1. **Synaptic C1q drives complement-dependent pruning, while microglial surface-associated C1q biases phagocyte state through receptor-specific signaling**
**Mechanism:** C1q deposited on weak or stressed synapses preferentially nucleates the classical complement cascade (`C1q -> C4 -> C3`), generating opsonins that engage microglial CR3/ITGAM-ITGB2 and promote engulfment. In contrast, C1q bound directly to microglial receptors or pericellular ligands may alter microglial transcriptional state without requiring full downstream complement activation. This would make localization mechanistically decisive because synaptic C1q mainly marks substrate, while microglial C1q mainly modulates effector state.
**Target:** `C1QA/C1QB/C1QC`, `C4A/C4B`, `C3`, `ITGAM/ITGB2 (CR3)`, microglia, excitatory synapses
**Supporting evidence:** Synaptic pruning roles for C1q/C3 in development and disease are well established; microglial CR3 is implicated in engulfment. Confident PMIDs: **18083105**, **24012419**.
**Falsifiable experiment:** In human iPSC neuron-microglia co-cultures, tether C1q selectively to synapses versus microglial membranes using engineered binders. Measure C3 deposition, CR3-dependent engulfment, and microglial RNA-seq. Prediction: synaptic targeting increases C3 deposition and spine loss; microglial targeting alters inflammatory/phagocytic state even if C3 deposition is limited.
**Confidence:** 0.78
2. **C1q outcomes are determined more by binding partner identity than by location: pentraxins and misfolded-protein ligands bias C1q toward distinct effector programs**
**Mechanism:** C1q may act as a context decoder whose function depends on the molecular scaffold it binds. When associated with synaptic pentraxins or apoptotic-like surface cues, it promotes complement activation and pruning. When bound to aggregated `Aβ`, phosphatidylserine-rich debris, or extracellular matrix ligands, it may favor microglial clearance programs or chronic inflammatory activation. Under this model, “synaptic versus microglial” differences are secondary to the identity of the C1q-bound complex.
**Target:** `C1Q`, neuronal pentraxins (`NPTX1`, `NPTX2`), `APP/Aβ`, phosphatidylserine-remodeling pathways, complement cascade
**Supporting evidence:** C1q binds diverse ligands including aggregated proteins and apoptotic structures; complement activation in AD around plaques is well supported. I am not confident enough to attach specific PMIDs for the binding-partner hierarchy claim.
**Falsifiable experiment:** Reconstitute purified human C1q with defined ligands (`Aβ oligomers`, synaptic membrane fractions enriched for neuronal pentraxins, apoptotic blebs, ECM proteins) and expose microglia plus complement-sufficient serum. Quantify complement activation, receptor usage, cytokines, and engulfment. Prediction: ligand identity predicts downstream program better than bead location alone.
**Confidence:** 0.66
3. **C1q bound to astrocyte-derived thrombospondin-rich extracellular matrix creates a “silent tagging” state distinct from inflammatory plaque-associated C1q**
**Mechanism:** Astrocytes secrete synaptogenic ECM proteins such as thrombospondins. C1q associated with ECM near synapses may stabilize low-grade synapse tagging without fully activating inflammatory complement, whereas plaque-associated or damage-associated C1q complexes may recruit robust classical cascade activation. This would explain why C1q can correlate both with remodeling and with overt neurotoxicity.
**Target:** `C1Q`, `THBS1/THBS2`, astrocytes, perisynaptic ECM, classical complement pathway
**Supporting evidence:** Astrocytic ECM strongly shapes synaptic maintenance; C1q accumulates at vulnerable synapses and plaques in neurodegeneration. I am not confident enough to cite a precise PMID for the thrombospondin-C1q interaction hypothesis.
**Falsifiable experiment:** Use organotypic brain slice cultures or iPSC tri-cultures with astrocyte-specific depletion of `THBS1/2`, then quantify C1q localization, C4/C3 deposition, and synapse elimination after `Aβ` or tau stress. Prediction: loss of thrombospondin-rich ECM redistributes C1q away from perisynaptic silent tagging and toward more inflammatory complement activation states.
**Confidence:** 0.51
4. **ApoE isoform shifts C1q function by changing its binding landscape on synapses and plaques**
**Mechanism:** `APOE4` may alter lipid, membrane, or aggregate surfaces so that C1q is preferentially recruited into pro-complement, pro-pruning, or plaque-associated inflammatory complexes, whereas `APOE3` permits more efficient debris packaging and clearance. The relevant determinant would be C1q’s biochemical partners shaped by ApoE-dependent lipidation states, not C1q alone.
