Recent Highlights

Publications after retirement

On October 1, 2011, Dr. Sieghart (at that time, age 66) officially retired from his position as Head of the Department of Biochemistry and Molecular Biology of the Nervous System, at the Center for Brain Research, Medical University Vienna, and became Guest Professor at this Center.

Nevertheless he continued to direct his Department until July 25, 2013, when his successor, Tibor Harkany, was appointed Professor at the Center for Brain Research and Head of the Department. At that time, the Department was renamed into „Department of Molecular Neurosciences“.

In the meantime, several scientific papers were published by Dr. Sieghart that were initiated and partially completed before his final retirement on July 25, 2013. The most important of these papers are listed in the section „Main contribution to research„.

Other important papers, however, were completed by Dr. Sieghart only after July 25, 2013. They deal with three different topics:

a) GABAA receptor subunit distribution in the human brain

b) alpha6-GABAA receptors and neuropsychiatric diseases

c) up-to-date reviews of the GABAA receptor field

 

a) GABAA receptor subunit distribution in the human brain

The excellent and highly specific antibodies against GABAA receptor subunits generated by Dr. Sieghart, resulted in multiple important and highly cited papers on the regional, cellular, and subcellular distribution of GABAA receptor subunits in the brain of rats and mice before his final retirement in 2013 (see „Main contribution to research„).

In the last years before his retirement, Ivan Milenkovic and Harald Stefanits, two MD-PhD students of Dr. Sieghart, investigated whether GABAA receptor subunit-specific antibodies suitable for immunohistochemistry in the rodent brain, can also be used in the human brain. Some of the antibodies were suitable for that purpose, others were not. This resulted in major efforts of Dr. Sieghart and his students to newly generate and characterize GABAA receptor-selective antibodies suitable for immunohistochemistry in the human brain. The first applications of these antibodies resulted in the publications

– Stojanovic et al., (2016) J Comp Neurol 524, 1805-1824 (16 citations)

This paper for the first time investigated the expression pattern of several GABAA receptor subunits in human cerebellum and adjacent brain structures not only during brain development but also in the adult human brain. This study supported previous hints that the distribution of GABAA receptor subunits might be partially different in rodents and humans.

This important finding partially explains some of the discrepancies observed in the behavioral effects of GABAA receptor subtype-selective drugs in rodents and humans. In addition, these data emphasize that the regional and cellular distribution of GABAA receptor subtypes in the human brain, but not that in the mouse or rat brain, should be used as a guideline for understanding the human GABAergic system.

Stefanits et al., (2018) J Comp Neurol 526, 324-348

This is the first detailed investigation of the immunohistochemical distribution of seven GABAA receptor subunits in the human amygdala. This important brain region regulates anxiety, stress, reward, mnestic functions, addiction and is involved in epilepsy. Again there were differences in the GABAA receptor subunit expression between human and rodent amygdala, again indicating that the human and not the rodent subunit distribution should be considered when developing subunit-selective drugs that can be used to modulate anxiety without inducing sedation!

 

b) alpha6-GABAreceptors and neuropsychiatric diseases

The identification of the alpha+beta- interface of GABAA receptors as a novel target for subtype-selective drugs by the group of Dr. Sieghart

Ramerstorfer et al., 2011, J. Neurosci. 31, 870-877 (81 citations)

resulted in the identification of the first ligands that were highly selective positive allosteric modulators of a6 subunit-containing GABAA receptors

Varagic et al., 2013, Br. J. Pharmacol. 169, 384-399 (34 citations)

 

At that time, the biological function of alpha6-containing GABAA receptors in the brain was not known. Nevertheless, the identification of alpha6-GABAA receptor-selective ligands was the starting point for an exciting collaboration with Lih-Chu Chiou from the University of Taiwan. LC Chiou had some evidence that the flavonoid hispidulin isolated from a local herb that remitted a patient´s intractable motor tics, might exert its action via alpha6-containing GABAreceptors. She read our article and contacted the group of Dr. Sieghart for a possible collaboration. Together with the group of Jim Cook, who synthesized the required highly selective positive modulators of alpha6-containing GABAreceptors, Dr. Sieghart started a collaboration with LC Chiou.

Already the first experiments of LC Chiou supported the conclusion that the alpha6-receptor-selective compounds, that in contrast to the classical benzodiazepines were devoid of any sedative, hypnotic, anxiolytic and muscle relaxant properties, are able to excert actions similar to those of hispidulin in assays mimicking tic-diseases. This indicated that alpha6-GABAreceptor-selective compounds could possibly be used for the treatment of tic-disorders. In contrast to the highly alpha6-selective compounds identified by Varagic et al., 2013, hispidulin is quite non-selective and also able to interact with most of the other GABAreceptor subtypes. It was thus decided to concentrate on alpha6-GABAreceptor-selective compounds for a future drug development.

