Consequences of mutations in the genes of the ER export machinery COPII in vertebrates

Defective anterograde protein-trafficking contributes to endoplasmic reticulum-stress in a CLN1 disease model

Lysosomal storage disorders (LSDs) represent 70 inherited metabolic diseases, in most of which neurodegeneration is a devastating manifestation. The CLN1 disease is a fatal neurodegenerative LSD, caused by inactivating mutations in the CLN1 gene encoding palmitoyl-protein thioesterase-1 (PPT1). S-palmitoylation, a reversable posttranslational modification by saturated fatty acids (generally palmitate) facilitates endosomal trafficking of many proteins, especially in the brain. While palmitoyl-acyltransferases (called ZDHHCs) catalyze S-palmitoylation, depalmitoylation is mediated by palmitoyl-protein thioesterases (PPTs). We previously reported that in Cln1-/- mice, which mimic human CLN1-disease, endoplasmic reticulum (ER)-stress leads to unfolded protein response (UPR) contributing to neurodegeneration. However, the mechanism underlying ER-stress has remained elusive. The anterograde (ER to Golgi) protein-trafficking is mediated via COPII (coat protein complex II) vesicles, whereas the retrograde transport (Golgi to ER) is mediated by COPI vesicles. We hypothesized that dysregulated anterograde protein-trafficking causing stagnation of proteins in the ER leads to ER-stress in Cln1-/- mice. We found that the levels of five COPII vesicle-associated proteins (i.e. Sar1, Sec23, Sec24, Sec13 and Sec31) are significantly higher in the ER-fractions of cortical tissues from Cln1-/- mice compared with those from their WT littermates. Remarkably, all COPII proteins, except Sec13, undergo S-palmitoylation. Moreover, CLN8, a Batten disease-protein, requires dynamic S-palmitoylation (palmitoylation-depalmitoylation) for ER-Golgi trafficking. Intriguingly, Ppt1-deficiency in Cln1-/- mice impairs ER-Golgi trafficking of Cln8-protein along with several other COPII-associated proteins. We propose that impaired anterograde trafficking causes excessive accumulation of proteins in the ER causing ER-stress and UPR contributing to neurodegeneration in CLN1 disease.

SEC24C deficiency causes trafficking and glycosylation abnormalities in an epileptic encephalopathy with cataracts and dyserythropoeisis

As a major component of intracellular trafficking, the coat protein complex II (COPII) is indispensable for cellular function during embryonic development and throughout life. The 4 SEC24 proteins (A-D) are essential COPII components involved in cargo selection and packaging. A human disorder corresponding to alterations of SEC24 function is currently known only for SEC24D. Here, we reported that biallelic loss of SEC24C leads to a syndrome characterized by primary microcephaly, brain anomalies, epilepsy, hearing loss, liver dysfunction, anemia, and cataracts in an extended consanguineous family with 4 affected individuals. We showed that knockout of sec24C in zebrafish recapitulated important aspects of the human phenotype. SEC24C-deficient fibroblasts displayed alterations in the expression of several COPII components as well as impaired anterograde trafficking to the Golgi, indicating a severe impact on COPII function. Transcriptome analysis revealed that SEC24C deficiency also affected the proteasome and autophagy pathways. Moreover, a shift in the N-glycosylation pattern and deregulation of the N-glycosylation pathway suggested a possible secondary alteration of protein glycosylation, linking the described disorder with the congenital disorders of glycosylation.

Compromised COPII vesicle trafficking leads to glycogenic hepatopathy

Being a vital cellular process, coat protein complex II (COPII) vesicle trafficking has been found to play a crucial role in liver metabolism. However, its functions and the underlying mechanisms in systemic metabolic homeostasis have not been fully understood. Here, with a newly identified gene trap zebrafish line (sec31anju221), we show that compromised COPII vesicle trafficking leads to biphasic abnormal hepatic metabolism. During the larval stage, deficiency of COPII-mediated trafficking leads to activation of the unfolded protein response and the development of hepatic steatosis. By using epistasis analysis, we found that the eIF2α-ATF4 pathway serves as the primary effector for liver steatosis. In adult sec31anju221 fish, the hepatosteatosis was reversed and the phenotype switched to glycogenic hepatopathy. Proteomic profiling and biochemical assays indicate that sec31anju221 fish are in a state of hypothyroidism. Moreover, our study shows that thyroid hormone treatment alleviates the metabolic defects. This study provides insights into processes of liver diseases associated with vesicle trafficking impairments and expands our understanding of the pathological interplay between thyroid and liver.

