Hematopoietic stem cell transplantation can deliver therapeutic proteins to the central nervous system (CNS) through transplant-derived microglia-like cells. However, current conditioning approaches result in low and slow engraftment of transplanted cells in the CNS. Here we optimized a brain conditioning regimen that leads to rapid, robust, and persistent microglia replacement without adverse effects on neurobehavior or hematopoiesis. This regimen combines busulfan myeloablation and six days of Colony-stimulating factor 1 receptor inhibitor PLX3397. Single-cell analyses revealed unappreciated heterogeneity of microglia-like cells with most cells expressing genes characteristic of homeostatic microglia, brain-border-associated macrophages, and unique markers. Cytokine analysis in the CNS showed transient inductions of myeloproliferative and chemoattractant cytokines that help repopulate the microglia niche. Bone marrow transplant of progranulin-deficient mice conditioned with busulfan and PLX3397 restored progranulin in the brain and eyes and normalized brain lipofuscin storage, proteostasis, and lipid metabolism. This study advances our understanding of CNS repopulation by hematopoietic-derived cells and demonstrates its therapeutic potential for treating progranulin-dependent neurodegeneration.

Hematopoietic stem cell transplantation can deliver therapeutic proteins to the CNS through donor-derived hematopoietic cells that become microglia-like cells. However, using standard conditioning approaches, hematopoietic stem cell transplantation is currently limited by low and slow engraftment of microglia-like cells. We report an efficient conditioning regimen based on Busulfan and a six-day course of microglia depletion using the colony-stimulating factor receptor 1 inhibitor PLX3397. Combining Busulfan-myeloablation and transient microglia depletion results in robust, rapid, and persistent microglia replacement by bone marrow-derived microglia-like cells throughout the CNS. Adding PLX3397 does not affect neurobehavior or has adverse effects on hematopoietic reconstitution. Through single-cell RNA sequencing and high-dimensional CyTOF mass cytometry, we show that microglia-like cells are a heterogeneous population and describe six distinct subpopulations. Though most bone-marrow-derived microglia-like cells can be classified as homeostatic microglia, their gene signature is a hybrid of homeostatic/embryonic microglia and border associated-macrophages. Busulfan-myeloablation and transient microglia depletion induce specific cytokines in the brain, ultimately combining myeloid proliferative and chemo-attractive signals that act locally to repopulate microglia from outside the niche. Importantly, this conditioning approach demonstrates therapeutic efficacy in a mouse model of GRN deficiency. Transplanting wild-type bone marrow into Grn^-/- mice conditioned with Busulfan plus PLX3397 results in high engraftment of microglia-like cells in the brain and retina, restoring GRN levels and normalizing lipid metabolism.

Background: Osteoporosis and low bone mineral density are highly prevalent among patients undergoing spinal fusion, contributing to higher rates of hardware failure, revision surgery, and poor postoperative outcomes. Despite clear risks, perioperative screening and treatment for osteoporosis remain inconsistent. Bone Health Clinics (BHCs) and Fracture Liaison Services (FLSs) have emerged as multidisciplinary models to address this care gap. We describe the implementation of a dedicated BHC at a single academic center and evaluate perioperative pharmacotherapy patterns, treatment barriers, and surgical outcomes among spine patients. Methods: We retrospectively reviewed 174 consecutive perioperative spine patients referred to our institutional BHC between October 2019 and April 2024. Demographics, surgical characteristics, bone health status, laboratory and DXA results, pharmacologic management, contraindications, insurance-related barriers, and medication sequencing were collected. Surgical outcomes included hardware failure and revision surgery. Bone health response was assessed by follow-up DXA scans. Results: The cohort was predominantly female (78.2%) with a mean age of 71.9 years. Most patients were referred by neurosurgery (53.4%) or orthopedics (41.4%). Based on DXA and fragility fracture history, 27.0% had osteopenia, 56.3% osteoporosis, and 13.8% severe osteoporosis. Pharmacotherapy was prescribed in 146 patients (83.9%), most commonly romosozumab (32.9%), denosumab (22.6%), and abaloparatide (21.2%). Twenty-eight patients (16.1%) did not receive medication, primarily due to contraindications. Insurance-related barriers disproportionately affected anabolic agents, whereas denosumab had the lowest denial rate (15.2%, p = 0.0124). Sequential therapy was common (32.1%), most frequently romosozumab followed by denosumab. Hardware failure occurred in 11.5% of patients, with 5.7% requiring revision surgery. Among the 80 patients (46.0%) with follow-up DXA, 60% demonstrated improved bone mineral density, with an average T-score gain of 0.6 ± 0.5. Conclusions: Integration of a multidisciplinary BHC into perioperative spine care was feasible, associated with high rates of pharmacotherapy initiation, and demonstrated favorable early surgical outcomes compared to published complication rates in this population. Insurance and contraindications remain key barriers to anabolic therapy access, driving prescribing toward denosumab. Prospective studies are needed to confirm the impact of perioperative bone optimization on surgical durability, cost-effectiveness, and patient-centered outcomes.