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Background This study aimed to evaluate the association of circular RNA La‐related RNA‐binding protein 4 (circ‐LARP4) with clinical features and prognosis in osteosarcoma patients, and further explore its effect on chemosensitivity in osteosarcoma cells. Methods Seventy‐two osteosarcoma patients with Enneking stage IIA‐IIB who underwent resection were consecutively enrolled, and then, tumor tissues and non‐tumor tissues were obtained. Circ‐LARP4 in tumor tissue/non‐tumor tissue was detected by quantitative polymerase chain reaction. After circ‐LARP4 overexpression and negative control overexpression plasmid transfection, relative cell viability (%) was evaluated by Cell Counting Kit‐8 in MG63 cells treated by different concentrations of cisplatin, methotrexate, and doxorubicin, and IC50 was calculated. Results Circ‐LARP4 was downregulated in tumor tissue compared with non‐tumor tissue and had a good value in distinguishing tumor tissue from non‐tumor tissue with an area under curve of 0.829 (95% CI: 0.762‐0.859). Meanwhile, tumor circ‐LARP4 was negatively correlated with the Enneking stage. After resection, circ‐LARP4 high expression patients showed an increased tumor cell necrosis rate to adjuvant chemotherapy compared to circ‐LARP4 low expression patients, and circ‐LARP4 high expression correlated with prolonged disease‐free survival and overall survival. In vitro experiments revealed that circ‐LARP4 overexpression elevated the chemosensitivity of MG63 cells to cisplatin and doxorubicin but not methotrexate, with decreased cisplatin IC50 and doxorubicin IC50 concentrations than negative control. Besides, miR‐424 overexpression attenuated the chemosensitivity in circ‐LARP4 overexpression‐treated MG63 cells. Conclusion Circ‐LARP4 high expression correlates with decreased Enneking stage and prolonged survival profiles, and it elevates chemosensitivity to cisplatin and doxorubicin via sponging miR‐424 in osteosarcoma.

Autophagosome-lysosome fusion is a common critical step in various forms of macroautophagy/autophagy including mitophagy, the selective degradation of mitochondria. Regulations of this fusion process remain poorly defined. Here we have determined the role of SIGMAR1, a unique endoplasmic reticulum membrane protein. Knockout of Sigmar1 impaired mitochondrial clearance without altering the PINK1-PRKN/Parkin signaling, in mouse retinal explants and cultured cells treated with carbonyl cyanide m-chlorophenyl hydrazone (CCCP) for induction of mitophagy. SIGMAR1 depletion also caused accumulation of autophagosome markers LC3-II and SQSTM1, but did not change the levels of BECN1 and ATG7, proteins associated with autophagosome biogenesis. Lysosomal pH and protease activities were not negatively affected. However, sigmar1 knockout partially compromised autophagosome-lysosome fusion in CCCP-treated NSC34 cells, as revealed by reduced GFP fluorescence quenching of GFP-RFP-LC3-II puncta and co-localization of lysosomes with mitochondria. Furthermore, SIGMAR1 co-immunoprecipitated with ATG14, STX17, and VAMP8 (but not SNAP29), proteins key to autophagosome-lysosome membrane fusion. Re-expressing SIGMAR1 in the null background rescued clearance of mitochondria and autophagosomes. In summary, we started out finding that sigmar1 knockout impaired the clearance of mitochondria and autophagosomes, and then narrowed down the SIGMAR1 modulation to the autophagosome-lysosome fusion step. This study may shed new light on understanding autophagy-associated cyto-protection and disease mechanisms. Abbreviations : APEX2, a genetically engineered peroxidase; BiFC, bimolecule fluorescence complementation; CCCP, a mitophagy inducing compound; CRISPR, clustered regularly interspaced short palindromic repeats; EM, electron microscopy; ER, endoplasmic reticulum; MAP1LC3/LC3, microtubule-associated protein 1 light chain 3; SIGMAR1, sigma non-opioid intracellular receptor 1.

The biogenesis of photosystem I (PSI), cytochrome b6f (Cytb6f) and NADH dehydrogenase (NDH) complexes relies on the spatially and temporally coordinated expression and translation of both nuclear and chloroplast genes. Here we report the identification of photosystem biogenesis regulator 1 (PBR1), a nuclear-encoded chloroplast RNA-binding protein that regulates the concerted biogenesis of NDH, PSI and Cytb6f complexes. We identified Ycf1, one of the two largest chloroplast genome-encoded open reading frames as the direct downstream target protein of PBR1. Biochemical and molecular analyses reveal that PBR1 regulates Ycf1 translation by directly binding to its mRNA. Surprisingly, we further demonstrate that relocation of the chloroplast gene Ycf1 fused with a plastid-transit sequence to the nucleus bypasses the requirement of PBR1 for Ycf1 translation, which sufficiently complements the defects in biogenesis of NDH, PSI and Cytb6f complexes in PBR1-deficient plants. Remarkably, the nuclear-encoded PBR1 tightly controls the expression of the chloroplast gene Ycf1 at the translational level, which is sufficient to sustain the coordinated biogenesis of NDH, PSI and Cytb6f complexes as a whole. Our findings provide deep insights into better understanding of how a predominant nuclear-encoded factor can act as a migratory mediator and undergoes selective translational regulation of the target plastid gene in controlling biogenesis of photosynthetic complexes.