Procollagen Transport Research Articles

Circadian control of the secretory pathway maintains collagen homeostasis.

Collagen is the most abundant secreted protein in vertebrates and persists throughout life without renewal. The permanency of collagen networks contrasts with both the continued synthesis of collagen throughout adulthood and the conventional transcriptional/translational homeostatic mechanisms that replace damaged proteins with new copies. Here, we show circadian clock regulation of endoplasmic reticulum-to-plasma membrane procollagen transport by the sequential rhythmic expression of SEC61, TANGO1, PDE4D and VPS33B. The result is nocturnal procollagen synthesis and daytime collagen fibril assembly in mice. Rhythmic collagen degradation by CTSK maintains collagen homeostasis. This circadian cycle of collagen synthesis and degradation affects a pool of newly synthesized collagen, while maintaining the persistent collagen network. Disabling the circadian clock causes abnormal collagen fibrils and collagen accumulation, which are reduced in vitro by the NR1D1 and CRY1/2 agonists SR9009 and KL001, respectively. In conclusion, our study has identified a circadian clock mechanism of protein homeostasis wherein a sacrificial pool of collagen maintains tissue function.

CXCL12/CXCR4-Mediated Procollagen Secretion Is Coupled To Cullin-RING Ubiquitin Ligase Activation

Tissue fibrosis is mediated by the actions of multiple pro-fibrotic proteins that can induce myofibroblast phenoconversion through diverse signaling pathways coupled predominantly to Smads or MEK/Erk proteins. The TGFβ/TGFβR and CXCL12/CXCR4 axes induce myofibroblast phenoconversion independently through Smads and MEK/Erk proteins, respectively. To investigate these mechanisms at the genetic level, we have now elucidated the TGFβ/TGFβR and CXCL12/CXCR4 transcriptomes in human fibroblasts. These transcriptomes are largely convergent, and up-regulate transcripts encoding proteins known to promote myofibroblast phenoconversion. These studies also revealed a molecular signature unique to CXCL12/CXCR4 axis activation for COPII vesicle formation, ubiquitination, and Golgi/ER localization/targeting. In particular, both CUL3 and KLHL12, key members of the Cullin-RING (CRL) ubiquitin ligase family of proteins involved in procollagen transport from the ER to the Golgi, were highly up-regulated in CXCL12-, but repressed in TGFβ-, treated cells. Up-regulation of CUL3 and KLHL12 was correlated with higher procollagen secretion by CXCL12-treated cells, and this affect was ablated upon treatment with inhibitors specific for CXCR4 or CUL3 and repressed by TGFβ/TGFβR axis activation. The results of these studies show that activation of the CXCL12/CXCR4 axis uniquely facilitates procollagen I secretion through a COPII-vesicle mediated mechanism to promote production of the ECM characteristic of fibrosis.

Abstract 3712: The relationship of Procollagen alpha 1 type 1 (Col1A1) / DDR2 signaling in malignant glioma and sensitivity to STAT 3/5 inhibitor

