Direct conversion of fibroblasts into functional astrocytes by defined transcription factors

Abstract

Direct cell reprogramming enables direct conversion of fibroblasts into functional neurons and oligodendrocytes using a minimal set of cell-lineage-specific transcription factors. This approach is rapid and simple, generating the cell types of interest in one step. However, it remains unknown whether this technology can be applied to convert fibroblasts into astrocytes, the third neural lineage. Astrocytes play crucial roles in neuronal homeostasis, and their dysfunctions contribute to the origin and progression of multiple human diseases. Herein, we carried out a screening using several transcription factors involved in defining the astroglial cell fate and identified NFIA, NFIB, and SOX9 to be sufficient to convert with high efficiency embryonic and postnatal mouse fibroblasts into astrocytes (iAstrocytes). We proved both by gene-expression profiling and functional tests that iAstrocytes are comparable to native brain astrocytes. This protocol can be then employed to generate functional iAstrocytes for a wide range of experimental applications.

Graphical abstract

Figure 1

NFIA, NFIB, and SOX9 Efficiently Reprogram Mouse Fibroblasts into iAstrocytes (A) Schematic representation of the one-step differentiation protocol. DIV, days in vitro. (B–D) Analysis of GFAP immunoreactivity in reprogrammed and control (noninfected [NI]) fibroblasts (d, days after infection). (E–G) Analysis of S100B immunoreactivity in reprogrammed and control fibroblasts. (H) Coimmunostaining of GFAP and S100B in iAstrocytes. (I) Immunocytochemical analysis for GLT-1 in iAstrocytes. (J) Quantification of GFAP, S100B, and GLT-1-positive cells in iAstrocytes (ABS9) and control fibroblasts. Cells nuclei are stained with DAPI. Data are expressed as means ± SEM. ^∗∗p < 0.01. The scale bar represents 20 μm (C, F, H, and I) and 100 μm (B, D, E, and G). Five independent experiments are represented in (J). See also Figures S1–S4.

Figure 2

Transcriptional Profiling of iAstrocytes (A) Microarray analysis comparison among noninfected MEFs (Fibro), induced astrocytes (iAstro), and primary cortical astrocytes (Astro). (B) Hierarchical clustering of the analyzed samples shows a strong degree of correlation between iAstro and primary astrocytes. (C–E) Scatterplot comparison between fibroblasts and iAstro shows that most of the astrocytic markers are increased in reprogrammed cells (C), whereas other neural markers are unaltered (D) and some fibroblasts markers are silenced (E). (F) Real-time RT-PCR analysis confirms that the expression of most astrocytic markers is comparable between iAstro and primary astrocytes. Data are expressed as means ± SEM. ^∗p < 0.05; ^∗∗p < 0.01. Three independent experiments are represented in (F).

Figure 3

Functional Characterization of Induced Astrocytes (A) Representative traces of ionic currents evoked in mouse-cultured cortical astrocytes (Astro), iAstrocytes (iAstro), and fibroblasts (Fibro) using a ramp (left) and voltage-step protocol (right). The stimulation protocols are depicted as insets and described in Experimental Procedures. (B) Graph of resting membrane potential values for the three cell types recorded before the membrane current activation. (C) Percentage of ramp current decrease measured at +100 mV by application of 4-AP (500 μM) in iAstrocytes, cortical astrocytes, and fibroblasts. (D) Representative Fura-2 responses triggered by thrombin (3.5 U/ml) application over time in astrocytes (gray trace), iAstrocytes (red trace), and fibroblasts (black trace). (E) The peak amplitudes of Fura-2 responses and the τ values of Ca²⁺ decay are shown as means ± SEM of three independent experiments (n > 50). (F and G) Glutamate uptake assay. [³H]L-glutamate cell content (F) and corresponding glutamate levels in the medium (G). ^∗∗p < 0.01; ^∗∗∗p < 0.001. Data are expressed as means ± SEM. Five cells recorded in three independent experiments are represented in (C) and (E). Three independent experiments are represented in (F) and (G).

Figure 4

Activation of iAstrocytes by Cytokines (A and B) Immunocytochemical analysis shows that GFAP expression (A) is higher in interleukin-1-stimulated (iAstro+Il1) than in untreated iAstrocytes (iAstro), whereas S100B expression (B) is not affected. (C and D) Quantification of GFAP- and S100B-positive cells (C) and staining intensity (D) in control and Il1-treated cells. AU, arbitrary units. (E) Real-time RT-PCR analysis shows that NF-kB pathway is significantly activated in induced astrocytes after Il1 treatment. Cells nuclei are stained with DAPI. Data are expressed as means ± SEM. ^∗p < 0.05; ^∗∗p < 0.01. The scale bar represents 60 μm (A) and 30 μm (B). Four independent experiments are represented in (C)–(E).

Figure 5

Conversion of Human Fibroblasts into iAstrocytes (A–C) Immunocytochemical analysis of GFAP and S100B in human neonatal fibroblasts reprogrammed with NFIA, NFIB, and SOX9 (ABS9) for 3 weeks. The insets show higher magnification of typical GFAP or S100B staining signals. (D) Quantification of GFAP- and S100B-positive cells in ABS9 or NI cells. Cells nuclei are stained with DAPI. Data are expressed as means ± SEM. ^∗∗p < 0.01. The scale bar represents 50 μm. Three independent experiments are represented in (D).