Transcript Document

MOLECULAR REGULATION
OF DEVELOPMENT
GROWTH FACTOR SIGNALING,
HOX GENES AND THE BODY PLAN
Overview
 Pattern formation by morphogen gradients of
growth factors and the dorsal-ventral (D-V)
axis.
 B) Hox genes and the antero-posterior (A-P)
axis.
- colinearity
- activation by retinoic acid
- Hox genes in Evolution and Development
During development groups of inducing cells called organizing
centers secrete graded growth factor signals. The concentration
gradient of a morphogen can induce multiple cell differentiation
choices.
D-V patterning can be
best studied in the frog
egg.
D-V cell differentiation depends of a gradient of BMP signaling.
Secreted antagonists such as Chordin and Noggin can bind to
growth factors in the extracellular space and prevent binding to cell
surface receptors. This inhibitory mechanism is used in
development to generate morphogen gradients. Chordin establishes
a BMP4 gradient at gastrula.
Chordin
inhibits
Signal transduction: membrane receptors transduce the signal so that
transcription factors are activated through phosphorylation. TGFβ
family members (30 different ligands in humans) activate cell surface
receptors (serine-threonine kinases).
Xnr
BMP4
Bone morphogenetic proteins (BMPs) are multi-functional growth factors that belong
to the transforming growth factor b (TGFb) superfamily. To date, around 20 BMP
family members have been identified and characterized. BMPs signal through
serine/threonine kinase receptors, composed of type I and II subtypes. Three type I
receptors have been shown to bind BMP ligands, type IA and IB BMP receptors
(BMPR-IA or ALK-3 and BMPR-IB or ALK-6) and type IA
activin receptor (ActR-IA or ALK-2) (Koenig, B.B. et al. (1994)
Epidermal ectoderm
Neural ectoderm
Dorsal
Ventral
Mesoderm
Endoderm
Spemann’s
Organizer
At gastrula a gradient of BMP4 (a TGFβ-like growth factor) is established by
a ventral source of BMP4, and a dorsal source of Chordin and Noggin, two
secreted BMP antagonists produced in the dorsal organizing center.
a morphogen gradient can be generated by a source of growth factor
(such as BMP) or by a localized source of inhibitor (such as Chordin).
This is how organizing centers work in embryonic induction.
Embryogenesis is controlled by surprisingly
different signal transduction pathways:
1)
2)
3)
4)
5)
6)
7)
TGFβ/BMP Serine/Threonine kinase receptors
Receptor Tyrosine kinases such as FGF, EGF, IGF, Insulin
Wnts
Sonic Hedgehog
Notch
G protein-coupled receptors (7-transmembrane receptors)
Nuclear hormone receptors
Different signaling pathways pattern are active in the
embryo, the same signals can trigger different types
of cell differentiation responses in cells of different
developmental history thus generating hundreds of
differentiated cell types in the human body.
A-P patterning outline:
2a) Hox genes: colinearity between the
body plan and gene order in genomic
DNA
2b) Hox genes and Retinoic acid
2c) Hox genes in the Evolution of
Development
Homeotic Mutations
Cloning of the homeobox (nucleic acid).
Homeodomain refers to protein.
The homeodomain is a 60 aa
Define Hox, homeobox
DNA-binding domain that is very
conserved during evolution. It fits
into the major groove of the DNA.
The term homeobox is reserved for the nucleic acid sequences that
encode homeodomains. Since they are highly conserved, they can be
detected by low-stringency hybridization across species.
Homeotic genes specify body segment identity in Drosophila.
Hox complexes are conserved between Drosophila and mammals (from De Robertis et al.,
Scientific American, 1990)
Vertebrates have four Hox complexes, with about 10 genes
each. They can be aligned in 13 groups of paralogues.
They display colinearity:
a) Temporal colinearity: genes on one end of the complex are
expressed first, those on the other (posterior) end are turned
on last.
b) Spatial colinearity: the more anteriorly expressed genes are
in one end, the more posterior ones at the other end of the
gene complex.
c) Anterior Hox genes are activated sequentially by retinoic
acid.
