How does the segmentation clock work? The first somite oscillator genes to be discovered were Hes genes, which are key components of the Notch signaling pathway. They are directly regulated by the activated form of Notch, and they code for inhibitory transcription regulators that inhibit the expression of other genes, including Delta. As well as regulating other genes, the products of Hes genes can directly regulate their own expression, creating a remarkably simple negative feedback loop. Autoregulation of certain specific Hes genes (depending on species) is thought to be the basic generator of the oscillations of the somite clock. Although the machinery has been modified in various ways in different species, the underlying principle seems to be conserved. When the key Hes gene is transcribed, the amount of Hes protein product builds up until it is sufficient to block Hes gene transcription; synthesis of the protein ceases; the protein then decays, permitting transcription to begin again; and so on, cyclically (Figure 21–39). The period of oscillation, which determines the size of each somite, depends on the delay in the feedback loop. This equals the sum of the gestation delays and accumulation delays (that is, the molecular lifetimes) of the Hes mRNA and protein molecules, according to the additive principle discussed earlier. Mathematical modeling (see Chapter 8) allows us to relate these basic molecular parameters to the cycle time of the segmentation clock: to a first approximation, the cycle period is simply equal to twice the total delay in the negative feedback loop, and thus twice the sum of the delays occurring at each step of the loop.
The feedback loop just described is intracellular, and each cell in the presomitic mesoderm can generate oscillations on its own. But these oscillations at the single-cell level are somewhat erratic and imprecise, reflecting the fundamentally noisy, stochastic nature of the control of gene expression, as discussed in Chapter 7. A mechanism is needed to keep all the cells in the presomitic mesoderm that will form a particular somite oscillating in synchrony. This is achieved through cell–cell communication via the Notch signaling pathway, to which the Hes genes are coupled. The gene regulatory circuitry is such that in this context Notch signaling does not drive neighboring cells to be different, as in lateral inhibition, but does just the opposite: it keeps them in unison. In mutants where Notch signaling
Figure 21–39 Delayed negative feedback giving rise to oscillating gene expression.
(a) a single gene, coding for a transcription regulator that inhibits its own expression, can behave as an oscillator. For oscillation to occur, there must be a delay (or several delays) in the feedback circuit, and the lifetimes of the mrNa and protein (which contribute to the delay) must be short compared with the total delay. the total delay determines the period of oscillation. It is thought that a feedback circuit like this, based on a pair of redundantly acting genes called Her1 and Her7 in the zebrafish—or their counterpart, Hes7, in the mouse—is the pacemaker of the segmentation clock governing somite formation. (B) the predicted oscillation of Her1 and Her7 mrNa and protein, computed using rough estimates of the feedback circuit parameters appropriate to this gene in the zebrafish. concentrations are measured as numbers of molecules per cell. the predicted period is close to the observed period, which is 30 minutes per somite in the zebrafish (depending on temperature).
fails, including mutants defective in Delta or Notch itself, the cells drift out of synchrony and somite segmentation is again disrupted. This leads to gross deformity of the vertebral column—an extraordinary display of the consequences of the noisy temporal control of gene expression at the single-cell level, writ large in the structure of the vertebrate body as a whole.
Intracellular Developmental programs can help Determine the time-course of a cell’s Development
Although signaling between cells plays an essential part in driving the progress of development, this does not mean that cells always need signals from other cells to prod them into changing their character as development proceeds. Some of these changes are intrinsic to the cell (like the ticking of the segmentation clock) and depend on intracellular developmental programs that can operate even when the cell is removed from its normal environment.