Extracellular-vesicle type of volume transmission and tunnelling-nanotube type of wiring transmission add a new dimension to brain neuro-glial networks

Mapping GABAergic projections that mediate feeding

Neuroscience offers important insights into the pathogenesis and treatment of obesity by investigating neural circuits underpinning appetite and feeding. Gamma-aminobutyric acid (GABA), one of the most abundant neurotransmitters in the brain, and its associated receptors represent an array of pharmacologically targetable mediators of appetite signalling. Targeting the GABAergic system is therefore an increasingly investigated approach to obesity treatment. However, the many GABAergic projections that control feeding have yet to be collectively analysed. This review provides a comprehensive analysis of the relationship between GABAergic signalling and appetite by examining both foundational studies and the results of newly emerging chemogenetic/optogenetic experiments. A current snapshot of these efforts to map GABAergic projections influencing appetite is provided, and potential avenues for further investigation are provided.

The role of mitochondrial transfer via tunneling nanotubes in the central nervous system: A review

Tumour necrosis factor alpha-induced protein 2 (TNFAIP2) is a gene induced by tumor necrosis factor in endothelial cells. TNFAIP2 has important functions in physiological and pathological processes, including cell proliferation, adhesion, migration, angiogenesis, inflammation, tunneling nanotube (TNT) formation and tumorigenesis. Moreover, TNFAIP2 is the key factor in the formation of TNTs. TNTs are related to signal transduction between different cell types and are considered a novel means of cell-to-cell communication. Mesenchymal stem cells (MSCs) are pluripotent cells that exhibit self-renewal, multidirectional differentiation, paracrine function and immune-regulating ability. MSCs can transfer mitochondria through TNTs to improve the functions of target cells. This review revealed that TNFAIP2 promotes the formation of TNTs and that MSCs rely on TNTs for mitochondrial transfer to ameliorate cell dysfunction.

Tryptophan metabolism as a 'reflex' feature of neuroimmune communication: Sensor and effector functions for the indoleamine-2, 3-dioxygenase kynurenine pathway

Although the central nervous system (CNS) and immune system were regarded as independent entities, it is now clear that immune system cells can influence the CNS, and neuroglial activity influences the immune system. Despite the many clinical implications for this 'neuroimmune interface', its detailed operation at the molecular level remains unclear. This narrative review focuses on the metabolism of tryptophan along the kynurenine pathway, since its products have critical actions in both the nervous and immune systems, placing it in a unique position to influence neuroimmune communication. In particular, since the kynurenine pathway is activated by pro-inflammatory mediators, it is proposed that physical and psychological stressors are the stimuli of an organismal protective reflex, with kynurenine metabolites as the effector arm co-ordinating protective neural and immune system responses. After a brief review of the neuroimmune interface, the general perception of tryptophan metabolism along the kynurenine pathway is expanded to emphasize this environmentally driven perspective. The initial enzymes in the kynurenine pathway include indoleamine-2,3-dioxygenase (IDO1), which is induced by tissue damage, inflammatory mediators or microbial products, and tryptophan-2,3-dioxygenase (TDO), which is induced by stress-induced glucocorticoids. In the immune system, kynurenic acid modulates leucocyte differentiation, inflammatory balance and immune tolerance by activating aryl hydrocarbon receptors and modulates pain via the GPR35 protein. In the CNS, quinolinic acid activates N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors, whereas kynurenic acid is an antagonist: the balance between glutamate, quinolinic acid and kynurenic acid is a significant regulator of CNS function and plasticity. The concept of kynurenine and its metabolites as mediators of a reflex coordinated protection against stress helps to understand the variety and breadth of their activity. It should also help to understand the pathological origin of some psychiatric and neurodegenerative diseases involving the immune system and CNS, facilitating the development of new pharmacological strategies for treatment.

An integrated cytokine and kynurenine network as the basis of neuroimmune communication

Two of the molecular families closely associated with mediating communication between the brain and immune system are cytokines and the kynurenine metabolites of tryptophan. Both groups regulate neuron and glial activity in the central nervous system (CNS) and leukocyte function in the immune system, although neither group alone completely explains neuroimmune function, disease occurrence or severity. This essay suggests that the two families perform complementary functions generating an integrated network. The kynurenine pathway determines overall neuronal excitability and plasticity by modulating glutamate receptors and GPR35 activity across the CNS, and regulates general features of immune cell status, surveillance and tolerance which often involves the Aryl Hydrocarbon Receptor (AHR). Equally, cytokines and chemokines define and regulate specific populations of neurons, glia or immune system leukocytes, generating more specific responses within restricted CNS regions or leukocyte populations. In addition, as there is a much larger variety of these compounds, their homing properties enable the superimposition of dynamic variations of cell activity upon local, spatially limited, cell populations. This would in principle allow the targeting of potential treatments to restricted regions of the CNS. The proposed synergistic interface of 'tonic' kynurenine pathway affecting baseline activity and the superimposed 'phasic' cytokine system would constitute an integrated network explaining some features of neuroimmune communication. The concept would broaden the scope for the development of new treatments for disorders involving both the CNS and immune systems, with safer and more effective agents targeted to specific CNS regions.

Tunneling nanotubes and mesenchymal stem cells: New insights into the role of melatonin in neuronal recovery

Efficient cell-to-cell communication is essential for tissue development, homeostasis, and the maintenance of cellular functions after injury. Tunneling nanotubes (TNTs) have emerged as a new important method of cell-to-cell communication. TNTs are primarily established between stressed and unstressed cells and can transport a variety of cellular components. Mitochondria are important trafficked entities through TNTs. Transcellular mitochondria transfer permits the incorporation of healthy mitochondria into the endogenous network of recipient cells, changing the bioenergetic profile and other functional properties of the recipient and may allow the recipient cells to recuperate from apoptotic processes and return to a normal operating state. Mesenchymal cells (MSCs) can form TNTs and transfer mitochondria and other constituents to target cells. This occurs under both physiological and pathological conditions, leading to changes in cellular energy metabolism and functions. This review summarizes the newly described capacity of melatonin to improve mitochondrial fusion/fission dynamics and promote TNT formation. This new evidence suggests that melatonin's protective effects could be attributed to its ability to prevent mitochondrial damage in injured cells, reduce senescence, and promote anastasis, a natural cell recovery phenomenon that rescues cells from the brink of death. The modulation of these new routes of intercellular communication by melatonin could play a key role in increasing the therapeutic potential of MSCs.

