For better or worse, complement proteins tag synapses for elimination by microglia. A paper published July 13 in Nature Neuroscience identifies a neuronal protein that keeps this process in check. Researchers led by Gek-Ming Sia at the University of Texas in San Antonio report that neurons spit out SRPX2, a protein that directly binds the complement protein C1q and halts the complement cascade. The researchers found that SRPX2 limits synaptic pruning by microglia during brain development in mice. They proposed that it could play a part much later in life, in neurodegeneration, where complement-mediated pruning has been implicated in the destruction of synapses. Autoantibodies to SRPX2 have been found in the cerebrospinal fluid of Alzheimer’s disease patients (Lim et al., 2019).
- SRPX2 binds C1q, inhibiting the classic complement cascade.
- Without SRPX2, microglia engulf more synapses.
- The visual and somatosensory systems seem most sensitive.
In the developing brain, the complement system tags weakened or redundant synapses for consumption by microglia (Dec 2007 news; Schafer et al., 2012). However, overzealous pruning has been implicated in driving synaptic destruction in neurodegenerative disease (Aug 2013 news; Nov 2015 conference news; Jul 2019 conference news). Though scientists have identified numerous complement inhibitors outside of the brain, they have thus far detected none within the CNS.
Previously, the researchers hypothesized that sushi repeat protein X-linked 2 (SRPX2) could fit the bill. Secreted by neurons, the protein enhances synaptic signaling, and variants in its gene have been tied to language and social behavior disorders (Sia et al., 2013; Chen et al., 2017; Soteros et al., 2018). Furthermore, sushi domains are common among complement inhibitors (Gialeli et al., 2018).
Co-first authors Qifei Cong and Breeanne Soteros started by investigating whether SRPX2 interacts with C1q—the complement protein that binds synapses and sets off their pruning. In lysates from mouse brain, the researchers detected SRPX2 and C1q in complexes. In cell culture experiments, they found that SRPX2 directly bound C1q, but only when cell membranes were present. This suggested that SRPX2 likely binds C1q immobilized on the cell surface.
Complement-mediated elimination of synapses sculpts neuronal circuitry during brain development, particularly in the somatosensory cortex and the dorsal lateral geniculate nucleus (dLGN), a region in the thalamus that receives visual input from the retina. Neurons in these regions expressed SRPX2, while microglia, astrocytes, and oligodendrocytes did not. Using immunohistochemistry, Cong and colleagues spotted SRPX2 mingling with C1q, sometimes in excitatory pre-synapses. SRPX2 knockout mice produced normal levels of C1q but an excess of C3, a protein in the complement cascade downstream of C1q that is required to trigger microglia to engulf synapses. This suggested that SRPX2 blocks the function of C1q, halting the cascade.
Neuronal Defender. SRPX2 mRNA (red) spotted in neurons (left), but not microglia (Iba), astrocytes (GFAP), or oligodendrocytes (Olig2) of the dorsal lateral geniculate nucleus (top) and layer 4 of the somatosensory cortex (bottom). [Courtesy of Cong et al., Nature Neuroscience, 2020.]
What were the consequences of taking the brakes off the cascade? Using mice devoid of SRPX2, C3, or both, the researchers pieced together the role of the complement inhibitor in the development of retinal axons and the somatosensory cortex. In both, SRPX2 transiently kept synaptic pruning in check, guiding the growth of neural circuitry. Without SRPX2, microglia excessively thinned dendritic spines. C3 was required for pruning in SRPX2 knockouts, directly implicating the complement cascade. SRPX2 knockout mice had fewer synapses in their somatosensory cortices and this persisted beyond the pruning stage.
Sia told Alzforum that his lab is now exploring if SRPX2 protects synapses during aging, or during neurodegenerative disease. He thinks SRPX2 expression might track with that of C1q, which spikes during development and then creeps up again with age.
Shane Liddelow of New York University would not be surprised if SRPX2 rose and fell with complement. He proposed that weak SRPX2 expression or secretion might hasten synaptic deterioration during aging and disease. Along those lines, Sia wondered whether SRPX2 could turn out to be a resilience factor, shielding against synaptic damage inflicted by Aβ plaques or tau tangles, for example.
“Since the classical complement pathway, which is initiated by C1q, seems to be overactivated in schizophrenia, Alzheimer's disease, and other CNS diseases, it is intriguing to speculate that expression or activity of SRPX2 or other endogenous complement inhibitors might regulate synapse loss and neuronal injury in these diseases,” wrote Borislav Dejanovic of the Broad Institute of MIT and Harvard. He added that it will be interesting to see if other sushi-domain-containing proteins expressed in the brain also inhibit complement, and to investigate which types of neurons and synapses are affected by these pathways.—Jessica Shugart
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- Cong Q, Soteros BM, Wollet M, Kim JH, Sia GM. The endogenous neuronal complement inhibitor SRPX2 protects against complement-mediated synapse elimination during development. Nat Neurosci. 2020 Jul 13; PubMed.