Cortical Bone Loss and Fragility in a 2-Month Triple Transgenic Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) and osteoporosis frequently co-occur in the elderly; however, the pathophysiological link between these two diseases remains unclear. This study investigates skeletal alterations in a triple transgenic 3xTg-AD mouse model of AD (3xTg-AD), which harbors mutations in β-amyloid precursor protein (βAPP_Swe), presenilin-1 (PS1_M146V), and tau_P301L, and recapitulates key aspects of AD pathology, including age-dependent β-amyloid plaque accumulation and cognitive decline. To assess early skeletal changes, we analyzed femurs and tibiae of 2-month-old male non-Tg and 3xTg-AD mice (n = 9/group) using micro-CT. Despite the absence of β-amyloid plaques at this stage, 3xTg-AD mice showed significant cortical bone loss, with reduced bone surface, periosteal and endosteal perimeters, total and cortical cross-sectional area, and polar moment of inertia. The 3-point-bending test confirmed compromised mechanical properties, including reduced maximum load-to-fracture and stiffness. Histological analyses highlighted an increased number of Empty Osteocyte Lacunae, reduced TRAP⁺ osteocytes, and an elevated number of osteoclasts; such evidence indicates impaired osteocyte function and increased bone resorption. These findings indicate that cortical bone loss and compromised mechanical properties occur before detectable neuropathological hallmarks in this AD model.