CXCL12 engages cortical inhibitory neurons to enhance dendritic spine plasticity and structured network activity

: The chemokine CXCL12 is a highly conserved peptide that regulates homeostatic processes in the brain throughout life. Recent work shows CXCL12 increases dendritic spine density in cortical neurons, which requires activation of CXCL12's receptor CXCR4. This same pathway reverses cortical dendritic spine deficits and cognitive impairment in an animal model of neuroHIV. However, it remained unclear if CXCL12 simply preserved existing spines or also engaged spine plasticity processes that drove network-level adaptations. We therefore tested if CXCL12 could regulate dendritic spine turnover, maturation, clustering, and neuronal network activity in primary rat cortical neurons of either sex using live-cell imaging, confocal microscopy, and multi-electrode arrays. Intriguingly, CXCL12-treated neurons formed significantly more new spines than controls, and this outcome was blocked by the CXCR4 antagonist AMD3100. CXCL12 also increased the density of thin spines expressing post-synaptic markers, including postsynaptic density protein 95 (PSD-95), phospho-PSD-95Ser295 and GluA1, and allowed neurons to better maintain synaptic PSD-95 puncta size. Thin spines were modestly closer together after CXCL12 treatment, suggesting a possible effect on anatomical spine clustering. These effects translated to structured network activity, as CXCL12 increased spike frequency within network bursts in multi-electrode array cultures. Finally, a targeted knockdown of CXCR4 in inhibitory neurons, which mostly lack dendritic spines, prevented CXCL12 from increasing spine density on excitatory neurons. Overall, our findings suggest CXCL12/CXCR4 signaling engages inhibitory neurons along with multiple aspects of spine dynamics and remodeling to shape how broader neuronal networks function.Significance Statement Several neurological disorders accelerate cognitive decline, and there are few effective treatments to slow or reverse cognitive symptoms. Though these disorders often have distinct underlying mechanisms, they typically reduce the density of dendritic spines in brain regions that facilitate learning and memory. We previously reported that the homeostatic chemokine CXCL12 restored dendritic spine density and improved cognitive performance in a rodent model of HIV-associated neurocognitive disorder (HAND), suggesting the pathway holds broadly applicable therapeutic targets. Here, we further uncovered that CXCL12 regulates spine plasticity processes that help spines stabilize and integrate into neuronal networks. These results shed further light on chemokines as intrinsic neuromodulators and their potential to help identify therapeutic targets to restore neuronal function.