[Todos] Seminario Especial conjunto IFIBYNE-DFBMC-Sebastian Kadener -Martes 19 de julio a las 12 hs (RECORDATORIO)

[Patricia Iwanczyszyn]

Martes 19 de julio a las 12 hs - en el aula de seminarios.

Sebastian Kadener, PhD The Hebrew University of Jerusalem

Título: "Understanding the robustness of the 
circadian clock: from gene expression to neuronal networks and behavior"

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 Drosophila, 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.

Circadian clocks are extraordinarily robust 
systems; 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.

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) what are the 
mechanism that mediate this robustness at the 
cell-autonomous (molecular-cellular) level? and 
2) How does the neural network structure of the 
brain circadian clock contribute to this robustness?

1) By performing fluorescent real-time monitoring 
of single cells carrying reporters we found that 
post-transcriptional regulation of the central 
circadian transcription factor clk is key for 
robust circadian gene expression. Briefly, we 
found that regulation of clk (mainly by miRNAs) 
sets a threshold for the amount of clk mRNA that 
will be translated into protein. At low 
transcriptional levels, no protein is produced, 
however at higher levels of transcription, CLK 
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 
could explain how coherent cycles of CLK-driven 
transcription are generated in the living animal. 
We have demonstrated that this regulation is also 
highly relevant in vivo. We are currently 
determining whether this regulation also 
diminishes the transcriptional noise of the system.

2) By utilizing an inducible tool to perturb 
CLK-driven transcription in vivo, 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.

Host: Alejandro Colman Lerner


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