Advancing age alters the expression of the ryanodine receptor 3 isoform in adult rat superior cervical ganglia

Aging alters spontaneous and neurotransmitter-mediated Ca2+ signaling in smooth muscle cells of mouse mesenteric arteries

OBJECTIVE

Aging impairs MA dilation by reducing the ability of sensory nerves to counteract sympathetic vasoconstriction. This study tested whether altered SMC Ca²⁺ signals to sympathetic (NE) and sensory (CGRP) neurotransmitters underlie aging-related deficits in vasodilation.

METHODS

MAs from young and old mice were pressurized and loaded with Fluo-4 dye for confocal measurement of SMC Ca²⁺ sparks and waves. Endothelial denudation resolved the influence of ECs. SMCs were immunolabeled for RyR isoforms and compared with transcript levels for RyRs and CGRP receptor components.

RESULTS

SMCs from young vs old mice exhibited more spontaneous Ca²⁺ spark sites with no difference in Ca²⁺ waves. NE reduced spark sites and increased waves for both groups; addition of CGRP restored sparks and reduced waves only for young mice. Endothelial denudation attenuated Ca²⁺ responses to CGRP for young but not old mice, which were already attenuated, suggesting a diminished role for ECs with aging. CGRP receptor expression was similar between ages with increased serum CGRP in old mice, where RyR1 expression was replaced by RyR3.

CONCLUSION

With aging, we suggest that altered RyR expression in SMCs contributes to impaired ability of sensory neurotransmission to restore Ca²⁺ signaling underlying vasomotor control during sympathetic activation.

Evidence of functional ryanodine receptors in rat mesenteric collecting lymphatic vessels

In the current study, the potential contributions of ryanodine receptors (RyRs) to intrinsic pumping and responsiveness to substance P (SP) were investigated in isolated rat mesenteric collecting lymphatic vessels. Responses to SP were characterized in lymphatic vessels in the absence or presence of pretreatment with nifedipine to block L-type Ca²⁺ channels, caffeine to block normal release and uptake of Ca²⁺ from the sarcoplasmic reticulum, ryanodine to block all RyR isoforms, or dantrolene to more selectively block RyR1 and RyR3. RyR expression and localization in lymphatics was also assessed by quantitative PCR and immunofluorescence confocal microscopy. The results show that SP normally elicits a significant increase in contraction frequency and a decrease in end-diastolic diameter. In the presence of nifedipine, phasic contractions stop, yet subsequent SP treatment still elicits a strong tonic contraction. Caffeine treatment gradually relaxes lymphatics, causing a loss of phasic contractions, and prevents subsequent SP-induced tonic contraction. Ryanodine also gradually diminishes phasic contractions but without causing vessel relaxation and significantly inhibits the SP-induced tonic contraction. Dantrolene treatment did not significantly impair lymphatic contractions nor the response to SP. The mRNA for all RyR isoforms is detectable in isolated lymphatics. RyR2 and RyR3 proteins are found predominantly in the collecting lymphatic smooth muscle layer. Collectively, the data suggest that SP-induced tonic contraction requires both extracellular Ca²⁺ plus Ca²⁺ release from internal stores and that RyRs play a role in the normal contractions and responsiveness to SP of rat mesenteric collecting lymphatics.NEW & NOTEWORTHY The mechanisms that govern contractions of lymphatic vessels remain unclear. Tonic contraction of lymphatic vessels caused by substance P was blocked by caffeine, which prevents normal uptake and release of Ca²⁺ from internal stores, but not nifedipine, which blocks L-type channel-mediated Ca²⁺ entry. Ryanodine, which also disrupts normal sarcoplasmic reticulum Ca²⁺ release and reuptake, significantly inhibited substance P-induced tonic contraction. Ryanodine receptors 2 and 3 were detected within the smooth muscle layer of collecting lymphatic vessels.

Targeted neuronal nitric oxide synthase transgene delivery into stellate neurons reverses impaired intracellular calcium transients in prehypertensive rats

