Spatial and developmental heterogeneity of calcium signaling in olfactory ensheathing cells

Role of P2Y receptors in astrocyte physiology and pathophysiology

Astrocytes are active constituents of the brain that manage ion homeostasis and metabolic support of neurons and directly tune synaptic transmission and plasticity. Astrocytes express all known P2Y receptors. These regulate a multitude of physiological functions such as cell proliferation, Ca²⁺ signalling, gliotransmitter release and neurovascular coupling. In addition, P2Y receptors are fundamental in the transition of astrocytes into reactive astrocytes, as occurring in many brain disorders such as neurodegenerative diseases, neuroinflammation and epilepsy. This review summarizes the current literature addressing the function of P2Y receptors in astrocytes in the healthy brain as well as in brain diseases.

Norepinephrine-Induced Calcium Signaling and Store-Operated Calcium Entry in Olfactory Bulb Astrocytes

It is well-established that astrocytes respond to norepinephrine with cytosolic calcium rises in various brain areas, such as hippocampus or neocortex. However, less is known about the effect of norepinephrine on olfactory bulb astrocytes. In the present study, we used confocal calcium imaging and immunohistochemistry in mouse brain slices of the olfactory bulb, a brain region with a dense innervation of noradrenergic fibers, to investigate the calcium signaling evoked by norepinephrine in astrocytes. Our results show that application of norepinephrine leads to a cytosolic calcium rise in astrocytes which is independent of neuronal activity and mainly mediated by PLC/IP3-dependent internal calcium release. In addition, store-operated calcium entry (SOCE) contributes to the late phase of the response. Antagonists of both α1- and α2-adrenergic receptors, but not β-receptors, largely reduce the adrenergic calcium response, indicating that both α-receptor subtypes mediate norepinephrine-induced calcium transients in olfactory bulb astrocytes, whereas β-receptors do not contribute to the calcium transients.

Insights into olfactory ensheathing cell development from a laser-microdissection and transcriptome-profiling approach

Olfactory ensheathing cells (OECs) are neural crest-derived glia that ensheath bundles of olfactory axons from their peripheral origins in the olfactory epithelium to their central targets in the olfactory bulb. We took an unbiased laser microdissection and differential RNA-seq approach, validated by in situ hybridization, to identify candidate molecular mechanisms underlying mouse OEC development and differences with the neural crest-derived Schwann cells developing on other peripheral nerves. We identified 25 novel markers for developing OECs in the olfactory mucosa and/or the olfactory nerve layer surrounding the olfactory bulb, of which 15 were OEC-specific (that is, not expressed by Schwann cells). One pan-OEC-specific gene, Ptprz1, encodes a receptor-like tyrosine phosphatase that blocks oligodendrocyte differentiation. Mutant analysis suggests Ptprz1 may also act as a brake on OEC differentiation, and that its loss disrupts olfactory axon targeting. Overall, our results provide new insights into OEC development and the diversification of neural crest-derived glia.

Dopamine-induced calcium signaling in olfactory bulb astrocytes

It is well established that astrocytes respond to the major neurotransmitters glutamate and GABA with cytosolic calcium rises, whereas less is known about the effect of dopamine on astroglial cells. In the present study, we used confocal calcium imaging in mouse brain slices of the olfactory bulb, a brain region with a large population of dopaminergic neurons, to investigate calcium signaling evoked by dopamine in astrocytes. Our results show that application of dopamine leads to a dose-dependent cytosolic calcium rise in astrocytes (EC₅₀ = 76 µM) which is independent of neuronal activity and mainly mediated by PLC/IP₃-dependent internal calcium release. Antagonists of both D₁- and D₂-class dopamine receptors partly reduce the dopaminergic calcium response, indicating that both receptor classes contribute to dopamine-induced calcium transients in olfactory bulb astrocytes.

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.

