Bio

Supervisors


Honors & Awards


  • NARSAD Young Investigator Award, Brain & Behavior Research Foundation (2014-2015)
  • HFSP Long Term Fellow, Human Frontier Science Program (2009-2012)
  • Dean's Fellowship, Stanford University (2009)
  • EMBL Predoctoral Fellow, European Molecular Biology Laboratory (2003-2007)

Education & Certifications


  • Ph.D., University of Heidelberg and European Molecular Biology Laboratory, Neuroscience (2007)
  • M.Sc., University of Patras, Industrial Pharmacy (2001)
  • B.Sc., University of Patras, Biology (1999)

Publications

Journal Articles


  • Rab3B protein is required for long-term depression of hippocampal inhibitory synapses and for normal reversal learning PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Tsetsenis, T., Younts, T. J., Chiu, C. Q., Kaeser, P. S., Castillo, P. E., Suedhof, T. C. 2011; 108 (34): 14300-14305

    Abstract

    Rab3B, similar to other Rab3 isoforms, is a synaptic vesicle protein that interacts with the Rab3-interacting molecule (RIM) isoforms RIM1? and RIM2? as effector proteins in a GTP-dependent manner. Previous studies showed that at excitatory synapses, Rab3A and RIM1? are essential for presynaptically expressed long-term potentiation (LTP), whereas at inhibitory synapses RIM1? is required for endocannabinoid-dependent long-term depression (referred to as "i-LTD"). However, it remained unknown whether i-LTD also involves a Rab3 isoform and whether i-LTD, similar to other forms of long-term plasticity, is important for learning and memory. Here we show that Rab3B is highly enriched in inhibitory synapses in the CA1 region of the hippocampus. Using electrophysiological recordings in acute slices, we demonstrate that knockout (KO) of Rab3B does not alter the strength or short-term plasticity of excitatory or inhibitory synapses but does impair i-LTD significantly without changing classical NMDA receptor-dependent LTP. Behaviorally, we found that Rab3B KO mice exhibit no detectable changes in all basic parameters tested, including the initial phase of learning and memory. However, Rab3B KO mice did display a selective enhancement in reversal learning, as measured using Morris water-maze and fear-conditioning assays. Our data support the notion that presynaptic forms of long-term plasticity at excitatory and inhibitory synapses generally are mediated by a common Rab3/RIM-dependent pathway, with various types of synapses using distinct Rab3 isoforms. Moreover, our results suggest that i-LTD contributes to learning and memory, presumably by stabilizing circuits established in previous learning processes.

    View details for DOI 10.1073/pnas.1112237108

    View details for Web of Science ID 000294163500084

    View details for PubMedID 21844341

  • alpha-Synuclein Promotes SNARE-Complex Assembly in Vivo and in Vitro SCIENCE Burre, J., Sharma, M., Tsetsenis, T., Buchman, V., Etherton, M. R., Suedhof, T. C. 2010; 329 (5999): 1663-1667

    Abstract

    Presynaptic nerve terminals release neurotransmitters repeatedly, often at high frequency, and in relative isolation from neuronal cell bodies. Repeated release requires cycles of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-complex assembly and disassembly, with continuous generation of reactive SNARE-protein intermediates. Although many forms of neurodegeneration initiate presynaptically, only few pathogenic mechanisms are known, and the functions of presynaptic proteins linked to neurodegeneration, such as ?-synuclein, remain unclear. Here, we show that maintenance of continuous presynaptic SNARE-complex assembly required a nonclassical chaperone activity mediated by synucleins. Specifically, ?-synuclein directly bound to the SNARE-protein synaptobrevin-2/vesicle-associated membrane protein 2 (VAMP2) and promoted SNARE-complex assembly. Moreover, triple-knockout mice lacking synucleins developed age-dependent neurological impairments, exhibited decreased SNARE-complex assembly, and died prematurely. Thus, synucleins may function to sustain normal SNARE-complex assembly in a presynaptic terminal during aging.

