Can the Liver Regrow After Losing Two-Thirds? Mitochondrial NAD+ Is the Real Key to Regeneration
How remarkable is the liver’s regenerative ability? Even when two-thirds of a healthy liver is surgically removed, it can restore its original size within just a few weeks. However, under conditions such as disease or aging, this remarkable “self-healing superpower” declines dramatically. In mild cases, recovery after surgery becomes difficult; in severe cases, it can even lead to liver failure.
For decades, the fundamental mechanism that drives liver regeneration has remained a major mystery in medical science.
Recently, a groundbreaking study published in the leading journal Nature Metabolism finally provided a crucial answer. A research team led by Professor Joseph A. Baur at the University of Pennsylvania confirmed that the level of nicotinamide adenine dinucleotide (NAD+) within liver-cell mitochondria acts as a key “rate-limiting switch” that regulates the speed of liver regeneration.
This discovery is the first to clearly identify the regulatory role of a specific molecule within a specific cellular organelle, opening an entirely new direction for developing precision therapies that promote liver regeneration.
NAD+, often referred to as a “longevity molecule,” is a core cofactor in cellular energy metabolism. It participates in hundreds of biochemical reactions, including DNA repair, gene expression regulation, and energy production. Supplementing NAD+ precursors such as NR or NMN has also been shown to improve metabolic health and potentially slow aging.
What is less widely known, however, is that NAD+ is not evenly distributed inside the cell. Instead, it exists in different “working zones,” including the cytoplasm, nucleus, and mitochondria, where it performs distinct functions.
Among these compartments, mitochondria—the cell’s “power plants”—depend heavily on NAD+ to maintain efficient energy production. Yet the way mitochondria obtain NAD+ contains an important secret.
A major breakthrough came with the discovery of the SLC25A51 gene several years ago. Mitochondria cannot synthesize NAD+ on their own. Instead, they rely on SLC25A51, a specialized transporter that actively imports NAD+ from the cytoplasm.
This discovery made it possible to precisely regulate mitochondrial NAD+ levels and opened the door to investigating its role in liver regeneration.
To determine whether mitochondrial NAD+ is truly essential for liver regeneration, researchers conducted a bold experiment using genetic engineering.
They found that completely deleting the SLC25A51 gene in mice was lethal, highlighting its essential role in life. As a result, the researchers created heterozygous mice (Slc25a51+/-) that carried only one functional copy of the gene.
Although these mice appeared normal on the surface, deeper analysis revealed something remarkable: the total NAD+ level in the liver remained unchanged, yet the mitochondrial NAD+ content was significantly reduced. This confirmed that SLC25A51 functions as the key regulator controlling mitochondrial NAD+ supply.
The true test came during a liver regeneration experiment. After undergoing two-thirds partial hepatectomy, these mice showed markedly slower liver regeneration compared with normal mice. Their liver-to-body weight ratio recovered poorly, and signs of hepatocyte proliferation were minimal.
In other words, simply reducing NAD+ supply within the mitochondria—the cell’s small “energy room”—was enough to slow the entire liver repair process. At that moment, the central importance of mitochondrial NAD+ became unmistakably clear.
If reducing the transport pathway slows regeneration, could increasing it accelerate the process? The researchers decided to test this hypothesis.
Using adeno-associated virus (AAV) as a vector, they specifically overexpressed the SLC25A51 gene in liver cells—essentially installing a “booster pump” for mitochondrial NAD+ uptake.
The results were striking. Mice with increased SLC25A51 expression showed a significant rise in mitochondrial NAD+ levels.
After undergoing the same two-thirds liver resection, these mice demonstrated remarkably enhanced regenerative capacity. Liver regrowth was significantly faster, and hepatocyte proliferation was highly active. The regenerative effect was comparable to that observed with systemic supplementation of NAD+ precursors.
Even more importantly, detailed cell-fractionation analysis showed that this accelerated regeneration resulted specifically from increased mitochondrial NAD+ levels, while NAD+ levels in the cytoplasm and nucleus remained unchanged.
This finding delivered a decisive conclusion: boosting mitochondrial NAD+ alone is sufficient to enhance liver regeneration, and this mechanism likely explains how NAD+ precursor supplementation exerts its regenerative effects.
Why is mitochondrial NAD+ so crucial?
Through metabolomic and proteomic analyses, researchers found that liver regeneration is an extremely energy-intensive process requiring the synthesis of large amounts of new cellular components such as lipids and proteins.
