Park and Lee Experimental & Molecular Medicine (2020) 52:1028–1038 https://doi.org/10.1038/s12276-020-0468-z Experimental & Molecular Medicine REVIEW ARTICLE Open Access Deciphering the star codings: astrocyte manipulation alters mouse behavior Keebum Park1 andSungJoongLee2 Abstract Astrocytes occupy a vast area within the central nervous system (CNS). Despite their abundance, the functional role of astrocytes in vivo has only begun to be uncovered. Astrocytes were typically thought to be involved in pathophysiological states. However, recent studies have shown that astrocytes are actively involved in cell signaling in normal physiological states; manipulating various aspects of astrocytic cell signaling in vivo has revealed that astrocytes are key players in controlling healthy behavior in the absence of pathophysiology. Unfortunately, the study of astrocyte function is often limited by the number of approaches available due to our lack of understanding of cell physiology. This review summarizes recent studies in which altered astrocyte signaling capacity resulted in dramatic changes in behavior. We not only discuss the methodologies available to manipulate astrocytes but also provide insights into the behavioral roles of astrocytes in the CNS. Introduction physiological activities. Recent studies have also recog- Astrocytes, or “star-shaped” cells, which are as abundant nized the importance of the functional and regulatory 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; in our CNS as neurons, densely and homogeneously roles that astrocytes play under normal physiological populate the brain, spinal cord, and retina1,2. Although conditions. The final outputs that produce one’s behavior astrocytes are abundantly distributed across the entire are generally accepted to be neuronal; however, astrocytes brain, they have traditionally been regarded as “support often critically modulate this final output. This relation- cells” that play metabolic and homeostatic roles for ship requires dynamic cell signaling between neurons and nearby neurons3. The first evidence that neurons and astrocytes. Morphologically, astrocytes have numerous astrocytes exchange functional signaling was presented processes that are in close proximity to neurons, other – ~30 years ago4 6. Later studies found that the star-shaped astrocytes, and blood vessels12. Astrocytic processes make cells are particularly active in processing pain and contact with neuronal synapses and form tripartite – inflammation7 9. Since then, studies have mainly focused synapses. A single astrocyte can make as many as 140,000 on the role of astrocytes within the context of the brain’s contacts with neuronal synapses13. Astrocytes actively pathophysiological states10,11. However, their role in exchange signals with nearby cells to complement and physiological brain function in vivo has not been inves- modulate neuronal communications in their vicinity. tigated for a while. They often alter the electrophysiological properties of the Over the last decade, there has been accumulating evi- surrounding area14. Therefore, it would not be unex- dence that astrocytes actively participate in the brain’s pected for some of the astrocyte’s modulatory effects to influence behavioral outcomes in vivo. The hypothesis that astrocytes are directly involved in brain activity is Correspondence: Keebum Park ([email protected])or further supported by recent studies showing how these Sung Joong Lee ([email protected]) cells are transcriptionally, translationally, morphologi- 1 Interdisciplinary Program in Neuroscience, College of Natural Sciences, cally, and functionally diverse across brain regions15. This Seoul National University, Seoul, Republic of Korea 2Department of Physiology and Neuroscience, Dental Research Institute, evidence is important for understanding the more active Seoul National University School of Dentistry, Seoul, Republic of Korea © The Author(s) 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a linktotheCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Official journal of the Korean Society for Biochemistry and Molecular Biology Park and Lee Experimental & Molecular Medicine (2020) 52:1028–1038 1029 roles of astrocytes under various physiological states. While traditional GECIs allowed researchers to only + Efforts to uncover the effects of astrocytes have yielded observe a single Ca2 dynamics, recent technological valuable insights and tools for researchers. Consequently, advancements have enabled multilayered approaches to + astrocytes are no longer regarded as passive auxiliary imaging and measuring Ca2 dynamics. Modified support cells that function uniformly throughout GECIs24 keep similar wavelengths from interfering with the CNS. one another and are less sensitive to artificial stimuli such Despite such advances and endeavors, however, a large as light. Researchers are also able to mark neurons and portion of astrocyte physiology remains vague, and it is astrocytes simultaneously using multicolored GECIs25 to unclear how astrocytes process information to affect deduce temporal associations between regional cell types. + behavior16. Therefore, studies are needed to characterize Astrocytic Ca2 arises from multiple sources. A major + astrocyte functions as well as their roles in the modulation source of Ca2 elevation is the endoplasmic reticulum, in + of various behaviors. In this review, we highlight recent which Ca2 release is triggered via activated inositol tri- 26 studies in which artificial disruption or enhancement of phosphate (IP3) receptors . Interestingly, both Gq- astrocyte cell signaling capacity in vivo resulted in coupled and Gi-coupled G-protein coupled receptors + immediate behavioral changes. These studies used (GPCRs) have been shown to elevate Ca2 levels in – experimental designs including but not limited to phar- astrocytes27 29. While several studies that targeted the 2+ macological, genetic, optogenetic, and chemogenetic GPCR-IP3 pathway to control astrocytic Ca activity approaches to specifically target astrocyte signaling pro- have failed to identify any noticeable physiological − − cesses. Additionally, we summarize the evidence for and effects30,31, other studies performed with Ip3r2 / mice provide current perspectives on the roles that astrocytes have found that this receptor is mainly responsible for + + play in cell signaling and their critical impacts on healthy somatic Ca2 signals but not for Ca2 signaling in cell behaviors in rodents. processes32. This implies that there are other factors + contributing to Ca2 elevation in astrocytes, particularly Astrocytic cell signaling in cell processes. Furthermore, the presence of sponta- + Manipulating astrocytes requires a profound under- neous and seemingly random patches of Ca2 in astro- standing of their signaling processes because each step cytes is mediated by ion influx through transient receptor may be important in understanding astrocyte function. potential (TRP) A1 and V1 channels33,34. The TRPA1 + Astrocytes are not electrically excitable cells; nevertheless, channel is also responsible for maintaining resting Ca2 they express wide range of functional neurotransmitter levels33. More recently, it was shown that mitochondria + receptors17. One of the most prominent responses that mediate localized Ca2 transients in astrocyte micro- astrocytes have to an external input is elevating intracel- domains35. Taken together, these findings demonstrate + + lular Ca2 levels. Following the first report of astrocytic that astrocyte Ca2 signaling is highly dynamic. Assessing + Ca2 elevation18, there have been multiple attempts to the implications of astrocyte function requires analysis understand the dynamics, origins, and implications of this with more spatiotemporal specificity, as well as compar- process. These attempts have included the use of fluor- isons of spontaneous and evoked, fast and slow, and + escent Ca2 indicator dyes, such as fura-2, rhod-2, or fluo- global and focal signaling36. Nevertheless, studies that + 4, and genetically encoded calcium indicators (GECIs), address the sources of astrocytic Ca2 provide important such as the calmodulin-containing GCaMP series. Fluor- information on potential targets for manipulating the + + escent Ca2 indicators have been used to uncover several astrocytic Ca2 response. important physiological properties of astrocytes19,20 and After astrocytes receive signals and internally process + are still the primarily used indicators in in vitro studies. them (primarily through Ca2 elevation), they often emit However, some studies have reported difficulties in signals to nearby cells. There is strong evidence that maintaining uniform levels of the dyes across the entire astrocytes are able to release signaling substances16 and + cell