New way of targeting proteins to exosomes - potential for melanoma, Alzheimer's disease

Researchers at the University of Pennsylvania School of Medicine have discovered how a protein called Pmel17 is sorted by pigment cells in the skin and eye to make a fiber matrix that eventually sequesters melanin, the dark pigment found in skin, hair, and eyes.

Understanding the molecular steps prior to fiber formation - and when this process goes awry - may lead to a better understanding of melanoma and Alzheimer's disease. Pmel17 is a major target within the immune system in current anti-melanoma immunotherapies. Michael S. Marks, PhD, Associate Professor of Pathology and Laboratory Medicine, and colleagues published their findings in the March issue of Developmental Cell.

Marks studies protein sorting - determining how proteins are delivered to the correct organelle, or subcompartments, within the cell. He investigates this basic process in pigment cells, particularly sorting to the melanin storage compartment called the melanosome. Melanin is normally stored by the cell in melanosomes because its build-up outside the melanosome can lead to cell death.

In the pigment-producing cell, called the melanocyte, melanin is laid down on a fibrous matrix made from Pmel17. Other work from the Marks lab and collaborators showed that the structure of Pmel17 is similar to amyloid protein, one of the hallmarks of Alzheimer's disease plaques. Using mouse and human melanoma cells, the Marks lab also studies melanocytes for pathological conditions associated with mutations along the protein-sorting process.

"There's no evidence that Pmel17 per se will initiate pathological cellular structures, but recent research from our lab shows that if we look at the structure of the fibers made up of Pmel17, it has all the biophysical properties of amyloid," explains Marks. "Pmel17 is functioning in a physiological capacity the same way that amyloid functions in a pathological capacity."

Before the fibers are laid down, the researchers found in the Developmental Cell study that Pmel17 passes through a series of compartments called endosomes, much the way proteins that are tagged for degradation do. They determined that this process also happens in non-pigment cells. This discovery indicates that sorting is not a melanocyte-specific process; the sorting phenomenon is a general one.

Other researchers have found that the Alzheimer's precursor protein, the prion protein (responsible for Creuztfeldt-Jakob's Disease, Mad Cow disease, and Kuru), and the precursors for several familial amyloid diseases all pass through one type of endosome. "This may be a general property of a class of amyloids - and the fact that the process happens in non-pigment cells means that it can also happen in neurons or epithelial cells where these amyloids cause problems," says Marks.

Pmel17 and other proteins of melanocytes are well-known tumor antigens in melanoma patients. "What's unique about these proteins, as opposed to other tumor antigens, is that there's good evidence in melanoma patients that - via Pmel17 - you can stimulate helper T cells, whose antigens are also processed within the cell by protein- sorting mechanisms," says Marks.

Exosomes are the special membranes with which the antigens associate in the protein-sorting process and are derived from endosome membranes. Hence, if the antigens get to the right endosome, they will be incorporated on exosomes. Once released outside the cell, the exosomes themselves get targeted to dendritic cells. Then exosomes ferry Pmel17 and other melanoma antigens from the melanoma tumor cell to the dendritic cell.

"Exosomes are a very hot topic now in cancer immunotherapy because dendritic cells are good at taking them up, processing the associated antigens, and presenting them to helper T cells, which then rally the immune system to fight the tumor."

Marks says that understanding how and why the sorting process is required for Pmel17 fiber formation will likely provide researchers with the chance to interfere with this process, and may thus provide some therapeutic or preventative treatments for diseases like Alzheimer's and the prion diseases.

"We've also shown a new way of targeting proteins to exosomes," says Marks. "If we learn more about how this process works, we may be able to better manipulate tumor antigen access to dendritic cells and perhaps their ability to stimulate T cells."

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