Synaptic-like Vesicles Facilitate Pioneer Axon Invasion

Piezo1-mediated spontaneous calcium transients in satellite glia impact dorsal root ganglia development

Spontaneous Ca2+ transients of neural cells is a hallmark of the developing nervous system. It is widely accepted that chemical signals, like neurotransmitters, contribute to spontaneous Ca2+ transients in the nervous system. Here, we reveal an additional mechanism of spontaneous Ca2+ transients that is mechanosensitive in the peripheral nervous system (PNS) using intravital imaging of growing dorsal root ganglia (DRG) in zebrafish embryos. GCaMP6s imaging shows that developing DRG satellite glia contain distinct spontaneous Ca2+ transients, classified into simultaneous, isolated, and microdomains. Longitudinal analysis over days in development demonstrates that as DRG satellite glia become more synchronized, isolated Ca2+ transients remain constant. Using a chemical screen, we identify that Ca2+ transients in DRG glia are dependent on mechanical properties, which we confirmed using an experimental application of mechanical force. We find that isolated spontaneous Ca2+ transients of the glia during development is altered by manipulation of mechanosensitive protein Piezo1, which is expressed in the developing ganglia. In contrast, simultaneous Ca2+ transients of DRG satellite glia is not Piezo1-mediated, thus demonstrating that distinct mechanisms mediate subtypes of spontaneous Ca2+ transients. Activating Piezo1 eventually impacts the cell abundance of DRG cells and behaviors that are driven by DRG neurons. Together, our results reveal mechanistically distinct subtypes of Ca2+ transients in satellite glia and introduce mechanobiology as a critical component of spontaneous Ca2+ transients in the developing PNS.

Advances in the study of axon-associated vesicles

The central nervous system is the most important and difficult to study system in the human body and is known for its complex functions, components, and mechanisms. Neurons are the basic cellular units realizing neural functions. In neurons, vesicles are one of the critical pathways for intracellular material transport, linking information exchanges inside and outside cells. The axon is a vital part of neuron since electrical and molecular signals must be conducted through axons. Here, we describe and explore the formation, trafficking, and sorting of cellular vesicles within axons, as well as related-diseases and practical implications. Furthermore, with deepening of understanding and the development of new approaches, accumulating evidence proves that besides signal transmission between synapses, the material exchange and vesicular transmission between axons and extracellular environment are involved in physiological processes, and consequently to neural pathology. Recent studies have also paid attention to axonal vesicles and their physiological roles and pathological effects on axons themselves. Therefore, this review mainly focuses on these two key nodes to explain the role of intracellular vesicles and extracellular vesicles migrated from cells on axons and neurons, providing innovative strategy for future researches.

The embryonic zebrafish brain is seeded by a lymphatic-dependent population of mrc1+ microglia precursors

Microglia are the resident macrophages of the CNS that serve critical roles in brain construction. Although human brains contain microglia by 4 weeks gestation, an understanding of the earliest microglia that seed the brain during its development remains unresolved. Using time-lapse imaging in zebrafish, we discovered a mrc1a+ microglia precursor population that seeds the brain before traditionally described microglia. These early microglia precursors are dependent on lymphatic vasculature that surrounds the brain and are independent of pu1+ yolk sac-derived microglia. Single-cell RNA-sequencing datasets reveal Mrc1+ microglia in the embryonic brains of mice and humans. We then show in zebrafish that these early mrc1a+ microglia precursors preferentially expand during pathophysiological states in development. Taken together, our results identify a critical role of lymphatics in the microglia precursors that seed the early embryonic brain.