Hypertension is associated with the early onset of cardiac sympathetic hyperresponsiveness and enhanced intracellular Ca(2+) concentration [Ca(2+)](i) in sympathetic neurons from both prehypertensive and hypertensive, spontaneously hypertensive rats (SHRs). Oxidative stress is a hallmark of hypertension, therefore, we tested the hypothesis that the inhibitory action of the nitric oxide-cGMP pathway on [Ca(2+)](i) transients is impaired in cardiac sympathetic neurons from the SHR. Stellate ganglia were isolated from young prehypertensive SHRs and age-matched normotensive Wistar-Kyoto rats. [Ca(2+)](i) was measured by ratiometric fluorescence imaging. Neurons from the prehypertensive SHR ganglia had a significantly higher depolarization evoked [Ca(2+)](i) transient that was also associated with decreased expression of neuronal nitric oxide synthase (nNOS), β1 subunit of soluble guanylate cyclase and cGMP when compared with the Wistar-Kyoto rat ganglia. Soluble guanylate cyclase inhibition or nNOS inhibition increased [Ca(2+)](i) in the Wistar-Kyoto rats but had no effect in SHR neurons. A nitric oxide donor decreased [Ca(2+)](i) in both sets of neurons, although this was markedly less in the SHR. A novel noradrenergic cell specific vector (Ad.PRSx8-nNOS/Cherry) or its control vector (Ad.PRSx8-Cherry) was expressed in sympathetic neurons. In the SHR, Ad.PRSx8-nNOS/Cherry-treated neurons had a significantly reduced peak [Ca(2+)](i) transient that was associated with increased tissue levels of nNOS protein and cGMP concentration compared with gene transfer of Ad.PRSx8-Cherry alone. nNOS inhibition significantly increased [Ca(2+)](i) after Ad.PRSx8-nNOS/Cherry expression. We conclude that artificial upregulation of stellate sympathetic nNOS via targeted gene transfer can directly attenuate intracellular Ca(2+) and may provide a novel method for decreasing enhanced cardiac sympathetic neurotransmission.

Abnormal intracellular calcium homeostasis in sympathetic neurons from young prehypertensive rats

Hypertension is associated with cardiac noradrenergic hyperactivity, although it is not clear whether this precedes or follows the development of hypertension itself. We hypothesized that Ca(2+) homeostasis in postganglionic sympathetic neurons is impaired in spontaneously hypertensive rats (SHRs) and may occur before the development of hypertension. The depolarization-induced rise in intracellular free calcium concentration ([Ca(2+)](i); measured using fura-2-acetoxymethyl ester) was significantly larger in cultured sympathetic neurons from prehypertensive SHRs than in age matched normotensive Wistar-Kyoto rats. The decay of the [Ca(2+)](i) transient was also faster in SHRs. The endoplasmic reticulum Ca(2+) content and caffeine-induced [Ca(2+)](i) amplitude were significantly greater in the young SHRs. Lower protein levels of phospholamban and more copies of ryanodine receptor mRNA were also observed in the young SHRs. Depleting the endoplasmic reticulum Ca(2+) store did not alter the difference of the evoked [Ca(2+)](i) transient and decay time between young SHRs and Wistar-Kyoto rats. However, removing mitochondrial Ca(2+) buffering abolished these differences. A lower mitochondrial membrane potential was also observed in young SHR sympathetic neurons. This resulted in impaired mitochondrial Ca(2+) uptake and release, which might partly be responsible for the increased [Ca(2+)](i) transient and faster decay in SHR sympathetic neurons. This Ca(2+) phenotype seen in early development in cardiac stellate and superior cervical ganglion neurons may contribute to the sympathetic hyperresponsiveness that precedes the onset of hypertension.

The cranial cervical ganglion and its branches in the White yak (Bos grunniens)

The heads of 12 White yaks (four castrated, four male and four female, 3-8 years old) were dissected to study the shape, location and branches of the cranial cervical ganglion macroscopically. The ganglion was a greyish arciform structure, with a mean length of 17.3 mm, a width of 8.0 mm and a thickness of 3.9 mm, located on the rostrolateral surface of m. longus capitis. Approximately 5% of the ganglion was covered laterally by the tympanic bulla and the rest by the m. stylohyoideus. The branches of the cranial cervical ganglion included the internal and external carotid nerves, the sympathetic trunk and communicating branches to the glossopharyngeal nerve, the pharyngeal branch of the vagus nerve and the hypoglossal nerve. In one specimen, the left cranial cervical ganglion was fusiform and only covered by the m. stylohyoideus. Gender differences of the cranial cervical ganglion in the White yak were not observed.

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

Maturation and long-term hypoxia alters Ca2+-induced Ca2+ release in sheep cerebrovascular sympathetic neurons

The contribution of sympathetic nerves arising from the superior cervical ganglia (SCG) toward the growth and function of cerebral blood vessels is pertinent throughout maturation as well as in response to cardiovascular stress imposed by high-altitude long-term hypoxia (LTH). The function of SCG sympathetic neurons is dependent on intracellular Ca2+ concentration ([Ca2+]i) signaling, which is strongly influenced by a process known as Ca(2+)-induced Ca2+ release (CICR) from the smooth endoplasmic reticulum (SER). In this study, we used the sheep SCG neuronal model to test the hypotheses that maturation decreases CICR and high-altitude LTH depresses CICR in fetal SCG neurons but not in those of the adult. We found that the contribution of CICR to electric field stimulation (EFS)-evoked [Ca2+]i transients was greatest in SCG cells from normoxic fetuses and was abolished by LTH. The decline in CICR was associated with a reduction in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in fetal SCG cells during LTH, reducing SER Ca2+ levels below the threshold needed for the coupling of Ca2+ influx and CICR. With respect to the maturation from the fetus to adult, the decrease in CICR may reflect both a reduction in the levels of ryanodine receptor isoforms 2 and 3 and SERCA function. In response to LTH and in contrast to the fetus, CICR function in adult SCG cells is maintained and may reflect alterations in other mechanisms that modulate the CICR process. As CICR is instrumental in the function of sympathetic neurons within the cerebrovasculature, the loss of this signaling mechanism in the fetus may have consequences for the adaptation to LTH in terms of fetal susceptibility to vascular insults.

