Blockade of mGluR5 in astrocytes derived from human iPSCs modulates astrocytic function and increases phagocytosis

Astrocytes are central players in maintaining brain homeostasis, including the regulation of synaptic remodeling and inflammatory responses. Their function in neurodegenerative diseases is highly complex, as they can adopt reactive phenotypes that are either beneficial or harmful depending on the environmental triggers. One key regulator of astrocytic activity is the metabotropic glutamate receptor 5 (mGluR5). While mGluR5 is often upregulated in neurodegenerative conditions, its role in human astrocytes remains debated. This study aimed to elucidate the influence of mGluR5 on the inflammatory and phagocytic responses of human induced pluripotent stem cell (hiPSC)-derived astrocytes when exposed to the pro-inflammatory stimulus tumor necrosis factor-alpha (TNF-α).

The researchers used hiPSC-derived astrocytes, which were matured for five weeks to ensure physiological relevance. These cells were characterized to confirm the expression of GFAP, S100β, and mGluR5. The team investigated the effects of mGluR5 blockade using CTEP—a highly specific, potent, and long-acting negative allosteric modulator (NAM)—in both basal conditions and under stimulation with recombinant TNF-α (rTNF-α). The investigation included measuring the secretion of inflammatory cytokines (IL-6 and IL-8), the expression of reactive astrocyte markers (such as SERPINA3), the rate of synaptoneurosome phagocytosis, and the production of synaptogenic molecules (BDNF, GPC4, and TSP1).

The researchers discovered that mGluR5 blockade via CTEP exerts a dual effect on astrocyte function. First, it significantly reduced the secretion of the pro-inflammatory cytokines IL-6 and IL-8 in the short term (4 hours) following rTNF-α stimulation, though this effect diminished after 24 hours. Second, CTEP treatment, either alone or in combination with rTNF-α, enhanced the phagocytosis of synaptic material. This increase in phagocytic capacity was associated with the regulation of specific receptors like MERTK and ITGAV. However, the study also identified potential downsides: mGluR5 inhibition did not reduce the expression of classic reactive astrocyte markers like SERPINA3 (and in fact, exacerbated it) and led to a decrease in the expression of key synaptogenic factors such as BDNF and TSP1.