New approaches for direct conversion of patient fibroblasts into neural cells

Abstract

Recent landmark studies have demonstrated the production of disease-relevant human cell types by two different methods; differentiation of stem cells using external morphogens or lineage conversion using genetic factors. Directed differentiation changes embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) into a desired cell type by providing developmental cues in an in vitro environment. Direct reprogramming is achieved by the introduction of exogenous lineage specific transcription factors to convert any somatic cell type into another, thereby bypassing an intermediate pluripotent stage. A variety of somatic cell types such as blood, keratinocytes and fibroblasts can be used to derive iPSC cells. However, the process is time consuming,laborious, expensive and gives rise to cells with reported epigenetic heterogeneity even amongst different iPSC lines from same patient which could propagate phenotypic variability. A major concern with the use of pluripotent cells as starting material for cell replacement therapy is their incomplete differentiation and their propensity to form tumors following transplantation. In comparison, transcription factor mediated reprogramming offers a direct route to target cell types. This could allow for rapid comparison of large cohorts of patient and control samples at a given time for disease modeling. Additionally, transcription factors that drive maturation may yield more functionally mature cells than directed differentiation. Several studies have demonstrated the feasibility of generating of cell types such as cardiomyocytes, hepatocytes, and neurons from fibroblasts. Here, we will discuss recent advances and key challenges regarding direct reprogramming of somatic cell types into diverse neural cells. This article is part of a Special Issue entitled SI: Exploiting human neurons.

Keywords: Direct conversion; Disease modeling; Induced neuron; Lineage conversion; Neurological disease; Reprogramming.

Figure 1. Different approaches for somatic cell reprogramming into neuronal lineages

Novel strategies are required to successfully increase the yield of homogeneous population of target cells and efficiency of reprogramming. Different aspects of reprogramming can be manipulated to obtain that goal like defined transcription factor cocktail, small molecules, non-coding RNAs and/or epigenetic modulation of gene regulatory network. While most neural lineages (neural stem/progenitor cells, different subtypes of neurons, glial cells like astrocytes and oligodendrocytes) has been successfully generated from patient fibroblasts using one or a combination of these methods, some like microglial cells are yet to be made.

Figure 2. Common pathways targeted by different cellular reprogramming approaches

Most methods that dictate the cellular conversion of somatic cells into neural cells have a mechanistic convergence. Diverse approaches like transcription factor cocktail, small molecules, miRNAs, siRNA or shRNA knockdown and/or protein repression usually result in effective chromatin remodeling through robust activation of neural genes and repression of fibroblastic genes. These are the areas that can be exploited for improving reprogramming efficiency.