I think this is a great paper that unravels a novel role for VAMP/synaptobrevin-associated protein B (VAPB) with great precision. The paper again illustrates the power of Drosophila and C. elegans as animal models for identifying functional roles of proteins and illuminating potential disease pathways.

The questions raised by the P56S-VAPB mutation are essentially the same as those raised by mutations in genes linked to other ALS forms, as well as to other neurodegenerative disorders: What is the function of the mutated protein? Does the disease result from a loss of its normal function or a gain of toxic activity? What is the role of protein aggregation? What determines the delayed onset of disease, and to what extent do disease pathways overlap with those in other ALS forms? And, most importantly, how to stop or prevent disease?

Several lines of evidence indicate that mutant VAPB, at least in part, may operate in a dominant-negative way by recruiting wild-type VAPB to inclusions. the paper by Han et al. provides one potential mechanism by which loss of...  Read more

I think this is a great paper that unravels a novel role for VAMP/synaptobrevin-associated protein B (VAPB) with great precision. The paper again illustrates the power of Drosophila and C. elegans as animal models for identifying functional roles of proteins and illuminating potential disease pathways.

The questions raised by the P56S-VAPB mutation are essentially the same as those raised by mutations in genes linked to other ALS forms, as well as to other neurodegenerative disorders: What is the function of the mutated protein? Does the disease result from a loss of its normal function or a gain of toxic activity? What is the role of protein aggregation? What determines the delayed onset of disease, and to what extent do disease pathways overlap with those in other ALS forms? And, most importantly, how to stop or prevent disease?

Several lines of evidence indicate that mutant VAPB, at least in part, may operate in a dominant-negative way by recruiting wild-type VAPB to inclusions. the paper by Han et al. provides one potential mechanism by which loss of VAPB function in motor neurons results in reduced performance of skeletal muscle.

This work follows a provocative study by the same groups published in Cell in 2008, indicating that a fragment of the Drosophila VAPB homologue containing the major sperm protein (MSP) domain may act as a paracrine factor released by motor neurons at the neuromuscular junctions. Now, Han et al. show that loss of VAPB in motor neurons in both Drosophila and C. elegans results in structural and functional mitochondrial abnormalities in the target muscles. Importantly, the same effect was also observed after mutant VAP overexpression in the motor neurons, supporting the dominant-negative mode of action of mutant VAP.

In an elegant series of experiments, the authors further show that the MSP domain of VAPB binds to a Robo and a Lar-receptor to control mitochondrial localization via the regulation of the actin skeleton. As indicated in the conclusion scheme by the authors (Fig. 8C), one major unresolved question is how the VAP-MSP fragment reaches the extracellular space. Another question is whether a similar mechanism operates in vertebrates and, in particular, in mammals. This question awaits the analysis of VAPB-knockout mice.

Another problem is that the authors’ model predicts that the functional defects resulting from VAPB deficiency in motor neurons occur in the muscle fibers. However, EMG and muscle biopsy (Marques et al., 2006) point to a neurogenic basis of the disease, consistent with the diagnosis of ALS, and implying functional loss and degeneration of motor neurons or their axons (rather than muscle fibers) as the prime event in the disease. Nevertheless several mechanisms can be envisaged by which loss of motor neuron VAPB contributes to muscle weakness not only in VAPB-ALS, but also in other ALS forms.

Remarkably, a recent study has suggested a mechanism by which mutant VAPB may cause mitochondrial abnormalities in a cell-autonomous way in motor neurons (De Vos et al., 2011). This study shows that VAPB may interact with the mitochondrial protein PTPIP51 to regulate its interaction with the endoplasmic reticulum and calcium homeostasis. According to this study, mutant VAPB disturbs this interaction and alters calcium homeostasis. In regard to that study, I would like to point out that we do not see ultrastructural mitochondrial abnormalities in motor neurons of our transgenic mice that overexpress mutant VAPB. The mice develop numerous VAPB "aggregates" in motor neurons, but the aggregates seem to be rather harmless, which is consistent with data from another study with VAPB transgenic mice (Tudor et al., 2010).

