Glial Cells Pass on Metabolites to Neurons

Photo: Glial cells

Electron microscope cross-section
image of the nerve fibres (axons)
of the optic nerve. Axons are
surrounded by oligodendrocytes;
© MPG

Around 100 billion neurons in the human brain enable us to think, feel and act. They transmit electrical impulses to remote parts of the brain and body via long nerve fibres known as axons. This communication requires enormous amounts of energy, which the neurons are thought to generate from sugar. Axons are closely associated with glial cells which, on the one hand, surround them with an electrically insulating myelin sheath and, on the other hand support their long-term function.

Oligodendrocytes are a group of highly specialised glial cells in the central nervous system. They are responsible for the formation of the fat-rich myelin sheath that surrounds the nerve fibres as an insulating layer. The comparison with the coating on electricity cables is an obvious one; however, myelin can do much more than act as the insulating layer on electricity cables: it increases the transmission speed of the axons and also reduces ongoing energy consumption. The extreme importance of myelin for a functioning nervous system is shown by the diseases that arise from a defective insulating layer, such as multiple sclerosis.

Interestingly, the function of the oligodendrocytes goes far beyond the mere provision of myelin. Nave and his team already succeeded in demonstrating years ago that healthy glial cells are also essential for the long-term function and survival of the axons themselves, irrespective of myelination. “The way in which the oligodendrocytes functionally support their associated axons was not clear to us up to now,” says Nave. In a new study, the researchers were able to show that the glial cells are involved in, among other things, the replenishment of energy in the nerve fibres. “They could be described as the petrol stations on the data highway of the axons,” says Nave, explaining the results.

But how does the energy refuelling work? Is there a metabolic connection between the oligodendrocytes and axons? To find out, Ursula Fünfschilling generated genetically modified mice: the function of the mitochondria was deliberately disrupted in the oligodendrocytes through the inactivation of the Cox10 gene. This affects the final stages of sugar breakdown taking place in the mitochondria where energy is harnessed – a process known as the respiratory chain.

And this is precisely what the scientists observed in their mice: the animals’ myelin was initially formed in the normal way. The loss of the mitochondrial respiratory chain, which started at this point, did not appear to affect the glial cells in the central nervous system. Even one year later, there were no neurodegenerative changes in the brain to be observed. The scientists assume that in the early weeks of life – a phase characterised by maximum energy requirement – the mutated oligodendrocytes still rely on many intact mitochondria.

MEDICA.de; Source: Max Planck Institute for Experimental Medicine