The application of in vitro-derived human neurons in neurodegenerative disease modeling

Abstract

The development of safe and effective treatments for age-associated neurodegenerative disorders is an on-going challenge faced by the scientific field. Key to the development of such therapies is the appropriate selection of modeling systems in which to investigate disease mechanisms and to test candidate interventions. There are unique challenges in the development of representative laboratory models of neurodegenerative diseases, including the complexity of the human brain, the cumulative and variable contributions of genetic and environmental factors over the course of a lifetime, inability to culture human primary neurons, and critical central nervous system differences between small animal models and humans. While traditional rodent models have advanced our understanding of neurodegenerative disease mechanisms, key divergences such as the species-specific genetic background can limit the application of animal models in many cases. Here we review in vitro human neuronal systems that employ stem cell and reprogramming technology and their application to a range of neurodegenerative diseases. Specifically, we compare human-induced pluripotent stem cell-derived neurons to directly converted, or transdifferentiated, induced neurons, as both model systems can take advantage of patient-derived human tissue to produce neurons in culture. We present recent technical developments using these two modeling systems, as well as current limitations to these systems, with the aim of advancing investigation of neuropathogenic mechanisms using these models.

Keywords: Alzheimer's disease; aging; direct conversion; iNs; iPSCs; induced neuron; induced pluripotent stem cells; neurodegenerative disease; stem cells.

Figure 1.. Schematic depicting conceptual steps involved in the generation of in vitro models for human neurodegenerative disease research.

(A) Human induced pluripotent-derived neurons (hiPSC-Ns) and (B) induced neurons (iNs). (A) Human primary fibroblasts, or other somatic primary cell source from living or deceased subjects, are (1) reprogrammed back to the fetal state into hiPSCs through the introduction of defined transcription factors, (2) induction of the neural fate is achieved through dual SMAD signaling inhibition, resulting in neural stem cells (NSCs) and (3) neurons are differentiated with the aid of neurotrophic factors in addition to other small molecules. hiPSC-Ns capture a unique patient’s genome in a dish and can recapitulate many factors in neurodegenerative disease processes, especially those with a genetic driver. Other CNS cell types can be generated this in-vitro technology as well; hiPSCs can be differentiated into microglia by driving towards a hematopoietic lineage with small molecules, and NSCs can give rise to astrocytes and oligodendrocytes. (B) The same starting fibroblast cells can be directly converted to iNs by the introduction of neuronal-specific transcription factors and small molecule cocktails that include SMAD pathway inhibitors and cell cycle blockers. Cells undergoing direct conversion to iNs bypass the pluripotent fetal state and may retain more of the epigenetic signature of the starting donor cells, rendering them to more likely represent phenotypes which may occur with cellular age.