Background

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease that is caused by ongoing degeneration of motor neurons due to the expression of a mutant gene known as the mutant superoxide dismutase 1 (SOD1).¹¹ This gene functions as an antioxidant enzyme by guarding the cells from reactive oxygen species toxicity.⁷ Mutations in this gene loses the ability to protect neurons from cytotoxicity causing neurodegenerative disorders such as ALS.⁷ Toxic gain of the mutated gene (SOD1) results in progressive paralysis due to the degenerations of the motor neurons followed by death within couple of years of diagnosis.⁷ The exact mechanisms of the progression of ALS is currently unknown, however, current research indicates that a group of cells called Natural Killer (NK) cells may be involved in the mechanism of ALS.¹ NK cells are heterogeneous and versatile cells that, through their cytokine production function, help regulate substances for the adaptive and innate immune responses.⁸ They also have regulatory functions to avert autoimmunity by eliminating autologous immune cells.⁹ Even though their exact role in ALS and other neurodegenerative disorders is unknown, this recent study provides evidence that support their involvement in ALS mice models and ALS patients.¹¹ Studies show that patients diagnosed with ALS, show an abundant amount of NK cells in their Central Nervous System (CNS) which create a cytotoxic environment in motor neurons of the CNS.²

Additional studies demonstrate that a continuous release of NK cells triggers unregulated activation of microglia and astrocytes release in the affected CNSA area of ALS. This causes a positive feedback loop that causes a release of a flood of anti- and proinflammatory cytokines such as IFN-γ to fight the inflammation.⁵ In other ALS studies, the pro-Inflammatory cytokine IFN-γ was observed to release a chemokine ligand-2 (CCL2), which is a proinflammatory molecule set to act as a neuroprotection against glutamate induced excitotoxicity.⁴ Research shows that an increased release of (CCL2) causes damage in spinal tissue observed in ALS mice. In normal motor neurons, Foxp3+ (Treg cells) act as microglia toxicity suppressors while in ALS it was observed that IFN-γ cytokines attack the Foxp3+ cells resulting in failure of toxicity suppressors recruitment in hSOD1 mice. On the other hand, the depletion of IFN-γ cells decreased disease progression and regulated microglia activation in ALS mice.⁶ 

Methods

In this study both mice and human participants were investigated. Transgenic mice with mutant ALS gene hSOD1 were used for the mice treatment. To observe the effects of absent NK cells in ALS mutated mice NK cells depletion was completed. The NK cell depletion was performed by using the blocking Ab against NK1.1. The sections of mice spinal cord and brains were stained with the fluorophore-conjugated antibody and Hoechst for nuclei visualization and consequently analyzed using a fluorescence microscope.¹¹

In the human treatment, CNS tissues and blood of 15 sporadic ALS patients (sALS) were analyzed for the presence and types of NK cells. Similarly, the presence of NK cells was analyzed in postmortem tissues obtained from 12 ALS patients who died of the disease and compared to the control that included postmortem spinal cord tissues from patients that died of non-neurological disease. By using the ALS patients and the mice model with the mutated gene the researchers were able to conclude the presence of NK cells in ALS disease and compare it to the patients and mice that did not have ALS.¹¹

 Results

Elevated levels NK cells were found in the spinal cord and cerebral motor cortex of the 15 ALS participants, postmortem patients, and also in hSOD1 mice. In comparison to the non-depleted NK cell mice, the depleted mice showed a greater increase in survival time and delay in paralysis. NK cells were found to be involved in triggering cytotoxicity of NKG2D receptors that express the NK cells. The hSOD1 NK cells expressed similar cytotoxic activity against healthy neurons. Results also showed reduced soma area and longer branches of the microglia of hSOD1 NK cell-depleted mice in comparison to the microglia of the control mice. Additionally, microglia of hSOD1 mice expressed genes that caused an inflammatory microenvironment, affecting the motor neurons, while in the NK cell-depleted hSOD1 mice, the expression of the exact genes was strongly reduced, and an increase of anti-inflammatory markers were observed.¹¹

Higher levels of IFNγ molecule were observed in hSOD1 mice compared to control mice. The immuno-depletion of the IFNγ molecule prolonged the life span of hSOD1 mice, delayed onset paralysis, and showed higher scores in reflex and rotarod tests. Results also indicated that IFNγ molecule stimulated CCL227 release which played a role in increasing NK cell accumulation in the CNS. When the IFNγ was depleted in cells, CCL227 levels dropped, and the NK concentration drastically decreased showing the direct effect of IFNγ on NK concentration. NK cell-depleted in hSOD1 mice also showed a higher expression of microglial CCL227 cells compared to mice with NK cells. IFNγ also reduced T cell (Tregs) in ALS patients. An immunofluorescence analysis of FOXP3+ cells (Treg cells) in the spinal cord of NK cells-depleted hSOD1mice showed higher numbers of Treg cells. When the IFNγ was blocked, the increase of foxp3 expression was observed in hSOD1 mice.¹¹ 

Discussion

In this study, familial ALS was examined through the implication of Natural Killer (NK) cells in hSOD1 mice, postmortem tissue of ALS patients as well as in live ALS patients.¹¹ The exact implication of NK cells in ALS is currently unknown but studies show that these cells that function to protect neurons by responding to inflammation or neuronal damage, might play a role in cytotoxicity of motor neurons in ALS. This study looked at a possible mechanism NK cells contribute to ALS. The results indicated that both ALS patients and hSOD1 mice displayed increased NK cells in CNS regions along with other molecules that are correlated with the presence of NK cells. The presence of NK cells in these mice contributed to ALS like symptoms while depletion of these cells alleviated the ALS symptoms by showing the direct implication of NK cells in this disease¹¹. Previous studies also showed abundant numbers of NK cells in the blood of ALS patients and in the spinal cord of hSOD1 mice while the absence of NK cells resulted in an increased life span and lighter symptoms of hSOD1 mice.³ A similar ALS study also showed an increased CCL2 levels in spinal cord and skeletal muscles of ALS patient's autopsies and hSOD1 mutated mice.⁴ In conclusion, this study showed the role NK cells play in facilitating a cascade of events that triggers cytotoxicity, demyelination, and ultimately death of MN in ALS. This study is significant because it provides a deeper understanding of NK cells implication in ALS by comparing ALS in human and in mice models. But most importantly, this study may take us closer to the development of a plausible therapeutic for the treatment of ALS. 

[+] References

[+] Other Work By Virginia Buracioc