Signaling pathways crucial in differentiation initiation and terminal differentiation during lens development
Precise regulation of signaling pathways that are involved in cell differentiation is crucial for the proper development of a tissue. Here we studied the regulatory mechanisms that control the initiation of differentiation in lens epithelial cells and terminal differentiation of lens fiber cells. Previously the caspase-3 protease, when activated at low levels downstream to the canonical mitochondrial death pathway, was shown to be a requisite "non-apoptotic" signal for lens epithelial cell differentiation initiation. However, it was not known how this caspase-3 signal is regulated and maintained at the low levels of activation necessary for its function as a differentiation and not a death signal. From our studies we discovered that a coordinated α6 integrin/IGF-1R survival signal mediated the level of caspase-3 activation. This α6 integrin/IGF-1R pathway activates NFκB, a transcription factor that induces expression of survival proteins in both the Bcl-2 and Inhibitor of Apoptosis (IAP) families, which in turn regulate caspase-3 activity. When IGF-1R activation is blocked, NFκB signaling is compromised and, both the Bcl-2 and IAP families fail to be induced. α6 integrin was found to be responsible for transactivation of IGF-1R and induction of its downstream effectors. Releasing these cellular controls on caspase-3 results in high levels of caspase-3, blocking lens cell differentiation and instead inducing cell death. Investigation of the signal responsible for activating the mitochondrial death pathway for its non-apoptotic function in differentiation-initiation suggests that generation of endogenous hydrogen peroxide (H2O2), a reactive oxygen species (ROS), is a good candidate for inducing caspase-activation in the developing lens. We also investigated the signals that lead to tightly programmed elimination of nuclei and organelles from the lens during terminal differentiation to form the organelle free zone (OFZ). We have discovered that the formation of the organelle free zone (OFZ) involves induction of an autophagic pathway. The mechanism responsible for inducing the autophagy process that leads to OFZ formation involves inhibition of a c-Jun NH2-terminal kinases (JNK)-regulated mTOR signal. mTOR, the mechanistic target of rapamycin, is a classical regulator of autophagy induction. Inhibition of JNK or mTOR signaling induces premature formation of OFZ by autophagy, with JNK signaling upstream of mTOR. This role for JNK as negative regulator of autophagy in the context of development represents a paradigm shift for the known signaling roles for JNK in autophagy. Collectively, our studies revealed essential regulatory mechanisms that control lens differentiation and development and are expected to have a wide applicability to other cell types.
Basu, Subhasree, "Signaling pathways crucial in differentiation initiation and terminal differentiation during lens development" (2013). ETD Collection for Thomas Jefferson University. AAI3599564.