Most of the biochemical understanding of MDD is a product of the first study conducted by Takahashi (1977); this landmark paper was the first paper to correlate elevated levels of monoamine oxidases (MAOs) with MDD. MAOs are flavin containing enzymes responsible for the catalytic activity of breaking down monoamines. There are two isoforms of MAO: MAO-A and MAO-B. MAO-A is the isoform that is able to bind to monoamine neurotransmitters such as serotonin, dopamine and norepinephrine; MAO-B also binds to dopamine, but primarily targets phenylethylamine and benzylamine as substrates (Duncan et al. 2012). When considering MDD, the isoform of MAO that is studied is MAO-A and not MAO-B, since MAO-A is the one responsible for the breakdown of monoamine neurotransmitters.
As previously stated, both MAOs are responsible for the breakdown of monoamines. This breakdown of monoamines is accomplished in the reaction shown in Figure 5. Monoamines lose their NH3+ ground and produce H2O2as well as NH4+. This has been shown to be the reaction that both MAO-A and MAO-B are involved in. The generation of H2O2 is generally bad for the cell, as an increase in H2O2 can lead to the generation of reactive oxygen species (ROS) and can also lead to neuronal/mitochondrial death (Duncan et al. 2012). It is logical to assume that in MDD where there is a higher amount of MAOs (both MAO-A and MAO-B) leading to a higher amount of produced ROS. Even though the overall reaction of MAOs are understood, the mechanism by with MAO-A performs this reaction is still not entirely understood. Mechanisms and crystal structures have been suggested for MAO-B; however the difference in binding substrates of the two isoforms of the enzyme suggests that the structure and mechanism of both MAOs might not be identical.
Structural Analysis of MAO-A
In order to see a possible mechanism of action for MAO-A, a study conducted by Ma et al. (2004) attempted to crystalize MAO-A from rats. This research was important to the study of MDD because there had not been a previously crystalized MAO-A structure; the researches would be able to compare structural similarities and differences between crystal structures for rat MAO-B and human MAO-B. After obtaining a crystal structure with a resolution of 3.2 angstroms, the researchers were able to identify a few key structural features of this enzyme. The first structural feature identified was the dimerization of MAO-A. In order for the enzyme to be catalytically active, the enzyme must bind to an identical enzyme to create the dimer. This information is useful for understanding of MAO-A works and how it could potentially be inhibited, however the researchers were not able to identify specific residues that were involved in the dimerization. The authors suggest that the overall folding of the dimer structure is similar to previously shown MAO-B structures. The researchers were also able to identify the binding region for FAD on MAO-A, and were able to identify that MAO-A binds FAD at a cysteine residue, a similar reaction seen in MAO-B. The binding of an inhibitor to MAO-A was shown to be a product of the inhibitor binding to FAD in the enzyme. The authors were able to identify two residues that form covalent bonds to the FAD (Cys323 and Thr326). Superimposing previously determined crystal structures for MAO-B shows that the binding pocket of inhibitors in these enzymes is located in the same place (although the binding itself is different). This gathered information is very useful for understanding how an inhibitor will bind to MAO-A. Being able to identify residues in MAO-A allows for further research to target these areas. This work is not done entirely, as there is still some work to be desired. Ma et al. (2004) were not able to crystalize the enzyme with its substrate, so any interaction between a monoamine and MAO-A is still not understood. This is unfortunate, as knowing what kinds of interactions MAO-A has with monoamine neurotransmitters could lead to a production of MAO-A specific drugs. Hopefully, more crystal structure research will become available to understand the actual mechanism of MAO-A; for now, the mechanism is still not fully understood.
Transcription of MAO-A
Even though the mechanism of MAO-A is not fully understood, research within the past 4 years has been more focused on why there is more MAO-A production. Johnson et al. 2011 suggests a possible reason for increased levels of MAO-A instead of just saying that MAO-A is elevated. After the discovery of a novel repressor protein of MAO-A, R1, the scientific community was still not entirely sure that this repressor protein was involved in MDD.
As shown in Figure 6, the researchers were able to show that patients that had MDD and patients that had MDD and were being treated with antidepressants not only had lower amounts of R1 but also had an elevated amount of MAO-A. Further statistical analysis shows that there is a negative correlation between R1 and MAO-A. This research is some of the first research in the field to offer an explanation as to why there is an increase in MAO-A, also allowing for a new target of research. More work with R1 could result in a decrease in MAO-A in patients with MDD.
Although the study by Johnson et al. (2011) is one of the first studies to show why there is more MAO-A, another study by Grunewald et al. (2012) suggests another transcription factor associated with overexpression of MAO-A. KLF11 was identified in their study to be a direct transcriptional activator in times of stress.
In Figure 7 we see that (through western blot and RT-PCR) increased levels of KLF11 increase both MAO-A production and catalytic activity. By inhibiting KLF11 using siRNA, the researchers were able to also show that there is a decrease in MAO-A production as well as MAO-A activity. A hat protein p300 was also shown to have a positive effect on MAO-A transcription and activity. This brilliant study was able to show the effects of KLF11 on MAO-A in a human neuroblastoma cell. Probably one of the best MAO-A papers to come out in a while, this work is extremely important in understanding the reason why MAO-A is overexpressed in MDD. Work with R1 could reveal the actual mechanism of the disease, giving a target to treatment research and stopping the overexpression of MAO-A.
Other Substrates for MAO-A
Some of the most recent work involving MAO-A has been focusing not on why more of it is made, but on what kinds of reactions MAO-A is doing beside the deamination of serotonin, dopamine and norepinephrine. In a recent study by Goldberg et al. (2014), the effects of MAO-A on GABA (a neurotransmitter associated with anxiety and insomnia). The monoamine GABA was shown to be dominated by MAO-A using high performance liquid chromatography (HPLC). The reason why GABA might be able to bind to MAO-A is because of the existence of a very lipophilic version of GABA at the pH of human cerebral spinal fluid (CSF); this lipophilic version might bind to MAO because it mimics the lipophilic moieties of other monoamine neurotransmitters. This preliminary data suggests another target of MAO-A, and an overexpression might be deaminating this neurotransmitter as well as serotonin, norepinephrine and dopamine. The authors suggest that the reason drugs that focus on the reuptake of serotonin and norepinephrine might fail is because MAO-A is still able to deaminate GABA, leading to a decrease in that neurotransmitter followed by MDD.
Although there has been no defined mechanism associated with MAO-A, its effects on MDD are still being studied today. Understanding the regulation of MAO-A production as well as the targets of MAO-A can provide useful mechanical information on how this disease works.