Three-dimensional cell-cell interactions promote direct reprogramming of patient fibroblasts into functional and transplantable neurons.

Direct reprogramming of somatic cells into induced neurons (iNs) has become an attractive strategy for the generation of patient-specific neurons for disease modeling and regenerative neuroscience. To this end, adult human dermal fibroblasts (hDFs) present one of the most relevant cell sources. However, iNs generated from adult hDFs using two-dimensional cultures are difficult to maintain in vitro and face challenges in survival upon transplantation into the adult brain, thus imposing constraints on biomedical applications of iN technology. Here, we present a platform for direct in vitro reprogramming of adult hDFs inside three-dimensional suspension microcultures (3D-iNs). We show that the 3D environment favors neuronal over fibroblast cellular identity to yield more robust conversion into functional neurons with extended culturing span. The 3D reprogramming approach also provides a platform for fusion into induced assembloids. 3D-iNs can be gently harvested and transplanted into the adult rodent brain to reproducibly generate neuron-rich grafts, thus eliminating a major bottleneck in the direct reprogramming field.