The ER and ageing II: calcium homeostasis

Increase in intracellular Ca(2+) concentration occurs by Ca(2+) influx through the plasma membrane and by Ca(2+) release from intracellular stores. The ER is the most important Ca(2+) store. Its stress, characterized by the impairment of Ca(2+) homeostasis and by the accumulation of misfolded proteins, can be induced by different factors. In turn, it induces defense mechanisms such as unfolded protein response, and when it is severe and prolonged, activation of the apoptotic pathway. Damage to the ER, impairment of its function, and a decreased level of its Ca(2+)-handling proteins might all play a role in physiological ageing by handicapping the ER stress response. Thus, healthy ageing is accompanied by subtle alterations of Ca(2+) homeostasis and signaling, including alterations in the ER Ca(2+) load and release. The expression and/or function of ryanodine receptors, IP3 receptors, and SERCA Ca(2+) pumps located in the ER membrane, and Ca(2+)-binding proteins within ER lumen all seem to be affected in aged cells. Data are presented on age-dependent, tissue-specific changes in ER-related Ca(2+) homeostasis in skeletal, cardiac and smooth muscles, as well as in the nervous and immune systems. Disturbances of Ca(2+) homeostasis and of signaling are potential targets for intervention in aged humans.

Advancing age alters the contribution of calcium release from smooth endoplasmic reticulum stores in superior cervical ganglion cells

In superior cervical ganglion (SCG) neurons calcium-induced calcium release (CICR), mediated by ryanodine receptors (RyRs), contributes to stimulation-evoked intracellular calcium ([Ca²⁺]_i) transients. Hypothesis: The contribution of CICR to electrical field stimulation (EFS)–evoked [Ca²⁺]_i transients in SCG cells declines with senescence and may be partially recovered in the presence of caffeine. We measured EFS-evoked [Ca²⁺]_i transients in isolated fura-2–loaded SCG cells from Fischer-344 rats aged 6, 12, and 24 months with either the RyR antagonist ryanodine to block the contribution of CICR to [Ca²⁺]_i transients or caffeine to sensitize CICR to EFS. EFS-evoked [Ca²⁺]_i transients increased from 6 to 12 months and declined at 24 months and ryanodine decreased [Ca²⁺]_i transients in SCG cells from 6- and 12-month-old animals only. Caffeine significantly increased EFS-evoked [Ca²⁺]_i transients in all age groups. These data suggest that CICR declines with senescence and residual CICR function may be reclaimed in senescent cells with caffeine.

Ca2+ extrusion in aged smooth muscle cells

We investigated the effects of aging in Ca(2+) extrusion mechanisms in smooth muscle bladder cells from 4 and 20-24-month-old guinea pigs using fluorescence microscopy and fura-2. Cells were challenged with a pulse of KCl immediately before perfusion with a Ca(2+) free solution containing no inhibitors (control, untreated cells) or inhibitors of plasma membrane Ca(2+) pump (PMCA, 1mM La(3+)), Na(+)/Ca(2+) exchanger (NCX, 1 microM SEA0400) or the sarcoendoplasmic Ca(2+) pump (SERCA, 1 microM thapsigargin). Treatment of young adult cells with the inhibitors allowed estimating a relative contribution of 55% for NCX, 27% for PMCA and 31% for SERCA. Combination of two inhibitors at the same time showed the presence of interaction between extrusion mechanisms. In aged cells the [Ca(2+)](i) extrusion was impaired due to decrease of PMCA activity, as revealed by the loss of effect of La(3+), and to inhibitory interactions between NCX and SERCA activities, indicated by acceleration of decay in response to their respective inhibitors. In conclusion, in smooth muscle cells aging decreases the overall Ca(2+) extrusion activity and modifies the interactions between the activities of the main Ca(2+) removing mechanisms.

