For the first time, a compound was able to restore the health of diseased upper motor neurons — nerve cells that carry voluntary movement signals from the brain to the spinal cord — in the brain of amyotrophic lateral sclerosis (ALS) mouse models, a study found.
These results support the further development of this compound, called NU-9, which may help a broad spectrum of ALS patients and others with upper motor neuron degeneration.
The study, “Improving mitochondria and ER stability helps eliminate upper motor neuron degeneration that occurs due to mSOD1 toxicity and TDP‐43 pathology,” was published in the journal Clinical and Translational Medicine.
ALS is characterized by the death of motor neurons, including upper motor neurons — movement-initiating nerve cells in the brain that send messages from the brain to the spinal cord — and lower motor neurons, which send signals from the spinal cord to muscles.
A majority of ALS cases are caused by the buildup and clumping of one of two disease-causing proteins – SOD1 and TDP-43 – in motor neurons.
These comprise two distinct disease mechanisms that only rarely overlap. That is why developing therapies that improve the health of diseased upper motor neurons caused by these two different mechanisms may help a broad spectrum of ALS patients.
NU-9 is a compound that, in cell-based tests, reduced SOD1 aggregation, had low toxicity, and was able to cross the blood-brain barrier, which is a selective membrane that allows some substances into the brain but prevents others.
Researchers at Northwestern University in Illinois now have demonstrated that NU-9 was able to improve the health of upper motor neurons that degenerated due to SOD1 and TDP-43 toxicity in ALS mouse models.
“Even though the upper motor neurons are responsible for the initiation and modulation of movement, and their degeneration is an early event in ALS, so far there has been no treatment option to improve their health,” Hande Ozdinler, PhD, lead investigator, said in a press release. “We have identified the first compound that improves the health of upper motor neurons that become diseased.”
First, the team validated two mouse models for ALS, in which one carried a mutated version of human SOD1 (hSOD-G93A) and the other a variant of TDP-43 (TDP-43-A315T).
Both mouse models developed progressive loss of upper motor neurons and displayed similar defects at a cellular level, as seen in postmortem tissue samples from ALS patients. These defects included damage to the mitochondria, the energy-producing structures within cells, and the endoplasmic reticulum, the protein synthesis site.
Next, NU-9 was delivered to hSOD1-G93A mice (and healthy control mice) daily starting 60 days after birth, when mice began to show symptoms and upper motor neuron cellular defects. NU-9 treatment led to significant improvements in the structure and integrity of mitochondria and endoplasmic reticulum of diseased upper motor neurons.
The total number of mitochondria in the neurons of hSOD-G93A mice significantly increased after NU-9 treatment compared with untreated controls, to a level comparable with healthy mice. Furthermore, NU-9 treatment significantly increased the percentage of healthy mitochondria in upper motor neurons of these mice.
While normal mice did not show misfolded SOD1, upper motor neurons of hSOD1-G93A mice had high levels of misfolded SOD1. In contrast, NU-9 treatment significantly reduced levels of misfolded SOD1, especially in diseased motor neurons.
Most untreated hSOD1-G93A upper motor neurons show damaged and disintegrating dendrites, which are branch-like extensions from nerve cells that are key to proper function. NU-9 treatment significantly improved the integrity of dendrites in hSOD-1G93A mice in a dose‐dependent manner.
NU-9 treatment also prevented the loss of upper motor neurons in the motor cortex, the outer brain layer, whose numbers were significantly higher compared to untreated hSOD-1G93A mice. Importantly, the average number of upper motor neurons present in these mice’s motor cortex treated 60 days with NU-9 was almost comparable to those seen in healthy mice.
The team then turned their attention to mice with TDP-43 pathology. Like SOD1 mutant mice, TDP-43-A315T mice treated with NU-9 showed improvements in both mitochondria and endoplasmic reticulum of upper motor neurons.
Notably, the number of mitochondria after NU-9 treatment in TDP-43A-315T mice became comparable to that of healthy mice, with the average percentage of healthy mitochondria increased significantly by 87% when compared to diseased motor neurons.
Moreover, the average number of upper motor neurons in the motor cortex of TDP-43-A315T mice treated with NU-9 was increased significantly compared to untreated mutant mice, to levels comparable to those of healthy mice.
As both SOD1 and TDP-43 pathology impact lower motor neurons, the team investigated the lower spinal cords of the mutant mice. While there was no major loss of lower motor neurons in TDP-43-A315T mice, there was a reduction in the numbers of these neurons in hSOD1-G93A mice. However, NU-9 treatment was not sufficient to eliminate ongoing lower motor neurons degeneration in these animals, “revealing neuroprotective effects of NU-9 to be selective for UMNs [upper motor neurons],” the team wrote.
Finally, motor function tests were conducted to measure the impact of NU-9 treatment. While diseased hSOD1-G93A and TDP-43A-315T mice performed worse than healthy animals in the hanging wire test, in which mutant mice failed to grab and hold on to the wire as disease progressed, their performance became comparable to healthy mice after treatment with NU-9.
NU-9 treatment did not significantly improve the rotarod test, which measured endurance on a rotating rod, in hSOD1-G93A animals, but did so in TDP-43A-315T mice.
“Our findings mark the identification of the first compound that improves the health of diseased UMNs, and lay the foundation for future mechanism‐focused and cell‐based drug discovery studies,” the investigators concluded.
Before beginning clinical trials in humans, the team will focus on more detailed toxicology and properties of the potential medicine, they said.
“Improving the health of brain neurons is important for ALS and other motor neuron diseases,” Ozdinler added.
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