Neurons burdened with fractured DNA may not suffer in silence. They may call for help. That’s the conclusion reached by researchers at MIT’s Picower Institute for Learning and Memory. They discovered that in Alzheimer’s disease, neurons that accumulate double-stranded breaks (DSBs) in their DNA secrete immune signals. Ultimately, these signals activate microglia, contributing to the neuroinflammation associated with Alzheimer’s disease.
To “hear” the faint neuronal cries, the researchers used single-nucleus, bulk, and spatial transcriptomic techniques to characterize DSB-bearing neurons from mouse models of Alzheimer’s disease and from postmortem brain samples from Alzheimer’s patients. By listening so intently, the researchers found that neurons can engage microglia through the secretion of chemotactic and pro-inflammatory factors. Neurons, then, participate in neuroinflammation and disease progression.
Detailed findings appeared in Science Advances, in an article titled, “Neurons burdened by DNA double-strand breaks incite microglia activation through antiviral-like signaling in neurodegeneration.” The article describes a previously undefined role for neurons in the context of neurodegenerative disease. In addition, the article reports a mechanistic link between genomic fragility and senescence in neurons and microglia activation.
“This is a novel concept in neuroscience: the idea that neurons can be activating inflammatory activity in response to DNA damage,” said Gwyneth Welch, PhD, the article’s lead author and a scientist in the laboratory of Picower’s Li-Huei Tsai, PhD, the article’s senior author. “The general idea was that neurons have a more passive relationship with microglia regarding age-associated neuroinflammation.”
Although neurons have not been known to signal the brain’s immune system in Alzheimer’s disease, Welch, Tsai, and colleagues determined otherwise. They learned that neurons with mounting DSBs try to fix their fractured DNA but fail. Then the neurons call for help. They send molecular signals to microglia, which respond by taking on a more inflammatory state.
“DSB-bearing neurons enter a late-stage DNA damage response marked by nuclear factor κB (NFκB)–activated senescent and antiviral immune pathways,” the scientists reported. They also found that by interrupting the immune signaling, they could prevent microglia from entering an activated state and degrading synapses.
“Spatial transcriptomics reveal that regions of [our mouse model’s] brain tissue dense with DSB-bearing neurons harbor signatures of inflammatory microglia, which is ameliorated by NFκB knockdown in neurons,” the scientists continued. “Inhibition of NFκB in DSB-bearing neurons also reduces microglia activation in organotypic mouse brain slice culture.”
“We now know that DNA-damaged neurons exhibit senescent phenotypes and play an active role in eliciting an immune response from microglia and perhaps astrocytes,” Tsai stated. “Moreover, we identified two cytokines secreted by damaged neurons to recruit microglia and elicit microglial response. Importantly, we show that inhibition of NFκB rescued synaptic loss in neurodegeneration, further elucidating of impact of neuroimmune response on synaptic integrity and cognitive function.”
Finally, the scientists identified CCL2 and CXCL10 as primary signaling molecules secreted from DSB-bearing neurons to recruit and activate microglia: “In [our mouse model], DSB-bearing neurons are the first cell type to express CXCL10 and CCL2. Mainly astrocytes and microglia express these chemokines at a later time point, presumably in response to DSB-bearing neurons.”
To explore the role of CCL2 and CXCL10 in neuroinflammation, the scientists conducted experiments in which primary neuron cultures were treated with etoposide (ETP), a chemical that induces DSBs. “[Immunodepletion] of CCL2 or CXCL10 in conditioned medium from ETP-treated neurons was able to prevent branch shortening and end-point reduction in microglia in acute slice culture,” the scientists observed. “Notably, increased levels of both of these cytokines are implicated in the pathogenesis of AD and affect blood-brain barrier permeability to aid the infiltration of peripheral monocytes.”
The scientists noted that CCL2 and CXCL10 are crucial for effective microglia recruitment and activation, but they also suggested that imbalance in these signaling axes have detrimental effects on cognition and pathology clearance.
“Our data posit that neurons play meaningful roles in neuroinflammation, which historically has been thought to be driven largely by glial cells,” the scientists concluded. “Crucially, this axis of neuron-microglia communication is mediated by DNA damage accumulation in neurons, revealing that two hallmarks of AD, genome fragility and neuroinflammation, are mechanistically linked.”