A Subset of Oligodendrocyte Lineage Cells Interact With the Developing Dorsal Root Entry Zone During Its Genesis

Oligodendrocytes are the myelinating cell of the CNS and are critical for the functionality of the nervous system. In the packed CNS, we know distinct profiles of oligodendrocytes are present. Here, we used intravital imaging in zebrafish to identify a distinct oligodendrocyte lineage cell (OLC) that resides on the dorsal root ganglia sensory neurons in the spinal cord. Our profiling of OLC cellular dynamics revealed a distinct cell cluster that interacts with peripheral sensory neurons at the dorsal root entry zone (DREZ). With pharmacological, physical and genetic manipulations, we show that the entry of dorsal root ganglia pioneer axons across the DREZ is important to produce sensory located oligodendrocyte lineage cells. These oligodendrocyte lineage cells on peripherally derived sensory neurons display distinct processes that are stable and do not express mbpa. Upon their removal, sensory behavior related to the DRG neurons is abolished. Together, these data support the hypothesis that peripheral neurons at the DREZ can also impact oligodendrocyte development.

Identification of astroglia-like cardiac nexus glia that are critical regulators of cardiac development and function

Glial cells are essential for functionality of the nervous system. Growing evidence underscores the importance of astrocytes; however, analogous astroglia in peripheral organs are poorly understood. Using confocal time-lapse imaging, fate mapping, and mutant genesis in a zebrafish model, we identify a neural crest-derived glial cell, termed nexus glia, which utilizes Meteorin signaling via Jak/Stat3 to drive differentiation and regulate heart rate and rhythm. Nexus glia are labeled with gfap, glast, and glutamine synthetase, markers that typically denote astroglia cells. Further, analysis of single-cell sequencing datasets of human and murine hearts across ages reveals astrocyte-like cells, which we confirm through a multispecies approach. We show that cardiac nexus glia at the outflow tract are critical regulators of both the sympathetic and parasympathetic system. These data establish the crucial role of glia on cardiac homeostasis and provide a description of nexus glia in the PNS.

From whole organism to ultrastructure: progress in axonal imaging for decoding circuit development

Since the pioneering work of Ramón y Cajal, scientists have sought to unravel the complexities of axon development underlying neural circuit formation. Micrometer-scale axonal growth cones navigate to targets that are often centimeters away. To reach their targets, growth cones react to dynamic environmental cues that change in the order of seconds to days. Proper axon growth and guidance are essential to circuit formation, and progress in imaging has been integral to studying these processes. In particular, advances in high- and super-resolution microscopy provide the spatial and temporal resolution required for studying developing axons. In this Review, we describe how improved microscopy has revolutionized our understanding of axonal development. We discuss how novel technologies, specifically light-sheet and super-resolution microscopy, led to new discoveries at the cellular scale by imaging axon outgrowth and circuit wiring with extreme precision. We next examine how advanced microscopy broadened our understanding of the subcellular dynamics driving axon growth and guidance. We finally assess the current challenges that the field of axonal biology still faces for imaging axons, and examine how future technology could meet these needs.

Emerging roles of MT-MMPs in embryonic development

Membrane-type matrix metalloproteinases (MT-MMPs) are cell membrane-tethered proteinases that belong to the family of the MMPs. Apart from their roles in degradation of the extracellular milieu, MT-MMPs are able to activate through proteolytic processing at the cell surface distinct molecules such as receptors, growth factors, cytokines, adhesion molecules, and other pericellular proteins. Although most of the information regarding these enzymes comes from cancer studies, our current knowledge about their contribution in distinct developmental processes occurring in the embryo is limited. In this review, we want to summarize the involvement of MT-MMPs in distinct processes during embryonic morphogenesis, including cell migration and proliferation, epithelial-mesenchymal transition, cell polarity and branching, axon growth and navigation, synapse formation, and angiogenesis. We also considered information about MT-MMP functions from studies assessed in pathological conditions and compared these data with those relevant for embryonic development.

Pioneer Axons Utilize a Dcc Signaling-Mediated Invasion Brake to Precisely Complete Their Pathfinding Odyssey