Role of Extracellular Vesicles in Glia-Neuron Intercellular Communication

Cross talk between glia and neurons is crucial for a variety of biological functions, ranging from nervous system development, axonal conduction, synaptic transmission, neural circuit maturation, to homeostasis maintenance. Extracellular vesicles (EVs), which were initially described as cellular debris and were devoid of biological function, are now recognized as key components in cell-cell communication and play a critical role in glia-neuron communication. EVs transport the proteins, lipids, and nucleic acid cargo in intercellular communication, which alters target cells structurally and functionally. A better understanding of the roles of EVs in glia-neuron communication, both in physiological and pathological conditions, can aid in the discovery of novel therapeutic targets and the development of new biomarkers. This review aims to demonstrate that different types of glia and neuronal cells secrete various types of EVs, resulting in specific functions in intercellular communications.

Cell Heterogeneity Uncovered by Single-Cell RNA Sequencing Offers Potential Therapeutic Targets for Ischemic Stroke

Ischemic stroke is a detrimental neurological disease characterized by an irreversible infarct core surrounded by an ischemic penumbra, a salvageable region of brain tissue. Unique roles of distinct brain cell subpopulations within the neurovascular unit and peripheral immune cells during ischemic stroke remain elusive due to the heterogeneity of cells in the brain. Single-cell RNA sequencing (scRNA-seq) allows for an unbiased determination of cellular heterogeneity at high-resolution and identification of cell markers, thereby unveiling the principal brain clusters within the cell-type-specific gene expression patterns as well as cell-specific subclusters and their functions in different pathways underlying ischemic stroke. In this review, we have summarized the changes in differentiation trajectories of distinct cell types and highlighted the specific pathways and genes in brain cells that are impacted by stroke. This review is expected to inspire new research and provide directions for investigating the potential pathological mechanisms and novel treatment strategies for ischemic stroke at the level of a single cell.

Modulating brain integrative actions as a new perspective on pharmacological approaches to neuropsychiatric diseases

A critical aspect of drug development in the therapy of neuropsychiatric diseases is the "Target Problem", that is, the selection of a proper target after not simply the etiopathological classification but rather the detection of the supposed structural and/or functional alterations in the brain networks. There are novel ways of approaching the development of drugs capable of overcoming or at least reducing the deficits without triggering deleterious side effects. For this purpose, a model of brain network organization is needed, and the main aspects of its integrative actions must also be established. Thus, to this aim we here propose an updated model of the brain as a hyper-network in which i) the penta-partite synapses are suggested as key nodes of the brain hyper-network and ii) interacting cell surface receptors appear as both decoders of signals arriving to the network and targets of central nervous system diseases. The integrative actions of the brain networks follow the "Russian Doll organization" including the micro (i.e., synaptic) and nano (i.e., molecular) levels. In this scenario, integrative actions result primarily from protein-protein interactions. Importantly, the macromolecular complexes arising from these interactions often have novel structural binding sites of allosteric nature. Taking G protein-coupled receptors (GPCRs) as potential targets, GPCRs heteromers offer a way to increase the selectivity of pharmacological treatments if proper allosteric drugs are designed. This assumption is founded on the possible selectivity of allosteric interventions on G protein-coupled receptors especially when organized as "Receptor Mosaics" at penta-partite synapse level.

Nanotube-like processes facilitate material transfer between photoreceptors

Neuronal communication is typically mediated via synapses and gap junctions. New forms of intercellular communication, including nanotubes (NTs) and extracellular vesicles (EVs), have been described for non-neuronal cells, but their role in neuronal communication is not known. Recently, transfer of cytoplasmic material between donor and host neurons ("material transfer") was shown to occur after photoreceptor transplantation. The cellular mechanism(s) underlying this surprising finding are unknown. Here, using transplantation, primary neuronal cultures and the generation of chimeric retinae, we show for the first time that mammalian photoreceptor neurons can form open-end NT-like processes. These processes permit the transfer of cytoplasmic and membrane-bound molecules in culture and after transplantation and can mediate gain-of-function in the acceptor cells. Rarely, organelles were also observed to transfer. Strikingly, use of chimeric retinae revealed that material transfer can occur between photoreceptors in the intact adult retina. Conversely, while photoreceptors are capable of releasing EVs, at least in culture, these are taken up by glia and not by retinal neurons. Our findings provide the first evidence of functional NT-like processes forming between sensory neurons in culture and in vivo.

Potential Role of Exercise Induced Extracellular Vesicles in Prostate Cancer Suppression

Physical exercise is increasingly recognized as a valuable treatment strategy in managing prostate cancer, not only enhancing supportive care but potentially influencing disease outcomes. However, there are limited studies investigating mechanisms of the tumor-suppressive effect of exercise. Recently, extracellular vesicles (EVs) have been recognized as a therapeutic target for cancer as tumor-derived EVs have the potential to promote metastatic capacity by transferring oncogenic proteins, integrins, and microRNAs to other cells and EVs are also involved in developing drug resistance. Skeletal muscle has been identified as an endocrine organ, releasing EVs into the circulation, and levels of EV-containing factors have been shown to increase in response to exercise. Moreover, preclinical studies have demonstrated the tumor-suppressive effect of protein and microRNA contents in skeletal muscle-derived EVs in various cancers, including prostate cancer. Here we review current knowledge of the tumor-derived EVs in prostate cancer progression and metastasis, the role of exercise in skeletal muscle-derived EVs circulating levels and the alteration of their contents, and the potential tumor-suppressive effect of skeletal muscle-derived EV contents in prostate cancer. In addition, we review the proposed mechanism of exercise in the uptake of skeletal muscle-derived EVs in prostate cancer.

