Roles of mitochondria and temperature in the control of intracellular calcium in adult rat sensory neurons

TRPV4 acts as a mitochondrial Ca2+-importer and regulates mitochondrial temperature and metabolism

TRPV4 is associated with the development of neuropathic pain, sensory defects, muscular dystrophies, neurodegenerative disorders, Charcot Marie Tooth and skeletal dysplasia. In all these cases, mitochondrial abnormalities are prominent. Here, we demonstrate that TRPV4, localizes to a subpopulation of mitochondria in various cell lines. Improper expression and/or function of TRPV4 induces several mitochondrial abnormalities. TRPV4 is also involved in the regulation of mitochondrial numbers, Ca²⁺-levels and mitochondrial temperature. Accordingly, several naturally occurring TRPV4 mutations affect mitochondrial morphology and distribution. These findings may help in understanding the significance of mitochondria in TRPV4-mediated channelopathies possibly classifying them as mitochondrial diseases.

Presynaptic GCaMP expression decreases vesicle release probability at the calyx of Held

Synaptic vesicle (SV) exocytosis is intimately dependent on free local Ca²⁺ near active zones. Genetically encoded calcium indicators (GECIs) have become an indispensable tool to monitor calcium dynamics during physiological responses, and they are widely used as a proxy to monitor activity in neuronal ensembles and at synaptic terminals. However, GECIs’ ability to bind Ca²⁺ at physiologically relevant concentration makes them strong candidates to affect calcium homeostasis and alter synaptic transmission by exogenously increasing Ca²⁺ buffering. In the present study, we show that genetically expressed GCaMP6m modulates SV release probability at the mouse calyx of Held synapse. GCaMP6m expression for approximately three weeks decreased initial SV release for both low‐frequency stimulation and high‐frequency stimulation trains, and slowed presynaptic short‐term depression. However, GCaMP6m does not affect quantal events during spontaneous activity at this synapse. This study emphasizes the careful use of GECIs as monitors of neuronal activity and inspects the role of these transgenic indicators which may alter calcium‐dependent physiological responses.

Divergent effects of painful nerve injury on mitochondrial Ca(2+) buffering in axotomized and adjacent sensory neurons

Mitochondria critically regulate cytoplasmic Ca(2+) concentration ([Ca(2+)]c), but the effects of sensory neuron injury have not been examined. Using FCCP (1µM) to eliminate mitochondrial Ca(2+) uptake combined with oligomycin (10µM) to prevent ATP depletion, we first identified features of depolarization-induced neuronal [Ca(2+)]c transients that are sensitive to blockade of mitochondrial Ca(2+) buffering in order to assess mitochondrial contributions to [Ca(2+)]c regulation. This established the loss of a shoulder during the recovery of the depolarization (K(+))-induced transient, increased transient peak and area, and elevated shoulder level as evidence of diminished mitochondrial Ca(2+) buffering. We then examined transients in Control neurons and neurons from the 4th lumbar (L4) and 5th lumbar (L5) dorsal root ganglia after L5 spinal nerve ligation (SNL). The SNL L4 neurons showed decreased transient peak and area compared to control neurons, while the SNL L5 neurons showed increased shoulder level. Additionally, SNL L4 neurons developed shoulders following transients with lower peaks than Control neurons. Application of FCCP plus oligomycin elevated resting [Ca(2+)]c in SNL L4 neurons more than in Control neurons. Whereas application of FCCP plus oligomycin 2s after neuronal depolarization initiated mitochondrial Ca(2+) release in most Control and SNL L4 neurons, this usually failed to release mitochondrial Ca(2+) from SNL L5 neurons. For comparable cytoplasmic Ca(2+) loads, the releasable mitochondrial Ca(2+) in SNL L5 neurons was less than Control while it was increased in SNL L4 neurons. These findings show diminished mitochondrial Ca(2+) buffering in axotomized SNL L5 neurons but enhanced Ca(2+) buffering by neurons in adjacent SNL L4 neurons.

Ca(2+) homeostasis, Ca(2+) signalling and somatodendritic vasopressin release in adult rat supraoptic nucleus neurones

Multiple mechanisms that maintain Ca(2+) homeostasis and provide for Ca(2+) signalling operate in the somatas and neurohypophysial nerve terminals of supraoptic nucleus (SON) neurones. Here, we examined the Ca(2+) clearance mechanisms of SON neurones from adult rats by monitoring the effects of the selective inhibition of different Ca(2+) homeostatic molecules on cytosolic Ca(2+) ([Ca(2+)](i)) transients in isolated SON neurones. In addition, we measured somatodendritic vasopressin (AVP) release from intact SON tissue in an attempt to correlate it with [Ca(2+)](i) dynamics. When bathing the cells in a Na(+)-free extracellular solution, thapsigargin, cyclopiazonic acid (CPA), carbonyl cyanide 3-chlorophenylhydrazone (CCCP), and the inhibitor of plasma membrane Ca(2+)-ATPase (PMCA), La(3+), all significantly slowed down the recovery of depolarisation (50 mM KCl)-induced [Ca(2+)](i) transients. The release of AVP was stimulated by 50 mM KCl, and the decline in the peptide release was slowed by Ca(2+) transport inhibitors. In contrast to previous reports, our results show that in the fully mature adult rats: (i) all four Ca(2+) homeostatic pathways, the Na(+)/Ca(2+) exchanger, the endoplasmic reticulum Ca(2+) pump, the plasmalemmal Ca(2+) pump and mitochondria, are complementary in actively clearing Ca(2+) from SON neurones; (ii) somatodendritic AVP release closely correlates with intracellular [Ca(2+)](i) dynamics; (iii) there is (are) Ca(2+) clearance mechanism(s) distinct from the four outlined above; and (iv) Ca(2+) homeostatic systems in the somatas of SON neurones differ from those expressed in their terminals.

Calcium-dependent control of temporal processing in an auditory interneuron: a computational analysis

Sensitivity to acoustic amplitude modulation in crickets differs between species and depends on carrier frequency (e.g., calling song vs. bat-ultrasound bands). Using computational tools, we explore how Ca(2+)-dependent mechanisms underlying selective attention can contribute to such differences in amplitude modulation sensitivity. For omega neuron 1 (ON1), selective attention is mediated by Ca(2+)-dependent feedback: [Ca(2+)](internal) increases with excitation, activating a Ca(2+)-dependent after-hyperpolarizing current. We propose that Ca(2+) removal rate and the size of the after-hyperpolarizing current can determine ON1's temporal modulation transfer function (TMTF). This is tested using a conductance-based simulation calibrated to responses in vivo. The model shows that parameter values that simulate responses to single pulses are sufficient in simulating responses to modulated stimuli: no special modulation-sensitive mechanisms are necessary, as high and low-pass portions of the TMTF are due to Ca(2+)-dependent spike frequency adaptation and post-synaptic potential depression, respectively. Furthermore, variance in the two biophysical parameters is sufficient to produce TMTFs of varying bandwidth, shifting amplitude modulation sensitivity like that in different species and in response to different carrier frequencies. Thus, the hypothesis that the size of after-hyperpolarizing current and the rate of Ca(2+) removal can affect amplitude modulation sensitivity is computationally validated.