In a remarkable series of experiments on a fungus that causes cryptococcal meningitis, a deadly infection of the membranes that cover the spinal cord and brain, investigators at UC Davis have isolated a protein that appears to be responsible for the fungus’ ability to cross from the bloodstream into the brain.
Normally the brain is protected from bacterial, viral and fungal pathogens in the bloodstream by a tightly packed layer of endothelial cells lining capillaries within the central nervous system — the so-called blood-brain barrier. Relatively few organisms — and drugs that could fight brain infections or cancers — can breach this protective barrier.
The fungus studied in this research causes cryptococcal meningoencephalitis, a usually fatal brain infection that annually affects some 1 million people worldwide, most often those with an impaired immune system. People typically first develop an infection in the lungs after inhalation of the fungal spores of C. neoformans in soil or pigeon droppings. The pathogen then spreads to the brain and other organs.
In an effort to discover how C. neoformans breaches the blood-brain barrier, the investigators isolated candidate proteins from the cryptococcal cell surface. One was a previously uncharacterized metalloprotease that they named Mpr1. (A protease is an enzyme — a specialized protein — that promotes a chemical reaction; a metalloprotease contains a metal ion — in this case zinc — that is essential for its activity.) The M36 class of metalloproteases to which Mpr1 belongs is unique to fungi and does not occur in mammalian cells.
The investigators next artificially generated a strain of C. neoformans that lacked Mpr1 on the cell surface. Unlike the normal wild-type C. neoformans, the strain without Mpr1 could not cross an artificial model of the human blood-brain barrier.
They next took a strain of common baking yeast — Saccharomyces cerevisiae — that does not cross the blood-brain barrier and does not normally express Mpr1, and modified it to express Mpr1 on its cell surface. This strain then gained the ability to cross the blood-brain barrier model.
Investigators then infected mice with either the C. neoformans that lacked Mpr1 or the wild-type strain by injecting the organisms into their bloodstream. Comparing the brain pathology of mice 48 hours later, they found numerous cryptococci-filled cysts throughout the brain tissue of mice infected with the wild-type strain; these lesions were undetectable in those infected with the strain lacking Mpr1. In another experiment, after 37 days of being infected by the inhalation route, 85 percent of the mice exposed to the wild-type C.neoformans had died, while all of those given the fungus without Mpr1 were alive.