New Function of Astrocytes
A recent study has discovered a new role for astrocytes. Astrocytes are the most abundant cell found in the human brain. Formerly, it was considered that astrocytes are the help cells of the neurons. Some of their functions are biochemical support of endothelial cells, maintaining the extracellular ion balance and providing nutrients for the nervous tissue. A research team led by Dr Katsuhiko Mikoshiba from the RIKEN (Rikagaku Kenkyujo – The Institute of Physical and Chemical Research) Brain Science Institute in Wako, Japan, has discovered that astrocytes have another role, that of transmitting chemical signals to neurons, vascular cells and other astrocytes.
Astrocytes are cells shaped like stars, having a central ‘soma’ and many extensions that connect the astrocyte to the neighboring cells that it regulates. Healthy astrocytes use their extensions (know as processes) to send individual Ca2+ signals. This process was previously discovered to be regulated by a receptor found in the cellular membrane – mGluR5 (metabotropic glutamate receptor 5). However, researchers weren’t certain about the way each signal was confined to an individual extension. Understanding this process could be important for the future therapy of Alzheimer’s disease or epilepsy. Astrocytes from patients suffering from these conditions send out global signals, instead of sending out individual signals.
In order to understand the regulation of astrocyte signaling, researchers used quantum dots (semiconductors that emit light when excited) to mark the mGluR5 receptors. Researchers observed that the marked receptors didn’t pass from the extension to the soma. Normal astrocytes contain a mGluR5-selective barrier that allowed the extensions to regulate their signals through Ca2+ dissociation.
Dr. Mikoshiba’s team used an over-expression of the mGluR5 receptor in order to overwhelm the barrier and note its characteristics. They concluded that the barrier is made of proteins that interact with the mGluR5 receptor. Each protein contained by the barrier only interacts with a single molecule of mGluR5 receptor, thus preventing it from crossing to the soma. Because the number of barrier proteins is limited, an abundance of mGluR5 causes an overflow, thus some receptors cross the barrier into the soma. This leads to the global propagation of signals sent by the astrocyte.
A series of experimental models of Alzheimer’s and epilepsy have shown an increased concentration of mGluR5 in astrocytes. The team of scientists suggest that comprehending the molecular characteristics of the barrier can offer new treatment methods for the aforementioned conditions. “We are very curious to know the effect of global astrocytic Ca2+ signaling on the neuronal network and neuro-vascular coupling”, says Dr Mikoshiba.