Primary cortical neurons form calcium-phosphate sheets with a bone-precursor-like ultrastructure.

Calcium-phosphate is a critical component in healthy bone and teeth formation, but its pathologic buildup in brain can occur in dyshomeostatic calcium disorders like Alzheimer's disease and Leigh syndrome. Extracellular calcium-phosphate in the nervous system is not well understood, but prior evidence suggests mitochondria as a potential source. Recently, large unidentified double-membrane-bound hyper-electron dense sheet aggregates of unknown content were reported in Huntington's disease model neurons using cryo-ET that were absent in wild-type control neurons. We use a combination of cryo-ET, cryo-CLEM, and LDSAED to demonstrate that these sheet aggregates are generated by wild-type cortical neurons isolated from embryonic day 18 rat embryos and have an octacalcium phosphate-like structure. They appear to be derived from mitochondria and are extruded at least in part by migrasomes. These findings reveal an important link for how mitochondria can serve as reservoirs for intracellular and extracellular calcium-phosphate highlighting them as potential therapeutic targets in neurological disorders characterized by pathological calcification, such as Alzheimer's disease.