A systemic circadian nicotinic acid riboside (NaR) signal engages the unfolded protein response and adipogenesis via the prefoldin complex

This study identifies nicotinic acid riboside (NaR) as a liver clock-controlled circulating metabolite that engages the prefoldin complex to modulate unfolded protein response signaling and adipogenesis in a time-dependent manner, thereby linking systemic circadian rhythms to tissue-specific physiological outcomes.

The Gist

The Big Picture: The Body's Master Clock and a "Time-Traveling" Molecule

Imagine your body is a massive, bustling city. Every neighborhood (your liver, your fat tissue, your brain) has its own local clock that tells it when to wake up, eat, and sleep. But for the city to function smoothly, all these neighborhoods need to talk to each other.

This paper discovers a new "messenger" that travels through the bloodstream to help these neighborhoods coordinate. This messenger is a molecule called Nicotinic Acid Riboside (NaR).

The researchers found that NaR isn't just a random chemical floating around; it acts like a metronome. Its levels go up and down in a perfect 24-hour rhythm, driven by the liver's internal clock. When the liver's clock breaks, this rhythm stops, and the messenger gets stuck at a constant high level, which causes confusion in other parts of the body.

The Story of the Fat Cell (Adipocyte)

To understand what this messenger does, the scientists looked at fat cells. Think of a fat cell as a warehouse that stores energy (lipids).

1. The Delivery: The fat cells can grab this NaR messenger from the blood.
2. The Alarm System: When the fat cell gets NaR, it doesn't just use it for fuel. Instead, it triggers a specific "alarm system" inside the cell called the Unfolded Protein Response (UPR).
   • Analogy: Imagine the UPR is a quality control team in a factory. If the machines (proteins) start making mistakes or getting tangled, the UPR team rushes in to fix them.
3. The Target: The NaR messenger specifically targets a machine in the factory called the Prefoldin Complex.
   • Analogy: The Prefoldin Complex is like a folding robot that helps new proteins get the right shape. The researchers found that NaR actually makes this robot wobble or shake a bit (destabilize it).
   • Why is this good? Paradoxically, making the robot wobble seems to make the factory more alert. It signals the quality control team (UPR) to get ready, which helps the cell prepare to store energy efficiently.

The Twist: Timing is Everything

This is the most fascinating part of the discovery. The fat cells react differently depending on how they receive the NaR message.

• Scenario A: The Constant Shout (Chronic Exposure)
  Imagine someone standing next to the factory door shouting "NaR! NaR! NaR!" 24 hours a day without stopping.

  • Result: The factory gets overwhelmed. The quality control team gets confused, the folding robot breaks down, and the warehouse stops storing energy. The fat cell refuses to fill up.

• Scenario B: The Rhythmic Knock (Pulsed Exposure)
  Now, imagine the messenger knocks on the door for 8 hours, then leaves for 16 hours, then knocks again. This mimics the natural day/night cycle.

  • Result: The factory loves this rhythm. The quality control team stays sharp, the folding robot works efficiently, and the warehouse fills up with energy (lipid accumulation).

Why Does This Matter?

1. The Liver is the Conductor: The liver acts like the conductor of an orchestra. It controls the rhythm of the NaR messenger. If the liver's clock is broken (like in shift workers or people with jet lag), the NaR rhythm breaks. The fat cells get confused, stop storing energy properly, and this could lead to metabolic diseases.

2. It's Not Just About "How Much": For a long time, scientists thought it only mattered how much of a chemical was in your blood. This paper shows that timing matters just as much. A chemical can be good for you if it arrives in a rhythm, but bad for you if it arrives constantly.

3. Aging and Health: As we get older, our natural rhythms get weaker, and levels of NaR drop. The authors suggest that losing this rhythmic "knock" might be why older people have trouble managing their weight and metabolism. Restoring this rhythm could be a new way to treat metabolic issues.

Summary in One Sentence

The liver sends a rhythmic chemical signal (NaR) to fat cells that acts like a metronome; when the fat cells hear this signal in a steady beat, they efficiently store energy, but if the signal becomes a constant, unending noise, the cells shut down their storage capabilities.

Technical Summary

1. Problem Statement

While the molecular architecture of the circadian clock (the BMAL1/CLOCK feedback loop) is well-defined in individual tissues, the mechanisms by which circadian timing information is communicated between organs to coordinate systemic metabolism remain incompletely understood. Specifically:

• It is unclear which circulating metabolites act as systemic signals conveying circadian information from peripheral clocks (like the liver) to distant tissues.
• The functional role of specific NAD⁺ pathway intermediates in inter-organ crosstalk is poorly defined.
• How temporal patterns of metabolic signals (oscillatory vs. sustained) influence cellular differentiation and proteostasis is not fully characterized.

2. Methodology

The study employed a multi-omics approach combining in vivo mouse models, cell culture, transcriptomics, proteomics, and structural biology:

