Involvement of the calcium channel beta3 subunit in olfactory signal transduction

Mechanism of copper nanoparticle toxicity in rainbow trout olfactory mucosa

Chemosensory perception is crucial for fish reproduction and survival. Direct contact of olfactory neuroepithelium to the surrounding environment makes it vulnerable to contaminants in aquatic ecosystems. Copper nanoparticles (CuNPs), which are increasingly used in commercial and domestic applications due their exceptional properties, can impair fish olfactory function. However, the molecular events underlying olfactory toxicity of CuNPs are largely unexplored. Our results suggested that CuNPs were bioavailable to olfactory mucosal cells. Using RNA-seq, we compared the effect of CuNPs and copper ions (Cu²⁺) on gene transcript profiles of rainbow trout (Oncorhynchus mykiss) olfactory mucosa. The narrow overlap in differential gene expression between the CuNP- and Cu²⁺-exposed fish revealed that these two contaminants exert their effects through distinct mechanisms. We propose a transcript-based conceptual model that shows that olfactory signal transduction, calcium homeostasis, and synaptic vesicular signaling were affected by CuNPs in the olfactory sensory neurons (OSNs). Neuroregenerative pathways were also impaired by CuNPs. In contrast, Cu²⁺ did not induce toxicity pathways and rather upregulated regeneration pathways. Both Cu treatments reduced immune system pathway transcripts. However, suppression of transcripts that were associated with inflammatory signaling was only observed with CuNPs. Neither oxidative stress nor apoptosis were triggered by Cu²⁺ or CuNPs in mucosal cells. Dysregulation of transcripts that regulate function, maintenance, and reestablishment of damaged olfactory mucosa represents critical mechanisms of toxicity of CuNPs. The loss of olfaction by CuNPs may impact survival of rainbow trout and impose an ecological risk to fish populations in contaminated environments.

The effects of diltiazem on growth, reproduction, energy reserves, and calcium-dependent physiology in Daphnia magna

With the growth of both the pharmaceutical industry and the human population and longevity, more drugs are used and processed each day. Inevitably, these pharmaceuticals enter wastewater through human excretion and improper disposal of leftovers. One such medication, diltiazem, a calcium channel blocker, is of importance due to its widespread consumption, and prevalence in aquatic environments. To study the sub-lethal effects of diltiazem on aquatic animals, we investigated its impacts no feeding behaviour, heart rate, respiration, growth, and reproduction of a bioindicator species, Daphnia magna. When exposed to environmentally relevant concentrations, D. magna increased their heart rate by 12% and oxygen consumption by 48%. However, exposure did not have any effects on thoracic limb movement frequency or peristalsis (i.e. feeding behaviour). Individuals exposed to diltiazem for a longer duration (16 days) showed a 44% decrease in lipid reserves and produced between 17 and 28% fewer neonates which were 10-12% larger. Our study demonstrated that exposure to diltiazem creates an energy imbalance in D. magna which could, in the long run, influence their populations.

Purinergic Signaling Pathway in Human Olfactory Neuronal Precursor Cells

Extracellular ATP and trophic factors released by exocytosis modulate in vivo proliferation, migration, and differentiation in multipotent stem cells (MpSC); however, the purinoceptors mediating this signaling remain uncharacterized in stem cells derived from the human olfactory epithelium (hOE). Our aim was to determine the purinergic pathway in isolated human olfactory neuronal precursor cells (hONPC) that exhibit MpSC features. Cloning by limiting dilution from a hOE heterogeneous primary culture was performed to obtain a culture predominantly constituted by hONPC. Effectiveness of cloning to isolate MpSC-like precursors was corroborated through immunodetection of specific protein markers and by functional criteria such as self-renewal, proliferation capability, and excitability of differentiated progeny. P2 receptor expression in hONPC was determined by Western blot, and the role of these purinoceptors in the ATP-induced exocytosis and changes in cytosolic Ca²⁺ ([Ca²⁺]_i) were evaluated using the fluorescent indicators FM1-43 and Fura-2 AM, respectively. The clonal culture was enriched with SOX2 and OCT3/4 transcription factors; additionally, the proportion of nestin-immunopositive cells, the proliferation capability, and functionality of differentiated progeny remained unaltered through the long-term clonal culture. hONPC expressed P2X receptor subtypes 1, 3-5, and 7, as well as P2Y2, 4, 6, and 11; ATP induced both exocytosis and a transient [Ca²⁺]_i increase predominantly by activation of metabotropic P2Y receptors. Results demonstrated for the first time that ex vivo-expressed functional P2 receptors in MpSC-like hONPC regulate exocytosis and Ca²⁺ signaling. This purinergic-triggered release of biochemical messengers to the extracellular milieu might be involved in the paracrine signaling among hOE cells.

