Midkine Overexpression Promotes Functional Recovery After Spinal Cord Injury by Enhancing Microglial Efferocytosis Via LRP-1.

OBJECTIVE: Traumatic spinal cord injury (SCI) induces neuronal apoptosis and neuroinflammation, which exacerbate secondary damage and hinder functional recovery. Efficient clearance of apoptotic cells and modulation of the inflammatory microenvironment of spinal cord are essential for promoting tissue repair. This study aimed to investigate whether Midkine (MDK), a heparin-binding growth factor, facilitates functional recovery after SCI and explores the underlying mechanisms. METHODS: A rat model of moderate SCI was established using Allen's impact method. Lentiviral vectors were used to overexpress MDK in the spinal cord. Behavioral assessments, including BBB score and gait analysis, were performed to evaluate motor function recovery. Motor evoked potentials (MEPs) serve as a neurophysiological tool for evaluating the functional integrity of the corticospinal tract. In vivo and in vitro experiments were conducted to assess microglial efferocytosis and elucidate the underlying molecular mechanisms. RESULTS: Transcriptomic bioinformatic analysis suggests that SCI is characterized by pronounced accumulation of apoptotic cells and robust neuroinflammatory responses, whereas single-cell analysis implicates MDK as a key contributor to neurorepair after SCI. MDK expression is dynamically regulated following SCI, with an early upregulation followed by a gradual decline over time, its location predominantly observed around microglial cells. Functionally, MDK overexpression significantly enhances motor recovery after SCI, accompanied by reduced neuroinflammation, decreased neuronal apoptosis, and improved neuroprotection. Mechanistically, MDK promotes microglial efferocytosis both in vivo and in vitro, activates the AKT/mTOR signaling pathway, upregulates BDNF and LRP-1 expression, and facilitates microglial polarization toward an anti-inflammatory M2 phenotype. Notably, inhibition of LRP-1 with receptor-associated protein (RAP) abolished the efferocytic and neuroprotective effects of recombinant MDK, highlighting LRP-1 as a key mediator of MDK's actions in microglia. CONCLUSION: Our study unveils the MDK/LRP-1/efferocytosis axis as a previously unrecognized therapeutic target for SCI. By orchestrating apoptotic cell clearance, dampening neuroinflammation, and fostering neuroprotection, this axis critically shapes the post-injury microenvironment to facilitate recovery. These findings suggest that MDK-centered therapy may represent a strategy for spinal cord repair, with LRP-1 modulation offering precise control over microglial responses.