Role of Oligodendrocytes in Amyotrophic Lateral Sclerosis

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Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease which causes death of upper and lower motor neurons, leading to a progressive loss of motor functions [1]. The degeneration of motor neurons has been linked with the dysfunction of neuroglial cells such as oligodendrocytes. Oligodendrocytes main function is to provide support and insulation to the axons in the central nervous system (CNS), by creating the myelin sheat. They have been also found to provide metabolic support to the neurons by supplying energy metabolites such as glucose and lactate [2]. Since the most abundant transporter of lactate in oligodendrocytes is monocarboxylate transporter 1 (MCT1), their disruption results in axonal damage and therefore neural loss. It has been found that oligodendrocyte progenitors (NG2+ cells) have an enhanced proliferation and differentiation, but even though new oligodendrocytes are being formed, they fail to mature, resulting in a progressive demyelination of axons. Experiments in mouse models in which the mutated gene that results in ALS, SOD1, was selectively removed, lead to a delay in the disease onset and increased the survival of mice. This proposes that mutations are sufficient to produce death of motor neurons [3].

1.Oligodendrocyte progenitors (NG2+ cells)

1.1Genetic fate tracing of oligodendrocytes

When motor neurons are lost in SOD1 (G93A) mice, there are changes in behavior of NG2+ cells. NG2+ cells proliferate at about 20 fold faster than control mice by end stage of disease. To study how NG2+ cells proliferate throughout the disease, Kang et al [3] gave mice BrdU, which is a marker for proliferating cells, and measured the overall amount after 5 days, in the lumbar region of spinal cord. In control mice, the amount of NG2+ cells decreased with age, while in SOD1 (G93A) mice, NG2+ cells continued proliferating into adulthood. There was high proliferating rate in ventral gray matter before the disease onset and even a higher rate as the disease progressed. These results suggest that the behavior of NG2+ cells is being altered even before the onset of the disease, but they noticed that eventhough there is an increase in NG2+ cells their density is not enhanced before the end stage, due to the fact that old oligodendrocytes are dying and being replaced during early disease.
Furthermore, to determine the fate of NG2+ cells at early stage of disease, genetic lineage tracing was performed by giving 4-Hydroxytamoxifen (4-HT) to mice, to induce EGFP expression. Ninety days later there were more EGFP+ cells in the ventral gray matter of SOD1 (G93A) mice than in control, and by the end stage, the density of EGFP+ cells in SOD1 (G93A) mice was even higher than control mice.
By end stage disease, NG2+ cells proliferate and differentiate more often into oligodendrocytes in the ventral grey matter of SOD1 (G93A) mice. To determine which NG2+ cells go on differentiating into mature oligodendrocytes, they followed EGFP+ cells which were inmmunoreactive to antibody to APC (CC1), a marker of differentiated oligodendrocytes, in Pdgfra-creER-Z/EG-SOD1 (G93A) and Pdgfra-creER-Z/EG (control). In control mice there were few EGFP+ CC1+ cells, while there was a significant increase of newly generated oligodendrocytes in SOD1 (G93A) mice ventral gray matter. Eventhough there is an increase in oligodendrocytes in SOD1 mice, their density is the same as in control mice, which suggest there must be some death of old oligodendrocytes as the disease progresses.
Philips et al [1], also examined CC1 oligodendrocytes in ventral gray matter and noticed morphological changes in SOD1 (G93A) mice, which increased in number as the disease progressed. They went further into investigating if these morphological changes, such as the thickening of cell body and elongated reactive morphology, could predict the death of these oligodendrocytes. They performed double inmmunolabelling, CC1 and anti-cleaved caspase-3 (a marker for apoptotic cell death). In the spinal cord of SOD1 (G93A) mice, cleaved caspase-3 was present in CC1+ oligodendrocytes, but was not present in control mice. This demonstrated that oligodendrocytes are indeed involved in the disease progress.
