Key Takeaways:

  • This paper demonstrates that the profilin1 model recapitulates key ALS features and can be used as an effective tool for testing ALS treatments.
  • The profilin1 model can complement the SOD1 mouse model, which for decades has been the main animal model used to test ALS treatments.
  • Profilin1 mice demonstrate an element of human ALS not seen in the SOD1 mouse, TDP-43 pathology.
  • More animal models of ALS are essential to reflect the diversity of the disease in humans and find effective treatments for everyone with ALS.

For decades, the SOD1 G93A transgenic mouse model of ALS has been the primary tool used by ALS researchers for preclinical drug testing in animals. These mice are genetically altered to harbor multiple copies, or “transgenes,” of a mutant human SOD1 gene that is responsible for some cases of familial ALS. This genetic alteration causes mice to show symptoms that are similar to those in ALS patients and demonstrate a moderate disease progression allowing enough time for therapeutic intervention—two crucial characteristics for a successful disease model.

However, ALS is an extremely diverse disease with many different genetic and sporadic forms. Because of this, discovering new models to better reflect more disease forms has long been a leading priority of the ALS research community, including the ALS Therapy Development Institute (ALS TDI). Recently, ALS TDI researchers authored a paper, published in the journal Neurobiology of Disease, validating a newer mouse model of ALS with a transgenic version of the human ALS-related mutation in the profilin1 gene, marking a major milestone in this effort.

The Significance of a New Model of ALS

Dr. Theo Hatzipetros, ALS TDI’s Senior Director of Pharmacology, says that this paper was the result of more than five years of effort by ALS TDI scientists in collaboration with researchers from Brown University

“We’ve comprehensively characterized an ALS mouse model that, in many ways, matches the SOD1 model in recapitulating key features of ALS,” says Dr. Hatzipetros.  “Additionally, it also presents with TDP-43 pathology, a hallmark of ALS that the SOD1 model lacks.”

TDP-43 is an RNA binding protein, with many roles in cellular function that is normally found in the nucleus of cells. However, in the majority of ALS cases, TDP-43 accumulates abnormally in the cytoplasm of motor neurons, forming aggregates. This mislocalization and aggregation are thought to contribute to the development and progression of ALS symptoms. TDP-43 pathology has been found in both familial and sporadic ALS cases, as well as in frontotemporal dementia (FTD).

ALS patients with SOD1 or profilin1 mutations represent a relatively small number of the total ALS population. Gene therapies designed to directly target the biology of SOD1 or profilin1, hold significant potential for patients carrying these specific mutations and can be effectively tested in these models. Additionally, since they share common downstream disease mechanisms seen across ALS, these models can also serve as a valuable tools for screening drugs that may apply to the wider ALS population.

“With the SOD1 and PFN1 models we can now interrogate two different pathways that have been shown to play roles in ALS,” says Kaly Mueller, a Senior Associate Scientist at ALS TDI and lead author of the paper, “With the SOD1 model we can focus on oxidative stress and free radicals, and with profilin1 we can focus on cytoskeletal dynamics, giving us more target options for therapeutics.”

Characterizing the Profilin1 Mouse Model

Characterizing a mouse model involves assessing how the disease manifests both physically and biologically, as well as evaluating the variability of these changes between individual transgenic mouse. An ideal model should closely replicate key symptoms of the human disease, enabling researchers to determine whether potential treatments can reverse those symptoms. It must also have attributes suitable for reliable and efficient drug testing.

In the case of ALS, relevant model characteristics include:

  • Symptoms such as muscle weakness and paralysis, loss of neuromuscular function, reduction in motor neuron count, body weight loss, and progressive symptom worsening leading to reduced survival.
  • A disease onset and progression timeline that is neither too rapid nor too prolonged, and remains consistent across animals — ensuring mice live long enough to allow therapeutic testing while keeping experiments feasible within a practical timeframe.

Over five years, ALS TDI researchers closely observed and documented the behavioral characteristics and disease progression, as well as biomarkers such as neurofilament light chain (NfL) in the profilin1 mouse. They found that the model clearly demonstrated symptoms including weakness in the limbs, trouble walking, weight loss, and increased levels of NfL.

Additionally, the team conducted thorough histological testing, observing the microscopic structure of the model’s cells and tissues. These experiments confirmed the presence of key disease features including TDP-43 pathology and neuroinflammation. Finally, in collaboration with the lab of Gregorio Valdez at Brown University, the breakdown of neuromuscular junctions in this model was described.

This research was a large, time-consuming undertaking, utilizing over 250 individual mice. However, according to Dr. Hatzipetros, these efforts are essential to ensuring that the model consistently behaves as expected.

"If the model isn’t consistent each time you test a therapeutic, you can’t draw meaningful conclusions from the results.”, he says. “Characterizing a mouse model is painstaking work, but it’s a necessary investment to ensure the data you generate is reliable and informative."

Drug Testing in the Profilin1 Mouse

While these results indicated that the profilin1 mouse has the potential to be a useful tool in addition to its SOD1 counterpart, there are also some key differences that necessitate different approaches for their use. Perhaps the most significant is that, while the profilin1 mouse demonstrates many of the same symptoms as a SOD1 mouse, they take much longer to develop and progress more slowly. The life expectancy for profilin1 mice is, consequently, several months longer than SOD1 mice.

To help account for this and other differences in the two models, the ALS TDI team included several recommendations for other labs looking to use this model in their paper, to make their drug testing strategy more efficient. Still, this increased lifespan makes testing drugs in the model a longer and more expensive process. It is therefore unlikely that this model will replace SOD1 mice as the primary model for drug testing in ALS.

Profilin1 Mice at ALS TDI—and Beyond

With these considerations in mind, the ALS TDI pharmacology team is already beginning to adapt its drug testing strategy to utilize this new model, says Mueller.

“We may choose to retest some of the treatments previously evaluated in the SOD1 mouse model in the Profilin1 model,” she says. “If a treatment works in both models, it may have a better chance of translating to the clinic. If it’s effective in only one, it likely targets a pathway specific to either SOD1 or profilin1 and may not be suitable for all ALS patients.”

By publishing this paper, Dr. Hatzipetros and Mueller also say they hope that others will learn from their work and begin using this model to test their own potential treatments.

“As an institute committed to finding treatments for everybody with ALS,” says Dr. Hatzipetros, “it's our responsibility to do this work and share it, so others can build on what we’ve learned about this model and test their drugs effectively without having to recharacterize the models themselves. Even if we’re not the ones to discover a new drug using this model, if our work helps someone else get there, it still serves our mission.”

Additional Models of ALS

In addition to their work on the profilin 1 mouse, ALS TDI researchers are also working to validate other models of ALS that will further encompass the diversity of the disease.

“We're constantly working on the next model,” says Dr. Hatzipetros. “Currently, we're characterizing mice, and zebrafish, with TDP-43 and C9orf72 ALS-linked mutations. We're putting these through the same process of characterization that we did for the profilin1 model. The goal is the same—to assess whether they can be ideal models to test drugs and better reflect the diversity of the disease.”

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