A negative feedback loop of transcription factors that controls stem cell pluripotency and self-renewal.

Embryonic stem (ES) cells possess the ability to renew themselves while maintaining the capacity to differentiate into virtually all cell types of the body. Current evidence suggests that ES cells maintain their pluripotent state by expressing a battery of transcription factors including Oct4 and Nanog. However, little is known about how ES cells maintain the expression of these pluripotent factors in ES cells. Here we present evidence that Oct4, Nanog, and FoxD3 form a negative feedback loop to maintain their expression in pluripotent ES cells. First, Oct4 maintains Nanog activity by directly activating its promoter at sub-steady-state concentration but repressing it at or above steady-state levels. On the other hand, FoxD3 behaves as a positive activator of Nanog to counter the repressive effect of Oct4. The expression of Oct4 is activated by FoxD3 and Nanog but repressed by Oct4 itself, thus, exerting an important negative feedback loop to limit its own activity. Indeed, overexpression of either FoxD3 or Nanog in ES cells failed to increase the concentration of Oct4 beyond the steady-state concentration, whereas knocking down either FoxD3 or Nanog reduces the expression of Oct4 in ES cells. Finally, overexpression of Oct4 or Nanog failed to compensate the loss of Nanog or Oct4, respectively, suggesting that both are required for ES self-renewal and pluripotency. Our results suggest the FoxD3-Nanog-Oct4 loop anchors an interdependent network of transcription factors that regulate stem cell pluripotency.