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Published on: October 4, 2017
Widespread aggregation of mutant VAPB associated with ALS does not cause motor neuron degeneration or modulate mutant
Linghua Qiu1, Tao Qiao, Melissa Beers
1Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01602, USA. linghua.qiu@umassmed.edu
Background:
A proline-to-serine substitution at position-56 (P56S) of vesicle-associated membrane protein-associated protein B (VAPB) causes a form of dominantly inherited motor neuron disease (MND), including typical and atypical amyotrophic lateral sclerosis (ALS) and a mild late-onset spinal muscular atrophy (SMA). VAPB is an integral endoplasmic reticulum (ER) protein and has been implicated in various cellular processes, including ER stress, the unfolded protein response (UPR) and Ca2+ homeostasis. However, it is unclear how the P56S mutation leads to neurodegeneration and muscle atrophy in patients. The formation of abnormal VAPB-positive inclusions by mutant VAPB suggests a possible toxic gain of function as an underlying mechanism. Furthermore, the amount of VAPB protein is reported to be reduced in sporadic ALS patients and mutant SOD1G93A mice, leading to the hypothesis that wild type VAPB plays a role in the pathogenesis of ALS without VAPB mutations.
Results:
To investigate the pathogenic mechanism in vivo, we generated human wild type (wtVAPB) and mutant VAPB (muVAPB) transgenic mice that expressed the transgenes broadly in the CNS. We observed robust VAPB-positive aggregates in the spinal cord of muVAPB transgenic mice. However, we failed to find an impairment of motor function and motor neuron degeneration. We also did not detect any change in the endogenous VAPB level or evidence for induction of the unfolded protein response (UPR) and coaggregation of VAPA with muVAPB. Furthermore, we crossed these VAPB transgenic mice with mice that express mutant SOD1G93A and develop motor neuron degeneration. Overexpression of neither wtVAPB nor muVAPB modulated the protein aggregation and disease progression in the SOD1G93A mice.
Conclusion:
Overexpression of VAPBP56S mutant to approximately two-fold of the endogenous VAPB in mouse spinal cord produced abundant VAPB aggregates but was not sufficient to cause motor dysfunction or motor neuron degeneration. Furthermore, overexpression of either muVAPB or wtVAPB does not modulate the course of ALS in SOD1G93A mice. These results suggest that changes in wild type VAPB do not play a significant role in ALS cases that are not caused by VAPB mutations. Furthermore, these results suggest that muVAPB aggregates are innocuous and do not cause motor neuron degeneration by a gain-of-toxicity, and therefore, a loss of function may be the underlying mechanism.
Insights
Vesicle-associated membrane protein-associated protein B (VAPB) P56S mutation causes motor neuron disease, but overexpression of mutant VAPB in mice did not lead to neurodegeneration. This suggests a loss-of-function mechanism rather than toxic gain of function for VAPB-related motor neuron disease.
Area of Science:
- Neuroscience
- Genetics
- Cell Biology
Background:
- Vesicle-associated membrane protein-associated protein B (VAPB) mutations, specifically P56S, are linked to inherited motor neuron diseases like amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA).
- VAPB, an endoplasmic reticulum (ER) protein, is involved in ER stress, unfolded protein response (UPR), and calcium homeostasis, but the precise mechanism of P56S mutation-induced neurodegeneration remains unclear.
- The formation of VAPB aggregates suggests a potential toxic gain-of-function, though reduced VAPB levels in other ALS forms hint at a possible loss-of-function role.
Purpose of the Study:
- To investigate the in vivo pathogenic mechanisms of VAPB mutations in motor neuron disease.
- To determine if VAPB aggregates cause neurodegeneration through a toxic gain-of-function mechanism.
- To assess the role of wild-type VAPB in amyotrophic lateral sclerosis (ALS) pathogenesis.
Main Methods:
- Generation of transgenic mice expressing human wild-type (wtVAPB) and mutant (muVAPB) VAPB broadly in the central nervous system (CNS).
- Assessment of motor function, motor neuron degeneration, and VAPB aggregation in muVAPB transgenic mice.
- Crossing VAPB transgenic mice with SOD1G93A mutant mice to evaluate the modulation of ALS progression and protein aggregation.
Main Results:
- Transgenic mice overexpressing muVAPB exhibited robust VAPB-positive aggregates in the spinal cord but showed no motor impairment or motor neuron degeneration.
- No changes in endogenous VAPB levels, UPR induction, or VAPA coaggregation were detected in muVAPB mice.
- Overexpression of either wtVAPB or muVAPB did not alter protein aggregation or disease progression in SOD1G93A mutant mice.
Conclusions:
- Overexpression of mutant VAPB (P56S) leading to VAPB aggregates is insufficient to cause motor dysfunction or neurodegeneration in mice.
- The presence of muVAPB aggregates does not appear to cause motor neuron degeneration via a gain-of-toxicity mechanism.
- These findings suggest that a loss of VAPB function, rather than a toxic gain of function from aggregates, may underlie VAPB-related motor neuron diseases, and wild-type VAPB changes are not significant in non-VAPB mutation ALS cases.
