Temperature-dependent calcium-induced calcium release via InsP3 receptors in mouse olfactory ensheathing glial cells

Astrocytic uptake of posttranslationally modified amyloid-β leads to endolysosomal system disruption and induction of pro-inflammatory signaling

The disruption of astrocytic catabolic processes contributes to the impairment of amyloid-β (Aβ) clearance, neuroinflammatory signaling, and the loss of synaptic contacts in late-onset Alzheimer's disease (AD). While it is known that the posttranslational modifications of Aβ have significant implications on biophysical properties of the peptides, their consequences for clearance impairment are not well understood. It was previously shown that N-terminally pyroglutamylated Aβ3(pE)-42, a significant constituent of amyloid plaques, is efficiently taken up by astrocytes, leading to the release of pro-inflammatory cytokine tumor necrosis factor α and synapse loss. Here we report that Aβ3(pE)-42, but not Aβ1-42, gradually accumulates within the astrocytic endolysosomal system, disrupting this catabolic pathway and inducing the formation of heteromorphous vacuoles. This accumulation alters lysosomal kinetics, lysosome-dependent calcium signaling, and upregulates the lysosomal stress response. These changes correlate with the upregulation of glial fibrillary acidic protein (GFAP) and increased activity of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Treatment with a lysosomal protease inhibitor, E-64, rescues GFAP upregulation, NF-κB activation, and synapse loss, indicating that abnormal lysosomal protease activity is upstream of pro-inflammatory signaling and related synapse loss. Collectively, our data suggest that Aβ3(pE)-42-induced disruption of the astrocytic endolysosomal system leads to cytoplasmic leakage of lysosomal proteases, promoting pro-inflammatory signaling and synapse loss, hallmarks of AD-pathology.

ThermoCyte: an inexpensive open-source temperature control system for in vitro live-cell imaging

Live-cell imaging is a common technique in microscopy to investigate dynamic cellular behaviour and permits the accurate and relevant analysis of a wide range of cellular and tissue parameters, such as motility, cell division, wound healing responses and calcium (Ca2+) signalling in cell lines, primary cell cultures and ex vivo preparations. Furthermore, this can occur under many experimental conditions, making live-cell imaging indispensable for biological research. Systems which maintain cells at physiological conditions outside of a CO2 incubator are often bulky, expensive and use proprietary components. Here we present an inexpensive, open-source temperature control system for in vitro live-cell imaging. Our system 'ThermoCyte', which is constructed from standard electronic components, enables precise tuning, control and logging of a temperature 'set point' for imaging cells at physiological temperature. We achieved stable thermal dynamics, with reliable temperature cycling and a standard deviation of 0.42°C over 1 h. Furthermore, the device is modular in nature and is adaptable to the researcher's specific needs. This represents simple, inexpensive and reliable tool for laboratories to carry out custom live-cell imaging protocols, on a standard laboratory bench, at physiological temperature.

Sublamina-specific organization of the blood brain barrier in the mouse olfactory nerve layer

Astrocytes constitute the main glial component of the mammalian blood brain barrier (BBB). However, in the olfactory bulb (OB), the olfactory nerve layer (ONL) is almost devoid of astrocytes, raising the question which glial cells are part of the BBB. We used mice expressing EGFP in astrocytes and tdTomato in olfactory ensheathing cells (OECs), a specialized type of glial cells in the ONL, to unequivocally identify both glial cell types and investigate their contribution to the BBB in the olfactory bulb. OECs were located exclusively in the ONL, while somata of astrocytes were located in deeper layers and extended processes in the inner sublamina of the ONL. These processes surrounded blood vessels and contained aquaporin-4, an astrocytic protein enriched at the BBB. In the outer sublamina of the ONL, in contrast, blood vessels were surrounded by aquaporin-4-negative processes of OECs. Transcardial perfusion of blood vessels with lanthanum and subsequent visualization by electron microscopy showed that blood vessels enwrapped by OECs possessed intact tight junctions. In acute olfactory bulb preparations, injection of fluorescent glucose 6-NBDG into blood vessels resulted in labeling of OECs, indicating glucose transport from the perivascular space into OECs. In addition, Ca²⁺ transients in OECs in the outer sublamina evoked vasoconstriction, whereas Ca²⁺ signaling in OECs of the inner sublamina had no effect on adjacent blood vessels. Our results demonstrate that the BBB in the inner sublamina of the ONL contains astrocytes, while in the outer ONL OECs are part of the BBB.