Also, they followed oligodendrocytes as the disease progressed, to demonstrate that oligodendrocytes in SOD1 (G93A) spinal cord increased their turnover. PLP-YFP- SOD1 (G93A) transgenic mice were used. Proteolipid Protein (PLP) is a structural protein of myelin sheats and will be present only in mature oligodendrocytes. After treatment with tamoxifen, YFP in CC1+ was found in gray and white matter oligodendrocytes, but was not detected in NG2+ cells. Therefore CC1+ oligodendrocytes which are YFP- are the new oligodendrocytes from NG2+, which suggest there is an increase in the rate of production of new oligodendrocytes in PLP-YFP- SOD1 (G93A) mice.
To determine oligodendrocyte survival as the disease progressed, genetic tracing was performed in PLP-creER, ROSA26-EYFP, SOD1 (G93A) mice [1]. 4-HT was administered and the number of EYFP+ oligodendrocytes in the spinal cord at 40 and 70 days after labelling. In SOD1 (G93A) mice, EYFP+ oligodendrocytes were reduced by 20% at 40 days, and by end stage disease there was a further decrease that resulted in 65% reduction of the initial labelled oligodendrocytes. This demonstrated there is prominent loss of early born oligodendrocytes in SOD1 (G93A) mice ventral gray matter near motor neurons, as mice show signs of disease.
As oligodendrocytes degenerate in the spinal cord, some EGFP oligodendrocytes expressed active caspase-3 at end stage mice and the formation of EGFP+ clusters and active microglia; which are attracted by apoptotic cells. This shows that oligodendrocytes are dying via apoptotic cell death.
They wanted to know if the loss of motor neurons was enough to recruit NG2+ cells proliferation and differentiation. They proceeded into ablating motor neurons, but NG2+ cells did not proliferate, suggesting that the loss of motor neurons is not enough to recruit NG2+ cells.
To determine when oligodendrocytes start to show abnormalities, their morphology was examined in Mobp-EGFP, SOD1 (G93A) mice. In control mice, oligodendrocyte somata (EGFP+ Olig2+) in spinal cord has a nice oval shape, while SOD1 (G93A) mice at early and end stage have an irregularly shaped EGFP+ structures, lack nuclei and look like the fragments that result from apoptotic cell death. In spite of these differences EGFP+ Olig2+ density was the same as control mice. This demonstrates that pathological changes in oligodendrocytes is widespread and occur when mice start to show behavioral manifestations of the disease.
Similarly, Philips induced expression of YFP in NG2+ cells and their mature progeny in PDGFRa-YFP- SOD1 mice and control mice (PDGFRa-YFP). In the control group, about 20% of NG2+ cells were labelled, but there was a higher proliferation of NG2+ cells in SOD1 (G93A) mice. Also YFP+ CC1+ expression in SOD1 (G93A) mice was double than control, but most of these newly differentiated oligodendrocytes had altered morphology. Therefore this demonstrates that as the disease progresses, NG2+ cells generate new oligodendrocytes but they are dysmorphic.
Also the expression of myelin basic protein (MBP), myelin associated glycoprotein (MAG) and monocarboxylate transporter 1 (MCT1), was examined to investigate if the decrease in metabolic support and the increased rate of oligodendrocyte contributed to degeneration of motor neurons in SOD1 (G93A) mice. MCT1 and MBP were reduced in SOD1 (G93A) mice, and even mote decrease in MBP expression was seen in the ventral horn gray and white matter, which suggest there may be a defect in gray matter oligodendrocytes.
NG2+ cells proliferate and differentiate at higher rates when there is loss of oligodendrocytes. Data shows that oligodendrocytes degenerate in the spinal cord of SOD1 (G93A) mice. These mice present oligodendrocytes with cleaved caspase-3, suggesting these cells become apoptotic. There is increased generation of oligodendrocytes from NG2+ cells, but these cells are dysmorphic, and the expression of mature oligodendrocytes markers does not mean that the cell is functional, as with the expression of CC1+, myelin formation was compromised because there was a decrease in MBP.

