Several different mutations in the tiny profilin-1 are responsible for some familial forms of amyotrophic lateral sclerosis. According to computational models published in the August 30 Scientific Reports, these variants restrict the protein’s flexibility, straining its relationship with actin and potentially many other partners. The mutations also make the protein more hydrophobic, coaxing it to aggregate. The authors, led by Mahmoud Kiaei at the University of Arkansas for Medical Sciences in Little Rock, propose that the structural constraints lead to a toxic mix of misfolding, aggregation, and loss of function that eventually fell motor neurons during aging.
Mutations in profilin-1 (PFN1) account for 1 to 2 percent of familial ALS cases (see Jul 2012 news; Jun 2015 news). At just 140 amino acids in length, the protein contains only two interaction domains. With one, it binds actin monomers and plays an essential role in their polymerization and in the formation of the cytoskeleton. With the other, PFN1 latches onto proteins that contain poly-L-proline (PLP) motifs. These proteins carry out a variety of functions. Profilin binds upward of 50 partners, suggesting its influence stretches well beyond the cytoskeleton. Cell culture and mouse models indicate that ALS-causing mutations in the PFN1 gene destabilize the protein, promote its aggregation, and even instigate the aggregation of TDP-43, another player in ALS pathogenesis (see Oct 2016 news; Tanaka et al., 2016; Fil et al., 2017). However, researchers have yet to nail down which of these mechanisms ultimately steers motor neurons toward their demise (see April 2017 news).
Kiaei and colleagues investigated how two different mutations changed the protein’s shape. Using a previously published crystal structure of PFN1 in complex with actin and PLP, they simulated the structural and functional consequences of the glycine-118-valine (G118V) and threonine-109-methionine (T109M) mutations (Ferron et al., 2007). According to the crystal structure, G118 interacts directly with actin via a hydrogen bond, and the simulations indicated that switching this residue to valine, a larger nonpolar amino acid, would restrict PFN1’s flexibility and weaken its association with actin. Furthermore, the conformational changes caused by the valine substitution led to greater exposure of an adjacent valine residue—V119. Together, these exposed sticky residues boosted PFN1’s overall hydrophobicity, a property known to increase the likelihood of aggregation.
The T109 residue resides adjacent to, but is not part of, the PLP binding site. However, the computational simulations revealed that substituting the threonine with the bulkier methionine would shift the conformation of residues that do contact PLP, leading to weaker binding. Similar to the scenario with G118V, the T109M mutation also made PFN1 more rigid and hydrophobic. The findings suggest that a combination of rigidity and hydrophobicity would weaken the protein’s associations with its binding partners, and promote its aggregation.
How might these structural shifts lead to motor neuron demise that occurs only with aging? Kiaei explained that in the G118V mouse model he developed, aggregates of PFN1 start accumulating in motor neurons prior to the onset of motor symptoms. He proposed that early in life, neurons manage to recognize and dispose of the misfolded protein translated from the mutant copy of PFN1. During this time, the cells make do with their single good copy of the gene. However, at some point in the aging process, motor neurons fail to clear the accumulating misfolded protein, which then aggregates and ensnares other proteins, perhaps including the normal copy of PFN1. The mutant protein may also wreak havoc by virtue of weakened associations with its binding partners, he added, but he thinks toxic gain of function primarily drives ALS in these familial cases.
Kiaei noted that as part of the protein’s normal life cycle, PFN1 is tagged for disposal via ubiquitin. He speculated that the increased rigidity of the protein caused by the mutations may limit its efficient labeling and disposal.
In his lab and through a company he founded called Rock Gen Therapeutics, Kiaei is testing whether small molecules that stabilize PFN1 can prevent the toxic cascade. He is using a similar strategy to stabilize proteins that aggregate in other neurodegenerative diseases as well.
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