Homeobox (Hox) genes were originally discovered in the fruit fly
Drosophila, where they function through a conserved
omeodomain as transcriptional regulators to control embryonic
morphogenesis. Since then over 1000 homeodomain proteins have
been identified in several species. In vertebrates, 39 Hox genes
have been identified as homologs of the original Drosophila
complex, and like their Drosophila counterparts they are
organized within chromosomal clusters. Vertebrate Hox genes
have also been shown to play a critical role in embryonic
development as transcriptional regulators.
In vertebrates, more than 200 homeobox-containing genes have
been identified
In Drosophila, a single homeotic complex (HOM-C) comprised of two separate clusters [Bithorax complex (BX-C) and
Antennapedia complex (ANT-C)] is located on chromosome 3
In mice the Hox complex is comprised of 39 genes that are arranged into four separate chromosomal clusters
designated Hox A, B, C, and D
Vertebrate Hox and Drosophila HOM-C homeobox clusters is spatial colinearity, which is the expression of the Hox
genes along the anteroposterior axis of the embryo in congruence with their arrangement along the chromosome .
Thus,
the
3’
genes within a
chromosomal
cluster
are
expressed more
anteriorly than
the 5’ genes with
respect to the
rostrocaudal axis
of the embryo. In
general,
vertebrate Hox
genes
also
display temporal
colinearity and
colinear
sensitivity to RA,
a known inducer
of Hox gene
expression
Ems empty spiracles (Emx in mammalian), transcription factors belonging to
the orthodenticle family (Otx1, Otx2) play an important role during
early and later events required for proper brain development.
Otx1 is involved in corticogenesis, sense organ
development and pituitary functions, while Otx2 is necessary earlier in
development, for the correct anterior neural plate specification and
organisation of the primitive streak.
The first few processes that vertebrate embryos undergo - the division of the
egg cytoplasm into cells (cleavage), the formation of the primary tissue layers
(gastrulation) and the designation of embryonic coordinates (axis formation)
do not seem to require the involvement of Hox genes and other anteriorposterior patterning genes. However, after the initiation of gastrulation, the
subdivision of the embryo along the anterior-posterior axis begins.
Concurrently, genes like the Hox genes, orthodenticle-related genes, and other
homeobox-containing genes, start to be expressed in specific anterior-posterior
domains. The Hox genes are expressed from the hindbrain posteriorly and turn
on in an ordered sequential fashion, posterior to the domainof Otx2, an
orthodenticle homolog.
Hox-C6 protein is seen in eight thoracic
segments of the mouse embryo.
Translation is blocked in the tail region,
probably through the action of
microRNAs. The inset shows that HoxC6 mRNA is expressed all the way to
the tip of the tail.
From De Robertis, Cell 132, 185-195 (2008)
Fig. 18
Conservations between Drosophila and the four human
Hox complexes
High RA response
Low RA response
From De Robertis, E.M. Evo-Devo: Variations on Ancestral themes. Cell 132, 185-195 (2008)
Temporal and spatial colinearity: order of Hox genes in DNA
follows the antero-posterior body axis.
Why have Hox genes stayed together in a complex?
2b) Retinoic acid activates HOX genes sequentially in
cultured human teratocarcinoma cells
Retinoic acid receptor is a DNA-binding protein that works as a ligandactivated transcription factor. Many hydrophobic hormone receptors
work in this way. Nuclear receptors work very differently from cell
surface receptors.
(RA)
RA
Hox complexes have a retinoic acid receptor response element (RARE) in the
DNA before paralogue 1. This DNA enhancer element controls expression of
many genes in the complex. In retinoic acid teratogenesis, Hox gene expression
borders move into more anterior regions.
RARE
Hox genes and the development of body plans
Homeotic transformations in humans. A cervical vertebra transformed
into a thoracic one with ribs.
Hox knockouts in mice cause homeotic transformations, in this case an
extra rib in the lumbar region (HoxC-8 mutant). Treatment with retinoic
Fig. 11
acid can also cause lumbar ribs.
Numb vs Notch: drosophila vs Uomo