Functional maturation of human iPSC-derived pyramidal neurons in vivo is dependent on proximity with the host tissue.

Human induced pluripotent stem cells (hiPSCs) have been used extensively in vitro to model early events in neurodevelopment. Because of a number of shortcomings, previous work has established a potential to use these cells in vivo after transplantation into the mouse brain. Here, we describe a systematic approach for the analysis of transplanted hiPSC-derived neurons and glial cells over time in the mouse brain. Using functional two-photon imaging of GCaMP6f- expressing human neural cells, we define and quantify the embryonic-like features of their spontaneous activity. This is substantiated by detailed electron microscopy (EM) of the graft. We relate this to the synaptic development the neurons undergo up to 7 months in vivo. This system can now be used further for the genetic or experimental manipulation of developing hiPSC-derived cells addressing neurodevelopmental diseases like schizophrenia or Autism Spectrum Disorder.

A chimeric mouse model to study human iPSC-derived neurons: the case of a truncating SHANK3 mutation.

Using human induced pluripotent stem cells (iPSC), recent studies have shown that the events underlying autism spectrum disorders (ASD) can occur during neonatal development. We previously analyzed the iPSC-derived pyramidal cortical neurons of a subset of patients with ASD carrying de novo heterozygous mutations in postsynaptic SHANK3 protein, in culture. We reported altered spinogenesis of those neurons. The transplantation of human iPSC-derived neuronal precursors into mouse brain represents a novel option for in vivo analysis of mutations affecting the human brain. In this study, we transplanted the neuronal precursor cells (NPC) into the cortex of newborn mice to analyze their integration and maturation at early stages of development and studied axonal projections of transplanted human neurons into adult mouse brain. We then co-transplanted NPC from a control individual and from a patient carrying a de novo heterozygous SHANK3 mutation. We observed a reduction in cell soma size of selective neuronal categories and in axonal projections at 30 days post-transplantation. In contrast to previous in vitro studies, we did not observe any alteration in spinogenesis at this early age. The humanized chimeric mouse models offer the means to analyze ASD-associated mutations further and provide the opportunity to visualize phenotypes in vivo.

Long-term development of human iPSC-derived pyramidal neurons quantified after transplantation into the neonatal mouse cortex.

One of the main obstacles for studying the molecular and cellular mechanisms underlying human neurodevelopment in vivo is the scarcity of experimental models. The discovery that neurons can be generated from human induced pluripotent stem cells (hiPSCs) paves the way for novel approaches that are stem cell-based. Here, we developed a technique to follow the development of transplanted hiPSC-derived neuronal precursors in the cortex of mice over time. Using post-mortem immunohistochemistry we quantified the differentiation and maturation of dendritic patterns of the human neurons over a total of six months. In addition, entirely hiPSC-derived neuronal parenchyma was followed over eight months using two-photon in vivo imaging through a cranial window. We found that transplanted hiPSC-derived neuronal precursors exhibit a "protracted" human developmental programme in different cortical areas. This offers novel possibilities for the sequential in vivo study of human cortical development and its alteration, followed in "real time".