Lysophosphatidylcholine augments Ca(v)3.2 but not Ca(v)3.1 T-type Ca(2+) channel current expressed in HEK-293 cells

Protein Kinase C Regulates Expression and Function of the Cav3.2 T-Type Ca2+ Channel during Maturation of Neonatal Rat Cardiomyocyte

Two distinct isoforms of the T-type Ca2+ channel, Cav3.1 and Cav3.2, play a pivotal role in the generation of pacemaker potentials in nodal cells in the heart, although the isoform switches from Cav3.2 to Cav3.1 during the early neonatal period with an unknown mechanism. The present study was designed to investigate the molecular system of the parts that are responsible for the changes of T-type Ca2+ channel isoforms in neonatal cardiomyocytes using the whole-cell patch-clamp technique and mRNA quantification. The present study demonstrates that PKC activation accelerates the Ni2+-sensitive beating rate and upregulates the Ni2+-sensitive T-type Ca2+ channel current in neonatal cardiomyocytes as a long-term effect, whereas PKC inhibition delays the Ni2+-sensitive beating rate and downregulates the Ni2+-sensitive T-type Ca2+ channel current. Because the Ni2+-sensitive T-type Ca2+ channel current is largely composed of the Cav3.2-T-type Ca2+ channel, it is accordingly assumed that PKC activity plays a crucial role in the maintenance of the Cav3.2 channel. The expression of Cav3.2 mRNA was highly positively correlated with PKC activity. The expression of a transcription factor Nkx2.5 mRNA, possibly corresponding to the Cav3.2 channel gene, was decreased by an inhibition of PKCβII. These results suggest that PKC activation, presumably by PKCβII, is responsible for the upregulation of CaV3.2 T-type Ca2+ channel expression that interacts with a cardiac-specific transcription factor, Nkx2.5, in neonatal cardiomyocytes.

An intricate dance: Life experience, multisystem resiliency, and rate of telomere decline throughout the lifespan

Accumulation of life stressors predicts accelerated development and progression of diseases of aging. Telomere length, the DNA-based biomarker indicating cellular aging, is a mechanism of disease development, and is shortened in a dose response fashion by duration and severity of life stressor exposures. Telomere length captures the interplay between genetics, life experiences and psychosocial and behavioral factors. Over the past several years, psychological stress resilience, healthy lifestyle factors, and social connections have been associated with longer telomere length and it appears that these factors can protect individuals from stress-induced telomere shortening. In the current review, we highlight these findings, and illustrate that combining these `multisystem resiliency' factors may strengthen our understanding of aging, as these powerful factors are often neglected in studies of aging. In naturalistic studies, the effects of chronic stress exposure on biological pathways are rarely main effects, but rather a complex interplay between adversity and resiliency factors. We suggest that chronic stress effects can be best understood by directly testing if the deleterious effects of stress on biological aging processes, in this case the cell allostasis measure of telomere shortening, are mitigated in individuals with high levels of multisystem resiliency. Without attending to such interactions, stress effects are often masked and missed. Taking account of the cluster of positive buffering factors that operate across the lifespan will take us a step further in understanding healthy aging. While these ideas are applied to the telomere length literature for illustration, the concept of multisystem resiliency might apply to aging broadly, from cellular to systemic health.

alpha1G-dependent T-type Ca2+ current antagonizes cardiac hypertrophy through a NOS3-dependent mechanism in mice

In noncontractile cells, increases in intracellular Ca2+ concentration serve as a second messenger to signal proliferation, differentiation, metabolism, motility, and cell death. Many of these Ca2+-dependent regulatory processes operate in cardiomyocytes, although it remains unclear how Ca2+ serves as a second messenger given the high Ca2+ concentrations that control contraction. T-type Ca2+ channels are reexpressed in adult ventricular myocytes during pathologic hypertrophy, although their physiologic function remains unknown. Here we generated cardiac-specific transgenic mice with inducible expression of alpha1G, which generates Cav3.1 current, to investigate whether this type of Ca2+ influx mechanism regulates the cardiac hypertrophic response. Unexpectedly, alpha1G transgenic mice showed no cardiac pathology despite large increases in Ca2+ influx, and they were even partially resistant to pressure overload-, isoproterenol-, and exercise-induced cardiac hypertrophy. Conversely, alpha1G-/- mice displayed enhanced hypertrophic responses following pressure overload or isoproterenol infusion. Enhanced hypertrophy and disease in alpha1G-/- mice was rescued with the alpha1G transgene, demonstrating a myocyte-autonomous requirement of alpha1G for protection. Mechanistically, alpha1G interacted with NOS3, which augmented cGMP-dependent protein kinase type I activity in alpha1G transgenic hearts after pressure overload. Further, the anti-hypertrophic effect of alpha1G overexpression was abrogated by a NOS3 inhibitor and by crossing the mice onto the Nos3-/- background. Thus, cardiac alpha1G reexpression and its associated pool of T-type Ca2+ antagonize cardiac hypertrophy through a NOS3-dependent signaling mechanism.

