Shinya Yamanaka's 2006 discovery of induced pluripotent stem cells (iPSCs) ignited a revolution in the field of stem cell biology (1). For the first time, nearly all human somatic tissues could be produced from iPSCs reprogrammed from blood or skin cells, in a process that took only weeks. This advance was particularly crucial for obtaining surrogate tissues from cell types that are otherwise difficult to procure and do not readily expand in vitro, such as cardiac or neural cells. Additionally, many ethical concerns are avoided, because this technology uses a patient's own genetic material to create iPSCs rather than relying on embryonic stem cells. In the aftermath of Yamanaka's discovery, entire biomedical industries have developed around the promise of using human iPSCs (hiPSCs) and their derivatives for in vitro disease modeling, drug screening, and cell therapy (2).

The hiPSC technology has had a particularly notable impact in cardiac regenerative medicine, a field where scientists and clinicians have been working to devise new methods to better understand how cardiovascular disease manifests and how to restore cardiovascular function after disease strikes (3). The heart is limited in its ability to regenerate lost cardiomyocytes (beating heart muscle cells), following an adverse event such as a heart attack (4). Cardiomyocytes derived from hiPSCs (hiPSC-CMs) may represent a potential replacement option for dead cells in such a scenario. However, certain issues remain to be addressed, such as whether hiPSC-CMs can integrate with host myocardial tissue in the long term (5).

While using hiPSC-CMs for in vivo cell therapy may become practical in the future, employing hiPSC-CMs for high-throughput drug discovery and screening is becoming a reality in the present (6). Cardiovascular diseases can be recapitulated in a dish with patient-specific hiPSC-CMs. For example, if a patient exhibits a cardiac arrhythmia caused by a genetic abnormality in a sarcomeric protein or ion channel, that same rhythm problem can be recapitulated in vitro (7). Thanks to advances in hiPSC differentiation protocols, hiPSC-CMs can now be mass-produced to study cardiovascular disease mechanisms in vitro (8).

My graduate thesis in the laboratories of Joseph Wu and Sean Wu at Stanford University focused on in vitro applications of hiPSC-CMs for cardiovascular disease modeling and for high-throughput screening of chemotherapeutic compounds to predict cardiotoxicity. I initially embarked on a project using hiPSC-CMs to model viral myocarditis, a viral infection of the heart, caused by the B3 strain of coxsackievirus (9). I began by demonstrating that hiPSC-CMs express the receptors necessary for viral internalization and subsequently found that hiPSC-CMs were highly susceptible to coxsackievirus infection, exhibiting viral cytopathic effect within hours of infection. I also identified compounds that could alleviate coxsackievirus infection on hiPSC-CMs, a translationally relevant finding, as there remains a shortage of treatments for viral myocarditis.

Using a genetically modified variant of coxsackievirus B3 expressing luciferase, I developed a screening platform for assessing the efficacy of antiviral compounds. Pretreatment with interferon-, ribavirin, or pyrrolidine dithiocarbamate markedly suppressed viral replication on hiPSC-CMs by activating intracellular antiviral response and viral protein clearance pathways. These compounds alleviated viral replication in a dose-dependent fashion at low concentrations without causing cellular toxicity.

I next sought to use hiPSC-CMs to screen anticancer chemotherapeutic compounds for their off-target cardiovascular toxicities (10). Cardiotoxicity represents a major cause of drug withdrawal from the pharmaceutical market, and several chemotherapeutic agents can cause unintended cardiovascular damage (11). Using cultured hiPSC-CMs, I evaluated 21 U.S. Food and Drug Administrationapproved tyrosine kinase inhibitors (TKIs), commonly prescribed anticancer compounds, for their cardiotoxic potential. HiPSC-CMs express the major tyrosine kinase receptor proteins such as the insulin, insulin-like growth factor (IGF), vascular endothelial growth factor (VEGF), and platelet-derived growth factor (PDGF) receptors, lending validity to this cellular model.

Initially, human induced pluripotent stem cells (hiPSCs) can be produced by reprogramming skin or blood cells by nonviral or viral reprogramming methods. Cardiac differentiation protocols allow for the creation of cardiomyocytes derived from hiPSCs (hiPSC-CMs) for downstream applications, including in vitro disease modeling, drug screening, and regenerative cell therapy.

With data from a battery of cellular apoptosis, contractility, electrophysiology, and signaling assays, I generated a cardiac safety index to help align in vitro toxicity data to clinical drug safety guidelines (12). From the safety index, I determined that a subclass of VEGF receptor 2/PDGF receptorinhibiting tyrosine kinase inhibitors, some of which exhibit toxicity clinically, also elicited cardiotoxicities in hiPSC-CMs. These manifested as substantial alterations in cellular electrophysiology, contractility, and viability when administered at clinically relevant concentrations. I also discovered that cotreatment with either IGF or insulin partially rescued TKI-induced toxicity by up-regulating antiapoptotic signaling pathways. This work could prove useful for groups aiming to develop effective screening platforms to assess new chemotherapeutic compounds for cardiotoxic side effects.

I also collaborated with the Center for the Advancement of Science in Space (CASIS) to send a sample of hiPSC-CMs to the International Space Station. As humankind ventures beyond our home planet, it is imperative that we better understand how the heart functions for long periods of time in microgravity. Analysis of these hiPSC-CMs revealed microgravity-induced alterations in metabolic gene expression and calcium handling (13).

In recent years, the stem cell field has experienced an explosion of studies using hiPSC-CMs as a model cellular system to study cardiovascular biology. As improvements in hiPSC-CM mass production continue, we will see a rise in studies using these cells for disease modeling and drug screening. Thus, although hiPSC-CM technology is in its infancy, it holds great potential to improve cardiovascular health.

PHOTO: COURTESY OF A. SHARMA

FINALIST

Arun Sharma

Arun Sharma received his undergraduate degree from Duke University and a Ph.D. from Stanford University. Having completed a postdoctoral fellowship at the Harvard Medical School, Sharma is now a senior research fellow jointly appointed at the Smidt Heart Institute and Board of Governors Regenerative Medicine Institute at the Cedars-Sinai Medical Center in Los Angeles. His research seeks to develop in vitro platforms for cardiovascular disease modeling and drug cardiotoxicity assessment. http://www.sciencemag.org/content/367/6483/1206.1

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Stem cells to help the heart - Science Magazine

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