NIDA-supported investigators have identified a pair of proteins that direct the formation of cell-to-cell connections—called synapses—that control the flow of information through the brain. An understanding of normal synapse formation may lead to improved treatment of conditions, such as epilepsy and drug addiction, that are characterized in part by too many synapses.
Research by Dr. Ben Barres, Dr. Karen Christopherson, and colleagues at Stanford University shows that proteins called thrombospondins, which are produced by cells called glia, are essential for the development of the circuitry that connects the brain's nerve cells, or neurons. Although glial cells comprise 90 percent of the cells in an adult brain, their role remains largely a mystery, Dr. Barres says. "We knew that neurons would not survive without glia to support them. Now we see that very few synapses will form without the thrombospondins produced by glial cells." Thrombospondins are produced most abundantly during mammalian brain development, especially during the surge in brain growth that begins after birth and continues through infancy. Low levels of the protein in the mature brain correspond with a poor ability to form new synapses.
To study the role of glial cells in neuron growth and maintenance, the investigators first identified a type of nerve cell that would grow in a laboratory dish without a supporting culture of glial cells. They discovered that one type of rat neuron—retinal ganglion cells, which transmit visual information from the retina to visual centers of the brain—survived and divided under these conditions. The researchers then added a layer of glial cells to the culture, and noted the changes. They found that retinal cells formed very few synapses when no glial cells were present, but synaptic formation increased seven-fold when glial cells were added.
The researchers next isolated individual proteins produced by glial cells. When they added two of the proteins, called thrombospondin 1 and thrombospondin 2, to retinal cell cultures with no supporting glial cells present, the retinal cells formed synapses just as efficiently as when they were exposed to a supporting culture of whole glial cells.
Reproduced with permission from Christopherson et al, 2005.Proteins Produced by Glial Cells Are Crucial to Development of Synapses. Rat retinal nerve cells grown in a culture containing no glial cells (Control) develop very few synaptic structures (indicated by bright staining). Nerve cells grown in culture containing glial cells and in cultures containing glial proteins thrombospondin 1 (TSP1) and thrombospondin 2 (TSP2) developed extensive synaptic structures.To confirm the role of thrombospondins in synaptic formation in living animals, the investigators developed a strain of mice lacking the genes necessary to produce thrombospondins 1 and 2. When investigators compared the adult brains of these mice to those of normal mice, they found 40 percent fewer synapses.
It is likely that other, as-yet-unidentified factors are necessary for the development of fully functional synapses, Dr. Barres says. A functional synapse consists of structures on two neurons: One is a sending (pre-synaptic) neuron that releases messenger chemicals called neurotransmitters; the second (postsynaptic) neuron is a receiver that detects the neurotransmitter and responds to the chemical message. The synapses Dr. Barres and his colleagues developed by exposure to thrombospondins 1 and 2 were structurally normal but functionally "silent"—the postsynaptic cells did not respond to neurotransmitters. Research continues with a search for factors that activate the postsynaptic terminals, Dr. Barres says. "We know that glia produce at least one other protein, which we have not yet identified, that is necessary for a fully functional synapse."
"Addictive drugs disrupt communication in the brain in part by altering the synapses," observes Dr. Jonathan Pollock of NIDA's Division of Basic Neuroscience and Behavioral Research. "Identifying and fully understanding the contribution made by thrombospondins could make possible the development of thrombospondin-based therapies to act on synapses and reverse or control the effects of drug addiction or other neurologic disorders."
Source
- Christopherson, K.S., et al. Thrombospondins are astrocyte-secreted proteins that promote CNS synaptogenesis. Cell 120(3):421-33, 2005. [Abstract]