Boosting Brain Blood Flow Rescues the Brain's Waste-Clearance System During Sepsis

The glymphatic system — the brain's perivascular waste-clearance network driven by the water channel AQP4 on astrocyte endfeet — is increasingly recognized as central to brain health and neurodegeneration. Disruption of this system has been implicated in Alzheimer's disease, traumatic brain injury, and stroke. Sepsis-associated encephalopathy (SAE), which affects up to 70% of severe sepsis patients and carries high mortality and long-term cognitive consequences, had been suspected to involve glymphatic dysfunction, but the temporal dynamics, mechanisms, and therapeutic implications were previously unknown.

This study used a mouse model of systemic inflammation (intraperitoneal LPS at 5 mg/kg) and developed novel near-infrared II (NIR-II) fluorescent probes — including a 66.9 kDa biomimetic fluorescent protein (BSA@IR-780) — to non-invasively image CSF flow through the glymphatic system in living mice. Imaging was performed at 1, 3, 24, and 72 hours post-LPS. At 1 hour, glymphatic influx around the middle cerebral artery (MCA) was significantly reduced compared to controls. By 3 hours, influx reversed and became progressively elevated, peaking at 24 hours before returning to baseline at 72 hours (p<0.05 to p<0.001 across timepoints). Simultaneously, CSF efflux through the olfactory bulb/nasopharyngeal lymphatic plexus was continuously suppressed from 1 to 72 hours. Brain homogenate quantification at 30 minutes post-tracer confirmed significantly elevated tracer levels in LPS-treated mice at 24 hours. Critically, at the 5 mg/kg dose used, Evans Blue extravasation assays confirmed BBB integrity was preserved, demonstrating that elevated glymphatic influx was not an artifact of barrier breakdown.

Cerebral blood flow (CBF) measured by laser Doppler showed an inverse relationship with glymphatic influx trends after LPS: CBF dropped significantly at 3 hours (matching the influx surge) and remained depressed. The investigators then tested levosimendan — a calcium-sensitizing cardiac inotrope and vasodilator approved in many countries for acute heart failure — as a means to restore CBF. Levosimendan treatment significantly increased CBF, normalized glymphatic influx and efflux dynamics, and restored amyloid-beta clearance (assessed with a small-molecule NIR-II probe mimicking Aβ). Crucially, when levosimendan was combined with bilateral carotid artery stenosis surgery to re-induce hypoperfusion, its glymphatic benefits were completely abolished — definitively establishing cerebral hypoperfusion as the causal mediator of LPS-induced glymphatic dysfunction.

Mechanistically, AQP4 normally concentrates at astrocytic endfeet surrounding blood vessels (perivascular polarization). LPS caused significant AQP4 depolarization — redistribution away from endfeet — which would impair the directional CSF-ISF exchange the glymphatic system depends on. Levosimendan's restoration of CBF prevented this depolarization, maintaining AQP4 at endfeet. When AQP4 was pharmacologically inhibited with TGN-020, levosimendan's benefits on Aβ clearance and neuroinflammation suppression were completely blocked, confirming AQP4 as the essential downstream effector. Neuroinflammation markers (microglial activation, pro-inflammatory cytokines) were elevated by LPS and significantly attenuated by levosimendan in an AQP4-dependent manner. Behavioral testing showed levosimendan also significantly ameliorated LPS-induced neurological deficits.

The findings establish a mechanistic chain: systemic inflammation → cerebral hypoperfusion → AQP4 depolarization → glymphatic dysfunction → impaired waste clearance and neuroinflammation → SAE. This is the first study to temporally characterize glymphatic dynamics across 72 hours of systemic inflammation using in vivo NIR-II imaging, and the first to causally link hypoperfusion to glymphatic failure and test a clinically available drug to reverse it. The identification of levosimendan as a potential SAE therapeutic is particularly notable given its existing clinical safety profile.