Type-3 ryanodine receptor involved in Ca2+-induced Ca2+ release and transmitter exocytosis at frog motor nerve terminals

The Putative Drosophila TMEM184B Ortholog Tmep Ensures Proper Locomotion by Restraining Ectopic Firing at the Neuromuscular Junction

TMEM184B is a putative seven-pass membrane protein that promotes axon degeneration after injury. TMEM184B mutation causes aberrant neuromuscular architecture and sensory and motor behavioral defects in mice. The mechanism through which TMEM184B causes neuromuscular defects is unknown. We employed Drosophila melanogaster to investigate the function of the closely related gene, Tmep (CG12004), at the neuromuscular junction. We show that Tmep is required for full adult viability and efficient larval locomotion. Tmep mutant larvae have a reduced body contraction rate compared to controls, with stronger deficits in females. In recordings from body wall muscles, Tmep mutants show substantial hyperexcitability, with many postsynaptic potentials fired in response to a single stimulation, consistent with a role for Tmep in restraining synaptic excitability. Additional branches and satellite boutons at Tmep mutant neuromuscular junctions are consistent with an activity-dependent synaptic overgrowth. Tmep is expressed in endosomes and synaptic vesicles within motor neurons, suggesting a possible role in synaptic membrane trafficking. Using RNAi knockdown, we show that Tmep is required in motor neurons for proper larval locomotion and excitability, and that its reduction increases levels of presynaptic calcium. Locomotor defects can be rescued by presynaptic knockdown of endoplasmic reticulum calcium channels or by reducing evoked release probability, further suggesting that excess synaptic activity drives behavioral deficiencies. Our work establishes a critical function for Tmep in the regulation of synaptic transmission and locomotor behavior.

Effects of intravesicular loading of a Ca2+ chelator and depolymerization of actin fibers on neurotransmitter release in frog motor nerve terminals

Ca²⁺ -induced Ca²⁺ release (CICR) via type-3 ryanodine receptor enhances neurotransmitter release in frog motor nerve terminals. To test a possible role of synaptic vesicle in CICR, we examined the effects of loading of EGTA, a Ca²⁺ chelator, into synaptic vesicles and depolymerization of actin fibers. Intravesicular EGTA loading via endocytosis inhibited the ryanodine sensitive enhancement of transmitter release induced by tetanic stimulation and the associated rises in intracellular-free Ca²⁺ ([Ca²⁺ ]_i : Ca²⁺ transients). Latrunculin A, a depolymerizer of actin fibers, enhanced both spontaneous and stimulation-induced transmitter release, but inhibited the enhancement of transmitter release elicited by successive tetanic stimulation. The results suggest a possibility that the activation of CICR from mobilized synaptic vesicles caused the enhancement of neurotransmitter release.

Contextual Fear Memory Formation and Destabilization Induce Hippocampal RyR2 Calcium Channel Upregulation

Hippocampus-dependent spatial and aversive memory processes entail Ca2+ signals generated by ryanodine receptor (RyR) Ca2+ channels residing in the endoplasmic reticulum membrane. Rodents exposed to different spatial memory tasks exhibit significant hippocampal RyR upregulation. Contextual fear conditioning generates robust hippocampal memories through an associative learning process, but the effects of contextual fear memory acquisition, consolidation, or extinction on hippocampal RyR protein levels remain unreported. Accordingly, here we investigated if exposure of male rats to contextual fear protocols, or subsequent exposure to memory destabilization protocols, modified the hippocampal content of type-2 RyR (RyR2) channels, the predominant hippocampal RyR isoforms that hold key roles in synaptic plasticity and spatial memory processes. We found that contextual memory retention caused a transient increase in hippocampal RyR2 protein levels, determined 5 h after exposure to the conditioning protocol; this increase vanished 29 h after training. Context reexposure 24 h after training, for 3, 15, or 30 min without the aversive stimulus, decreased fear memory and increased RyR2 protein levels, determined 5 h after reexposure. We propose that both fear consolidation and extinction memories induce RyR2 protein upregulation in order to generate the intracellular Ca2+ signals required for these distinct memory processes.

