Abstract
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.
Highlights
Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease caused by the selective degeneration of spinal and corticospinal motor neurons, resulting in muscle weakness and atrophy along with spastic paralysis [1,2]
We found that deficiency in alsin led to a significant accumulation of enlarged Rab5-associated endosomes in neurons, which was correlated with a dramatic decrease of endosomal motility and may contribute to the increased lysosome-dependent degradation of internalized glutamate receptors in amyotrophic lateral sclerosis 2 (ALS2)-/- neurons
Accumulation of enlarged Rab5-associated endosomes in ALS2-/- hippocampal neurons To investigate whether alsin-deficiency affects the morphology of early endosomes, we expressed green fluorescent protein (GFP)-Rab5 in hippocampal neurons derived from wild-type and ALS2-/mice
Summary
Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease caused by the selective degeneration of spinal and corticospinal motor neurons, resulting in muscle weakness and atrophy along with spastic paralysis [1,2]. One form of inherited juvenile-onset amyotrophic lateral sclerosis (ALS2) is caused by loss of function mutations in the ALS2 gene [3,4,5,6]. Almost all reported mutations in the ALS2 gene result in the loss of the VPS9 domain [4,6,10,11,12,13], suggesting that the proposed Rab GEF activity of alsin plays a critical role in protecting motor neurons from degeneration
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