US researchers have found that electronic cigarettes, often targeted at youth and pregnant women, produce a stress response in brain stem cells.

Present throughout life, stem cells become specialised cells with more specific functions, such as brain cells, blood cells, or bone.

"Although originally introduced as safer, ECs, such as Vuse and JUUL, are not harmless," said Atena Zahedi from the University of California.

"Even short-term exposure can stress cells in a manner that may lead, with chronic use, to cell death or disease. Our observations are likely to pertain to any product containing nicotine," Zahedi said.

Using cultured mouse neural stem cells, the researchers identified the mechanism underlying EC-induced stem cell toxicity as "stress-induced mitochondrial hyperfusion," or SIMH.

"SIMH is a protective, survival response," said Prue Talbot, a professor in the University of California.

"Our data show that exposure of stem cells to e-liquids, aerosols, or nicotine produces a response that leads to SIMH," Talbot said.

Electronic cigarettes, or ECs, are nicotine-delivery devices that aerosolize nicotine and flavour chemicals through heating.

The high levels of nicotine in ECs lead to a nicotine flooding of special receptors in the neural stem cell membrane," Zahedi said.

"Nicotine binds to these receptors, causing them to open up. Calcium and other ions begin to enter the cell. Eventually, a calcium overload follows," she said.

Zahedi explained that too much calcium in the mitochondria is harmful. The mitochondria then swell, changing their morphology and function.

They can even rupture and leak molecules that lead to cell death, according to the study published in the journal iScience.

"If the nicotine stress persists, SIMH collapses, the neural stem cells get damaged and could eventually die," Zahedi said.

"If that happens, no more specialised cells -- astrocytes and neurons, for example -- can be produced from stem cells," she said.

Damaged stem cell mitochondria could accelerate ageing and lead to neurodegenerative diseases, she noted.

The researchers said that youth and pregnant women need to pay especially close attention to their results.

"Nicotine exposure during prenatal or adolescent development can affect the brain in multiple ways that may impair memory, learning, and cognition," Talbot said.

"Furthermore, addiction and dependence on nicotine in youth are pressing concerns. It's worth stressing that it is the nicotine that is doing damage to neural stem cells and their mitochondria," she added.