Differential expression of ryanodine receptor isoforms after spinal cord injury

Ryanodine receptors (RyRs) are highly conductive intracellular Ca²⁺ release channels and are widely expressed in many tissues, including the central nervous system. RyRs have been implicated in intracellular Ca²⁺ overload which can drive secondary damage following traumatic injury to the spinal cord (SCI), but the spatiotemporal expression of the three isoforms of RyRs (RyR1-3) after SCI remains unknown. Here, we analyzed the gene and protein expression of RyR isoforms in the murine lumbar dorsal root ganglion (DRG) and the spinal cord lesion site at 1, 2 and 7 d after a mild contusion SCI. Quantitative RT PCR analysis revealed that RyR3 was significantly increased in lumbar DRGs and at the lesion site at 1 and 2 d post contusion compared to sham (laminectomy only) controls. Additionally, RyR2 expression was increased at 1 d post injury within the lesion site. RyR2 and -3 protein expression was localized to lumbar DRG neurons and their spinal projections within the lesion site acutely after SCI. In contrast, RyR1 expression within the DRG and lesion site remained unaltered following trauma. Our study shows that SCI initiates acute differential expression of RyR isoforms in DRG and spinal cord.

Exploring the central modulation hypothesis: do ancient memory mechanisms underlie the pathophysiology of trigger points?

A myofascial trigger point (TrP) is a point of focal tenderness, associated with a taut band of muscle fibers, that can develop in any skeletal muscle. TrPs are a common source of pain and motor dysfunction in humans and other vertebrates. There is no universally accepted pathophysiology to explain the etiology, symptomatology and treatment of TrPs. This article reviews and extends the author's previously published hypothesis for the pathophysiology of TrPs, "Trigger Points and Central Modulation-A New Hypothesis." The author proposes that central nervous system-maintained global changes in α-motoneuron function, resulting from sustained plateau depolarization, rather than a local dysfunction of the motor endplate, underlie the pathogenesis of TrPs.

Presynaptic CaMKIIα modulates dopamine D3 receptor activation in striatonigral terminals of the rat brain in a Ca²⁺ dependent manner

CaMKIIα is expressed at high density in the nucleus accumbens where it binds to postsynaptic D3 receptors inhibiting their effects. In striatonigral projections, activation of presynaptic D3 receptors potentiates D1 receptor-induced stimulation of cAMP production and GABA release. In this study we examined whether the presynaptic effects of D3 receptor stimulation in the substantia nigra reticulata (SNr) are modulated by Ca²⁺ activation of CaMKIIα. In SNr synaptosomes two procedures that increase cytoplasmic Ca²⁺, ionomycin and K⁺-depolarization, blocked the additional stimulation of cAMP accumulation produced by coactivating D3 and D1 dopamine receptors. The selective CaMKIIα inhibitor KN-62 reversed the blockade produced by ionomycin and K⁺-depolarization. Incubation in either Ca²-free solutions or with the selective Ca²⁺ blocker nifedipine, also reversed the blocking effects of K⁺-depolarization. Immunoblot studies showed that K⁺-depolarization increased CaMKIIα phosphorylation in a KN-62 sensitive manner and promoted CaMKIIα binding to D3 receptors. In K⁺-depolarized tissues, D3 receptors potentiated D1 receptor-induced stimulation of [³H]GABA release only when CaMKIIα was blocked with KN-62. In the presence of this inhibitor, the selective D3 agonist PD 128,907 reduced the ED₅₀ for the D1 agonist SKF 38393 from 56 to 4 nM. KN-62 also enhanced the effects of dopamine on depolarization induced [³H]GABA release. KN-62 changed ED₅₀ for dopamine from 584 to 56 nM. KN-62 did not affect D1 and D4 receptor responses. These experiments show that in striatonigral projections, CaMKIIα inhibits the action of D3 receptors in a Ca²⁺ dependent manner blocking their modulatory effects on GABA release. These findings suggest a mechanism through which the frequency of action potential discharge in presynaptic terminals regulates dopamine effects.

