Antibody That Opens Blood-Brain Barrier Might Also Ferry Lifesaving Drugs
By Deborah Borfitz
March 25, 2022 | A future where single-chain nanobodies and biologically active proteins might voyage through the blood-brain barrier to treat neurological conditions—including Alzheimer’s disease and brain tumors—recently came closer into view with the discovery of an antibody that succeeded in temporarily opening and resealing the crucial filtering mechanism in mice. It was the latest milestone to come out of the lab of Anne Eichmann, Ph.D., professor of cardiology and cellular and molecular physiology at Yale School of Medicine, who plans to also explore the antibody as a drug delivery platform.
The endothelium is the key component of the blood-brain barrier because it carries the previously found “Wnt signaling” that is important for regulating a gene expression program ensuring vital nutrients reach the brain but circulating toxins or pathogens are kept out, she says. Unc5B is an endothelial membrane receptor which, as her lab observed years ago, is both specific to the vascular system and indispensable to its development. “When you can’t establish circulation, you die.”
As her team most recently learned, Unc5B functions as an upstream regulator of the Wnt signaling pathway and it can be blocked for several hours with the novel antibody that prevents it from binding to ligand Netrin-1 responsible for the barrier effect. This essentially creates an on-demand blood-brain barrier opening, as recently described in Nature Communications (DOI: 10.1038/s41467-022-28785-9).
Opening of the blood-brain barrier allows molecules a “window of opportunity” to be ferried to their destination, Eichmann explains. Interestingly, only bioactive molecules up to a certain molecular weight (40 kilodaltons) can be delivered to the brain using anti-Unc5B, a list that excludes immunoglobulins (e.g., antibodies) but includes small molecule drugs and the nanobodies—which can be aimed at multiple targets, including amyloid beta—and growth factors such as brain-derived neurotrophic factor (BDNF)—potentially useful in treating a variety of neurodegenerative diseases.
So far, the effects of nanobodies or BDNF delivered into the brains of mice with the antibody have not been tested, says Eichmann. The efficacy and potential toxicity of anti-Unc5B penetrating the blood-brain barrier will now be evaluated. A second antibody having a similar effect will be simultaneously investigated, she adds.
Yale postdoctoral associate Kevin Boyé initiated study of Unc5B in Eichmann’s lab five years ago, to follow up on the finding that when the receptor was knocked out in mice they died early in their embryonic development because their vasculature failed to form properly, says Eichmann. A protein known as Claudin5, involved in creating the tight junctions between endothelial cells of the blood-brain barrier, was also significantly reduced.
What exactly that receptor was doing remained a mystery until Eichmann’s lab subsequently teamed up with Susan Ackerman, Ph.D. (University of California, San Diego), to repeat the experiment on conditional neonatal mice where the receptor could be knocked out with a drug. The mice again died, but not before developing seizures—a sign that the blood-brain barrier might have opened, Eichmann says.
When Unc5B was disrupted in adult mice, the blood-brain barrier could again be seen opening, she continues. The team then found the Wnt signaling pathway and proceeded to engineer the anti-Unc5B antibody to temporarily block the molecule.
The focus has now shifted to defining the transient period for shuttling a therapeutic to the brain using anti-Unc5B and the other candidate antibody, says Eichmann. “Safety is of course an area of great concern because when you think about treating a patient with a neurological disease, the last thing you want to do is potentially make the disease worse by opening the barrier.”
Any of the various components of the blood-brain barrier—endothelial cells as well as pericytes, microglial cells, and astrocytes—are potential drug targets. The spinal cord and eyes, as well as the brain, have this protective shield. “The eyes and cerebellum share one particular Wnt signaling pathway, whereas the rest of the brain uses a different Wnt signaling pathway, and we believe our [Unc5B] antibody targets both of these.”
Future studies will explore anti-UNC5B as a delivery platform for brain-penetrating drugs, which might include chemotherapy for treating brain tumors as well as growth factors that may be useful in the management of neurodegenerative disorders and perhaps depression, she says. Preclinical experimentation will be done using rodent models, at least until someone figures out how to create profuse vasculature in a human brain organoid.