Peaks of differentiation inhibitors

According to the paradigm of inhibition of differentiation by sequestration of class A bHLH proteins, the quantity of binding partner should be low prior to differentiation, and the competition parameter $\alpha$ introduced earlier should thus be high. Relieving inhibition of differentiation, by increasing the quantity of binding partner, and thus decreasing competition, cannot move the bHLH dimerisation network from a state where it supports coexpression of many switch elements, to a state where only one is expressed. Also, increasing the transcription strength of the network $\sigma$ does not destabilise equilibria.

It is thus impossible to account for the selection of one differentiation outcome by increasing the availability of class A proteins (for example by downregulation of Id proteins). However, it is still possible that the competition strength, even in the undifferentiated state, is sufficiently low for many switch elements to be co-expressed. A potential mechanism to select 1 element and extinguish all others is then to transiently increase the competition strength, for example by transient up-regulation of Id proteins, just prior to differentiation commitment (a corresponding simulation is shown in Figure 6). This is in fact what has been experimentally observed on independent occasions, on a short time scale, during in vitro differentiation of osteoblasts (Ogata et al., 1993), neurons (Nagata and Todokoro, 1994), myeloid cells (Ishiguro et al., 1996), astrocytes (Andres-Barquin et al., 1997), Schwann cells (Stewart et al., 1997), keratinocytes (Langlands et al., 2000), germ cells (Houldsworth et al., 2001), and fibroblasts (Chambers et al., 2003). No rationale for these transient effects had been proposed so far.

When Id proteins are not up-regulated, other proteins could play the same role of increasing competition in the bHLH network. For example, Hes-1, which also has class A sequestering activity (Sasai et al., 1992), is transiently upregulated upon differentiation of an immortalised hair cell line (Rivolta et al., 2002), an immortalised neural cell line (Ohtsuka et al., 1998), and neuroblastoma (Grynfeld et al., 2000) (although its role in the latter case is complicated by the fact that it also binds Id proteins and interferes with Id2/E2-2 complex formation, Jögi et al., 2002); the transient Hes-1 expression is concomitant with downregulation of the bHLH protein HASH-1. Hes genes are dominant repressors with many targets (Barolo and Levine, 1997), and could also directly repress many elements of the network, which can be modeled by a decrease in the transcription strength $\sigma$, and has the same effect of destabilising equilibria where many elements are coexpressed.

Finally, erythroid-specific genes have been observed to be transiently downregulated upon induced, in-vitro differentiation (Lister et al., 1995), which could also be explained by transiently-increased competition in a bHLH dimerisation network.