Association Between Microglia, Inflammatory Factors, and Complement with Loss of Hippocampal Mossy Fiber Synapses
Andrew D Kraft1,2, Christopher A McPherson1, G Jean Harry3
1Neurotoxicology Group, National Toxicology Program Laboratory, National Institute of Environmental Health Sciences, National Institutes of Health, P.O. Box 12233, Mail Drop E1-07, Research Triangle Park, NC, 27709, USA.
Insights
Trimethyltin (TMT) induces neuronal apoptosis, activating microglia and complement factors. This study reveals a link between microglia reactivity, inflammatory gene induction, and synaptic degeneration in the hippocampus.
Area of Science:
- Neuroscience
- Immunology
- Cell Biology
Background:
- Complement factors play roles in neuroinflammation and tissue repair.
- Microglia are key immune cells in the central nervous system involved in clearing debris.
- Understanding complement activation during neurodegeneration is crucial for therapeutic development.
Purpose of the Study:
- To investigate complement activation and microglial response during the clearance of degenerating neuronal components.
- To characterize the temporal dynamics of inflammatory and anti-inflammatory gene expression following neuronal apoptosis.
Main Methods:
- Hippocampal neurons were ablated using trimethyltin (TMT) in mice.
- Analysis of neuronal apoptosis, microglial morphology, and gene/protein expression (mRNA, C1q, synaptophysin).
- Quantitative assessment of inflammatory markers (TNFα, IL-1β, IL-6) and microglial markers (Arginase-1, IL-10).
Main Results:
- TMT induced neuronal apoptosis, amoeboid microglia, and elevated pro-inflammatory cytokines (TNFα, IL-1β, IL-6) and C1q protein.
- Reactive microglia showed co-localization with synaptic fragments, indicating clearance activity.
- Concurrent induction of anti-inflammatory genes (Arginase-1, IL-10, TGF-β1, Ym1) was observed alongside pro-inflammatory markers.
- Complement component mRNA levels (C1qa, C1qb, C3, Cr3a, Cr3b) were elevated, correlating with synaptic degeneration.
Conclusions:
- Microglial reactivity and complement factor expression are associated with synaptic degeneration following apoptotic neuronal loss.
- The study highlights a complex interplay between pro-inflammatory and anti-inflammatory responses during microglial-mediated clearance.
- These findings provide insights into the molecular mechanisms underlying neuroinflammation and synaptic damage.
Abstract:
Complement-associated factors are implicated in pathogen presentation, neurodegeneration, and microglia resolution of tissue injury. To characterize complement activation with microglial clearance of degenerating mossy fiber boutons, hippocampal dentate granule neurons were ablated in CD-1 mice with trimethyltin (TMT; 2.2 mg/kg, i.p.). Neuronal apoptosis was accompanied by amoeboid microglia and elevations in tumor necrosis factor [Tnfa], interleukin 1β [Il1b], and Il6 mRNA and C1q protein. Inos mRNA levels were unaltered. Silver degeneration and synaptophysin staining indicated loss of synaptic innervation to CA3 pyramidal neurons. Reactive microglia with thickened bushy morphology showed co-localization of synaptophysin+ fragments. The initial response at 2 days post-TMT included transient elevations in Tnfa, Il1b, Il6, and Inos mRNA levels. A concurrent increase at 2 days was observed in arginase-1 [Arg1], Il10, transforming growth factor β1 [Tgfb1], and chitinase 3 like-3 [Ym1] mRNA levels. At 2 days, C1q protein was evident in the CA3 with elevated C1qa, C1qb, C3, Cr3a, and Cr3b mRNA levels. mRNA levels remained elevated at 5 days, returning to control by 14 days, corresponding to silver degeneration. mRNA levels for pentraxin3 (Ptx3) were elevated on day 2 and Ptx1 was not altered. Our data suggest an association between microglia reactivity, the induction of anti-inflammatory genes concurrent with pro-inflammatory genes and the expression of complement-associated factors with the degeneration of synapses following apoptotic neuronal loss.