Abstract We have previously shown that Procollagen alpha 1 type 1 (Col1A1) is found in low and intermediate grade glioma and in less aggressive glioblastoma multiforme (GBM) (Cancer Invest. 23:577, 2005). However, these tumor cells are more prone to ER stress -inducing agents such as Befeldine (BFA) which block the transport of Procollagen to the cell surface. We also found that GBM cells which possess Col1A1 also express DDR2. The role of Col1A1/ DDR2 signaling in glioma is not known. We have used three GBM cell lines: Glioma1 established in our laboratory from a patient who progressed from grade 3 to grade 4, U-118, A-172 and U-373 to study this signaling pathway. Both Glioma1 and U-118 express Col1A1 and DDR1 and 2. A-172 and U-373 do not express Col1A1 but express both DDR2. U-373 does not express HSP47 which is an essential protein to fold collagen. In order to define the function of DDR2/ Col1A1 in GBM, we have used knock down COL1A1 and DDR2. Silencing Col1A1 leads to a significant increase in invasiveness by Matrigel assay which indicated that Col1A1 is important in preventing invasion in GBM and hence is found more in low and intermediate grade glioma. However, it has minimal effect on cell cycle or cell proliferation. Next, we have silenced DDR2 in all 4 cell lines and studied the biochemical changes. We found that silencing DDR2 does not affect cell proliferation, cell death or cell cycle. However, it does affect the sensitivity to BFA. In cell lines which possess Col1A1, the cell viability increased by 20 -30% (p <0.05). In contrast, cell lines which do not express Col1A1, the cell viability decreased by 25-30% (p<0.01). However, there is no difference in sensitivity to the DNA damaging agent cisplatin. Thus, DDR2 may function differently depending on the presence of Col1A1. To explore this further, we have performed a limited phoshoprotein kinase array in Glioma 1 with siCol1A1 or siDDR2. We found increase in STAT3, 5 and STAT6 upon silencing either Col1A1 or DDR2 which is further confirmed by immunoblot in Glioma 1 for pSTAT 3 and 5. We then tested the antitumor effect of STAT3/5 inhibitor (SH4-54) a benzoic acid based inhibitor (provided by Dr. James Turkson) which interferes with the SH2 and DNA binding domains as well as tyr705 phosphorylation in Glioma1 and U-118 w/wo SiCol1A1. At 0.5 uM the cell viability in Glioma 1 is 55.65 + 1.35 and 74.2 + 4.6 in U-118 while SiCol1A1 it decreased to 44.5 + 3.53% in Glioma 1 and 58.2+ 1.9, respectively, with minimal activity in A-172 which does not possess Col1A1. In contrast, there is no effect with another commercially available STAT 3 inhibitor S31-201 which only interferes with DNA binding domain. Overall, our data suggest that Col1A1/DDR2/ STAT signaling may be important in certain GBM cell lines and can be exploited for future treatment in brain tumor (Supported by Wanfang Hospital-Taipei Medical University Fellowship and VA Research Fund). Citation Format: Shumei Chen, Chunjing Wu, Ying-Ying Li, Medhi Wangpaichitr, Ronald J. Benveniste, James Turkson, Niramol Savaraj, Lynn G. Feun. The relationship of Procollagen alpha 1 type 1 (Col1A1) / DDR2 signaling in malignant glioma and sensitivity to STAT 3/5 inhibitor. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3712.

Biochemical and Structural Characterization of Neocartilage Formed by Mesenchymal Stem Cells in Alginate Hydrogels

A popular approach to make neocartilage in vitro is to immobilize cells with chondrogenic potential in hydrogels. However, functional cartilage cannot be obtained by control of cells only, as function of cartilage is largely dictated by architecture of extracellular matrix (ECM). Therefore, characterization of the cells, coupled with structural and biochemical characterization of ECM, is essential in understanding neocartilage assembly to create functional implants in vitro. We focused on mesenchymal stem cells (MSC) immobilized in alginate hydrogels, and used immunohistochemistry (IHC) and gene expression analysis combined with advanced microscopy techniques to describe properties of cells and distribution and organization of the forming ECM. In particular, we used second harmonic generation (SHG) microscopy and focused ion beam/scanning electron microscopy (FIB/SEM) to study distribution and assembly of collagen. Samples with low cell seeding density (1e7 MSC/ml) showed type II collagen molecules distributed evenly through the hydrogel. However, SHG microscopy clearly indicated only pericellular localization of assembled fibrils. Their distribution was improved in hydrogels seeded with 5e7 MSC/ml. In those samples, FIB/SEM with nm resolution was used to visualize distribution of collagen fibrils in a three dimensional network extending from the pericellular region into the ECM. In addition, distribution of enzymes involved in procollagen processing were investigated in the alginate hydrogel by IHC. It was discovered that, at high cell seeding density, procollagen processing and fibril assembly was also occurring far away from the cell surface, indicating sufficient transport of procollagen and enzymes in the intercellular space. At lower cell seeding density, the concentration of enzymes involved in procollagen processing was presumably too low. FIB/SEM and SHG microscopy combined with IHC localization of specific proteins were shown to provide meaningful insight into ECM assembly of neocartilage, which will lead to better understanding of cartilage formation and development of new tissue engineering strategies.

