Download - Gene Regulatory Networks
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Gene Regulatory Networks
slides adapted from
Shalev Itzkovitz’s talk
given at IPAM UCLA on July 2005
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Protein networks - Protein networks - optimizedoptimized molecular computersmolecular computers
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E. coli – a model organismE. coli – a model organism
Single cell, 1 micron length
Contains only ~1000 protein types at any given moment
still :still : Amazing technologyAmazing technology
sensors enginecomputer
Communication bus
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Can move toward food and away from Can move toward food and away from toxinstoxins
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Flagella assemblyFlagella assembly
•Composed of 12 types of proteins
•Assembled only when there is an environmental need for motility
•Built in an efficient and precise temporal order
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Proteins are encoded by DNA Proteins are encoded by DNA
DNA – same inside every cell, the instruction manual, 4-letter chemical alphabet – A,G,T,C
E. Coli – 1000 protein types at any given moment
>4000 genes (or possible protein types) – need regulatory mechanism to select the active set
DNA
RNA
Protein
transcription
translation
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Gene RegulationGene Regulation
proteinprotein Inducer(external signal)
•Proteins are encoded by the DNA of the organism.
protein
promoter regionACCGTTGCAT
Coding regionDNA
•Proteins regulate expression of other proteins by interacting with the DNA
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INCREASED TRANSCRIPTION
X X*
Sx
X*
Y
Y
ActivatorX
YY
X binding sitegene Y
X Y
Bound activator
Activators increase gene productionActivators increase gene production
No transcription
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Bound repressor X Y
X X*
Sx
No transcription
X*
Unbound repressor
X
Bound repressor
Y
YY
Y
Repressors decrease gene productionRepressors decrease gene production
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X1 X2 X3
Signal 1 Signal 2 Signal 3 Signal 4 Signal N
Xm
gene 1 gene 2 gene 3 gene 4 gene 5 gene 6 ...gene k
Environment
Transcription factors
genes
...
...
An environmental sensing mechanismAn environmental sensing mechanism
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Gene Regulatory NetworksGene Regulatory Networks
•Nodes are proteins )or the genes that encode them(
X Y
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•shallow network, few long cascades.
•compact in-degree )promoter size limitation(
The gene regulatory network of E. coliThe gene regulatory network of E. coli
Shen-Orr et. al. Nature Genetics 2002
•modular
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Asymmetric degree distribution due toAsymmetric degree distribution due toPromoter size limitationPromoter size limitation
protein
promoter regionACCGTTGCAT
Coding regionDNA
X
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What logical function do the nodes represent?What logical function do the nodes represent?
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Example – Energy source utilizationExample – Energy source utilization
lacZ is a protein needed to break down lactose into carbonThe E. coli prefers glucose
lacZ
2 possible energy sources
How will the E. coli decide when to create this protein?
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Proteins have a costProteins have a cost
•E. Coli creates ~106 proteins during its life time
•~1000 copies on average for each protein type
E. Coli will grow 1/1000 slower,Enough for evolutionary pressure
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AND gate encoded by proteins and DNAAND gate encoded by proteins and DNA
lacZ gene is controlled by 2 “sensory” proteins :
TTGACA…TATAAT
TTGACA…TATAAT
TTGACA…TATAAT
TTGACA…TATAAT
Jacob & Monod, J. Mol. Biol. 1961
lactose
~glucose
LacZ Production
lactose sensor
Unbinds when senses lactose
glucose absence sensor
binds when senses no glucose
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Experimental measurement of input Experimental measurement of input functionfunction
GFP
promoter….ctgaagccgcttt….
The bacteria becomes greenin proportion to the production rate
E.Coli
Glu
cose
Lactose
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The input function of the lactose operon The input function of the lactose operon is more elaborate than a simple AND gateis more elaborate than a simple AND gate
Setty et. al. PNAS 2003
Lac
tos
e (IP
TG
)
glucose(cAMP)
glucose
(cAMP)
lactose(IPTG)
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E. Coli can modify the input function by E. Coli can modify the input function by small changes in the promoter DNA small changes in the promoter DNA
…AAGGCCT…
…AAGTCCT…
…AAGTCTT…
AND gateAND gate
OR gateOR gate
LacZ gateLacZ gate
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Input function is optimally tuned Input function is optimally tuned to the environmentto the environment
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XA
XAK
Negative autoregulationNegative autoregulation
Simple regulation
Negative autoregulation
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Blue nodes have self-edges
N=420 NodesE=520 EdgesEs=40 self-edges
Negative autoregulationNegative autoregulation is a hugely is a hugely statistically significant patternstatistically significant pattern
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A protein with negative autoregulation is A protein with negative autoregulation is a recurring pattern with a defined functiona recurring pattern with a defined function
Are there larger recurring patternsAre there larger recurring patternswhich play a defined functional role ?which play a defined functional role ?
