Endoplasmic reticulum stress in chondrodysplasias caused by mutations in collagen types II and X

Abstract

The endoplasmic reticulum is primarily recognized as the site of synthesis and folding of secreted, membrane-bound, and some organelle-targeted proteins. An imbalance between the load of unfolded proteins and the processing capacity in endoplasmic reticulum leads to the accumulation of unfolded or misfolded proteins and endoplasmic reticulum stress, which is a hallmark of a number of storage diseases, including neurodegenerative diseases, a number of metabolic diseases, and cancer. Moreover, its contribution as a novel mechanistic paradigm in genetic skeletal diseases associated with abnormalities of the growth plates and dwarfism is considered. In this review, I discuss the mechanistic significance of endoplasmic reticulum stress, abnormal folding, and intracellular retention of mutant collagen types II and X in certain variants of skeletal chondrodysplasia.

Keywords: Chondrodysplasia; Collagen; Endoplasmic reticulum stress; Mechanism; Mutation; Unfolded protein response.

Fig. 1

Endoplasmic reticulum signaling triggers unfolded protein response. Under physiological conditions, glucose-regulated protein 78 kDa (Grp78/BiP), protein disulfide isomerase (PDI), heat shock protein 47 (HSP47), and other molecular chaperones are present in the lumen of endoplasmic reticulum (ER). Additionally, Grp78 binds the ER luminal domains of the three ER stress receptors, i.e., protein kinase RNA-like endoplasmic reticulum kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme 1 (IRE1). Accumulation of unfolded proteins in the ER, e.g., as a result of mutation in the collagen gene, induces sequential dissociation of Grp78 from PERK (1), ATF6 (2), and IRE1 (3), respectively, and their activation. Dissociated Grp78 molecules and other chaperones are mobilized to form complexes with unfolded proteins aggregated in the ER. Activated PERK (via dimerization and autophosphorylation) phosphorylates eukaryotic initiation factor 2α (eIF2α). This phosphorylation suppresses general protein synthesis, thus decreasing the entry of newly synthesized proteins into the ER and enabling translation of ATF4. ATF4 translocates to the nucleus and induces the transcription of genes required to restore ER homeostasis including that for CCAAT/enhancer binding protein homologous protein (CHOP). ATF6 is activated by limited proteolysis after its translocation from the ER to the Golgi apparatus. It is cleaved by site 1 and site 2 proteases (S1P, S2P) releasing the cytoplasmic 50kDa domain (ATF6₅₀) which is an active transcription factor. ATF6₅₀ regulates the expression of genes involved in the unfolded protein response (UPR), including chaperones, CHOP, and X-box binding protein 1 (XBP1) . Additionally, the activation of XBP1 is carried out by IRE1. Activated IRE1 produces an unconventional splice in cytoplasmic XBP1 mRNA. Spliced XBP1 protein (sXBP1) translocates to the nucleus and upregulates the transcription of genes encoding chaperones to increase the protein folding capacity of the ER and genes controlling the endoplasmic reticulum-associated degradation (ERAD) system, a mechanism by which misfolded protein is retrotranslocated into the cytoplasm and degraded in the proteasome. This complex action aims to restore ER homeostasis by blocking unfolded protein aggregation, inducing degradation of aggregated proteins and enhancing folding capacity

Fig. 2

Quality control interplay in the “ER-Golgi-lysosomes/extracellular space axis.” Extracellular space proteins, e.g., collagens, are synthesized by ribosomes and translocated into the endoplasmic reticulum (ER). In the ER, proteins accomplish their native form (folding, assembling) under strict quality control mechanisms. Appropriate folding/structure of the proteins enables their transport and modification in the Golgi, followed by transport to the extracellular space and/or to lysosomes. Accelerated aggregation of unfolded/misfolded proteins which overload the folding capacity within the ER induces the ER stress sensors, i.e., protein kinase RNA-like endoplasmic reticulum kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme 1 (IRE1) which further activate the unfolded protein response (UPR) signaling. Unfolded/misfolded proteins are retained and directed to degradation by endoplasmic reticulum-associated degradation (ERAD) system, apoptosis, or autophagy (reciprocal regulation, yellow arrows). Under the quality control mechanisms, the ER stress aims to restore the homeostasis within ER through regulation of protein entry into the ER, folding, and degradation. Black arrows show the alternative pathways of the ER-Golgi-lysosomes/extracellular space axis depending on the proper folding, unfolding, or misfolding of protein. Red, intermittent arrows depict homeostatic control pathways (plus sign stimulatory, minus sign inhibitory)

Fig. 3

Histology of the tibial growth plates of the 10-week-old WT^ProGFP(−) (a, e), WT^ProGFP(+) (b, f), R992C^ProGFP(−) (c, g), and R992C^ProGFP(+) (D, H) mice maintained in the absence (A–D) or presence (E–H) of Dox. In contrast to chondrocytes seen in the growth plates of the 10-week-old WT^ProGFP(−) (a), WT^ProGFP(+) (b), and R992C^ProGFP(−) (c) mice, the columnar organization of chondrocytes in the R992C^ProGFP(+) littermates was altered (d). For instance, in the R992C^ProGFP(+) mice, such alterations were indicated by the presence of disorganized columns whose continuity of the typical palisade-like arrangement was often interrupted by extended areas in which the chondrocytes were absent (d). Switching off the expression of the R992C ProGFP in the ^DoxR992C^ProGFP(+) mice maintained in Tet-off conditions resulted in developing growth plates in which chondrocytes were organized correctly (h). Growth plates from these mice had a normal morphology comparable to that seen in the ^DoxR992C^ProGFP(−) littermates (g) as well as their ^DoxWT^ProGFP(−) and ^DoxWT^ProGFP(+) counterparts maintained in Tet-off conditions (e, f) (reprinted from the American Journal of Pathology (Arita et al. 2015) with permission from Elsevier)