha-synuclein transgenic mice. Moreover, we found that LRRK2 promoted the abnormal aggregation and somatic accumulation of alpha-synuclein in A53T mice, which likely resulted from the impairment of microtubule dynamics, Golgi organization, and the ubiquitin-proteasome pathway. Conversely, genetic ablation of LRRK2 preserved the Golgi structure and suppressed the aggregation and somatic accumulation of alpha-synuclein, and thereby delayed the progression of neuropathology in A53T mice. These findings demonstrate that overexpression of LRRK2 enhances alpha-synuclein-mediated cytotoxicity and suggest inhibition of LRRK2 expression as a potential therapeutic option for ameliorating alpha-synuclein-induced neurodegeneration.