MND and Me Foundation Research Grant – Update Nov 2017

Recipient:  Prof David Pow

Much of the signaling between the nerve cells in our brain is performed by the release and uptake of an amino acid called glutamate. However, if there is too much glutamate the nerve cells can be overstimulated and ultimately die.

This effect is called excitotoxicity which is one of the things that goes wrong in motor neurone disease (MND).

Funding from MNDRIA has enabled us to study a novel protein called EAAT5 that acts both as a glutamate transporter (removing glutamate from places where it acts as a neurotransmitter) and as an “off switch” to neurotransmission (a molecular switch that stops the release of glutamate from nerve cells).

In our initial experiments, we observed a loss of EAAT5 in the spinal cords of SOD1 mutant mice, which are a recognised mouse model of MND. We proposed that a loss of EAAT5 would lead to nerve cells becoming overstimulated due to elevated levels of glutamate – an event that would likely lead to the death of these cells.

We have now moved our studies to an examination of human brains. We have been excited to gain access to brain tissues from deceased donors who have had either no neurological abnormalities (controls) or those who have had MND.

This very rare resource has allowed us to look at the expression of EAAT5 in humans, to now determine if the levels of the protein or the messenger RNA that codes for the protein, are abnormal in people with MND.

We have performed molecular biology experiments and demonstrated for the first time that EAAT5 messenger RNA is indeed abundantly expressed in the human brain (Figure 1).

This is a novel discovery in itself since the literature universally describes EAAT5 as being restricted to the retina. We have also shown that EAAT5 is abundantly expressed at the protein level in the motor cortex of control brains, however, significant losses of this protein was observed in MND subjects (Fig 2).

This suggests that regulation of glutamate levels in the MND brain is likely to be impaired, leading to excitotoxicity and death of motor neurons.

We are currently examining the anatomical localisation of this protein in human brains, including tissues from donors afflicted with MND, and performing experiments to get an understanding of the way EAAT5 operates at the molecular level in normal and diseased states. Our initial observations on human motor cortex show that EAAT5 is localised to populations of neurons including the large pyramidal neurons.

Fig. 3 Human pyramidal neuron expressing immunoreactivity (brown stain) for EAAT5.

This expression of EAAT5 by these glutamatergic neurons that provide the long-range signals from the cortex makes the protein ideally placed to regulate the outputs from the motor cortex.

Our future thinking centres around understanding the physiology of cortical EAAT5 – seeking ways to modulate EAAT5, and hopefully influencing functional outcomes.