Dendrimer Delivered shRNA Targeting the CCL20-CCR6 Axis Suppresses Complement-Mediated Microglial Synaptic Pruning and Ameliorates Chronic Neuroinflammation After Repetitive Traumatic Brain Injury

Repetitive traumatic brain injury (rTBI) leads to lasting microglial activation and chronic inflammation, but the underlying causes of long-term synaptic damage are still unknown. This research identifies the CCL20-CCR6 chemokine pathway as a crucial factor in sustaining microglial activation and complement-induced synaptic loss following rTBI.

Proteomic analysis at 30 days post-injury (dpi) revealed a general normalization of complement-related inflammatory and synaptic processes in the cortex and hippocampus, indicating a direct connection between chemokine signaling, microglial activation, and synaptic vulnerability. A novel dendrimer-based shRNA system (shCombo-DPX) was created to target this pathway simultaneously by silencing both CCL20 and CCR6.

When administered via intranasal or intravenous routes in rTBI mice, shCombo-DPX reduced CCL20-CCR6 expression, decreased chronic microgliosis and astrogliosis, and inhibited complement activation. The treatment prevented microglial engulfment of synapses, preserved synaptic proteins, restored brain-derived neurotrophic factor (BDNF) levels, and improved motor, anxiety-related, and cognitive functions.

In studies with microglia-neuron cultures, silencing CCL20 decreased LPS-induced complement signaling, avert synaptic loss, neuronal cell death, and depletion of BDNF. In contrast, introducing recombinant CCL20 triggered dendritic degeneration, caspase-3 activation, microglial reactivity, imbalance in complement regulation, and synaptic injury both in laboratory settings and in live animals.

Overall, these results pinpoint CCL20-CCR6 as a primary initiator of chronic complement-mediated synaptic damage in rTBI and support the use of dendrimer-delivered shRNA therapy as a targeted approach to lessen prolonged neurodegeneration.