Axons navigate through the embryo to construct a functional nervous system. A missing part of the axon navigation puzzle is how a single axon traverses distinct anatomic choice points through its navigation. The dorsal root ganglia (DRG) neurons experience such choice points. First, they navigate to the dorsal root entry zone (DREZ), then halt navigation in the peripheral nervous system to invade the spinal cord, and then reinitiate navigation inside the CNS. Here, we used time-lapse super-resolution imaging in zebrafish DRG pioneer neurons to investigate how embryonic axons control their cytoskeleton to navigate to and invade at the correct anatomic position. We found that invadopodia components form in the growth cone even during filopodia-based navigation, but only stabilize when the axon is at the spinal cord entry location. Further, we show that intermediate levels of DCC and cAMP, as well as Rac1 activation, subsequently engage an axon invasion brake. Our results indicate that actin-based invadopodia components form in the growth cone and disruption of the invasion brake causes axon entry defects and results in failed behavioral responses, thereby demonstrating the importance of regulating distinct actin populations during navigational challenges.SIGNIFICANCE STATEMENT Correct spatiotemporal navigation of neuronal growth cones is dependent on extracellular navigational cues and growth cone dynamics. Here, we link dcc-mediated signaling to actin-based invadopodia and filopodia dynamics during pathfinding and entry into the spinal cord using an in vivo model of dorsal root ganglia (DRG) sensory axons. We reveal a molecularly-controlled brake on invadopodia stabilization until the sensory neuron growth cone is present at the dorsal root entry zone (DREZ), which is ultimately essential for growth cone entry into the spinal cord and behavioral response.

Cyclic Stretch of Either PNS or CNS Located Nerves Can Stimulate Neurite Outgrowth

The central nervous system (CNS) does not recover from traumatic axonal injury, but the peripheral nervous system (PNS) does. We hypothesize that this fundamental difference in regenerative capacity may be based upon the absence of stimulatory mechanical forces in the CNS due to the protective rigidity of the vertebral column and skull. We developed a bioreactor to apply low-strain cyclic axonal stretch to adult rat dorsal root ganglia (DRG) connected to either the peripheral or central nerves in an explant model for inducing axonal growth. In response, larger diameter DRG neurons, mechanoreceptors and proprioceptors showed enhanced neurite outgrowth as well as increased Activating Transcription Factor 3 (ATF3).

Comparative Connectomics Reveals How Partner Identity, Location, and Activity Specify Synaptic Connectivity in Drosophila

The mechanisms by which synaptic partners recognize each other and establish appropriate numbers of connections during embryonic development to form functional neural circuits are poorly understood. We combined electron microscopy reconstruction, functional imaging of neural activity, and behavioral experiments to elucidate the roles of (1) partner identity, (2) location, and (3) activity in circuit assembly in the embryonic nerve cord of Drosophila. We found that postsynaptic partners are able to find and connect to their presynaptic partners even when these have been shifted to ectopic locations or silenced. However, orderly positioning of axon terminals by positional cues and synaptic activity is required for appropriate numbers of connections between specific partners, for appropriate balance between excitatory and inhibitory connections, and for appropriate functional connectivity and behavior. Our study reveals with unprecedented resolution the fine connectivity effects of multiple factors that work together to control the assembly of neural circuits.

Sensory Neurons: The Formation of T-Shaped Branches Is Dependent on a cGMP-Dependent Signaling Cascade

Axon bifurcation - a specific form of branching of somatosensory axons characterized by the splitting of the growth cone - is mediated by a cGMP-dependent signaling cascade composed of the extracellular ligand CNP (C-type natriuretic peptide), the transmembrane receptor guanylyl cyclase Npr2 (natriuretic peptide receptor 2), and the kinase cGKI (cGMP-dependent protein kinase I). In the absence of any one of these components, the formation of T-shaped axonal branches is impaired in neurons from DRGs (dorsal root ganglia), CSGs (cranial sensory ganglia) and MTNs (mesencephalic trigeminal neurons) in the murine spinal cord or hindbrain. Instead, axons from DRGs or from CSGs extend only either in an ascending or descending direction, while axons from MTNs either elongate within the hindbrain or extend via the trigeminal ganglion to the masseter muscles. Collateral formation from non-bifurcating stem axons is not affected by impaired cGMP signaling. Activation of Npr2 requires both binding of the ligand CNP as well as phosphorylation of serine and threonine residues at the juxtamembrane regions of the receptor. The absence of bifurcation results in an altered shape of termination fields of sensory afferents in the spinal cord and resulted in impaired noxious heat sensation and nociception whereas motor coordination appeared normal.