Advances in neurobiology最新文献

Electron tomography (ET) has emerged as a critical tool for visualizing the three-dimensional ultrastructure of biological specimens at nanometer resolution. This chapter focuses on the application of ET in studying synaptic ultrastructure, in particular postsynaptic density, providing a detailed overview of the techniques and methodologies used to achieve high-resolution three-dimensional reconstruction of synapses. Beginning with an introduction to electron tomography, this chapter delves into the principles of electron tomography, including sample preparation, data collection, and image processing. Special emphasis is placed on the application of cryogenic electron tomography, which allows for the visualization of biological samples in their near-native state. This chapter also reviews the historical context of postsynaptic ultrastructure studies using conventional electron microscopy and explores the significant insights gained using electron tomography, particularly in understanding the nanoscale organization and structural complexity of excitatory synapses. This chapter concludes by discussing the future potential of electron tomography in advancing our knowledge of synaptic biology and its implications for neuroscience.

The presynaptic active zone (AZ) is a precisely organized nanoscale domain where synaptic vesicle exocytosis and neurotransmitter release are governed by tightly regulated protein networks. This review synthesizes recent insights from electron microscopy (EM) and super-resolution fluorescence microscopy that have deepened our understanding of active zone architecture in mammalian central nervous system synapses and at the Drosophila neuromuscular junction (NMJ). These imaging techniques have elucidated the spatial organization of key active zone proteins relative to one another and to the plasma membrane, which is notably well-ordered at the Drosophila NMJ. Here, we present a detailed overview of the nanometer-scale positioning of AZ proteins across the two types of synapses. In parallel, the idea that active zone nanostructures may form through liquid-liquid phase separation has emerged as a potential organizing principle. The transient and dynamic interactions characteristic of phase-separated protein condensates contrast with models that attribute nanodomain organization to specific, stable protein-protein interactions, raising the question of how the active zone's stable core architecture is reconciled with its capacity for dynamic plasticity.

Synaptic transmission between pre- and postsynaptic neurons occurs when the presynaptic neuron terminal is temporarily depolarized upon action potential arrival, opening voltage-gated Ca²⁺ channels at synapses. Ca²⁺ will flow into the presynapse, and it will trigger the fusion of neurotransmitter-filled synaptic vesicles with the presynaptic membrane in less than a millisecond. Neurotransmitter molecules are then released into the synaptic cleft and bind to receptors in the postsynaptic membrane. Understanding the mechanisms that underlie this complex cellular process requires detailed knowledge of the spatial and structural organization of the macromolecular components of the synapse. This chapter focuses on recent structural insights into the presynaptic machinery and the spatial relationship between synaptic vesicles, presynaptic factors, and postsynaptic receptors.

Synapses are the fundamental units of communication and information processing in the nervous system. They show remarkable functional autonomy, as well as speed of signaling, bidirectional plasticity, and diversity. Synapses shape complex network functions, such as learning, memory, pattern separation, pattern completion, and many others. Synaptic dysfunction is responsible for a wide range of neurological and psychiatric diseases. Many of these diseases are emerging as synaptopathies, but the precise disease mechanisms are unknown. Given their micron-scale small size and seemingly compact structure, synapses were generally thought to be functionally indivisible structures. However, with the advent of high-resolution electrophysiology and imaging techniques, it is increasingly clear that synapses harbor functionally specialized nanomodules. This expanding nanobiology of the synapse provides a new perspective on synaptic signaling. The novel insight gained from this work is critical to understand disease mechanisms and to guide the development of appropriate therapeutic strategies for major brain diseases ranging from neurodegenerative and neurodevelopmental disorders to neuropsychiatric disorders such as depression and schizophrenia.

Glia, which are nonneuronal cells in the central nervous system, include astrocytes, microglia, and oligodendrocyte lineage cells. Historically, their passive roles in maintaining central nervous system function-such as supplying substrates for neuronal energy, buffering neurotransmitters, and improving neural conductance-have been primarily highlighted. Importantly, recent research has revealed that glial cells express genes directly related to controlling synapse nano-organization. In particular, astrocytes have the ability to secrete molecules that induce synapse formation and maturation. They also modulate the structure of synapses by expressing proteins that make direct contact with the synaptic membrane. Moreover, astrocytes can actively eliminate synapses through their phagocytic machinery during development and adulthood, thereby establishing circuit homeostasis. Microglia can also assist in the integration of synapses into the neural circuit and regulate synapse formation, maintenance, and elimination. Here, we review key findings on the mechanisms of glial contributions to synapse nano-organization. We will also discuss how different glial cells contribute to the development and homeostasis of synapses through distinct cellular and molecular pathways in both health and disease.

