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The brain’s Twitter system: Neuronal extracellular vesicles
Twitter, a microblogging and social networking service, has become a “go-to’” for conversations, updates, breaking news, and sharing the more mundane aspects of our lives. Tweets, which were lengthened from 140 to 280 characters in 2017, rapidly communicate and disseminate information to a wide audience. Generally, tweets are visible to everyone, though users can mute and block other users from viewing their tweets. Spikes in tweets and tweeting frequency reflect hyper-current events: the last minutes of the Super Bowl, certification of an election, or a new movie release. In fact, social scientists have analyzed tweet frequencies to examine the impact of local and national events. However, few are aware that like celebrities, politicians, influencers, and ordinary citizens, the human brain also tweets.
In this article, we describe the components of the brain’s “Twitter” system, how it works, and how it might someday be used to improve the diagnosis and treatment of psychiatric disorders.
Brain tweets
The brain’s Twitter system involves extracellular vesicles (EVs), tiny (<1 µm) membrane-bound vesicles that are released from neurons, glia, and other neuronal cells (Table). These EVs cross the blood-brain barrier and facilitate cell-to-cell communication within and among tissues (Figure 1).
First described in the 1980s,1 EVs are secreted by a diverse array of cells: mast cells reticulocytes, epithelial cells, immune cells, neurons, glia, and oligodendrocytes. Like tweets, EVs rapidly disseminate packets of information throughout the brain and body and direct the molecular activity of recipient cells in both health and disease. These “brain tweets” contain short, circumscribed messages, and the characters are the EV cargos: RNAs, proteins, lipids, and metabolites. Like a Twitter feed, EVs cast a wide communication net across the body, much of which finds its way to the blood. As neuroscientists, we can follow these tweets by isolating tissue-derived EVs in plasma and examining their surface molecules and cargo. By following this Twitter feed, we can tap into important molecular communications and identify “trending” (evolving) pathological processes, and perhaps use the brain Twitter feed to improve diagnosis and treatments. We can pinpoint, in the blood, signals from CNS processes, down to the level of identifying EV cargos from specific brain cell types.
Within the CNS, EVs are secreted by neurons, where they may modulate synaptic plasticity and transfer molecular cargo among neurons. EVs also facilitate communication between neurons and glia, maintain homeostasis, trigger neuroprotective processes, and even regulate synaptic transmission.2
What’s in a brain tweet?
To discuss what’s in a brain tweet, we must first understand how a brain tweet is composed. EVs are pinched off from membranes of intercellular structures (eg, golgi or endoplasmic reticulum) or pinched off directly from cell membranes, where upon release they become EVs. There is a complex cellular machinery that transports what ultimately becomes an EV to the cell membrane.3 EVs contain unique mixtures of lipids, proteins, and nucleic acids (eg, microRNA [miRNA], mRNA, and noncoding RNA).4 To date, nearly 10,000 proteins, 11,000 lipids, 3,500 mRNAs, and 3,000 miRNAs have been identified as cargos in extracellular vesicles (Figure 1). Similar to how the release of EVs is dependent on complex intracellular machinery, the packing of these contents into what will become the EV involves a parallel set of complex machinery that is largely directed by endosomal sorting complexes required for transport (ESCRT) proteins.5 Of interest, when viruses attack cells, they hijack this EV packaging system to package and release new viruses. EVs vary in size, shape, and density; this variation is related to the cell origin, among other things. EVs also differ in their membrane lipid composition and in terms of transmembrane proteins as well as the proteins that facilitate EV binding to target cells (Figure 2).6 Ultimately, these exosomes are taken up by the recipient cells.
EV-facilitated neuron-to-neuron tweets have been implicated in neuronal growth and differentiation.7 EV-driven communication between cells also can decrease dendrite growth and can trigger microglia to prune synapses.8 EVs from glial cells may promote neuronal integrity, directly boost presynaptic glutamate release,9 or even, through miRNAs, change the expression of glutamate receptors.10 EVs from astrocytes transport proteins that enable neuronal repair, while EVs from microglia regulate neuronal homeostasis. EV cargos—lipids, proteins, and miRNAs—from neurons modify signal transduction and protein expression in recipient cells. Taken together, data suggest that EVs facilitate anterograde and retrograde transfer of signals across synapses,7,11 a putative mechanism for driving synaptic plasticity,12 which is a process implicated in the therapeutic efficacy of psychotropic medications and psychotherapies.
Continue to: #Targets and #neuron
#Targets and #neuron
Adding a hashtag to a tweet links it to other tweets, just as membrane features of EVs direct how EVs link to target cells. When these EVs bind to target cells, they fuse and release their cargo into the target cell (Figure 2). These directed cargo—whether mRNA, proteins, or other molecules—can direct the recipient cell to modify its firing rate (in the case of neurons), alter transmitter release, and increase or decrease expression of various genes. The targeting process is complex, and our understanding of this process is evolving. Briefly, integrin, lipid composition, glycans (eg, polysaccharides), and tetraspanin components of EVs influence their affinity for specific target cells.13 Recently, we have been able to read these hashtags and isolate cell-specific, neuron-derived EVs. Immunoadsorption techniques that leverage antibodies against L1 cell adhesion molecule protein (L1CAM(+)), primarily expressed in neurons, can identify neuronally-derived EVs (Figure 3). The specific EVs contain cargos of neuronal origin and provide a “window” into molecular processes in the brain by way of the blood (or other peripheral fluids). In following the neuronal tweets, we can follow molecular measures of important brain molecules in biofluids outside the CNS, including saliva and potentially urine (Figure 1B and 1C). In following these specific neuronal Twitter feeds, we can gain critical insights into specific brain processes.
