Noncanonical autophagy at ER exit sites regulates procollagen turnover.

Type I collagen is the main component of bone matrix and other connective tissues. Rerouting of its procollagen precursor to a degradative pathway is crucial for osteoblast survival in pathologies involving excessive intracellular buildup of procollagen that is improperly folded and/or trafficked. What cellular mechanisms underlie this rerouting remains unclear. To study these mechanisms, we employed live-cell imaging and correlative light and electron microscopy (CLEM) to examine procollagen trafficking both in wild-type mouse osteoblasts and osteoblasts expressing a bone pathology-causing mutant procollagen. We found that although most procollagen molecules successfully trafficked through the secretory pathway in these cells, a subpopulation did not. The latter molecules appeared in numerous dispersed puncta colocalizing with COPII subunits, autophagy markers and ubiquitin machinery, with more puncta seen in mutant procollagen-expressing cells. Blocking endoplasmic reticulum exit site (ERES) formation suppressed the number of these puncta, suggesting they formed after procollagen entry into ERESs. The punctate structures containing procollagen, COPII, and autophagic markers did not move toward the Golgi but instead were relatively immobile. They appeared to be quickly engulfed by nearby lysosomes through a bafilomycin-insensitive pathway. CLEM and fluorescence recovery after photobleaching experiments suggested engulfment occurred through a noncanonical form of autophagy resembling microautophagy of ERESs. Overall, our findings reveal that a subset of procollagen molecules is directed toward lysosomal degradation through an autophagic pathway originating at ERESs, providing a mechanism to remove excess procollagen from cells.

Transforming growth factor-ß1/Smad3-independent epithelial-mesenchymal transition in type I collagen glomerulopathy.

The glomerulofibrotic Col1a2-deficient mouse model demonstrates glomerular homotrimeric type I collagen deposition in mesangial and subendothelial spaces. In this report, we investigate the role of transforming growth factor ß1 (TGF-ß1) in myofibroblast activation and epithelial-mesenchymal transition (EMT) in this glomerulopathy. Immunohistochemical analyses of glomerular α-sma, desmin, vimentin, and proliferating cell nuclear antigen demonstrated parietal epithelial cell proliferation and EMT in late stages of the glomerulopathy in the Col1a2-deficient mice. Glomerular TGF-ß1 RNA and protein were not elevated in 1- and 3-month-old mice as determined by quantitative reverse transcriptase-polymerase chain reaction and protein immunoassay analyses. To investigate further whether TGF-ß1 plays a role in the glomerulopathy outside of the 1- and 3-month time periods, the Col1a2-deficient mice were bred with Smad3 knockout mice. If the glomerular fibrosis in the Col1a2-deficient mice is mediated by the TGF-ß1/Smad3 transcription pathway, it was hypothesized that the resultant Col1a2-deficient/Smad3-deficient mice would exhibit attenuated glomerular homotrimer deposition. However, the Col1a2-deficient/Smad3-deficient kidneys were similarly affected as compared to age-matched Col1a2-deficient kidneys, suggesting that homotrimeric type I collagen deposition in the Col1a2-deficient mouse is independent of TGF-ß1/Smad3 signaling. Deposition of homotrimeric type I collagen appears to be the initiating event in this glomerulopathy, providing evidence that EMT and myofibroblast activation occur following initiation, consistent with a secondary wound-healing response independent of TGF-ß1.

Osteoblast Malfunction Caused by Cell Stress Response to Procollagen Misfolding in α2(I)-G610C Mouse Model of Osteogenesis Imperfecta.

Glycine (Gly) substitutions in collagen Gly-X-Y repeats disrupt folding of type I procollagen triple helix and cause severe bone fragility and malformations (osteogenesis imperfecta [OI]). However, these mutations do not elicit the expected endoplasmic reticulum (ER) stress response, in contrast to other protein-folding diseases. Thus, it has remained unclear whether cell stress and osteoblast malfunction contribute to the bone pathology caused by Gly substitutions. Here we used a mouse with a Gly610 to cysteine (Cys) substitution in the procollagen α2(I) chain to show that misfolded procollagen accumulation in the ER leads to an unusual form of cell stress, which is neither a conventional unfolded protein response (UPR) nor ER overload. Despite pronounced ER dilation, there is no upregulation of binding immunoglobulin protein (BIP) expected in the UPR and no activation of NF-κB signaling expected in the ER overload. Altered expression of ER chaperones αB crystalline and HSP47, phosphorylation of EIF2α, activation of autophagy, upregulation of general stress response protein CHOP, and osteoblast malfunction reveal some other adaptive response to the ER disruption. We show how this response alters differentiation and function of osteoblasts in culture and in vivo. We demonstrate that bone matrix deposition by cultured osteoblasts is rescued by activation of misfolded procollagen autophagy, suggesting a new therapeutic strategy for OI. © 2016 American Society for Bone and Mineral Research.

Deficient degradation of homotrimeric type I collagen, α1(I)3 glomerulopathy in oim mice.

Col1a2-deficient (oim) mice synthesize homotrimeric type I collagen due to nonfunctional proα2(I) collagen chains. Our previous studies revealed a postnatal, progressive type I collagen glomerulopathy in this mouse model, but the mechanism of the sclerotic collagen accumulation within the renal mesangium remains unclear. The recent demonstration of the resistance of homotrimeric type I collagen to cleavage by matrix metalloproteinases (MMPs), led us to investigate the role of MMP-resistance in the glomerulosclerosis of Col1a2-deficient mice. We measured the pre- and post-translational expression of type I collagen and MMPs in glomeruli from heterozygous and homozygous animals. Both the heterotrimeric and homotrimeric isotypes of type I collagen were equally present in whole kidneys of heterozygous mice by immunohistochemistry and biochemical analysis, but the sclerotic glomerular collagen was at least 95-98% homotrimeric, suggesting homotrimeric type I collagen is the pathogenic isotype of type I collagen in glomerular disease. Although steady-state MMP and Col1a1 mRNA levels increased with the disease progression, we found these changes to be a secondary response to the deficient clearance of MMP-resistant homotrimers. Increased renal MMP expression was not sufficient to prevent homotrimeric type I collagen accumulation.