Hox genes and pattern development of vertebrates

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Hox genes and pattern development of vertebrates
Pattern formation harmonious arrays of different
elements, such as the array of fingers on the
hand, body pattern (head, trunk, and tail), or
limb patterns. Pattern formation is best
understood in Drosophila, where most genes that
contribute to the body plan are described.
Anteroposterior axis in vertebrates is specified
by a group of genes called homeobox genes. There
are many similarities to Drosophila. The
dorsoventral axis in vertebrates is also
specified by genes that have counterparts in
Drosophila. Interestingly, although vertebrates
and invertebrates share a similar body plan, it
is inverted. Vertebrate limb development is
controlled by multiple genes, including homeobox
genes. Development depends on the process of
reciprocal interaction between ectoderm and
mesenchyme.
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Homeobox genes
The homeobox many genes that control pattern
formation share a consensus sequence of 180
nucleotides. Genes with a homeobox are highly
conserved in plants and animals and are called
homeobox genes. Homeodomain the protein encoded
by a homeobox gene contains a 60 amino acid
domain encoded by the homeobox. All homeodomain
containing proteins are transcription factors
that bind to gene specific promoters or enhancers.
The homeodomain has three alpha helices. The
recognition helix (a3) aligns in the major groove
of DNA TALE proteins (Exd) an atypical family
of homeodomain proteins that help to target
specific homeodomain transcription factors to
their correct cis elements. They bind to DNA
first, they alter the DNA conformation slightly,
and this assists in binding of other homeodomain
transcription factors. TALE proteins also
stabilize bound homeodomain proteins.
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Homeobox genes are often clustered on one
chromosome
Homeobox genes exhibit two patterns of
localization 1. Some are scattered throughout
the genome. 2. Hox genes other homeobox genes
are clustered within a small region and in a very
specific sequence (Hox genes form a Hox complex).
These Hox genes are highly conserved in organisms
ranging from flies to humans.
How did Hox genes evolve? Hox complexes arose
by repeated duplication and mutation of an
ancestral homeobox gene. This formed an ancestral
HOX complex. In some organisms, including most
vertebrates, the HOX complex has been duplicated
four times.
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Hox genes are organized into paralogy and
orthology groups
Paralogy group are simply the natural clusters
of Hox genes on the chromosome. Vertebrates have
4, flies have 1. They are named a, b, c, and
d. Orthology group sequence comparisons of each
gene shows that certain genes are closely
related. These are the genes in most vertical
columns (for example, Hox a9, b9, c9 and d9).
Orthologous Hox genes from different species can
replace one another in function (Hox d4
substitutes for Dfd in Drosophila).
Genes of any orthology group have corresponding
positions within each complex (if clusters are
aligned in rows, the orthology groups form
columns. Genes are numbered consecutively from
1-13.
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Hox genes specify anteroposterior body pattern
The physical order of Hox genes within the
complex is related to their order of expression
along the anteroposterior axis of the
embryo!! Genes at the 3 end are expressed in
the anterior and genes at the 5 end are
expressed progressively further
posteriorly. Hoxb cluster is expressed in the
central nervous system. Each gene is first
expressed at a sharply defined point and
expression continues posteriorly and gradually
tapers off. Rhombomeres the pattern of Hox gene
expression often coincide with repetitive bulges
in the sides of the rhomencephalon.
mouse
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What accounts for the pattern of Hox gene
expression?
mouse
Enhancer sharing the sequential order of Hox
genes within the complex may result from the fact
that all genes in the complex share a common
enhancer element. If any gene is removed, or if
the complex is broken apart, the removed genes
may not be expressed properly. Thus, the
sequential order has been preserved during
evolution of different organisms.
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The dorsoventral body plan are frogs just upside
down flies?
The dorsoventral body pattern of vertebrates and
invertebrates are similar but inverted.
Invertebrates, such as lobsters and flies, have
the central nervous system on the ventral side
and the heart is positioned dorsally.
How is the dorsoventral axis established? Vertebr
ates goosecoid stimulates noggin and chordin,
which help induce the dorsal pattern. BMP-4 and
xolloid induce the ventral pattern. Invertebrates
decapentaplegic (dpp) and tolloid induce dorsal
development such as heart. Short gastrulation
protein (sog) induces ventral development of
CNS. Dpp and BMP-4 are similar and belong to the
TGF-b family. Sog and chordin also share sequence
similarity.
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Are sog and chordin interchangable in flies and
frogs?