P/Q Type Calcium Channel Cav2.1 Defines a Unique Subset of Glomeruli in the Mouse Olfactory Bulb

Voltage-gated calcium (Cav) channels are a prerequisite for signal transmission at the first olfactory sensory neuron (OSN) synapse within the glomeruli of the main olfactory bulb (MOB). We showed previously that the N-type Cav channel subunit Cav2.2 is present in the vast majority of glomeruli and plays a central role in presynaptic transmitter release. Here, we identify a distinct subset of glomeruli in the MOB of adult mice that is characterized by expression of the P/Q-type channel subunit Cav2.1. Immunolocalization shows that Cav2.1+ glomeruli reside predominantly in the medial and dorsal MOB, and in the vicinity of the necklace glomerular region close to the accessory olfactory bulb. Few glomeruli are detected on the ventral and lateral MOB. Cav2.1 labeling in glomeruli colocalizes with the presynaptic marker vGlut2 in the axon terminals of OSNs. Electron microscopy shows that Cav2.1+ presynaptic boutons establish characteristic asymmetrical synapses with the dendrites of second-order neurons in the glomerular neuropil. Cav2.1+ glomeruli receive axonal input from OSNs that express molecules of canonical OSNs: olfactory marker protein, the ion channel Cnga2, and the phosphodiesterase Pde4a. In the main olfactory epithelium, Cav2.1 labels a distinct subpopulation of OSNs whose distribution mirrors the topography of the MOB glomeruli, that shows the same molecular signature, and is already present at birth. Together, these experiments identify a unique Cav2.1+ multiglomerular domain in the MOB that may form a previously unrecognized olfactory subsystem distinct from other groups of necklace glomeruli that rely on cGMP signaling mechanisms.

Voltage-Activated Calcium Channels as Functional Markers of Mature Neurons in Human Olfactory Neuroepithelial Cells: Implications for the Study of Neurodevelopment in Neuropsychiatric Disorders

In adulthood, differentiation of precursor cells into neurons continues in several brain structures as well as in the olfactory neuroepithelium. Isolated precursors allow the study of the neurodevelopmental process in vitro. The aim of this work was to determine whether the expression of functional Voltage-Activated Ca²⁺ Channels (VACC) is dependent on the neurodevelopmental stage in neuronal cells obtained from the human olfactory epithelium of a single healthy donor. The presence of channel-forming proteins in Olfactory Sensory Neurons (OSN) was demonstrated by immunofluorescent labeling, and VACC functioning was assessed by microfluorometry and the patch-clamp technique. VACC were immunodetected only in OSN. Mature neurons responded to forskolin with a five-fold increase in Ca²⁺. By contrast, in precursor cells, a subtle response was observed. The involvement of VACC in the precursors’ response was discarded for the absence of transmembrane inward Ca²⁺ movement evoked by step depolarizations. Data suggest differential expression of VACC in neuronal cells depending on their developmental stage and also that the expression of these channels is acquired by OSN during maturation, to enable specialized functions such as ion movement triggered by membrane depolarization. The results support that VACC in OSN could be considered as a functional marker to study neurodevelopment.

