Homeobox protein GBX-2 is a protein that in humans is encoded by the GBX2 gene. [5] [6] [7]
Homeobox protein GBX-2 is a protein that in humans is encoded by the GBX2 gene. [5] [6] [7]
Gastrulation Brain Homeobox 2, or commonly known as GBX2, is a type of transcription factor that aids in the arranging of the midbrain and hindbrain during gastrulation. The hindbrain is broken up into seven or eight rhombomeres during gastrulation and GBX2 is responsible for rhombomeres one through three. GBX2 also takes part in the signaling and expression of other genes. It has been said that there is a threshold requirement for the amount of GBX2, so what occurs depends on the amount expressed. For example, specific amounts of GBX2 is needed for regulating the anterior and posterior patterning in the hindbrain. If the specific threshold is not met, then the patterning does not occur. This is the case with the other genes it helps to express like FGF8. With every gene comes some diseases associated with it. Colon Small Cell Carcinoma and Optiz-Gbbb Syndrome, which causes abnormalities throughout the midline of the body, are the diseases most closely associated with the GBX2 gene. [8]
Gastrulation Brain Homeobox 2 (GBX2) is a homeobox gene involved in the normal development of rhombomeres 1-3 which is the mid/hindbrain region. This gene is a dosage dependent transcription factor involved in the regulation of proper expression of other genes. GBX2 expression occurs during gastrulation and continues to be expressed in the later stages of embryogenesis. During these different stages, GBX2 is responsible for several important processes. In the neural plate stage GBX2 is required in order for the anterior hindbrain precursors to survive and form correctly. Also at this stage in development GBX2 is required for the proper regulation of different gene expression needed for the early establishment of A/P patterning in the neural plate. In the early stages of brain morphogenesis GBX2 is required for both the normal development of the anterior hindbrain and the proper formation of the mid/hindbrain organizer. Because of the effects on the mid/hindbrain organizer, GBX2 is involved in the positioning of the expression domain for isthmic FGF8. Since this is a dosage dependent gene, the different amounts of gene present in certain location can cause different outcomes. FGF8 is affected by the different dosages in the location it is expressed. The absence of GBX2 causes FGF8 expression is shifted caudally and over expression of GBX2 causes FGF8 expression to be shifted rostrally. Not all of the rhombomeres GBX2 is expressed in require the same strictness of dose regulation. Of the three, rhombomere 2 has the most strict dose requirements.
The Gbx-2 gene is thought to be involved in neural crest cell patterning and differentiation during fetal growth as a transcription factor. As a zygote becomes a gastrula, the embryonic ectoderm differentiates into these neural crest cells, which give rise to many structures including muscle, neurons, and bone later in development. In order to test Gbx-2's role in neural crest cell development, the region of neural cell proliferation was injected with the morphogen Wnt8. In response, Snail2 was expressed, which led to transcription factor activation and subsequent protein formation. When Wnt8 was inhibited in the same region, no transcription factors were expressed and neural cell growth was inhibited or not seen. There have also been studies that showed that when the reverse occurred, with the absence or presence of Snail2 affecting Wnt8, similar effects to transcription factor activation occurred. Such research has shown that transcription factor activation is dependent on both genes. Wnt is thought to be a neural crest cell inducer, and Gbx-2 is one of the gene targets involved. [9] In cases where Gbx-2 mRNA is in excess, an expansion of neural crest cells is seen, whereas when there is a lack of mRNA, neural crest cells fail to proliferate. If neural crest cells are unable to proliferate, many of the body's important organs and processes will be inhibited and could likely lead to miscarriage of the fetus.
Knockout of the GBX2 gene causes the failure of many structures to form, such as the isthmic nuclei, the cerebellum, motor nerve V and many other derivatives of rhombomeres 1-3. GBX2 gene knockout embryos will continue to develop and will reach full term pregnancy. The babies are born but if there is a lack of GBX2 expression all will die soon after birth. [10] [11]
Knockdown of the gbx2 gene leads to a truncated anterior hindbrain as well as abnormal clusters of cell bodies in r2 and r3 which are associated with problems in cranial nerve V. It has been shown that any structures derived from r1-r3 will be adversely affected by mutations or deficiencies in gbx2. These structures include the aortic arch and right Subclavian artery which, when improperly developed, can lead to cardiovascular defects in addition to craniofacial defects from improper development of cranial nerve V. [12]
In the vertebrate embryo, a rhombomere is a transiently divided segment of the developing neural tube, within the hindbrain region in the area that will eventually become the rhombencephalon. The rhombomeres appear as a series of slightly constricted swellings in the neural tube, caudal to the cephalic flexure. In human embryonic development, the rhombomeres are present by day 29.
Neural crest cells are a temporary group of cells that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia.
The zona limitans intrathalamica (ZLI) is a lineage-restriction compartment and primary developmental boundary in the vertebrate forebrain that serves as a signaling center and a restrictive border between the thalamus and the prethalamus.
The limb bud is a structure formed early in vertebrate limb development. As a result of interactions between the ectoderm and underlying mesoderm, formation occurs roughly around the fourth week of development. In the development of the human embryo the upper limb bud appears in the third week and the lower limb bud appears four days later.
