By Anna Truong, Neurobiology, Physiology, and Behavior, ’22
Author’s Note: I wrote this piece of work for an assignment through my UWP 104F course, and felt very connected with it. I decided my topic to be about a disease known as ALS because my father was diagnosed when I was at a young age. At the age of nine, I did not understand the gravity of becoming sick, and how much the world can change when someone important in your life passes away. I did not understand how impactful a disease was until I had the experience as a family member. ALS became a topic of interest to me since then from class presentations about interesting scientific topics to college research papers and literature reviews. This literature review is something that I am proud of because it encompasses ALS as the disease that has involved me and my family. From this review, I hope readers learn more about ALS and how the current research can pave a way for future research in the treatment of ALS.
Introduction
Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease that is characterized by progressive degeneration of motor neurons in the spinal cord and brain [1-5]. Motor neuron degeneration inhibits the ability of the brain to send signals to the muscles to control movement. There are two types of motor neurons responsible for this communication: the upper and lower motor neurons. Lesions in upper motor neurons prevent the signal cascade to the lower motor neurons that send another signal responsible for muscle movement. This can lead to muscular atrophy, paralysis, and eventually death [1-5]. Various injuries such as damage to the spinal cord or strokes, as well as other factors like oxidative stress induced by free radicals, contribute to the destruction of motor neurons [6]. Approximately 5 of every 100,000 individuals will be affected by ALS and the average life expectancy after diagnosis is between 2-5 years [2].
Many studies have focused on identifying the cause for motor neuron cell death and the genes involved in the development of the disease. Although bodily mechanisms by which motor neurons degenerate remain unclear, they are thought to encompass a non-cell autonomous process [3]. The purpose of this literature review is to analyze the current and potential treatments that can be effective toward individuals experiencing ALS. This article will focus on a current drug treatment called Edaravone, followed by potential treatments, astrocyte-based therapy and cell-based therapies.
Drug Treatments
Edaravone is a free radical-scavenging drug that functions to protect motor neurons from free radicals and oxidative stress damage in the central nervous system (CNS) [2,6]. Edaravone effectively acts on oxidative stress by reducing the number of free radicals to slow disease progression. With the absence of a cure, such treatment options have mainly contributed to prolonged survival [6].
ALS Functional Rating Scale
In this section, we will analyze the effect of Edaravone on disease progression through scoring of motor function by the revised ALS Functional Rating Scale (ALSFRS-R). The ALSFRS-R is an instrument designed for the clinical evaluation of functional status of ALS patients and efficacy of clinical trials [6]. It measures 12 aspects of physical function such as swallowing, breathing, and walking, scoring functioning ability from 4 (normal) to 0 (no ability) with a maximum total score of 48 and a minimum of 0.
Edaravone treatment on ALS patients
During normal disease progression, it is assumed that decline in functioning scores is almost linear [6]. When comparing ALSFRS-R scores between patients who received either placebo treatment or Edaravone treatment, there was a significantly faster decline in functional scores for those who received the placebo. This indicates a considerable loss in the ability to perform everyday tasks [2]. In conjunction with these results, a further study has shown greater improvements in ALSFRS-R scores for patients after beginning Edaravone treatment compared to the pre-treatment period [7]. The pre-treatment period lost an average of 4.7 points on the ALSFRS-R whereas the treatment period showed a smaller average loss of 2.3 points over the same time duration [7]. This indicates possible clinical efficacy for Edaravone due to its ability to effect a more gradual decline.
In addition, compared to placebo, Edaravone remains effective for up to a year, after which survival rates start to decline [2]. Edaravone’s effectiveness is also more prevalent in the early stages of ALS progression, but long-term effects of Edaravone are not yet fully evaluated so results past a year are unclear. Further limitations to these studies, including a nonlinear difference in decline between functional rates of early stages of ALS and end stages of ALS, require more research before affirming the long-term health benefits through Edaravone [2,6,7]. Therefore, as a marketed drug, it is difficult to be sure of its full effectiveness from the lack of positive results in life expectancy of the target population. On the other hand, no detrimental effects or worsening of symptoms due to Edaravone were analyzed during patient trials besides a few side effects including bruising, headaches, and hypoxia [6]. Due to these factors, Edaravone remains a partially beneficial drug.
Potential Therapeutic Options
Although Edaravone’s effectiveness is still actively being deciphered, there have been studies on whether other types of cellular targets within the brain and stem cells, such as astrocytes, could help slow down or halt disease progression and thus be effective treatments for ALS [1,3,4]. Astrocytes are a type of glial cell within the CNS that is inflamed under the diseased state [1]. Most of the following research involves the SOD1-G93A transgenic mice expressing the human SOD1 gene with G93A mutation. It is an important mouse model for studying ALS as it presents many of the pathological symptoms experienced by patients, including motor impairment and motor neuron death, allowing for an analogous simulation [1].
