How astrocytes help neurons survive
Energy utilization by brain tissue
In the absence of starvation the cells of the brain rely almost entirely on blood glucose for energy capture in the form of adenosine triphosphate (ATP). The human brain consumes a large portion of the body’s oxygen and blood glucose supply. It represents only 2% of the body’s total mass, but it consumes 20% of oxygen and glucose supplied by blood.
While both neurons and astrocytes take up glucose from blood, co-operation between these two cell types is essential for its efficient use in support of neuronal synapse transmission. But, before we discuss mutual energy support systems in operation between neurons and astrocytes we will talk about the location of astrocytes in the brain.
Where in the brain astrocytes are found
All central nervous system (CNS) regions are uniformly populated with non-overlapping astrocytes.
Astrocytes outnumber neurons by over five-fold and their turnover rate is low. Astrocytes are divided into two main types, protoplasmic and fibrous.
Protoplasmic astrocytes are located in brain grey matter. Fibrous astrocytes are found in the white matter of the brain.
Protoplasmic astrocytes have several stem like branches that give rise in turn to many finely branching processes that form a globe-like structure. In contrast fibrous astrocytes appear as an assembly of long filaments. The very large cell bodies in the right part of the brain tissue section displayed above are neurons. The smaller cell bodies near the neurons are astrocytes, and the very tiny cell bodies are microglia.
In the adult brain, in the grey matter of the central brain structure known as the hippocampus and in the brain cortex many finely branching processes from a single protoplasmic astrocyte are estimated to contact up to 600 dendrites of multiple neurons enveloping 100,000 or more synapses.
Fibrous astrocytes in the white matter of the brain make contact with nerve axons at the Nodes of Ranvier. Both types of astrocytes also make extensive contact with blood arterioles and capillaries.
Astrocytes surrounding neuronal synapses
Some protoplasmic astrocytes blanket synapses so closely that they contribute to a structure called the “tripartite synapse”.
At this type of neuronal synapse astrocytes receive neurotransmitter input at their plasma membrane proteins. Expression of specific neurotransmitter binding proteins on astrocyte plasma membrane is matched to the neurotransmitter released in their immediate environment.
This arrangement allows astrocytes to monitor activity and energy needs of the neurons it is supporting, and to maintain the fluid, ion, and pH level of the synaptic interstitial fluid critical for brain homeostasis.
Networks of connected astrocytes
Another important feature of astrocyte structure is that the most distal tips of their long processes can meet and be joined together by gap junctions. Gap junctions directly link the cytoplasm of multiple astrocyte cells forming a network through which ions and molecules can pass freely between cells.
In various parts of the brain large cellular networks of astrocytes can reach a millimeter in size, encompassing hundreds to thousands of astrocytes. Astrocyte networks are organized into anatomical and functional compartments in different brain areas similar to neuronal networks.
Energy co-operation between astrocytes and neurons
A major role played by astrocytes is to capture nutrient molecules from blood and convert them to a form that supports the high energy needs of neuronal synapses.
Both astrocytes and neurons can take up glucose from blood. However, it is astrocytes that regulate blood flow to neurons. They do this by releasing molecules that cause either vasoconstriction or vasodilation of the blood vessels.
Only astrocytes can store glucose that they extract from blood in the form of glycogen. Glycogen is a storage form of glucose available at times of intense activity or fasting. However, the energy management system at work between neurons and astrocytes is more complicated than mere uptake and storage of glucose.
The best studied system for energy management in brain is the dynamic collaboration between astrocytes and excitatory glutamate neurons. The neurotransmitter glutamate released at neuronal synapses is mainly retrieved from the synapse by astrocytes rather than by the neurons.
The primary astrocyte glutamate transporters in human brain are the ATP dependent Na+/glutamate co-transporters EAAT1 and EAAT2. These two transporters consume ATP and co-transport 1 glutamate molecule with 3 Na+ ions into astrocytes, an energy expensive process.
To compensate for the loss of glutamate from the neuron, astrocytes synthesize glutamine de novo from pyruvate, the end product of glucose breakdown in the cytosol, and export it to the neuron. About 1/3 of glutamate taken up by astrocytes is also converted to glutamine by astrocytes and returned to the neuron in that form. Within the neuron glutamine is converted to glutamate and repackaged into vesicles for synaptic release.
One theory is that this arrangement provides an energy advantage for neurons. The uptake of glutamate from the synapse is an energy expensive process. Also the de novo synthesis of glutamate requires a drain of intermediates from the tricarboxylic acid (TCA) energy cycle in mitochondria.
Without TCA cycle intermediates the amount of ATP that can be made from glucose by the neuron is reduced. A neuron in this co-operative arrangement with astrocytes maintains its TCA energy cycle components and can use all its own glucose for aerobic ATP production.
White matter fibrous astrocytes are located close to oligodendrocytes and they have different tasks than brain grey matter protoplasmic astrocytes that surround neuronal dendrites. The energy demands of white matter of the brain are much lower than those of brain grey matter. Some studies have measured differences in energy demand between these two regions as high as 12-fold.
White matter fibrous astrocytes make extensive contact with blood vessels and also form networks with gap junctions. They supply glucose to the axon and maintain extracellular fluid properties of the white matter.
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Margaret Thompson Reece PhD, physiologist, former Senior Scientist and Laboratory Director at academic medical centers in California, New York and Massachusetts and CSO at Serometrix LLC is now CEO at Reece Biomedical Consulting LLC.
Dr. Reece is passionate about helping students, online and in person, pursue careers in life sciences. Her books “Physiology: Custom-Designed Chemistry” (2012), “Inside the Closed World of the Brain” (2015) and the workbook (2017) companion to her online course “30-Day Challenge: Craft Your Plan for Learning Physiology” are written for those new to life science. More about her books can be found at amazon/author/margaretreece.
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