Induced neuronal cells: how to make and define a neuron

Abstract

Cellular plasticity is a major focus of investigation in developmental biology. The recent discovery that induced neuronal (iN) cells can be generated from mouse and human fibroblasts by expression of defined transcription factors suggested that cell fate plasticity is much wider than previously anticipated. In this review, we summarize the most recent developments in this nascent field and suggest criteria to help define and categorize iN cells that take into account the complexity of neuronal identity.

Figure 1. Summary of all iN cell studies to date

Figure 2. Direct versus indirect reprogramming

Long expression of the 4 Yamanaka factors lead to iPS cell formation when grown in ES cell media. Short expression induces a transient, unstable pluripotent state that can be quickly differentiated into neural precursors or cardiomyocytes depending on the media components. Direct reprogramming (e.g. iN cell reprogramming) does not involve a pluripotent intermediate stage. Red arrows: reprogramming, grey arrows: differentiation.

Figure 3. Neuronal properties in order of stringency (maturation/ extent of reprogramming)

Abbreviations: NT neurotransmitter, MAP2 microtubule associated protein 2

Figure 4. Examples of morphological and electrophysiological criteria of iN cells

A, Tuj1-positive fibroblasts extending one or more fairly thin cellular process. B, Immature iN cells extending one or two long branching neuritis from their soma. C, More mature iN cell morphologies characterized by multiple, long, branching processes extending from the cell body. Often iN cells sit on top of a dense network of neuritis derived from surrounding cells. D, A voltage clamp recording of spontaneous PSCs. The baseline is fairly tight and there is only little noise detectable. Both clusters of spikes (region 1, black) and separated spikes (region 2, red) represent most likely postsynaptic events as seen by higher time resolution (lower black and red traces). PSCs are typically of asymmetric shape with a fast deviation from the baseline followed by a slow rectification. E, A trace in the same recording mode with a much noisier baseline. Region 1 shows a group of spikes with similar amplitude deviation as in D. Higher resolution (lower black trace) reveals high levels of baseline fluctuation precluding the identification of PSCs. Other areas in the same trace (e.g. region 2, red trace) contain spikes that most likely represent synaptic currents.