Synapses form trillions of connections between billions of neurons in the brain to establish neural circuits that allow us to sense, think, act, learn, and remember. Synaptic weight is a crucial concept to understand the nervous system, yet its clear definition remains elusive, despite more than a century of searching. This NeuroNex Network assembles world experts to study synapses from molecules to behavior, in order to answer this fundamental and ambitious question: What constitutes synaptic weight and what role does it play in shaping neural circuits?

Answering this question requires a major shift away from thinking about synapses as isolated entities. Traditionally, synapses have been treated as on or off switches, that is, one-bit machines. Recent models, based on synapse size as a proxy for synaptic weight, show that this assumption is wrong. In fact, the information content can be much higher, for example, being >4 bits at hippocampal synapses. Synaptic weight is controlled over broad temporal and spatial scales that are dynamically regulated by activity in neural circuits. New evidence points to subcellular resources (endoplasmic reticulum, mitochondria, endosomes, ribosomes) that broker and drive synaptic efficacy and plasticity through mechanisms that regulate local protein synthesis. Thus, an understanding of synaptic weights needs to be addressed at both subcellular and circuit levels.

Key research resources being developed and disseminated

To achieve these goals, new technologies are needed to bridge multiple scales in image resolution and to collect sufficiently large tissue volumes to perform circuit level analyses. An important new approach involves conical tilt tomography on the scanning electron microscope operating in the transmission mode (tomoSEM). TomoSEM aims to fill the current resolution-to- volume gap between methods used for structural biology (high resolution, small volumes) and connectomics (relatively low resolution, large volumes). TomoSEM provides the axial (≤15 nm) and in-plane (≤2 nm) resolution needed to identify and quantify subcellular components in large volumes. It eliminates major artifacts of other electron microscopy methods, while reducing human effort by more than tenfold and lowering cost by more than half.

Our Network comprises world experts in protein chemistry, cell biology, connectomics, and behavior who will provide new understanding about the relationships between structure, function, and plasticity of synaptic weights in circuits. Experts in electron microscopy will implement, validate, and deploy tomoSEM. Experts in image analysis, geometry, statistics, machine learning, and multilevel modeling will create platforms to search data for hidden order. These strategies will overcome limits of accumulating data locally one synapse at a time.

Our work lays the foundation to propel deeper understanding of brain function and regulation from nanoscale to circuit levels. Moreover, we will ensure a long-lasting, far-reaching impact by leveraging work from this NeuroNex Network, our NSF Neurotechnology Hub, NSF Innovation grants, and BRAIN Initiative projects, to enable acquisition and sharing of the new knowledge.  Newly developed concepts, tools, and resources will be posted to 3DEM.org.  In addition, workshops (virtual and in-person) will be conducted annually and will include updates on project resources, demonstrations on the latest tools being developed, and hands-on instructions on enhancements to analyze tools.  We envision that future application, beyond the brain, of the knowledge and tools developed here will give rise to data that will address fundamental and potentially novel principles of complex self-organizing systems.

Key Personnel

Participating Investigators
IRG1: Molecular determinants and manifestations of defined synapse states

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1. Silvio O. Rizzoli, co-PI, Professor, Dept. Neuro- and Sensory Physiology, Univ. Med. Ctr., Göttingen & Erin M. Schuman, Director, Max Planck Institute for Brain Research (Partners, Ger)
2.  A. Radu Aricescu, Professor, Program Leader, MRC Senior Fellow, MRC Laboratory of Molecular Biology, Cambridge (UK)
3. Alice Yen-Ping Ting, Professor, Dept. of Genetics, Biology, and Chemistry, Stanford (US)
4. Volker Haucke, Director & Professor, Dept. Molecular Pharm., Leibniz FMP (Ger)
5. Stephan Sigrist, Professor, Institute for Biology, Neurogenetics Freie Universität, Berlin (Ger)
6. Paul De Koninck, Director, Cell. & Molecular Neurosci. Div., CERVO Brain Research Center & Professor, Dept. of Biochemistry, Microbiology, and Bioinformatics, Université Laval (Can)
7. Ruben Fernandez-Busnadiego, Professor Univ. of Göttingen Med Ctr (Ger, Partner: Helmstaedter, IRG3)

IRG2: Subcellular constituents and manifestations of defined synapse states

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1. Kristen M. Harris (Lead PI), Professor, Dept. of Neuroscience, Univ. of Texas, Austin (US)
2. Mark H. Ellisman (Lead IRG2), Distinguished Professor, Dept. of Neurosciences and Bioengineering, Univ. of California, San Diego (US), Senior Fellow HHMI, Janelia Campus
3. Bryan W. Jones, Research Associate Professor (Moran Eye Center) and Adjunct Associate Professor, Dept. of Ophthalmology and Visual Sciences, Univ. of Utah (US)
4. Erik M. Jorgensen, Distinguished Professor, Univ. of Utah, School of Biological Sciences (US)
5. Christian Rosenmund, Professor, NeuroCure, Inst. for Neurophysiology, Charité & Nils Brose, Director, Dept. of Molecular Neurobiology, MPI of Experimental Medicine (Ger Partners)