View all comments by Dick Jaarsma

John Landers and colleagues have successfully used exome capture followed by deep sequencing to identify novel mutations in the profilin gene (PFN1) that cause familial amyotrophic lateral sclerosis (FALS). The application of this methodology has greatly speeded up the identification of pathogenic mutations. Data obtained from affected members of two kindreds revealed different coding mutations, C71G and M114T, both being present in PFN1, that segregated with disease and had not been previously reported in available SNP databases. Subsequent screening of this gene for mutations in 273 further FALS and 816 sporadic ALS cases revealed two more FALS index cases with the C71G mutation and one other case with the M114T mutation. Two new mutations, G118V and E117G, were found in familial cases, and the E117G mutation was also found in two sporadic cases. The E117G mutation also occurred in three out of 7,560 controls and must be viewed with caution. No coding changes in PFN2 and PFN3 were seen in FALS cases.

Quite a lot of research has been performed on profilin, and it is known to...  Read more

John Landers and colleagues have successfully used exome capture followed by deep sequencing to identify novel mutations in the profilin gene (PFN1) that cause familial amyotrophic lateral sclerosis (FALS). The application of this methodology has greatly speeded up the identification of pathogenic mutations. Data obtained from affected members of two kindreds revealed different coding mutations, C71G and M114T, both being present in PFN1, that segregated with disease and had not been previously reported in available SNP databases. Subsequent screening of this gene for mutations in 273 further FALS and 816 sporadic ALS cases revealed two more FALS index cases with the C71G mutation and one other case with the M114T mutation. Two new mutations, G118V and E117G, were found in familial cases, and the E117G mutation was also found in two sporadic cases. The E117G mutation also occurred in three out of 7,560 controls and must be viewed with caution. No coding changes in PFN2 and PFN3 were seen in FALS cases.

Quite a lot of research has been performed on profilin, and it is known to function in the regulation of actin structure, transforming a globular monomer, known as G-actin, to long helical polymer, F-actin. This transformation is crucial in cytoskeletal dynamics, important in neurite outgrowth, growing axons, and synapse formation. Furthermore, the FALS-associated mutations lie in close proximity to the actin binding residues. The mutations were convincingly shown in this study to reduce axon outgrowth. Growth cone size and morphology were also affected by the mutations, with greatly reduced F-actin-rich filopodia being present.

Functional studies carried out by the authors showed that the three novel mutations had a propensity to aggregate and, when transfected into a neuronal cell line (Neuro2A) or primary motor neurons, formed ubiquitinated aggregates. Whilst these aggregates did not show co-aggregation with FUS or SMN, co-staining of aggregates with PFN1 and TDP-43 occurred in about one-third of cells. However, no abnormal PFN pathology was seen in spinal cord sections from sporadic cases of ALS.

Abnormalities in a number of cytoskeletal proteins are found in motor neuron diseases, but tend to be quite rare. At present, no autopsy cases harboring these PFN1 mutations are available, but information about the neuropathological features of this condition will be extremely valuable. The pathogenic mechanism remains to be elucidated and could involve effects on actin polymerization, or may yet result from one of the other numerous protein interactions reported for PFN.

View all comments by J. de Belleroche

There remain many families with ALS where the causative genes are yet to be discovered. In our Department of Neurology at the University of Tokyo, causative genes are unidentified in approximately half of ALS families.

In PFN1-associated familial ALS (FALS), the mutations were found in seven out of 274 families, meaning the frequency of families with mutations in PFN1 is rare. Nonetheless, I think this finding is important. Surely, most of the FALS families may have mutations in orphan genes. We need to identify all the causative genes for FALS, which should bring insight into sporadic ALS, which is more common compared to FALS.

I believe this discovery provides better understanding of the pathophysiology of ALS. Abnormality in conversion of monomeric (G)-actin to filamentous (F)-actin is a new mechanism in the disease.

We are pursuing similar approaches, and hope to contribute to better understanding ALS in the very near future.

View all comments by Shoji Tsuji