Diseases and Disorders

Amyotrophic Lateral Sclerosis: Membralin-Boosting Gene Modification

Srikar Chintala


Abstract 

Amyotrophic lateral sclerosis, or ALS, is a progressive nervous system disease that causes the death of motor neurons in the brain and spinal cord which leads to a loss of voluntary muscle control. Currently, there is no cure for this fatal disease. However, novel research has deduced that the deficiency of membralin proteins present in nerve cells may be the cause of death of the motor neurons in ALS. Since all proteins are encoded by genes, researchers can target and modify the CI9ORF6 gene that codes for the membralin protein. In doing so, astrocytes would no longer be deficient in membralin, possibly curing individuals of ALS.
 

Introduction

Amyotrophic lateral sclerosis (ALS) is a progressive, degenerative disease that destroys the nerve cells in control of voluntary muscle movement. These nerve cells are called “motor neurons”. Motor neurons initiate their pathway in the brain, travel through the brainstem, and signal responses to muscles that control voluntary and involuntary movement such as those in the arms, legs, chest, throat, mouth, etc [3]. When the motor neurons die, the muscles that are supposed to respond gradually atrophy.

Generally ALS is sorted into two categories: upper and lower motor neuron disease. In upper motor neuron disease, nerves of the brain are affected. In lower motor neuron disease, nerves from the brainstem are affected. Yet, regardless of the type of motor neuron disease, the motor neurons still become damaged and die. This is eventually fatal because most people with ALS die from respiratory failure, which occurs when people cannot get enough oxygen from their lungs into their blood or when they cannot properly remove carbon dioxide from their blood.  This happens because the disease can eventually lead to paralysis of the muscles that control breathing. Based on US population studies, a little more than 5,600 people in the United States are diagnosed with ALS each year. Some of the common symptoms include “weakness in muscles of the hands, arms or legs, impairment in the use of arms and legs, twitching and cramping of muscles, weakness in the muscles that control speech, swallowing, or breathing, and slurred speech” [3]. There is no current cure for ALS, but much research is being conducted to find the role of possible genetic and environmental factors. One of the novel research findings involves the discovery of the membralin protein present in nerve cells [2].

 

Current Research on Links Between Membralin and ALS

Until recently, researchers were not aware of the membralin protein’s function. Membralin (TMEM259) is an evolutionarily conserved endoplasmic reticulum (ER) membrane protein. It is a novel component of the ER-associated degradation machinery, the targeting of misfolded proteins and the subsequent degradation of those proteins [4]. It has been previously noted that membralin mutations or deficiency in mouse models induce ER stress, rendering neurons more vulnerable to cell death [5]. 

More recently though, researchers at Sanford Burnham Prebys in San Diego identified that membralin may play a key role in ALS. To understand the role of membralin in neurodegenerative diseases, the researchers designed mice that lacked this protein in all the cells of the body. Then, the scientists also designed mice that specifically lacked the protein in different brain cells, such as motor neurons and glial cells (cells that surround the neurons to provide support). These glial cells include astrocytes, microglia, and oligodendrocytes. The investigators found that lack of membralin in astrocytes led to a loss of motor neurons in the spinal cord and caused motor defects, mimics of the defects that could occur in people suffering from ALS. It is believed that the membralin deletion in astrocytes may result in the accumulation of neurotoxic components in the extracellular environment. More specifically, the astrocytes are associated with an elevation in extracellular glutamate and reduced EAAT2 expression. Astrocytes play an essential role in the homeostatic regulation of extracellular glutamate, and the glutamate transporter EAAT2, which is the primary mediator of extracellular glutamate uptake. Given glutamate’s excitotoxicity feature, an excess of glutamate can lead to the motor neuron damage present in ALS due to the overactivation of receptors for glutamate. Given that these are all features of membralin deletions in nerve cells, the researchers also ensured to test whether or not membralin expression could ameliorate lethality and the pathological effects associated with ALS. The membralin injected mice proved that membralin expression could reverse neurotoxic effects, extend lifespan, and reduce gliosis. Although these results are currently specific to mice, the researchers did find a strong correlation between EAAT2 expression and membralin levels in both human and mice models [2].  This proves that there is a good chance that such a treatment could also work for humans suffering from ALS.

