Stem cell-derived motor neurons from people with familial amyotrophic lateral sclerosis (ALS) have mutation-specific alterations in glutamate receptors and calcium signals, which may alter nerve signaling and play a role in the disease, a study found.
These findings may lead to future therapeutic strategies for ALS that are best tailored to each patient population, the researchers say.
The study, titled “Altered calcium dynamics and glutamate receptor properties in iPSC derived motor neurons from ALS patients with C9orf72, FUS, SOD1 or TDP43 mutations,” was published in the journal Human Molecular Genetics.
ALS is marked by the loss of motor neurons, the specialized nerve cells that send electrical signals from the central nervous system — the brain and spinal cord — to the muscles, controlling their function. Until now, the reason why the motor neurons of ALS patients start to die is unclear.
One of the possibilities is the excitotoxicity theory, which is based on the accumulation of toxic levels of glutamate — one of the most important excitatory neurotransmitters in the central nervous system.
Scientists believe that elevated glutamate leads to overexcitation and calcium overload of motor neurons, leading to their death.
Glutamate is one of the messengers responsible for nerve signaling. It is produced by certain nerve cells and sent as signal that will pass on a message to other nerve cells. It exerts its effects by binding to specific receptors on nerve cells. When this happens, these receptors let in large amounts of calcium ions, an important mediator of glutamate’s effects inside nerve cells.
The neurotransmitter can become toxic in two general ways. First, too much glutamate leads to the overstimulation (overexcitement) of the receiving nerve cell. Second, the receptors on the receiving nerve cell can be oversensitive, so that low glutamate amounts are enough to excite that nerve cell.
Riluzole, the first FDA-approved treatment for ALS — sold as Rilutek, by Sanofi, and as Tiglutik by ITF Pharma — is believed to work by inhibiting the release of glutamate and preventing calcium overload within motor neurons.
However, there have been contradictory results showing that nerve cell stimulation can be protective against ALS. That raises the question of whether approaches like riluzole are beneficial — or if it’s better, on the contrary, to induce nerve cell activity.
To gather more insights on this question, researchers now examined the molecular alterations linked with glutamate signaling present at the motor neurons of people with familial ALS.
Specifically, they looked at glutamate receptor properties and calcium dynamics in motor neurons derived in laboratory cultures (petri dishes) from patients’ pluripotent stem cells (iPSC). Pluripotent stem cells are cells that can develop into nearly all cells of the adult body,
When analyzed, these motor neurons were found to carry common familial ALS mutations. These were specifically found in genes C9orf72 (four cell lines), FUS (nine), SOD1 (three) or TARDP (TDP-43 protein) (three). The researchers compared them with motor neurons from healthy controls (seven cell lines).
Using calcium-specific fluorescent dyes, the researchers imaged the calcium dynamics inside each of these motor neuron lines under the microscope, finding several mutation-specific alterations.
Motor neurons with a C9orf72 mutation had spontaneous transient elevations of calcium more frequently, while TARDP-mutated neurons showed higher basal levels of calcium, compared with healthy controls.
To analyze glutamate receptor properties, the researchers also stimulated motor neurons with agents specific to different types of receptors. With this approach, they saw that TDP-43 mutant neurons had greater calcium peaks, and responded more frequently to AMPA and metabotropic receptors, respectively, compared with other mutations and healthy controls.
Additionally, neurons with FUS mutations produced more AMPA and kainate receptors than healthy neurons. The expression of kainate receptors and voltage gated calcium channels in mutant C9orf72, as well as metabotropic receptors in SOD1 mutant neurons, also was significantly elevated compared with controls.
“In conclusion, our iPSC study showed mutation-specific alterations of glutamate receptor properties and calcium dynamics in ALS patient-derived MNs [motor neurons] that might contribute to the development of novel translational strategies with individual stratification of neuroprotective ALS therapies,” the researchers concluded.