Recurring patternRecurring pattern Defined functionDefined functionlogic networklogic network
XORXOR
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Network motifs
Subgraphs which occur in the real network significantly more than in a
suitable random ensemble of networks.
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3-node subgraph
Basic terminologyBasic terminology
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Basic terminologyBasic terminology
4-node subgraph
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x
y
z
x
y
z
Feed-forward loop 3-node feedback loop (cycle)
Two examples of 3-node subgraphsTwo examples of 3-node subgraphs
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13 directed connected 3-node subgraphs13 directed connected 3-node subgraphs
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199 4-node directed connected subgraphs
And it grows pretty fast for larger subgraphs : 9364 5-node subgraphs,
1,530,843 6-node…
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Real = 5 Rand=0.5±0.6
Zscore (#Standard Deviations)=7.5
5
6 13
1
2 16
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Network motifs
Subgraphs which occur in the real network significantly more than in a suitable random
ensemble of networks.
Algorithm : 1) count all n-node connected subgraphs in the real network.
3) generate an ensemble of random networks- networks which preserve the degree sequence of the real network
4) Repeat 1) and 2) on each random network
•Subgraphs with a high Z-score are denoted as network motifs.
rand
randreal NNZ
2) Classify them into one of the possible n-node isomorphic subgraphs
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Network motifs in E. coli Network motifs in E. coli transcription networktranscription network
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Only one 3-node network motif – the Only one 3-node network motif – the feedforward loopfeedforward loop
Nreal=40
Nrand=7±3
Z Score (#SD) =10
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Blue nodes=
x
y
zFFL
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X
Y
Z
AND
Sx
Sy
The coherent FFL circuitThe coherent FFL circuit
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Threshold for activating Y
Coherent FFL – a sign sensitive filterCoherent FFL – a sign sensitive filter
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OFF pulse
Feedforward loop is a sign-sensitive filterFeedforward loop is a sign-sensitive filter
Vs.
=lacZYA =araBAD
Mangan et. al. JMB
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X
Y
Z
AND
Sx
Sy
Kyz
-1 -0.5 0 0.5 1 1.5 2 2.5
0
0.5
1
-1 -0.5 0 0.5 1 1.5 2 2.50
0.5
1
-1 -0.5 0 0.5 1 1.5 2 2.50
0.2
0.4
Kyz
Sx
Y*Y*
Z
Time
Incoherent FFL – a pulser circuitIncoherent FFL – a pulser circuit
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A motif with 4 nodes :A motif with 4 nodes : bi-fanbi-fan
Nreal=203
Nrand=47±12
Z Score=13
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bifans extend to formbifans extend to form Dense-Overlapping-RegulonsDense-Overlapping-Regulons
Array of gates for hard-wired decision making
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Another motif :Another motif : Single Input ModuleSingle Input Module
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Single Input Module motifs can control timing Single Input Module motifs can control timing of gene expressionof gene expression
Shen-Orr et. al. Nature Genetics 2002
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The order of gene expression matches the order of the pathway
Fluorescence
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Glutamate
N-Ac-Glutamate
N-Ac-glutamyl-p
N-Ac-Ornithine
N-Ac-glutamyl-SA
Ornithine
Arginine
argA
argR
argB
argC
argD
argE
(min)0 20 40 60 80 100
Zaslaver et. al. Nature Genetics 2004
argR
arg
A
arg
B
arg
E
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Single Input Module motif is responsible for Single Input Module motif is responsible for exact timing in the flagella assemblyexact timing in the flagella assembly
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Kalir et. al., science,2001
Single Input Module motif is responsible for Single Input Module motif is responsible for exact timing in the flagella assemblyexact timing in the flagella assembly
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The gene regulatory network of E. coliThe gene regulatory network of E. coli
Shen-Orr et. al. Nature Genetics 2002
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Gene regulation networks can be simplified Gene regulation networks can be simplified in terms of recurring building blocksin terms of recurring building blocks
Network motifs are functional building blocks of these information
processing networks.
Each motif can be studied theoretically and experimentally.
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Efficient detection of larger motifs?
• The presented motif detection algorithm is exponential in the number of nodes of the motif.
• More efficient algorithms are needed to look for larger motifs in higher-order organism that have much larger gene-regulatory networks.
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More information :More information :
http://www.weizmann.ac.il/mcb/UriAlon/
PapersPapersmfinder – network motif detection softwaremfinder – network motif detection softwareCollection of complex networksCollection of complex networks