When mitochondrial NAD+ levels are high, mitochondrial respiration becomes more efficient—particularly Complex I activity and fatty-acid oxidation, both of which depend on NAD+. This allows mitochondria to generate greater amounts of ATP to support regeneration.
At the same time, livers overexpressing SLC25A51 showed upregulated lipid-metabolism pathways. This suggests that mitochondrial NAD+ not only provides “energy currency” (ATP), but may also influence intracellular signaling pathways that promote the production of essential “building materials” such as fatty acids.
During regeneration stress, mitochondria in control mice rapidly consumed many metabolic intermediates. In contrast, cells overexpressing SLC25A51 experienced less depletion and maintained a more stable metabolic state.
In this sense, high mitochondrial NAD+ levels act like a metabolic buffer pool, helping liver cells cope with the enormous metabolic demands of regeneration.
This study elevates our understanding of NAD+ biology to a new level. It identifies the mitochondrial NAD+ pool as a core therapeutic target for liver regeneration, while also explaining a long-standing clinical puzzle: why overall NAD+ levels sometimes appear unchanged even when physiological functions deteriorate.
The answer may lie in the depletion of NAD+ within specific cellular compartments. This insight suggests that evaluating NAD+ metabolism in the future will require more precise analysis of its subcellular distribution.
Perhaps the most exciting implication is its potential for clinical translation and drug development.
Currently, supplementation with NAD+ precursors such as NR and NMN represents a broad but relatively non-targeted strategy. This research, however, points toward a precision approach.
In the future, scientists may develop drugs that act like molecular “keys,” specifically activating the SLC25A51 transporter in liver cells. Such therapies could rapidly “recharge” mitochondrial NAD+ when needed—such as after liver surgery or drug-induced liver injury—greatly enhancing regenerative capacity while avoiding unnecessary systemic effects.
Of course, this study was conducted only in male mice, and further research is needed to determine whether the findings apply to females and whether mitochondrial NAD+ plays a similar role in the repair of other organs.
Nevertheless, we now hold one of the key “keys” to unlocking the mystery of liver regeneration—bringing new hope for the future treatment of liver diseases.
For decades, the fundamental mechanism that drives liver regeneration has remained a major mystery in medical science.
Recently, a groundbreaking study published in the leading journal Nature Metabolism finally provided a crucial answer. A research team led by Professor Joseph A. Baur at the University of Pennsylvania confirmed that the level of nicotinamide adenine dinucleotide (NAD+) within liver-cell mitochondria acts as a key “rate-limiting switch” that regulates the speed of liver regeneration.
This discovery is the first to clearly identify the regulatory role of a specific molecule within a specific cellular organelle, opening an entirely new direction for developing precision therapies that promote liver regeneration.
PART 01
The Secret Inside Cells: NAD+ “Compartmentalization” and Its Dedicated Transport Channel
NAD+, often referred to as a “longevity molecule,” is a core cofactor in cellular energy metabolism. It participates in hundreds of biochemical reactions, including DNA repair, gene expression regulation, and energy production. Supplementing NAD+ precursors such as NR or NMN has also been shown to improve metabolic health and potentially slow aging.
What is less widely known, however, is that NAD+ is not evenly distributed inside the cell. Instead, it exists in different “working zones,” including the cytoplasm, nucleus, and mitochondria, where it performs distinct functions.
Among these compartments, mitochondria—the cell’s “power plants”—depend heavily on NAD+ to maintain efficient energy production. Yet the way mitochondria obtain NAD+ contains an important secret.
A major breakthrough came with the discovery of the SLC25A51 gene several years ago. Mitochondria cannot synthesize NAD+ on their own. Instead, they rely on SLC25A51, a specialized transporter that actively imports NAD+ from the cytoplasm.
This discovery made it possible to precisely regulate mitochondrial NAD+ levels and opened the door to investigating its role in liver regeneration.
PART 02
The Truth Revealed by Gene Editing: Fewer “Pipelines,” Slower Regeneration
To determine whether mitochondrial NAD+ is truly essential for liver regeneration, researchers conducted a bold experiment using genetic engineering.
They found that completely deleting the SLC25A51 gene in mice was lethal, highlighting its essential role in life. As a result, the researchers created heterozygous mice (Slc25a51+/-) that carried only one functional copy of the gene.