In the current state of medication development, the SOD1-G93A transgenic mice are utilized for their relation to astrocytes, a promising target for effective treatments. The increasing number of studies performed on astrocytes show them to be crucially involved in ALS through their influence on motor neuron fate and disease progression. The studies discussed will present multiple experiments on the SOD1-G93A transgene, and explain how elimination and/or alteration of this gene can help slow prominent signs of disease and extend lifespan [1,5].
Role of Astrocytes in ALS
Upon a specific signal within the CNS, astrocytes can transform into either their reactive A1 state characterized by promotion of neurodegeneration and toxicity or their neuroprotective A2 state which promotes healing and repair of injury [3,9]. Among ALS patients, the reactive A1 astrocytes are dominant along with the mutant transgenic SOD1-G93A, contributing toxic components that participate in ALS pathology [1].
Amongst these pathologies, researchers investigate neuroinflammation, characterized by inflammation of the nervous tissue, to prove its benefits for minimizing reactive astrocytes [1]. Neuroinflammatory stimuli like lipopolysaccharide (LPS) lead to a signal transduction cascade that can secrete immunologically active molecules like IL-1α, TNFα, and C1q that transform resting astrocytes to their neurotoxic A1 state [1]. These reactive astrocytes will lose regular functions and secrete factors toxic to neurons [1,3]. Moreover, isolated astrocytes from ALS patients were found to be toxic to healthy, cultured motor neurons [3]. This indicates the involvement of astrocytes in motor neuron death that can lead to a progressive decline of motor function [3]. Lowering the prevalence of neuroinflammation may contribute to a decrease in motor neuron death, and therefore delayed progression of ALS.
Healthy individual’s communication between the motor neuron, astrocytes, and immune cells compared to those of an ALS individual
Astrocyte-Based Therapy
To minimize neuroinflammatory effects of ALS, Guttenplan et al. determined that knockout, or the genetically modified absence of IL-1α, TNFα, and C1q in SOD1-G93A mice did not produce any reactive astrocytes [1]. This triple knockout was also linked to the possibility of neuroinflammatory reactive astrocytes becoming a therapeutic target for ALS. The triple knockout mice presented with lower levels of reactive astrocyte marker C3 and had a significantly extended lifespan of over 50% compared to regular SOD1-G93A mice [1]. Treatments that implement this mechanism of lowering neuroinflammation can contribute to a turning point in increasing efficacy rates of therapies involved in ALS.
In addition, diagnosis is primarily followed after the presence of symptoms [1]. An approach to treatment included restoring normal functionality of endogenous astrocytes through the transplantation of healthy astrocytes in patients [3]. These transplanted healthy astrocytes can provide neuroprotection through reduction of misfolded proteins in motor neurons. However, they can also transform into neurotoxic A1 astrocytes when in the diseased environment of the CNS [3]. The mechanisms through which transplanted astrocytes act continue to be thoroughly understood, yet provide a promising target for an ALS targeted therapy [1,3]. Delay in disease progression may be more effective with a combination of therapies attacking both reactive astrocytes and motor neurons compared to individual therapies [1].
Cell-Based Therapy
Another approach that has been studied as a potential therapeutic target for ALS is through stem cells. Mesenchymal stem cells (MSCs) are adult multipotent precursors that can be prompted to release neurotrophic factors released by A2 astrocytes and have shown to be beneficial in the regeneration of healthy cells [3]. Transplantation of the same individual’s MSCs induced to secrete neurotrophic factors showed early signs of safety and treatment effectiveness [3].
Furthermore, a specific stem cell therapy “Neuro-Cells”, a combination of MSCs and hematopoietic stem cells (HSCs), along with anti-inflammatory measures was administered to both SOD1-G93A mice and FUS-tg mice, a variant of the standard SOD1-G93A strain [4]. In SOD-1 and FUS-tg, inflammation contributes to disease progression, allowing for comparison investigation in these two mutations [4]. When tested on rats subjected to spinal cord injury, this mixture had an anti-inflammatory effect, thus improving motor function and decreasing concentrations of proinflammatory cytokines in the cerebrospinal fluid [4]. Muscle degeneration among FUS-tg mice was also compared during Neuro-Cell injections. Muscular atrophy was noticed to be partly rescued by the mixed stem cell therapy. To verify these results, Neuro-Cells were administered to SOD1-G93A mice. Results showed an indication of improved motor function similar to that of the FUS-tg mice, thus providing further evidence of disease counteraction [4]. These signs of efficacy and preclinical studies of transplantation of MSCs and HSCs are indicators of beneficial treatments from the usage of stem cells through reduction of motor neuron death, prolonged survival and improved motor performances [3]. Coincidingly, according to de Munter et al., stem cell therapy should be utilized as a part of the cell-based treatment of ALS due to the knowledge present already in this field [4]. Even so, more research is needed to define the anti-inflammatory mechanisms in ALS pathology and other effects that “Neuro-Cells” have on ALS.
