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Guest Post: Astrocytes in Rett Syndrome: why we shouldn’t ignore them?

Marianne King, PhD Student

Guest Post: Astrocytes in Rett Syndrome: why we shouldn’t ignore them?

There is ever increasing data on the important role of astrocytes and microglia in maintaining the homeostasis and health of neural pathways. Although the neurons are the primary effector cell, the support network cells, and their dysregulation are of great interest in finding therapeutic interventions for diseases like Alzheimer’s Disease (AD).

Of particular focus is the role of Triggering Receptor expressed on myeloid cells 2 (TREM2), a receptor expressed by microglia. It has recently been shown in mice that loss of function mutations in Trem2 in Alzheimer’s Disease models with beta-amyloid pathology can advance the rate of brain atrophy.

Due to the scarcity of primary human CNS tissue, iPSC models are important tools to help assay development and R&D in drug discovery. The extensive datasets we are accumulating at Axol Biosciences from functional assays that probe the phagocytic activity, cytokine release and chemotaxis of our microglia, alongside mRNA profiling, provide robust assays by which to test compounds or mutant cell lines.

The functional differences we see in different genetic models shows the value and relevance of the cellular models we provide. For example, we have observed a distinct dysregulation of chemotaxis and phagocytosis in some of the TREM2 mutant microglial cell lines we offer (Figure 1).

Rett Syndrome

Rett Syndrome is a neurodevelopmental disorder that affects around 1 in 12,000 girls. Characterised by a period of normal development symptoms only start to appear after 6-18 months. These symptoms include seizures, cardiac and breathing problems, repetitive hand movements and communication difficulties. Whilst patients can expect a normal lifespan they are dependent on 24-hour care. There are currently no treatments for this disease.

The disease is caused by mutations in the MeCP2 gene, which is located on the X chromosome. During development one X chromosome in each somatic cell is randomly inactivated, resulting in a mosaic expression of both mutant and healthy MeCP2 alleles. It for this reason that MeCP2 mutations are lethal in boys, as they do not have a compensatory healthy MeCP2 gene.

How does MeCP2 affect development in Rett Syndrome?

MeCP2 binds to chromatin and can recruit factors such as HDACs that remodel it into an inactive state. In normal development, neurogenesis occurs first and the switch to astrogenesis is tightly controlled. During neurogenesis MeCP2 binds to methylated portions of astrocyte-specific gene promoters, such as GFAP (figure 1). As development progresses this methylation decreases and MeCP2 no longer binds to the promoter, allowing gene transcription.

This controlled timing results in the correct number of neurons and astrocytes being generated. However, in Rett Syndrome the switch to astrogenesis occurs too quickly, as MeCP2 is mutated and cannot remodel the promoters’ chromatin into an inactive state. Therefore, astrocyte genes can be transcribed too early.

Interestingly, iPSCs from Rett Syndrome patients differentiate more readily into GFAP-positive cells than controls, further corroborated by an increase in GFAP staining in Rett Syndrome brains.

Figure 1: MeCP2 binds to methylated portions of genes, indicated by the red wavy line. It then recruits chromatin remodellers that remodel chromatin into an inactive state, preventing astrocyte-specific genes from being transcribed during neurogenesis. During neurogenesis this methylation decreases, therefore MeCP2 cannot bind and the chromatin remodels into an active state. This allows transcription of astrocyte-specific genes, and the commencement of astrogenesis.

What are astrocytes doing? Or what are they not doing…

In mice

Astrocytes taken from female heterozygous MeCP2-/+ mice showed a significant reduction in the secretion of inflammatory markers IL-6 and IL-1β when treated with the immunostimulant lipopolysaccharide (LPS) in comparison to healthy astrocytes. In addition, the MAPK pathway in the MeCP2-/+ mice was found to be hyperphosphorylated, implying that the pathway was partially activated despite the lack of external stimulation. Consequently, this may have resulted in a smaller response to LPS compared to the healthy astrocytes. The dampened response of pro-inflammatory signalling may suggest that astrocytes in Rett Syndrome are not as capable of triggering an appropriate immune response when needed, consequently impacting upon the brains’ ability to detect harmful stimuli.

Under control conditions healthy astrocytes promote dendritic growth when co-cultured with healthy neurons. However, co-cultures involving astrocytes from the MeCP2-/+ mice and healthy hippocampal neurons resulted in shorter dendrites and somas. It was also noted that siRNAs targeted against MeCP2 in healthy astrocytes also resulted in a reduction in dendritic outgrowth, validating that this effect is due to the MeCP2 deficiency in astrocytes.

MeCP2-deficient murine astrocytes from another study also demonstrated reduced glutamate clearance. This could further impact neurons as excess glutamate in the synaptic cleft can cause excitotoxicity in the postsynaptic neuron.

In iPSCs

Human iPSC-derived astrocytes generated from Rett Syndrome donors have a detrimental impact on the morphology of healthy iPSC-derived interneurons; with a reduction in soma size and shorter dendrites. Functionally this resulted in reduced frequency of miniature excitatory postsynaptic currents (mEPSCs) but did not affect the amplitude. Such a reduction in soma size and dendritic length recapitulates what has been observed in animal models of Rett Syndrome. The mechanism behind the reduced frequency on mEPSCs is not fully understood.

Overall MeCP2 has been found to affect cytokine production, glutamate clearance, neuronal morphology and electrical signalling which all contribute to abnormalities in neurodevelopment.

Can these phenotypes be rescued in models of Rett Syndrome?

Human iPSC-derived astrocytes from Rett Syndrome patients had a detrimental effect on WT iPSC-derived interneuron morphology, but healthy iPSC-derived astrocytes had a positive impact on Rett Syndrome iPSC-derived interneurons, compared to cultures with Rett Syndrome astrocytes. This further highlights the impact astrocytes can have on neurons in disease.

Reintroduction of MeCP2 specifically into astrocytes ameliorates symptoms in MeCP2 null mice. Additionally, astrocyte conditioned media has a beneficial effect on MeCP2-/- murine neurons, improving their dendritic length.

This shows that it is possible to reduce some of the negative effects in Rett Syndrome models by targeting astrocytes. In conjunction with other therapies it could be that treatments targeting astrocytes would result in a positive effect.

Conclusion

It has been shown that astrocytes have a role to play in Rett Syndrome and their function in this disease is worth studying. In order to generate the most translational data possible it is important to consider multiple cell types when studying neurological disease.

Induced pluripotent stem cells provide us with the potential to generate vast amounts of different human cells which would previously have been very difficult to obtain. This means we can create cell-based models of neurological diseases without resorting to using animal cells or the limited supply of primary human brain cells. Human iPSC-derived cells offer translational models with low variability and increased consistency which have the potential to help us discover novel treatments and improve the understanding of Rett Syndrome and other neurological diseases.

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