Emerging role of tumor microenvironment derived exosomes in therapeutic resistance and metastasis through epithelial-to-mesenchymal transition

The tumor microenvironment (TME) constitutes multiple cell types including cancerous and non-cancerous cells. The intercellular communication between these cells through TME derived exosomes may either enhance or suppress the tumorigenic processes. The tumor-derived exosomes could convert an anti-tumor environment into a pro-tumor environment by inducing the differentiation of stromal cells into tumor-associated cells. The exosomes from tumor-associated stromal cells reciprocally trigger epithelial-to-mesenchymal transition (EMT) in tumor cells, which impose therapeutic resistance and metastasis. It is well known that these exosomes contain the signals of EMT, but how these signals execute chemoresistance and metastasis in tumors remains elusive. Understanding the significance and molecular signatures of exosomes transmitting EMT signals would aid in developing appropriate methods of inhibiting them. In this review, we focus on molecular signatures of exosomes that shuttle between cancer cells and their stromal populations in TME to explicate their impact on therapeutic resistance and metastasis through EMT. Especially Wnt signaling is found to be involved in multiple ways of exosomal transport and hence we decipher the biomolecules of Wnt signaling trafficked through exosomes and their potential in serving as therapeutic targets.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Proteostatic imbalance and protein spreading in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder whose exact causative mechanisms are still under intense investigation. Several lines of evidence suggest that the anatomical and temporal propagation of pathological protein species along the neural axis could be among the main driving mechanisms for the fast and irreversible progression of ALS pathology. Many ALS-associated proteins form intracellular aggregates as a result of their intrinsic prion-like properties and/or following impairment of the protein quality control systems. During the disease course, these mutated proteins and aberrant peptides are released in the extracellular milieu as soluble or aggregated forms through a variety of mechanisms. Internalization by recipient cells may seed further aggregation and amplify existing proteostatic imbalances, thus triggering a vicious cycle that propagates pathology in vulnerable cells, such as motor neurons and other susceptible neuronal subtypes. Here, we provide an in-depth review of ALS pathology with a particular focus on the disease mechanisms of seeding and transmission of the most common ALS-associated proteins, including SOD1, FUS, TDP-43, and C9orf72-linked dipeptide repeats. For each of these proteins, we report historical, biochemical, and pathological evidence of their behaviors in ALS. We further discuss the possibility to harness pathological proteins as biomarkers and reflect on the implications of these findings for future research.

Microvesicles from Schwann-Like Cells as a New Biomaterial Promote Axonal Growth

Schwann cells promote axonal regeneration following peripheral nerve injury. However, in terms of clinical treatment, the therapeutic effects of Schwann cells are limited by their source. The transmission of microvesicles from neuroglia cells to axons is a novel communication mechanism in axon regeneration.To evaluate the effect of microvesicles released from Schwann-like cells on axonal regeneration, neural stem cells derived from human embryonic stem cells differentiated into Schwann-like cells, which presented a typical morphology and characteristics similar to those of schwann cells. The glial markers like MBP, P0, P75^NTR, PMP-22, GFAP, HNK-1 and S100 were upregulated, whereas the neural stem markers like NESTIN, SOX1 and SOX2 were significantly downregulated in schwann-like cells. Microvesicles enhanced axonal growth in dorsal root ganglia neurons and regulated GAP43 expression in neuron-like cells (N2A and PC12) through the PTEN/PI3 K/Akt signaling pathway. A 5 mm section of sciatic nerve was transected in Sprague-Dawley rats. With microvesicles transplantation, regenerative nerves were evaluated after 6 weeks. Microvesicles increased sciatic function index scores, delayed gastrocnemius muscle atrophy and elevated βIII-tubulin-labeled axons in vivo. Schwann-like cells serve as a convenient source and promote axonal growth by secreting microvesicles, which may potentially be used as bioengineering materials for nerve tissue repair.

Neural versus alternative integrative systems: molecular insights into origins of neurotransmitters

Transmitter signalling is the universal chemical language of any nervous system, but little is known about its early evolution. Here, we summarize data about the distribution and functions of neurotransmitter systems in basal metazoans as well as outline hypotheses of their origins. We explore the scenario that neurons arose from genetically different populations of secretory cells capable of volume chemical transmission and integration of behaviours without canonical synapses. The closest representation of this primordial organization is currently found in Placozoa, disk-like animals with the simplest known cell composition but complex behaviours. We propose that injury-related signalling was the evolutionary predecessor for integrative functions of early transmitters such as nitric oxide, ATP, protons, glutamate and small peptides. By contrast, acetylcholine, dopamine, noradrenaline, octopamine, serotonin and histamine were recruited as canonical neurotransmitters relatively later in animal evolution, only in bilaterians. Ligand-gated ion channels often preceded the establishment of novel neurotransmitter systems. Moreover, lineage-specific diversification of neurotransmitter receptors occurred in parallel within Cnidaria and several bilaterian lineages, including acoels. In summary, ancestral diversification of secretory signal molecules provides unique chemical microenvironments for behaviour-driven innovations that pave the way to complex brain functions and elementary cognition. This article is part of the theme issue 'Basal cognition: multicellularity, neurons and the cognitive lens'.

Potential role of exosome in post-stroke reorganization and/or neurodegeneration

Currently, stroke is a common and devastating condition, which is sometimes associated with permanent cerebral damages. Although in early time after stroke, the related treatments are mainly focused on the restoration of cerebral blood flow (CBF), at the same time, some changes are commencing that continue for a long time and need to be specially noticed. Previous studies have proposed several molecular mechanisms in these post-stroke events. Exosomes are a type of vesicle, which are formed and secreted by most cells as a mean to transfer cellular constituents such as proteins, DNA and/or RNA to distant cells. Therefore, they are considered as a novel mechanism of cellular communication. Herein, we reviewed the current knowledge on cascades, which are activated after stroke and consequently lead to the reorganization and/or continuance of tissue damage and development of other disorders such as Neurodegenerative diseases (ND). Thereafter, we summarized the latest proofs about the possible participation of exosomes in transferring some components such as proteins and micro-RNAs (miRs), from the affected areas to other parts of the brain and eventually cause the above-mentioned post-stroke events.

Tissue Micro-channels Formed by Collagen Fibers and their Internal Components: Cellular Evidence of Proposed Meridian Conduits in Vertebrate Skin

In order to clarify fine structures of the hypothetical meridian conduits of Chinese traditional medicine (CTM) in the skin, the present study used light and transmission electron microscopy to examine fasciae in different vertebrate species. Collagen fiber bundles and layers were arranged in a crisscross pattern, which developed into a special tissue micro-channel (TMC) network, in a manner that was analogs to the proposed skin meridian conduits. It was further revealed that tissue fluid in lateral TMC branches drained into wide longitudinal channels, which were distinctly different from lymphatic capillary. Mast cells, macrophages, and extracellular vesicles such as ectosomes and exosomes were distributed around telocytes (TCs) and their long processes (Telopodes, Tps) within the TMC. Cell junctions between TCs developed, which could enable the communication between contiguous but distant Tps. On the other hand, winding free Tps without cell junctions were also uncovered inside the TMC. Tissue fluid, cell junctions of TCs, mast cells, macrophages, and extracellular vesicles within the TMC corresponded to the circulating "" ("Qi-Xue", i.e., information, message, and energy) of meridian conduits at the cytological level. These results could provide morphological evidence for the hypothesis that "meridians are the conduit for Qi-Xue circulation" in CTM.

