Challenging the Dogma: Reevaluating the Role of Astrocyte Calcium Signaling in Physiology

Gliotransmission represents one of the most important conceptual shifts in neuroscience in the past several decades. Gliotransmission refers to the process whereby glial cells release specific neurotransmitters that modulate synaptic transmission via pre- and postsynaptic activation of G-protein coupled (GPCR) and ionotropic receptors. Astrocytes release glutamate, ATP or D-serine upon activation of a wide range of Gq-GPCRs to modulate neuronal activity in the numerous regions of the brain. These "gliotransmitters" modulate spontaneous and evoked neuronal activity at both excitatory and inhibitory synapses, and affect heterosynaptic depression and long term potentiation in the hippocampus. The theory of astrocyte Ca2+-dependent modulation of neuronal activity is built upon the hypothesis that activation of Gq-GPCRs on astrocytes leads to Inositol trisphosphate (IP3) receptor-mediated calcium increases that trigger the release of gliotransmitters. However, evidence supporting this hypothesis has primarily relied on non-physiological methods for increasing or decreasing glial Ca2+. For example, uncaging of Ca2+ or chelation of intracellular Ca2+ in astrocytes affects synaptic transmission. However the usage of these methods does not specifically target IP3 receptor-mediated Ca2+ increases, considered the primary source of Ca2+ changes in astrocytes for gliotransmission. To probe the role of IP3 receptors (IP3R) in Ca2+-dependent release of gliotransmitters from astrocytes, we have used the IP3R type 2 (IP3R2) knockout mouse model. Through a combination of Ca2+ imaging and electrophysiology experiments we found that deletion of IP3R2 in astrocytes blocks intracellular Ca2+ increases in astrocytes in response to Gq GCPR activation. Further, that lack of IP3R-mediated Ca2+ increases does not affect excitatory synaptic transmission of both CA1 and CA3 pyramidal neurons in the hippocampus. Analysis of IP3R2 conditional knockout mice reveals specific behavioral changes in acoustic startle response and spatial learning in the Morris Water Maze. These novel findings represent a departure from the established theory of gliotransmission and are a significant step forward in our understanding of the role of astrocytic IP3R-mediated Ca2+ increases in physiology

Molecular approaches for manipulating astrocytic signaling in vivo

Astrocytes are the predominant glial type in the central nervous system and play important roles in assisting neuronal function and network activity. Astrocytes exhibit complex signaling systems that are essential for their normal function and the homeostasis of the neural network. Altered signaling in astrocytes is closely associated with neurological and psychiatric diseases, suggesting tremendous therapeutic potential of these cells. To further understand astrocyte function in health and disease, it is important to study astrocytic signaling in vivo. In this review, we discuss molecular tools that enable the selective manipulation of astrocytic signaling, including the tools to selectively activate and inactivate astrocyte signaling in vivo. Lastly, we highlight a few tools in development that present strong potential for advancing our understanding of the role of astrocytes in physiology, behavior, and pathology

Astrocyte IP3R2-dependent Ca2+ signaling is not a major modulator of neuronal pathways governing behavior

Calcium-dependent release of gliotransmitters by astrocytes is reported to play a critical role in synaptic transmission and be necessary for long-term potentiation (LTP), long-term depression (LTD) and other forms of synaptic modulation that are correlates of learning and memory. Further, physiological processes reported to be dependent on Ca2+ fluxes in astrocytes include functional hyperemia, sleep, and regulation of breathing. The preponderance of findings indicate that most, if not all, receptor dependent Ca2+ fluxes within astrocytes are due to release of Ca2+ through IP3 receptor/channels in the endoplasmic reticulum. Findings from several laboratories indicate that astrocytes only express IP3 receptor type 2 (IP3R2) and that a knockout of IP3R2 obliterates the GPCR-dependent astrocytic Ca2+ responses. Assuming that astrocytic Ca2+ fluxes play a critical role in synaptic physiology, it would be predicted that elimination of astrocytic Ca2+ fluxes would lead to marked changes in behavioral tests. Here, we tested this hypothesis by conducting a broad series of behavioral tests that recruited multiple brain regions, on an IP3R2 conditional knockout mouse model. We present the novel finding that behavioral processes are unaffected by lack of astrocyte IP3R-mediated Ca2+ signals. IP3R2 cKO animals display no change in anxiety or depressive behaviors, and no alteration to motor and sensory function. Morris water maze testing, a behavioral correlate of learning and memory, was unaffected by lack of astrocyte IP3R2-mediated Ca2+-signaling. Therefore, in contrast to the prevailing literature, we find that neither receptor-driven astrocyte Ca2+ fluxes nor, by extension, gliotransmission is likely to be a major modulating force on the physiological processes underlying behavior

What Is the Role of Astrocyte Calcium in Neurophysiology?