Possible stem cell therapies that could be used to treat ALS
Discussion/Conclusion
While there are currently effective drug treatments available for ALS, there is still research being conducted on these drugs to better ensure quality and effectiveness. Edaravone’s ability to slow disease progression remains minimal or ineffective to patients who are past the beginning stages of ALS progression, and toward end-stage ALS, respectively. Decline in patients experiencing ALS occurs non-linearly with a rapid decline toward the end-stage, and so clinical effects of Edaravone may not be beneficial. Its therapeutic effects are yet to be better understood and whether or not their effectiveness is due in part to the drug. Although it is currently being used as a treatment option, Edaravone could be further improved for efficacy.
In association, potential treatment options of astrocyte-based therapies and cell-based therapies play an important role in the future of ALS. Targeting astrocytes and neuroinflammation, and utilizing stem cell therapy can provide benefits to slowing disease progression. However, much like the current drugs, there is still much to understand about other subpopulations of astrocytes and stem cells that could contribute to ALS pathology. The intertwined participation of therapies is important to note as it could provide greater benefits to patients seeking out future treatments. Although options of treatment are currently limited, these studies suggest potential therapeutic approaches that can be optimized to halt or slow disease progression. Currently, stem cells are encouraged to be part of treatment in ALS patients, suggesting its potential in reducing inflammation and therefore can be highly effective in minimizing motor neuron death. Additionally, astrocytes are becoming a major direction in the studies of ALS, due to its direct involvement in motor neuron death associated with the disease. Astrocytes may become the center of research in the near future and lead to a more efficient slowing of disease progression compared to the currently approved drugs. With more studies, the cellular mechanisms contributing to the deterioration of motor neurons involved in ALS can lead to promising treatments with greater efficacy against the disease. In due time, we can hope to see an increase in the average life expectancy of 2-5 years to much longer.
References:
- Guttenplan, K. A., Weigel, M. K., Adler, D. I., et al. (2020). Knockout of reactive astrocyte activating factors slows disease progression in an ALS mouse model. Nature communications, 11(1), 3753. https://doi.org/10.1038/s41467-020-17514-9
- Shefner, J., Heiman-Patterson, T., Pioro, E. P., Wiedau-Pazos, M., Liu, S., Zhang, J., Agnese, W., & Apple, S. (2020). Long-term edaravone efficacy in amyotrophic lateral sclerosis: Post-hoc analyses of Study 19 (MCI186-19). Muscle & nerve, 61(2), 218–221. https://doi.org/10.1002/mus.26740
- Izrael, M., Slutsky, S. G., & Revel, M. (2020). Rising Stars: Astrocytes as a Therapeutic Target for ALS Disease. Frontiers in neuroscience, 14, 824. https://doi.org/10.3389/fnins.2020.00824
- de Munter, J., Shafarevich, I., Liundup, A., et al. (2020). Neuro-Cells therapy improves motor outcomes and suppresses inflammation during experimental syndrome of amyotrophic lateral sclerosis in mice. CNS neuroscience & therapeutics, 26(5), 504–517. https://doi.org/10.1111/cns.13280
- Apolloni, S., Amadio, S., Fabbrizio, P., Morello, G., Spampinato, A. G., Latagliata, E. C., Salvatori, I., Proietti, D., Ferri, A., Madaro, L., Puglisi-Allegra, S., Cavallaro, S., & Volonté, C. (2019). Histaminergic transmission slows progression of amyotrophic lateral sclerosis. Journal of cachexia, sarcopenia and muscle, 10(4), 872–893. https://doi.org/10.1002/jcsm.12422
- Luo, L., Song, Z., Li, X., Huiwang, Zeng, Y., Qinwang, Meiqi, & He, J. (2019). Efficacy and safety of edaravone in treatment of amyotrophic lateral sclerosis-a systematic review and meta-analysis. Neurological sciences : official journal of the Italian Neurological Society and of the Italian Society of Clinical Neurophysiology, 40(2), 235–241. https://doi.org/10.1007/s10072-018-3653-2
- Sawada H. (2017). Clinical efficacy of edaravone for the treatment of amyotrophic lateral sclerosis. Expert opinion on pharmacotherapy, 18(7), 735–738. https://doi.org/10.1080/14656566.2017.1319937
- Liu, J., & Wang, F. (2017). Role of Neuroinflammation in Amyotrophic Lateral Sclerosis: Cellular Mechanisms and Therapeutic Implications. Frontiers in immunology, 8, 1005. https://doi.org/10.3389/fimmu.2017.01005
- Liddelow, S., & Barres, B. (2015). SnapShot: Astrocytes in Health and Disease. Cell, 162(5), 1170–1170.e1. https://doi.org/10.1016/j.cell.2015.08.029