The brain is the most important part of the central nervous system in the human body. It is the center of consciousness and controls all voluntary motor activity of the body. Mechanical trauma sustained to the head during a car accident, fall, or sports injury can lead to a traumatic brain injury (TBI) that may have long ranging and sustained physical, cognitive and emotional effects. TBI is the most common form of brain injury and it contributes to one third of all injury related deaths in the United States. The Stabenfeldt lab aims to develop regenerative strategies that will harness inherent endogenous repair mechanisms in traumatic brain injury to improve functional outcomes in cognitive and motor functions. Previous research has demonstrated that the acute inflammatory response after TBI releases soluble cytokines that mediate regeneration after TBI. One of such soluble signal is stromal cell derived factor-1α (SDF-1α) and its receptor CXCR4. The SDF-1α/CXCR4 signaling axis directs the migration and organization of neural progenitor/ stem cells which is important in the regeneration of the injury area. In this study, we probed this paradigm by injecting bolus and nanoparticle exogenous SDF-1α intracortically into mice then sacrificing at 1, 3, and 7 days’ post-injection. Increased CXCR4 positive cells were expressed around the SDF-1α injection area. This study specifically focused on characterizing microglia and macrophage population in the brains that expressed CXCR4 via immunohistochemistry. Data from this study showed that the bolus group initiated microglial activation within the injection tract particularly at day 3 post injection which was resolved by day 7. However, the nanoparticle group initiated the activation of microglial/macrophages as early as day 1 post injection which proceeded to day 7. This shows that the nanoparticle groups initiated an
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inflammatory reaction in the injection tract irrespective of SDF-1α since the blank nanoparticle (nanoparticle with no SDF-1α) group exhibited the identical trend.

Stromal cell-derived factor-1α (SDF-1α) and its key receptor, CXCR4 are ubiquitously expressed in systems across the body (e.g. liver, skin, lung, etc.). This signaling axis regulates a myriad of physiological processes that range from maintaining of organ homeostasis in adults to, chemotaxis of stem/progenitor and immune cell types after injury. Given its potential role as a therapeutic target for diverse applications, surprisingly little is known about how SDF-1α mediated signaling propagates through native tissues. This limitation ultimately constrains rational design of interventional biomaterials that aim to target the SDF-1α/CXCR4 signaling axis. One application of particular interest is traumatic brain injury (TBI) for which, there are currently no means of targeting the underlying biochemical pathology to improve prognosis.

Growing evidence suggests a relationship between SDF-1α/CXCR4 signaling and endogenous neural progenitor/stem cells (NPSC)-mediated regeneration after neural injury. Long-term modulation of the SDF-1α/CXCR4 signaling axis is thus hypothesized as a possible avenue for harnessing and amplifying endogenous regenerative mechanisms after TBI. In order to understand how the SDF-1α/CXCR4 signaling can be modulated in vivo, we first developed and characterized a sustained protein delivery platform in vitro. We were the first, to our knowledge, to demonstrate that protein release profiles from poly(D,L,-lactic-co-glycolic) acid (PLGA) particles can be tuned independent of particle fabrication parameters via centrifugal fractioning. This process of physically separating the particles altered the average diameter of a particle population, which is in turn was correlated to critical release characteristics. Secondly, we demonstrated sustained release of SDF-1α from PLGA/fibrin composites (particles embedded in fibrin) with tunable burst release as a function of fibrin concentration. Finally, we contrasted the spatiotemporal localization of endogenous SDF-1α and CXCR4 expression in response to either bolus or sustained release of exogenous SDF-1α. Sustained release of exogenous SDF-1α induced spatially diffuse endogenous SDF-1/CXCR4 expression relative to bolus SDF-1 administration; however, the observed effects were transient in both cases, persisting only to a maximum of 3 days post injection. These studies will inform future systematic evaluations of strategies that exploit SDF-1α/CXCR4 signaling for diverse applications.