Conversion of mouse and human fibroblasts into functional spinal motor neurons

Abstract

The mammalian nervous system comprises many distinct neuronal subtypes, each with its own phenotype and differential sensitivity to degenerative disease. Although specific neuronal types can be isolated from rodent embryos or engineered from stem cells for translational studies, transcription factor-mediated reprogramming might provide a more direct route to their generation. Here we report that the forced expression of select transcription factors is sufficient to convert mouse and human fibroblasts into induced motor neurons (iMNs). iMNs displayed a morphology, gene expression signature, electrophysiology, synaptic functionality, in vivo engraftment capacity, and sensitivity to degenerative stimuli similar to those of embryo-derived motor neurons. We show that the converting fibroblasts do not transit through a proliferative neural progenitor state, and thus form bona fide motor neurons via a route distinct from embryonic development. Our findings demonstrate that fibroblasts can be converted directly into a specific differentiated and functional neural subtype, the spinal motor neuron.

Figure 1. Generation of Hb9∷GFP+ Induced Motor Neurons by 7 Factors

(A) Experimental outline. 11 candidate transcription factors include eight developmental genes in addition to the three iN factors. (B) Hb9∷GFP+ neurons express Tuj1 (purple). Scale bars represent 40 μm. (C) iMNs generated with 10 factors (without Isl1) express endogenous Islet (red). Scale bars represent 40 μm. (D) Isl1 is dispensable for generating iMNs. Scale bar represents 200 μm. (E) Reprogramming efficiency is greater with Hb9 or Isl1 on top of 4 factors (Lhx3, Ascl1, Brn2 and Myt1l) at day 21 post-transduction. Error bars indicate ±s.d. *P <0.05 (Student's t-test, two-tailed). (F) Addition of Ngn2 to the 6-factor pool (Hb9, Isl1, Lhx3, Ascl1, Brn2 and Myt1l) greatly enhances reprogramming efficiency as seen 10 days after transduction. Error bars indicate ±s.d. ***P<0.001; **P <0.01 (Student's t-test, two-tailed). (G) The 7 iMN factors convert adult tail tip fibroblasts into motor neurons. Scale bar represents 100 μm. See also Figure S1.

Figure 2. iMNs Possess Gene Expression Signatures of Motor Neurons

(A) Global transcriptional analysis of FACS-purified Hb9∷GFP+ motor neurons. iMNs cluster with control motor neurons and away from MEFs. (B–D) Pairwise gene expression comparisons show that iMNs are highly similar to embryo-derived motor neurons and dissimilar from the starting MEFs; black labeling denotes genes expressed in motor neurons, red labeling denotes genes expressed in fibroblasts, and the red lines indicate the diagonal and 2-fold changes between the sample pairs. (E) qRT-PCR data showing expression of endogenous transcripts of the 7 iMN factors relative to their levels in ES-MNs. Error bars indicate ± s.d. See also Figure S2 and Table S1.

Figure 3. iMNs Express Neuronal and Motor Neuron Proteins

(A) iMNs express the pan-neuronal marker Map2 (red). Scale bars represent 100 μm. (B) iMNs express synapsin (red). Scale bars represent 20 μm. (C) iMNs express vesicular cholineacetyltransferase (vChAT, red). Scale bars represent 40 μm. (D) iMNs express the motor neuron-selective transcription factor Hb9 (red). Scale bars represent 80 μm. See also Figure S2.

Figure 4. Electrophysiological Activity and In Vitro Functionality of iMNs

(A) iMNs express functional sodium channels. (B) iMNs express functional sodium and potassium channels. (C) iMN sodium channel activity is appropriately blocked by tetrodotoxin (TTX). (D) iMNs fire a single action potential upon depolarization. (E) iMNs fire multiple action potentials upon depolarization. (F) 100 μM GABA induces inward currents in iMNs. (G) 100 μM glycine induces inward currents in iMNs. (H) 100 μM kainate induces inward currents in iMNs. (I) iMN-induced contractions of C2C12 myotubes are blocked by 50 μM curare. The arrow indicates the timing of curare addition. (J) iMNs cultured with chick myotubes form NMJs with characteristic α-bungarotoxin (α-BTX, red) staining. The dotted line outlines the boundaries of a myotube. Scale bar represents 5 μm. See also Figure S3, Table S2 and Movie S1.

Figure 5. In Vivo Functionality and In Vitro Utility of iMNs

(A) Diagram showing the injection of iMNs into the neural tube of the stage 17 chick embryo. (B) Transverse sections of iMN-injected chick neural tube 5 day after transplantation. Arrows in both panels indicate the same axon of an iMN exiting the spinal cord through the ventral root. D: dorsal, V: ventral, VR: ventral root. (C) FACS-purified Hb9∷GFP+ iMNs co-cultured with wild-type or the mutant SOD1G93A-overexpressing glia for 10 days. Scale bars reperesent 5 μm. (D) Quantification of (C). Error bars indicate ±s.d. **P <0.01 (Student's t-test, two-tailed). (E) SOD1G93A iMNs exhibit reduced survival in culture with wild-type glia. Error bars indicate ±s.d. **P <0.01 (Student's t-test, two-tailed). (F) Changes in iMN number after 9 days of culture in the presence or absence of neurotrophic factors (GDNF, BDNF and CNTF). Error bars indicate ±s.d. **P <0.01 (Student's t-test, two-tailed). See also Figure S4.

Figure 6. Transdifferention Does Not Occur Through a Nestin+ Neural Progenitor State

(A) Percentage of iMNs that have incorporated BrdU. (B) Outline of the lineage tracing experiment using Nestin∷CreER; LOX-STOP-LOX-H2B-mCherry; Hb9∷GFP iPSCs or MEFs. To detect Nestin+ intermediates, cultures were treated with 1–2 μM 4-OHT during directed diffentiation of iPSCs (positive control) or during transdifferentiation of fibroblasts by the 7 factors. (C) FACS-purified, mCherry+ Hb9∷GFP+ motor neurons derived from the triple transgenic iPSCs in the presence of 1 μM 4-OHT. Expression of mCherry was observed in 3% of Hb9∷GFP+ cells (n > 2,000) and indicates the activation of Nestin∷CreER during directed differentiation. Scale bars represent 40 μm. (D) mCherry- Hb9∷GFP+ iMNs generated from the triple transgenic MEFs by transdifferentiation in the presence of 2 μM 4-OHT. mCherry+ iMNs were never observed (n > 5,000), suggesting a Nestin+ state is not accessed during reprogramming. Scale bars represent 40 μm.

Figure 7. Human iMNs Are Generated by 8 Transcription Factors

(A) An Hb9∷GFP+ neuron generated from a HEF culture by 8 transcription factors. Scale bars represent 80 μm. (B) Quantification of human iMN reprogramming efficiency at day 30 post-transduction. (C) Human iMNs express vesicular choline acetyltransferase (vChAT, red). Scale bars represent 80 μm. (D) Human iMNs express functional sodium and potassium channels. (E) Human iMNs fire action potentials upon depolarization. (F) 100 μM kainate induces inward currents in human iMNs. (G) 100 μM GABA induces inward currents in human iMNs.