AMPA Receptor-Mediated Ca2+ Transients in Mouse Olfactory Ensheathing Cells

Ca²⁺ signaling in glial cells is primarily triggered by metabotropic pathways and the subsequent Ca²⁺ release from internal Ca²⁺ stores. However, there is upcoming evidence that various ion channels might also initiate Ca²⁺ rises in glial cells by Ca²⁺ influx. We investigated AMPA receptor-mediated inward currents and Ca²⁺ transients in olfactory ensheathing cells (OECs), a specialized glial cell population in the olfactory bulb (OB), using whole-cell voltage-clamp recordings and confocal Ca²⁺ imaging. By immunohistochemistry we showed immunoreactivity to the AMPA receptor subunits GluA1, GluA2 and GluA4 in OECs, suggesting the presence of AMPA receptors in OECs. Kainate-induced inward currents were mediated exclusively by AMPA receptors, as they were sensitive to the specific AMPA receptor antagonist, GYKI53655. Moreover, kainate-induced inward currents were reduced by the selective Ca²⁺-permeable AMPA receptor inhibitor, NASPM, suggesting the presence of functional Ca²⁺-permeable AMPA receptors in OECs. Additionally, kainate application evoked Ca²⁺ transients in OECs which were abolished in the absence of extracellular Ca²⁺, indicating that Ca²⁺ influx via Ca²⁺-permeable AMPA receptors contribute to kainate-induced Ca²⁺ transients. However, kainate-induced Ca²⁺ transients were partly reduced upon Ca²⁺ store depletion, leading to the conclusion that Ca²⁺ influx via AMPA receptor channels is essential to trigger Ca²⁺ transients in OECs, whereas Ca²⁺ release from internal stores contributes in part to the kainate-evoked Ca²⁺ response. Endogenous glutamate release by OSN axons initiated Ca²⁺ transients in OECs, equally mediated by metabotropic receptors (glutamatergic and purinergic) and AMPA receptors, suggesting a prominent role for AMPA receptor mediated Ca²⁺ signaling in axon-OEC communication.

Purinergic Signaling in the Vertebrate Olfactory System

Adenosine 5'-triphosphate (ATP) is an ubiquitous co-transmitter in the vertebrate brain. ATP itself, as well as its breakdown products ADP and adenosine are involved in synaptic transmission and plasticity, neuron-glia communication and neural development. Although purinoceptors have been demonstrated in the vertebrate olfactory system by means of histological techniques for many years, detailed insights into physiological properties and functional significance of purinergic signaling in olfaction have been published only recently. We review the current literature on purinergic neuromodulation, neuron-glia interactions and neurogenesis in the vertebrate olfactory system.

Panglial gap junctions between astrocytes and olfactory ensheathing cells mediate transmission of Ca2+ transients and neurovascular coupling

