Immunomodulation With Local, Sustained Delivery of Pituitary Adenylate Cyclase Activating Polypeptide Results in Improved Functional Recovery in Stroke-Injured Mice.

Abstract

Following ischemic stroke, astrocytes and microglia become activated and create a hostile microenvironment that can exacerbate brain damage, yet these cells also contribute to tissue regeneration. Pituitary adenylate cyclase activating polypeptide (PACAP) is a promising neuroprotective peptide that modulates microglia toward a pro-reparative phenotype, however, its short half-life in vivo and dose-limited off-target effects have made systemic delivery untenable. Local delivery presents a promising alternative. To this end, we developed a hydrogel-nanoparticle composite for the minimally invasive, local delivery of PACAP to the brain and tested this strategy in chemically-induced, endothelin-1 stroke-injured mice. We demonstrate that prolonged delivery of PACAP improved the physical strength and mobility of mice for up to 28 days after stroke. The treatment decreased the number of apoptotic neurons in the stroke microenvironment, increased neuron survival at 28 days post-stroke, and attenuated reactive astrogliosis and microglia activation. PACAP stimulation resulted in increased Iba1⁺Arg1⁺ pro-reparative microglia and decreased Iba1⁺CD86⁺ pro-inflammatory microglia. Furthermore, PACAP stimulation significantly decreased pro-inflammatory GFAP⁺LCN2⁺ and GFAP⁺S100β⁺ astrocytes versus controls. This phenotypic shift in microglia and astrocytes may account for the functional improvements post stroke and paves the way for local delivery of new therapeutic strategies targeting the immune response for stroke treatment.