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Martes 19 de julio a las 12 hs - en el aula de seminarios.<br><br>
Sebastian Kadener, PhD The Hebrew University of Jerusalem<br><br>
Título: "Understanding the robustness of the circadian clock: from
gene expression to neuronal networks and behavior"<br><br>
One of the key quests of modern biology is to disentangle the molecular
and cellular bases of behavior. Circadian (24hs) rhythms in locomotor
activity are one of the best-characterized behaviors at the molecular,
cellular and neural levels. Despite that, our understanding of how these
rhythms are generated is still limited. The current model postulates that
circadian clocks keep time through a complex
transcriptional-translational negative feedback loop that takes place in
the so-called “clock cells”. Each one of these clock cells has been
proposed to function autonomously (each cell is its own oscillator). In
<i>Drosophila</i>, the master gene CLK and CYC activate the circadian
system by promoting rhythmic transcription of several key clock genes.
Three of these target gene products, PER, TIM and CWO repress CLK-CYC
mediated transcription in a timely manner. These cycles of
transcriptional activation and repression lead to 24 hours molecular
oscillations, which ultimately generate the behavioral rhythms. <br><br>
Circadian clocks are extraordinarily <b>robust systems</b>; they are able
to keep time accurately without any timing cues. In addition, and despite
their biochemical nature they are resilient to big variations in
environmental conditions (i.e. temperature). This is likely the result of
possessing multiple layers of regulation, which assure accurate
timekeeping and buffering of stochastic changes into the molecular
clockwork. Recent evidence suggests that these layers of regulation
extend even beyond the single cell level. Circadian neurons in the brain
are organized in a network that is believed to synchronize the individual
neuronal oscillators thereby contributing to a coherent and robust
behavioral output.<br><br>
Our current work focuses on the study of mechanisms that contribute to
the robustness of the circadian clocks. For doing so we aim to tackle two
different issues: 1) <b>what are the mechanism that mediate this
robustness at the cell-autonomous (molecular-cellular) level?</b> and 2)
<b>How does the neural network structure of the brain circadian clock
contribute to this robustness? <br><br>
</b>1) By performing fluorescent real-time monitoring of single cells
carrying reporters we found that post-transcriptional regulation of the
central circadian transcription factor <i>clk </i>is key for robust
circadian gene expression. Briefly, we found that regulation of <i>clk
</i>(mainly by miRNAs) sets a threshold for the amount of <i>clk </i>mRNA
that will be translated into protein. At low transcriptional levels, no
protein is produced, however at higher levels of transcription, CLK<i>
</i>protein is produced in a dose dependent way. In reporters lacking
this regulation, leaking transcription is prevalent. As CLK is a very
powerful transcriptional activator, these small changes are greatly
amplified when we analyzed the effect of losing this regulation at the
single cell level on a CLK-driven transcriptional reporter (Tim-YFP).
This amplification is bigger than 10 fold for low levels of transcription
and <b>could explain how coherent cycles of CLK-driven transcription are
generated in the living animal</b>. We have demonstrated that this
regulation is also highly relevant <i>in vivo.</i> We are currently
determining whether this regulation also diminishes the transcriptional
noise of the system. <br><br>
2) By utilizing an inducible tool to perturb CLK-driven transcription
<i>in vivo</i>, we are studying the response of the circadian network to
perturbations in circadian transcription in different sets of neurons.
Indeed we found that while a well-studied peripheral circadian oscillator
is deeply affected by the disruption of CLK-driven molecular
oscillations, the brain circadian clock is more resilient to this
perturbation. We are currently determining which characteristics of the
brain neuronal network are responsible for this resistance.<br><br>
Host: Alejandro Colman Lerner<br><br>
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