1. dNTP metabolism in early
Drosophila melanogaster embryos
Debopriyo Biswas, Yonghyun Song, and Stanislav Y. Shvartsman
Lewis Sigler Institute for Integrative Genomics, Princeton University
Ribonucleotide reductase (RNR) is responsible for synthesis of
deoxyribonucleoside triphosphates (dNTPs). The precise balance of dNTP is
required for DNA replication and repair accuracy. Misregulation of dNTPs
content by mutations in RNR leads to high mutation rates which can cause
genomic instability and disease. Drosophila melanogaster embryos are used as
a system to investigate RNR activity. In the first 2 hours after fertilization, the
embryo undergoes 13 rapid synchronous nuclear divisions. By the end, the
embryo has around 6000 nuclei. For these divisions to happen, the embryo
needs abundant supply of deoxyribonucleoside triphosphates (dNTPs) for DNA
replication. We show, with in vivo experiments, that in early embryogenesis of
Drosophila melanogaster, RNR provides large amounts of dNTP while keeping
cellular dNTP concentrations balanced. We show that this regulation of balance
is in agreement with allosteric regulatory principles that have been assessed
previously in cell-culture and in vitro experiments.
Abstract
Background
100μm
Cycle 14Cycle 10 Cycle 11 Cycle 12 Cycle 13
• Young embryos (stage 1 and 2) are picked out and
dechorionated.
• Embryos are partially desiccated in preparation
for injection.
• Embryos are injected with 70 pL of chemical
solution (which is roughly 1% of embryo volume).
• Nuclear divisions are imaged using fluorescent
confocal microscopy, as embryos have histone
tagged with GFP.
Methods
• Ribonucleotide reductase (RNR)
catalyzes the synthesis of dNTPs
(deoxyribonucleoside
triphosphate) from NDPs
(nucleoside diphosphate).
• dNTP regulates enzyme activity
allosterically by product-inhibition.
(dNTP = dATP + dCTP + dTTP + dGTP)
• HU injection causes cell cycle failure at cycle 12.
• dNTP co-injection rescues HU induced cell cycle failure.
• dATP injected embryos fail at the same cell cycle as HU.
• dGTP injected embryos fail at cycle 13.
• dCTP and dTTP co-injection rescues dGTP-induced cell cycle failure.
• dCTP and dTTP injected embryos mostly complete all cycles normally.
Results
• RNR is responsible for dNTP synthesis in early embryos – injecting with
hydroxyurea (inhibitor of RNR) stops cell cycles.
• RNR follows allosteric regulatory principles established in in vitro
experiments.
• dGTP has delayed effect on cell cycle failure – dGTP injection drives
reduction of ADP to dATP, which in turn induces cell cycle failure.
Future direction: use mass spectrometry to quantify dNTP concentrations post
dGTP injection.
Conclusion
References and Acknowledgements
• D. melanogaster embryos undergo
13 rapid, synchronous nuclear
divisions in 2 hours, making about
6000 nuclei.
• If the mother loaded all dNTP
monomers required for all 13
divisions, the dNTP concentration in
the embryo would be over 200 μM,
which is 10x higher concentration
than that in most eukaryotic cells.
Such concentration would cause
mutations via replication errors.
• This suggests that dNTP is de novo
synthesized.
Hypothesis: Early D. melanogaster embryos satisfy rapid dNTP synthesis at low
dNTP concentrations via RNR enzyme, whose activity is product-inhibited.
Results
Count number of nuclei Measure area
• Nuclear densities are computed as
function of time (on MATLAB).
• If division fails, record time of failure.
Hofer, Anders, et al. "DNA building blocks: keeping control of manufacture."
Critical reviews in biochemistry and molecular biology 47.1 (2012): 50-63.
Huge thanks to everyone in Shvartsman lab for all the help, advice and memories.
Special thanks to Paul for feedback on the poster.
Thanks to Dr. Laevsky for help and training with the microscope.
This research was made possible by the generous support of the Lewis-Sigler
Institute for Integrative Genomics.
Dedicated to the countless fruit flies that die every day for the sake of biological research.
Cycle 9,
t = 8 mins
Division,
t = 13 mins
Cycle 10,
t = 21 mins
Failed division,
t = 58 mins
Dying embryo,
t = 70 mins
RNR Enzyme
Allosteric regulation in RNR
All embryos died
All embryos
passed the cycle
Half the embryos
passed the cycle
70% embryos pass
this cycle
Legend
Embryos after dATP (20 mM) injection, under confocal microscopy
Cell cycle success rate post injection
Nuclear divisions at the surface in early embryos
Source: IGTRCN