The gastrointestinal tract initially forms as a tube inside the embryo. Accessory digestive organs sprout from this tube in response to inductive signals from the surrounding mesoderm. Both the pancreas and the liver form at about the same time (4th week after fertilization) and at about the same place in the embryonic gut (the junction between the foregut and the midgut).
The pancreas forms as ventral and dorsal outgrowths that eventually fuse together when the gut rotates. The liver forms from the “hepatic diverticulum” that grows from the gut about 23-26 days after fertilization. These liver bud cells work with surrounding tissues to form the liver.
What determines whether an endodermal cell becomes a liver or pancreatic precursor cell?
Wolfram Goessling and Trista North from the Harvard Stem Cell Institute (HSCI) have identified a gradient of the molecule prostaglandin E (PGE) in zebrafish embryos that acts as a liver/pancreas switch.
Postdoctoral researcher Sahar Nissim in the Goessling laboratory has uncovered how PGE toggles endodermal cells between the liver-pancreas fate. Nissim has shown that endodermal cells exposed to more PGE become liver cells and those exposed to less PGE become pancreas. This is the first time that prostaglandins have been reported as the factor that can switch cell identities from one fate to another.
After completing these experiments, HSCI scientists collaborated with colleague Richard Mass to determine if their PGE-mediated cell fate switch also occurred in mammals. Here again, Richard Sherwood from the Mass established that mouse endodermal cells became liver if exposed to PGE and pancreas if exposed to less PGE. Sherwood also demonstrated that PGE enhanced liver growth and regeneration.
Goessling become interested in PGE in 2005, when a chemical screen identified PGE as an agent that amplified blood stem cell populations in zebrafish embryos. Goessling that transitioned this work to human patients, and a phase 1b clinical trial that uses PGE to increase umbilical cord blood transplants has just been completed.
PGE might be useful for instructing pluripotent human stem cells that have been differentiated into endodermal cells to form completely functional, mature liver cells that can be used to treatment patients with liver disease.
Skeletal muscle – that type of voluntary muscle that allows movement – has proven difficult to grow in the laboratory. While particular cells can be differentiated into skeletal muscle cells, forming a coherent, structurally sound skeletal muscle is a tough nut to crack from a research perspective.
Another problem dogging muscle research is the difficulty growing new muscle in patients with muscle diseases such as muscular dystrophy or other types of disorders that weaken and degrade skeletal muscle.
Now research groups at the Boston Children’s Hospital Stem Cell Program have reported that they can boost the muscle mass and even reverse the disease of mice that suffer from a type of murine muscular dystrophy. To do this, this group use a combination of three different compounds that were identified in a rapid culture system.
This ingenious rapid culture system uses the cells of zebrafish (Danio rerio) embryos to screen for these muscle-inducing compounds. These single cells are placed into the well of a 96-well plate, and then treated with various compounds to determine if those chemical induce the muscle formation. To facilitate this process, the zebrafish embryo cells express a very special marker that consists of the myosin light polypeptide 2 gene fused to a red-colored protein called “cherry.” When cells become muscle, they express the myosin light polypeptide 2 gene at high levels. Therefore, any embryo cell that differentiates into muscle should glow a red color.
Once a cocktail of muscle-inducing chemicals were identified in this assay, those same chemicals were used to treat induced pluripotent stem cells made from cells taken from patients with muscular dystrophy. Those iPSCs were treated with the combination of chemicals identified in the zebrafish embryo screen as muscle inducing agents.
The results were outstanding. Leonard Zon from the Division of Hematology/Oncology, Children’s Hospital Boston and Dana-Farber Cancer Institute and his colleagues showed that a combination of basic Fibroblast Growth Factor, an adenylyl cyclase activator called forskolin, and the GSK3β inhibitor BIO induced skeletal muscle differentiation in human induced pluripotent stem cells (iPSCs). Furthermore, these muscle cells produced engraftable myogenic progenitors that contributed to muscle repair when implanted into mice with a rodent form of muscular dystrophy.
Zon hopes that clinical trials can being soon in order to translate these remarkable results into patients with muscle loss within the next several years. Zon and his co-workers are also screening compounds to address other types of disorders beyond muscular dystrophy.
This paper represents the application of shear and utter genius. However, there is one caveat. The mice into which the muscles were injected were immunodeficient mice whose immune systems are unable to reject transplanted tissues. In human patients with muscular dystrophy, an immune response against dystrophin, the defective protein, has been an enduring problem (for a review of this, see T. Okada and S. Takeda, Pharmaceuticals (Basel). 2013 Jun 27;6(7):813-836). While there have been some technological developments that might circumvent this problem, transplanting large quantities of muscle cells might be beyond the pale. Muscular dystrophy results from disruption of an important junction between the muscle and substratum to which the muscle is secured. This connection is mediated by the “dystrophin-glycoprotein complex.” Structural disruptions of this complex (shown below) lead to unanchored muscle that cannot contract properly, and eventually atrophies and degrades.
This is a remarkable advance, but until the host immune response issue is satisfactorily addressed, it will remain a problem.