**Target:** `APOE`, `C1Q`, `TREM2`, plaque-associated microglia, dystrophic synapses
**Supporting evidence:** ApoE genotype strongly influences complement-rich AD pathology and microglial state, but the exact C1q-partner mechanism remains unresolved. No PMID attached because I am not fully confident about a direct C1q-ApoE mechanistic citation.
**Falsifiable experiment:** Compare `APOE3` and `APOE4` human iPSC neuron-astrocyte-microglia cultures after `Aβ` exposure. Map C1q interactomes by proximity labeling or immunoprecipitation-mass spectrometry in synaptosome and plaque-like fractions. Prediction: `APOE4` shifts C1q toward complexes enriched for complement activators and phagocytic ligands, with greater synapse loss.
**Confidence:** 0.62
5. **Microglial TREM2 state determines whether C1q-tagged material is cleared adaptively or drives chronic synaptotoxic inflammation**
**Mechanism:** C1q tagging may be upstream of two divergent microglial responses. In a competent `TREM2` state, microglia clear C1q-opsonized debris efficiently. In `TREM2`-impaired states, the same C1q-tagged substrates persist and sustain complement amplification, cytokine production, and bystander synapse elimination. Thus, the apparent effect of C1q may depend on the receiving microglial state more than the initial site of deposition.
**Target:** `TREM2`, `TYROBP`, `C1Q`, `C3`, microglia
**Supporting evidence:** TREM2 is a major regulator of microglial response to neurodegenerative pathology, and complement activation is coupled to synapse loss in AD models. I am not confident enough to assign a specific PMID to the integrated C1q-TREM2 model.
**Falsifiable experiment:** In `TREM2` wild-type versus knockout microglia co-cultured with neurons, apply identical amounts of synapse-bound C1q and measure clearance kinetics, inflammatory transcriptional programs, and secondary spine loss. Prediction: `TREM2` loss converts a clearance-dominant response into persistent complement-associated injury.
**Confidence:** 0.69
6. **Therapeutically sparing synaptic C1q recognition while blocking C1q-C1r/C1s activation could preserve beneficial debris sensing and prevent destructive complement amplification**
**Mechanism:** If C1q has useful surveillance or cargo-recognition roles that depend on ligand binding but pathology requires downstream classical pathway activation, then inhibiting the `C1q-C1r-C1s` protease step should be safer than pan-C1q depletion. This predicts that harmful effects arise when C1q is converted from recognition molecule to cascade trigger.
**Target:** `C1Q`, `C1R`, `C1S`, classical complement pathway
**Supporting evidence:** Classical pathway activation downstream of C1q is implicated in synapse loss and neuroinflammation; the distinction between recognition and activation is biologically plausible but still unresolved in CNS disease. No PMID attached because I am not fully confident in a CNS-specific citation for this exact therapeutic framing.
**Falsifiable experiment:** Compare pan-C1q neutralization versus selective `C1r/C1s` inhibition in AD-model neuron-microglia co-cultures and in vivo. Readouts: synapse density, plaque compaction, debris clearance, microglial state, and cognition. Prediction: downstream blockade preserves more homeostatic clearance while reducing synaptotoxic complement activity.
**Confidence:** 0.74
7. **Subcellularly restricted C1q at inhibitory synapses has a distinct role from excitatory synaptic C1q, creating circuit-specific vulnerability**
**Mechanism:** C1q may not act uniformly across all synapses. Differential localization to inhibitory versus excitatory terminals, combined with distinct molecular partners, could produce opposite network consequences. Loss of inhibitory synapses would promote hyperexcitability, while loss of excitatory synapses would promote disconnection. This may explain heterogeneous effects across AD, tauopathy, and synucleinopathy circuits.
**Target:** `C1Q`, inhibitory synapse markers (`GAD1/GAD2`, `SLC6A1`), excitatory synapse markers (`VGLUT1/SLC17A7`), microglial complement receptors
**Supporting evidence:** Complement-mediated synapse loss is established, but synapse-class selectivity for C1q remains insufficiently resolved. No PMID attached because I am not confident of a precise citation.
**Falsifiable experiment:** Perform super-resolution imaging and synaptosome profiling from disease models and human tissue to quantify C1q occupancy on inhibitory versus excitatory synapses, then test selective vulnerability in co-culture. Prediction: one synapse class shows preferential C1q tagging and greater complement-dependent elimination, correlated with network phenotype.
**Confidence:** 0.57