serotonerging signaling in stress related disorders

SEROTONERGIC SIGNALING AND EXTRACELLULAR MATRIX IN THE SYNAPTIC PLASTICITY AND STRESS RELATED DISORDERS 

"Serotonin-dependent trans-synaptic mechanism of extracellular matrix modifier release” was implemented as part of the Homing program, funded by the European Regional Development Fund under the Operational Programme Intelligent Development 2014 - 2020 (PO IR), Axis IV: Increasing the scientific-research potential, Measure 4.4: Enhancing the staffing potential of the R&D sector.

 Amount of funding awarded: 798 647,00 PLN 

Our research focuses on the investigation of the relationship between serotonergic signaling and extracellular matrix in the synaptic plasticity underlying stress-related disorders. Chronic stress, a major risk factor for depression, leads to pathological forms of synaptic plasticity, atrophy of dendrites and synapses. The function and morphology of dendritic spines can be modulated both by serotonin-mediated signaling and by extracellular matrix (ECM) proteins. However, the precise mechanisms of possible coordinated interplay between ECM and serotonin-mediated signaling in neurons under physiological and pathological conditions were poorly understood. We described the unknown signaling pathway that linked these processes. We have found that 5-HT7R stimulation increases local MMP-9 activity, triggering dendritic spines remodeling, synaptic pruning and impairment of long-term potentiation (LTP). The underlying molecular machinery involves 5-HT7R-mediated activation of MMP-9, which leads to CD44 cleavage followed by Cdc42 activation. We have shown that the membrane protein CD44 is a new neuronal MMP-9 substrate. Pharmacological/genetic suppression of this pathway rescues the 5-HT7R-induced synaptic changes and the deficit in LTP. The results of these studies have been published in Cell Reports in May 2017 (Bijata et al., 2017, Cell Reports) and were highlighted by the editor on the journal issue cover. The above mentioned project was carried out in the framework of the following research grants: National Science Centre Project “Preludium 5” (UMO-2013/09/N/NZ3/03542) and National Science Centre – “Harmonia 3” (2012/06/M/NZ3/00163)).  

After describing the new signaling pathway in vitro, it was natural consequence to study this pathway in an in vivo model in the brain of adult animals. The obtained results show existence of the above-mentioned signaling pathway also in the brain of adult animals, as well as its enormous effect on the development of depression. Therefore, we decided to check whether this pathway is over-activated during depression. In order to select the most optimal animal model, a review and meta-analysis of the available literature was carried out. The results of our meta-analysis were included in the paper that was published in December 2018 (Antoniuk S., Bijata M. et al., Neurosci Biobehav Rev). The best depression model so far described in mice found to be the chronic unpredictable stress model. Using animal model of depression, we have observed a strong activation of the components associated with the 5-HT7R receptor signaling pathway and remodeling of dendritic spines in CA1 subregion of hippocampus similar to that caused by the activation of the 5-HT7R receptor. At the same time, we have observed that the hippocampal dentate gyrus (DG) subregion in response to 5-HT7R stimulation exhibits structural changes in the opposite direction (spines maturation). Additionally, such changes did not occur in the DG after CUS. The results obtained by us in the project were included in the manuscript which is currently under review. This line of research we plan also to further investigate during of implementation of SONATA grant. 

The proposed research focuses on explaining the mechanisms of serotonin receptor 5 HT7-mediated structural plasticity in specific subregions of the hippocampus. Our research focus on: 

(1) Investigating the 5-HT7R-mediated profiles of Cdc42/RhoA activation in different hippocampal subregions and determining their role in structural plasticity; 

(2) Characterizing the types of neurons expressing the 5-HT7R in different hippocampal subregions and examining changes in CA1 and DG synaptic plasticity following 5-HT7R activation.

Interestingly, recently we have found also that another serotonin receptor, 5-HT4R, leads to maturation of dendritic spines and enhancement of LTP. This effect appears to be exactly the opposite to effect that we observed in response to 5-HT7R receptor stimulation on structural and functional plasticity. This result intrigued us very much because both receptors activate MMP-9. Utilizing both fluorescents techniques as well as biochemical assays, we demonstrated that stimulation of 5-HT4R leads to RhoA activation, which in turn causes inactivation of cofilin by phosphorylation, what increases actin polymerization and stabilization. Moreover, we have studied molecular details of this phenomenon and associate the observed morphological changes of dendritic spines with biochemical processes at the level of single dendritic spines. We were able to monitor the high-resolution spatiotemporal pattern of 5-HT4R-dependent RhoA activation with simultaneous monitoring of F-actin dynamics and morphological effects in living neurons. Furthermore, it was possible to correlate observed changes with immunostaining pattern (e.g. PSD-95) in exactly the same dendritic spine after fixation (Fig. 3). The results of this study were published in February 2020 (Schill*, Bijata* et al., 2020, Communications Biology *- equally contributed). Study of crosstalk between 5-HT4R and 5-HT7R during physiological and pathological conditions is both the subject of our current project entitled “Serotonin-dependent trans-synaptic mechanism of extracellular matrix modifier release” within the framework of  “HOMING” program of Foundation for Polish Science (PI Monika Bijata https://www.fnp.org.pl/assets/Monika-Bijata_opis.pdf ; https://www.youtube.com/watch?v=t732dS3jm-I ). This project is based on the hypothesis that elevated levels of corticosterone during chronic stress results in an increase in MAOs activity, resulting in faster decay of 5-HT (and its lower concentration in the synapse). Postsynaptic receptors, 5-HT7R and 5-HT4R undergo dynamic palmitoylation, which is dependent on agonist (5-HT) concentration. Palmitoylation of those receptors changes its agonist-independent constitutive activity. Non-palmitoylated receptors possess increased constitutive activity towards Gαs-mediated signaling. Stimulation of 5-HT7R and 5-HT4R causes activation of MMP-9 and changes in dendritic spines morphology, but it is unknown which Gα proteins are involved in this process. 

The results obtained within the HOMING project funded by Foundation for Polish Science grant POIR.04.04.00-00-43BC/17-00: