Astrocytic Complement Regulator Release Modulates Synaptic Vulnerability (CD55/…

Molecular Mechanism and Rationale

Astrocytic release of soluble complement regulators CD55 and CD46, rather than membrane-bound expression, represents a dynamic mechanism for spatiotemporal control of complement-mediated synaptic pruning. Astrocytes constitutively express high levels of CD55 and CD46, which undergo metalloproteinase-mediated shedding via ADAM10 and ADAM17 cleavage at defined juxtamembrane sites, generating soluble forms (sCD55, sCD46) that retain full complement regulatory activity through preserved CCP domains. This shedding process is regulated by astrocytic calcium signaling and ATP release, with P2Y1 receptor activation triggering ADAM protease mobilization and enhanced CD55/CD46 release within minutes.

The spatial gradient of soluble complement regulators creates zones of differential synaptic protection based on proximity to astrocytic processes. Tripartite synapses with intimate astrocytic contact receive continuous sCD55/sCD46 delivery, maintaining robust complement inhibition through local concentrations reaching 50-100 nM. Conversely, synapses located >2 μm from astrocytic processes experience dramatically reduced soluble regulator availability due to rapid diffusion and enzymatic degradation by synaptic proteases. This creates a vulnerability gradient where distant excitatory synapses become preferentially susceptible to complement activation.

During anesthesia-induced astrocytic depolarization, ATP depletion impairs P2Y1 signaling and reduces ADAM protease activity, leading to diminished sCD55/sCD46 release. Simultaneously, anesthesia enhances microglial migration toward complement-unprotected synapses through C3a gradient formation. The resulting complement cascade preferentially targets synapses with insufficient soluble regulator coverage, enabling precise spatial control of synaptic elimination. This astrocyte-dependent mechanism explains regional variations in anesthesia-induced synaptic loss and provides a novel therapeutic target through modulation of complement regulator shedding.