The evolving biology of small molecules: controlling cell fate and identity

Glycogen synthase kinase-3 inhibition promotes proliferation and neuronal differentiation of human-induced pluripotent stem cell-derived neural progenitors

Human-induced pluripotent stem cell-derived neural progenitors (hiPSC-NPs) have the ability to self-renew and differentiate into glial and neuronal lineages, which makes them an invaluable source in cell replacement therapy for neurological diseases. Therefore, their enhanced proliferation and neuronal differentiation are pivotal features that can be used in repairing neurological injuries. One of the main regulators of neural development is Wnt signaling, which results in the inhibition of glycogen synthase kinase 3 (GSK-3). Here, we assess the impact of GSK-3 inhibition by the small molecule CHIR99021 on the expansion and differentiation of hiPSC-NPs in an adherent condition and a defined medium. Cell proliferation analyses have revealed that inhibition of GSK-3 in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) increased the proliferation of hiPSC-NPs across 10 passages. The inhibition of β-catenin signaling by XAV and NOTCH signaling by DAPT reversed CHIR impact on hiPSC-NPs proliferation. The target genes of β-catenin, C-MYC and CYCLIN D1 as well as NOTCH target genes, HES1 and HES5 were upregulated. The treatment of NPs by CHIR in the absence of bFGF and EGF resulted in an increase of neuronal differentiation rather than proliferation by stabilization of β-catenin regardless of the NOTCH pathway. Thus, GSK-3 inhibition has been shown to promote proliferation of the NPs by activating β-catenin and NOTCH-related cell cycle genes in the presence of bFGF and EGF. Additionally, during GSK-3 inhibition, an absence of these growth factors allows for the switch to neuronal differentiation with a bias toward a dopaminergic fate. This may provide desired cells that can be used in therapeutic applications and offer insights into the etiology of some neurological disorders.

The reciprocal relationship between primordial germ cells and pluripotent stem cells

Primordial germ cells (PGCs) are induced in the epiblast early in mammalian development. They develop their specific fate separate from somatic cells by the generation of a unique transcriptional profile and by epigenetic modifications of histones and DNA. PGCs are related to pluripotent cells in many respects, both on a molecular and a cell biological level. Mimicking their in vivo development, PGCs can be derived in culture from pluripotent cells. Vice versa, PGCs can be converted in vitro into pluripotent embryonic germ cells. Recent evidence indicates that the derivation of pluripotent embryonic stem cells from explanted inner cell mass cells may pass through a germ cell-like state, but that this intermediate is not obligatory. In this review, we discuss PGC development and its relevance to pluripotency in mammalian embryos. We outline possibilities and problems connected to the application of in vitro-derived germ cells in reproductive medicine.

Mechanisms and techniques of reprogramming: using PDX-1 homeobox protein as a novel treatment of insulin dependent diabetes mellitus

Homeobox proteins are key regulators of stem cell proliferation and differentiation which function as transcription factors and regulate cell fate decisions. Pancreatic Duodenal Homeobox-1 (PDX-1) is a homeobox protein which acts as a key regulator in the development of b cells in the Islets of Langerhans. It plays an important role in maintaining the identity and function of the Islets of Langerhans, and in the development of the pancreas. There is strong evidence that PDX-1 plays a role in activating the insulin promoter and increasing insulin levels in response to glucose. PDX-1 also binds to sequences within β cells and regulates the promoter activity of a number of islet genes including insulin, glut-2 and neurogenin 3. When fused with the VP16 activation sequence, transfection of the PDX-1 gene has been shown to transform liver cells into insulin producing cells. Because homeobox proteins are able to passively translocate through cell membranes, due to an intrinsic transduction domain (penetratin), the use of these proteins to reprogram target cells may help overcome the limiting supply of β cells and be a potential future treatment for Type 1 diabetes.

Concise Review: Alchemy of Biology: Generating Desired Cell Types from Abundant and Accessible Cells

A major goal of regenerative medicine is to produce cells to participate in the generation, maintenance, and repair of tissues that are damaged by disease, aging, or trauma, such that function is restored. The establishment of induced pluripotent stem cells, followed by directed differentiation, offers a powerful strategy for producing patient-specific therapies. Given how laborious and lengthy this process can be, the conversion of somatic cells into lineage-specific stem/progenitor cells in one step, without going back to, or through, a pluripotent stage, has opened up tremendous opportunities for regenerative medicine. However, there are a number of obstacles to overcome before these cells can be widely considered for clinical applications. Here, we focus on induced transdifferentiation strategies to convert mature somatic cells to other mature cell types or progenitors, and we summarize the challenges that need to be met if the potential applications of transdifferentiation technology are to be achieved.

Cellular reprogramming: a new technology frontier in pharmaceutical research

Induced pluripotent stem cells via cellular reprogramming are now finding multiple applications in the pharmaceutical research and drug development pipeline. In the pre-clinical stages, they serve as model systems for basic research on specific diseases and then as key experimental tools for testing and developing therapeutics. Here we examine the current state of cellular reprogramming technology, with a special emphasis on approaches that recapitulate previously intractable human diseases in vitro. We discuss the technical and operational challenges that must be tackled as reprogrammed cells become incorporated into routine pharmaceutical research and drug discovery.

The evolving biology of cell reprogramming

Modern stem cell biology has achieved a transformation that was thought by many to be every bit as unattainable as the ancient alchemists' dream of transforming base metals into gold. Exciting opportunities arise from the process known as 'cellular reprogramming' in which cells can be reliably changed from one tissue type to another. This is enabling novel approaches to more deeply investigate the fundamental basis of cell identity. In addition, new opportunities have also been created to study (perhaps even to treat) human genetic and degenerative diseases. Specific cell types that are affected in inherited disease can now be generated from easily accessible cells from the patient and compared with equivalent cells from healthy donors. The differences in cellular phenotype between the two may then be identified, and assays developed to establish therapies that prevent the development or progression of disease symptoms. Cellular reprogramming also has the potential to create new cells to replace those whose death or dysfunction causes disease symptoms. For patients suffering from inherited cases of degenerative diseases like Parkinson's disease or amyotrophic lateral sclerosis (also known as motor neuron disease), the future realization of such cell-based therapies would truly be worth its weight in gold. However, before this enormous potential can become a reality, several significant biological and technical challenges must be overcome. Furthermore, to maintain the credibility of the scientific community with the general public, it is important that hope-inspiring advances are not over-hyped. The papers in this issue of the Philosophical Transactions of the Royal Society B: Biological Sciences cover many areas relevant to this topic. In this Introduction, we provide an overall context in which to consider these individual papers.