Alpha-Synuclein Aggregates Associated with Mitochondria in Tunnelling Nanotubes

The interaction of α-synuclein with mitochondria in both typical and atypical Parkinson's disease is a critical component of degeneration. The mechanism of cell-to-cell propagation of pathological α-synuclein in synucleinopathies is unclear. Intercellular exchange of mitochondria along tunnelling nanotubes has been described in other diseases, such as cancer; however, its role in synucleinopathies is unknown. Pathological α-synuclein species have been demonstrated previously to move from cell to cell via tunnelling nanotubes. This process was further explored using co-culture and monoculture systems to determine if α-synuclein binds to migrating mitochondria within tunnelling nanotubes. Super-resolution analysis via stimulated emission depletion microscopy showed interaction between α-synuclein with the mitochondrial outer membrane and the presence of alpha-synuclein associated with mitochondria in tunnelling nanotubes between 1321N1, differentiated THP-1 and SH-SY5Y cell types. siRNA knockdown of Miro1, a critical protein-bridging mitochondria to the motor adaptor complex, had no effect on mitochondrial density or α-synuclein association with mitochondria in tunnelling nanotubes. The results show that α-synuclein aggregates associate with mitochondria in intercellular tunnelling nanotubes, suggesting that mitochondria-mediated α-synuclein transfer between cells may contribute to cell-to-cell spread of α-synuclein aggregates and disease propagation.

Serotonergic Axons as Fractional Brownian Motion Paths: Insights Into the Self-Organization of Regional Densities

All vertebrate brains contain a dense matrix of thin fibers that release serotonin (5-hydroxytryptamine), a neurotransmitter that modulates a wide range of neural, glial, and vascular processes. Perturbations in the density of this matrix have been associated with a number of mental disorders, including autism and depression, but its self-organization and plasticity remain poorly understood. We introduce a model based on reflected Fractional Brownian Motion (FBM), a rigorously defined stochastic process, and show that it recapitulates some key features of regional serotonergic fiber densities. Specifically, we use supercomputing simulations to model fibers as FBM-paths in two-dimensional brain-like domains and demonstrate that the resultant steady state distributions approximate the fiber distributions in physical brain sections immunostained for the serotonin transporter (a marker for serotonergic axons in the adult brain). We suggest that this framework can support predictive descriptions and manipulations of the serotonergic matrix and that it can be further extended to incorporate the detailed physical properties of the fibers and their environment.

Exosomes as Actively Targeted Nanocarriers for Cancer Therapy

In recent years, it has been found that exosomes can be used as nanocarriers, which can be used in the treatment of tumors by carrying contents. The exosomes are derived from the secretion of the organism's own cells and are characterized by a phospholipid bilayer structure and a small particle size. These characteristics guarantee that the exosomes can carry a wide range of tumor drugs, deliver the drug to the cancer, and reduce or eliminate the tumor drug band. The toxic side effects were significantly eliminated; meanwhile, the therapeutic effects of the drug on the tumor were remarkably improved. This paper reviewed the strategies and drugs presented by different scholars for the treatment of tumors based on the drugs carried by exosomes.

Cell-to-Cell Communication in Learning and Memory: From Neuro- and Glio-Transmission to Information Exchange Mediated by Extracellular Vesicles

Most aspects of nervous system development and function rely on the continuous crosstalk between neurons and the variegated universe of non-neuronal cells surrounding them. The most extraordinary property of this cellular community is its ability to undergo adaptive modifications in response to environmental cues originating from inside or outside the body. Such ability, known as neuronal plasticity, allows long-lasting modifications of the strength, composition and efficacy of the connections between neurons, which constitutes the biochemical base for learning and memory. Nerve cells communicate with each other through both wiring (synaptic) and volume transmission of signals. It is by now clear that glial cells, and in particular astrocytes, also play critical roles in both modes by releasing different kinds of molecules (e.g., D-serine secreted by astrocytes). On the other hand, neurons produce factors that can regulate the activity of glial cells, including their ability to release regulatory molecules. In the last fifteen years it has been demonstrated that both neurons and glial cells release extracellular vesicles (EVs) of different kinds, both in physiologic and pathological conditions. Here we discuss the possible involvement of EVs in the events underlying learning and memory, in both physiologic and pathological conditions.

α-Synuclein Trafficking in Parkinson's Disease: Insights From Fly and Mouse Models

Protein aggregation and accumulation are common pathological hallmarks in neurodegenerative diseases. To efficiently clear and eliminate such aggregation becomes an important cellular strategy for cell survival. Lewy bodies inclusion and aggregation of α-Synuclein (α-Syn) during the pathogenesis of Parkinson's disease (PD) serve as a good example and are potentially linked to other pathological PD features such as progressive dopaminergic neuron cell death, behavioral defects, and nonmotor symptoms like anosmia, cognitive impairment, and depression. Years of research have revealed a variety of mechanisms underlying α-Syn aggregation, clearance, and spread. Particularly, vesicular routes associated with the trafficking of α-Syn, leading to its aggregation and accumulation, have been shown to play vital roles in PD pathogenesis. How α-Syn proteins propagate among cells in a prion-like manner, either from or to neurons and glia, via means of uptake or secretion, are questions under active investigation and have been of central interest in the field of PD study. This review covers components and pathways of possible vesicular routes involved in α-Syn trafficking. Events including but not limited to exocytosis and endocytosis will be discussed within the context of an overall cellular trafficking theme. Recent advances on α-Syn trafficking mechanisms and their significance in mediating PD pathogenesis will be thoroughly reviewed, ending with a discussion on the advantages and limitations of different animal PD models.

On the G Protein-Coupled Receptor Neuromodulation of the Claustrum

G protein-coupled receptors modulate the synaptic glutamate and GABA transmission of the claustrum. The work focused on the transmitter–receptor relationships in the claustral catecholamine system and receptor–receptor interactions between kappa opioid receptors (KOR) and SomatostatinR2 (SSTR2) in claustrum. Methods used involved immunohistochemistry and in situ proximity ligation assay (PLA) using confocal microscopy. Double immunolabeling studies on dopamine (DA) D1 receptor (D1R) and tyrosine hydroxylase (TH) immunoreactivities (IR) demonstrated that D1R IR existed in almost all claustral and dorsal endopiriform nucleus (DEn) nerve cell bodies, known as glutamate projection neurons, and D4R IR in large numbers of nerve cell bodies of the claustrum and DEn. However, only a low to moderate density of TH IR nerve terminals was observed in the DEn versus de few scattered TH IR terminals found in the claustrum. These results indicated that DA D1R and D4R transmission in the rat operated via long distance DA volume transmission in the rat claustrum and DEn to modulate claustral-sensory cortical glutamate transmission. Large numbers of these glutamate projection neurons also expressed KOR and SSTR2 which formed KOR-SSTR2 heteroreceptor complexes using PLA. Such receptor–receptor interactions can finetune the activity of the glutamate claustral-sensory cortex projections from inhibition to enhancement of their sensory cortex signaling. This can give the sensory cortical regions significant help in deciding on the salience to be given to various incoming sensory stimuli.

