One of the most devastating diseases in existence is amyotrophic lateral sclerosis (ALS), an incurable, progressive nervous system disease that affects nerve cells in the brain and spinal cord, causing loss of muscle control, is incurable.
Often called Lou Gehrig’s disease after the American baseball player who was diagnosed with it, the cruel disorder affects as many as 30,000 in the US alone, many of them young adults at diagnosis, with 5,000 new cases diagnosed each year. Estimates suggest that ALS is diagnosed in one out of 400 people and responsible for as many as five of every 100,000 deaths in people aged 20 or older.
Early symptoms of ALS include stiff muscles, muscle twitches and gradual increasing weakness and muscle wasting. Half of the victims develop at least mild difficulties with thinking and behavior, and about 15% develop frontotemporal dementia. Most people experience pain.
The affected muscles are responsible for chewing food, speaking, and walking. Motor neuron loss continues until the ability to eat, speak, move, and finally the ability to breathe is lost. ALS eventually causes paralysis and early death, usually from respiratory failure. The average life expectancy after diagnosis of victims is currently only about three years.
But now, a research group from the Sackler Faculty of Medicine and the Sagol School of Neuroscience at Tel Aviv University (TAU) has for the first time uncovered the biological mechanism causing nerve destruction in ALS.
The groundbreaking study, led by Prof. Eran Perlson and doctoral students Topaz Altman and Ariel Ionescu, suggests that the course of this fatal disease can be delayed and even reversed in its early stages. It was conducted in collaboration with Dr. Amir Dori, director of the clinic for neuro-muscular diseases at Sheba Medical Center at Tel Hashomer (near Tel Aviv).
The study is an international collaboration with leading scientists from Germany, France, England and the US, with the assistance of Tal Gardus Perry and Amjad Ibraheem from Perlson’s lab.
The results of the study were published in the prestigious journal Nature Communication under the title “Axonal TDP-43 condensates drive neuromuscular junction disruption through inhibition of local synthesis of nuclear encoded mitochondrial proteins.”
“To this day, it is unclear what causes the disease,” said Perlson. “Only about 10% of the patients carry a familial background with known genetic mutations, but the remaining 90% are a mystery. The paralysis caused by the disease results from damage to the motor neurons, which leads to the degeneration nerve endings and to the loss of muscle innervation. This consequently leads to the degeneration of the nerve and the death of motor neurons in the spinal cord, however until now we could not understand the basic biological mechanism causing the initial damage behind this vicious cascade.”
To solve the mystery, the TAI researchers focused on a protein called TDP-43, which had been shown in earlier studies to accumulate in unusual amounts and localization in the brains of about 95% of all ALS patients. The team revealed a novel biological link between the protein’s accumulation and the degeneration of the synapses between the motor neuron endings and the muscles called neuromuscular junctions that translate neural commands into physical movements.
Taking muscle biopsies from ALS patients, the researchers found that the toxic protein accumulates also in high proximity to these neuromuscular junctions during the early stages of the disease and before patients develop any serious symptoms. In a series of experiments performed by the researchers, both in cells of ALS patients and in genetically modified model animals, they discovered that the accumulation of the TDP-43 protein in the neuromuscular junction inhibits the ability to locally synthesize proteins that are essential to mitochondrial activity, which provides the power of fundamental cellular processes.
The dysfunction of mitochondria in nerve terminals leads to neuromuscular junction disruption and ultimately to the death of the motor neurons. “It’s important first to understand the spatial complexity of motor neurons,” continued Perlson.
“The motor neurons are found in the spinal cord and need to reach every muscle in the body to operate it. One can imagine, for example, an extension cable coming out of the spinal cord and reaching the muscles in the little toe in our foot,” he said. “These extensions can be as long as one meter in adults and are called axons. In earlier studies, we have shown that to maintain this complex organization motor neuron axons require an increased amount of energy, particularly in the most remote parts, the neuromuscular junctions.”
In the current study, they focused on a pathological change in TDP-43 protein that takes place in these axons and at neuromuscular junctions. In a normal motor neuron, this protein is mainly found in the nucleus. They showed that in ALS this protein exits the nucleus and accumulates throughout the entire cell and particularly in the neuromuscular junction.
As the function of motor neurons depends on these neuromuscular junctions located on the remote end of the “extension cable,” the researchers realized that this finding could be of critical importance. “We discovered that the accumulations formed by the TDP-43 protein in neuromuscular junctions trap RNA molecules and prevent the synthesis of essential proteins to mitochondrial function. Mitochondria are organelles found in cells and are the main energy providers for numerous cellular processes, including neural transmission,” Perlson went on. “he condensation of TDP-43 protein in neuromuscular junctions results in a severe energy depletion, prevents mitochondrial repair and consequently leads to the disruption of these junctions, degeneration of the entire ‘extension cable’ and to the death of motor neurons in the spinal cord.”
To confirm their findings, they decided to use an experimental molecule recently published by a group of researchers from the US for developed for another purpose – enhancing neural regeneration after injury by the disassembly of protein condensates in neural extensions. The researchers proved that this molecule could also disassemble the axonal TDP-43 protein condensates in cells from ALS patients and that this process improved the ability to produce essential proteins, enhanced mitochondrial activity, and prevented neuromuscular junction degeneration.
In addition, the researchers showed in model animals that by reversing TDP-43 accumulation in nerves and neuromuscular junction enabled recovery of degenerated neuromuscular junctions and to rehabilitate the diseased model animals almost completely.
“The moment we induced the disassembly of TDP-43 protein condensates, the nerves’ ability to produce proteins was recovered, particularly the synthesis of proteins essential to mitochondrial activity. All this made it possible for the nerves to regenerate,” concluded Perlson.
“We were able to prove, through pharmacological as well as genetic means, that motor nerves can regenerate and that patients can have hope. In fact, we located the basic mechanism, as well as the proteins responsible for the disruption of the nerves from the muscles and for their degeneration. This discovery can lead to the development of new therapies that could either dissolve the TDP-43 protein condensates or increase the production of proteins essential to mitochondrial function, and thereby heal the nerve cells before the irreversible damage that occurs in the spinal cord. We are tackling the problem on the other end – in the neuromuscular junction. And if in the future we could diagnose and intervene early enough, maybe it will be possible to inhibit the destructive degeneration in ALS patients’ muscles.”