EVs in psychiatric disorders
EVs are implicated in neuroinflammation,14 neurogenesis, synaptic plasticity, and epigenetic regulation—all processes that are involved in the pathophysiology of psychiatric disorders. Postmortem research suggests that EVs in the brain carry proinflammatory molecules from microglia, as well as secretions of regulatory miRNA that are responsible for synaptic plasticity and dendritic growth in depression, bipolar disorder, schizophrenia, and addiction. In addition, second-generation antipsychotics change the composition of EV cargos in the brain, altering their RNA, protein, and lipid content, often reflecting profound changes in gene expression in various cells in the CNS. In our lab, we have identified several molecules in plasma EVs, both lipids and miRNA, that can potentially predict the response to treatment of pediatric anxiety with selective serotonin reuptake inhibitors as well as opiate addiction.15
Further, given our increasing understanding of the way in which EV cargo reflects neuronal physiology as well as the potential pathophysiologic states of cells (including neurons), studying EVs’ molecular content can identify molecular messages—in blood—that are derived from the neurons in the brain. Having the tools to examine molecular brain regulators or other markers of disease progression (eg, beta amyloid) or brain health (eg, brain-derived neurotrophic factor) may advance our understanding and treatment of psychiatric disorders and create opportunities for precision medicine driven by biological rather than ethnologic and phenomenological markers. Whereas in the not-too-distant past molecular processes in the brain were only accessible through invasive measures—such as brain biopsy or through a lumbar puncture—studying CNS-derived EVs in blood offers us an opportunity to gain access to brain molecular signatures with relative ease. Often, these molecular signatures predate clinical changes by years or months, allowing us the prospects of potentially identifying and treating CNS disorders early on, possibly even before the onset of symptoms.
Therapeutic use of the Twitter feed
EV may be used to alter brain receptor structures in a targeted way to facilitate treatment of various psychiatric disorders. One example is a proof-of-concept study in mice in which administration of artificially manufactured EVs led to a decrease of opioid receptor mu.16 This was done by constructing EVs that carry neuron-specific rabies viral glycoprotein (RVG) peptide on the membrane surface to deliver mu opioid receptor small interfering RNA into the brain. This resulted in downregulation of mu opioid receptor and a decrease in morphine relapse.16
Additional ways in which EVs can be used therapeutically is via targeted drug delivery CNS methods. EVs may represent the next generation of treatment by allowing not only medication transport into the CNS,17 but also by facilitating directed CNS transport. What if we could use a molecular hashtag to send a dopaminergic agent to the substantia nigra of a patient with Parkinson disease but avoid sending that same treatment to the limbic cortex, where it might produce perceptual disturbances or hallucinations? In the future, EVs may help clinicians access the CNS, which is traditionally restricted by the blood brain barrier, and make it easier to achieve CNS concentrations of medications13 while decreasing medication exposure in other parts of the body. The therapeutic potential of EVs for medication delivery and regenerative medicine is awe-inspiring. Several studies have modified EVs to improve their therapeutic properties and to target delivery to specific cells13 by leveraging EV surface markers.18
Future directions for EVs
A better understanding of neuron-derived EVs may eventually help us abandon nosology-based diagnostic criteria and adopt molecular-based diagnostic approaches in psychiatry. It may allow us to consider a molecular synaptic etiology of psychiatric disorders, and diagnose patients based on synaptic pathology utilizing “neuron-derived EV liquid biopsies.” Such a shift would align psychiatry with other medical fields in which diagnosis and treatment are often based on biopsies and blood tests. Because proteins in EVs often exist in their native states, intact with their posttranslational modifications, they provide a window into testing their actual in vivo functioning. EVs have an immense potential to revolutionize psychiatric diagnosis, facilitate precision treatment, predict response, and discover much-needed novel therapeutics.
Bottom Line
Much like a tweet, extracellular vesicles (EVs) encode short messages that are transmit ted efficiently throughout the CNS and body. They may represent a reservoir for CNS-specific biomarkers that can be is olated from plasma to guide psychiatric diagnosis and treatment. EVs represent a new frontier in the molecular study of psychiatric illness.
