Altered calcium signaling in Bergmann glia contributes to spinocerebellar ataxia type-1 in a mouse model of SCA1

Fingolimod Prevents Neuroinflammation but Has a Limited Effect on the Development of Ataxia in a Mouse Model for SCA1

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disorder that predominantly affects the Purkinje cells (PCs) of the cerebellum, leading to cerebellar degeneration, motor dysfunction, and cognitive impairment. Sphingosine-1-phosphate (S1P) signaling, known to modulate neuroinflammation, has been identified as a potential therapeutic target in SCA1. To investigate the therapeutic efficacy of the S1P modulator fingolimod, we treated a mouse model for SCA1, ATXN1[82Q]/+ mice during three different periods with fingolimod and assessed the effects. Potential therapeutic effects were monitored by tracking locomotion during the treatment period and examining PC morphology, connectivity, and markers for neuroinflammation post-mortem. Fingolimod treatment reduced astrocyte and microglial activation during all three treatment periods. We found no effect on calbindin levels or the thickness of the molecular layer, but fingolimod did improve the extent of the synaptic input of climbing fibers to PCs. While fingolimod improved important aspects of cellular pathology, we could only detect signs of improvement in the locomotion phenotype when treatment started at a later stage of the disease. In conclusion, fingolimod is able to mitigate neuroinflammation, preserve aspects of PC function in SCA1, and remediate part of the ataxia phenotype when treatment is appropriately timed. Although behavioral benefits were limited, targeting S1P pathways represents a potential therapeutic strategy for SCA1. Further studies are needed to optimize treatment regimens and assess long-term outcomes.

Estradiol Mediates Astrocyte-Neuron Communication in the Hippocampus

Accumulating evidence has revealed the existence of functional astrocyte-neuron communication based on the ability of astrocytes to respond to neurotransmitters and release gliotransmitters. However, little is known about how other signaling molecules, such as hormones, impact astrocyte function. Estradiol (E2) is an important hormone known to regulate neuronal activity, synaptic transmission, plasticity, and animal behavior. However, whether E2 specifically signals to astrocytes in situ and the functional consequences on astrocyte-neuron communication remain unknown. Therefore, we investigated the impact of estradiol on astrocyte activity and astrocyte-neuron communication in the mouse hippocampus. Using an RNAscope approach, we determined that estrogen receptors (ERα and ERβ) are expressed in astrocytes in both female and male mice. In both sexes, confocal imaging of hippocampal slices determined that astrocytes respond to locally applied E2 with calcium elevations. In pyramidal neurons, slow inward currents (SICs) are mediated by the activation of extrasynaptic NMDA receptors and indicate gliotransmission. Electrophysiological recordings of hippocampal neurons determined that E2 increases the frequency, but not the amplitude, of SICs. We also recorded excitatory synaptic transmission evoked by Schaffer collateral stimulation. Here, only in females, did E2 produce a reduction in excitatory synaptic transmission. The E2-induced effects on the astrocyte calcium signal and gliotransmission were prevented by the broad estrogen receptor antagonist ICI 182,780. Taken together, these results demonstrate the existence of estradiol-mediated astrocyte-neuron communication in both female and male mice. They reveal that E2 can signal to astrocytes and, through this signaling, E2 may regulate neuronal activity and synaptic transmission.

A spatial threshold for astrocyte calcium surge

Astrocytes are active cells involved in brain function through the bidirectional communication with neurons, in which astrocyte calcium plays a crucial role. Synaptically evoked calcium increases can be localized to independent subcellular domains or expand to the entire cell, i.e., calcium surge. Because a single astrocyte may contact ~100,000 synapses, the control of the intracellular calcium signal propagation may have relevant consequences on brain function. Yet, the properties governing the spatial dynamics of astrocyte calcium remains poorly defined. Imaging subcellular responses of cortical astrocytes to sensory stimulation in mice, we show that sensory-evoked astrocyte calcium responses originated and remained localized in domains of the astrocytic arborization, but eventually propagated to the entire cell if a spatial threshold of >23% of the arborization being activated was surpassed. Using Itpr2-/- mice, we found that type-2 IP3 receptors were necessary for the generation of astrocyte calcium surge. We finally show using in situ electrophysiological recordings that the spatial threshold of the astrocyte calcium signal consequently determined the gliotransmitter release. Present results reveal a fundamental property of astrocyte physiology, i.e., a spatial threshold for astrocyte calcium propagation, which depends on astrocyte intrinsic properties and governs astrocyte integration of local synaptic activity and subsequent neuromodulation.

A spatial threshold for astrocyte calcium surge

Astrocytes are active cells involved in brain function through the bidirectional communication with neurons, in which the astrocyte calcium signal plays a crucial role. Synaptically-evoked calcium increases can be localized to independent subcellular domains or expand to the entire cell, i.e., calcium surge. In turn, astrocytes may regulate individual synapses by calcium-dependent release of gliotransmitters. Because a single astrocyte may contact ∼100,000 synapses, the control of the intracellular calcium signal propagation may have relevant consequences on brain function by regulating the spatial range of astrocyte neuromodulation of synapses. Yet, the properties governing the spatial dynamics of the astrocyte calcium signal remains poorly defined. Imaging subcellular responses of cortical astrocytes to sensory stimulation in mice, we show that sensory-evoked astrocyte calcium responses originated and remained localized in domains of the astrocytic arborization, but eventually propagated to the entire cell if a spatial threshold of >23% of the arborization being activated was surpassed. Using transgenic IP 3 R2 -/- mice, we found that type-2 IP 3 receptors were necessary for the generation of the astrocyte calcium surge. We finally show using in situ electrophysiological recordings that the spatial threshold of the astrocyte calcium signal consequently determined the gliotransmitter release. Present results reveal a fundamental property of astrocyte calcium physiology, i.e., a spatial threshold for the astrocyte intracellular calcium signal propagation, which depends on astrocyte intrinsic properties and governs the astrocyte integration of local synaptic activity and the subsequent neuromodulation.

One-Sentence Summary

There is a spatial threshold for the astrocyte intracellular calcium signal propagation that is determined by astrocyte intrinsic properties and controls gliotransmission.