A Regulator of Secretory Vesicle Size, Kelch-Like Protein 12, Facilitates the Secretion of Apolipoprotein B100 and Very-Low-Density Lipoproteins—Brief Report

One of the major risk factors for atherosclerosis is the plasma level of low-density lipoprotein (LDL), which is a product of very-low-density lipoprotein (VLDL). Hepatic apolipoprotein B100 (apoB100) is the essential component that provides structural stability to VLDL particles. Newly translated apoB100 is partially lipidated in the endoplasmic reticulum (ER), forming nascent apoB100-VLDL particles. These particles are further modified to form fully mature VLDLs in the Golgi apparatus. Therefore, the transport of nascent VLDL from the ER to the Golgi represents a critical step during VLDL maturation and secretion and in regulating serum LDL cholesterol levels. Our previous studies showed that apoB100 exits the ER in coat complex II vesicles (COPII), but the cohort of related factors that control trafficking is poorly defined. Expression levels of Kelch-like protein 12 (KLHL12), an adaptor protein known to assist COPII-dependent transport of procollagen, were manipulated by using a KLHL12-specific small interfering RNA and a KLHL12 expression plasmid in the rat hepatoma cell line, McArdle RH7777. KLHL12 knockdown decreased the secreted and intracellular pools of apoB100, an effect that was attenuated in the presence of an autophagy inhibitor. KLHL12 knockdown also significantly reduced secretion of the most lipidated apoB100-VLDL species and led to the accumulation of apoB100 in the ER. Consistent with these data, KLHL12 overexpression increased apoB100 recovery and apoB100-VLDL secretion. Images obtained from confocal microscopy revealed colocalization of apoB100 and KLHL12, further supporting a direct link between KLHL12 function and VLDL trafficking from the ER. KLHL12 plays a critical role in facilitating the ER exit and secretion of apoB100-VLDL particles, suggesting that KLHL12 modulation would influence plasma lipid levels.

Sar1 assembly regulates membrane constriction and ER export

The guanosine triphosphatase Sar1 controls the assembly and fission of COPII vesicles. Sar1 utilizes an amphipathic N-terminal helix as a wedge that inserts into outer membrane leaflets to induce vesicle neck constriction and control fission. We hypothesize that Sar1 organizes on membranes to control constriction as observed with fission proteins like dynamin. Sar1 activation led to membrane-dependent oligomerization that transformed giant unilamellar vesicles into small vesicles connected through highly constricted necks. In contrast, membrane tension provided through membrane attachment led to organization of Sar1 in ordered scaffolds that formed rigid, uniformly nonconstricted lipid tubules to suggest that Sar1 organization regulates membrane constriction. Sar1 organization required conserved residues located on a unique C-terminal loop. Mutations in this loop did not affect Sar1 activation or COPII recruitment and enhanced membrane constriction, yet inhibited Sar1 organization and procollagen transport from the endoplasmic reticulum (ER). Sar1 activity was directed to liquid-disordered lipid phases. Thus, lipid-directed and tether-assisted Sar1 organization controls membrane constriction to regulate ER export.

Actin Filaments Are Required for Fibripositor-mediated Collagen Fibril Alignment in Tendon

Cells in tendon deposit parallel arrays of collagen fibrils to form a functional tissue, but how this is achieved is unknown. The cellular mechanism is thought to involve the formation of intracellular collagen fibrils within Golgi to plasma membrane carriers. This is facilitated by the intracellular processing of procollagen to collagen by members of the tolloid and ADAMTS families of enzymes. The carriers subsequently connect to the extracellular matrix via finger-like projections of the plasma membrane, known as fibripositors. In this study we have shown, using three-dimensional electron microscopy, the alignment of fibripositors with intracellular fibrils as well as an orientated cable of actin filaments lining the cytosolic face of a fibripositor. To demonstrate a specific role for the cytoskeleton in coordinating extracellular matrix assembly, cytochalasin was used to disassemble actin filaments and nocodazole or colchicine were used to disrupt microtubules. Microtubule disruption delayed procollagen transport through the secretory pathway, but fibripositor numbers were unaffected. Actin filament disassembly resulted in rapid loss of fibripositors and a subsequent disappearance of intracellular fibrils. Procollagen secretion or processing was not affected by cytochalasin treatment, but the parallelism of extracellular collagen fibrils was altered. In this case a significant proportion of collagen fibrils were found to no longer be orientated with the long axis of the tendon. The results suggest an important role for the actin cytoskeleton in the alignment and organization of the collagenous extracellular matrix in embryonic tendon.

