Advances in neuro-imaging and molecular biology have unearthed a subtle, previously unknown layer in the brain. As described in the journal Science, the newly discovered layer forms a previously unknown component of brain anatomy that acts as both a protective barrier and platform from which immune cells monitor the brain for infection and inflammation.
“The discovery of a new anatomic structure that segregates and helps control the flow of cerebrospinal fluid (CSF) in and around the brain now provides us much greater appreciation of the sophisticated role that CSF plays not only in transporting and removing waste from the brain, but also in supporting its immune defenses,” said Maiken Nedergaard, co-director of the Center for Translational Neuromedicine at University of Rochester and the University of Copenhagen. Nedergaard and her colleagues have made significant findings in the field of neuroscience, including detailing the many critical functions of previously overlooked cells in the brain called glia and the brain’s unique process of waste removal, which the lab named the glymphatic system.
The study focuses on the series of membranes that encase the brain, creating a barrier from the rest of the body and keeping the brain bathed in CSF. The traditional understanding of what is collectively called the meningeal layer identifies the three individual layers as dura, arachnoid, and pia matter.
This new layer discovered by the international research team further divides the space between the arachnoid and pia layers, the subarachnoid space, into two compartments, separated by the newly described layer, which the researchers name SLYM (Subarachnoidal LYmphatic-like Membrane). While the paper mostly describes the function of SLYM in mice, it also reports its presence in the adult human brain as well.
SLYM is a type of membrane that lines other organs in the body, including the lungs and heart, called mesothelium. These membranes typically surround and protect organs, and harbour immune cells.
The new membrane is very thin and delicate, consisting of only a few cells in thickness. Yet SLYM is a tight barrier, allowing only very small molecules to transit and it also seems to separate “clean” and “dirty” CSF. This last observation hints at the likely role played by SLYM in the glymphatic system, which requires a controlled flow and exchange of CSF, allowing the influx of fresh CSF while flushing the toxic proteins associated with Alzheimer’s and other neurological diseases from the central nervous system. This discovery will help researchers more precisely understand the mechanics of the glymphatic system.
The SLYM also appears important to the brain’s defences. The central nervous system has its own native population of immune cells, and the membrane’s integrity prevents outside immune cells from entering. In addition, the membrane appears to host its own population of central nervous system immune cells that use SLYM as an observation point close to the surface of the brain from which to scan passing CSF for signs of infection or inflammation.
Discovery of the SLYM opens the door for further study of its role in brain disease. For example, the researchers note that larger and more diverse concentrations of immune cells congregate on the membrane during inflammation and aging. Furthermore, when the membrane was ruptured during traumatic brain injury, the resulting disruption in the flow of CSF impaired the glymphatic system and allowed non-central nervous system immune cells to enter the brain.
These and similar observations suggest that diseases as diverse as multiple sclerosis, central nervous system infections, and Alzheimer’s might be triggered or worsened by abnormalities in SLYM function. They also suggest that the delivery of drugs and gene therapeutics to the brain may be impacted by SLYM, which will need to be considered as new generations of biologic therapies are being developed.
