Label-Free Quantitative Proteomics Reveals Survival Mechanisms Developed by Hypertrophic Chondrocytes under ER Stress

Emerging
evidence implicates ER stress caused by unfolded mutant
proteins in chondrocytes as the underlying pathology of chondrodysplasias.
ER stress is triggered in hypertrophic chondrocytes (HCs) in a mouse
model (13del) of metaphyseal chondrodysplasia type Schmid (MCDS) caused
by misfolded mutant collagen X proteins, but the HCs do not undergo
apoptosis; rather chondrocyte differentiation is altered, causing
skeletal abnormality. How 13del HCs can escape from apoptosis and
survive ER stress is not understood. Here we compared the proteomes
of HCs isolated from 13del growth plates with normal HCs using a label-free
quantitative mass spectrometry approach. Pathway enrichment analyses
of differentially expressed proteins showed significant changes in
glycolysis and ER-mitochondria pathways in 13del HCs as well as in
ATDC5 cell lines expressing wt and 13del collagen X. In vivo, we showed
expression of mitochondrial calcium channels was reduced while mitochondrial
membrane polarity was maintained in 13del chondrocytes, while in vitro,
glucose uptake was maintained. We propose 13del HCs survive by a mechanism
whereby changes in ER-mitochondria communication reduce import of
calcium coupled to maintenance of mitochondrial membrane polarity.
These findings provide the initial insights into our understanding
of growth plate changes caused by protein misfolding in the pathogenesis
of chondrodysplasias