Brain Peptide CCK Blocks Microglia From Destroying Memory Synapses in Sepsis

Sepsis-associated encephalopathy affects up to 70% of sepsis patients and is the most common form of encephalopathy in intensive care units, yet effective neuroprotective treatments remain elusive. Cognitive deficits following sepsis are associated with abnormal glial activation and synaptic loss, but the precise molecular mediators linking peripheral inflammation to hippocampal memory circuits have been poorly understood.

Researchers from Peking University and collaborating institutions used a well-established LPS injection model (5 mg/kg i.p.) in 3-month-old male C57BL/6 mice to mimic SAE, alongside LPS-stimulated BV2 microglial cell cultures as an in vitro model. They tested whether cholecystokinin (CCK), specifically the CCK8 isoform, could reverse cognitive impairment using two complementary approaches: direct bilateral infusion of CCK8 into the dorsal hippocampal CA1 region via implanted cannulas, and chemogenetic activation (DREADD hM3Dq) of CCK-positive neurons in CA1 using CCK-Cre transgenic mice. Cognitive outcomes were assessed with the Morris water maze (MWM) and fear conditioning test (FCT). RNA sequencing, Western blot, immunofluorescence, and whole-cell electrophysiological recordings were employed to dissect mechanisms.

Both exogenous CCK8 delivery and endogenous CCK release via chemogenetic neuron activation significantly improved spatial memory and fear-conditioned learning in SAE mice. Mechanistically, LPS-induced SAE caused marked upregulation of complement C1q at hippocampal synapses, driving microglia-mediated phagocytosis of excitatory (PSD-95-positive) synapses and reducing excitatory synaptic transmission as measured by electrophysiology. CCK8 treatment substantially suppressed C1q expression and microglial engulfment of synapses, preserving excitatory synaptic density and long-term potentiation-related plasticity. CCK also inhibited the conversion of astrocytes to a neurotoxic A1 phenotype that is known to further damage synapses. In vitro experiments with BV2 cells confirmed that these anti-phagocytic effects are mediated specifically through the CCK2-type receptor (CCK2R) on microglia, as CCK2R antagonism blocked the neuroprotective effects.

The study also showed that RNA sequencing of hippocampal tissue revealed complement pathway dysregulation as the dominant transcriptomic signature in SAE, and that CCK reversed these transcriptomic changes. Reduced synaptic complement tagging, fewer microglia-engulfed synaptic puncta inside Iba1-positive cells, and restored mEPSC frequency collectively confirmed synaptic preservation.

These findings identify a novel neuroprotective axis in which hippocampal CCK signaling restrains complement-driven, microglial-mediated synapse elimination during systemic inflammation. The dual approach of pharmacological CCK8 administration and circuit-level chemogenetic neuron activation both proved effective, broadening potential therapeutic translation strategies for SAE.