Transcript Document

17. Developmental Regulation
Developmental Regulation
• Many inherited diseases result from
mutations in genes regulating development
• Treatments/therapies may be discerned by
understanding regulatory mechanisms
• Three basic points of control:
1. Transcriptional regulation
2. Polarity within the cell
3. Extracellular signaling
Transcriptional Regulatory
Cascades
• Transcription proteins and cofactors –
activators and repressors
• Cis-acting elements
• Locus arrangement (gene order)
• Chromatin structure – domains,
methylation, acetylation
• Cell polarity and signal transduction
Cis-acting transcriptional regulatory elements:
Definitions
Locus Control Region (LCR)
1. Opens locus chromatin domains
2. Insulates against effects of surrounding positive or negative
chromatin
3. Has cell lineage-specific enhancer activity
4. Influences timing of replication and choice of origin utilized
Enhancer
1. Stimulates transcription in an orientation-independent manner
2. Classical enhancers function independently of orientation and
distance
Promoter
1. Region of DNA at which RNA polymerase binds and initiates
transcription
Response elements
1. Causes a gene to respond to a regulatory transcription factor
Cis-acting transcriptional regulatory elements:
Definitions (continued)
Insulator
1. Creates independent functional domain without enhancement
or activation function by blocking effects of surrounding
positive or negative chromatin
2. Interrupts communication between a promoter and another
regulatory element when placed between them
Matrix attachment region (MAR) or scaffold attachment region
(SAR)
1. DNA segment that may bind the nuclear scaffold
2. A DNA loop between two MARs may form an independent
chromosomal domain
3. Has no enhancer activity
Insulator
HS
HSs
HS
Promoters
Enhancers
Response elements
(Regulatory promoter)
MAR
Insulator
MAR
LCR
Action of trans-acting transcription factors
Transcriptional environment is dynamic and changes as development proceeds.
Control of human b-globin gene expression – a model of
developmental transcription regulation
a-globin
•
•
•
•
Carries O2 and CO2
Tetramer (a2b2)
Allosteric regulation
Tetramer composition varies with
age
b-globin
Synthesis of hemoglobin is
coordinately regulated
Pattern of globin chain synthesis during development
The human globin genes
• Each gene cluster is under the control of
a major enhancer (HS –40) or locus
control region (LCR)
a-like genes on chr 16
z
a
b
b
a
a2 a1
HS-40
b-like genes on chr 11
e
LCR
Gg
Ag
d
b
b-globin chain
The Locus Control Region
• The LCR is DNAse-hypersensitive in cells expressing the globin genes
• Sensitivity to DNAse reflects a relaxed chromatin structure that allows
binding of transcription factors
• The LCR regulates the entire gene cluster permitting it to be further
regulated on a gene-by-gene basis
+1
LCR
promoter
transcribed region
DNAsehypersensitive
sites
Sites that are less-sensitive to DNAse
between nucleosomes and at other
regulatory regions adjacent to genes
The LCR and the globin genes bind common protein factors
Prototypical b-like globin gene
Promoter
Enhancer
Models proposed for LCR-globin gene interaction
A) Looping
B) Tracking
C) Facilitated tracking
D) Linking
Cellular Polarity
• Morphogen gradient
A morphogen is a protein whose local
concentration (or activity) causes the surrounding
region to take up a particular structure or fate.
• Patterning
Partitioning of the cell or embryo into “zones”
destined to develop into different tissues or
structures.
A series of events resulting from the initial asymmetry within
the egg is translated into the control of gene expression so that
specific regions of the egg acquire different properties.
(top)
Anterior
(head)
Posterior
(tail)
(underneath)
At the start of development, gradients are established in the egg
along two axes, anterior-posterior and dorsal-ventral.
Regulatory cascades - concept
1. Maternal gene
products, called
morphogens,
establish gradients in
early embryogenesis.
2. Anterior-posterior
development uses
localized gene
regulators.
3. Dorsal-ventral
development uses
localized receptorligand interactions.
Expressed during oogenesis
by the mother. Act upon or
within the maturing oocyte.
Expressed after fertilization.
Mutations in these genes alter
the number or polarity of
segments. Three groups of
segmentation genes act
sequentially to define
increasingly smaller regions of
the embryo.
Control the identity of a
segment, but do not affect the
number, polarity or size of
segments. Mutations in these
genes cause one body part to
develop the phenotype of
another part.
Homeotic (Hox) Genes
Lab pb Zen Dfd Scr Antp Ubx Abd-AAbd-B
1. Impose the
program that
determines the
unique
differentiation
of each
segment.
2. Complex loci
found as gene
clusters.
3. Spatial colinearity – Hox gene order strictly
corresponds to their expression domains along the body
of the embryo.
Hox gene clusters are conserved between species.
Drosophila
lab
pb
ANT-C
Dfd Scr Ant
p
UbxAbd-AAbd-B
A4
A7
Chromosome
BX-C
3
Mouse
A1
A2
A3
A5
A6
A9
A10 A11
A13
Evx1
6
HoxA
B1
B2
B3
B4
B5
B6
B7
B8
B9
B13
11
HoxB
C4 C5
C6
C8
C9 C10 C11 C12 C13
15
HoxC
D1
D3
D4
D8
D9 D10 D11 D12 D13
Evx2
HoxD
anterior 3’
5’ posterior
Human and Mouse: Four gene clusters (A, B, C,
D) that are organized into 13 paralogue groups.
2
Spatial colinearity in mouse
B1
B2
B3
B4
B5
B6
B7
B8
B9
B13
Adapted from Hunt et al. (1991)
Development Supp.: 187-197.
Comparison of spatial colinearity between fruit fly and mouse.
Humans have the same number of Hox gene clusters as mice and
spatial colinearity is conserved between species.
Combinations of Hox genes specify the development of
the anterior-posterior axis
Temporal colinearity in mouse
1
2
3
anterior 3’
earliest
4
5
6
7
9
10
11
13
5’ posterior
latest
direction of opening of
chromatin in complex
during gastrulation
Temporal colinearity: As the body plan develops in an anteriorposterior direction during gastrulation, there is a sequential
expression of the homeotic complex.
Diseases associated with Hox gene mutations
Polydactyly
1. Hand-foot-genital syndrome
(Hoxa deletion)
2. Synpolydactyly (Hoxd
deletion)
3. Cleft palate
4. Brain abnormalities
5. Uterine abnormalities
6. Retinoic acid, which causes
birth defects, affects Hox
genes
Extracellular Signaling
• Communication of environmental stimuli to
intracellular milieu, thus effecting appropriate
cellular (developmental) response
• Four types
–
–
–
–
Endocrine
Paracrine
Neuronal
Contact-dependent