IRG3: Ultrastructural motifs of synapses in neural circuits with defined states

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1. R. Clay Reid (Lead IRG3), Senior Investigator, Neural Coding, The Allen Institute (US)
2. Davi Bock, Associate Professor, Dept. Neurological Sciences, Larner College of Medicine, Univ. of Vermont (US)
3. Gregory S.X.E. Jefferis, Programme Leader, MRC Laboratory of Molecular Biology and Director of Research, Department of Zoology, University of Cambridge (UK)
4. Moritz Helmstaedter, Director (MPI), Dept. of Connectomics (Partner: Fernandez-Busnadiego, IRG1, Ger)
5. Narayanan Kasthuri, Neuroscientist, Bio Division, Argonne National Laboratory, Adjunct Assistant Professor, Dept. of Neurobiology, Univ. of Chicago (US)
6. Linnaea E. Ostroff, Assistant Professor of Physiol. and Neurobiol., Univ. CT-Storrs (US)

IRG4: Cyberinfrastructure to enable multiscale discovery in diverse neural circuits

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1. Terrence J. Sejnowski (Lead IRG4), Francis Crick Professor, The Salk Institute (US)
2. Uri Manor, Core Director, Staff Scientist, The Salk Institute (US)
3. Joshua T. Vogelstein, Assist. Professor, Dept. of Biomedical Engineering, Core Faculty, Institute for Computational Medicine & Center for Imaging Science, Johns Hopkins Univ. (US)
4. Flavie Lavoie-Cardinal, Independent Researcher, CERVO Brain Research Center & Assistant Professor, Dept. of Psychiatry and Neuroscience, Université Laval (Can)
5. James P. Carson, Director of the 3DEM portal, Manager of Research Acceleration in Life Sciences Computing, TACC, Univ. of Texas, Austin (US)
6. Viren Jain, Staff Research Scientist and Tech Lead/Manager, Google (US)

IRG1 website links
http://rizzoli-lab.de/
https://brain.mpg.de/research/schuman-department/
https://www2.mrc-lmb.cam.ac.uk/group-leaders/a-to-g/radu-aricescu/
http://www.tinglab.org/
https://www.leibniz-fmp.de/haucke
https://www.bcp.fu-berlin.de/en/biologie/arbeitsgruppen/genetik/ag_sigrist/index.html
https://cervo.ulaval.ca/en/paul-de-koninck
https://neuropathologie.umg.eu/forschung/arbeitsgruppen-labore/ag-fernandez-busnadiego/

IRG2 website links
https://3dem.org/
https://synapseweb.clm.utexas.edu/
https://ncmir.ucsd.edu/mark
http://marclab.org/outreach/people/bryan-w-jones/
https://www.thejorgensenlab.org/
http://www.rosenmundlab.de/en/people/3
https://www.uni-goettingen.de/en/57921.html

IRG3 website links
https://alleninstitute.org/what-we-do/brain-science/about/team/staff-profiles/r-clay-reid/
http://med.uvm.edu/neuro/research_basicscience
http://flybrain.mrc-lmb.cam.ac.uk/jefferislabwebsite/
https://brain.mpg.de/research/helmstaedter-department.html
https://www.anl.gov/profile/narayanan-kasthuri
https://ostrofflab.pnb.uconn.edu/

IRG4 website links
https://cnl.salk.edu/
https://www.salk.edu/scientist/uri-manor/
https://jovo.me/
https://cervo.ulaval.ca/en/flavie-lavoie-cardinal
http://virenjain.org/

Reaching Out to the Community

Many of our collaborators and investigators have established programs within their communities and across the world. These programs include, but are not limited to, coursework for high school, undergraduate, and graduate students; public outreach programs; programs to aid under-represented minorities; and mentoring of peers and students on a global level.  Drs. Jorgensen, Sejnowski, Bock, Carson, Fernandez-Busnadiego, Harris, Kasthuri, De Koninck and Lavoie-Cardinal have all developed and participated in course instructions. The University of Göttingen (Rizzoli, Brose, and Fernandez-Busnadiego) hosts outreach activities, such as the “DenkBar”, in which conferences are organized in clubs or bars, for a wide audience, composed of both young students and older participants, and the “Night of Science” (“Nacht des Wissens”), organized bi-annually by the University of Göttingen, and several other institutes of the Göttingen Campus, and allows access to the institutes for the general public, offers talks, open-lab activities, and workshops.

Other outreach programs include Harris (World Science Festivals; Sound of Science, Memory Matters); Fernandez-Busnadiego (public articles and webinars; Twitter: ruferbus); Haucke (Long Night of Science, public articles & lectures for the general public); Jorgensen (Science Night Live at a Pub, OSHER Inst. adult education, Brain Awareness Week); Rosenmund (Long night of Sciences, Berlin); Lavoie-Cardinal (public conferences in high schools and colleges, Canadian Light Microscopy Course, Frontiers in Neurophotonics Summer School); Jefferis (Royal Society Public Lecture Series, Cambridge Science Festival, blog named Fly Connectomics); Aricescu (contribution to podcasts/blogs/interviews, Twitter account, lab open days); Brose (Theme Concerts Munich State Opera, public articles); Vogelstein (public website: Open Connectome Project with >30 datasets and >100,000 visitors, public twitter feed with >5,800 followers); Bock (hosted CATMAID tracing environment; helped develop curriculum for high school students and teachers in Ector County Texas, a rural and economically disadvantaged population, to trace neuronal circuits in the adult fly brain; educational talks at high school and undergraduate level; interviews with lay press); De Koninck (4-5 presentations and interviews per year at high schools, lay public audiences and media); and Carson (public 3DEM website; press release articles posted to TACC twitter and Facebook). Lastly, several investigators developed curriculums to serve and mentor underrepresented minorities in high schools within their community.  These are several examples of current activities our principal investigators have developed to educate and reach out to the public and share their love of science and the brain.