Do metabolites regulate embryo development?

Changes in cellular metabolites have been shown to regulate embryonic stem cell development at the earliest stages of life. Metabolites are simple compounds generated during life-sustaining chemical activities in cells.

The recent findings should improve scientists' ability to use embryonic stem cells to grow new tissues and organs to replace those damaged by disease or injury. The findings also could lead to new treatments for common disorders ranging from infertility to cancer.

The researchers reported on their study in the Nov. 16 issue of the journal Nature Cell Biology.

After fertilization, a human egg begins to travel down the fallopian tube. As it does, it begins to divide to form a ball of embryonic cells. Each of these cells, called naive pre-implantation embryonic cells, has the capacity to develop into any cell type in the human body — an ability called pluripotency.

When the developing embryo enters the uterus, it must implant into the uterine lining if the pregnancy is to proceed. When this occurs, the naive stem cells undergo a critical change as they take the first step toward differentiating into specific cell types, such as gut, muscle or nerve cells. Such cells are called primed embryonic stem cells.

"Implantation to a mother's uterus is arguably one of the hardest things we ever have to do in life. In fact, most embryos fail to successfully implant and the pregnancy ends."

Hannele Ruohola-Baker PhD, Department of Biochemistry,
Institute for Stem Cell and Regenerative Medicine, University of Washington, Seattle, Washington, USA, and corresponding author.

Scientists in the field of tissue regeneration are particularly interested in this shift from naive to primed embryonic stem cells. Although primed, post-implantation embryonic stem cells can still turn into any type of human cell, they are more difficult to work with than pre-implantation or naive cells.

To find out more about the differences between naive and primed pluripotent cells, the UW researchers first compared their gene profiles. This work conducted by Yuliang Wang, now a senior research associate at Oregon Health & Science University, uncovered intriguing differences in genes affecting cell metabolism.

"The expression of metabolic genes, particularly those related to the function of mitochondria, was much higher in naive cells.

"There was also a big difference in gene expression of a specific enzyme called nicotinamide N-methyltransferase [or NNMT]."

Yuliang Wang PhD, Computational Biology Program, Department of Biomedical Engineering, Computational Biology Program, School of Medicine, Oregon Health & Science University, Portland, Oregon, USA

To determine the effect of these changes, Henrik Sperber, a graduate student in the Ruohola-Baker laboratory, used a technique called mass spectroscopy to compare levels within cells of the metabolites.

The approach, called metabolomic analysis, is a 'chemical snapshot' that reveals in great detail what is going on within cells at a specific moment.

Just by looking at the metabolomic profiles of cells, researchers saw it was possible to distinguish between naive and primed pluripotent cells.

The telltale metabolite found to be enriched in naive cells was methylnicotinamide, abbreviated MNA and a product of the metabolic enzyme whose levels increase in many cancers — nicotinamide N-methyltransferase or abbreviated as NNMT.

When active, NNMT consumes a methyl group from a compound called S-adenosyl methionine. This methyl group is normally used in a gene regulation process called epigenetic histone methylation.

Without an adequate supply of the S-adenosyl methionine, regulation by histone methylation — and therefore correct gene expression — cannot take place.

Researchers found that in naive cells, NNMT was active and behaved as a metabolic 'methyl-sink' — lowering the level of methyl groups available.

It therefore limits the repression of genes by a process called histone methylation - where a methal group is added to the histone around which DNA is wrapped and alters how the DNA is either activated or repressed.

All of this occurs outside of the genetic blueprint or epigenetically.

In the primed cells, on the other hand, NNMT activity was low. As a result, S-adenosyl methionine was available for these epigenetic modifications that are required for a cell to enter the primed state.

In fact, by knocking out specific genes through CRISPR gene-editing technology, Julie Mathieu, acting instructor in Ruohola-Baker laboratory, demonstrated that it was possible to stabilize the cells in either the primed or naive state by manipulating NNMT activity alone.

"Our findings indicate that metabolites alone appear to be able drive many of the key changes in cell function and differentiation. In addition to advancing our understanding of human embryo development, the findings suggest we may be able to use metabolites - relatively simple compounds - to alter cell fate in the treatment of common disorders."

Hannele Ruohola-Baker PhD

For example, such an approach might eventually form the basis for treating the most common cause of infertility — failure of the embryo to successfully implant — or for affecting cell changes leading to cancer.

Abstract
For nearly a century developmental biologists have recognized that cells from embryos can differ in their potential to differentiate into distinct cell types. Recently, it has been recognized that embryonic stem cells derived from both mice and humans exhibit two stable yet epigenetically distinct states of pluripotency: naive and primed. We now show that nicotinamide N-methyltransferase (NNMT) and the metabolic state regulate pluripotency in human embryonic stem cells (hESCs). Specifically, in naive hESCs, NNMT and its enzymatic product 1-methylnicotinamide are highly upregulated, and NNMT is required for low S-adenosyl methionine (SAM) levels and the H3K27me3 repressive state. NNMT consumes SAM in naive cells, making it unavailable for histone methylation that represses Wnt and activates the HIF pathway in primed hESCs. These data support the hypothesis that the metabolome regulates the epigenetic landscape of the earliest steps in human development.

This work was supported by the American Heart Association, The Ellison Medical Foundation, the Schultz Fellowship for Health Sciences, the National Institutes of Health, and the National Heart Lung and Blood Institute Progenitor Cell Biology Consortium.

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Nov 17, 2015   Fetal Timeline   Maternal Timeline   News   News Archive   

Embryonic stem cells (ESCs) are pluripotent and derived from the inner cell mass (ICM)
of blastocyst embryos. Although identified over 30 years ago, they are still not
entirely understood.

Image Credit: TRENDS in Cell Biology