Targeting Protein Could Protect Nerve Cells via Blood-CNS Barrier
ARF6 activity in MS behind damaging barrier 'leaks' into central nervous system
The protective barrier that prevents cells and large molecules from crossing into the central nervous system (CNS) is known to be unusually “leaky” in multiple sclerosis (MS), but targeting a protein called ARF6 can help to stabilize this barrier, a study in mouse models of MS found.
Such a treatment approach could work to prevent abnormal immune cells from reaching the brain and spinal cord to cause nerve cell damage, without the side effects of current MS therapies.
Study findings “indicate that ARF6 plays a critical role in [brain-CNS barrier] function … and provide preclinical rationale for targeting ARF6 as a therapeutic approach to treat MS,” its researchers wrote.
Blocking ARF6 may treat MS without systemwide immunosuppression
As its name suggests, the blood-CNS barrier is a wall of blood vessel cells that surrounds the central nervous system — the brain and spinal cord — and helps to regulate which substances and cells can pass from circulating blood into the CNS. This helps to keep delicate nerve tissue safe from infections and to maintain an environment where nerve cells can optimally function.
Problems in this barrier have been observed in a number of neurological disorders, including MS. Its breakdown is thought to permit more inflammatory cells and molecules to enter the CNS, driving the nerve damage that causes MS.
“We think the breakdown of the blood-CNS barrier is causing a real problem,” Shannon Odelberg, PhD, research associate professor of internal medicine at the University of Utah and study co-author, said in a university press release. “So, if you can stabilize the vasculature and reduce that leak, you can reduce those proteins that do the damage, and you can reduce the inflammatory response.”
Scientists conducted a battery of experiments to examine the barrier’s dysfunction in mice with experimental autoimmune encephalitis (EAE), a common mouse model of MS.
Normally, the blood-CNS barrier is composed of endothelial cells, the cells that line blood vessels and form tight connections with each other. In EAE mice, however, many of these cells are seen to have undergone a process called endothelial-to-mesenchymal transition (EndoMT), taking on the characteristics of more loosely connected mesenchymal cells.
The barrier’s endothelial cells “lose those tight junctions when they convert to a mesenchymal cell type, allowing the fluid and the proteins to pass into the tissue where damage can occur,” Odelberg said.
Notably, this transition occurred before symptom onset in the mice, suggesting that “EndoMT is likely a major catalyst for the development of [brain-CNS barrier] dysfunction and neuroinflammation in MS,” the researchers wrote.
Through further tests, the researchers determined that the protein ARF6 plays a central role in coordinating EndoMT in barrier cells. Increasing ARF6 activity was sufficient to induce EndoMT in these cells, while blocking its activity stopped the initiation of EndoMT by a signaling molecule called interleukin 1-beta.
Researchers then engineered mice to lack the ARF6 protein — either throughout their body or only in specific cell types — before EAE was induced. Tests showed that lower ARF6 levels in endothelial cells, but not in other cell types, significantly reduced clinical severity scores in these mice.
Knocking out ARF6 in endothelial cells also led to fewer inflammatory T-cells in the CNS, but not elsewhere in the body. This is particularly noteworthy because most current MS therapies work by broadly suppressing the immune system, meaning throughout the body, which can prevent MS progression but also increase the risk of infections.
Further tests using a small molecule that blocks ARF6 activity, called NAV-2729, yielded similar results. Treating mice with severe disease, evident in clinical scores that had almost reached a maximum, significantly eased the animals’ symptoms, which was accompanied by fewer CNS-infiltrating T-cells and lesser evidence of brain-CNS barrier leaks.
“Treatment with NAV-2729, even after the establishment of severe EAE, markedly reduces clinical symptoms, stabilizes the BCNSB [brain-CNS barrier], reverses EndoMT, and deceases demyelination without immunocompromising the host,” the team wrote.
“These results suggest that pharmacologic inhibition of ARF6 could provide a novel approach for treating MS while still preserving an intact immune system for fighting viral reactivation,” the scientists concluded.