An Improved Method to Generate Human Induced Astrocytes

Direct cell reprogramming has the major advantage of maintaining the aging signature of the cell (Mertens et al, 2018), allowing to study age-related features of disorders on a patient-specific basis. For the past decade, several methods have been developed and optimized to efficiently convert patient-derived dermal fibroblasts to induced neurons (iNs) (Drouin-Ouellet et al, 2017; Ladewig et al, 2012; Mertens et al, 2015; Pang et al, 2011; Yoo et al, 2011). However, and as it has historically often been the case in the field of neuroscience, the main focus of direct neural reprogramming has been neurons, leaving the development of efficient methods to generate glia from human somatic cells scarce and slow.

Glia, including astrocytes, are major contributors of neurodegenerative disorders such as amyotrophic lateral sclerosis, Alzheimer's, and Parkinson's disease (Habib et al, 2020; Kam et al, 2020; Yamanaka and Komine, 2018). Astrocytes undergo major phenotypic and functional changes in the aging mammal (Palmer and Ousman, 2018), although how this applies to human astrocytes and whether this contributes to the development of age-related neurodegenerative disorders remain elusive due to the lack of systems in which to study the function of astrocytes in the context of aging on the scale of the human lifespan.

A new study published in Stem Cell Reports describes an efficient method to directly reprogram human fibroblasts to functional induced astrocytes (iAs) using the forced expression of two gliogenic transcription factors, NFIB and Sox9 (Quist et al, 2022). This method builds on recent work from the same group achieving forward programming of human induced pluripotent stem cells (iPSCs) using the same set of transcription factors (Canals et al, 2018).

With their improved protocol, converted cells express several classic astrocyte markers including S100B and GFAP, reach a stellate astrocytic morphology and perform many essential functions attributed to astrocytes, including glutamate uptake and the ability to form gap junctions, in addition to having similar electrophysiological properties as human fetal primary astrocytes, to express ATP-induced calcium signaling and to respond to inflammatory stimuli. The report also describes the first coculture system of iNs and iAs fully generated from human fibroblasts, which incidentally substantially improves iNs maturation.

Although direct reprogramming of mouse and human fibroblasts has previously been reported (Caiazzo et al, 2015; Tian et al, 2016), this new method reports an improvement of 120% over the previous transcription factor-based method (Caiazzo et al, 2015), and describes for the first time a successful generation of iAs from fibroblasts derived from 82- and 96-year-old donors. Moreover, iAs can be obtained in as early as 21 days (as compared with iPSC-to-astrocytes differentiation methods that are frequently taking >2 months), and may yield a similar level of genetic mosaicism than that found in fibroblasts, which would better recapitulate genetic heterogeneity.

These, combined with the likeliness of directly reprogrammed astrocytes to maintain the aging signature of the parental fibroblasts and the generation of iN/iA cocultures, constitutes a major advance to study the role of astrocytes and non-cell autonomous interaction in the context of aging and age-associated neurodegenerative disorders (Fig. 1). The method by Quist et al (2022) to generate iAs derived from human fibroblasts covering the whole human lifespan thus potentially facilitates the beginning of profound investigations on the interplay between astrocyte-driven pathophysiologies and aging.

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FIG. 1.

The potential of human iAs to study age-associated neurodegenerative disorders. The direct conversion of fibroblasts from human elderly donors to astrocytes using the transcription factors Sox9 and Nfib is likely to maintain the aging signature of the parental cell, thus allowing the study of age-associated changes leading to neurodegenerative disorders, including changes in senescence, inflammaging, metabolism, epigenetics, and pathological protein expression. Furthermore, the development of iN/iA cocultures will allow to study neuron–astrocyte interactions as well as neurodegeneration-inducing astrocytic factors and potential spread between cell type of pathological proteins. Part of the figure was created with BioRender.com. iAs, induced astrocytes; iNs, induced neurons.