The microtubular cytoskeleton of olfactory neurons derived from patients with schizophrenia or with bipolar disorder: Implications for biomarker characterization, neuronal physiology and pharmacological screening

Schizophrenia (SZ) and Bipolar Disorder (BD) are highly inheritable chronic mental disorders with a worldwide prevalence of around 1%. Despite that many efforts had been made to characterize biomarkers in order to allow for biological testing for their diagnoses, these disorders are currently detected and classified only by clinical appraisal based on the Diagnostic and Statistical Manual of Mental Disorders. Olfactory neuroepithelium-derived neuronal precursors have been recently proposed as a model for biomarker characterization. Because of their peripheral localization, they are amenable to collection and suitable for being cultured and propagated in vitro. Olfactory neuroepithelial cells can be obtained by a non-invasive brush-exfoliation technique from neuropsychiatric patients and healthy subjects. Neuronal precursors isolated from these samples undergo in vitro the cytoskeletal reorganization inherent to the neurodevelopment process which has been described as one important feature in the etiology of both diseases. In this paper, we will review the current knowledge on microtubular organization in olfactory neurons of patients with SZ and with BD that may constitute specific cytoskeletal endophenotypes and their relation with alterations in L-type voltage-activated Ca(2+) currents. Finally, the potential usefulness of neuronal precursors for pharmacological screening will be discussed.

Absence of plateau potentials in dLGN cells leads to a breakdown in retinogeniculate refinement

The link between neural activity and the refinement of projections from retina to the dorsal lateral geniculate nucleus (dLGN) of thalamus is based largely on studies that disrupt presynaptic retinogeniculate activity. Postsynaptic mechanisms responsible for implementing the activity-dependent remodeling in dLGN remain unknown. We tested whether L-type Ca(2+) channel activity in the form of synaptically evoked plateau potentials in dLGN cells is needed for remodeling by using a mutant mouse that lacks the ancillary β3 subunit and, as a consequence, has highly reduced L-type channel expression and attenuated L-type Ca(2+) currents. In the dLGNs of β3-null mice, glutamatergic postsynaptic activity evoked by optic tract stimulation was normal, but plateau potentials were rarely observed. The few plateaus that were evoked required high rates of retinal stimulation, but were still greatly attenuated compared with those recorded in age-matched wild-type mice. While β3-null mice exhibit normal stage II and III retinal waves, their retinogeniculate projections fail to segregate properly and dLGN cells show a high degree of retinal convergence even at late postnatal ages. These structural and functional defects were also accompanied by a reduction in CREB phosphorylation, a signaling event that has been shown to be essential for retinogeniculate axon segregation. Thus, postsynaptic L-type Ca(2+) activity plays an important role in mediating the refinement of the retinogeniculate pathway.

Contribution of protein kinase Cα in the stimulation of insulin by the down-regulation of Cavβ subunits

Voltage-gated calcium (Cav) channels and protein kinase C (PKC) isozymes are involved in insulin secretion. In addition, Cavβ, one of the auxiliary subunits of Cav channels, also regulates the secretion of insulin as knockout of Cavβ3 (β3(-/-)) subunits in mice led to efficient glucose homeostasis and increased insulin levels. We examined whether other types of Cavβ subunits also have similar properties. In this regard, we used small interfering RNA (siRNA) of these subunits (20 μg each) to down-regulate them and examined blood glucose, serum insulin and PKC translocation in isolated pancreatic β cells of mice. While the down-regulation of Cavβ2 and β3 subunits increased serum insulin levels and caused efficient glucose homeostasis, the down-regulation of Cavβ1 and β4 subunits failed to affect both these parameters. Examination of PKC isozymes in the pancreatic β-cells of Cavβ2- or β3 siRNA-injected mice showed that three PKC isozymes, viz., PKC α, βII and θ, translocated to the membrane. This suggests that when present, Cavβ2 and β3 subunits inhibited PKC activation. Among these three isozymes, only PKCα siRNA inhibited insulin and increased glucose concentrations. It is possible that the activation of PKCs βII and θ is not sufficient for the release of insulin and PKCα is the mediator of insulin secretion under the control of Cavβ subunits. Since Cavβ subunits are present intracellularly, it is possible that they (1) inhibited the translocation of PKC isozymes to the membrane and (2) decreased the interaction between Cav channels and PKC isozymes and thus the secretion of insulin.