• In Vivo Models:
  • Metabolomics: Re-analysis of serum and liver metabolomics data from Wild-Type (WT), whole-body Bmal1 knockout (KO), and hepatocyte-specific Bmal1 reconstituted (LRE) mice under time-restricted feeding.
  • Genetic Models: Utilization of LRE mice to determine if the liver clock alone is sufficient to restore rhythmicity in circulating metabolites.
• Cell Culture Models:
  • Differentiating 3T3-L1 adipocytes, C2C12 myotubes, and HT-22 neuronal cells.
  • Treatment with Nicotinic Acid Riboside (NaR) at varying concentrations (1–100 µM) and durations (1h, 6h, 8 days).
  • Temporal Protocols: Comparison of continuous (sustained) NaR exposure vs. pulsatile (8-hour daily pulse) exposure.
• Omics & Profiling:
  • Transcriptomics: RNA-seq (Smart-seq3) to identify differentially expressed genes (DEGs) and pathway enrichment (UPR, adipogenesis).
  • Proteome Integral Solubility Alteration (PISA): A thermal proteome profiling technique to identify direct protein targets of NaR by measuring changes in protein thermal stability.
  • Proteomics: LC-MS/MS with TMT labeling for quantitative analysis of protein stability.
• Molecular & Functional Assays:
  • siRNA Knockdown: Targeting Pfdn6 (a prefoldin subunit) to assess its role in UPR and adipogenesis.
  • Pharmacological Inhibition: Use of specific inhibitors for UPR branches (PERK, IRE1, ATF6) to dissect signaling pathways.
  • Structural Biology: AlphaFold-based structure prediction, pocket prediction (PrankWeb), and molecular docking (SwissDock) to model NaR binding to prefoldin subunits.
  • Metabolite Quantification: Targeted metabolomics to track intracellular NaR and NAD⁺ kinetics.

3. Key Contributions & Results

A. Identification of NaR as a Liver-Controlled Circadian Signal

• Clock Dependence: Re-analysis of serum metabolomics revealed that Nicotinic Acid Riboside (NaR) and Quinolinic Acid are the only metabolites in the tryptophan-NAD⁺ pathway that exhibit strict clock-dependent rhythmicity (rhythmic in WT, arrhythmic in Bmal1 KO).
• Liver Sufficiency: In hepatocyte-specific Bmal1 reconstituted (LRE) mice, NaR rhythmicity in the serum was restored, demonstrating that the liver clock is sufficient to generate systemic NaR oscillations.
• Kinetics: Adipocytes rapidly uptake exogenous NaR (peak intracellular levels at 1h), which subsequently drives a gradual increase in NAD⁺ levels (4–6h).

B. NaR Engages the Unfolded Protein Response (UPR) via the Prefoldin Complex

• Transcriptional Response: NaR treatment in differentiating adipocytes selectively induced UPR-associated genes (Manf, Pdia6, Xbp1) without broadly activating general stress pathways. This response was independent of downstream NAD⁺ metabolites (NMN treatment did not mimic the effect).
• Direct Target Identification (PISA): Proteome-wide stability profiling revealed that NaR treatment destabilizes the Prefoldin complex (specifically subunits Pfdn1, Pfdn5, Pfdn6).
  • Docking analysis predicted NaR binding pockets in all prefoldin subunits, with the strongest interaction predicted for Pfdn6.
  • Lysate-based PISA confirmed NaR preferentially binds Pfdn1 and Pfdn6, suggesting the destabilization of the assembled complex is a secondary consequence of direct binding.
• Mechanism: Knockdown of Pfdn6 abolished the NaR-induced expression of UPR genes, confirming the prefoldin complex is required for this signaling axis.

C. Regulation of Adipogenesis and CEBPA

• Convergence on CEBPA: NaR signaling converges on the master adipogenic transcription factor CEBPA.
  • NaR induces Cebpa expression.
  • This induction requires both the Prefoldin complex (Pfdn6) and the ATF6 branch of the UPR.
  • Interestingly, while ATF6 inhibition blocked UPR gene induction, it did not fully block Cebpa induction unless IRE1 was also inhibited, suggesting a synergistic requirement for ATF6 and IRE1 in regulating CEBPA levels.
• Time-Dependent Effects (The "Temporal Paradox"):
  • Pulsatile NaR (Mimicking Circadian Rhythm): Daily 8-hour pulses significantly increased CEBPA protein levels and enhanced lipid accumulation (adipogenesis).
  • Sustained NaR (Constant Exposure): Continuous treatment suppressed lipid deposition and failed to increase CEBPA protein levels.
  • This indicates that adipocytes interpret the temporal structure of the signal, not just the metabolite concentration.

4. Significance

• New Signaling Paradigm: The study identifies NaR not merely as a metabolic precursor for NAD⁺, but as a time-encoded systemic signal that conveys circadian information from the liver to adipose tissue.
• Proteostasis-Adipogenesis Link: It establishes a novel mechanistic link between the Prefoldin chaperone complex, the UPR, and adipocyte differentiation, suggesting that controlled proteostatic stress is a prerequisite for healthy fat cell development.
• Temporal Biology: The findings highlight that the pattern of metabolic exposure (oscillatory vs. constant) dictates physiological outcomes. This challenges static experimental designs and suggests that therapeutic interventions targeting metabolic pathways (e.g., NAD⁺ boosters) must consider circadian timing to avoid maladaptive effects (e.g., suppressing lipid storage vs. promoting it).
• Aging Implications: Given that circulating NaR levels decline with age, the loss of NaR rhythmicity may contribute to age-related metabolic inflexibility and adipose dysfunction.

5. Limitations

• Binding Affinity: While PISA and docking suggest interaction, direct biophysical measurements of NaR-prefoldin binding affinity were not performed.
• In Vivo Causality: The study did not directly manipulate systemic NaR rhythmicity in vivo to prove causality in adipose physiology (e.g., via temporal NaR administration in mice).
• Tissue Specificity: The determinants of why NaR induces UPR in neurons/adipocytes but not muscle cells require further investigation.

Conclusion

This paper elucidates a mechanism where the liver clock controls the rhythmic secretion of NaR, which acts as a systemic signal to modulate adipocyte proteostasis via the prefoldin complex and UPR, ultimately regulating lipid storage in a time-dependent manner. This reveals a critical layer of circadian regulation where the temporal dynamics of a metabolite are as physiologically significant as its abundance.