Genetic Signaling: Transcription Factor Cascades and Segmentation

Figure 1: Bicoid mRNA in a fruit fly (Drosophila) embryo.

A fresh Drosophila egg shows a concentration of bicoid (bcd) expression in its anterior end.

© 2007 Nature Publishing Group Irion, U. & St. Johnston, D. bicoid RNA localization requires specific binding of an endosomal sorting complex. Nature 445, 554–558 (2007) doi:10.1038/nature05503. All rights reserved.

Within most differentiated cells, several levels of induction produce a multitude of transcription factors. Moreover, during embryonic development, the actual process of differentiation itself requires an integrated system of transcription factors that turn gene expression on and off with strict precision and timing. This is particularly true in the case of segmentation.

Segmentation is the term applied to the differentiation patterns of vertebrates and arthropods, in which serial repetition of similar anatomical modules creates a body axis. In vertebrates, segmentation results in the development of the spine, head, and limbs; in insects, it results in formation of the abdomen, head, and thorax. Segmentation is believed to be the result of the rhythmic activation and deactivation of certain signals that dictate specific relevant cellular processes. This oscillating process is called the segmentation clock. As scientists learn more about the timing and mechanisms of the segmentation clock, they also continue to uncover information about regulatory interactions and signal cascades in general.

A large portion of the existing research on genetic control of segmentation has been conducted using fruit flies. For instance, over the past 25 years, more than 40 genes have been identified as playing a role in segmentation in Drosophila melanogaster (Peel et al., 2005). Segmentation in fruit flies is controlled by a cascade of transcription factors, some of which have been found in other arthropods. Typically, cascade systems occur within individual cells; however, during segmentation processes, signal cascades are carried across multiple cells within specific regions of a developing embryo.

Figure 4: A cyclical wave of hairy1 mRNA expression occurs during the formation of each somite in chick embryos.

The expression pattern of the hairy1 transcription factor correlates with the formation of each individual somite. At the top row, microscopy images show hairy1 mRNA expression in chicken embryos with 15, 16, and 17 somites (dark blue). The lower row shows drawings of those same areas that clarify how hairy1 expression moves as a wave during somite formation, and embryo maturation. Hairy1 mRNA, shown in blue, starts broadly in the posterior (bottom). With time, the band of expression progresses upward and narrows as it approaches the anterior end. Eventually, it forms a narrow stripe at the posterior end of the newly formed somite (SI or SII). This entire process takes approximately 90 minutes. Scale bar 200μm.

© 1997 Elsevier Inc. Palmeirim, I. et al. Avian hairy gene expression identifies a molecular clock linked to vertebrate segmentation and somitogenesis. Cell 91, 639–648 (1997) doi:http://dx.doi.org/10.1016/S0092-8674(00)80451-1. All rights reserved.

Figure Detail

In vertebrates, the development of the precursor to an animal's skeletal system is called somitogenesis. Because the vertebrate spine consists of repeating units, the development of these units can be compared to the segmentation process in arthropods. Spinal development, however, is controlled by an oscillating process that is dependent on a group of genes in the Notch pathway, only some of which have homologues in the Drosophila segmentation control machinery.

During vertebrate spinal development, oscillations in gene expression occur on a temporal basis. Specifically, a signal in the Notch pathway is "on" when the spinal precursor, or somite, begins to form, but is "off" by the time somite formation is complete. Because Notch serves as a transcription factor, it alters the concentrations of its targets at the front of the somite as compared to the rear, thus establishing different segments.

The Notch pathway is activated when a protein ligand signal from adjacent cells, called Delta, binds to Notch on a cell's surface. In this regard, Notch serves as a transmembrane receptor. Upon Delta binding, the part of Notch that is found in the cell's cytoplasm is released from the membrane. This part of Notch serves as a transcription factor, and it plays a major role in the segmentation clock. A cascade then activates several downstream genes, including the hairy/enhancer of split (hes) gene, which encodes another transcription factor. Indeed, the hairy1 transcription factor was the first such factor that was determined to be regulated in a temporal fashion; in investigations using chickens, researchers noted that the expression of this protein was consistent with the time it takes to form a single somite (90 minutes) (Figure 4; Palmeirim et al., 1997). The negative feedback loop of the Notch pathway results in waves of gene expression and, ultimately, the patterns of body development observed during embryonic growth. Notch signaling appears to be primarily found in vertebrates, and there is no evidence that it plays a role in early insect segmentation (Peel et al., 2005).

Several models have been proposed to explain the oscillation of gene expression in different organisms. In zebrafish, it seems that the Her1 and Her7 transcription factors can repress their own expression periodically, thus creating an oscillating effect on gene expression (Dequeant & Pourquie, 2008). A similar negative feedback system controlled by the HES protein family has also been observed in mice. However, in mice, expression DNA microarray analyses also identified a host of additional genes that are expressed in a cyclical pattern and appear to be associated with the segmentation clock. Whether these genes are controlled directly by the HES family of transcription factors or indirectly through HES-mediated induction of transcription factors acting downstream is not yet entirely clear.

It appears that the role of maternal factors in fruit flies is not universal among all insects, nor is it common to vertebrates. However, later in the cascades of nearly all insects and vertebrates, the Hox genes play a role in identifying each segment and defining what part of the body each segment will become (Figure 5). In fruit flies, the discovery of Hox genes was facilitated by observations of spontaneous mutants. Mutations in specific Hox genes caused legs to grow out of a fly's head, wings to grow where they shouldn't be, and a number of other physical deformities.

Figure 5: Genes define segments in fruit flies.

Two gene clusters (top) are expressed in particular regions of a developing fruit fly embryo (center). These genes define segments in the body of the adult fly (bottom).

© 2013 Nature Education All rights reserved.

Figure Detail

Thanks to extensive experimentation with fruit flies, much is known about the genetic factors that control body patterns. Similar experimentation with vertebrates is much more complicated and introduces a host of ethical issues. For these reasons, there is still little data about vertebrate segmentation and the oscillating clock. However, information regarding certain patterns, such as those discussed above, might help researchers develop strategies to treat diseases caused by abnormal segmental patterning during human development.