Disturbed neurotransmitter homeostasis in ether lipid deficiency

Dorninger F, Kӧnig T, Scholze P, Berger ML, Zeitler G, Wiesinger C, Gundacker A, Pollak DD, Huck S, Just WW, Forss-Petter S, Pifl C, and Berger J. (2019) Disturbed neurotransmitter homeostasis in ether lipid deficiency. Human Molecular Genetics

Neurotransmission requires neurotransmitters to be released from a presynaptic axon into the synaptic cleft where they can bind to receptors on the dendrites of a postsynaptic neuron. Within the presynaptic neuron, neurotransmitters are first packaged within a vesicle that attaches to and fuses with the membrane of the presynaptic axon, releasing the neurotransmitters into the synapse. Although there are many proteins involved in this process, the lipid composition of the neuronal membrane also has an important role. Specifically, a class of lipids containing a vinyl-ether bond called plasmalogens are known to be involved in membrane structure and vesicular fusion. A plasmalogen deficiency caused by the inability to synthesize vinyl-ether lipids produces a class of disorders called rhizomelic chondrodysplasia punctata (RCDP). With a prevalence of 1:100 000 live births and less than 100 known cases in North America, RCDP is a very rare disorder. To analyze the role of plasmalogens in neurotransmitter homeostasis, Dorninger et al used a glycerophosphate O-acyltransferase (Gnpat) genetic knockout (KO) mouse model. These animals are unable to synthesize plasmalogens due to a mutation in one of the initial proteins in the biosynthesis pathway.

To confirm whether the plasmalogen deficiency in the Gnpat KO mice caused a reduction in neurotransmitter levels, the team used high-performance liquid chromatography (HPLC) to determine the levels of dopamine, norepinephrine, serotonin, gamma aminobutyric acid (GABA), glycine, glutamate, and taurine. Dopamine was found at the highest levels in the tissues but was decreased by 24% in the Gnpat KO brain tissue; norepinephrine and serotonin were decreased similarly at a reduction of 28% and 23%, respectively, compared to the wild-type tissues. Although GABA showed a decrease of 20% in Gnpat KO, glycine and glutamate did not show significant differences between the two genotypes. Interestingly, taurine showed an increase of 12% in the Gnpat KO brain tissues compared to the wild-type samples. However, since it is released from the cell soma and not vesicles, it is unsurprising that it did not follow a similar trend to the other neurotransmitters. To determine whether these reductions were consistent across the entire brain or occurred in specific regions, GABA, taurine, norepinephrine, and serotonin levels were detected within parietal cortex, striatum, and substantia nigra. GABA was decreased within the parietal cortex and substantia nigra by ~20%, while serotonin and norepinephrine were reduced in the parietal cortex by 37% and 46%. No significant difference was found in these neurotransmitters within the striatum. Unlike these neurotransmitters where the differences were found to be region-dependent, taurine showed a 15-20% increase in all areas of the brain studied.

To determine whether the decrease in neurotransmitter levels is due to an increase in turnover of the monoamines, a metabolite/transmitter ratio was analyzed. Turnover was studied in both the mouse tissues and in human post-mortem brain tissues from individuals born with ether lipid deficiency (either RCDP or Zellweger spectrum disorder, a group of peroxisomal disorders where various peroxisomal metabolic pathways are affected, including those involved in plasmalogen biosynthesis) or NIH NeuroBioBank controls. Within both the mouse and human samples, a trend of increased monoamine turnover of serotonin and dopamine was present within the parietal cortex of the lipid deficient tissues compared to the wild-type controls. Interestingly, glutamate was increased within the parietal cortex and the caudate nucleus of the lipid-deficient patients, which could be the result of an increase of astrocytosis in Zellweger spectrum patients. Glutamate is known to be present at high amounts in astrocytes, therefore, if there is an increase in the production of astrocytes, this could explain an increase in glutamate.

Vesicles are crucial for the transport of neurotransmitters and the ability of the vesicular monoamine transporter (VMAT2) to package neurotransmitters from the cytoplasm into vesicles to then be transported out of the cell is essential. To investigate whether the reduction in neurotransmitters seen was the result of decreased levels of vesicular uptake, the selective ligand dihydrotetrabenazine (DTBZ) was used in a radioactive binding assay to measure levels of VMAT2. DTBZ binding was found to be reduced by 50% in the Gnpat KO mice compared to the wild-type mice, indicating a decrease in VMAT2 and a reduction in the uptake of cytosolic neurotransmitters. Impaired binding is unsurprising as plasmalogen membrane content is known to affect the activity of membrane associated proteins, like VMAT2.

Dorninger et al have demonstrated that there is a distinct reduction in many neurotransmitters throughout the brains from plasmalogen deficient mice. Neurotransmitter levels are influenced by many factors and because of this, the complete cause for this decrease is not known. This study shows that some of these decreases are region-specific or due to an increase in monoamine turnover. Further, the reduction of VMAT2 in Gnpat KO mice would prevent the neurotransmitters from being packaged into vesicles adequately, impairing neurotransmission. In addition to the biochemical assays described, behavioral assays showed that the Gnpat KO mice were more hyperactive, displayed stereotypy, had reduced social interaction, and did not show the same length of freezing time after being presented with the fear stimulus. These behavioral tests provide further evidence that the Gnpat KO mice are unable to maintain neurotransmitter homeostasis by demonstrating these downstream results of improper neurotransmitter signaling. Due to the importance of plasmalogens in maintaining normal membrane structure, vesicle formation, and vesicular fusion, this decrease in neurotransmitters is unsurprising. Further research investigating the downstream effects of a plasmalogen deficiency will provide more information about the roles plasmalogens have in different disorders.