Based on these preliminary results, an international consortium was assembled, consisting of the groups of Jim Cook (University of Wisconsin, Milwaukee, Wisconsin, USA), LC Chiou (National Taiwan University, Taipei, Taiwan), and Werner Sieghart and Margot Ernst (Center for Brain Research, Vienna, Austria). Margot Ernst, appointed by Tibor Harkany, was the successor of W. Sieghart as director of the GABAreceptor research group at the Center for Brain Research. The group of Jim Cook synthesized deuterated alpha6-selective compounds to increase their metabolic stability and bioavailability. Margot Ernst investigated their alpha6-selectivity in electrophysiological experiments at various recombinant GABAA receptor subtypes. LC Chiou investigated whether these compounds could ameliorate a disrupted prepulse inhibition of the startle response (PPI), an animal model mimicking tic-disorders.

alpha6-containing GABAA receptors might also be involved in other neuropsychiatric disorders:

Soon it became clear that a disruption in the prepulse inhibition of the startle response (PPI) not only is involved in tic-disorders, but is also manifested in patients with other neuropsychiatric disorders, such as obsessive-compulsive disorder, attention deficit disorder, panic disorder, nocturnal enuresis, Huntington´s disease, premenstrual dysphoric disorder, mania in bipolar disorder, antisocial personality disorder and schizophrenia. Especially in patients with schizophrenia, PPI disruption is a well-known endophenotype manifestation.

In addition, other evidence indicated that GABAreceptors containing alpha6-subunits also might play a role in neuropathic pain and in migraine, and possibly also in depression. Experiments investigating a possible role of alpha6-GABAreceptors in neuropathic pain led to the inclusion of Miroslav M. Savic, Dept. Pharmacology, Faculty of Pharmacy, University of Belgrade, Belgrade, Serbia, into the consortium. Experiments investigating a possible role of alpha6-GABAreceptors in migraine, led to the inclusion of Pi-Chuan Fan, Division of Neurology, Department of Pediatrics, National Taiwan University Hospital, Taipei, Taiwan, into the consortium.

Finally, the group of Marko M. Mihovilovic from the Institute of Applied Synthetic Chemistry of the Vienna University of Technology also joined the consortium. Based on the results of Varagic et al., 2013, mentioned above, Margot Ernst and Marko Mihovilovic had some idea on how to generate additional compounds that might be selective modulators of alpha6-GABAreceptors. W. Sieghart financed an initial pilot study to enable their synthesis by the group of Marko Mihovilovic. These and subsequent synthetic efforts led to some other novel alpha6-selective positive allosteric modulators, that later on were included into the patent application.

To coordinate all these different groups, to clarify and communicate their mutual requirements, to determine priorities and ensure a smooth development of the project, Dr. Sieghart was elected as the coordinator of this consortium, aiming for a future patent application on alpha6-selective ligands and their possible clinical application.

The provisional patent application was filed June 3, 2016, in the USA

PCT/US2016/035761

The patent application was then published on May 17, 2018, Pub. No.: US 2018/0134698 A1

020871-9110-US02 Published Application — 23433488 v1

 

So far, work on this patent application resulted in three publications with Dr. Sieghart as a co-author. Dr. Sieghart together with the respective first author was significantly involved in writing of these three papers and strongly contributed to the development of the hypotheses put forward to explain the action of the positive allosteric modulators of alpha6-GABAreceptors in the respective animal models.

Knutson et al., (2018) J Med Chem 61, 2422-2446

In this paper, the design, synthesis, basic pharmacology, and behavioral actions of novel deuterated ligands functionally selective for alpha6-GABAreceptors, is described. Depending on the position of the introduced Deuterium, compounds with improved metabolic stability and enhanced bioavailability were obtained in a non-predictable way.

Chiou et al., (2018) Br J Pharmacol 175, 2414-2427

Here, for the first time evidence is provided indicating that cerebellar alpha6-GABAreceptors are novel therapeutic targets for ameliorating disrupted prepulse inhibition in neuropsychiatric disorders. In this important paper, for the very first time also a function of alpha6-containing receptors in cerebellum is described and a hypothesis is put forward for the molecular mechanism of action of alpha6-GABAreceptors in cerebellar function and in disrupted prepulse inhibition.

Fan et al., (2018) Neuropharmacol 140, 1-13

Here, it is demonstrated that enhancing the activity of alpha6-GABAA receptors can inhibit a trigeminal vascular system activation that is involved in the pathogenesis of migraine in a way similar to topiramate, a cliniclly effective anti-migraine agent. These results suggest that alpha6-GABAreceptor-selective positive allosteric modulators have the potential to be developed as a novel pharmacotherapy for migraine.

 

c) Important recent reviews of the GABAreceptor field

Sieghart W (2014) GABAreceptors in the brain: exciting targets for the development of clinically important drugs. Magyar Tudományos Akademia 2014. ISSN 1419-8959, ISBN 978-963-508-779-2.

This is a historical description on how and why Dr. Sieghart entered the field of GABAreceptor research and how he developed his own research line

This review discusses the so far identified drug binding sites at GABAA receptors and for the first time also available evidence that most if not all drugs interacting with GABAreceptors can do that via multiple binding sites at the same receptor.

free reprints can be downloaded from

http://pharmrev.aspetjournals.org/cgi/reprint/70/4/836.pdf?ijkey=wtW8XYX1J4hQsDs&keytype=finite

This review critically discusses the methodology used to investigate GABAreceptor subtype-selectivity and provides the chemical structures and published concentration-response curves from electrophysiological experiments of more than 40 compounds claimed to be GABAreceptor subtype-selective. Evidence is provided that most of these compounds are not receptor subtype-selective at all under in vivo conditions. The discrepancy in the behavior of some of these compounds between rodents and man is discussed and tentatively explained and strategies for the development of compounds with much better receptor subtype-selectivity are proposed.

Finally, this comprehensive dataset can now be used for selecting combinations of suitable compounds that can help to draw valid conclusions on the function of GABAreceptor subtypes in the brain.

This article has been recommended in F1000Prime as being of special significance in its field by F1000 Faculty Member Hanns Möhler.

https://f1000.com/prime/734202313?bd=1