Expansion of the calcium hypothesis of brain aging and Alzheimer's disease: minding the store

Evidence accumulated over more than two decades has implicated Ca²⁺ dysregulation in brain aging and Alzheimer's disease (AD), giving rise to the Ca²⁺ hypothesis of brain aging and dementia. Electrophysiological, imaging, and behavioral studies in hippocampal or cortical neurons of rodents and rabbits have revealed aging-related increases in the slow afterhyperpolarization, Ca²⁺ spikes and currents, Ca²⁺ transients, and L-type voltage-gated Ca²⁺ channel (L-VGCC) activity. Several of these changes have been associated with age-related deficits in learning or memory. Consequently, one version of the Ca²⁺ hypothesis has been that increased L-VGCC activity drives many of the other Ca²⁺-related biomarkers of hippocampal aging. In addition, other studies have reported aging- or AD model-related alterations in Ca²⁺ release from ryanodine receptors (RyR) on intracellular stores. The Ca²⁺-sensitive RyR channels amplify plasmalemmal Ca²⁺ influx by the mechanism of Ca²⁺-induced Ca²⁺ release (CICR). Considerable evidence indicates that a preferred functional link is present between L-VGCCs and RyRs which operate in series in heart and some brain cells. Here, we review studies implicating RyRs in altered Ca²⁺ regulation in cell toxicity, aging, and AD. A recent study from our laboratory showed that increased CICR plays a necessary role in the emergence of Ca²⁺-related biomarkers of aging. Consequently, we propose an expanded L-VGCC/Ca²⁺ hypothesis, in which aging/pathological changes occur in both L-type Ca²⁺ channels and RyRs, and interact to abnormally amplify Ca²⁺ transients. In turn, the increased transients result in dysregulation of multiple Ca²⁺-dependent processes and, through somewhat different pathways, in accelerated functional decline during aging and AD.

Age-dependent changes in Ca2+ homeostasis in peripheral neurones: implications for changes in function

Calcium ions represent universal second messengers within neuronal cells integrating multiple cellular functions, such as release of neurotransmitters, gene expression, proliferation, excitability, and regulation of cell death or apoptotic pathways. The magnitude, duration and shape of stimulation-evoked intracellular calcium ([Ca2+]i) transients are determined by a complex interplay of mechanisms that modulate stimulation-evoked rises in [Ca2+]i that occur with normal neuronal function. Disruption of any of these mechanisms may have implications for the function and health of peripheral neurones during the aging process. This review focuses on the impact of advancing age on the overall function of peripheral adrenergic neurones and how these changes in function may be linked to age-related changes in modulation of [Ca2+]i regulation. The data in this review suggest that normal aging in peripheral autonomic neurones is a subtle process and does not always result in dramatic deterioration in their function. We present studies that support the idea that in order to maintain cell viability peripheral neurones are able to compensate for an age-related decline in the function of at least one of the neuronal calcium-buffering systems, smooth endoplasmic reticulum calcium ATPases, by increased function of other calcium-buffering systems, namely, the mitochondria and plasmalemma calcium extrusion. Increased mitochondrial calcium uptake may represent a 'weak point' in cellular compensation as this over time may contribute to cell death. In addition, we present more recent studies on [Ca2+]i regulation in the form of the modulation of release of calcium from smooth endoplasmic reticulum calcium stores. These studies suggest that the contribution of the release of calcium from smooth endoplasmic reticulum calcium stores is altered with age through a combination of altered ryanodine receptor levels and modulation of these receptors by neuronal nitric oxide containing neurones.

Amyloid-beta-(1-42) increases ryanodine receptor-3 expression and function in neurons of TgCRND8 mice

Disruption of intracellular calcium homeostasis precedes the neurodegeneration that occurs in Alzheimer disease (AD). Of the many neuronal calcium-regulating proteins, we focused on endoplasmic reticulum (ER)-resident ryanodine receptors (RyRs) because they are increased in the hippocampus of mice expressing mutant presenilin-1 and are associated with neurotoxicity. Others have observed that ryanodine binding is elevated in human postmortem hippocampal regions suggesting that RyR(s) are involved in AD pathogenesis. Here we report that extracellular amyloid-beta(Abeta)-(1-42) specifically increased RyR-3, but not RyR-1 or RyR-2, gene expression in cortical neurons from C57Bl6 mice. Furthermore, endogenously produced Abeta-(1-42) increased RyR-3 mRNA and protein in cortical neurons from transgenic (Tg)CRND8 mice, a mouse model of AD. Increased RyR-3 mRNA and protein was also observed in brain tissue from 4- to 4.5-month-old Tg animals compared with non-Tg littermate controls. In experiments performed in nominal extracellular calcium, neurons from Tg mice had significant increases in intracellular calcium following ryanodine or glutamate treatment compared with littermate controls, which was abolished by treatment with small interfering RNA directed to RyR-3, indicating that the higher levels of calcium originated from RyR-3-regulated stores. Taken together, these observations suggest that Abeta-(1-42)-mediated changes in intracellular calcium homeostasis is regulated in part through a direct increase of RyR-3 expression and function.