LncRNA NR_030777/DRP1 negatively regulates intercellular mitochondrial transfer in cobalt nanoparticles-induced neuronal damage

Technical Summary: LncRNA NR_030777/DRP1 Negatively Regulates Intercellular Mitochondrial Transfer in Cobalt Nanoparticles-Induced Neuronal Damage

1. Problem Statement

Cobalt nanoparticles (CoNPs) are widely utilized in medical implants (e.g., metal-on-metal joint prostheses) and the renewable energy sector (e.g., lithium-ion batteries). Despite their utility, CoNPs are established neurotoxicants capable of crossing biological barriers and inducing neuronal damage, apoptosis, and oxidative stress. While previous research identified that intercellular mitochondrial transfer from astrocytes (or glial cells) to neurons via tunneling nanotubes (TNTs) serves as a protective mechanism against CoNPs-induced toxicity, the molecular regulators governing this process remain poorly understood. Specifically, the role of long non-coding RNAs (lncRNAs) and mitochondrial dynamics proteins, such as Dynamin-related protein 1 (DRP1), in modulating TNT-mediated mitochondrial transfer during CoNPs exposure was unknown.

2. Methodology

The study employed a neuron-glia co-culture model using mouse hippocampal neuronal cells (HT22) and mouse glioma cells (Glioma261). Glioma261 cells were engineered to stably express mitochondria-targeted enhanced green fluorescent protein (mito-EGFP) to visualize mitochondrial transfer.

• Experimental Treatments: Cells were exposed to varying concentrations of CoNPs (0–30 µg/mL) for 24 hours.
• Inhibition/Activation Strategies:
  • TNT Inhibition: Cells were pretreated with Latrunculin B (LAT-B) or nocodazole (actin/microtubule inhibitors) or GW4869 (extracellular vesicle inhibitor) to distinguish TNT-mediated transfer from vesicle-mediated transfer.
  • Genetic Manipulation: Glioma261 cells were transfected with plasmids to overexpress lncRNA NR_030777 or DRP1, or treated with siRNA to knockdown NR_030777.
  • Pharmacological Modulation: The DRP1 inhibitor Mdivi-1 was used to suppress DRP1 activity.
• Rescue Experiments: A combination of NR_030777 overexpression and Mdivi-1 treatment was performed to determine if DRP1 inhibition could reverse the effects of NR_030777.
• Assessment Techniques:
  • Mitochondrial Transfer & TNTs: Quantified via confocal microscopy (counting transferred mitochondria and TNT structures).
  • Cellular Health: Apoptosis (Annexin V/7-AAD), oxidative stress (intracellular O₂⁻ via DHE), mitochondrial ROS (MitoSOX), and mitochondrial membrane potential (MMP via TMRE).
  • Senescence: Measured via SA-β-gal activity.
  • Molecular Analysis: Western blotting for DRP1 and phosphorylated DRP1 (Ser616); qRT-PCR for NR_030777 expression.

3. Key Results

• TNTs as the Primary Transfer Route: CoNPs treatment induced mitochondrial transfer from Glioma261 to HT22 cells. This transfer was significantly suppressed by LAT-B and nocodazole (TNT inhibitors) but only weakly affected by GW4869 (vesicle inhibitor), confirming TNTs as the dominant pathway. Blocking TNTs exacerbated CoNPs-induced apoptosis, superoxide production, mitochondrial ROS, and loss of membrane potential.
• DRP1 as a Negative Regulator: CoNPs treatment reduced total DRP1 protein levels while increasing phosphorylated DRP1 (Ser616). Functional assays revealed that DRP1 acts as a negative regulator of TNT-mediated mitochondrial transfer:
  • Inhibition of DRP1 (via Mdivi-1) increased mitochondrial transfer and TNT formation.
  • Overexpression of DRP1 suppressed mitochondrial transfer and reduced TNT numbers between heterotypic cells.
• NR_030777 Upregulates DRP1 and Suppresses Transfer: CoNPs exposure downregulated NR_030777 expression.
  • Overexpression: Artificially increasing NR_030777 levels upregulated DRP1 protein, suppressed mitochondrial transfer via TNTs, and worsened neuronal damage (increased apoptosis, ROS, and senescence).
  • Knockdown: Reducing NR_030777 levels decreased DRP1, enhanced mitochondrial transfer, and mitigated CoNPs-induced toxicity.
• Mechanistic Validation (Rescue): In cells overexpressing NR_030777, the subsequent inhibition of DRP1 using Mdivi-1 reversed the suppression of mitochondrial transfer. This restored mitochondrial movement, reduced apoptosis, and improved mitochondrial function, confirming that NR_030777 exerts its detrimental effects specifically through the upregulation of DRP1.

4. Key Contributions

• Identification of a Novel Epigenetic Mechanism: The study establishes lncRNA NR_030777 as a critical epigenetic regulator of intercellular mitochondrial communication in the context of environmental neurotoxicity.
• Clarification of DRP1's Role in TNTs: While DRP1 is traditionally known for regulating mitochondrial fission, this paper demonstrates its specific role as a negative regulator of TNT-mediated mitochondrial transfer between glial cells and neurons.
• Elucidation of the CoNPs Toxicity Pathway: The research defines a specific pathway where CoNPs exposure leads to the dysregulation of the NR_030777/DRP1 axis, resulting in the inhibition of protective mitochondrial transfer and subsequent neuronal damage.

5. Significance and Claims

The authors claim this study provides the first evidence linking lncRNA-mediated epigenetic regulation to intercellular mitochondrial transfer in the context of environmental toxicant-induced neurotoxicity.

• Mechanistic Insight: The findings uncover a novel mechanism where the lncRNA NR_030777 modulates DRP1 expression to inhibit TNT-mediated mitochondrial transfer, thereby exacerbating CoNPs-induced neurotoxicity.
• Toxicological Implications: The study positions CoNPs as an environmental epigenetic toxicant capable of disrupting protective cellular communication processes.
• Therapeutic Potential: By identifying the NR_030777/DRP1 axis as a regulator of mitochondrial transfer, the authors suggest these components as potential targets for mechanistic research and therapeutic strategies aimed at preventing or controlling neural damage caused by environmental hazards.

The authors note that while the study provides robust in vitro evidence, the downstream mechanisms connecting DRP1 to TNT formation require further exploration using in vivo models.