Cellular stress pathways in cartilage biology and disease

Abstract

Cells experience many intrinsic and extrinsic stress factors and respond by activating specific pathways for survival. In many human diseases and normal developmental processes, cross talks between cellular stress pathways are part of the regulatory system determining cell fate. It is well established that the presence of misfolded proteins in the endoplasmic reticulum (ER) activate the unfolded protein response (UPR) as part of the ER-stress signal for cell survival. Furthermore, other stress pathways in cells utilize the UPR as part of the cascade of events for cell survival. For example, while hypoxia activates HIF proteins, components of UPR are also utilized for cell survival. However, whether UPR mediating from the ER stress has a direct relationship with the hypoxia response or the HIF pathway is not known.

Here, using a mouse model for metaphyseal chondrodyspalsia type Schmid (MCDS) with activation of ER-stress in hypertrophic chondrocyte due to the expression of misfolded collagen X in the developing growth plate, we showed that the hypoxia pathway could indeed be utilized by the UPR response, with the PERK sensor and ATF4 in the stabilization of HIF proteins for cell survival. Specifically, we showed that accumulation of mutant collagen type X (13del) proteins in the ER of hypertrophic chondrocytes resulted in a stabilization of HIF proteins and nuclear localization. This event is a cell autonomous confirmed in mouse chimeras containing a mixture of both normal and 13del MCDS hypertrophic chondrocytes.

While the mechanism of HIF protein stabilization is still not clear, it is likely to be a stabilization of HIF proteins already present in chondrocytes, as the growth plate cartilage is avascular. Thus, the HIF proteins are captured by hypertrophic chondrocytes under ER stress as part of the survival mechanism. That HIF proteins are needed for survival of MCDS hypertrophic chondrocyte is consistent with our finding that a conditional inactivation of the Hif-1α gene in hypertrophic chondrocytes results in a mild induction of apoptosis. However, the role of Hif2α still needs to be addressed, as it is likely to have a redundant role with Hif1α.

A link to ATF4 was provided through the generation of Col10a1-ATF transgenic mice with ectopic expression of ATF4 in hypertrophic chondrocytes under the regulation of the Col10a1 promoter. The result is an expanded zone of hypertrophic chondrocytes similar to 13del-MCDS mice and more interestingly, a stabilization of HIF proteins that are localized in the nucleus. However, the mechanism and precise link in this instance require further study of the molecular links and validation of interplay.

Finally, we showed that hypertrophic chondrocytes under ER stress induced a change in the metabolic process that is consistent with a hypoxia condition. It is likely that the metabolic changes require more oxygen than is available in the environment, such that the cellular changes mimic a hypoxia outcome, providing a positive signal using EF5 as an indicator. My findings have provided further insights into the interplay between stress signals in cells and the disease mechanism for MCDS patients.