Adenosine heteroreceptor complexes in the basal ganglia are implicated in Parkinson's disease and its treatment

The adenosine homo, iso and heteroreceptor complexes in the basal ganglia play a highly significant role in modulating the indirect and direct pathways and the striosomal projections to the nigro-striatal DA system. The major adenosine receptor complexes in the striato-pallidal GABA neurons can be the A2AR-D2R and A2AR-D2R-mGluR5 receptor complexes, in which A2AR protomers and mGluR5 protomers can allosterically interact to inhibit D2R protomer signaling. Through a reorganization of these heteroreceptor complexes upon chronic dopaminergic treatment a pathological and prolonged inhibition of D2R receptor protomer signaling can develop with motor inhibition and wearing off of the therapeutic effects of levodopa and dopamine receptor agonists. The direct pathway is enriched in D1R in and around glutamate synapses enhancing the ability of these GABA neurons to be activated and increase motor initiation. The brake on these GABA neurons is in this case exerted by A1R forming A1R-D1R heteroreceptor complexes in which they allosterically inhibit D1R signaling and thereby reduce motor initiation. Upon chronic levodopa treatment a reorganization of the D1R heteroreceptor complexes develops with the formation of putative A1R-D1R-D3 in addition to D1R-D3R complexes in which D3R enhances D1R protomer signaling and may make the A1R protomer brake less effective. Alpha-synuclein monomers-dimers are postulated to form complexes with A2AR homo and heteroprotomers in the plasma membrane enhancing alpha-synuclein aggregation and toxicity. The alpha-synuclein fibrils formed in the A2AR enriched dendritic spines of the striato-pallidal GABA neurons may reach the surrounding DA terminals via extracellular-vesicle-mediated volume transmission involving internalization of the vesicles and their cargo (alpha-synuclein fibrils) into the vulnerable DA terminals, enhancing their degeneration followed by retrograde flow of these fibrils in the DA axons to the vulnerable nigral DA nerve cells.

Decoding epigenetic cell signaling in neuronal differentiation

Neurogenesis is the process by which new neurons are generated in the brain. Neural stem cells (NSCs) are differentiated into neurons, which are integrated into the neural network. Nowadays, pluripotent stem cells, multipotent stem cells, and induced pluripotent stem cells can be artificially differentiated into neurons utilizing several techniques. Specific transcriptional profiles from NSCs during differentiation are frequently used to approach and observe phenotype alteration and functional determination of neurons. In this context, the role of non-coding RNA, transcription factors and epigenetic changes in neuronal development and differentiation has gained importance. Epigenetic elucidation has become a field of intense research due to distinct patterns of normal conditions and different neurodegenerative disorders, which can be explored to develop new diagnostic methods or gene therapies. In this review, we discuss the complexity of transcription factors, non-coding RNAs, and extracellular vesicles that are responsible for guiding and coordinating neural development.

Tau Secretion

The pathological propagation of Tau protein is a hallmark of multiple neurodegenerative disorders, collectively referred to tauopathies with Alzheimer's disease (AD) being most prevalent, but including a range of frontotemporal dementias (FTDs). The extracellular Tau is important during the progression of tauopathies, although Tau is mainly expressed intracellularly for physiological functions. Extracellular Tau could be actively secreted by one cell then taken up by adjacent cells, leading to the cell-to-cell transmission of Tau. Accumulating evidence has demonstrated that Tau propagation is not only by the trans-synaptic spreading but also via exo-synaptic spreading pathways especially under the pathological conditions. Among these, exosomes, microvesicles and tunneling nanotubes (TNTs) are proposed exo-synaptic pathways for the spread of Tau pathology. These findings have led to the idea that extracellular Tau could be a novel therapeutic target to halt the propagation of Tau pathology. From this perspective, this charter focuses on recent advances in understanding the mechanisms of Tau secretion and discusses the role of such mechanisms in the development of Tau pathology.

Molecular Communication of a Dying Neuron in Stroke

When a main artery of the brain occludes, a cellular response involving multiple cell types follows. Cells directly affected by the lack of glucose and oxygen in the neuronal core die by necrosis. In the periphery surrounding the ischemic core (the so-called penumbra) neurons, astrocytes, microglia, oligodendrocytes, pericytes, and endothelial cells react to detrimental factors such as excitotoxicity, oxidative stress, and inflammation in different ways. The fate of the neurons in this area is multifactorial, and communication between all the players is important for survival. This review focuses on the latest research relating to synaptic loss and the release of apoptotic bodies and other extracellular vesicles for cellular communication in stroke. We also point out possible treatment options related to increasing neuronal survival and regeneration in the penumbra.

A stochastic approach to serotonergic fibers in mental disorders

Virtually all brain circuits are physically embedded in a three-dimensional matrix of fibers that release 5-hydroxytryptamine (5-HT, serotonin). The density of this matrix varies across brain regions and cortical laminae, and it is altered in some mental disorders, including Major Depressive Disorder and Autism Spectrum Disorder. We investigate how the regional structure of the serotonergic matrix depends on the stochastic behavior of individual serotonergic fibers and introduce a new framework for the quantitative analysis of this behavior. In particular, we show that a step-wise random walk, based on the von Mises-Fisher probability distribution, can provide a realistic and mathematically concise description of these fibers. We also consider other stochastic models, including the fractional Brownian motion. The proposed approach seeks to advance the current understanding of the ascending reticular activating system (ARAS) and may also support future theory-guided therapeutic approaches.