Microtubule organization and L-type voltage-activated calcium current in olfactory neuronal cells obtained from patients with schizophrenia and bipolar disorder

Olfactory neuroepithelial cells in culture have been proposed as a model to study the physiopathology of psychiatric disorders and biomarker characterization for diagnosis. In patients with schizophrenia (SZ) and bipolar disorder (BD) diminished microtubule-associated proteins expression occurs, which might lead to aberrant microtubular organization and which in turn may affect Ca(2+) voltage-activated currents. The aim of this work was to characterize of microtubule organization as well as of the L-type Ca(2+) current in neuronal precursors obtained from nasal exfoliates of patients with SZ and BD. Microtubule organization was studied by immunofluorescence with a specific anti-III β-tubulin antibody and by quantification of globular and assembled tubulin by Western blot. L-type current recording was performed by whole-cell patch-clamp technique and nifedipine superfusion. The results showed differential altered microtubular organization in neuronal precursors of SZ and BD. Short microtubules were observed in BD neurons, while extensive, unstained subcellular areas and disorganized microtubules were evident in SZ neuronal precursors. Patients with BD showed a decrease in amounts of tubulin in total homogenates and 40% decrease in the globular fraction. However, L-type current in BD was similar to that in healthy subjects (HS). In contrast, this current in SZ was 50% lower. These reduction in L-type current in SZ together with differential microtubule alterations are potential biomarkers that may differentiates SZ and BD.

Requirements for synaptically evoked plateau potentials in relay cells of the dorsal lateral geniculate nucleus of the mouse

In developing cells of the mouse dorsal lateral geniculate nucleus (dLGN), synaptic responses evoked by optic tract (OT) stimulation give rise to long-lasting, high-amplitude depolarizations known as plateau potentials. These events are mediated by L-type Ca2+ channels and occur during early postnatal life, a time when retinogeniculate connections are remodelling. To better understand the relationship between L-type activity and dLGN development we used an in vitro thalamic slice preparation which preserves the retinal connections and intrinsic circuitry in dLGN and examined how synaptic responses evoked by OT stimulation lead to the activation of plateau potentials. By varying the strength and temporal frequency of OT stimulation we identified at least three factors that contribute to the developmental regulation of plateau activity: the degree of retinal convergence, the temporal pattern of retinal stimulation and the emergence of feed-forward inhibition. Before natural eye opening (postnatal day 14), the excitatory synaptic responses of relay cells receiving multiple retinal inputs summated in both the spatial and temporal domains to produce depolarizations sufficient to activate L-type activity. After eye opening, when inhibitory responses are fully developed, plateau activity was rarely evoked even with high temporal rates of OT stimulation. When the bulk of this inhibition was blocked by bath application of bicuculline, the incidence of plateau activity increased significantly. We also made use of a transgenic mouse that lacks the β3 subunit of the L-type Ca2+ channel. These mutants have far fewer membrane-bound Ca2+ channels and attenuated L-type activity. In β3 nulls, L-type plateau activity was rarely observed even at young ages when plateau activity prevails. Thus, in addition to the changing patterns of synaptic connectivity and retinal activity, the expression of L-type Ca2+ channels is a requisite component in the manifestation of plateau activity.

Ablation of Ca2+ channel beta3 subunit leads to enhanced N-methyl-D-aspartate receptor-dependent long term potentiation and improved long term memory

The beta subunits of voltage-dependent Ca(2+) channels (VDCCs) have marked effects on the properties of the pore-forming alpha(1) subunits of VDCCs, including surface expression of channel complexes and modification of voltage-dependent kinetics. Among the four different beta subunits, the beta(3) subunit (Ca(v)beta3) is abundantly expressed in the hippocampus. However, the role of Ca(v)beta3 in hippocampal physiology and function in vivo has never been examined. Here, we investigated Ca(v)beta3-deficient mice for hippocampus-dependent learning and memory and synaptic plasticity at hippocampal CA3-CA1 synapses. Interestingly, the mutant mice exhibited enhanced performance in several hippocampus-dependent learning and memory tasks. However, electrophysiological studies revealed no alteration in the Ca(2+) current density, the frequency and amplitude of miniature excitatory postsynaptic currents, and the basal synaptic transmission in the mutant hippocampus. On the other hand, however, N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic currents and NMDAR-dependent long term potentiation were significantly increased in the mutant. Protein blot analysis showed a slight increase in the level of NMDAR-2B in the mutant hippocampus. Our results suggest a possibility that, unrelated to VDCCs regulation, Ca(v)beta3 negatively regulates the NMDAR activity in the hippocampus and thus activity-dependent synaptic plasticity and cognitive behaviors in the mouse.