Extracellularly delivered single-stranded viral RNA causes neurodegeneration dependent on TLR7

Area of Science:

• Neuroimmunology
• Molecular Neuroscience

Background:

• Innate immune receptors, like Toll-like receptors (TLRs), detect pathogens and host factors, mediating rapid responses.
• While TLRs are implicated in CNS disorders, the specific ligands causing TLR activation and subsequent CNS damage remain incompletely understood.
• TLR7, predominantly in immune cells, is also found in CNS neurons and microglia, suggesting a role in brain immunity.

Purpose of the Study:

• To investigate the role of single-stranded RNA (ssRNA) and TLR7 in central nervous system (CNS) neurodegeneration and neuroinflammation.
• To elucidate the mechanisms of ssRNA-induced neuronal damage, including the involvement of adapter molecules and cell death pathways.
• To validate the in vivo relevance of TLR7-mediated neuroinflammation and neurodegeneration.

Main Methods:

• In vitro studies using ssRNA40 (an HIV-derived TLR7 ligand) on primary neuronal cultures and microglia.
• Assessment of neuronal cell death, caspase-3 activation, and inflammatory responses.
• In vivo studies involving intrathecal administration of ssRNA40 in wild-type and TLR7-deficient mice.

Main Results:

• Extracellular ssRNA40 induces TLR7-dependent neuronal cell death in vitro, involving MyD88 and caspase-3 activation.
• Microglia amplify ssRNA40-induced neuronal damage by mounting a TLR7-mediated inflammatory response.
• Intrathecal ssRNA40 administration causes widespread neurodegeneration in wild-type mice but not in TLR7(-/-) mice.

Conclusions:

• Extracellular ssRNA is a potent inducer of neurodegeneration and neuroinflammation in the CNS.
• TLR7 plays an obligatory role in ssRNA-mediated CNS damage, acting both in neurons and microglia.
• These findings reveal a critical pathway through which ssRNA can contribute to CNS pathology.