Astrocytes comprise approximately half of the volume of the adult mammalian brain and are the primary neuronal structural and trophic supportive elements. Astrocytes are organized into distinct nonoverlapping domains and extend elaborate and dense fine processes that interact intimately with synapses and cerebrovasculature. The recognition in the mid 1990s that astrocytes undergo elevations in intracellular calcium concentration following activation of G protein-coupled receptors by synaptically released neurotransmitters demonstrated not only that astrocytes display a form of excitability but also that astrocytes may be active participants in brain information processing. The roles that astrocytic calcium elevations play in neurophysiology and especially in modulation of neuronal activity have been intensely researched in recent years. This review will summarize the current understanding of the function of astrocytic calcium signaling in neurophysiological processes and discuss areas where the role of astrocytes remains controversial and will therefore benefit from further study

Astrocyte IP3R2-dependent Ca[superscript 2+] signaling is not a major modulator of neuronal pathways governing behavior

Calcium-dependent release of gliotransmitters by astrocytes is reported to play a critical role in synaptic transmission and be necessary for long-term potentiation (LTP), long-term depression (LTD) and other forms of synaptic modulation that are correlates of learning and memory. Further, physiological processes reported to be dependent on Ca[superscript 2+] fluxes in astrocytes include functional hyperemia, sleep, and regulation of breathing. The preponderance of findings indicate that most, if not all, receptor dependent Ca[superscript 2+] fluxes within astrocytes are due to release of Ca[superscript 2+] through IP3 receptor/channels in the endoplasmic reticulum. Findings from several laboratories indicate that astrocytes only express IP3 receptor type 2 (IP3R2) and that a knockout of IP3R2 obliterates the GPCR-dependent astrocytic Ca[superscript 2+] responses. Assuming that astrocytic Ca[superscript 2+] fluxes play a critical role in synaptic physiology, it would be predicted that elimination of astrocytic Ca[superscript 2+] fluxes would lead to marked changes in behavioral tests. Here, we tested this hypothesis by conducting a broad series of behavioral tests that recruited multiple brain regions, on an IP3R2 conditional knockout mouse model. We present the novel finding that behavioral processes are unaffected by lack of astrocyte IP3R-mediated Ca[superscript 2+] signals. IP3R2 cKO animals display no change in anxiety or depressive behaviors, and no alteration to motor and sensory function. Morris water maze testing, a behavioral correlate of learning and memory, was unaffected by lack of astrocyte IP3R2-mediated Ca[superscript 2+]-signaling. Therefore, in contrast to the prevailing literature, we find that neither receptor-driven astrocyte Ca[superscript 2+] fluxes nor, by extension, gliotransmission is likely to be a major modulating force on the physiological processes underlying behavior.National Institute of Neurological Disorders and Stroke (U.S.) (Grant NS020212)P30 HD0311

Astrocyte glutamate uptake coordinates experience‐dependent, eye‐specific refinement in developing visual cortex

The uptake of glutamate by astrocytes actively shapes synaptic transmission, however its role in the development and plasticity of neuronal circuits remains poorly understood. The astrocytic glutamate transporter, GLT1 is the predominant source of glutamate clearance in the adult mouse cortex. Here, we examined the structural and functional development of the visual cortex in GLT1 heterozygous (HET) mice using two-photon microscopy, immunohistochemistry and slice electrophysiology. We find that though eye-specific thalamic axonal segregation is intact, binocular refinement in the primary visual cortex is disrupted. Eye-specific responses to visual stimuli in GLT1 HET mice show altered binocular matching, with abnormally high responses to ipsilateral compared to contralateral eye stimulation and a greater mismatch between preferred orientation selectivity of ipsilateral and contralateral eye responses. Furthermore, we observe an increase in dendritic spine density in the basal dendrites of layer 2/3 excitatory neurons suggesting aberrant spine pruning. Monocular deprivation induces atypical ocular dominance plasticity in GLT1 HET mice, with an unusual depression of ipsilateral open eye responses; however, this change in ipsilateral responses correlates well with an upregulation of GLT1 protein following monocular deprivation. These results demonstrate that a key function of astrocytic GLT1 function during development is the experience-dependent refinement of ipsilateral eye inputs relative to contralateral eye inputs in visual cortex