The NLRX1-SLC39A7 complex orchestrates mitochondrial dynamics and mitophagy to rejuvenate intervertebral disc by modulating mitochondrial Zn2+ trafficking.

ABSTRACT

Intervertebral disc degeneration (IDD) is the most critical pathological factor in the development of low back pain. The maintenance of nucleus pulposus (NP) cell and intervertebral disc integrity benefits largely from well-controlled mitochondrial quality, surveilled by mitochondrial dynamics (fission and fusion) and mitophagy, but the outcome is cellular context-dependent that remain to be clarified. Our studies revealed that the loss of NLRX1 is correlated with NP cell senescence and IDD progression, which involve disordered mitochondrial quality. Further using animal and in vitro tissue and cell models, we demonstrated that NLRX1 could facilitate mitochondrial quality by coupling mitochondrial dynamic factors (p-DNM1L, L-OPA1S-OPA1, OMA1) and mitophagy activity. Conversely, mitochondrial collapse occurred in NLRX1-defective NP cells and switched on the compensatory PINK1-PRKN pathway that led to excessive mitophagy and aggressive NP cell senescence. Mechanistically, NLRX1 was originally shown to interact with zinc transporter SLC39A7 and modulate mitochondrial Zn2+ trafficking via the formation of an NLRX1-SLC39A7 complex on the mitochondrial membrane of NP cells, subsequently orchestrating mitochondrial dynamics and mitophagy. The restoration of NLRX1 function by gene overexpression or pharmacological agonist (NX-13) treatment showed great potential for regulating mitochondrial fission with synchronous fusion and mitophagy, thus sustaining mitochondrial homeostasis, ameliorating NP cell senescence and rejuvenating intervertebral discs. Collectively, our findings highlight a working model whereby the NLRX1-SLC39A7 complex coupled mitochondrial dynamics and mitophagy activity to surveil and target damaged mitochondria for degradation, which determines the beneficial function of the mitochondrial surveillance system and ultimately rejuvenates intervertebral discs.Abbreviations 3-MA 3-methyladenine; Baf-A1 bafilomycin A1; CDKN1A/p21 cyclin dependent kinase inhibitor 1A; CDKN2A/p16 cyclin dependent kinase inhibitor 2A; DNM1L/DRP1 dynamin 1 like; EdU 5-Ethynyl-2'-deoxyuridine; HE hematoxylin-eosin; IDD intervertebral disc degeneration; IL1B/IL-1ß interleukin 1 beta; IL6 interleukin 6; MAP1LC3B/LC3B microtubule associated protein 1 light chain 3 beta; MKI67/Ki67 marker of proliferation Ki-67; LBP low back pain; MMP mitochondrial membrane potential; MFN1 mitofusin 1; MFN2 mitofusin 2; MFF mitochondrial fission factor; NP nucleus pulposus; NLRX1 NLR family member X1; OMA1 OMA1 zinc metallopeptidase; OPA1 OPA1 mitochondrial dynamin like GTPase; PINK1 PTEN induced kinase 1; PRKN parkin RBR E3 ubiquitin protein ligase; ROS reactive oxidative species; SASP senescence-associated secretory phenotype; SA-GLB1/ß-gal senescence-associated galactosidase beta 1; SO safranin o; TBHP tert-butyl hydroperoxide; TP53/p53 tumor protein p53; SLC39A7/ZIP7 solute carrier family 39 member 7; TOMM20 translocase of outer mitochondrial membrane 20; TIMM23 translocase of inner mitochondrial membrane 23.