Unconventional Secretion and Intercellular Transfer of Mutant Huntingtin

The mechanism of intercellular transmission of pathological agents in neurodegenerative diseases has received much recent attention. Huntington’s disease (HD) is caused by a monogenic mutation in the gene encoding Huntingtin (HTT). Mutant HTT (mHTT) harbors a CAG repeat extension which encodes an abnormally long polyglutamine (polyQ) repeat at HTT’s N-terminus. Neuronal pathology in HD is largely due to the toxic gain-of-function by mHTT and its proteolytic products, which forms both nuclear and cytoplasmic aggregates that perturb nuclear gene transcription, RNA splicing and transport as well cellular membrane dynamics. The neuropathological effects of mHTT have been conventionally thought to be cell-autonomous in nature. Recent findings have, however, indicated that mHTT could be secreted by neurons, or transmitted from one neuronal cell to another via different modes of unconventional secretion, as well as via tunneling nanotubes (TNTs). These modes of transmission allow the intercellular spread of mHTT and its aggregates, thus plausibly promoting neuropathology within proximal neuronal populations and between neurons that are connected within neural circuits. Here, the various possible modes for mHTT’s neuronal cell exit and intercellular transmission are discussed.

Understanding the Role of Adenosine A2AR Heteroreceptor Complexes in Neurodegeneration and Neuroinflammation

Adenosine is a nucleoside mainly formed by degradation of ATP, located intracellularly or extracellularly, and acts as a neuromodulator. It operates as a volume transmission signal through diffusion and flow in the extracellular space to modulate the activity of both glial cells and neurons. The effects of adenosine are mediated via four adenosine receptor subtypes: A1R, A2AR, A2BR, A3R. The A2AR has a wide-spread distribution but it is especially enriched in the ventral and dorsal striatum where it is mainly located in the striato-pallidal GABA neurons at a synaptic and extrasynaptic location. A number of A2AR heteroreceptor complexes exist in the striatum. The existence of A2AR-D2R heteroreceptor complexes with antagonistic A2AR-D2R interactions in the striato-pallidal GABA neurons is well-known with A2AR activation inhibiting Gi/o mediated signaling of D2Rs. A2AR-mGluR5 heteroreceptor complexes were also found in with synergistic receptor-receptor interactions enhancing the inhibition of the D2R protomer signaling. They are located mainly in extrasynaptic regions of the striato-pallidal GABA neurons. Results recently demonstrated the existence of brain A2AR-A2BR heteroreceptor complexes, in which A2BR protomer constitutively inhibited the function of the A2AR protomer. These adenosine A2AR heteroreceptor complexes may modulate alpha-synuclein aggregation and toxicity through postulated bidirectional direct interactions leading to marked increases in A2AR signaling both in nerve cells and microglia. It is of high interest that formation of A2AR-A2ABR heteroreceptor complexes provides a brake on A2AR recognition and signaling opening up a novel strategy for treatment of A2AR mediated neurodegeneration.

Cocaine modulates allosteric D2-σ1 receptor-receptor interactions on dopamine and glutamate nerve terminals from rat striatum

The effects of nanomolar cocaine concentrations, possibly not blocking the dopamine transporter activity, on striatal D2-σ1 heteroreceptor complexes and their inhibitory signaling over Gi/o, have been tested in rat striatal synaptosomes and HEK293T cells. Furthermore, the possible role of σ1 receptors (σ1Rs) in the cocaine-provoked amplification of D2 receptor (D2R)-induced reduction of K+-evoked [3H]-DA and glutamate release from rat striatal synaptosomes, has also been investigated. The dopamine D2-likeR agonist quinpirole (10nM-1μM), concentration-dependently reduced K+-evoked [3H]-DA and glutamate release from rat striatal synaptosomes. The σ1R antagonist BD1063 (100nM), amplified the effects of quinpirole (10 and 100nM) on K+-evoked [3H]-DA, but not glutamate, release. Nanomolar cocaine concentrations significantly enhanced the quinpirole (100nM)-induced decrease of K+-evoked [3H]-DA and glutamate release from rat striatal synaptosomes. In the presence of BD1063 (10nM), cocaine failed to amplify the quinpirole (100nM)-induced effects. In cotransfected σ1R and D2LR HEK293T cells, quinpirole had a reduced potency to inhibit the CREB signal versus D2LR singly transfected cells. In the presence of cocaine (100nM), the potency of quinpirole to inhibit the CREB signal was restored. In D2L singly transfected cells cocaine (100nM and 10μM) exerted no modulatory effects on the inhibitory potency of quinpirole to bring down the CREB signal. These results led us to hypothesize the existence of functional D2-σ1R complexes on the rat striatal DA and glutamate nerve terminals and functional D2-σ1R-DA transporter complexes on the striatal DA terminals. Nanomolar cocaine concentrations appear to alter the allosteric receptor-receptor interactions in such complexes leading to enhancement of Gi/o mediated D2R signaling.

Local translation in neuronal compartments: how local is local?

Efficient neuronal function depends on the continued modulation of the local neuronal proteome. Local protein synthesis plays a central role in tuning the neuronal proteome at specific neuronal regions. Various aspects of translation such as the localization of translational machinery, spatial spread of the newly translated proteins, and their site of action are carried out in specialized neuronal subcompartments to result in a localized functional outcome. In this review, we focus on the various aspects of these local translation compartments such as size, biochemical and organelle composition, structural boundaries, and temporal dynamics. We also discuss the apparent absence of definitive components of translation in these local compartments and the emerging state-of-the-art tools that could help dissecting these conundrums in greater detail in the future.

Potential Modes of Intercellular α-Synuclein Transmission

Intracellular aggregates of the α-synuclein protein result in cell loss and dysfunction in Parkinson’s disease and atypical Parkinsonism, such as multiple system atrophy and dementia with Lewy bodies. Each of these neurodegenerative conditions, known collectively as α-synucleinopathies, may be characterized by a different suite of molecular triggers that initiate pathogenesis. The mechanisms whereby α-synuclein aggregates mediate cytotoxicity also remain to be fully elucidated. However, recent studies have implicated the cell-to-cell spread of α-synuclein as the major mode of disease propagation between brain regions during disease progression. Here, we review the current evidence for different modes of α-synuclein cellular release, movement and uptake, including exocytosis, exosomes, tunneling nanotubes, glymphatic flow and endocytosis. A more detailed understanding of the major modes by which α-synuclein pathology spreads throughout the brain may provide new targets for therapies that halt the progression of disease.