Related Resources
Vesiclepedia. www.microvesicles.org/
1. Harding C, Heuser J, Stahl P. Receptor-mediated endocytosis of transferrin and recycling of the transferrin receptor in rat reticulocytes. J Cell Biol. 1983;97(2):329-339. doi:10.1083/jcb.97.2.329
2. Huo L, Du X, Li X, et al. The emerging role of neural cell-derived exosomes in intercellular communication in health and neurodegenerative diseases. Front Neurosci. 2021;15:738442. doi:10.3389/fnins.2021
3. Raposo G, Stoorvogel W. Extracellular vesicles: exosomes, microvesicles, and friends. J Cell Biol. 2013;200(4):373-83. doi: 10.1083/jcb.201211138
4. Keerthikumar S, Chisanga D, Ariyaratne D, et al. ExoCarta: a web-based compendium of exosomal cargo. J Mol Biol. 2016;428(4):688-692. doi:10.1016/j.jmb.2015.09.019
5. Babst M. A protein’s final ESCRT. Traffic. 2005;6(1):2-9. doi:10.1111/j.1600-0854.2004.00246.x
6. Anakor E, Le Gall L, Dumonceaux J, et al. Exosomes in ageing and motor neurone disease: biogenesis, uptake mechanisms, modifications in disease and uses in the development of biomarkers and therapeutics. Cells. 2021;10(11)29-30. doi:10.3390/cells10112930
7. Chivet M, Javalet C, Hemming F, et al. Exosomes as a novel way of interneuronal communication. Biochem Soc Trans. 2013;41(1):241-244. doi:10.1042/BST20120266
8. Liu HY, Huang CM, Hung YF, et al. The microRNAs Let7c and miR21 are recognized by neuronal Toll-like receptor 7 to restrict dendritic growth of neurons. Exp Neurol. 2015;269:202-212. doi:10.1016/j.expneurol.2015.04.011
9. Antonucci F, Turola E, Riganti L, et al. Microvesicles released from microglia stimulate synaptic activity via enhanced sphingolipid metabolism. EMBO J. 2012;31(5):1231-1240. doi:10.1038/emboj.2011.489
10. Goncalves MB, Malmqvist T, Clarke E, et al. Neuronal RARβ signaling modulates PTEN activity directly in neurons and via exosome transfer in astrocytes to prevent glial scar formation and induce spinal cord regeneration. J Neurosci. 2015;35(47):15731-15745. doi:10.1523/JNEUROSCI.1339-15.2015
11. Korkut C, Li Y, Koles K, et al. Regulation of postsynaptic retrograde signaling by presynaptic exosome release. Neuron. 2013;77(6):1039-1046. doi:10.1016/j.neuron.2013.01.013
12. Chivet M, Javalet C, Laulagnier K, et al. Exosomes secreted by cortical neurons upon glutamatergic synapse activation specifically interact with neurons. J Extracell Vesicles. 2014;3(1):24722. doi:10.3402/jev.v3
13. Dickens AM, Tovar-Y-Romo LB, Yoo SW, et al. Astrocyte-shed extracellular vesicles regulate the peripheral leukocyte response to inflammatory brain lesions. Sci Signal. 2017;10(473). doi:10.1126/scisignal.aai7696
14. Strawn J, Levine A. Treatment response biomarkers in anxiety disorders: from neuroimaging to neuronally-derived extracellular vesicles and beyond. Biomark Neuropsychiatry. 2020;3:100024.
15. Liu Y, Li D, Liu Z, et al. Targeted exosome-mediated delivery of opioid receptor Mu siRNA for the treatment of morphine relapse. Sci Rep. 2015;5:17543. doi:10.1038/srep17543
16. Shahjin F, Chand S, Yelamanchili S V. Extracellular vesicles as drug delivery vehicles to the central nervous system. J Neuroimmune Pharmacol. 2020;15(3):443-458. doi:10.1007/s11481-019-09875-w
17. Murphy DE, de Jong OG, Brouwer M, et al. Extracellular vesicle-based therapeutics: natural versus engineered targeting and trafficking. Exp Mol Med. 2019;51(3):1-12. doi:10.1038/s12276-019-0223-5
18. Meng W, He C, Hao Y, et al. Prospects and challenges of extracellular vesicle-based drug delivery system: considering cell source. Drug Deliv. 2020;27(1):585-598. doi:10.1080/10717544.2020.1748758
Twitter, a microblogging and social networking service, has become a “go-to’” for conversations, updates, breaking news, and sharing the more mundane aspects of our lives. Tweets, which were lengthened from 140 to 280 characters in 2017, rapidly communicate and disseminate information to a wide audience. Generally, tweets are visible to everyone, though users can mute and block other users from viewing their tweets. Spikes in tweets and tweeting frequency reflect hyper-current events: the last minutes of the Super Bowl, certification of an election, or a new movie release. In fact, social scientists have analyzed tweet frequencies to examine the impact of local and national events. However, few are aware that like celebrities, politicians, influencers, and ordinary citizens, the human brain also tweets.
In this article, we describe the components of the brain’s “Twitter” system, how it works, and how it might someday be used to improve the diagnosis and treatment of psychiatric disorders.
Brain tweets
The brain’s Twitter system involves extracellular vesicles (EVs), tiny (<1 µm) membrane-bound vesicles that are released from neurons, glia, and other neuronal cells (Table). These EVs cross the blood-brain barrier and facilitate cell-to-cell communication within and among tissues (Figure 1).
First described in the 1980s,1 EVs are secreted by a diverse array of cells: mast cells reticulocytes, epithelial cells, immune cells, neurons, glia, and oligodendrocytes. Like tweets, EVs rapidly disseminate packets of information throughout the brain and body and direct the molecular activity of recipient cells in both health and disease. These “brain tweets” contain short, circumscribed messages, and the characters are the EV cargos: RNAs, proteins, lipids, and metabolites. Like a Twitter feed, EVs cast a wide communication net across the body, much of which finds its way to the blood. As neuroscientists, we can follow these tweets by isolating tissue-derived EVs in plasma and examining their surface molecules and cargo. By following this Twitter feed, we can tap into important molecular communications and identify “trending” (evolving) pathological processes, and perhaps use the brain Twitter feed to improve diagnosis and treatments. We can pinpoint, in the blood, signals from CNS processes, down to the level of identifying EV cargos from specific brain cell types.
