The Notch signaling pathway controls numerous cell fate decisions during development and adulthood through diverse mechanisms. Thus, whereas it functions as an oscillator during somitogenesis, it can mediate an all-or-none cell fate switch to influence pattern formation in various tissues during development. Furthermore, while in some contexts continuous Notch signaling is required, in others a transient Notch signal is sufficient to influence cell fate decisions. However, the signaling mechanisms that underlie these diverse behaviors in different cellular contexts have not been understood. Notch1 along with two downstream transcription factors hes1 and RBP-Jk forms an intricate network of positive and negative feedback loops, and we have implemented a systems biology approach to computationally study this gene regulation network. Our results indicate that the system exhibits bistability and is capable of switching states at a critical level of Notch signaling initiated by its ligand Delta in a particular range of parameter values. In this mode, transient activation of Delta is also capable of inducing prolonged high expression of Hes1, mimicking the "ON" state depending on the intensity and duration of the signal. Furthermore, this system is highly sensitive to certain model parameters and can transition from functioning as a bistable switch to an oscillator by tuning a single parameter value. This parameter, the transcriptional repression constant of hes1, can thus qualitatively govern the behavior of the signaling network. In addition, we find that the system is able to dampen and reduce the effects of biological noise that arise from stochastic effects in gene expression for systems that respond quickly to Notch signaling.This work thus helps our understanding of an important cell fate control system and begins to elucidate how this context dependent signaling system can be modulated in different cellular settings to exhibit entirely different behaviors.

Muscle stem (satellite) cells are relatively resistant to cell-autonomous aging. Instead, their endogenous signaling profile and regenerative capacity is strongly influenced by the aged P-Smad3, differentiated niche, and by the aged circulation. With respect to muscle fibers, we previously established that a shift from active Notch to excessive transforming growth factor-beta (TGF-beta) induces CDK inhibitors in satellite cells, thereby interfering with productive myogenic responses. In contrast, the systemic inhibitor of muscle repair, elevated in old sera, was suggested to be Wnt. Here, we examined the age-dependent myogenic activity of sera TGF-beta1, and its potential cross-talk with systemic Wnt. We found that sera TGF-beta1 becomes elevated within aged humans and mice, while systemic Wnt remained undetectable in these species. Wnt also failed to inhibit satellite cell myogenicity, while TGF-beta1 suppressed regenerative potential in a biphasic fashion. Intriguingly, young levels of TGF-beta1 were inhibitory and young sera suppressed myogenesis if TGF-beta1 was activated. Our data suggest that platelet-derived sera TGF-beta1 levels, or endocrine TGF-beta1 levels, do not explain the age-dependent inhibition of muscle regeneration by this cytokine. In vivo, TGF-beta neutralizing antibody, or a soluble decoy, failed to reduce systemic TGF-beta1 and rescue myogenesis in old mice. However, muscle regeneration was improved by the systemic delivery of a TGF-beta receptor kinase inhibitor, which attenuated TGF-beta signaling in skeletal muscle. Summarily, these findings argue against the endocrine path of a TGF-beta1-dependent block on muscle regeneration, identify physiological modalities of age-imposed changes in TGF-beta1, and introduce new therapeutic strategies for the broad restoration of aged organ repair.