Ca2+ dynamics within the neuronal endoplasmic reticulum upon synaptic input

Okubo, Y., Suzuki, J., Kanemaru, K., Nakamura, N., Shibata, T. and Iino, M.

Visualization of Ca2+ filling mechanisms upon synaptic inputs in the endoplasmic reticulum of cerebellar Purkinje cells.

J. Neurosci. 35(48):15837-15846, 2015 Abstract

Introduced in "This Week in The Journal" Crick here


We used G-CEPIA1er to visualize Ca2+ within the endoplasmic reticulum (ER) in cerebellar Purkinje cells. We found that the intraluminal diffusion of Ca2+, rather than Ca2+ reuptake, is the dominant mechanism for the replenishment of the local [Ca2+]ER depletion immediately following the parallel fiber stimulation. Thus, the neuronal ER functions as an intracellular tunnel to redistribute stored Ca2+ within the neurons. We also found that repetitive climbing fiber stimulation, which induces cytosolic Ca2+ spikes in PCs, cumulatively increased [Ca2+]ER. Thus, the ER also functions as a leaky integrator of Ca2+ spike-inducing synaptic inputs.

Ca2+ signaling regulates myelination

Daisuke Ino, Hiroshi Sagara, Junji Suzuki, Kazunori Kanemaru, Yohei Okubo and Masamitsu Iino

Neuronal Regulation of Schwann Cell Mitochondrial Ca2+ Signaling during Myelination

Cell Reports 2015 doi: 10.1016/j.celrep.2015.08.039 Article

We used microdialysis and in vivo transfection technique to demonstrate that PNS axons release ATP to regulate mitochondrial Ca2+ signaling in surrounding Schwann cells, and propose that this axon to Schwann cell signaling have an important role in proper myelination.

Finding Ca2+ twinkles in astrocytes in vivo

Kanemaru, K., Sekiya, K., Xu, M., Satoh, K., Kitajima, N., Yoshida, K., Okubo, Y., Sasaki, T., Moritoh, S., Hasuwa, H., Mimura, M., Horikawa, K., Matsui, K., Nagai, T., Iino, M., Tanaka, K.F.

In vivo visualization of subtle, transient and local activity of astrocytes using an ultrasensitive Ca2+ indicator

Cell Reports 2014 doi: 10.1016/j.celrep.2014.05.056. Abstract

Here, we have made it possible to image Ca2+ signals in the fine processes of individual astrocytes in vivo using transgenic mice that express an ultrasensitive genetically encoded Ca2+ indicator, YC-Nano50, in an astrocyte-specific manner. This method allowed us to find a previously unidentified mode of spontaneous astrocytic Ca2+ signals, Ca2+ twinkles, which are preferentially displayed in fine astrocytic processes in living mice brain. Moreover, a highly sensitive nature of astrocytic fine processes as a sensitive detector of neuronal activity was also revealed.

Imaging intraorganellar Ca2+ dynamics

Junji Suzuki, Kazunori Kanemaru, Kuniaki Ishii, Masamichi Ohkura, Yohei Okubo, Masamitsu Iino

Imaging intraorganellar Ca2+ at subcellular resolution using CEPIA

Nature Communications DOI 10.1038/ncomms5153. Abstract

Here, we describe a powerful technique that utilizes a family of genetically-encoded Ca2+ indicators, named CEPIA (for Calcium-measuring organelle-Entrapped Protein IndicAtors). CEPIA can be used to image ER and mitochondrial Ca2+ dynamics simultaneously with cytosolic Ca2+ concentration and other cellular processes at high spatiotemporal resolution.

Neuroprotection by reactive astrocytes

Kazunori Kanemaru, Jun Kubota, Hiroshi Sekiya, Kenzo Hirose, Yohei Okubo, Masamitsu Iino

Calcium-dependent N-cadherin upregulation mediates reactive astrogliosis and neuroprotection after brain injury

Proc Natl Acad Sci USA 110, 11612-11617, 2013. PubMed

Brain injury induces phenotypic changes in astrocytes, known as reactive astrogliosis, which may influence neuronal survival. Here we show that brain injury induces inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ signaling in astrocytes, and that the Ca2+ signaling is required for astrogliosis. We found that type 2 IP3 receptor knockout (IP3R2KO) mice deficient in astrocytic Ca2+ signaling have impaired reactive astrogliosis and increased injury-associated neuronal death. We identified N-cadherin and pumilio 2 (Pum2) as downstream signaling molecules, and found that brain injury induces up-regulation of N-cadherin around the injured site. This effect is mediated by Ca2+-dependent down-regulation of Pum2, which in turn attenuates Pum2-dependent translational repression of N-cadherin. Furthermore, we show that astrocyte-specific knockout of N-cadherin results in impairment of astrogliosis and neuroprotection. Thus, astrocytic Ca2+ signaling and the downstream function of N-cadherin play indispensable roles in the cellular responses to brain injury. These findings define a previously unreported signaling axis required for reactive astrogliosis and neuroprotection following brain injury.

New roles of NO signaling in the brain

Sho Kakizawa, Toshiko Yamazawa, Yili Chen, Akihiro Ito, Takashi Murayama, Hideto Oyamada, Nagomi Kurebayashi, Osamu Sato, Masahiko Watanabe, Nozomu Mori, Katsuji Oguchi, Takashi Sakurai, Hiroshi Takeshima, Nobuhito Saito and Masamitsu Iino

Nitric oxide-induced calcium release via ryanodine receptors regulates neuronal function

EMBO J. 31, 417-428, 2012. PubMed

Mobilization of intracellular Ca2+ stores regulates a multitude of cellular functions, but the role of intracellular Ca2+ release via the ryanodine receptor (RyR) in the brain remains incompletely understood. We found that nitric oxide (NO) directly activates RyRs, which induceCa2+ release from intracellular stores of central neurons, and thereby promote prolonged Ca2+ signalling in the brain. Reversible S-nitrosylation of type 1 RyR (RyR1) triggers this Ca2+ release. NO-induced Ca2+ release (NICR) is evoked by type 1 NO synthase-dependent NO production during neural firing, and is essential for cerebellar synaptic plasticity. NO production has also been implicated in pathological conditions including ischaemic brain injury, and our results suggest that NICR is involved in NO-induced neuronal cell death. These findings suggest that NICR via RyR1 plays a regulatory role in the physiological and pathophysiological functions of the brain.

New review paper (J. Physiol.)

Okubo, Y. and Iino, M.
Visualization of glutamate as a volume transmitter. 
J. Physiol. 589, 481-488, 2011.PubMed

Abstract: Glutamate is the major excitatory neurotransmitter in the central nervous system. Although glutamate mediates synaptically confined point-to-point transmission, it has been suggested that under certain conditions glutamate may escape from the synaptic cleft (glutamate spillover), accumulate in the extrasynaptic space, and mediate volume transmission to regulate important brain functions. However, the inability to directly measure glutamate dynamics around active synapses has limited our understanding of glutamatergic volume transmission. The recent development of a family of fluorescent glutamate indicators has enabled the visualization of extrasynaptic glutamate dynamics in brain tissues. In this topical review, we examine glutamate as a volume transmitter based on novel results of glutamate imaging in the brain.