Within the CNS, EVs are secreted by neurons, where they may modulate synaptic plasticity and transfer molecular cargo among neurons. EVs also facilitate communication between neurons and glia, maintain homeostasis, trigger neuroprotective processes, and even regulate synaptic transmission.2
What’s in a brain tweet?
To discuss what’s in a brain tweet, we must first understand how a brain tweet is composed. EVs are pinched off from membranes of intercellular structures (eg, golgi or endoplasmic reticulum) or pinched off directly from cell membranes, where upon release they become EVs. There is a complex cellular machinery that transports what ultimately becomes an EV to the cell membrane.3 EVs contain unique mixtures of lipids, proteins, and nucleic acids (eg, microRNA [miRNA], mRNA, and noncoding RNA).4 To date, nearly 10,000 proteins, 11,000 lipids, 3,500 mRNAs, and 3,000 miRNAs have been identified as cargos in extracellular vesicles (Figure 1). Similar to how the release of EVs is dependent on complex intracellular machinery, the packing of these contents into what will become the EV involves a parallel set of complex machinery that is largely directed by endosomal sorting complexes required for transport (ESCRT) proteins.5 Of interest, when viruses attack cells, they hijack this EV packaging system to package and release new viruses. EVs vary in size, shape, and density; this variation is related to the cell origin, among other things. EVs also differ in their membrane lipid composition and in terms of transmembrane proteins as well as the proteins that facilitate EV binding to target cells (Figure 2).6 Ultimately, these exosomes are taken up by the recipient cells.
EV-facilitated neuron-to-neuron tweets have been implicated in neuronal growth and differentiation.7 EV-driven communication between cells also can decrease dendrite growth and can trigger microglia to prune synapses.8 EVs from glial cells may promote neuronal integrity, directly boost presynaptic glutamate release,9 or even, through miRNAs, change the expression of glutamate receptors.10 EVs from astrocytes transport proteins that enable neuronal repair, while EVs from microglia regulate neuronal homeostasis. EV cargos—lipids, proteins, and miRNAs—from neurons modify signal transduction and protein expression in recipient cells. Taken together, data suggest that EVs facilitate anterograde and retrograde transfer of signals across synapses,7,11 a putative mechanism for driving synaptic plasticity,12 which is a process implicated in the therapeutic efficacy of psychotropic medications and psychotherapies.
Continue to: #Targets and #neuron
#Targets and #neuron
Adding a hashtag to a tweet links it to other tweets, just as membrane features of EVs direct how EVs link to target cells. When these EVs bind to target cells, they fuse and release their cargo into the target cell (Figure 2). These directed cargo—whether mRNA, proteins, or other molecules—can direct the recipient cell to modify its firing rate (in the case of neurons), alter transmitter release, and increase or decrease expression of various genes. The targeting process is complex, and our understanding of this process is evolving. Briefly, integrin, lipid composition, glycans (eg, polysaccharides), and tetraspanin components of EVs influence their affinity for specific target cells.13 Recently, we have been able to read these hashtags and isolate cell-specific, neuron-derived EVs. Immunoadsorption techniques that leverage antibodies against L1 cell adhesion molecule protein (L1CAM(+)), primarily expressed in neurons, can identify neuronally-derived EVs (Figure 3). The specific EVs contain cargos of neuronal origin and provide a “window” into molecular processes in the brain by way of the blood (or other peripheral fluids). In following the neuronal tweets, we can follow molecular measures of important brain molecules in biofluids outside the CNS, including saliva and potentially urine (Figure 1B and 1C). In following these specific neuronal Twitter feeds, we can gain critical insights into specific brain processes.
EVs in psychiatric disorders
EVs are implicated in neuroinflammation,14 neurogenesis, synaptic plasticity, and epigenetic regulation—all processes that are involved in the pathophysiology of psychiatric disorders. Postmortem research suggests that EVs in the brain carry proinflammatory molecules from microglia, as well as secretions of regulatory miRNA that are responsible for synaptic plasticity and dendritic growth in depression, bipolar disorder, schizophrenia, and addiction. In addition, second-generation antipsychotics change the composition of EV cargos in the brain, altering their RNA, protein, and lipid content, often reflecting profound changes in gene expression in various cells in the CNS. In our lab, we have identified several molecules in plasma EVs, both lipids and miRNA, that can potentially predict the response to treatment of pediatric anxiety with selective serotonin reuptake inhibitors as well as opiate addiction.15
Further, given our increasing understanding of the way in which EV cargo reflects neuronal physiology as well as the potential pathophysiologic states of cells (including neurons), studying EVs’ molecular content can identify molecular messages—in blood—that are derived from the neurons in the brain. Having the tools to examine molecular brain regulators or other markers of disease progression (eg, beta amyloid) or brain health (eg, brain-derived neurotrophic factor) may advance our understanding and treatment of psychiatric disorders and create opportunities for precision medicine driven by biological rather than ethnologic and phenomenological markers. Whereas in the not-too-distant past molecular processes in the brain were only accessible through invasive measures—such as brain biopsy or through a lumbar puncture—studying CNS-derived EVs in blood offers us an opportunity to gain access to brain molecular signatures with relative ease. Often, these molecular signatures predate clinical changes by years or months, allowing us the prospects of potentially identifying and treating CNS disorders early on, possibly even before the onset of symptoms.
