Krüppel

''Krüppel'' expression pathway

Krüppel is expressed in the center of the embryo during the cellular blastoderm stage of development. Its expression pattern is restricted to this domain largely through interactions with the maternal effect genes Bicoid and Nanos, and fellow gap gene Hunchback and Knirps.

Bicoid maternal transcripts are deposited at the anterior end of the embryo, while Nanos maternal transcripts are located at the posterior. Hunchback mRNA transcripts are present throughout the embryo. Bicoid and Nanos both encode morphogens that have the opposite effect on Hunchback mRNA translation – Bicoid activates translation, whereas Nanos represses it. As such, Hunchback mRNA is translated so that Hunchback protein is present in the concentration gradient which decreases along the anterior – posterior axis. This Hunchback gradient indirectly results in an anterior boundary for Knirps expression. Other factors induce a posterior boundary, so that Knirps is expressed in a stripe in the posterior region of the embryo.

Hunchback and Knirps are both transcription factors that regulate Krüppel expression. High levels of Hunchback inhibit expression, whereas low levels of Hunchback activate expression. Knirps acts as a repressor to inhibit expression. This results in Krüppel being expressed in a stripe in the center of the embryo's A-P axis, where Hunchback concentration has dropped to a low enough level so that it can act as an activator, but Knirps is not yet present to inhibit. In this way the initial gradients of morphogens can lead to the establishment of a specific region within the blastoderm. It can be compared to a narrow bandwidth filter in engineering.

Effects of ''Krüppel'' expression

The Krüppel protein is a transcription factor, and has been shown to act as a repressor. It functions in collaboration with other gap genes and their localized protein products to regulate the expression of the primary pair rule genes – even skipped (eve), hairy (h), and runt. It has been postulated that Krüppel inhibits eve expression to create the posterior boundary of eve stripe two, and evidence has also been found for Krüppel being a player specifically in the formation of hairy stripe 7. The expression patterns of pair rule gene will in turn regulate the segment polarity genes, making Krüppel essential for proper development along the anterior posterior axis and segment identity.

Clinical significance

Krüppel has shown homology to the mammalian Krüppel-like factors, which play key biological roles in the pathogenesis of many human diseases: cancer, obesity, inflammatory disorders and cardiovascular complications. Moreover, KLFs are known to be involved in inducible pluripotent stem cells generation, and preservation of the pluripotent state of embryonic stem cells.

References

References

1. (March 1988). "The GLI gene is a member of the Kruppel family of zinc finger proteins". Nature.
2. "FlyBase Gene Report: Dmel\Kr".
3. Hoy, Marjorie A.. (January 2019). "Insect Molecular Genetics". Academic Press.
4. (October 1980). "Mutations affecting segment number and polarity in Drosophila". Nature.
5. (September 1984). "Mutations affecting the pattern of the larval cuticle inDrosophila melanogaster: I. Zygotic loci on the second chromosome". Wilhelm Roux's Archives of Developmental Biology.
6. (July 1984). "Krüppel, a gene whose activity is required early in the zygotic genome for normal embryonic segmentation". Developmental Biology.
7. (December 1986). "A conserved family of nuclear proteins containing structural elements of the finger protein encoded by Krüppel, a Drosophila segmentation gene". Cell.
8. (July 2011). "Krüppel-like factor 4 inhibits tumorigenic progression and metastasis in a mouse model of breast cancer". Neoplasia.
9. (2012). "Krüppel-like factor 4, a tumor suppressor in hepatocellular carcinoma cells reverts epithelial mesenchymal transition by suppressing slug expression". PLOS ONE.
10. (April 2005). "Drastic down-regulation of Krüppel-like factor 4 expression is critical in human gastric cancer development and progression". Cancer Research.
11. (March 2010). "Tumor suppressor activity of KLF6 mediated by downregulation of the PTTG1 oncogene". FEBS Letters.
12. (July 1990). "Drosophila Krüppel protein is a transcriptional repressor". Nature.
13. (1985). "Spatial and temporal patterns of Krüppel gene expression in early Drosophila embryos". Nature.
14. (March 2010). "The bicoid morphogen system". Current Biology.
15. (April 1990). "Gradients of Krüppel and knirps gene products direct pair-rule gene stripe patterning in the posterior region of the Drosophila embryo". Cell.
16. (May 1991). "Transcriptional regulation of a pair-rule stripe in Drosophila". Genes & Development.
17. (December 1999). "hairy stripe 7 element mediates activation and repression in response to different domains and levels of Krüppel in the Drosophila embryo". Mechanisms of Development.
18. (April 2007). "Krüppel-like factor 4 exhibits antiapoptotic activity following gamma-radiation-induced DNA damage". Oncogene.
19. (October 2009). "Krüppel-like family of transcription factors: an emerging new frontier in fat biology". International Journal of Biological Sciences.
20. (May 2007). "Kruppel-like factor 4 regulates endothelial inflammation". The Journal of Biological Chemistry.
21. (February 2007). "TGFbeta inducible early gene-1 (TIEG1) and cardiac hypertrophy: Discovery and characterization of a novel signaling pathway". Journal of Cellular Biochemistry.
22. (March 2008). "A core Klf circuitry regulates self-renewal of embryonic stem cells". Nature Cell Biology.
23. (October 2009). "The role of Krüppel-like factors in the reprogramming of somatic cells to induced pluripotent stem cells". Histology and Histopathology.
24. (November 2007). "Induction of pluripotent stem cells from adult human fibroblasts by defined factors". Cell.