2. Monocarboxylate Transporter 1 (MCT1) down regulation

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed pellentesque ullamcorper ante quis vehicula. Aenean massa odio, placerat eget vehicula id, porta eget mauris. In hac habitasse platea dictumst. Aenean suscipit pellentesque ante. Etiam vel nunc a justo laoreet congue. Phasellus at ullamcorper quam. In mattis sem blandit lectus pharetra ac feugiat nibh ornare. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Suspendisse a risus a lacus feugiat tempor. Morbi sed risus turpis, quis viverra arcu. Fusce vel erat libero. Nam nunc ante, laoreet at tincidunt quis, tempor vel lorem. Integer dapibus pulvinar est, sit amet ultricies orci posuere et. Mauris euismod sagittis sapien quis vulputate. Maecenas mollis massa ac nibh ultricies egestas. Etiam tincidunt, lorem sed bibendum ultricies, orci metus feugiat lacus, sit amet gravida ante mi sit amet metus.

2.1 Motor Cortex of ALS patients

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed pellentesque ullamcorper ante quis vehicula. Aenean massa odio, placerat eget vehicula id, porta eget mauris. In hac habitasse platea dictumst. Aenean suscipit pellentesque ante. Etiam vel nunc a justo laoreet congue. Phasellus at ullamcorper quam. In mattis sem blandit lectus pharetra ac feugiat nibh ornare. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Suspendisse a risus a lacus feugiat tempor. Morbi sed risus turpis, quis viverra arcu. Fusce vel erat libero. Nam nunc ante, laoreet at tincidunt quis, tempor vel lorem. Integer dapibus pulvinar est, sit amet ultricies orci posuere et. Mauris euismod sagittis sapien quis vulputate. Maecenas mollis massa ac nibh ultricies egestas. Etiam tincidunt, lorem sed bibendum ultricies, orci metus feugiat lacus, sit amet gravida ante mi sit amet metus.

2.2 Spinal Cord of SOD1 mice

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed pellentesque ullamcorper ante quis vehicula. Aenean massa odio, placerat eget vehicula id, porta eget mauris. In hac habitasse platea dictumst. Aenean suscipit pellentesque ante. Etiam vel nunc a justo laoreet congue. Phasellus at ullamcorper quam. In mattis sem blandit lectus pharetra ac feugiat nibh ornare. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Suspendisse a risus a lacus feugiat tempor. Morbi sed risus turpis, quis viverra arcu. Fusce vel erat libero. Nam nunc ante, laoreet at tincidunt quis, tempor vel lorem. Integer dapibus pulvinar est, sit amet ultricies orci posuere et. Mauris euismod sagittis sapien quis vulputate. Maecenas mollis massa ac nibh ultricies egestas. Etiam tincidunt, lorem sed bibendum ultricies, orci metus feugiat lacus, sit amet gravida ante mi sit amet metus.

3. Selective removal of mutant SOD1

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed pellentesque ullamcorper ante quis vehicula. Aenean massa odio, placerat eget vehicula id, porta eget mauris. In hac habitasse platea dictumst. Aenean suscipit pellentesque ante. Etiam vel nunc a justo laoreet congue. Phasellus at ullamcorper quam. In mattis sem blandit lectus pharetra ac feugiat nibh ornare. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Suspendisse a risus a lacus feugiat tempor. Morbi sed risus turpis, quis viverra arcu. Fusce vel erat libero. Nam nunc ante, laoreet at tincidunt quis, tempor vel lorem. Integer dapibus pulvinar est, sit amet ultricies orci posuere et. Mauris euismod sagittis sapien quis vulputate. Maecenas mollis massa ac nibh ultricies egestas. Etiam tincidunt, lorem sed bibendum ultricies, orci metus feugiat lacus, sit amet gravida ante mi sit amet metus.

Bibliography
1. first full source reference
2. second full source reference

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