Therapeutic use of the Twitter feed
EV may be used to alter brain receptor structures in a targeted way to facilitate treatment of various psychiatric disorders. One example is a proof-of-concept study in mice in which administration of artificially manufactured EVs led to a decrease of opioid receptor mu.16 This was done by constructing EVs that carry neuron-specific rabies viral glycoprotein (RVG) peptide on the membrane surface to deliver mu opioid receptor small interfering RNA into the brain. This resulted in downregulation of mu opioid receptor and a decrease in morphine relapse.16
Additional ways in which EVs can be used therapeutically is via targeted drug delivery CNS methods. EVs may represent the next generation of treatment by allowing not only medication transport into the CNS,17 but also by facilitating directed CNS transport. What if we could use a molecular hashtag to send a dopaminergic agent to the substantia nigra of a patient with Parkinson disease but avoid sending that same treatment to the limbic cortex, where it might produce perceptual disturbances or hallucinations? In the future, EVs may help clinicians access the CNS, which is traditionally restricted by the blood brain barrier, and make it easier to achieve CNS concentrations of medications13 while decreasing medication exposure in other parts of the body. The therapeutic potential of EVs for medication delivery and regenerative medicine is awe-inspiring. Several studies have modified EVs to improve their therapeutic properties and to target delivery to specific cells13 by leveraging EV surface markers.18
Future directions for EVs
A better understanding of neuron-derived EVs may eventually help us abandon nosology-based diagnostic criteria and adopt molecular-based diagnostic approaches in psychiatry. It may allow us to consider a molecular synaptic etiology of psychiatric disorders, and diagnose patients based on synaptic pathology utilizing “neuron-derived EV liquid biopsies.” Such a shift would align psychiatry with other medical fields in which diagnosis and treatment are often based on biopsies and blood tests. Because proteins in EVs often exist in their native states, intact with their posttranslational modifications, they provide a window into testing their actual in vivo functioning. EVs have an immense potential to revolutionize psychiatric diagnosis, facilitate precision treatment, predict response, and discover much-needed novel therapeutics.
Bottom Line
Much like a tweet, extracellular vesicles (EVs) encode short messages that are transmit ted efficiently throughout the CNS and body. They may represent a reservoir for CNS-specific biomarkers that can be is olated from plasma to guide psychiatric diagnosis and treatment. EVs represent a new frontier in the molecular study of psychiatric illness.
Related Resources
Vesiclepedia. www.microvesicles.org/
Twitter, a microblogging and social networking service, has become a “go-to’” for conversations, updates, breaking news, and sharing the more mundane aspects of our lives. Tweets, which were lengthened from 140 to 280 characters in 2017, rapidly communicate and disseminate information to a wide audience. Generally, tweets are visible to everyone, though users can mute and block other users from viewing their tweets. Spikes in tweets and tweeting frequency reflect hyper-current events: the last minutes of the Super Bowl, certification of an election, or a new movie release. In fact, social scientists have analyzed tweet frequencies to examine the impact of local and national events. However, few are aware that like celebrities, politicians, influencers, and ordinary citizens, the human brain also tweets.
In this article, we describe the components of the brain’s “Twitter” system, how it works, and how it might someday be used to improve the diagnosis and treatment of psychiatric disorders.
Brain tweets
The brain’s Twitter system involves extracellular vesicles (EVs), tiny (<1 µm) membrane-bound vesicles that are released from neurons, glia, and other neuronal cells (Table). These EVs cross the blood-brain barrier and facilitate cell-to-cell communication within and among tissues (Figure 1).
First described in the 1980s,1 EVs are secreted by a diverse array of cells: mast cells reticulocytes, epithelial cells, immune cells, neurons, glia, and oligodendrocytes. Like tweets, EVs rapidly disseminate packets of information throughout the brain and body and direct the molecular activity of recipient cells in both health and disease. These “brain tweets” contain short, circumscribed messages, and the characters are the EV cargos: RNAs, proteins, lipids, and metabolites. Like a Twitter feed, EVs cast a wide communication net across the body, much of which finds its way to the blood. As neuroscientists, we can follow these tweets by isolating tissue-derived EVs in plasma and examining their surface molecules and cargo. By following this Twitter feed, we can tap into important molecular communications and identify “trending” (evolving) pathological processes, and perhaps use the brain Twitter feed to improve diagnosis and treatments. We can pinpoint, in the blood, signals from CNS processes, down to the level of identifying EV cargos from specific brain cell types.
Within the CNS, EVs are secreted by neurons, where they may modulate synaptic plasticity and transfer molecular cargo among neurons. EVs also facilitate communication between neurons and glia, maintain homeostasis, trigger neuroprotective processes, and even regulate synaptic transmission.2
What’s in a brain tweet?