Astrocytes are arranged in highly organized gap junction-coupled networks, communicating via the propagation of Ca²⁺ waves. Astrocytes are gap junction-coupled not only to neighboring astrocytes, but also to oligodendrocytes, forming so-called panglial syncytia. It is not known, however, whether glial cells in panglial syncytia transmit information using Ca²⁺ signaling. We used confocal Ca²⁺ imaging to study intercellular communication between astrocytes and olfactory ensheathing glial cells (OECs) in in-toto preparations of the mouse olfactory bulb. Our results demonstrate that Ca²⁺ transients in juxtaglomerular astrocytes, evoked by local photolysis of "caged" ATP and "caged" tACPD, led to subsequent Ca²⁺ responses in OECs. This transmission of Ca²⁺ responses from astrocytes to OECs persisted in the presence of neuronal inhibition, but was absent when gap junctional coupling was suppressed with carbenoxolone. When Ca²⁺ transients were directly evoked in OECs by puff application of DHPG, they resulted in delayed Ca²⁺ responses in juxtaglomerular astrocytes, indicating that panglial transmission of Ca²⁺ signals occurred in a bidirectional manner. In addition, panglial transmission of Ca²⁺ signals from astrocytes to OECs resulted in vasoconstriction of OEC-associated blood vessels in the olfactory nerve layer. Our results demonstrate functional transmission of Ca²⁺ signals between different classes of glial cells within gap junction-coupled panglial networks and the resulting regulation of blood vessel diameter in the olfactory bulb.

Normal cerebellar development in S100B-deficient mice

The calcium-binding protein S100B has been shown to support neuron proliferation, migration and neurite growth in vitro, while the significance of S100B for neuronal development in vivo is controversial. We have investigated the effect of S100B deficiency on cerebellar development in S100B knockout mice at an age of 5 and 10 days after birth (P5 and P10). This time range covers important developmental steps in the cerebellum such as granule cell proliferation and migration, as well as dendritic growth of Purkinje cells. Bergmann glial cells contain a particularly high concentration of S100B and serve as scaffold for both migrating granule cells and growing Purkinje cell dendrites. This renders the postnatal cerebellum ideal as a model system to study the importance of S100B for glial and neuronal development. We measured the length of Bergmann glial processes, the width of the external granule cell layer as a measure of granule cell proliferation, the decrease in width of the external granule cell layer between P5 and P10 as a measure of granule cell migration, and the length of Purkinje cell dendrites in wild-type and S100B knockout mice. None of these parameters showed significant differences between wild-type and knockout mice. In addition, wild-type and knockout mice performed equally in locomotor behaviour tests. The results indicate that S100B-deficient mice have normal development of the cerebellum and no severe impairment of motor function.

Piezoelectric Nanoparticle-Assisted Wireless Neuronal Stimulation

Tetragonal barium titanate nanoparticles (BTNPs) have been exploited as nanotransducers owing to their piezoelectric properties, in order to provide indirect electrical stimulation to SH-SY5Y neuron-like cells. Following application of ultrasounds to cells treated with BTNPs, fluorescence imaging of ion dynamics revealed that the synergic stimulation is able to elicit a significant cellular response in terms of calcium and sodium fluxes; moreover, tests with appropriate blockers demonstrated that voltage-gated membrane channels are activated. The hypothesis of piezoelectric stimulation of neuron-like cells was supported by lack of cellular response in the presence of cubic nonpiezoelectric BTNPs, and further corroborated by a simple electroelastic model of a BTNP subjected to ultrasounds, according to which the generated voltage is compatible with the values required for the activation of voltage-sensitive channels.

Microscopic heat pulse-induced calcium dynamics in single WI-38 fibroblasts

Temperature-sensitive Ca²⁺ dynamics occur primarily through transient receptor potential channels, but also by means of Ca²⁺ channels and pumps on the endoplasmic reticulum membrane. As such, cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyt) is re-equilibrated by changes in ambient temperature. The present study investigated the effects of heat pulses (heating duration: 2 s or 150 s) on [Ca²⁺]_cyt in single WI-38 fibroblasts, which are considered as normal cells. We found that Ca²⁺ burst occurred immediately after short (2 s) heat pulse, which is similar to our previous report on HeLa cells, but with less thermosensitivity. The heat pulses originated from a focused 1455-nm infrared laser light were applied in the vicinity of cells under the optical microscope. Ca²⁺ bursts induced by the heat pulse were suppressed by treating cells with inhibitors for sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) or inositol trisphosphate receptor (IP₃R). Long (150 s) heat pulses also induced Ca²⁺ bursts after the onset of heating and immediately after re-cooling. Cells were more thermosensitive at physiological (37°C) than at room (25°C) temperature; however, at 37°C, cells were responsive at a higher temperature (ambient temperature+heat pulse). These results strongly suggest that the heat pulse-induced Ca²⁺ burst is caused by a transient imbalance in Ca²⁺ flow between SERCA and IP₃R, and offer a potential new method for thermally controlling Ca²⁺-regulated cellular functions.