Differential control of presynaptic CaMKII activation and translocation to active zones

The release of neurotransmitters, neurotrophins, and neuropeptides is modulated by Ca(2+) mobilization from the endoplasmic reticulum (ER) and activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Furthermore, when neuronal cultures are subjected to prolonged depolarization, presynaptic CaMKII redistributes from the cytoplasm to accumulate near active zones (AZs), a process that is reminiscent of CaMKII translocation to the postsynaptic side of the synapse. However, it is not known how presynaptic CaMKII activation and translocation depend on neuronal activity and ER Ca(2+) release. Here these issues are addressed in Drosophila motoneuron terminals by imaging a fluorescent reporter of CaMKII activity and subcellular distribution. We report that neuronal excitation acts with ER Ca(2+) stores to induce CaMKII activation and translocation to a subset of AZs. Surprisingly, activation is slow, reflecting T286 autophosphorylation and the function of presynaptic ER ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP3Rs). Furthermore, translocation is not simply proportional to CaMKII activity, as T286 autophosphorylation promotes activation, but does not affect translocation. In contrast, RNA interference-induced knockdown of the AZ scaffold protein Bruchpilot disrupts CaMKII translocation without affecting activation. Finally, RyRs comparably stimulate both activation and translocation, but IP3Rs preferentially promote translocation. Thus, Ca(2+) provided by different presynaptic ER Ca(2+) release channels is not equivalent. These results suggest that presynaptic CaMKII activation depends on autophosphorylation and global Ca(2+) in the terminal, while translocation to AZs requires Ca(2+) microdomains generated by IP3Rs.

Dynasore, an inhibitor of dynamin, increases the probability of transmitter release

Dynasore was recently developed as a small molecule, selective non-competitive inhibitor of the protein dynamin. This inhibitor has been shown to block dynamin-dependent endocytosis and is now used commonly to study vesicular recycling at synapses. We have measured the effects of dynasore on spontaneous and evoked transmitter release at the frog neuromuscular junction and shown that, in addition to inhibiting endocytosis, dynasore increases the probability of transmitter release. Furthermore, we have shown that dynasore exposure leads to an increase in resting intra-terminal calcium, but this effect does not completely account for the dynasore-mediated increase in the probability of transmitter release. Therefore, in interpreting effects of the dynamin inhibitor dynasore at synapses, one must be alert to potential increases in presynaptic calcium concentration and transmitter release probability.

Digoxin facilitates neuromuscular transmission in mouse diaphragm

Low concentration of digoxin (3 nM) facilitated spontaneous and evoked release of neurotransmitter acetylcholine thereby increasing the frequency of miniature end-plate potentials, amplitude of single end-plate potentials, their quantum content and the plateau level in the bursts during stimulation of the phrenic nerve at rates of 4, 7, and 50 Hz. These effects were prevented by blockade of ryanodine receptors with ryanodine (10-20 microM).

A deletion mutation in Slc12a6 is associated with neuromuscular disease in gaxp mice

Giant axonopathy (gaxp), an autosomal recessive mouse mutation, exhibits ataxia of the hind legs with a slight side-to-side wobble while walking. Within the genomic region of the gaxp locus, a total of 94 transcripts were identified; the annotation of these genes using OMIM and PubMed yielded three potential candidate genes. By cDNA microarray analysis, 54 genes located on or near the gaxp locus were found to exhibit differential expression between gaxp and littermate controls. Based on microarray data and the known function of genes identified, Slc12a6 was selected as the primary candidate gene and analyzed using the Reveal technology of SpectruMedix. A 17-base deletion was detected from within exon 4 of Slc12a6. Reverse transcriptase polymerase chain reaction validated the difference in Slc12a6 expression in different types of mice at the mRNA level, revealing a marked reduction in gaxp mice. Western blot analysis indicated that the protein product of Slc12a6, the K(+)-Cl(-) cotransporter Kcc3, was not detectable in gaxp mice. The causative role of the exon 4 mutation within Slc12a6 in the gaxp phenotype was further confirmed by screening multiple inbred strains and by excluding the mutation of nearby genes within the gaxp locus.

