ALS, also known as Lou Gehrig’s disease, affects over 100,000 people worldwide. Currently, there are no effective treatments, largely due to a poor understanding of how the disease begins and progresses at the molecular level, so it is invariably fatal. Among the pivotal cellular phenomena, the researchers noted that the motor neurons of ALS patients contain excessive aggregation of a protein called TDP-43. Because the TDP-43 proteins blocked in these aggregates cannot perform their normal function, scientists believe this accumulation contributes to motor neuron degeneration, the hallmark of ALS. A few years ago it was understood why these aggregates appear: it is cellular stress due to the formation of free radicals (ROS). Although the formation of these intermediates is a normal phenomenon in all cells, in the case of ALS, scientists suspect that there are environmental toxins that over time lengthen the production of ROS, eventually causing the aggregation of the mutated TDP-43 protein.
It is possible, according to experts, that there is a threshold for which small aggregates of TDP-43 are not relatively toxic to motor neurons; it is only the passage of time that enlarges the aggregates and makes them a problem for the nerve cells. Furthermore, even after stress has been relieved, the agglutination of TDP-43 persists in ALS motor neurons, but not in healthy neurons. The team then analyzed and identified chemical compounds that prevent this persistent accumulation of TDP-43 induced by cellular stress. These compounds also increased neuron survival with TDP-43 proteins containing an ALS-associated mutation. Normally, TDP-43 proteins help process cellular messenger RNAs, which serve as genetic blueprints for making proteins (protein synthesis). But when they aggregate outside the nucleus, TDP-43 proteins cannot perform their normal duties and this can have a profound effect on many cellular functions. The first effect is that the production of specific cellular proteins cannot occur, since the RNAs are not processed.
By generating motor neurons from induced pluripotent stem cells (iPSCs) that had been converted from human skin cells, a team from the University of San Diego, California, a few years ago, to mimic the cellular aspects of ALS exposed these laboratory motor neurons to toxins. which stressed the cells. After the exposure, the cellular entanglements of TDP-43 appeared. Among the toxins was the old antibiotic puromycin, which also inhibits protein synthesis and triggers the so-called unfolded protein response (UPR). Researchers have tested thousands of chemical compounds for their effects on protein aggregation with RNAs. They were surprised to find compounds that not only reduced the overall amount of aggregation by up to 75%, but also varied the size and number of aggregates per cell. Some of the compounds tested were molecules with extensive aromatic regions (arms that allow them to insert into nucleic acids). TDP-43, in fact, must bind RNA to join the protein tangles associated with ALS.
This too is previously unrecognized information: TDP-43 aggregates are not just proteins, but trap partner RNAs that must be translated into proteins. According to the scientists, these compounds expand information to unravel the relationship between RNA-protein aggregations and neurological diseases. Many proteins that bind RNAs, in fact, use hydrophobic portions (called RRM, with the amino acids phenylalanine, glycine, valine and tryptophan) to pair with the bases of their nucleic acid. Therefore, it makes sense that a compound that interacts with RNA prevents TDP-43 from aggregating: it would act as a competitive base for base pairing. And the same information could open the basis for the synthesis of aromatic-structured drugs that prevent TDP-43 from aggregating and sequestering cellular RNAs away from normal protein synthesis. But hydrophobicity is apparently a problem that also afflicts the main mutant protein of ALS: superoxide dismutase (SOD1).
Researchers at Sastra Deemed University in India have verified that the SOD aggregates that occur in the disease are due to the fact that the mutation causes the protein to change its solubility: from hydrophilic it becomes more hydrophobic, and it is precisely the hydrophobic regions of the protein a cause the appearance of aggregates similar to those of TDP-43. Incidentally, SOD1 is an enzyme responsible for the removal of certain cellular ROS, a function that appears to be lost following the mutation. The problem is that it’s unclear whether it’s the SOD1 mutation that triggers her aggregation or it’s the normal cellular ROS that over time do this for her. Incidentally, this modification is associated with the formation of an internal molecular bridge to the protein that should not exist: a disulfide bridge (S-S) that simulates both the oxidation of the protein and reduces its solubility. To reverse the problem, there are research groups that are investigating chemical compounds that can act as selective antioxidants or change the “diseased” conformation of SOD1.
Particularly selective would be circular, enlarged-ring sulfur compounds, which seem to prefer to attach themselves to residue 111 of the protein. A prototype of these substances is called 1,2-dithian-1-oxide, which opens and forms a bridge between two mutant SOD1 monomers and prevents them from irreversibly aggregating. This compound was tested in 2018 by Dr. Donnelly’s group at the University of A second compound that would do the same job is called ebselen: it is an antioxidant molecule with an enzymatic-like action because it contains a selenium atom. This also opens up like the previous compound and prevents the diseased protein from clumping together. Not only that, ebselen contains two aromatic groups, which would make it a potential drug to target TDP-43 aggregates as well. A stroke of luck, defined as “killing two birds with one stone”. This possibility is being investigated by two separate research groups.
One of them, from the University of Arizona, led by Professor Khanna, has identified a partially aromatic compound (nTRD22) that prevents the TDP-43 protein from aggregating with a head-tail mechanism. This prevented the mutated protein from sequestering its partner RNA and improved motor coordination in an ALS model in Drosophila. A limitation that has delayed the engineering of molecules against ALS is the fact that, unfortunately, both SOD1 and TDP-43 do not have protein “pockets” in which to accommodate molecules of a certain diameter. Possibly this is also one of the factors for which the molecules tested to date are partially effective, but do not have a great selectivity. This is prompting experts to consider the underlying cellular processes of ALS as more worthy of targeting. The metabolic alterations, oxidative stress and inflammation that are known to intervene in the pathogenetic process seem to be more easily attacked.
Also because many proteins belonging to these processes have a structure in which there are molecular “pockets” and others have molecules or drugs already known and / or validated.
- edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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