Role(s) of p53/p63 in chondrocyte re-differentiation upon activation of ER stress

Abstract

Endoplasmic Reticulum (ER) stress signal is a cellular response to various insults including

abnormal protein folding load, activating the unfolded protein response. Under severe ER stress,

apoptosis will occur in most cell types. Interestingly, this does not happen in a disease model for

Metaphyseal chondrodysplasia type Schmid (MCDS), where ER stress was activated in the

hypertrophic zone of the growth plate where mutant collagen X proteins that cannot be folded

correctly is expressed. Instead of normal progression from proliferating chondrocytes (PCs) to

hypertrophic chondrocytes (HCs) and conversion to bone, HCs in MCDS mice undergo

re-differentiation to PCs as a survival strategy due to an activation of ER stress. Transcription

factors are known to be important in regulating differentiation. p53 family members, as

transcription factors, are known to play important roles in developmental processes including

cellular reprogramming, thus, we hypothesize that the ectopic expression of key transcription

factors, p53 and TAp63, which are activated by ER stress is involved in HC re-differentiation. p53

is normally expressed in late PCs, Pre-HCs, and upper HCs, while TAp63 is expressed in PCs and

Pre-HCs suggesting they may have roles in chondrocyte differentiation. p53 activated under ER

stress in HCs are nuclear localized in MCDS mice, but did not invoke the apoptotic programme.

In this project, using quantitative analyse to study the expression level of p53 and p63 isoforms, it

was confirmed that p53 and TAp63γ are in part transcriptionally activated upon ER stress. From

functional study by inactivating p53 in MCDS mice, it was shown that p53 alone was not sufficient

to mediate re-differentiation. Given that TAp63γ isoforms is also highly upregulated upon ER

stress, and the negative regulator, ΔNp63, is downregulated, this combination of change in gene

expression also need to be considered.

Furthermore, known regulators of p53 and p63 activity such as ASPP1 and iASPP are also

differentially expressed in HCs, and are altered upon activation of ER stress favouring cell survival.

Thus, it would be important to evaluate the combination of TAp63 in the re-differentiation process

from conditional inactivation of p63 or in combination with p53 to gain a clearer understanding of

the contribution and relationship of these transcription factors in the survival strategy of stressed

HCs.