The Features and Functions of Neuronal Assemblies: Possible Dependency on Mechanisms beyond Synaptic Transmission

"Neuronal assemblies" are defined here as coalitions within the brain of millions of neurons extending in space up to 1-2 mm, and lasting for hundreds of milliseconds: as such they could potentially link bottom-up, micro-scale with top-down, macro-scale events. The perspective first compares the features in vitro versus in vivo of this underappreciated "meso-scale" level of brain processing, secondly considers the various diverse functions in which assemblies may play a pivotal part, and thirdly analyses whether the surprisingly spatially extensive and prolonged temporal properties of assemblies can be described exclusively in terms of classic synaptic transmission or whether additional, different types of signaling systems are likely to operate. Based on our own voltage-sensitive dye imaging (VSDI) data acquired in vitro we show how restriction to only one signaling process, i.e., synaptic transmission, is unlikely to be adequate for modeling the full profile of assemblies. Based on observations from VSDI with its protracted spatio-temporal scales, we suggest that two other, distinct processes are likely to play a significant role in assembly dynamics: "volume" transmission (the passive diffusion of diverse bioactive transmitters, hormones, and modulators), as well as electrotonic spread via gap junctions. We hypothesize that a combination of all three processes has the greatest potential for deriving a realistic model of assemblies and hence elucidating the various complex brain functions that they may mediate.

Extracellular Vesicles in Physiology, Pathology, and Therapy of the Immune and Central Nervous System, with Focus on Extracellular Vesicles Derived from Mesenchymal Stem Cells as Therapeutic Tools

Extracellular vesicles (EVs) are membrane-surrounded structures released by most cell types. They are characterized by a specific set of proteins, lipids and nucleic acids. EVs have been recognized as potent vehicles of intercellular communication to transmit biological signals between cells. In addition, pathophysiological roles of EVs in conditions like cancer, infectious diseases and neurodegenerative disorders are well established. In recent years focus has been shifted on therapeutic use of stem cell derived-EVs. Use of stem cell derived-EVs present distinct advantage over the whole stem cells as EVs do not replicate and after intravenous administration, they are less likely to trap inside the lungs. From the therapeutic perspective, the most promising cellular sources of EVs are mesenchymal stem cells (MSCs), which are easy to obtain and maintain. Therapeutic activity of MSCs has been shown in numerous animal models and the beneficial paracrine effect of MSCs may be mediated by EVs. The various components of MSC derived-EVs such as proteins, lipids, and RNA might play a specific therapeutic role. In this review, we characterize the role of EVs in immune and central nervous system (CNS); present evidences for defective signaling of these vesicles in neurodegeneration and therapeutic role of EVs in CNS.

Potential Transfer of Polyglutamine and CAG-Repeat RNA in Extracellular Vesicles in Huntington's Disease: Background and Evaluation in Cell Culture

In Huntington's disease (HD) the imperfect expanded CAG repeat in the first exon of the HTT gene leads to the generation of a polyglutamine (polyQ) protein, which has some neuronal toxicity, potentially mollified by formation of aggregates. Accumulated research, reviewed here, implicates both the polyQ protein and the expanded repeat RNA in causing toxicity leading to neurodegeneration in HD. Different theories have emerged as to how the neurodegeneration spreads throughout the brain, with one possibility being the transport of toxic protein and RNA in extracellular vesicles (EVs). Most cell types in the brain release EVs and these have been shown to contain neurodegenerative proteins in the case of prion protein and amyloid-beta peptide. In this study, we used a model culture system with an overexpression of HTT-exon 1 polyQ-GFP constructs in human 293T cells and found that the EVs did incorporate both the polyQ-GFP protein and expanded repeat RNA. Striatal mouse neural cells were able to take up these EVs with a consequent increase in the green fluorescent protein (GFP) and polyQ-GFP RNAs, but with no evidence of uptake of polyQ-GFP protein or any apparent toxicity, at least over a relatively short period of exposure. A differentiated striatal cell line expressing endogenous levels of Hdh mRNA containing the expanded repeat incorporated more of this mRNA into EVs as compared to similar cells expressing this mRNA with a normal repeat length. These findings support the potential of EVs to deliver toxic expanded trinucleotide repeat RNAs from one cell to another, but further work will be needed to evaluate potential EV and cell-type specificity of transfer and effects of long-term exposure. It seems likely that expanded HD-associated repeat RNA may appear in biofluids and may have use as biomarkers of disease state and response to therapy.

The role of transmitter diffusion and flow versus extracellular vesicles in volume transmission in the brain neural-glial networks

Two major types of intercellular communication are found in the central nervous system (CNS), namely wiring transmission (point-to-point communication, the prototype being synaptic transmission with axons and terminals) and volume transmission (VT; communication in the extracellular fluid and in the cerebrospinal fluid (CSF)) involving large numbers of cells in the CNS. Volume and synaptic transmission become integrated inter alia through the ability of their chemical signals to activate different types of receptor protomers in heteroreceptor complexes located synaptically or extrasynaptically in the plasma membrane. The demonstration of extracellular dopamine (DA) and serotonin (5-HT) fluorescence around the DA and 5-HT nerve cell bodies with the Falck-Hillarp formaldehyde fluorescence method after treatment with amphetamine and chlorimipramine, respectively, gave the first indications of the existence of VT in the brain, at least at the soma level. There exist different forms of VT. Early studies on VT only involved spread including diffusion and flow of soluble biological signals, especially transmitters and modulators, a communication called extrasynaptic (short distance) and long distance (paraaxonal and paravascular and CSF pathways) VT. Also, the extracellular vesicle type of VT was demonstrated. The exosomes (endosome-derived vesicles) appear to be the major vesicular carriers for VT but the larger microvesicles also participate. Both mainly originate at the soma-dendritic level. They can transfer lipids and proteins, including receptors, Rab GTPases, tetraspanins, cholesterol, sphingolipids and ceramide. Within them there are also subsets of mRNAs and non-coding regulatory microRNAs. At the soma-dendritic membrane, sets of dynamic postsynaptic heteroreceptor complexes (built up of different types of physically interacting receptors and proteins) involving inter alia G protein-coupled receptors including autoreceptors, ion channel receptors and receptor tyrosine kinases are hypothesized to be the molecular basis for learning and memory. At nerve terminals, the presynaptic heteroreceptor complexes are postulated to undergo plastic changes to maintain the pattern of multiple transmitter release reflecting the firing pattern to be learned by the heteroreceptor complexes in the postsynaptic membrane.