If the body pattern of flies and frogs is
established by similar signals, one might expect
ectopic expression of sog from flies to act as
Spemanns organizer and to be able to organize
neural tube in a frog embryo. Injection of sog
RNA into the early frog gastrula causes formation
of a second blastopore. Frog embryos can be
completely ventralized by exposure to UV
radiation (no CNS develops). However, the
development of UV-treated embryos can be
completely rescued by injection of sog
RNA. Injection of noggin RNA from frogs into
Drosophila embryos prevented the development of
normal dorsal structures such as amnioserosa and
dorsal ectoderm, but induced a second set of
neurons. Thus, the signals for dorsoventral
pattern formation are similar in flies and frogs.
However, it is a mystery why they become inverted.
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Hox gene expression contributes to limb
development
Limb buds the first stage of limb development
occurs at 5 weeks in humans when small paddle
shaped limb buds form. Apical ectodermal ridge
(AER) a specialized structure formed by the
ectodermal covering of the limb bud. It is a
ridge that runs anterior to posterior. Progress
zone underneath the AER lies a zone of
mesenchyme that actively proliferates to form the
limb. Somites contribute mesenchyme to form
muscles and lateral plate mesoderm forms
cartilage and connective tissue.
The limb is formed by differential growth of
mesenchyme cells, by programmed cell death
(between digits), and specific patterns of
differentiation induced by Hox genes and other
factors. How does the limb know where to form?
How do arms become different than legs? How does
the limb develop its three axis?
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Limb position is determined by FGF and Hox genes
The position of limb development depends on
signals from other tissues. Fibroblast growth
factor (FGF) these growth factors are produced
by mesenchyme (FGF-10) and epidermis (FGF-8) to
induce limb formation. If a small bead containing
FGF is implanted under the skin, an extra limb
develops. If the bead is implanted in the flank
near the anterior, it forms a wing. If the bead
is implanted posteriorly, it forms a leg. Knock
out mice lacking FGF-10 fail to develop limbs and
have no apical ectodermal ridge or zone of
polarizing activity.
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Hox gene expression is directly changed by FGF
beads. Under normal conditions, wing expresses
Hox d9 Flank expresses Hox b9 and c9. Leg
expresses Hox b9, c9, and d9. Ectopic
development of wing on the flank results in loss
of Hox b9 and Hox c9, whereas, ectopic
development of leg is induced by induction of all
three Hox genes.
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Signals from somites and lateral plate
mesoderm specify the dorsoventral limb axis
Limb bud mesoderm is first induced by signals
from the adjacent somites. This mesoderm then
induces ectoderm to form the AER. The somite
continues to induce ectoderm to form the dorsal
surface of the limb. The lateral plate mesoderm
induces the ventral portion of limb ectoderm.
Radical fringe a gene expressed in the dorsal
ectoderm that provides a dorsalizing function and
helps to induce the AER. Wnt-7a another gene
expressed in the dorsal ectoderm. It stimulates
the underlying mesenchyme to produce Lmx-1, an
important signal for dorsal differentiation.
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Engrailed a gene expressed in the ventral
ectoderm that provides a ventralizing function.
It also inhibits expression of radical fringe and
Lmx-1. The AER develops at the junction between
areas that express radical fringe and engrailed.
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Sonic hedgehog induces the anteroposterior
pattern of fingers and toes
If a small piece of the chicken posterior wing
bud is grafted to the anterior portion of a
second embryo, it causes extra digits in a mirror
image pattern. Normally, II, III, and IV develop.
Now IV, III, II, III, IV form. Zone of
polarizing activity the ability of a tissue to
establish the posterior position of the
anteroposterior pattern (rear of limb bud).
Sonic hedgehog shh is expressed at high level in
the zone of polarizing activity, suggesting its
importance. To test this, purified shh DNA was
transfected into cells and the cells were
transplanted into the wing bud. The exact same
result occurred, thus, shh is important for
generating wing pattern.
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Reciprocal interaction is critical for wing
development
Interactions occur between the AER on the surface
and limb bud mesenchyme that lies underneath in
the progress zone. Limb bud mesenchyme induces
formation of the AER from limb bud ectoderm. If
the ectoderm of a limb bud is removed at an early
stage of development, the mesoderm induces a new
AER. If the AER is removed during a later stage,
the limb mesenchyme stops growing and the limb is
truncated. If the limb bud mesenchyme is removed
or replaced with other tissue, the AER quickly
degenerates. The AER serves a permissive rather
than an instructive role. If you reverse its
orientation, digits will develop normally. If you
replace wing AER with leg AER, the wing develops
normally. The mesenchyme is the instructive
influence for limb development.