Sequence-specific Recognition of Collagen Triple Helices by the Collagen-specific Molecular Chaperone HSP47

HSP47 is a molecular chaperone that plays an unknown role during the assembly and transport of procollagen. Our previous studies showed that, unlike most chaperones, HSP47 interacts with a correctly folded substrate. We suggested that HSP47 either stabilizes the correctly folded collagen helix from heat denaturation or prevents lateral aggregation prior to its transport from the endoplasmic reticulum. In this study we have addressed the role of triple helix stability in the binding of HSP47 to procollagen by expressing procollagen molecules with differing thermal stabilities and analyzing their ability to interact with HSP47 within the endoplasmic reticulum. Our results show that HSP47 interacts with thermostable procollagen molecules, suggesting that helix stabilization is not the primary function of HSP47 and that the interaction of HSP47 with procollagen depends upon the presence of a minimum of one Gly-X-Arg triplet within the triple helical domain. Interestingly, procollagen chains containing high proportions of stabilizing triplets formed triple helices and interacted with HSP47 even in the absence of proline hydroxylation, demonstrating that recognition does not depend upon this modification. Our results support the view that HSP47 functions early in the secretory pathway by preventing the lateral aggregation of procollagen chains.

Imaging of procollagen transport reveals COPI-dependent cargo sorting during ER-to-Golgi transport in mammalian cells.

We have examined the ER-to-Golgi transport of procollagen, which, when assembled in the lumen of the ER, is thought to be physically too large to fit in classically described 60-80 nm COPI- and COPII-coated transport vesicles. We found that procollagen exits the ER via COPII- coated ER exit sites and is transported to the Golgi along microtubules in defined transport complexes. These procollagen-containing transport complexes are, however, distinct from those containing other cargo proteins like ERGIC-53 and ts-045-G. Furthermore, they do not label for the COPI coat complex in contrast to those containing ts-045-G. Inhibition of COPII or COPI function before addition of ascorbate, which is required for the folding of procollagen, inhibits export of procollagen from the ER. Inactivation of COPI coat function after addition of ascorbate results in the localisation of procollagen to transport complexes that now also contain ERGIC-53 and are inhibited in their transport to the Golgi complex. These data reveal the existence of an early COPI-dependent, pre-Golgi cargo sorting step in mammalian cells.

Age-related attenuation of HSP47 heat response in fibroblasts

The collagen-binding heat shock protein of molecular weight 47 000 (HSP47), resident in the endoplasmic reticulum (ER), is assumed to play a specific role as a molecular chaperon in the processing of procollagen molecules. The present investigation of age-related alteration in the HSP47 heat response in cultured murine and human fibroblasts revealed expression in cells with a low population doubling level (PDL) derived from young mice and people more inducible by heat treatment than those from older mice and people. On the other hand, cells with a high PDL showed a very low heat response in terms of HSP47 expression regardless of the donor age. Northern blot analysis of HSP47 m-RNA indicated that the age related attenuation of HSP47 expression was regulated by transcriptional mechanisms. Furthermore, immunofluorescent analysis using a monoclonal antibody against the carboxylterminal propeptide of type I procollagen revealed far greater retention of procollagen molecules in the ER lumen of cells from old persons than in those from young persons. This was particularly prominent in heat-treated cells from old persons, indicating the possibility that the observed decrease in HSP47 heat response might cause blockage of procollagen transport to the Golgi and therefore secretion.

Kinetic studies of the intracellular transport of procollagen and fibronectin in human fibroblasts. Effects of the monovalent ionophore, monensin.