 

Discussion

Given that membralin expression is vital in astrocytes towards reducing the death and damage of motor neurons, gene therapy targeted at boosting membralin in astrocytes could potentially reverse the effects of ALS. In the study discussed, researchers used a form of gene therapy (injecting membralin) towards reversing the neurotoxic effects. Although the results did show an extended lifespan and reduced symptoms, CRISPR might potentially be better for curing ALS. CRISPR is regarded as a powerful tool in gene editing because it has made gene modification or editing very simple. Unlike traditional gene therapy where additional copies of the normal gene are introduced into cells, CRISPR repairs the defects on site by removing the problematic DNA or correcting it to restore normal gene functions. CRISPR has proven to improve genome-wide targeting accuracy and, thus, function [6]. Sequence analysis has revealed that there are no closely related genes for membralin, suggesting that membralin represents the sole member of a unique protein family [7]. Thus, there must be a specific gene that encodes for membralin. This gene was discovered to be the human gene CI9ORF6 which localizes to chromosome 19p13.3 [7]. The lack of membralin in astrocytes is likely caused by defective ribosomes, which can be caused by several factors. However, most defects in ribosomes are generally caused by mutations in or damage to the rRNA. This rRNA would have to be encoded by the CI9ORF6 gene, which means this gene has to be mutated itself. Hence, CRISPR could target the mutated or damaged CI9ORF6 gene and repair the defects to result in normal gene functions. By having these normal gene functions, membralin would be expressed at its normal and maximum capacity which could potentially reverse the effects of ALS. However, before this occurs, further research has to be conducted to discover what specific nucleotides are mutated in the CI9ORF6 gene to attribute to a lack of membralin in astrocytes. Solely from this research could the potential impact of this type of treatment become more evident.


References


  1. Amyotrophic lateral sclerosis (ALS). Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/amyotrophic-lateral-sclerosis/symptoms-causes/syc-20354022. Retrieved: 03/01/21.

  2. Jiang, Lu-Lin, et al. (21/05/2019). Membralin deficiency dysregulates astrocytic glutamate homeostasis, leading to ALS-like impairment. The Journal of Clinical Investigation. https://www.jci.org/articles/view/127695. Retrieved: 03/01/21.

  3. ALS. Hospital for Special Surgery. https://www.hss.edu/condition-list_amyotrophic-lateral-sclerosis.asp. Retrieved: 03/01/21.

  4. TMEM259. National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/gene/91304#gene-expression. Retrieved: 03/01/21.

  5. Yang, Bo, et al. (05/15/2015). The critical role of membralin in postnatal motor neuron survival and disease. National Center for Biotechnology Information. https://pubmed.ncbi.nlm.nih.gov/25977983/. Retrieved: 03/01/21.

  6. (09/30/2019). Rationally engineered Staphylococcus aureus Cas9 nucleases with high genome-wide specificity. Proceedings of the National Academy of Sciences. https://www.pnas.org/content/116/42/20969. Retrieved: 03/01/21.

  7. Andersson, Gabriel and Gabriel von Euler. (12/11/2002). Characterization and expression of the gene encoding membralin, an evolutionary conserved protein expressed in the central nervous system. Gene Expression Patterns. https://www.sciencedirect.com/science/article/pii/S1567133X02000194?via%3Dihubl. Retrieved: 03/01/21.

Srikar Chintala

Srikar Chintala


Hi! My name is Srikar Chintala, and I am a junior in high school. I have a strong passion for anything science-related, and I am always challenging myself to learn more. Science has this magic to be able to continuously drive me towards working harder, and I absolutely live for that feeling! Outside of school and learning, I love to box, workout, play video games, and hang out with friends!