To discuss what’s in a brain tweet, we must first understand how a brain tweet is composed. EVs are pinched off from membranes of intercellular structures (eg, golgi or endoplasmic reticulum) or pinched off directly from cell membranes, where upon release they become EVs. There is a complex cellular machinery that transports what ultimately becomes an EV to the cell membrane.3 EVs contain unique mixtures of lipids, proteins, and nucleic acids (eg, microRNA [miRNA], mRNA, and noncoding RNA).4 To date, nearly 10,000 proteins, 11,000 lipids, 3,500 mRNAs, and 3,000 miRNAs have been identified as cargos in extracellular vesicles (Figure 1). Similar to how the release of EVs is dependent on complex intracellular machinery, the packing of these contents into what will become the EV involves a parallel set of complex machinery that is largely directed by endosomal sorting complexes required for transport (ESCRT) proteins.5 Of interest, when viruses attack cells, they hijack this EV packaging system to package and release new viruses. EVs vary in size, shape, and density; this variation is related to the cell origin, among other things. EVs also differ in their membrane lipid composition and in terms of transmembrane proteins as well as the proteins that facilitate EV binding to target cells (Figure 2).6 Ultimately, these exosomes are taken up by the recipient cells.
EV-facilitated neuron-to-neuron tweets have been implicated in neuronal growth and differentiation.7 EV-driven communication between cells also can decrease dendrite growth and can trigger microglia to prune synapses.8 EVs from glial cells may promote neuronal integrity, directly boost presynaptic glutamate release,9 or even, through miRNAs, change the expression of glutamate receptors.10 EVs from astrocytes transport proteins that enable neuronal repair, while EVs from microglia regulate neuronal homeostasis. EV cargos—lipids, proteins, and miRNAs—from neurons modify signal transduction and protein expression in recipient cells. Taken together, data suggest that EVs facilitate anterograde and retrograde transfer of signals across synapses,7,11 a putative mechanism for driving synaptic plasticity,12 which is a process implicated in the therapeutic efficacy of psychotropic medications and psychotherapies.
Continue to: #Targets and #neuron
#Targets and #neuron
Adding a hashtag to a tweet links it to other tweets, just as membrane features of EVs direct how EVs link to target cells. When these EVs bind to target cells, they fuse and release their cargo into the target cell (Figure 2). These directed cargo—whether mRNA, proteins, or other molecules—can direct the recipient cell to modify its firing rate (in the case of neurons), alter transmitter release, and increase or decrease expression of various genes. The targeting process is complex, and our understanding of this process is evolving. Briefly, integrin, lipid composition, glycans (eg, polysaccharides), and tetraspanin components of EVs influence their affinity for specific target cells.13 Recently, we have been able to read these hashtags and isolate cell-specific, neuron-derived EVs. Immunoadsorption techniques that leverage antibodies against L1 cell adhesion molecule protein (L1CAM(+)), primarily expressed in neurons, can identify neuronally-derived EVs (Figure 3). The specific EVs contain cargos of neuronal origin and provide a “window” into molecular processes in the brain by way of the blood (or other peripheral fluids). In following the neuronal tweets, we can follow molecular measures of important brain molecules in biofluids outside the CNS, including saliva and potentially urine (Figure 1B and 1C). In following these specific neuronal Twitter feeds, we can gain critical insights into specific brain processes.
EVs in psychiatric disorders
EVs are implicated in neuroinflammation,14 neurogenesis, synaptic plasticity, and epigenetic regulation—all processes that are involved in the pathophysiology of psychiatric disorders. Postmortem research suggests that EVs in the brain carry proinflammatory molecules from microglia, as well as secretions of regulatory miRNA that are responsible for synaptic plasticity and dendritic growth in depression, bipolar disorder, schizophrenia, and addiction. In addition, second-generation antipsychotics change the composition of EV cargos in the brain, altering their RNA, protein, and lipid content, often reflecting profound changes in gene expression in various cells in the CNS. In our lab, we have identified several molecules in plasma EVs, both lipids and miRNA, that can potentially predict the response to treatment of pediatric anxiety with selective serotonin reuptake inhibitors as well as opiate addiction.15
Further, given our increasing understanding of the way in which EV cargo reflects neuronal physiology as well as the potential pathophysiologic states of cells (including neurons), studying EVs’ molecular content can identify molecular messages—in blood—that are derived from the neurons in the brain. Having the tools to examine molecular brain regulators or other markers of disease progression (eg, beta amyloid) or brain health (eg, brain-derived neurotrophic factor) may advance our understanding and treatment of psychiatric disorders and create opportunities for precision medicine driven by biological rather than ethnologic and phenomenological markers. Whereas in the not-too-distant past molecular processes in the brain were only accessible through invasive measures—such as brain biopsy or through a lumbar puncture—studying CNS-derived EVs in blood offers us an opportunity to gain access to brain molecular signatures with relative ease. Often, these molecular signatures predate clinical changes by years or months, allowing us the prospects of potentially identifying and treating CNS disorders early on, possibly even before the onset of symptoms.
