Intervertebral disc degeneration (IDD) is largely attributed to impaired endogenous repair. Nucleus pulposus-derived stem cells (NPSCs) senescence leads to endogenous repair failure. Small extracellular vesicles/exosomes derived from mesenchymal stem cells (mExo) have shown great therapeutic potential in IDD, while whether mExo could alleviate NPSCs senescence and its mechanisms remained unknown. We established a compression-induced NPSCs senescence model and rat IDD models to evaluate the therapeutic efficiency of mExo and investigate the mechanisms. We found that mExo significantly alleviated NPSCs senescence and promoted disc regeneration while knocking down thioredoxin (TXN) impaired the protective effects of mExo. TXN was bound to various endosomal sorting complex required for transport (ESCRT) proteins. Autocrine motility factor receptor (AMFR) mediated TXN K63 ubiquitination to promote the binding of TXN on ESCRT proteins and sorting of TXN into mExo. Knocking down exosomal TXN inhibited the transcriptional activity of nuclear factor erythroid 2-related factor 2 (NRF2) and activator protein 1 (AP-1). NRF2 and AP-1 inhibition reduced endogenous TXN production that was promoted by exosomal TXN. Inhibition of NRF2 in vivo diminished the anti-senescence and regenerative effects of mExo. Conclusively, AMFR-mediated TXN ubiquitination promoted the sorting of TXN into mExo, allowing exosomal TXN to promote endogenous TXN production in NPSCs via TXN/NRF2/AP-1 feed-forward circuit to alleviate NPSCs senescence and disc degeneration.

Objective:To investigate the role and molecular mechanism of sensory neuron-derived exosomes (nExo) in mediating intervertebral disc degeneration (IDD).Methods:A rat IDD model was constructed, with nExo injected into the intervertebral disc. After 4 weeks, the degenerative grades of operated discs were evaluated using histological staining, while the senescent phenotype of nucleus pulposus stem cells (NPSC) in the tissue was evaluated using immunofluorescence staining. For in vitro experiments, 24 hours after the treatment of nExo to NPSC, immunoblotting, flow cytometry, or senescence-associated β-galactosidase staining was applied to evaluate the senescent phenotype of NPSC. Transcriptomics analysis was applied to identify the key molecules that mediate nExo-induced cells senescence. After 4 weeks of injecting nExo and TXN into the rat tail disc degeneration model.Results:nExo increased the degenerative grades of IDD and increased the proportion of TEK +p16 + and TEK +p21 + cells (from 36.32% ±4.04%, 33.69% ±4.56% in IDD group to 56.41% ±5.26%, 50.14% ±8.49% in IDD+nExo group, respectively; t=7.420, P<0.001; t=4.184, P<0.0019, respectively) in the disc tissue. Besides, nExo promoted the expression of p16 and p21 in NPSC and increased the percentage of cells with positive senescence-associated β-galactosidase staining (from 7.32%±1.73% to 58.22%±11.38%, t=7.658, P=0.002), while the percentage of G2/M cells was downregulated (from 18.10%±1.32% to 1.60%±0.67%, t=19.290, P<0.001). Transcriptomic analysis showed that the differential genes of CTRL vs. nExo were closely related to cell senescence, and TXN was screened by intersecting the differential gene set with the cellular senescence gene sets from the published database. Furthermore, we verified that nExo decreased the content of TXN in NPSC, while exogenous TXN downregulated the expression of p16 and p21 in NPSC, reduced the positive cell rate of senescence-associated β-galactosidase staining (from 58.84%±3.99% to 21.68%±8.16%, t=7.048, P=0.021), increased the percentage of G2/M cells (from 1.21%±0.34% to 15.26%±2.60%, t=9.259, P=0.001). TXN significantly reduced the grade of disc tissue degeneration (histological score: 14.33±0.82 in the nExo group; 8.17±1.17 in the nExo+TXN group, t=10.590, P<0.001), significantly increased the content of extracellular matrix (from 10.94±4.35 μg/mg to 50.55±12.16 μg/mg, t=7.512, P<0.001), and reduced the proportion of TEK +p16 + and TEK +p21 + double-positive cells (from 54.92%±4.21% and 60.31%±9.02% to 27.93%±3.26% and 33.75%±8.07%, respectively; t=12.430, P<0.001; t=5.375, P<0.001, respectively). Conclusion:nExo promotes cell senescence and IDD by downregulating TXN in NPSC.

Squamosa promoter binding protein-like (SPL) family is a group of important transcription factors involved in the regulation of plant growth and development and the response to environmental stress, but there are few studies in perennial fruit trees such as citrus. In this study, Ziyang Xiangcheng (Citrus junos Sib.ex Tanaka), an important rootstock of Citrus, was used as the material for analysis. Based on plantTFDB transcription factor database and sweet orange genome database, 15 SPL family members were genome-widely identified and cloned from Ziyang Xiangcheng, and named CjSPL1-CjSPL15. Sequence analysis showed that the open reading frame (ORF) length of CjSPLs ranged from 393 bp to 2 865 bp, encoding 130-954 amino acids. Phylogenetic tree divided 15 CjSPLs into 9 subfamilies. Gene structure and conserved domain analysis predicted 20 different conserved motifs and SBP basic domains. Analysis of cis-acting promoter elements predicted 20 different promoter elements, including those related to plant growth and development, abiotic stress and secondary metabolites. The expression patterns of CjSPLs under drought, salt and low temperature stresses were analyzed by real-time fluorescence quantitative PCR (qRT-PCR), and many CjSPLs were significantly up-regulated after stress treatment. This study provides a reference for further study on the function of SPL family transcription factors in citrus and other fruit trees.
Phylogeny ; Transcription Factors/metabolism* ; Gene Expression Regulation, Plant ; Plant Proteins/metabolism* ; Multigene Family ; Stress, Physiological