COPII cage assembly factor Sec13 integrates information flow regulating endomembrane function in response to human variation

How information flow is coordinated for managing transit of 1/3 of the genome through endomembrane pathways by the coat complex II (COPII) system in response to human variation remains an enigma. By examining the interactome of the COPII cage-assembly component Sec13, we show that it is simultaneously associated with multiple protein complexes that facilitate different features of a continuous program of chromatin organization, transcription, translation, trafficking, and degradation steps that are differentially sensitive to Sec13 levels. For the trafficking step, and unlike other COPII components, reduction of Sec13 expression decreased the ubiquitination and degradation of wild-type (WT) and F508del variant cargo protein cystic fibrosis transmembrane conductance regulator (CFTR) leading to a striking increase in fold stability suggesting that the events differentiating export from degradation are critically dependent on COPII cage assembly at the ER Golgi intermediate compartment (ERGIC) associated recycling and degradation step linked to COPI exchange. Given Sec13's multiple roles in protein complex assemblies that change in response to its expression, we suggest that Sec13 serves as an unanticipated master regulator coordinating information flow from the genome to the proteome to facilitate spatial covariant features initiating and maintaining design and function of membrane architecture in response to human variation.

RNA interference-mediated silencing of coat protein II (COPII) genes affects the gut homeostasis and cuticle development in Locusta migratoria

The coat protein II (COPII) complex consists of five primary soluble proteins, namely the small GTP-binding protein Sar1, the inner coat Sec23/Sec24 heterodimers, and the outer coat Sec13/Sec31 heterotetramers. COPII is essential for cellular protein and lipid trafficking through cargo sorting and vesicle formation at the endoplasmic reticulum. However, the roles of COPII assembly genes remain unknown in insects. In present study, we identified five COPII assembly genes (LmSar1, LmSec23, LmSec24, LmSec13 and LmSec31) in Locusta migratoria. RT-qPCR results revealed that these genes showed different expression patterns in multiple tissues and developmental days of fifth-instar nymphs. Injection of double-stranded RNA against each LmCOPII gene induced a high RNAi efficiency, and considerably suppressed feeding, and increased mortality to 100 %. Results from the micro-sectioning and hematoxylin-eosin staining of midguts showed that the brush border was severely damaged and the number of columnar cells was significantly reduced in dsLmCOPII-injected nymphs, as compared with the control. The dilated endoplasmic reticulum phenotype of columnar cells was observed by transmission electron microscopy. RT-qPCR results further indicated that silencing any of the five genes responsible for COPII complex assembly repressed the expression of genes involved in insulin/mTOR-associated nutritional pathway. Therefore, COPII assembly genes could be promising RNAi targets for insect pest management by disrupting gut and cuticle development.

COP I and II dependent trafficking controls ER-associated degradation in mammalian cells

Misfolded proteins and components of the endoplasmic reticulum (ER) quality control and ER associated degradation (ERAD) machineries concentrate in mammalian cells in the pericentriolar ER-derived quality control compartment (ERQC), suggesting it as a staging ground for ERAD. By tracking the chaperone calreticulin and an ERAD substrate, we have now determined that the trafficking to the ERQC is reversible and recycling back to the ER is slower than the movement in the ER periphery. The dynamics suggest vesicular trafficking rather than diffusion. Indeed, using dominant negative mutants of ARF1 and Sar1 or the drugs Brefeldin A and H89, we observed that COPI inhibition causes accumulation in the ERQC and increases ERAD, whereas COPII inhibition has the opposite effect. Our results suggest that targeting of misfolded proteins to ERAD involves COPII-dependent transport to the ERQC and that they can be retrieved to the peripheral ER in a COPI-dependent manner.

