What's the science?
ALS is a devastating disease where motor neurons degenerate over time. Astrocytes which normally support neurons function abnormally in ALS and play a role in ongoing cell death. MicroRNAs are naturally occurring small pieces of non-coding RNA that regulate (and often inhibit) the production of proteins in cells. One type of microRNA called miR-218 has recently been shown to be higher in motor neurons in ALS and is released from the neuron into the cerebrospinal fluid. This microRNA, which is released from dying neurons, could communicate with astrocytes to cause dysfunction. This week in Brain Hoye and colleagues examine whether microRNA released from neurons are taken up by astrocytes and regulate their protein expression.
How did they do it?
They identified potential targets that microRNA miR-218 might bind to or regulate in astrocytes to affect their function. They did this by looking for genes with enriched (i.e. higher) expression in astrocytes. They identified EAAT2, a glutamate reuptake transporter that is enriched in healthy astrocytes but lost in ALS. They injected cells with either random microRNA or miR-218 (specifically released from motor neurons in ALS) to see whether it would affect EAAT2 expression. They then assessed whether miR-218 is taken up by astrocytes using a sensor they developed and whether it is free, protein bound, or encapsulated in vesicles. Finally, they tested whether any potentially pathological effects of miR-218 on astrocyte EAAT2 expression could be altered using antisense oligonucleotide therapy.
What did they find?
They found that miR-218 infected cells had reduced production of EAAT2 (measured with western blot) demonstrating that it can repress translation of this glutamate transporter in astrocytes. They then developed a ‘sensor’ to confirm that miR-218 is taken up by astrocytes. They found that the majority of miR-218 is protein bound in the cerebrospinal fluid (after it is released from the cell). They then tested whether they could block the miR-218 induced repression of EAAT2 expression in astrocytes using antisense oligonucleotides. They applied media from sporadic ALS patient iPSC-derived motor neurons to primary astrocytes that contained miR-218 with and without antisense oligonucleotides (a type of therapy for regulating protein production), and found that EAAT2 repression was blocked by antisense oligonucleotides. They then followed up by testing in a mouse model of ALS whether inhibiting miR-218 would block EAAT2 suppression. SOD1 mice (a mouse model of ALS) were treated with either antisense oligonucleotides or saline, and they found that the mice treated with the antisense oligonucleotides had less miR-218 activity in their brains. It also reduced astrogliosis (an abnormal increase in number of astrocytes) in ALS model mice.
What's the impact?
This is the first study to demonstrate that a microRNA (miR-218) release from dying motor neurons results in the repression of glutamate transporter (EAAT2) expression. Further, this study shows that blocking this repression using antisense oligonucleotides can reverse the effects of this microRNA on causing potentially damaging effects on astrocytes. Importantly, this study suggests that microRNAs play an important role in affecting astrocytes that contribute to ongoing neurodegeneration in ALS.
Hoye et al., Motor neuron-derived microRNAs cause astrocyte dysfunction in amyotrophic lateral sclerosis. Brain (2018). Access the original scientific publication here.