    View details for DOI 10.1126/science.1195227

    View details for Web of Science ID 000282098100047

    View details for PubMedID 20798282

  • A Neural Switch for Active and Passive Fear NEURON Gozzi, A., Jain, A., Giovanelli, A., Bertollini, C., Crestan, V., Schwarz, A. J., Tsetsenis, T., Ragozzino, D., Gross, C. T., Bifone, A. 2010; 67 (4): 656-666

    Abstract

    The central nucleus of the amygdala (CeA) serves as a major output of this structure and plays a critical role in the expression of conditioned fear. By combining cell- and tissue-specific pharmacogenetic inhibition with functional magnetic resonance imaging (fMRI), we identified circuits downstream of CeA that control fear expression in mice. Selective inhibition of a subset of neurons in CeA led to decreased conditioned freezing behavior and increased cortical arousal as visualized by fMRI. Correlation analysis of fMRI signals identified functional connectivity between CeA, cholinergic forebrain nuclei, and activated cortical structures, and cortical arousal was blocked by cholinergic antagonists. Importantly, inhibition of these neurons switched behavioral responses to the fear stimulus from passive to active responses. Our findings identify a neural circuit in CeA that biases fear responses toward either passive or active coping strategies.

    View details for DOI 10.1016/j.neuron.2010.07.008

    View details for Web of Science ID 000281534600014

    View details for PubMedID 20797541

  • GABA homeostasis contributes to the developmental programming of anxiety-related behavior BRAIN RESEARCH Depino, A. M., Tsetsenis, T., Gross, C. 2008; 1210: 189-199

    Abstract

    During development, when inhibitory and excitatory synapses are formed and refined, homeostatic mechanisms act to adjust inhibitory input in order to maintain neural activity within a normal range. As the brain matures, synaptogenesis slows and a relatively stable level of inhibition is achieved. Deficits in inhibitory neurotransmission are associated with increased anxiety-related behavior and drugs that potentiate GABA function, the major inhibitory neurotransmitter in the brain, are effective anxiolytics. These observations raise the possibility that transient perturbations in the activity of neural circuits during development might induce compensatory changes in inhibition that could persist into adulthood and contribute to changes in anxiety-related behavior. To test this hypothesis, we treated mice continuously during the major period of forebrain synaptogenesis (P14-28) with the GABA-A receptor positive modulator diazepam and assessed anxiety-related behavior in adulthood. Control experiments confirmed anxiolytic effects of the drug following one day of treatment and the development of tolerance following two weeks of treatment. When tested in adulthood, one month after the end of treatment, diazepam-treated mice exhibited significantly increased behavioral inhibition in the open-field, elevated-plus maze, and novel object behavioral paradigms. Levels of benzodiazepine binding sites in amygdala and frontal cortex were specifically decreased in diazepam-treated mice demonstrating that homeostatic adjustments in GABA function persist into adulthood. Our results show that increased GABAergic activity can affect the developmental programming of anxiety-related behavior.

    View details for DOI 10.1016/j.brainres.2008.03.006

    View details for Web of Science ID 000256507200019

    View details for PubMedID 18407251

  • Suppression of conditioning to ambiguous cues by pharmacogenetic inhibition of the dentate gyrus NATURE NEUROSCIENCE Tsetsenis, T., Ma, X., Lo Iacono, L., Beck, S. G., Gross, C. 2007; 10 (7): 896-902

    Abstract

    Serotonin receptor 1A knockout (Htr1a(KO)) mice show increased anxiety-related behavior in tests measuring innate avoidance. Here we demonstrate that Htr1a(KO) mice show enhanced fear conditioning to ambiguous conditioned stimuli, a hallmark of human anxiety. To examine the involvement of specific forebrain circuits in this phenotype, we developed a pharmacogenetic technique for the rapid tissue- and cell type-specific silencing of neural activity in vivo. Inhibition of neurons in the central nucleus of the amygdala suppressed conditioned responses to both ambiguous and nonambiguous cues. In contrast, inhibition of hippocampal dentate gyrus granule cells selectively suppressed conditioned responses to ambiguous cues and reversed the knockout phenotype. These data demonstrate that Htr1a(KO) mice have a bias in the processing of threatening cues that is moderated by hippocampal mossy-fiber circuits, and suggest that the hippocampus is important in the response to ambiguous aversive stimuli.