Distribution of and steroid hormone effects on calbindin-D9k in the immature rat brain

Calbindin-D_9k (CaBP-9k), one of the major calcium-binding and calcium-buffering proteins, is important in the physiological functioning of organs. The neuroanatomical localization of CaBP-9k in the rodent brain has not been reported; thus, this study investigated the neuroanatomical distribution of CaBP-9k and the regulation of CaBP-9k expression on steroid hormones in the immature rat brain. To confirm the influence of steroid hormones on CaBP-9k expression, immature female rats were injected for 5 days with estrogen (E2), progesterone (P4), dexamethasone (DEX), and their antagonists (ICI 182, 780 and RU 486). The localization and expression of the CaBP-9k protein in brain regions were identified by immunofluorescence and western blot assays, respectively. We observed that CaBP-9k expression was especially strong in hypothalamus, cerebellum, and brain stem. In addition, CaBP-9k was colocalized with mature-, GABAergic, dopaminergic, and oxytocinergic neurons. We also observed that the CaBP-9k protein level was significantly increased by P4 and reversed by antagonist RU 486 treatment in immature rat brain. In summary, CaBP-9k positive cells have a wide distribution in the immature rat brain, and CaBP-9k expression is regulated by P4. We suggest that CaBP-9k expression regulated by steroid hormone may serve as an important regulator of cytosolic calcium concentration in the brain.

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.

Selective Cre-mediated gene deletion identifies connexin 43 as the main connexin channel supporting olfactory ensheathing cell networks

Many functions of glial cells depend on the formation of selective glial networks mediated by gap junctions formed by members of the connexin family. Olfactory ensheathing cells (OECs) are specialized glia associated with olfactory sensory neuron axons. Like other glia, they form selective networks, however, the connexins that support OEC connectivity in vivo have not been identified. We used an in vivo mouse model to selectively delete candidate connexin genes with temporal control from OECs and address the physiological consequences. Using this model, we effectively abolished the expression of connexin 43 (Cx43) in OECs in both juvenile and adult mice. Cx43-deleted OECs exhibited features consistent with the loss of gap junctions including reduced membrane conductance, largely reduced sensitivity to the gap junction blocker meclofenamic acid and loss of dye coupling. This indicates that Cx43, a typically astrocytic connexin, is the main connexin forming functional channels in OECs. Despite these changes in functional properties, the deletion of Cx43 deletion did not alter the density of OECs. The strategy used here may prove useful to delete other candidate genes to better understand the functional roles of OECs in vivo.

Adenosine A1 Receptor-Mediated Attenuation of Reciprocal Dendro-Dendritic Inhibition in the Mouse Olfactory Bulb

It is well described that A₁ adenosine receptors inhibit synaptic transmission at excitatory synapses in the brain, but the effect of adenosine on reciprocal synapses has not been studied so far. In the olfactory bulb, the majority of synapses are reciprocal dendro-dendritic synapses mediating recurrent inhibition. We studied the effect of A₁ receptor activation on recurrent dendro-dendritic inhibition in mitral cells using whole-cell patch-clamp recordings. Adenosine reduced dendro-dendritic inhibition in wild-type, but not in A₁ receptor knock-out mice. Both NMDA receptor-mediated and AMPA receptor-mediated dendro-dendritic inhibition were attenuated by adenosine, indicating that reciprocal synapses between mitral cells and granule cells as well as parvalbumin interneurons were targeted by A₁ receptors. Adenosine reduced glutamatergic self-excitation and inhibited N-type and P/Q-type calcium currents, but not L-type calcium currents in mitral cells. Attenuated glutamate release, due to A₁ receptor-mediated calcium channel inhibition, resulted in impaired dendro-dendritic inhibition. In behavioral tests we tested the ability of wild-type and A₁ receptor knock-out mice to find a hidden piece of food. Knock-out mice were significantly faster in locating the food. Our results indicate that A₁ adenosine receptors attenuates dendro-dendritic reciprocal inhibition and suggest that they affect odor information processing.

