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Deep molecular diversity of mammalian synapses: why it matters and how to measure it

Nature Reviews Neuroscience volume 13, pages 365–379 (2012)Cite this article

Subjects

Key Points

  • Synapses of the mammalian CNS are highly complex structures composed of thousands of distinct proteins. Synaptic proteins are often expressed in variable patterns across different brain regions and neuronal populations, suggesting a high degree of molecular diversity in synapses.

  • Synapse types have long been discriminated on the basis of neurotransmitter molecules, but it is now necessary to recognize much deeper diversity of both synaptic protein composition and function within each traditional neurotransmitter type.

  • Intra-type synapse diversity reflects many factors, including diversity of presynaptic and postsynaptic parent neurons, activity histories and influences of third-party neurons and glial cells.

  • Intra-type functional diversity is reflected in strength, kinetics, voltage-dependence and plasticity of synaptic transmission. Improving knowledge of the relationships between molecular and functional aspects of diversity is sure to illuminate principals of synaptic circuit function.

  • Proteins identifiable at mature synapses include adhesion molecules that guide selective synaptogenesis during development. Such molecules may encode circuit context and connectivity in the mature brain and aid circuit reconstruction efforts.

  • Many mental and neurological disorders reflect mutations in synaptic proteins expressed in subsets of synapses. A better grasp of synapse molecular diversity is certain to contribute to our understanding of specific brain disorders.

  • New techniques that probe the combinatorial expression of proteins at the level of individual synapses within the brain are bolstering our efforts to link intra-type diversity to distinct physiological and structural properties of synaptic connections and brain circuits.

Abstract

Pioneering studies in the middle of the twentieth century revealed substantial diversity among mammalian chemical synapses and led to a widely accepted classification of synapse type on the basis of neurotransmitter molecule identity. Subsequently, powerful new physiological, genetic and structural methods have enabled the discovery of much deeper functional and molecular diversity within each traditional neurotransmitter type. Today, this deep diversity continues to pose both daunting challenges and exciting new opportunities for neuroscience. Our growing understanding of deep synapse diversity may transform how we think about and study neural circuit development, structure and function.

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Figure 1: Evidence for distinct patterns of synaptic protein expression.
Figure 2: Synaptic molecules and physiological diversity.
Figure 3: Synaptic proteins as a connectivity code.
Figure 4: Methods for probing the ultrastructure and molecular composition of single synapses.
Figure 5: Multidimensional views of single synapses.
Figure 6: A mechanistic classification of synapses on the basis of protein biosynthesis principles.

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Acknowledgements

The authors are grateful to F. Collman for many helpful discussions. The work was supported by the Gatsby Charitable Trust (London, UK), The Howard Hughes Medical Institute (Maryland, USA) (Collaborative Innovation Award #43667), the Mathers Foundation (New York, USA) and grants from the National Institute of Neurological Diseases and Stroke (Maryland, USA) (1R21NS063210, 1R01NS075252 and 1R01NS077601).

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  1. Department of Molecular and Cellular Physiology, Beckman Center, Stanford University School of Medicine, Stanford, 94305, California, USA

    Nancy A. O'Rourke, Nicholas C. Weiler, Kristina D. Micheva & Stephen J. Smith

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  1. Nancy A. O'Rourke
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Correspondence to Stephen J. Smith.

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Competing interests

Stanford has licensed a Smith and Micheva array tomography patent to Aratome, LLC, Menlo Park, California, USA. Both Smith and Micheva have founder's equity shares in Aratome. Nancy A. O'Rourke and Nicholas C. Weiler declare no competing interests.

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Glossary

Proteomic

Relating to the study of the proteome, which is the entire set of proteins expressed by an organism, tissue, cell or subcellular organelle. A variety of large-scale techniques are used, such as mass spectrometry, immunolabelling or tagging of proteins and yeast two-hybrid screens.

Synaptosomes

Artificially formed membranous structures that are generated by the subcellular fractionation of brain tissue homogenates. The synaptosome contains most of the presynaptic terminal, including synaptic vesicles and mitochondria, as well as the postsynaptic density and adjacent postsynaptic membrane.

Isoforms

Alternative forms of the same protein generated from either related genes or from alternate splicing of the same gene.

Short-term facilitation

Increase in the amplitude of synaptic transmission over multiple stimuli on the scale of milliseconds, thought to result from frequency-dependent build-up of presynaptic calcium, which increases the release probability for upcoming spikes.

Short-term depression

Decrease in the amplitude of synaptic transmission with repeated stimulation on the scale of milliseconds, thought to result from frequency-dependent depletion of fusion-ready vesicles, which decreases the release probability for upcoming spikes.

Release probability

The probability that a single presynaptic spike will result in the release of a vesicle of neurotransmitter into the synaptic cleft. Release probability is determined by multiple presynaptic factors.

Long-term potentiation

(LTP). Long-lasting increase in synaptic strength between neurons, usually resulting from synchronous or temporally coordinated pre- and postsynaptic activity.

Long-term depression

(LTD). Long-lasting weakening of synaptic strength between neurons, often resulting from asynchronous pre- and postsynaptic activity.

Pinceau synapses

Synapses shaped like a paintbrush ('pinceau' in French) that are formed at the base of the Purkinje cell axon by cerebellar basket cells.

Autism spectrum disorders

(ASDs). A group of complex neurodevelopmental disorders characterized by difficulties in social interaction, poor verbal and non-verbal communication and abnormal repetitive behaviours.

Golgi stain

A technique based on precipitations of metallic salts within cells and used for visualization of sparse subsets of neurons and glial cells in their entirety.

EM tomography

A method for the three-dimensional reconstruction of objects from a series of projection images that are recorded with a transmission electron microscope. It offers the opportunity to obtain spatial information on structural arrangements of cellular components.

Ultrathin sections

Histological sections of resin-embedded or frozen tissue with a thickness of 30–200nm. Such sections are typically used for electron microscopy.

Confocal microscopy

A fluorescence imaging technique that increases resolution through 'optical sectioning'. To attain optical sectioning, excitation light is scanned across an object, illuminating a single point at a time, and the emitted fluorescence light is detected through a pinhole aperture, limiting the light originating from outside the focal plane.

Two-photon microscopy

A form of microscopy in which a fluorochrome that would normally be excited by a single photon is stimulated quasi-simultaneously by two photons of lower energy. Under these conditions, fluorescence increases as a function of the square of the light intensity, and decreases as the fourth power of the distance from the focus. Because of this behaviour, only fluorochrome molecules near the plane of focus are excited, greatly reducing light scattering and photodamage of the sample.

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O'Rourke, N., Weiler, N., Micheva, K. et al. Deep molecular diversity of mammalian synapses: why it matters and how to measure it. Nat Rev Neurosci 13, 365–379 (2012). https://doi.org/10.1038/nrn3170

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