Global Research Trends of Exosomes in the Central Nervous System: A Bibliometric and Visualized Analysis

OBJECTIVE

Exosomes in the central nervous system (CNS) have become an attractive area of research with great value. However, few bibliometric analysis has been conducted. The study aimed to visualize the scientific trends and research hotspots of exosomes in the CNS by bibliometric analysis.

METHODS

All potential articles and reviews on exosomes in the CNS published in English from 2001 to 2021 were extracted from the Web of Science Core Collection. The visualization knowledge maps of critical indicators, including countries/regions, institutions, authors, journals, references, and keywords, were generated by CiteSpace and VOSviewer software. Besides, each domain's quantitative and qualitative analysis was also considered.

RESULTS

A total of 2,629 papers were included. The number of exosomes-related publications and citations regarding CNS increased yearly. These publications came from 2,813 institutions in 77 countries/regions, led by the United States and China. Harvard University was the most influential institution, while the National Institutes of Health was the most critical funding source. We identified 14,468 authors, among which Kapogiannis D had the most significant number of articles and the highest H-index, while Théry C was the most frequently co-cited. The cluster analysis of keywords generated 13 clusters. In summary, the topic of biogenesis, biomarker, and drug delivery will serve as hotspots in future research.

CONCLUSION

Exosomes-related CNS research has gained considerable attention in the past 20 years. The sources and biological functions of exosomes and their promising role in diagnosing and treating CNS diseases are considered hotspots in this field. The clinical translation of the results from exosomes-related CNS research will be of great importance in the future.

Exosome: The “Off-the-Shelf” Cellular Nanocomponent as a Potential Pathogenic Agent, a Disease Biomarker, and Neurotherapeutics

Exosomes are nanosized “off-the-shelf” lipid vesicles released by almost all cell types and play a significant role in cell–cell communication. Exosomes have already been proven to carry cell-specific cargos of proteins, lipids, miRNA, and noncoding RNA (ribonucleic acid). These vesicles can be selectively taken up by the neighboring cell and can regulate cellular functions. Herein, we have discussed three different roles of exosomes in neuroscience. First, we have discussed how exosomes play the role of a pathogenic agent as a part of cell–cell communication and transmit pathogens such as amyloid-beta (Aβ), further helping in the propagation of neurodegenerative and other neurological diseases. In the next section, the review talks about the role of exosomes in biomarker discovery in neurological disorders. Toward the end, we have reviewed how exosomes can be harnessed and engineered for therapeutic purposes in different brain diseases. This review is based on the current knowledge generated in this field and our comprehension of this domain.

Isolation of microglia-derived extracellular vesicles: towards miRNA signatures and neuroprotection

The functional preservation of the central nervous system (CNS) is based on the neuronal plasticity and survival. In this context, the neuroinflammatory state plays a key role and involves the microglial cells, the CNS-resident macrophages. In order to better understand the microglial contribution to the neuroprotection, microglia-derived extracellular vesicles (EVs) were isolated and molecularly characterized to be then studied in neurite outgrowth assays. The EVs, mainly composed of exosomes and microparticles, are an important cell-to-cell communication process as they exhibit different types of mediators (proteins, lipids, nucleic acids) to recipient cells. The medicinal leech CNS was initially used as an interesting model of microglia/neuron crosstalk due to their easy collection for primary cultures. After the microglia-derived EV isolation following successive methods, we developed their large-scale and non-targeted proteomic analysis to (i) detect as many EV protein markers as possible, (ii) better understand the biologically active proteins in EVs and (iii) evaluate the resulting protein signatures in EV-activated neurons. The EV functional properties were also evaluated in neurite outgrowth assays on rat primary neurons and the RNAseq analysis of the microglia-derived EVs was performed to propose the most representative miRNAs in microglia-derived EVs. This strategy allowed validating the EV isolation, identify major biological pathways in EVs and corroborate the regenerative process in EV-activated neurons. In parallel, six different miRNAs were originally identified in microglia-derived EVs including 3 which were only known in plants until now. The analysis of the neuronal proteins under the microglial EV activation suggested possible miRNA-dependent regulation mechanisms. Taken together, this combination of methodologies showed the leech microglial EVs as neuroprotective cargos across species and contributed to propose original EV-associated miRNAs whose functions will have to be evaluated in the EV-dependent dialog between microglia and neurons.