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.

Purinergic neurone-glia signalling in cognitive-related pathologies

Neuroglia, represented by astrocytes, oligodendrocytes, NG glia and microglia are homeostatic, myelinating and defensive cells of the brain. Neuroglial cells express various combinations of purinoceptors, which contribute to multiple intercellular signalling pathways in the healthy and diseased nervous system. Neurological diseases are invariably associated with profound neuroglial remodelling, which is manifest by reactive gliosis, pathological remodelling and functional atrophy of various types of glial cells. Gliopathology is disease and region specific and produces multiple glial phenotypes that may be neuroprotective or neurotoxic. In this review we summarise recent knowledge on the role of glial purinergic signalling in cognitive-related neurological diseases. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Voltage-dependent K+ currents contribute to heterogeneity of olfactory ensheathing cells

The olfactory nerve is permissive for axon growth throughout life. This has been attributed in part to the olfactory ensheathing glial cells that encompass the olfactory sensory neuron fascicles. Olfactory ensheathing cells (OECs) also promote axon growth in vitro and when transplanted in vivo to sites of injury. The mechanisms involved remain largely unidentified owing in part to the limited knowledge of the physiological properties of ensheathing cells. Glial cells rely for many functions on the properties of the potassium channels expressed; however, those expressed in ensheathing cells are unknown. Here we show that OECs express voltage-dependent potassium currents compatible with inward rectifier (Kir ) and delayed rectifier (KDR ) channels. Together with gap junction coupling, these contribute to the heterogeneity of membrane properties observed in OECs. The relevance of K(+) currents expressed by ensheathing cells is discussed in relation to plasticity of the olfactory nerve.

PACAP modulation of calcium ion activity in developing granule cells of the neonatal mouse olfactory bulb

Ca(2+) activity in the CNS is critical for the establishment of developing neuronal circuitry prior to and during early sensory input. In developing olfactory bulb (OB), the neuromodulators that enhance network activity are largely unknown. Here we provide evidence that pituitary adenylate cyclase-activating peptide (PACAP)-specific PAC1 receptors (PAC1Rs) expressed in postnatal day (P)2-P5 mouse OB are functional and enhance network activity as measured by increases in calcium in genetically identified granule cells (GCs). We used confocal Ca(2+) imaging of OB slices from Dlx2-tdTomato mice to visualize GABAergic GCs. To address whether the PACAP-induced Ca(2+) oscillations were direct or indirect effects of PAC1R activation, we used antagonists for the GABA receptors (GABARs) and/or glutamate receptors (GluRs) in the presence and absence of PACAP. Combined block of GABARs and GluRs yielded a 66% decrease in the numbers of PACAP-responsive cells, suggesting that 34% of OB neurons are directly activated by PACAP. Similarly, immunocytochemistry using anti-PAC1 antibody showed that 34% of OB neurons express PAC1R. Blocking either GluRs or GABARs alone indirectly showed that PACAP stimulates release of both glutamate and GABA, which activate GCs. The appearance of PACAP-induced Ca(2+) activity in immature GCs suggests a role for PACAP in GC maturation. To conclude, we find that PACAP has both direct and indirect effects on neonatal OB GABAergic cells and may enhance network activity by promoting glutamate and GABA release. Furthermore, the numbers of PACAP-responsive GCs significantly increased between P2 and P5, suggesting that PACAP-induced Ca(2+) activity contributes to neonatal OB development.