Regulation of neuronal excitability by release of proteins from glial cells

Effects of glial cells on electrical isolation and shaping of synaptic transmission between neurons have been extensively studied. Here we present evidence that the release of proteins from astrocytes as well as microglia may regulate voltage-activated Na(+) currents in neurons, thereby increasing excitability and speed of transmission in neurons kept at distance from each other by specialized glial cells. As a first example, we show that basic fibroblast growth factor and neurotrophin-3, which are released from astrocytes by exposure to thyroid hormone, influence each other to enhance Na(+) current density in cultured hippocampal neurons. As a second example, we show that the presence of microglia in hippocampal cultures can upregulate Na(+) current density. The effect can be boosted by lipopolysaccharides, bacterial membrane-derived stimulators of microglial activation. Comparable effects are induced by the exposure of neuron-enriched hippocampal cultures to tumour necrosis factor-α, which is released from stimulated microglia. Taken together, our findings suggest that release of proteins from various types of glial cells can alter neuronal excitability over a time course of several days. This explains changes in neuronal excitability occurring in states of thyroid hormone imbalance and possibly also in seizures triggered by infectious diseases.

Exosomes and other extracellular vesicles in neural cells and neurodegenerative diseases

The function of human nervous system is critically dependent on proper interneuronal communication. Exosomes and other extracellular vesicles are emerging as a novel form of information exchange within the nervous system. Intraluminal vesicles within multivesicular bodies (MVBs) can be transported in neural cells anterogradely or retrogradely in order to be released into the extracellular space as exosomes. RNA loading into exosomes can be either via an interaction between RNA and the raft-like region of the MVB limiting membrane, or via an interaction between an RNA-binding protein-RNA complex with this raft-like region. Outflow of exosomes from neural cells and inflow of exosomes into neural cells presumably take place on a continuous basis. Exosomes can play both neuro-protective and neuro-toxic roles. In this review, we characterize the role of exosomes and microvesicles in normal nervous system function, and summarize evidence for defective signaling of these vesicles in disease pathogenesis of some neurodegenerative diseases.

Exosomes as Novel Regulators of Adult Neurogenic Niches

Adult neurogenesis has been convincingly demonstrated in two regions of the mammalian brain: the sub-granular zone (SGZ) of the dentate gyrus (DG) in the hippocampus, and the sub-ventricular zone (SVZ) of the lateral ventricles (LV). SGZ newborn neurons are destined to the granular cell layer (GCL) of the DG, while new neurons from the SVZ neurons migrate rostrally into the olfactory bulb (OB). The process of adult neurogenesis persists throughout life and is supported by a pool of neural stem cells (NSCs), which reside in a unique and specialized microenvironment known as "neurogenic niche". Neurogenic niches are structured by a complex organization of different cell types, including the NSC-neuron lineage, glial cells and vascular cells. Thus, cell-to-cell communication plays a key role in the dynamic modulation of homeostasis and plasticity of the adult neurogenic process. Specific cell-cell contacts and extracellular signals originated locally provide the necessary support and regulate the balance between self-renewal and differentiation of NSCs. Furthermore, extracellular signals originated at distant locations, including other brain regions or systemic organs, may reach the niche through the cerebrospinal fluid (CSF) or the vasculature and influence its nature. The role of several secreted molecules, such as cytokines, growth factors, neurotransmitters, and hormones, in the biology of adult NSCs, has been systematically addressed. Interestingly, in addition to these well-recognized signals, a novel type of intercellular messengers has been identified recently: the extracellular vesicles (EVs). EVs, and particularly exosomes, are implicated in the transfer of mRNAs, microRNAs (miRNAs), proteins and lipids between cells and thus are able to modify the function of recipient cells. Exosomes appear to play a significant role in different stem cell niches such as the mesenchymal stem cell niche, cancer stem cell niche and pre-metastatic niche; however, their roles in adult neurogenic niches remain virtually unexplored. This review focuses on the current knowledge regarding the functional relationship between cellular and extracellular components of the adult SVZ and SGZ neurogenic niches, and the growing evidence that supports the potential role of exosomes in the physiology and pathology of adult neurogenesis.

Volume Transmission in Central Dopamine and Noradrenaline Neurons and Its Astroglial Targets

Already in the 1960s the architecture and pharmacology of the brainstem dopamine (DA) and noradrenaline (NA) neurons with formation of vast numbers of DA and NA terminal plexa of the central nervous system (CNS) indicated that they may not only communicate via synaptic transmission. In the 1980s the theory of volume transmission (VT) was introduced as a major communication together with synaptic transmission in the CNS. VT is an extracellular and cerebrospinal fluid transmission of chemical signals like transmitters, modulators etc. moving along energy gradients making diffusion and flow of VT signals possible. VT interacts with synaptic transmission mainly through direct receptor-receptor interactions in synaptic and extrasynaptic heteroreceptor complexes and their signaling cascades. The DA and NA neurons are specialized for extrasynaptic VT at the soma-dendrtitic and terminal level. The catecholamines released target multiple DA and adrenergic subtypes on nerve cells, astroglia and microglia which are the major cell components of the trophic units building up the neural-glial networks of the CNS. DA and NA VT can modulate not only the strength of synaptic transmission but also the VT signaling of the astroglia and microglia of high relevance for neuron-glia interactions. The catecholamine VT targeting astroglia can modulate the fundamental functions of astroglia observed in neuroenergetics, in the Glymphatic system, in the central renin-angiotensin system and in the production of long-distance calcium waves. Also the astrocytic and microglial DA and adrenergic receptor subtypes mediating DA and NA VT can be significant drug targets in neurological and psychiatric disease.

Diagnostic and therapeutic potentials of exosomes in CNS diseases

A newly discovered cell-to-cell communication system involves small, membrane-enveloped nanovesicles, called exosomes. We describe here how these extracellular nanoparticles were discovered and how it became gradually apparent that they play fundamental roles in regulation of physiological functions and pathological processes. Exosomes enable intercellular communication by transporting genetic material, proteins and lipids to cells in their vicinity or at distant sites, and subsequently regulating functions of targeted cells. Relatively recent experiments indicate that exosomes are released also by CNS cells, including cortical and hippocampal neurons, glial cells, astrocytes and oligodendrocytes, and that exosomes have significant impact on pathophysiology of the brain. How it is decided what individual exosomes will carry to their targets is not understood, but it appears that the contents may represent "signature cargos" that are characteristic for various conditions. Exploration of such characteristics could result in discovery of novel diagnostic biomarkers. Exosomes are also promising as a vehicle for therapeutic delivery of micro RNA or other compounds. How to deliver exosomes to selected sites has been a tantalizing question. Recent experiments revealed that at least some exosomes carry antibodies on their surface, suggesting that it may be feasible to deliver exosomes to unique sites based on the recognition of antigens by those antibodies. This discovery implies that rather precise targeting of both natural and engineered exosomes may be feasible. This would reduce distribution volume of therapeutics, and consequently minimize their side effects. This article is part of a Special Issue entitled Neuroimmunology in Health And Disease.