What are the mechanisms for analogue and digital signalling in the brain?

Synaptic transmission in the brain generally depends on action potentials. However, recent studies indicate that subthreshold variation in the presynaptic membrane potential also determines spike-evoked transmission. The informational content of each presynaptic action potential is therefore greater than initially expected. The contribution of this synaptic property, which is a fast (from 0.01 to 10 s) and state-dependent modulation of functional coupling, has been largely underestimated and could have important consequences for our understanding of information processing in neural networks. We discuss here how the membrane voltage of the presynaptic terminal might modulate neurotransmitter release by mechanisms that do not involve a change in presynaptic Ca(2+) influx.

Imaging analysis of neuron to glia interaction in microfluidic culture platform (MCP)-based neuronal axon and glia co-culture system

Proper neuron to glia interaction is critical to physiological function of the central nervous system (CNS). This bidirectional communication is sophisticatedly mediated by specific signaling pathways between neuron and glia(1,2) . Identification and characterization of these signaling pathways is essential to the understanding of how neuron to glia interaction shapes CNS physiology. Previously, neuron and glia mixed cultures have been widely utilized for testing and characterizing signaling pathways between neuron and glia. What we have learned from these preparations and other in vivo tools, however, has suggested that mutual signaling between neuron and glia often occurred in specific compartments within neurons (i.e., axon, dendrite, or soma)(3). This makes it important to develop a new culture system that allows separation of neuronal compartments and specifically examines the interaction between glia and neuronal axons/dendrites. In addition, the conventional mixed culture system is not capable of differentiating the soluble factors and direct membrane contact signals between neuron and glia. Furthermore, the large quantity of neurons and glial cells in the conventional co-culture system lacks the resolution necessary to observe the interaction between a single axon and a glial cell. In this study, we describe a novel axon and glia co-culture system with the use of a microfluidic culture platform (MCP). In this co-culture system, neurons and glial cells are cultured in two separate chambers that are connected through multiple central channels. In this microfluidic culture platform, only neuronal processes (especially axons) can enter the glial side through the central channels. In combination with powerful fluorescent protein labeling, this system allows direct examination of signaling pathways between axonal/dendritic and glial interactions, such as axon-mediated transcriptional regulation in glia, glia-mediated receptor trafficking in neuronal terminals, and glia-mediated axon growth. The narrow diameter of the chamber also significantly prohibits the flow of the neuron-enriched medium into the glial chamber, facilitating probing of the direct membrane-protein interaction between axons/dendrites and glial surfaces.

Lack of molecular-anatomical evidence for GABAergic influence on axon initial segment of cerebellar Purkinje cells by the pinceau formation

The axon initial segment (AIS) of cerebellar Purkinje cells (PCs) is embraced by ramified axons of GABAergic basket cells (BCs) called the pinceau formation. This unique structure has been assumed to be a device for the modulation of PC outputs through electrical and/or GABAergic inhibition. Electrical inhibition is supported by enriched potassium channels, absence of sodium channels, and developed septate-like junctions between BC axons. The neurochemical basis for GABAergic inhibition, however, has not been well investigated. Here we addressed this issue using C56BL/6 mice. First, we confirmed previous observations that typical synaptic contacts were rare and confined to proximal axonal portions, with the remaining portions being mostly covered by astrocytic processes. Then we examined the expression of molecules involved in GABAergic signaling, including GABA synthetic enzyme glutamic acid decarboxylase (GAD), vesicular GABA transporter vesicular inhibitory amino acid transporter (VIAAT), cytomatrix active zone protein bassoon, GABA receptor GABA(A)Rα1, and cell adhesion molecule neuroligin-2. These molecules were recruited to form a functional assembly at perisomatic BC-PC synapses and along the AIS of hippocampal and neocortical pyramidal cells. GAD and VIAAT immunogold labeling was five times lower in the pinceau formation compared with perisomatic BC terminals and showed no accumulation toward the AIS. Moreover, bassoon, neuroligin-2, and GABA(A)Rα1 formed no detectable clusters along the ankyrin-G-positive AIS proper. These findings indicate that GABAergic signaling machinery is organized loosely and even incompletely in the pinceau formation. Together, BCs do not appear to exert GABAergic synaptic inhibition on the AIS, although the mode of action of the pinceau formation remains to be explored.