KLHL12 can form large COPII structures in the absence of CUL3 neddylation

CUL3-RING ubiquitin ligases (CRL3s) are involved in various cellular processes through different Bric-a-brac, Tramtrack, and Broad-complex (BTB)-domain proteins. KLHL12, a BTB-domain protein, is suggested to play an essential role in the export of large cargo molecules such as procollagen from the endoplasmic reticulum (ER). CRL3^KLHL12 monoubiquitylates SEC31, leading to an increase in COPII vesicle dimension. Enlarged COPII vesicles can accommodate procollagen molecules. Thus, CRL3^KLHL12 is essential for the assembly of large COPII structures and collagen secretion. CRL3s are activated by CUL3 neddylation. Here, we evaluated the importance of CUL3 neddylation in COPII assembly and collagen secretion. Unexpectedly, the assembly of large COPII-KLHL12 structures persisted and cellular collagen levels decreased on treatment with MLN4924, a potent inhibitor of NEDD8-activating enzyme. When we introduced mutations into KLHL12 at the CUL3 interface, these KLHL12 variants did not interact with neddylated CUL3, but one of them (Mut A) still supported large COPII-KLHL12 structures. Overexpression of wild-type KLHL12, but not Mut A, lowered cellular collagen levels most likely via lysosomal degradation. Our results suggest that CUL3 neddylation is not necessary for the formation of large COPII-KLHL12 structures, but active CRL3^KLHL12 contributes to the maintenance of collagen levels in the cell.

Targeting coat protein II complex genes via RNA interference inhibits female adult feeding and reproductive development in the cabbage beetle Colaphellus bowringi

BACKGROUND

The cabbage beetle Colaphellus bowringi is a highly destructive cruciferous vegetable pest in Asia. This beetle is predominantly controlled by synthetic chemical pesticides, which leave pesticide residues on food and constitute a major hidden danger to human health. Based on preliminary research, we hypothesized that the coat protein II (COPII) complex, a primary coated vesicle that exports cargo molecules from the endoplasmic reticulum, is a promising novel target for the control of Colaphellus bowringi.

RESULTS

This study investigated whether disrupting COPII using RNA interference (RNAi) affects the growth and development of Colaphellus bowringi adults. The results showed that five COPII assembly genes, Sar1, Sec23, Sec24, Sec13, and Sec31, were uniformly expressed in multiple tissues of adult female Colaphellus bowringi. Injecting double-stranded RNA (dsRNA) against each gene induced a high RNAi efficiency by approximately 55-99%, and considerably inhibited yolk deposition and ovarian growth. Moreover, knockdown of Sar1, Sec23 and Sec24 suppressed feeding and increased mortality to 26.67%, 46.67%, and 42.22%, respectively. This was partially due to the down-regulation of insulin/mTOR-associated nutritional pathways. The results indicate that silencing any of the five genes responsible for COPII complex assembly represses Juvenile hormone and ecdysone signaling pathways, suggesting that vesicle transport plays a vital role in the endocrine regulation of Colaphellus bowringi females.

CONCLUSION

This study suggests that the COPII complex could be a promising RNAi target for the management of Colaphellus bowringi, which would reduce our dependence on chemical pesticides for pest control. © 2022 Society of Chemical Industry.

Collagen has a unique SEC24 preference for efficient export from the endoplasmic reticulum

SEC24 is mainly involved in cargo sorting during COPII vesicle assembly. There are four SEC24 paralogs (A-D) in vertebrates, which are classified into two subgroups (SEC24A/B and SEC24C/D). Pathological mutations in SEC24D cause osteogenesis imperfecta with craniofacial dysplasia in humans. sec24d mutant fish also recapitulate the phenotypes. Consistent with the skeletal phenotypes, the secretion of collagen was severely defective in mutant fish, emphasizing the importance of SEC24D in collagen secretion. However, SEC24D patient-derived fibroblasts show only a mild secretion phenotype, suggesting tissue-specificity in the secretion process. Using Sec24d KO mice and cultured cells, we show that SEC24A and SEC24B also contribute to endoplasmic reticulum (ER) export of procollagen. In contrast, fibronectin 1 requires either SEC24C or SEC24D for ER export. On the basis of our results, we propose that procollagen interacts with multiple SEC24 paralogs for efficient export from the ER, and that this is the basis for tissue-specific phenotypes resulting from SEC24 paralog deficiency.