The blood-brain barrier (BBB), a layer of endothelial cells lining the blood vessels leading to the brain, is one of the brain’s greatest assets. Responsible for determining what molecules are allowed to cross from the bloodstream into the brain, the BBB protects the brain from the invasion of infectious and toxic agents. However, this barrier is also one of the greatest obstacles for treating neurological disease and infection. Intended to keep out harmful pathogens, the BBB also obstructs access for therapeutic molecules meant to treat neurological conditions.
Two recent studies, from researchers at UC Davis and Mayo Clinic, shed light on the mechanisms molecules use to pass through the BBB and suggest novel methods for increasing access for therapeutic compounds. Many of the current methods used to cross the BBB function by chemically or physically disrupting the BBB and can be harmful and invasive. The two new studies propose more organic methods; one relying on a naturally occurring protein and the other a synthetic peptide carrier that mimics a normal physiological process.
Working to develop new treatments for cryptococcal meningitis, a team of researchers led by Angie Gelli at UC Davis, sought to discover how the pathogen, Cryptococcus neoformans breaches the blood-brain barrier. The researchers analyzed the extracellular proteome and identified a novel, secreted metalloprotease, Mpr1, as a possible candidate. Mpr1 belongs to a poorly characterized M36 class of fungalysins that are only expressed in some fungal species.
Next, the researchers generated a strain of C. neoformans lacking the gene encoding Mpr1 and tested its ability to cross the BBB. Not only was the strain lacking Mpr1 unable to pass through an in vitro model of the human BBB, but when the researchers modified a strain of common baking yeast to express Mpr1, it gained the ability to cross the BBB model.
The researchers observed similar results in vivo: mice infected with the Mpr1 null strain showed significant improvement in survival due to a reduced brain fungal burden and the pathology that is commonly seen in cryptococcal meningitis disease was absent.
The results, published in the June issue of mBio, fill a significant gap in the understanding of how C. neoformans crosses the blood-brain barrier and causes meningitis, says Angie Gelli, associate professor of pharmacology at UC Davis and principal investigator of the study.
According to Gelli, their discovery has significant therapeutic potential for treating meningitis as Mpr1 — or an aspect of its brain penetrating mechanism— could be a target for drug discovery. Furthermore, Mpr1 could be developed as part of a drug-delivery vehicle for numerous other brain infections and cancers. For example, an antibiotic or cancer-fighting drug that is unable to cross the blood-brain barrier on its own could be attached to a nanoparticle containing Mpr1, allowing it to hitch a ride and deliver its goods to where it is needed. Gelli’s group is currently pursuing such a nanoparticle drug-delivery system using Mpr1.
In another study, a group of researchers led by Gobinda Sarkar and Robert Jenkins at Mayo Clinic have demonstrated in a mouse model that their recently developed synthetic peptide carrier is a potential delivery vehicle for brain cancer chemotherapy drugs and other neurological medications. The findings are published in PLoS One.
The new study builds on previous research in which the peptide carrier K16ApoE was found to transport target proteins to the brain by mimicking a ligand-receptor system. Once injected into a vein, K16ApoE binds to proteins in the blood and creates apolipoprotein E (ApoE)-like entities. These complexes are recognized by the BBB receptor LDLR as near-normal ligands and transcytosis is initiated. The researchers believe that during ligand receptor- mediated transcytosis, transient pores are formed, through which various molecules can be transported to the brain. The researchers have now demonstrated the ability of K16ApoE to deliver eight different molecules (cisplatin, methotrexate, cetuximab, three different dyes, a synthetic peptide (Y8) and I-125) to the brain without requiring any chemical modification of the molecules.
One concern with increasing permeability of the BBB is that, in addition to therapeutic agents, undesirable molecules will also gain access to the brain. However, the researchers believe they have found a way to avoid such a problem. By premixing the peptide with the desired therapeutic agent, they are able to increase specificity and decrease the likelihood that other molecules in the bloodstream will bind to the peptide and be delivered to the brain.
The researchers believe that because their approach mimics a normal physiological process, it may affect the BBB less than methods which utilize physical or chemical means to open the barrier. Next, the researchers hope to focus on evaluating clinical efficacy of K16ApoE-mediate brain uptake of therapeutics for patients with brain cancer and other brain-associated disorders.
Researchers from both institutes are hopeful that their work will help to improve the treatment of neurological conditions. “The biggest obstacle to treating many brain cancers and infections is getting good drugs through the blood-brain barrier,” says Gelli. “If we could design an effective delivery system into the brain, the impact would be enormous for treating some of these terrible diseases.”