Early growth response protein 2 is a protein that in humans is encoded by the EGR2 gene. EGR2 is a transcription regulatory factor, containing three zinc finger DNA-binding sites, and is highly expressed in a population of migrating neural crest cells. It is later expressed in the neural crest derived cells of the cranial ganglion. The protein encoded by Krox20 contains two cys2his2-type zinc fingers. Krox20 gene expression is restricted to the early hindbrain development. It is evolutionarily conserved in vertebrates, humans, mice, chicks, and zebra fish. In addition, the amino acid sequence and most aspects of the embryonic gene pattern is conserved among vertebrates, further implicating its role in hindbrain development. When the Krox20 is deleted in mice, the protein coding ability of the Krox20 gene is diminished. These mice are unable to survive after birth and exhibit major hindbrain defects. These defects include but are not limited to defects in formation of cranial sensory ganglia, partial fusion of the trigeminal nerve (V) with the facial (VII) and auditory (VII) nerves, the proximal nerve roots coming off of these ganglia were disorganized and intertwined among one another as they entered the brainstem, and there was fusion of the glossopharyngeal (IX) nerve complex.
Paired-like homeodomain transcription factor 2 also known as pituitary homeobox 2 is a protein that in humans is encoded by the PITX2 gene.
Fibroblast growth factor 8(FGF-8) is a protein that in humans is encoded by the FGF8 gene.
Hematopoietically-expressed homeobox protein HHEX is a protein that in humans is encoded by the HHEX gene and also known as Proline Rich Homeodomain protein PRH.
POU domain, class 4, transcription factor 1 (POU4F1) also known as brain-specific homeobox/POU domain protein 3A (BRN3A), homeobox/POU domain protein RDC-1 or Oct-T1 is a protein that in humans is encoded by the POU4F1 gene.
Homeobox protein OTX2 is a protein that in humans is encoded by the OTX2 gene.
Homeobox protein Hox-A3 is a protein that in humans is encoded by the HOXA3 gene.
Heart- and neural crest derivatives-expressed protein 1 is a protein that in humans is encoded by the HAND1 gene.
Protein atonal homolog 1 is a protein that in humans is encoded by the ATOH1 gene.
Homeobox protein engrailed-1 is a protein that in humans is encoded by the EN1 gene.
Homeobox protein goosecoid(GSC) is a homeobox protein that is encoded in humans by the GSC gene. Like other homeobox proteins, goosecoid functions as a transcription factor involved in morphogenesis. In Xenopus, GSC is thought to play a crucial role in the phenomenon of the Spemann-Mangold organizer. Through lineage tracing and timelapse microscopy, the effects of GSC on neighboring cell fates could be observed. In an experiment that injected cells with GSC and observed the effects of uninjected cells, GSC recruited neighboring uninjected cells in the dorsal blastopore lip of the Xenopus gastrula to form a twinned dorsal axis, suggesting that the goosecoid protein plays a role in the regulation and migration of cells during gastrulation.
Neural crest cells are multipotent cells required for the development of cells, tissues and organ systems. A subpopulation of neural crest cells are the cardiac neural crest complex. This complex refers to the cells found amongst the midotic placode and somite 3 destined to undergo epithelial-mesenchymal transformation and migration to the heart via pharyngeal arches 3, 4 and 6.
The Cdx gene family, also called caudal genes, are a group of genes found in many animal genomes. Cdx genes contain a homeobox DNA sequence and code for proteins that act as transcription factors. The gene after which the gene family is named is the caudal or cad gene of the fruitfly Drosophila melanogaster. The human genome has three Cdx genes, called CDX1, CDX2 and CDX4. The zebrafish has no cdx2 gene, but two copies of cdx1 and one copy of cdx4. The Cdx gene in the nematode Caenorhabditis elegans is called pal-1.
Homeobox protein CDX-4 is a protein that in humans is encoded by the CDX4 gene. This gene is a member of the caudal-related homeobox transcription factor family that also includes CDX1 and CDX2.
Evx1 is a mammalian gene located downstream of the HoxA cluster, which encodes for a homeobox transcription factor. Evx1 is a homolog of even-skipped (eve), which is a pair-rule gene that regulates body segmentation in Drosophila. The expression of Evx1 is developmentally regulated, displaying a biphasic expression pattern with peak expression in the primitive streak during gastrulation and in interneurons during neural development. Evx1 has been shown to regulate anterior-posterior patterning during gastrulation by acting as a downstream effector of the Wnt and BMP signalling pathways. It is also a critical regulator of interneuron identity.
The isthmic organizer, or isthmus organizer, also known as the midbrain−hindbrain boundary (MHB), is a secondary organizer region that develops at the junction of the midbrain and metencephalon. The MHB expresses signaling molecules that regulate the differentiation and patterning of the adjacent neuroepithelium. This allows for the development of the midbrain and hindbrain as well as the specification of neuronal subtypes in these regions. The fact that the MHB is sufficient for the development of the mid and hindbrain was shown in an experiment where quail MHB cells transplanted into the forebrain of a chick were able to induce an ectopic midbrain and cerebellum.
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