Cell maintenance

An enzyme in blood-vessel linings sped up aging in mice by cutting an NAD-making protein

In mice, switching on a protein-cutting enzyme called AEP in the cells lining blood vessels damaged the vessels, made the animals frail and shortened their lives; blocking the enzyme or its cut largely reversed this. The work is in mice and cells, so it says nothing yet about people.

As we age, the tiniest blood vessels thin out and work less well, so tissues get less oxygen and nutrients. This is thought to feed into many problems of old age, from weaker muscles to poorer memory. A study in mice, from a team at the Shenzhen Institutes of Advanced Technology of the Chinese Academy of Sciences, proposes one molecular chain behind it, and tests ways to break that chain.

The chain starts in the endothelial cells, the thin layer of cells lining every blood vessel. The team's earlier work had found that a protein called C/EBPβ, which switches genes on, rises with age in nerve cells and turns on an enzyme called AEP. AEP is a protease: it cuts other proteins at particular spots. The new question was whether the same pair does damage in blood vessels.

A key player is NAD+, a small molecule that cells need for energy metabolism and repair and that falls with age. Cells make most of it with the help of an enzyme called NAMPT. Supplements that provide NAD+ building blocks, such as NMN, raise NAD+ levels in people, but the authors note that the health effects in trials have been mixed.

What they did

  • Looked for the pathway with age. They stained muscle and brain tissue from deceased young and old people and from young and old mice, and grew human endothelial cells from umbilical cords, pushing them into an aged, non-dividing state with hydrogen peroxide.
  • Turned the pathway up. They bred mice that make extra AEP, or extra human C/EBPβ, only in their endothelial cells, from birth. Some C/EBPβ mice also lacked the AEP gene.
  • Found the target. In test tubes and cells they checked which proteins AEP cuts.
  • Tried to undo it. From 10 months of age, C/EBPβ mice received a gene-therapy virus carrying either normal NAMPT or a version AEP can't cut, or were given NMN in their water or an AEP blocker called CP#11A in their food. CP#11A is an experimental compound developed by the same group.
  • Tested old normal mice. In 20-month-old mice, they lowered C/EBPβ or AEP in endothelial cells.

What they found

  • The pathway rises with age. Old human and mouse tissue had fewer endothelial cells and more C/EBPβ and AEP in them.
  • AEP cuts NAMPT. It cuts the enzyme at one spot, which crippled NAD+ production. In mice with the pathway turned up, NAD+ was lower in muscle and brain, and endothelial cells showed signs of senescence: they stopped dividing and released inflammatory signals.
  • Damaged vessels. These mice had blood vessels that widened poorly on demand, weaker blood-flow responses in the brain and a leakier blood–brain barrier, the seal that keeps many substances in the blood out of the brain.
  • Frailer, shorter-lived mice. They ran less, tired sooner on a treadmill, did worse on memory tasks and scored higher on a frailty index. Median lifespan of male mice with extra C/EBPβ was 390 days, against 804 days for normal littermates. Removing AEP brought it back to 754 days. Females showed the same pattern.
  • Blocking the cut helped. The uncuttable NAMPT raised NAD+ and improved vessels, behaviour and frailty more than normal NAMPT did.
  • NMN and CP#11A both helped. Both raised NAD+, improved the vessels, behaviour and frailty scores, and lengthened life in these fast-aging mice: median lifespan in males was 673 days with NMN and 718 with CP#11A, against 521 without treatment. CP#11A's effects were generally stronger. Tumours and other disease at death did not differ, and blood and urine tests showed no signs of liver or kidney damage from CP#11A.
  • Old normal mice. Lowering C/EBPβ or AEP in their endothelial cells raised NAD+ and improved movement and memory.

What to keep in mind

  • An extreme model. The authors point out that their mice overproduce C/EBPβ or AEP from birth, which does not mimic the slow rise of these proteins during human aging. Most lifespan results come from these fast-aging mice, not from normal ones.
  • Mostly mice and cells. The human evidence is limited to stained tissue samples from a small number of deceased donors; nothing was tested in living people.
  • Small groups for many tests. Many lab measurements used three animals or samples per group.
  • A mixed result for normal NAMPT. It lengthened life in females, but in males the difference from untreated mice was not statistically significant.
  • Not a treatment. CP#11A is an experimental compound, not an approved drug. NMN worked in these mice, but that does not show it does the same in people.

Why it matters

Falling NAD+ is one of the most discussed features of aging, but why it falls is less clear. This study offers one answer in mice: an enzyme in the blood-vessel lining that chops up the main NAD+-making enzyme. It also suggests why simply adding NAD+ building blocks might not be enough if the machinery that uses them is being cut. Whether the same thing happens in aging people, and whether blocking AEP would be safe or useful in humans, still has to be shown.

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