Within the single micron of the synapse, three distinct modes of neurotransmission, driven by synchronous, asynchronous, and spontaneous neurotransmitter release, occur concurrently. In this chapter, we discuss the synaptic nano-organization comprised of neurotransmitter release machinery, molecular platforms, scaffolding proteins, and liquid complexes that support the discrete signaling of these three modes of neurotransmission. This robust nano-organization supports unique functional roles for each discrete mode at both excitatory glutamatergic and inhibitory GABAergic synapses. Modular nanocolumn organization of excitatory synapses and largely single-domain organization of inhibitory synapses maintain homeostatic plasticity within neural circuits. These recent findings support a basic design principle where the single synapse is a highly ordered and compartmentalized unit that by the functional nano-segregation of distinct forms of neurotransmission shapes synaptic efficacy, determines neurotransmission reliability, and tunes plasticity. The development of novel tools will be instrumental in further elucidating the nano-environment of the synapse, essential to both uncovering mechanisms underlying neurological disorders as well as their treatment.

The complex nanostructure and spatiotemporal dynamics of central synapses remain among the fundamental mysteries of neurobiology. The resolution of traditional microscopy techniques-constrained by the intrinsic limits of light diffraction-is largely insufficient to study central synapses effectively. Conventional imaging can resolve areas roughly the size of a synapse's active zone, that is, severalfold larger than the size of synaptic vesicles. Recent advances have generated several super-resolution imaging modalities that overcome or bypass the light diffraction limit to support studies of synaptic nanostructure. In this chapter, we present the principles, features, and limitations of the most common super-resolution imaging tools. Though these advancements have greatly improved our understanding of synaptic architecture and dynamics, significant challenges remain. Difficulties of translating the existing tools to in vivo applications, and the inherent trade-off between spatial and temporal resolution, continue to limit studies of the function of central synapses in native tissue.

Synaptic cell adhesion molecules are critical components of the molecular programs underlying synapse formation and neural circuit assembly. Here, we discuss our current understanding of the functional roles of several of these synaptic adhesion molecules in the mammalian central nervous system. Emerging evidence, driven by advances in super-resolution approaches, supports that pre- and postsynaptic machinery are highly organized at the nanoscale level and that these precise sub-synaptic positions are important for synaptic transmission. We subsequently describe the nano-organization of several synaptic cell adhesion molecules and how these trans-synaptic complexes align release machinery to shape the synaptic function. Collectively, an understanding of the mechanistic roles of synaptic cell adhesion complexes will provide insights into how neural circuits assemble by vast numbers of diverse synaptic connections in the brain.

Dissecting the mechanisms of synaptic transmission touches on nearly all fields of neuroscience. Of particular recent importance is the discovery that protein distribution within single synapses is highly organized across multiple spatial scales, ranging from the nanoscale accumulation of just a few protein molecules to larger domains with unique multiprotein compositions. Here, we address recent data regarding postsynaptic molecular organization. We argue that the complexity of synaptic nanostructure generates functional capabilities that can fine-tune synaptic strength and that far exceed the classical limits of quantal synaptic transmission. We focus first on the critical scaffold protein PSD-95 as a case study for how to approach the emergent problem of describing and classifying forms of protein organization, including trans-synaptic "nanocolumn" relationships. Then, we discuss recent work identifying new features of NMDA receptor subsynaptic organization that appear likely to regulate the patterns of neural activity that can induce synaptic plasticity. Overall, we assert that these mechanisms of molecular coordination at scales of 20-150 nm enhance the synapse's ability to tune synaptic transmission, carry out detailed biochemical signaling, and allow more complex impacts on the cell.

Cell adhesion molecules (CAMs) play critical roles in mediating intercellular interactions in the context of the nervous system, such as guiding neuronal development, synapse formation and maturation, and synaptic plasticity. In addition to their extracellular adhesive roles, most CAMs induce intracellular signaling events and scaffold large protein complexes through intracellular domains. The molecular biology of how CAMs regulate synaptic development and function has been hugely advanced by decades of structural biology. These structures have illuminated multiple modes of CAM regulation, including how alternative splicing regulates CAM homotypic and heterotypic interactions. CAMs are diverse in size and contain a variety of adhesion domain classes such as immunoglobulin (Ig), leucine-rich repeats (LRR), and laminin G/neurexin/sex hormone (LNS). In this chapter, we detail structures of key synaptic adhesion complexes, including a mechanistic explanation of how these structures have informed functional work. Detailing the structural basis of synaptic adhesion provides a foundation for deciphering the complex interactions underlying neuronal connectivity and function in health and disease.