final ps has been posted variability in gene expression...
TRANSCRIPT
Final PS has been postedFinal PS has been posted(due 12/10 in class or before 3 PM in 68‐371B)
Last few lectures:Last few lectures:Developmental Systems Biology
‘Building an organismstarting from a single cell’starting from a single cell
Variability in gene expression underliesVariability in gene expression underliespartial penetrance in Caenorhabditis elegans
Nature, nurture, or just blind chance ?
Nature, nurture, or just blind chance ?Nature, nurture, or just blind chance ?
innate qualities (e.g. genome sequence) determinephenotype of an individual
Nature, nurture, or just blind chance ?Nature, nurture, or just blind chance ?
personal experiences (e.g. environment) determinephenotype of an individual
Nature, nurture, or just blind chance ?Nature, nurture, or just blind chance ?
E. coli budding yeast
Novick and Weiner (1957)Ozbudak et al. (2004)
Acar et al. (2005)
random variation (e.g. fluctuations in gene expression) d t i h t f i di id ldetermine phenotype of an individual
Introducing stochastic gene expression ‐ a thought experiment
mother
Introducing stochastic gene expression ‐ a thought experiment
daughter 1 daughter 2
Introducing stochastic gene expression ‐ a thought experiment
daughter 1 daughter 2
time time
d ht 1 d ht 2daughter 1 daughter 2
daughter 1
Introducing stochastic gene expression ‐ a thought experiment
daughter 1
cell division
grand daughter 1 grand daughter 2
To experimentally explorestochastic gene expression …g p
you would ideally count single… you would ideally count single molecules in single cells
Endogenous transcript counting in single cells Test in inducible systemsglucocorticoid receptor dependent transcription in human cells
No Dexamethasone 24 nM DexamethasoneNo Dexamethasone 24 nM DexamethasoneFKBP5 mRNA
A549 cells (human)
Test in inducible systemsOsmo‐shock induced gene expression in yeast
STL1 mRNA
No Salt 0.4 M NaCl
Yeast
mRNA transport in neurons
www.singlemoleculefish.comRaj et al. Nature Methods 5, 877 (2008).
What about stochastic gene expression in l i ll l i ?multicellular organisms?
Gene expression can be very noisy... … but the results often come out the same.
Dianne Arbus, Identical Twins, 1967Ozbudak et al. (2004)
Wild‐type organisms develop robustly
www.wormatlas.org
The C. elegans intestine is composed of E cell descendantscomposed of E cell descendants Intestinal fate is specified by a gene cascadeIntestinal fate is specified by a gene cascade
Cell lineage Gene expression Regulatory NetworkCell lineage
skn-1 skn-1
Gene expression Regulatory Network
med-1,2end-3 end-1
med-1,2end-3 end-1
OR
elt-2 elt-2
(Expresses throughoutrest of lifetime)
Intestine-specificgenes
(Morris Maduro, Joel Rothman, Jim McGhee)
Wild-type (N2)
DAPIend-3
DAPI(nuclei)
med-1,2med 1,2
elt-2elt 2end-1
Reconstructing the transcription dynamics ofendoderm specification in C. elegans (wild‐type)
Reconstructing the transcription dynamics ofendoderm specification in C. elegans (wild‐type)
Reconstructing the transcription dynamics ofendoderm specification in C. elegans (skn‐1 mutant)
(mutant)
Reconstructing the transcription dynamics ofendoderm specification in C. elegans (skn‐1 mutant)
(mutant)
(bimodal)
end‐1 expression is highly variable in skn‐1mutants
kskn‐1mutants
The number of cells expressing end‐1 and elt‐2 within mutant embryos varied greatly from embryo to embryo
Total elt‐2 expression correlates with number of expressing cells suggesting that variability in elt‐2 expression comes gg g y plargely from variable number of actively transcribing cells
The expression of end‐1 is quite variable even for a given total number of end 1 expressing cellsgiven total number of end‐1 expressing cells
Could end‐1 be a “controller” for elt‐2i ?expression?
Thresholding is observedin skn‐1mutants
num
ber
2tra
nscr
ipt n
end-1 transcript number
elt-2
400200 400200
Thresholding is observedin skn‐1mutants
How are these fluctuations
(mutant)
suppressed in wild‐type worm development?
0Number of nuclei
Deletion of end‐3 in wild‐type worms does increase end‐1/elt‐2 variability
Less redundancy leads to increased fluctuations
(mutant)(mutant)
(low noise) (mediumnoise)
(high noise)
wild type end-3 deletion skn-1 mutantyp3 transcription factors
controlling end-1
end 3 deletion2 transcription factors
controlling end-1
skn 1 mutant1 transcription factor
controlling end-1
Conclusions
We developed a method to count endogenous mRNA moleculesi fi d i di id l llin fixed individual cells www.singlemoleculefish.com
Raj et al. Nature Methods 5, 877 (2008)
In skn‐1mutants the embryo‐to‐embryo variability in the earlyIn skn 1mutants the embryo to embryo variability in the early transcription factors of endoderm development is much larger than in wild‐type embryos
In skn‐1mutants the endoderm fate is decided by thresholding on end‐1 levels
The less transcription factors controlling end‐1 the larger thevariability in end‐1 transcripts and endoderm fate decision