A newly discovered link between genes that contribute to hair colour and those involved in immunity and inflammation may explain why hair can turn grey in response to severe illness or chronic stress.
The finding, made using mouse models, is published in the journal PLOS Biology.
Hair fibres are primarily made of keratin, a long relatively colourless structural protein. In hair follicles, cells called melanocytes make the pigment melanin and transfer it to the cells that produce keratin. The melanin is then incorporated into the hair fibre.
The most common form of the pigment is eumelanin, which has a brown-black colour. Pheomelanin, however, is reddish. High levels of eumelanin lead to darker hair, while high levels of pheomelanin produce red hair. Variations in the ratios of these melanins produce the wide range of natural hair colours we’ve come to know.
Melanocytes only live for three to five years, but melanocyte stem cells produce new ones. As we lose these cells with age, new hair eventually appears whitish or grey.
It was known that a protein called melanogenesis associated transcription factor (MITF), plays an important role in the production and life cycle of melanocytes. Principally, it controls the activity of genes that make and deliver melanin. In mice, if MITF is absent during melanocyte development, their fur lacks pigment.
William Payan at the National Institutes of Health and Melissa Harris at the University of Alabama, both in the US, have now discovered that in addition to its role in melanin production MITF acts as an off-switch for certain genes involved in immune activity.
When a cell detects a foreign invader, such as a virus, it produces interferons – molecules that stimulate the activity of numerous immune-related genes, including those that shut down viral replication.
The findings reveal that mice with insufficient MITF had higher activity in several genes that are stimulated by interferon. In other words, MITF keeps a lid on their activity and when it is absent, that control is lost.
This suggested a link exists between hair pigmentation and immune responses. To find out, the researchers and their colleagues triggered a sustained immune response in mice already predisposed to greying fur. As a result, the mice lost many melanocytes and melanocyte stem cells, and grew significantly more grey hair.
“This new discovery suggests that genes that control pigment in hair and skin also work to control the innate immune system,” says Pavan. “These results may enhance our understanding of hair greying.”
Indeed, by revealing how immune activation negatively effects melanocytes and melanocyte stem cells, the findings help explain why a severe infection or sustained stress, which causes inflammation, can cause some people’s hair to go grey.
“More importantly, discovering this connection will help us understand pigmentation diseases with innate immune system involvement,” Pavan adds.