Therapeutic use of the Twitter feed
EV may be used to alter brain receptor structures in a targeted way to facilitate treatment of various psychiatric disorders. One example is a proof-of-concept study in mice in which administration of artificially manufactured EVs led to a decrease of opioid receptor mu.16 This was done by constructing EVs that carry neuron-specific rabies viral glycoprotein (RVG) peptide on the membrane surface to deliver mu opioid receptor small interfering RNA into the brain. This resulted in downregulation of mu opioid receptor and a decrease in morphine relapse.16
Additional ways in which EVs can be used therapeutically is via targeted drug delivery CNS methods. EVs may represent the next generation of treatment by allowing not only medication transport into the CNS,17 but also by facilitating directed CNS transport. What if we could use a molecular hashtag to send a dopaminergic agent to the substantia nigra of a patient with Parkinson disease but avoid sending that same treatment to the limbic cortex, where it might produce perceptual disturbances or hallucinations? In the future, EVs may help clinicians access the CNS, which is traditionally restricted by the blood brain barrier, and make it easier to achieve CNS concentrations of medications13 while decreasing medication exposure in other parts of the body. The therapeutic potential of EVs for medication delivery and regenerative medicine is awe-inspiring. Several studies have modified EVs to improve their therapeutic properties and to target delivery to specific cells13 by leveraging EV surface markers.18
Future directions for EVs
A better understanding of neuron-derived EVs may eventually help us abandon nosology-based diagnostic criteria and adopt molecular-based diagnostic approaches in psychiatry. It may allow us to consider a molecular synaptic etiology of psychiatric disorders, and diagnose patients based on synaptic pathology utilizing “neuron-derived EV liquid biopsies.” Such a shift would align psychiatry with other medical fields in which diagnosis and treatment are often based on biopsies and blood tests. Because proteins in EVs often exist in their native states, intact with their posttranslational modifications, they provide a window into testing their actual in vivo functioning. EVs have an immense potential to revolutionize psychiatric diagnosis, facilitate precision treatment, predict response, and discover much-needed novel therapeutics.
Bottom Line
Much like a tweet, extracellular vesicles (EVs) encode short messages that are transmit ted efficiently throughout the CNS and body. They may represent a reservoir for CNS-specific biomarkers that can be is olated from plasma to guide psychiatric diagnosis and treatment. EVs represent a new frontier in the molecular study of psychiatric illness.
Related Resources
Vesiclepedia. www.microvesicles.org/
1. Harding C, Heuser J, Stahl P. Receptor-mediated endocytosis of transferrin and recycling of the transferrin receptor in rat reticulocytes. J Cell Biol. 1983;97(2):329-339. doi:10.1083/jcb.97.2.329
2. Huo L, Du X, Li X, et al. The emerging role of neural cell-derived exosomes in intercellular communication in health and neurodegenerative diseases. Front Neurosci. 2021;15:738442. doi:10.3389/fnins.2021
3. Raposo G, Stoorvogel W. Extracellular vesicles: exosomes, microvesicles, and friends. J Cell Biol. 2013;200(4):373-83. doi: 10.1083/jcb.201211138
4. Keerthikumar S, Chisanga D, Ariyaratne D, et al. ExoCarta: a web-based compendium of exosomal cargo. J Mol Biol. 2016;428(4):688-692. doi:10.1016/j.jmb.2015.09.019
5. Babst M. A protein’s final ESCRT. Traffic. 2005;6(1):2-9. doi:10.1111/j.1600-0854.2004.00246.x
6. Anakor E, Le Gall L, Dumonceaux J, et al. Exosomes in ageing and motor neurone disease: biogenesis, uptake mechanisms, modifications in disease and uses in the development of biomarkers and therapeutics. Cells. 2021;10(11)29-30. doi:10.3390/cells10112930
7. Chivet M, Javalet C, Hemming F, et al. Exosomes as a novel way of interneuronal communication. Biochem Soc Trans. 2013;41(1):241-244. doi:10.1042/BST20120266
8. Liu HY, Huang CM, Hung YF, et al. The microRNAs Let7c and miR21 are recognized by neuronal Toll-like receptor 7 to restrict dendritic growth of neurons. Exp Neurol. 2015;269:202-212. doi:10.1016/j.expneurol.2015.04.011
9. Antonucci F, Turola E, Riganti L, et al. Microvesicles released from microglia stimulate synaptic activity via enhanced sphingolipid metabolism. EMBO J. 2012;31(5):1231-1240. doi:10.1038/emboj.2011.489
10. Goncalves MB, Malmqvist T, Clarke E, et al. Neuronal RARβ signaling modulates PTEN activity directly in neurons and via exosome transfer in astrocytes to prevent glial scar formation and induce spinal cord regeneration. J Neurosci. 2015;35(47):15731-15745. doi:10.1523/JNEUROSCI.1339-15.2015
11. Korkut C, Li Y, Koles K, et al. Regulation of postsynaptic retrograde signaling by presynaptic exosome release. Neuron. 2013;77(6):1039-1046. doi:10.1016/j.neuron.2013.01.013
12. Chivet M, Javalet C, Laulagnier K, et al. Exosomes secreted by cortical neurons upon glutamatergic synapse activation specifically interact with neurons. J Extracell Vesicles. 2014;3(1):24722. doi:10.3402/jev.v3
13. Dickens AM, Tovar-Y-Romo LB, Yoo SW, et al. Astrocyte-shed extracellular vesicles regulate the peripheral leukocyte response to inflammatory brain lesions. Sci Signal. 2017;10(473). doi:10.1126/scisignal.aai7696
14. Strawn J, Levine A. Treatment response biomarkers in anxiety disorders: from neuroimaging to neuronally-derived extracellular vesicles and beyond. Biomark Neuropsychiatry. 2020;3:100024.
