Human neural stem cells directly programmed from peripheral blood show functional integration into the adult mouse brain

Transplantation of induced pluripotent stem cell-derived neural cells represents a promising strategy for treating neurodegenerative diseases. However, reprogramming of somatic cells and their subsequent neural differentiation is complex and time-consuming, thereby impeding autologous applications. Recently, direct transcription factor-based conversion of blood cells into induced neural stem cells (iNSCs) has emerged as a potential alternative. However, little is known about the functionality of iNSC-derived neurons upon in vivo transplantation. Here, we grafted human iNSCs derived from adult peripheral blood by temporary overexpression of the transcription factors SOX2 and cMYC into the hippocampus or striatum of adult unlesioned immunodeficient Rag2^tm1FwaIl2rg^tm1Wjl mice of both sexes. Engrafted cells gave rise to stable transplants composed of mature neurons displaying extensive neurite outgrowth and dendritic spine formation. Functional analyses of acute slices using patch clamp recordings revealed that already after 12 weeks of in vivo maturation, most of iNSC-derived cells possess unique properties exclusive to neurons and exhibit voltage-dependent ion channel currents as well as action potential firing. Moreover, the formation of spontaneous inhibitory and excitatory postsynaptic currents, along with Rabies virus-based retrograde monosynaptic tracing data, strongly supports the structural and functional integration of graft-derived neurons. Taken together, our data demonstrate that iNSCs directly derived from peripheral blood cells have the inherent capacity to achieve full functional maturation in vivo, qualifying them as an alternative potential donor source for restorative applications and deserving further investigation.