Although these mice appeared normal on the surface, deeper analysis revealed something remarkable: the total NAD+ level in the liver remained unchanged, yet the mitochondrial NAD+ content was significantly reduced. This confirmed that SLC25A51 functions as the key regulator controlling mitochondrial NAD+ supply.
The true test came during a liver regeneration experiment. After undergoing two-thirds partial hepatectomy, these mice showed markedly slower liver regeneration compared with normal mice. Their liver-to-body weight ratio recovered poorly, and signs of hepatocyte proliferation were minimal.
In other words, simply reducing NAD+ supply within the mitochondria—the cell’s small “energy room”—was enough to slow the entire liver repair process. At that moment, the central importance of mitochondrial NAD+ became unmistakably clear.
PART 03
Reversing the Strategy: Increasing the “Pipeline” Accelerates Regeneration
If reducing the transport pathway slows regeneration, could increasing it accelerate the process? The researchers decided to test this hypothesis.
Using adeno-associated virus (AAV) as a vector, they specifically overexpressed the SLC25A51 gene in liver cells—essentially installing a “booster pump” for mitochondrial NAD+ uptake.
The results were striking. Mice with increased SLC25A51 expression showed a significant rise in mitochondrial NAD+ levels.
After undergoing the same two-thirds liver resection, these mice demonstrated remarkably enhanced regenerative capacity. Liver regrowth was significantly faster, and hepatocyte proliferation was highly active. The regenerative effect was comparable to that observed with systemic supplementation of NAD+ precursors.
Even more importantly, detailed cell-fractionation analysis showed that this accelerated regeneration resulted specifically from increased mitochondrial NAD+ levels, while NAD+ levels in the cytoplasm and nucleus remained unchanged.
This finding delivered a decisive conclusion: boosting mitochondrial NAD+ alone is sufficient to enhance liver regeneration, and this mechanism likely explains how NAD+ precursor supplementation exerts its regenerative effects.
PART 04
Mechanistic Insights: Dual Support of Energy and Biosynthetic Materials
Why is mitochondrial NAD+ so crucial?
Through metabolomic and proteomic analyses, researchers found that liver regeneration is an extremely energy-intensive process requiring the synthesis of large amounts of new cellular components such as lipids and proteins.
When mitochondrial NAD+ levels are high, mitochondrial respiration becomes more efficient—particularly Complex I activity and fatty-acid oxidation, both of which depend on NAD+. This allows mitochondria to generate greater amounts of ATP to support regeneration.
At the same time, livers overexpressing SLC25A51 showed upregulated lipid-metabolism pathways. This suggests that mitochondrial NAD+ not only provides “energy currency” (ATP), but may also influence intracellular signaling pathways that promote the production of essential “building materials” such as fatty acids.
During regeneration stress, mitochondria in control mice rapidly consumed many metabolic intermediates. In contrast, cells overexpressing SLC25A51 experienced less depletion and maintained a more stable metabolic state.
In this sense, high mitochondrial NAD+ levels act like a metabolic buffer pool, helping liver cells cope with the enormous metabolic demands of regeneration.
PART 05
Looking Ahead: From Broad Supplementation to Precision Targeting
This study elevates our understanding of NAD+ biology to a new level. It identifies the mitochondrial NAD+ pool as a core therapeutic target for liver regeneration, while also explaining a long-standing clinical puzzle: why overall NAD+ levels sometimes appear unchanged even when physiological functions deteriorate.
The answer may lie in the depletion of NAD+ within specific cellular compartments. This insight suggests that evaluating NAD+ metabolism in the future will require more precise analysis of its subcellular distribution.
Perhaps the most exciting implication is its potential for clinical translation and drug development.
Currently, supplementation with NAD+ precursors such as NR and NMN represents a broad but relatively non-targeted strategy. This research, however, points toward a precision approach.
In the future, scientists may develop drugs that act like molecular “keys,” specifically activating the SLC25A51 transporter in liver cells. Such therapies could rapidly “recharge” mitochondrial NAD+ when needed—such as after liver surgery or drug-induced liver injury—greatly enhancing regenerative capacity while avoiding unnecessary systemic effects.
Of course, this study was conducted only in male mice, and further research is needed to determine whether the findings apply to females and whether mitochondrial NAD+ plays a similar role in the repair of other organs.
Nevertheless, we now hold one of the key “keys” to unlocking the mystery of liver regeneration—bringing new hope for the future treatment of liver diseases.