Mosaic activity patterns and their relation to perceptual similarity: open discussions on the molecular basis and circuitry of odor recognition

Enormous advances have been made in the recent years in regard to the mechanisms and neural circuits by which odors are sensed and perceived. Part of this understanding has been gained from parallel studies in insects and rodents that show striking similarity in the mechanisms they use to sense, encode, and perceive odors. In this review, we provide a short introduction to the functioning of olfactory systems from transduction of odorant stimuli into electrical signals in sensory neurons to the anatomical and functional organization of the networks involved in neural representation of odors in the central nervous system. We make emphasis on the functional and anatomical architecture of the first synaptic relay of the olfactory circuit, the olfactory bulb in vertebrates and the antennal lobe in insects. We discuss how the exquisite and conserved architecture of this structure is established and how different odors are encoded in mosaic activity patterns. Finally, we discuss the validity of methods used to compare activation patterns in relation to perceptual similarity.

Purinergic signaling negatively regulates activity of an olfactory receptor in an odorant-dependent manner

Extracellular purines and pyrimidines are important signaling molecules that mediate diverse biological functions via cell surface purinergic receptors. Although purinergic modulation to olfactory activity has been reported, cell-specific expression and action of purinergic receptors deserve further exploration. We physiologically characterized expression of purinergic receptors in a set of olfactory sensory neurons that are responsive to both acetophenone and benzaldehyde (AB-OSNs). Sparsely distributed in the most ventral olfactory receptor zone, AB-OSNs were activated by P2 purinergic receptor agonists but not by P1 purinergic receptor agonist adenosine. Both P2X-selective agonist α,β-methylene ATP and P2Y-selective agonist uridine 5'-triphosphate (UTP) were stimulatory to AB-OSNs, indicating expression of both P2X and P2Y purinergic receptors in AB-OSNs. Pharmacological characterization of receptor specificity using various P2X and P2Y agonists and antagonists illustrated that P2X1 and P2Y2 receptors played major roles in purinergic signaling in AB-OSNs. Interestingly, the results of purinergic modulation to acetophenone-evoked responses were different from those to benzaldehyde-evoked responses within the same neurons. Activation of P2X1 receptors had more profound inhibitory effects on benzaldehyde-evoked intracellular calcium elevation than on acetophenone-evoked responses within the same neurons, and the reverse was true when P2Y2 receptors were activated. Cross-adaptation data showed that acetophenone and benzaldehyde bound to the same olfactory receptor. Thus, our study has demonstrated that purinergic signaling of P2X and P2Y receptors has different effects on olfactory transduction mediated by a defined olfactory receptor and the consequences of purinergic modulation of olfactory activity might depend on stereotypic structures of the odorant-receptor complex.

Transcription factor Runx1 inhibits proliferation and promotes developmental maturation in a selected population of inner olfactory nerve layer olfactory ensheathing cells

The olfactory system undergoes persistent regeneration throughout life. Olfactory ensheathing cells (OECs) are a specialized class of glia found exclusively in the olfactory system. OECs wrap olfactory sensory neuron axons and support their growth from the olfactory epithelium, and targeting to the olfactory bulb, during development and life-long regeneration. Because of this function and their ability to cross the boundary between central and peripheral nervous systems, OECs are attractive candidates for cell-based regenerative therapies to promote axonal repair in the injured nervous system. OECs are a molecularly, topologically and functionally heterogeneous group of cells and the mechanisms underlying the development and function of specific OEC subpopulations are poorly defined. This situation has affected the outcome and interpretation of OEC-based regenerative strategies. Here we show that the transcription factor Runx1 is selectively expressed in OECs of the inner olfactory nerve layer of the mouse olfactory bulb and in their precursors in the OEC migratory mass. Furthermore, we provide evidence that in vivo knockdown of mouse Runx1 increases the proliferation of the OECs in which Runx1 is expressed. Conversely, Runx1 overexpression in primary cultures of OECs reduces cell proliferation in vitro. Decreased Runx1 activity also leads to an increase in Runx1-expressing OEC precursors, with a parallel decrease in the number of more developmentally mature OECs. These results identify Runx1 as a useful new marker of a distinct OEC subpopulation and suggest that Runx1 is important for the development of this group of OECs. These observations provide an avenue for further exploration into the molecular mechanisms underlying the development and function of specific OEC subpopulations.