15. Liu Y, Li D, Liu Z, et al. Targeted exosome-mediated delivery of opioid receptor Mu siRNA for the treatment of morphine relapse. Sci Rep. 2015;5:17543. doi:10.1038/srep17543
16. Shahjin F, Chand S, Yelamanchili S V. Extracellular vesicles as drug delivery vehicles to the central nervous system. J Neuroimmune Pharmacol. 2020;15(3):443-458. doi:10.1007/s11481-019-09875-w
17. Murphy DE, de Jong OG, Brouwer M, et al. Extracellular vesicle-based therapeutics: natural versus engineered targeting and trafficking. Exp Mol Med. 2019;51(3):1-12. doi:10.1038/s12276-019-0223-5
18. Meng W, He C, Hao Y, et al. Prospects and challenges of extracellular vesicle-based drug delivery system: considering cell source. Drug Deliv. 2020;27(1):585-598. doi:10.1080/10717544.2020.1748758
1. Harding C, Heuser J, Stahl P. Receptor-mediated endocytosis of transferrin and recycling of the transferrin receptor in rat reticulocytes. J Cell Biol. 1983;97(2):329-339. doi:10.1083/jcb.97.2.329
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3. Raposo G, Stoorvogel W. Extracellular vesicles: exosomes, microvesicles, and friends. J Cell Biol. 2013;200(4):373-83. doi: 10.1083/jcb.201211138
4. Keerthikumar S, Chisanga D, Ariyaratne D, et al. ExoCarta: a web-based compendium of exosomal cargo. J Mol Biol. 2016;428(4):688-692. doi:10.1016/j.jmb.2015.09.019
5. Babst M. A protein’s final ESCRT. Traffic. 2005;6(1):2-9. doi:10.1111/j.1600-0854.2004.00246.x
6. Anakor E, Le Gall L, Dumonceaux J, et al. Exosomes in ageing and motor neurone disease: biogenesis, uptake mechanisms, modifications in disease and uses in the development of biomarkers and therapeutics. Cells. 2021;10(11)29-30. doi:10.3390/cells10112930
7. Chivet M, Javalet C, Hemming F, et al. Exosomes as a novel way of interneuronal communication. Biochem Soc Trans. 2013;41(1):241-244. doi:10.1042/BST20120266
8. Liu HY, Huang CM, Hung YF, et al. The microRNAs Let7c and miR21 are recognized by neuronal Toll-like receptor 7 to restrict dendritic growth of neurons. Exp Neurol. 2015;269:202-212. doi:10.1016/j.expneurol.2015.04.011
9. Antonucci F, Turola E, Riganti L, et al. Microvesicles released from microglia stimulate synaptic activity via enhanced sphingolipid metabolism. EMBO J. 2012;31(5):1231-1240. doi:10.1038/emboj.2011.489
10. Goncalves MB, Malmqvist T, Clarke E, et al. Neuronal RARβ signaling modulates PTEN activity directly in neurons and via exosome transfer in astrocytes to prevent glial scar formation and induce spinal cord regeneration. J Neurosci. 2015;35(47):15731-15745. doi:10.1523/JNEUROSCI.1339-15.2015
11. Korkut C, Li Y, Koles K, et al. Regulation of postsynaptic retrograde signaling by presynaptic exosome release. Neuron. 2013;77(6):1039-1046. doi:10.1016/j.neuron.2013.01.013
12. Chivet M, Javalet C, Laulagnier K, et al. Exosomes secreted by cortical neurons upon glutamatergic synapse activation specifically interact with neurons. J Extracell Vesicles. 2014;3(1):24722. doi:10.3402/jev.v3
13. Dickens AM, Tovar-Y-Romo LB, Yoo SW, et al. Astrocyte-shed extracellular vesicles regulate the peripheral leukocyte response to inflammatory brain lesions. Sci Signal. 2017;10(473). doi:10.1126/scisignal.aai7696
14. Strawn J, Levine A. Treatment response biomarkers in anxiety disorders: from neuroimaging to neuronally-derived extracellular vesicles and beyond. Biomark Neuropsychiatry. 2020;3:100024.
15. Liu Y, Li D, Liu Z, et al. Targeted exosome-mediated delivery of opioid receptor Mu siRNA for the treatment of morphine relapse. Sci Rep. 2015;5:17543. doi:10.1038/srep17543
16. Shahjin F, Chand S, Yelamanchili S V. Extracellular vesicles as drug delivery vehicles to the central nervous system. J Neuroimmune Pharmacol. 2020;15(3):443-458. doi:10.1007/s11481-019-09875-w
17. Murphy DE, de Jong OG, Brouwer M, et al. Extracellular vesicle-based therapeutics: natural versus engineered targeting and trafficking. Exp Mol Med. 2019;51(3):1-12. doi:10.1038/s12276-019-0223-5
18. Meng W, He C, Hao Y, et al. Prospects and challenges of extracellular vesicle-based drug delivery system: considering cell source. Drug Deliv. 2020;27(1):585-598. doi:10.1080/10717544.2020.1748758
