Astrocytic Complement Regulator Secretion Modulates Synaptic Vulnerability (CD5…

The secretion of soluble complement regulators CD55 and CD46 by reactive astrocytes represents a non-cell-autonomous mechanism for regulating complement-mediated synaptic pruning during neuroinflammation. Unlike membrane-bound forms, astrocyte-derived soluble CD55 (sCD55) and CD46 (sCD46) are generated through matrix metalloproteinase-mediated shedding, particularly by MMP-9 and ADAM17, which cleave near the GPI anchor site for CD55 and within the transmembrane domain for CD46. These soluble regulators retain their complement inhibitory functions but exhibit altered kinetics and spatial distribution, creating diffusible gradients that can protect synapses at distances up to 50-100 μm from the secreting astrocyte. Soluble CD55 maintains its ability to accelerate convertase decay but with reduced binding affinity (Kd ~200 nM vs 50 nM for membrane-bound), while sCD46 cofactor activity requires higher local concentrations of factor I for effective C3b/C4b cleavage. Astrocytic secretion is triggered by inflammatory cytokines (TNF-α, IL-1β) through NF-κB and STAT3 signaling pathways, with peak shedding occurring 6-12 hours post-stimulation. The secreted regulators preferentially accumulate at excitatory synapses through binding to neurexin-neuroligin complexes and heparan sulfate proteoglycans in the synaptic cleft. This creates a protective complement-regulatory shield that counteracts C1q deposition and prevents classical pathway activation. During chronic neuroinflammation, sustained astrocytic sCD55/sCD46 release shifts the balance from complement-mediated synaptic elimination toward synaptic preservation, potentially contributing to maladaptive circuit reorganization. The therapeutic implications suggest that modulating astrocytic complement regulator shedding could provide temporal control over synaptic pruning during neurodevelopmental disorders or neurodegeneration.