    View details for DOI 10.1038/nn1919

    View details for Web of Science ID 000247560200020

    View details for PubMedID 17558402

  • Human kallikrein 6 activity is regulated via an autoproteolytic mechanism of activation/inactivation BIOLOGICAL CHEMISTRY Bayes, A., Tsetsenis, T., Ventura, S., Vendrell, J., Aviles, F. X., Sotiropoulou, G. 2004; 385 (6): 517-524

    Abstract

    Human kallikrein 6 (protease M/zyme/neurosin) is a serine protease that has been suggested to be a serum biomarker for ovarian cancer and may also be involved in pathologies of the CNS. The precursor form of human kallikrein 6 (pro-hK6) was overexpressed in Pichia pastoris and found to be autoprocessed to an active but unstable mature enzyme that subsequently yielded the inactive, self-cleavage product, hK6 (D81-K244). Site-directed mutagenesis was used to investigate the basis for the intrinsic catalytic activity and the activation mechanism of pro-hK6. A single substitution R80 --> Q stabilized the activity of the mature enzyme, while substitution of the active site serine (S197 --> A) resulted in complete loss of hK6 proteolytic activity and facilitated protein production. Our data suggest that the enzymatic activity of hK6 is regulated by an autoactivation/autoinactivation mechanism. Mature hK6 displayed a trypsin-like activity against synthetic substrates and human plasminogen was identified as a putative physiological substrate for hK6, as specific cleavage at the plasminogen internal bond S460-V461 resulted in the generation of angiostatin, an endogenous inhibitor of angiogenesis and metastatic growth.

    View details for Web of Science ID 000222500200010

    View details for PubMedID 15255184

  • Emerging interest in the kallikrein gene family for understanding and diagnosing cancer ONCOLOGY RESEARCH Sotiropoulou, G., Rogakos, V., Tsetsenis, T., Pampalakis, G., Zafiropoulos, N., Simillides, G., Yiotakis, A., Diamandis, E. P. 2003; 13 (6-10): 381-391

    Abstract

    Kallikreins are proteolytic enzymes that constitute a subfamily of serine proteases. Novel kallikrein genes were cloned recently, and it was shown that the human kallikrein family contains 15 genes tandemly aligned on chromosomal locus 19q13.3-q13.4. Based on their altered expression in tumor cells, kallikreins may be involved in the pathogenesis and/or progression of cancer. Evidence is presented that certain kallikreins may be exploited as diagnostic cancer biomarkers. Although the function(s) of novel kallikreins is currently unknown, increasing evidence suggests that kallikreins may participate in regulatory enzymatic cascade(s). Elucidation of the function of novel kallikreins largely depends on the availability of active recombinant proteins. Here, the zymogen for kallikrein 13 was overexpressed in Pichia pastoris and biochemically characterized. It was shown that the kallikrein 13 zymogen displays intrinsic catalytic activity leading to autoactivation. A clipped form of kallikrein 13 was identified, indicating autocatalytic cleavage at the internal bond R114-S115. Mature kallikrein 13 displays trypsin-like activity with restricted specificity on synthetic and protein substrates. Combinatorial P1-Lys libraries of tetrapeptide fluorogenic substrates were synthesized and used for the profiling of the P2 specificity of selected kallikreins. Interestingly, it was shown that human kallikrein 13, similarly to PSA, could specifically cleave human plasminogen to generate angiostatin-like fragments, suggesting that specific kallikreins may have antiangiogenic actions. An understanding of the physiology of human kallikreins is emerging with potential clinical applications.

    View details for Web of Science ID 000182358200013

    View details for PubMedID 12725528

  • The structure of human prokallikrein 6 reveals a novel activation mechanism for the kallikrein family JOURNAL OF BIOLOGICAL CHEMISTRY Gomis-Ruth, F. X., Bayes, A., Sotiropoulou, G., Pampalakis, G., Tsetsenis, T., Villegas, V., Aviles, F. X., Coll, M. 2002; 277 (30): 27273-27281

    Abstract

    Zyme/protease M/neurosin/human kallikrein 6 (hK6) is a member of the human kallikrein family of trypsin-like serine proteinases and was originally identified as being down-regulated in metastatic breast and ovarian tumors when compared with corresponding primary tumors. Recent evidence suggests that hK6 may serve as a circulating tumor marker in ovarian cancers. In addition, it was described in the brain of Parkinson's disease and Alzheimer's disease patients, where it is implicated in amyloid precursor protein processing. It is thus a biomarker for these diseases. To examine the mechanism of activation of hK6, we have solved the structure of its proform, the first of a human kallikrein family member. The proenzyme displays a fold that exhibits chimeric features between those of trypsinogen and other family members. It lacks the characteristic "kallikrein loop" and forms the six disulfide bridges of trypsin. Pro-hK6 displays a completely closed specificity pocket and a unique conformation of the regions involved in structural rearrangements upon proteolytic cleavage activation. This points to a novel activation mechanism, which could be extrapolated to other human kallikreins.

    View details for DOI 10.1074/jbc.M201534200

    View details for Web of Science ID 000177055900074

    View details for PubMedID 12016211

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