Homeobox Protein HB24 is a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
that in humans is encoded by the ''HLX''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
.
Role in development
Hlx belongs to the class of homeobox transcription factors, initially cloned from a B-lymphocyte cell line.
Targeted knockout of the gene has demonstrated its vital role in liver and gut organogenesis.
Its expression is first noticed in embryonic day 9.5 (E9.5) in the
splanchnic mesoderm
The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow r ...
caudal to the level of the heart and foregut pocket, and in the branchial arches. Around E10- E12.5, the expression becomes more prominent in the
mesenchyme
Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every ...
of the visceral organs of the gut such as liver, intestines and gall bladder.
Hlx is essential for liver and gut expansion, but not for onset of their development. Heterozygous knockouts of Hlx (Hlx
+/−) are normal whereas homozygous knockouts (Hlx
−/–) develop severe hypoplasia of the liver and gut along with anaemia. Hlx controls the epithelial-mesenchymal interaction necessary for liver and gut expansion.
At E8.0, the primary liver bud is formed from the midgut
endoderm
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
in response to signals from the
cardiogenic mesoderm. This is followed by signals from the
septum transversum that induce epithelial-mesenchymal transition in the hepatic progenitors of the gut endoderm.
In a third stage, these signaling factors induce the liver endoderm to undergo proliferation and form liver cords. The same factor controls gut proliferation, and Hlx governs its expression. Although these mice develop anaemia, it is likely due to insufficient support from the liver in producing matrix component needed for hematopoiesis rather than an intrinsic defect in the hematopoietic cells.
References
Further reading
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External links
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Transcription factors
{{gene-1-stub