4-Chloro-m-cresol, an activator of ryanodine receptors, inhibits voltage-gated K(+) channels at the rat calyx of Held

4-Chloro-m-cresol (4-CmC) is thought to be a specific activator of ryanodine receptors (RyRs). Using this compound, we examined whether the RyR-mediated Ca(2+) release is involved in transmitter release at the rat calyx of Held synapse in brainstem slices. Bath application of 4-CmC caused a dramatic increase in the amplitude of excitatory postsynaptic currents (TIFCs) with the half-maximal effective concentration of 0.12 mm. By making direct patch-clamp whole-cell recordings from presynaptic terminals, we investigated the mechanism by which 4-CmC facilitates transmitter release. 4-CmC markedly prolonged the duration of action potentials, with little effect on their rise time kinetics. In voltage-clamp recordings, 4-CmC inhibited voltage-gated presynaptic K(+) currents (I(pK)) by 53% (at +20 mV) but had no effect on voltage-gated presynaptic Ca(2+) currents (I(pCa)). In simultaneous pre- and postsynaptic recordings, 4-CmC had no effect on the TIFC evoked by I(pCa). Although immunocytochemical study of the calyceal terminals showed immunoreactivity to type 3 RyRs, ryanodine (0.02 mm) had no effect on the 4-CmC-induced TIFC potentiation. We conclude that the facilitatory effect of 4-CmC on nerve-evoked transmitter release is mediated by its inhibitory effect on I(pK).

Ca(2+) sparks operated by membrane depolarization require isoform 3 ryanodine receptor channels in skeletal muscle

Stimuli are translated to intracellular calcium signals via opening of inositol trisphosphate receptor and ryanodine receptor (RyR) channels of the sarcoplasmic reticulum or endoplasmic reticulum. In cardiac and skeletal muscle of amphibians the stimulus is depolarization of the transverse tubular membrane, transduced by voltage sensors at tubular-sarcoplasmic reticulum junctions, and the unit signal is the Ca(2+) spark, caused by concerted opening of multiple RyR channels. Mammalian muscles instead lose postnatally the ability to produce sparks, and they also lose RyR3, an isoform abundant in spark-producing skeletal muscles. What does it take for cells to respond to membrane depolarization with Ca(2+) sparks? To answer this question we made skeletal muscles of adult mice expressing exogenous RyR3, demonstrated as immunoreactivity at triad junctions. These muscles showed abundant sparks upon depolarization. Sparks produced thusly were found to amplify the response to depolarization in a manner characteristic of Ca(2+)-induced Ca(2+) release processes. The amplification was particularly effective in responses to brief depolarizations, as in action potentials. We also induced expression of exogenous RyR1 or yellow fluorescent protein-tagged RyR1 in muscles of adult mice. In these, tag fluorescence was present at triad junctions. RyR1-transfected muscle lacked voltage-operated sparks. Therefore, the voltage-operated sparks phenotype is specific to the RyR3 isoform. Because RyR3 does not contact voltage sensors, their opening was probably activated by Ca(2+), secondarily to Ca(2+) release through junctional RyR1. Physiologically voltage-controlled Ca(2+) sparks thus require a voltage sensor, a master junctional RyR1 channel that provides trigger Ca(2+), and a slave parajunctional RyR3 cohort.

Enhancement of Ca2+-induced Ca2+ release by cyclic ADP-ribose in frog motor nerve terminals

Ca2+-induced Ca2+ release (CICR) occurs via activation of ryanodine receptors (RyRs) in frog motor nerve terminals after RyRs are primed for activation by repetitive Ca2+ entries, thereby contributing to synaptic plasticity. To clarify how the mechanism of CICR becomes activable by repetitive Ca2+ entries, we studied effects of a RyR modulator, cyclic ADP-ribose (cADPr), on CICR by Ca2+ imaging techniques. Use-dependent binding of fluorescent ryanodine and its blockade by ryanodine revealed the existence of RyRs in the terminals. Repetition of tetani applied to the nerve produced repetitive rises in intracellular Ca2+ ([Ca2+]i) in the terminals. The amplitude of each rise slowly waxed and waned during the course of the stimulation. These slow rises and decays were blocked by ryanodine, indicating the priming, activation and inactivation of CICR. Uncaging of caged-cADPr loaded in the terminals increased the amplitude of short tetanus-induced rises in [Ca2+]i and the amplitude, time to peak and half decay time of the slow waxing and waning rises in [Ca2+]i evoked by repetitive tetani. A cADPr blocker, 8-amino-cADPr, loaded in the terminals decreased the slow waxing and waning component of rises and blocked all the actions of exogenous cADPr. It is concluded that cADPr enhances the priming and activation of CICR. The four-state model for RyRs suggests that cADPr inhibits the inactivation of CICR and increases the activation efficacy of RyR.