Protein puts the brakes on melanin

Protein puts the brakes on melanin

The study in the Nature journal Scientific Reports shows that pigmentation is reduced by the activity of "TPC2," a protein that channels the flow of positive sodium ions out of the melanosomes, compartments that produce melanin in cells. When TPC2 lets those ions out, the inside of the melanosomes become more acidic, the researchers found, and that shuts down the enzyme that drives melanin production.

A lack of melanin production can be associated with albinism, visual impairment and a greater susceptibility to skin and eye cancer. Melanin protects DNA from ultraviolet radiation. For years, however, scientists have had little insight into how pigmentation is governed. In late 2014, the team discovered that melanosomes employed an ion channel, "OCA2," whose activity increases melanin production by reducing their acidity. OCA2 is named for the disease caused by mutations in the protein, oculocutaneous albinism type II.

The new study, therefore, finds that TPC2 and OCA2 counterbalance.

The team was able to prove exactly how TPC2 affects pigmentation. They worked in mouse skin cells and frog eye cells, which have larger melanosomes than human cells do. Otherwise all the same proteins and mechanisms are in place as in humans.

By making direct electrical measurements on the melanosomes, the team spotted a large inward flow of current (negative electrical charge) corresponding to positive ions flowing out. They showed that the current was independent of that regulated by OCA2 and depended on a lipid called PI(3,5)P2 being in the membrane of the melanosome.

The current was consistent with what's typically produced by TPC ion channels. When the researchers blocked TPC channels with the appropriate chemical called verapamil, the current stopped. Further testing showed that TPC2, rather than TPC1, is found all over the melanosome membrane.

Then came the smoking gun. The team deleted the TPC2 gene using CRISPR-Cas9 gene editing and not only found that doing so abolished the current inflow, but that adding back human TPC2 protein restored it.

They observed that cells with reduced TPC2 levels have more melanin, suggesting that TPC2 is a negative regulator of pigmentation.

From there they showed that melanosomes with TPC2 were a bit more acidic than those without it and that it indeed directly competes with OCA2. Acidity matters because the main enzyme that mediates melanin synthesis, tyrosinase, is only active at around neutral acidity.