The Role of Local Protein Degradation in Neurotransmitter Release and Homeostatic Plasticity

Project Information

SynDegrade

Grant agreement ID: 679881

DOI

10.3030/679881

Project closed

EC signature date 10 March 2016

Start date 1 May 2016

End date 30 April 2022

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 489 340,95

EU contribution

€ 1 489 340,95

Coordinated by

UNIVERSITAT ZURICH
Switzerland

Periodic Reporting for period 4 - SynDegrade (The Role of Local Protein Degradation in Neurotransmitter Release and Homeostatic Plasticity)

Reporting period: 2020-11-01 to 2022-04-30

Summary of the context and overall objectives of the project

Background:
Robust animal behavior presupposes stable neural function. Chemical synapses are the major sites of information transfer in the nervous system. While many synapses reliably transmit information for decades, the proteins determining synaptic transmission have half-lives of hours to months. It is currently unclear how stable synaptic transmission can be achieved in the light of constant protein turnover.
Several neurological diseases, such autism spectrum disorder, have been linked to unstable neural function. Moreover, maladaptive protein turnover has been implicated in a number of neural pathologies. Synaptic protein abundance and synaptic transmission are stabilized by homeostatic mechanisms. Yet, the relationship between the homeostatic control of protein abundance, or proteostasis, and the homeostatic maintenance of synaptic transmission remains enigmatic.

General objectives:
The major goal of this project was to discover molecular links between the mechanisms controlling local protein turnover at synapses and the homeostatic stabilization of synaptic transmission. To this end, we focused on the role of a major protein degradation pathway – the Ubiquitin Proteasome System (UPS) – in regulating neurotransmitter release and homeostatic stabilization of synaptic transmission.

Relevance:
The results of our basic research are expected to advance our understanding of the role of homeostatic stabilization of neural activity. In this regard, the implication of neuropsychiatric disease-related genes in the homeostatic control of synaptic transmission and protein abundance may inform us about the relationship between these genes and the respective pathophysiology. The translation of aspects of our work from Drosophila into the mammalian CNS constitutes a first important step towards bridging the gap between homeostatic regulation of synaptic transmission and neural physiology, as well as pathology in humans. Finally, our discovery of rapid stabilization of synaptic transmission in the mammalian CNS may have implications for neural network simulations, because their stability requires fast compensatory processes with temporal dynamics similar to the form of homeostatic plasticity discovered in this project.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

To systematically investigate the molecular mechanisms mediating UPS-dependent stabilization of synaptic transmission, we combined genetic screens with detailed electrophysiological quantification of synaptic transmission. This approach, which is currently only feasible in Drosophila, established several new molecular links between UPS-mediated protein degradation and homeostatic stabilization of synaptic transmission. In particular, we realized the first systematic analysis of the roles of E3 ubiquitin ligases, a class of enzymes regulating protein degradation-mediated proteostasis, in synaptic transmission. This screen identified several genes encoding for evolutionarily conserved E3 ubiquitin ligases that are required for homeostatic control of neurotransmitter release. Among those genes was thin, the Drosophila homolog of human tripartite motif-containing 32 (TRIM32), a gene linked to several neurological disorders, including autism spectrum disorder and schizophrenia. Furthermore, we revealed that thin acts through dysbindin, a gene linked to homeostatic plasticity in Drosophila and schizophrenia in humans.

Additionally, we studied the nano-architecture of synapses during homeostatic stabilization of synaptic transmission employing super-resolution light microscopy. We discovered that glutamatergic neurotransmitter receptors are arranged in stereotypic ‘nano-rings’ at the Drosophila neuromuscular junction (NMJ). Interestingly, these receptor rings aligned with presynaptic rings formed by proteins located with the nerve terminal. Moreover, these transsynaptically-aligned rings were modulated during homeostatic plasticity, and we identified the first gene that is required for homeostatic regulation of transsynaptic nano-architecture and homeostatic plasticity.

Finally, we provided the first evidence for rapid homeostatic stabilization of synaptic transmission in the mammalian central nervous system (CNS). Studying synaptic transmission in acute brain slices of the mouse cerebellum, we discovered that cerebellar synapses homeostatically regulate synaptic efficacy within minutes upon activity perturbation, at least an order of magnitude faster than previously thought. This is achieved by specific mechanisms that regulate neurotransmitter release from the nerve terminals.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We realized the first systematic investigation of E3 ubiquitin ligases in the context of synaptic transmission and homeostatic plasticity (Baccino-Calace et al., 2022). The results of this screen allow predicting the roles of those E3 ligases in synaptic function.

The discovery of rapid homeostatic compensation of synaptic transmission through neurotransmitter release regulation on the minute time scale in the mammalian CNS (Delvendahl et al. 2019, Kita et al., 2021) significantly advanced the field, because it was previously thought that (i) homeostatic mechanisms operate on a time scale of hours to days instead of minutes in the mammalian CNS, that (ii) homeostatic stabilization of synaptic transmission in the mammalian CNS is mainly expressed postsynaptically, and (iii) presynaptic homeostatic plasticity only acts in the peripheral nervous system, where it has been mainly studied in Drosophila (Delvendahl and Müller, 2019).

The unexpected discovery of transsynaptically aligned nano-rings and their homeostatic regulation is a major step, since it provides the first evidence for a stereotypic organization of transsynaptic nano-architecture, and because it links homeostatic release modulation to the regulation of transsynaptic nano-architecture (Muttathukunnel et al., https://doi.org/10.1101/2021.06.15.448550)

Cartoon summarizing the results of Baccino-Calace et al., 202