The Cardiovascular Research Center at Massachusetts General Hospital


Randall T Peterson

Randall T Peterson

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Science - Zebrafish Chemical Genetics: Discovering chemical modifiers of cell specification, differentiation, and patterning

Overview
Our lab is interested in understanding the molecules that govern formation of the organs of the body. Organs form from immature, undifferentiated cells through several developmental processes, including:

    1. Specification–undifferentiated cells with potential to adopt multiple cell fates are
        specified to become one particular cell type.

    2. Differentiation–the specified cells mature into the appropriate cell type capable of
        performing a specialized function.

    3. Patterning–cells migrate to appropriate locations and assemble into tissues that
        comprise the functional organ.

We are using a chemical genetic approach to identify small molecules that modify these processes in the intact zebrafish. The compounds we discover are then used as probes to study the processes they modify. Beyond contributing to our fundamental understanding of developmental biology, the ability to control these processes in a relevant in vivo context may facilitate the development of therapies aimed at organ regeneration and repair.

Why take a small molecule–based approach?
Thousands of genetic mutations have been identified in zebrafish, worms, fruit flies, and mice that are helping to elucidate the principles of developmental biology. Small molecules complement these genetic mutations in several ways. Whereas genetic mutations are often permanent, small molecules modifiers of a biological process can be added and removed at will, providing precise temporal control over the biological process.

This is especially valuable in developmental biology where it is often difficult to discriminate between the role of a gene at a later stage of development and secondary effects of the mutation acting at earlier stages. Small molecules also allow the extent of disruption to be titrated by changing the small molecule dose. In addition, some small molecules can serve as lead compounds for developing therapies for human diseases.

The zebrafish as a tool for small molecule discovery
The unique attributes of the zebrafish embryo allow 'chemical genetic' technologies to be applied to complex developmental processes. Unlike yeast, flies, and worms that are generally resistant to small molecule permeation, zebrafish embryos readily absorb small molecules from the surrounding medium. Furthermore, their transparency and small size enable screening on a scale that would be prohibitive for mice or other vertebrate model organisms. We frequently use whole zebrafish for high–throughput chemical screens to identify potent, specific small molecule modifiers of many aspects of vertebrate development and to discover novel compounds that suppress disease phenotypes.

Two types of zebrafish small molecule screen have been carried out. The first type is a simple developmental screen in which wild–type embryos are exposed to small molecules from a library of thousands of structurally diverse compounds, and small molecules that induce specific developmental defects are identified. Using screens of this type, we have discovered dozens of compounds that cause specific defects in hematopoesis, cardiac physiology, embryonic patterning, pigmentation, and morphogenesis of the heart, brain, ear, and eye.

A second type of zebrafish small molecule screen is the modifier screen in which small molecules capable of modifying a disease phenotype are identified. We recently demonstrated the feasibility of this approach by identifying a novel class of compounds capable of suppressing the gridlock mutation.

Zebrafish gridlock mutants exhibit a dysmorphogenesis of the aorta that prevents circulation to the trunk and tail and is a model of human coarctation of the aorta. Gridlock mutants were exposed to 5,000 compounds from a diverse small molecule library. Several compounds were identified that completely restore gridlock mutants to normal without causing additional developmental defects.



Angiograms highlighting the vasculature of a pair of mutant zebrafish.
Both fish are homozygous for the gridlock mutation, but the defect in the fish shown in the top image has been suppressed by treatment with the small molecule GS4012, as shown in the bottom image.

Determining small molecule mechanisms of action
We use the novel small molecules we discover to identify and characterize the key pathways regulating cell specification, differentiation, and patterning. This normally involves identifying the molecular target of the small molecule using a combination of biochemical and genomic techniques. Current efforts include:

    1. Determining the mechanism by which GS4012 influences the arterial–venous cell fate
        decision.

    2. Identifying the key pathways governing differentiation of myeloid cells in the blood.

    3. Determining the mechanism by which concentramide alters patterning of the heart

Collaborative Relationships
Randall Peterson's lab is part of the Chemical Biology and Developmental Biology Programs and works closely with Calum MacRae's lab.

Publications
Click here to view Randall Peterson's publications »


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