The monovalent ionophore, monensin, has been found to inhibit the secretion of both procollagen and fibronectin from human fibroblasts in cell culture. The kinetics of inhibition, as well as those for the release of inhibition, suggested that both proteins may be cotransported in the cell. In the present study, we have examined the intracellular translocation and release into the culture medium of procollagen and fibronectin, in the presence or absence of monensin. Pulse-chase studies were combined with subcellular fractionation of the fibroblasts to determine the rates of intracellular movement. We found that monensin did not significantly affect either the subcellular fractionation, or the distribution of organelle marker enzymes along the gradient, and that procollagen moved from a region of high buoyant density (primarily endoplasmic reticulum), through a mid-density region (primarily Golgi elements), to a region of low buoyant density before exiting from the cell. Monensin markedly decreased the rate of transit from one region to the other; kinetic analysis of the data showed a greater than 3-fold decrease in the rate constants for intracellular movement into the low buoyant density components and thence to the culture medium. The density gradient distribution of fibronectin was affected by monensin in similar fashion, indicating that it and procollagen probably follow the same intracellular route. Since monensin causes both proteins to accumulate in highest abundance in regions of the density gradient corresponding to Golgi and endoplasmic reticulum, the site of ionophore blockade appears to be within the Golgi apparatus. Thus, procollagen and fibronectin share a common intracellular route prior to entering the Golgi region of the cell.

The route of secretion of procollagen. The influence of alphaalpha'-bipyridyl, colchicine and antimycin A on the secretory process in embryonic-chick tendon and cartilage cells.

I. Embryonic-chick tendon cells were pulse-labelled for 4 min with [14C]proline and the 14C-labelled polypeptides were chased with unlabelled proline for up to 30 min. Isolation of subcellular fractions during the chase period and their subsequent analysis for bacterial collagenase-susceptible 14C-labelled peptides demonstrated the transfer of procollagen polypeptides from rough to smooth microsomal fractions and thence to the extracellular medium. Parallel analyses of Golgi-enriched fractions indicated the involvement of this organelle in the secretory pathway of procollagen. Sodium dodecylsulphate/polyacrylamide-gel electrophoresis of the 14C-labelled polypeptides present in the Golgi-enriched fractions demonstrated that the procollagen polypeptides were all present as disulphide-linked pro-gamma components. 2. When similar kinetic studies of the intracellular transport of procollagen were conducted with embryonic-chick cartilage cells almost identical results were obtained, but the rate of translocation of cartilage procollagen was significantly slower than that observed for tendon procollagen. 3. When hydroxylation of procollagen polypeptides was inhibited by alphaalpha'-bipyridyl, the nascent polypeptides accumulated in the rough microsomal fraction. 4. When cells were pulse-labelled for 4min with [14C)proline and the label was chased in the presence of colchicine, secretion of procollagen was inhibited and an intracellular accumulation of procollagen 14C-labelled polypeptides was observed in the Golgi-enriched fractions. 5. The energy-dependence of the intracellular transport of procollagen was demonstrated in experiments in which antimycin A was found to inhibit the transfer of procollagen polypeptides from rough to smooth endoplasmic reticulum. 6. It is concluded that procollagen follows the classical route of secretion taken by other extracellular proteins.

Ferritin-conjugated antibodies used for labeling of organelles involved in the cellular synthesis and transport of procollagen.

An improved procedure was used to conjugate ferritin to antibodies specific for the NH(2)-terminal extensions on the precursor form of collagen known as procollagen. The ferritin-antibody conjugates were then used to determine the localization of procollagen in fibroblasts isolated from chick-embryo tendons. Procollagen was found in the cisternae of the endoplasmic reticulum, indicating that the protein passes into this compartment early in its biosynthesis. Specific labeling of large Golgi vacuoles was also observed, suggesting that this compartment is also involved in the secretory process. When cells were incubated with colchicine so that the secretion of procollagen was delayed, there was an increase of large, smooth-surfaced vacuoles in the cells and these vacuoles were labeled with the ferritin-antibody conjugate.

Microtubules and Procollagen Transport

Microtubules and Procollagen Transport