Gene expression oscillations are widespread in biological systems. The segmentation clock is one such oscillator controlling segmentation of the vertebral column. Its disruption results in congenital vertebral defects in humans. We have built a mathematical pacemaker model that is based on a transcriptional-feedback loop. Our model predicts that a moderate increase in the stabilities of Her proteins or mRNAs should lengthen the period, while a further increase would abolish the oscillations. To establish the mechanisms of Her protein and mRNA turnover, and to elucidate the mechanism that transfers this periodic information to cells of the next- forming segments, we will: 1. Discover the post-translational mechanism that rapidly recycles oscillating proteins: We will perform loss-of- function experiments for proteins that are candidates to regulate rapid degradation of Her proteins. We will assess whether any of these genes regulate the degradation of the Her-family proteins. Finally, to test the prediction of our model, we will stabilize Her proteins to varying levels by reducing expression of proteins that trigger its degradation and measure corresponding changes in the segmentation process. 2. Discover the post-transcriptional mechanism that rapidly recycles oscillating RNAs: We will determine loss- of-function of which her-RNA-binding proteins stabilize her mRNAs and result in vertebral segmentation defects. We will test the prediction of our model by increasing the halflives of her mRNAs through reduced expression of RNA-binding proteins and determining how the segmentation process is affected. 3. Discover the information transfer mechanism from the segmentation clock to the segmentation machinery: We will perform time-resolved overexpression and loss-of-function experiments for the mesp gene to determine its impact on the segmentation process. We will determine the regulatory cascade starting with the segmentation clock, continuing through the dynamically expressed mesp transcription factor and ending with the formation of segment boundaries. Hes proteins also oscillate in neural progenitor cells, ovary cells and embryonic stem cells, where the oscillations appear to control the temporal switch from proliferation to differentiation. Gain-of-function of Hes proteins is correlated with cancer and their inhibition restores differentiation. Elucidating the dynamics of the Hes/Her oscillations during vertebral segmentation is significant not only for understanding and potentially preventing vertebral malformations, but also for developing approaches for controlled stem cell proliferation and differentiation in various tissues and therapies against cancer progression. Therefore, this application has strong relevance to the mission of the National Institute of Health.

Public Health Relevance

Vertebral segmentation in controlled by a gene expression oscillator functioning during embryonic development. The genetic bases of many vertebral defects are currently unknown. Discovery of genes involved in the oscillator mechanism is crucial as mutations in these genes are prime candidates for a causal role in birth defects of the vertebral column in humans.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
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Modeling and Analysis of Biological Systems Study Section (MABS)
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Lyster, Peter
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Albert Einstein College of Medicine
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