Oligodendrocyte lineage cells—including oligodendrocyte precursor cells (OPCs), myelinating oligodendrocytes, and transitional types in between—constitute a major group of human brain cells1 that actively participate in many critical functions for the proper operation and maintenance of our central nervous system throughout neural development and aging2,3,4. While oligodendrocytes are well known for producing myelin to facilitate neuronal activity transmission and support axonal health in white matter, OPCs are abundant (~5%) in gray matter where myelination is scarce and perform activity-dependent signaling functions to govern learning behavior and memory formation5,6,7,8. How oligodendroglial cells function and dysfunction in the pathogenesis of Alzheimer’s disease (AD) and other age-associated neurodegenerative conditions has been understudied9. The inadequacies of an appropriate model system and deficiencies in general knowledge to guide an experimental path forward are the major reasons for this gap.
In light of the latest breakthroughs in deriving human brain cells from pluripotent stem cells including embryonic stem (ES) and induced pluripotent stem (iPS) cells, such cellular models in conjunction with modern gene editing tools have emerged as robust tools to handle the intricate nexus of cellular interactions in the brain, and are capable of demonstrating human-specific disease manifestations10,11. Considering that individual brain cell types can exhibit distinct and even conflicting effects in the face of the same AD-promoting conditions12,13, this stem cell methodology uniquely offers cell type-specific information that has previously been missed using established in vivo or in vitro models that only provide aggregate readouts from collections of brain cell types. In the last decade, a good number of reliable protocols have been developed to generate human neurons from trans-differentiation of ES/iPS cells or direct conversion from other terminally differentiated cell types (e.g., fibroblasts)14,15. In particular, the application of key neurogenic transcription factors (e.g., neurogenin 2, Ngn2)16 to human pluripotent stem cells can generate a homogeneous population of well-characterized neuronal cell types for pure cultures without a need for coculturing with glial cells12,17,18. For induced human oligodendrocytes, there are a few published protocols that can generate functional cells highly resembling their primary counterparts, with a wide range of efficiency and demand in time and resources19,20,21,22,23,24,25,26,27,28. To date, none of these protocols have been applied to investigate how oligodendroglial cells respond to and affect AD pathogenesis.
Here, we describe our improved protocols for single and mixed cultures of human induced neurons (iNs) and OPCs/oligodendrocytes (iOPCs/iOLs). The iN protocol described here is based on the widely used Ngn2 approach16, and has the additional feature of being glia-free. The resultant iNs are homogenous and highly resemble the cortical layer 2/3 excitatory neurons, with characteristic pyramidal morphology, gene expression pattern and electrophysiological features17,18 (Figure 1). To overcome some of the fundamental barriers in directed differentiation of pluripotent stem cells, we have developed a simple and effective method of low-dose dimethyl sulfoxide (DMSO) pre-treatment29,30, and reported an enhanced propensity of human ES/iPS cells to transdifferentiate into iOPCs and iOLs31, based on a widely-adapted protocol by Douvaras and Fossati32. We have further simplified the protocol and incorporated a robust differentiation-promoting compound, clemastine7,33,34, to accelerate the process of oligodendroglial maturation. As a result (Figure 2), the iOPCs can be generated in 2 weeks (~95% positive for the marker O4) and iOLs in four weeks (expressing mature markers MBP and PLP1). Interestingly, we found iOPCs alone secrete a remarkable amount of amyloid-β (Aβ), consistent with the independent transcriptomic data showing the abundant expression of the amyloid precursor protein (APP) and the processing protease β-secretase (BACE1) in oligodendrocyte lineage cells35,36. Moreover, our iN-iOPC co-culture system promotes the ensheathing of axons by MBP-positive iOL processes and provides significant support for synapse formation (Figure 3). Thus, the protocols we have detailed below have technical and biological advantages over previously catalogued neuron-oligodendroglia co-culturing methods, and hold a promise in better modeling the neurodegeneration in AD.