TRPs in olfaction

The mammalian olfactory system has become an excellent model system to understand the function of transient receptor potential (TRP) channels within their native cellular and circuit environment. The discovery that the canonical TRP channel TRPC2 is highly expressed in sensory neurons of the vomeronasal organ (VNO) has led to major advances in our understanding of the cellular and molecular processes underlying signal transduction of pheromones and other molecular cues that play an essential role in the control of instinctive decisions and innate social behaviors. TRPC2 knockout mice provide a striking example that the loss of function of a single gene can cause severe alterations in a variety of social interactions including the display of aggression, social dominance, and sexual behaviors. There is mounting evidence that TRPC2 is not the only TRP channel expressed in cells of the olfactory system but that other TRP channel subtypes such as TRPC1, TRPC4, TRPC6, TRPM4, and TRPM5 could also play important functional roles in mammalian olfaction. Here, I review such findings and discuss future areas for investigation.

Temperature dependence of IP3-mediated local and global Ca2+ signals

We examined the effect of temperature (12-40°C) on local and global Ca2+ signals mediated by inositol trisphosphate receptor/channels (IP3R) in human neuroblastoma (SH-SY5Y) cells. The amplitudes and spatial spread of local signals arising from single IP3R (blips) and clusters of IP3R (puffs) showed little temperature dependence, whereas their kinetics (durations and latencies) were markedly accelerated by increasing temperature. In contrast, the amplitude of global Ca2+ waves increased appreciably at lower temperatures, probably as a result of the longer duration of IP(3)R channel opening. Several parameters, including puff and blip durations, puff latency and frequency, and frequency of repetitive Ca2+ waves, showed a biphasic temperature dependence on Arrhenius plots. In all cases the transition temperature occurred at ∼25°C, possibly reflecting a phase transition in the lipids of the endoplasmic reticulum membrane. Although the IP3-evoked Ca2+ signals were qualitatively similar at 25°C and 36°C, one should consider the temperature sensitivity of IP3-mediated signal amplitudes when extrapolating from room temperature to physiological temperature. Conversely, further cooling may be advantageous to improve the optical resolution of channel gating kinetics.

Spatial and developmental heterogeneity of calcium signaling in olfactory ensheathing cells

Olfactory ensheathing cells (OECs) are specialized glial cells in the mammalian olfactory system supporting growth of axons from the olfactory epithelium into the olfactory bulb. OECs in the olfactory bulb can be subdivided into OECs of the outer nerve layer and the inner nerve layer according to the expression of marker proteins and their location in the nerve layer. In the present study, we have used confocal calcium imaging of OECs in acute mouse brain slices and olfactory bulbs in toto to investigate physiological differences between OEC subpopulations. OECs in the outer nerve layer, but not the inner nerve layer, responded to glutamate, ATP, serotonin, dopamine, carbachol, and phenylephrine with increases in the cytosolic calcium concentration. The calcium responses consisted of a transient and a tonic component, the latter being mediated by store-operated calcium entry. Calcium measurements in OECs during the first three postnatal weeks revealed a downregulation of mGluR(1) and P2Y(1) receptor-mediated calcium signaling within the first 2 weeks, suggesting that the expression of these receptors is developmentally controlled. In addition, electrical stimulation of sensory axons evoked calcium signaling via mGluR(1) and P2Y(1) only in outer nerve layer OECs. Downregulation of the receptor-mediated calcium responses in postnatal animals is reflected by a decrease in amplitude of stimulation-evoked calcium transients in OECs from postnatal days 3 to 21. In summary, the results presented reveal striking differences in receptor responses during development and in axon-OEC communication between the two subpopulations of OECs in the olfactory bulb.