Intracellular Ca(2+)- and PKC-dependent upregulation of T-type Ca(2+) channels in LPC-stimulated cardiomyocytes

Lysophosphatidylcholine (LPC) accumulation in intracellular and/or interstitial space in cardiomyocytes may underlie as a mechanism for tachycardia and various arrhythmias during cardiac ischemia, which is usually accompanied by elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)). The present study was therefore designed to investigate possible mechanisms responsible for [Ca(2+)](i) elevation by LPC focusing on T-type Ca(2+) channel current (I(Ca.T)). LPC as well as phorbol 12-myristate 13-acetate (PMA) significantly accelerated the beating rates of neonatal rat cardiomyocytes. Augmentation of I(Ca.T) by LPC was dependent on the intracellular Ca(2+) concentration: an increase of I(Ca.T) was significantly larger in high [Ca(2+)](i) condition (pCa=7) than those in low [Ca(2+)](i) condition (pCa=11). In heterologous expression system by use of human cardiac Ca(V)3.1 and Ca(V)3.2 channels expressed in HEK293 cells, LPC augmented Ca(V)3.2 channel current (I(Cav3.2)) in a concentration-dependent manner but not Ca(V)3.1 channel current (I(Cav3.1)). Augmentation of I(Cav3.2) by LPC was highly [Ca(2+)](i) dependent: I(Cav3.2) was unchanged when pCa was 11 but was markedly increased when [Ca(2+)](i) was higher than 10(-10) M (pCa<or=10) by LPC application (10-50 microM). A specific inhibitor of protein kinase Calpha (Ro-32-0432) attenuated the increase of I(Cav3.2) by LPC. LPC stimulates I(Ca.T) in a [Ca(2+)](i)-dependent manner via PKCalpha activation, which may play a role in triggering arrhythmias in pathophysiological conditions of the heart.

Lysophosphatidyl choline modulates mechanosensitive L-type Ca2+ current in circular smooth muscle cells from human jejunum

The L-type Ca2+ channel expressed in gastrointestinal smooth muscle is mechanosensitive. Direct membrane stretch and shear stress result in increased Ca2+ entry into the cell. The mechanism for mechanosensitivity is not known, and mechanosensitivity is not dependent on an intact cytoskeleton. The aim of this study was to determine whether L-type Ca2+ channel mechanosensitivity is dependent on tension in the lipid bilayer in human jejunal circular layer myocytes. Whole cell currents were recorded in the amphotericin-perforated-patch configuration, and lysophosphatidyl choline (LPC), lysophosphatidic acid (LPA), and choline were used to alter differentially the tension in the lipid bilayer. Shear stress (perfusion at 10 ml/min) was used to mechanostimulate L-type Ca2+ channels. The increase in L-type Ca2+ current induced by shear stress was greater in the presence of LPC (large head-to-tail proportions), but not LPA or choline, than in the control perfusion. The increased peak Ca2+ current also did not return to baseline levels as in control conditions. Furthermore, steady-state inactivation kinetics were altered in the presence of LPC, leading to a change in window current. These findings suggest that changes in tension in the plasmalemmal membrane can be transmitted to the mechanosensitive L-type Ca2+ channel, leading to altered activity and Ca2+ entry in the human jejunal circular layer myocyte.

17 beta-estradiol modulates expression of low-voltage-activated Ca(V)3.2 T-type calcium channel via extracellularly regulated kinase pathway in cardiomyocytes

T-type Ca(2+) channel current (I(Ca,T)) plays an important role for spontaneous pacemaker activity and is involved in the progression of structural heart diseases. Estrogens are of importance for the regulation of growth and differentiation and function in a wide array of target tissues, including those in the cardiovascular system. The aim of this study was to elucidate the short-term and long-term effects of 17beta-estradiol (E(2)) on I(Ca,T) in cardiomyocytes. We employed in vivo and in vitro techniques to clarify E(2)-mediated modulation of heart rate (HR) in ovariectomized rats and I(Ca,T) in cardiomyocytes. Ovariectomy increased HR and E(2) supplement reduced HR in ovariectomized rats. Slowing of E(2)-induced HR was consistent with the deceleration of automaticity in E(2)-treated neonatal cardiomyocytes. Short-term application of E(2) did not have significant effects on I(Ca,T), whereas in cardiomyocytes treated with 10 nm E(2) for 24 h, estrogen receptor-independent down-regulation of peak I(Ca,T) and declination of Ca(V)3.2 mRNA were observed. Expression of a cardiac-specific transcription factor Csx/Nkx2.5 was also suppressed by E(2) treatment for 24 h. On the other hand, expression of Ca(V)3.1 mRNA was unaltered by E(2) treatment in this study. An ERK-1/2, 5 inhibitor, PD-98059, abolished the effects of E(2) on I(Ca,T) and Ca(V)3.2 mRNA as well as Csx/Nkx2.5 mRNA. These findings indicate that E(2) decreases Ca(V)3.2 I(Ca,T) through activation of ERK-1/2, 5, which is mediated by the suppression of Csx/Nkx2.5-dependent transcription, suggesting a genomic effect of E(2) as a negative chronotropic factor in the heart.

Transcription factors Csx/Nkx2.5 and GATA4 distinctly regulate expression of Ca2+ channels in neonatal rat heart

The cardiac transcription factors Csx/Nkx2.5 and GATA4 play important roles in vertebrate heart development. Although mutations of Csx/Nkx2.5 or GATA4 are associated with various congenital heart diseases, their mechanism of action on cardiomyocyte function is not completely elucidated. In this study, we therefore investigated the actions of these transcription factors on the electrophysiological features and expression of ion channels in cardiomyocytes. Genes for transcription factors Csx/Nkx2.5 and GATA4 were transfected into rat neonatal cardiomyocytes by adenoviral infection. Action potentials, L-, T-type Ca(2+) channels and hyperpolarization-activated cation current (I(h)) of rat neonatal myocytes were recorded by patch clamp technique after adenoviral infection. Expression of ion channels was confirmed by real-time PCR. In Csx/Nkx2.5 overexpression myocytes, the spontaneous beating rate was markedly increased with an up-regulation of the Ca(v)3.2 T-type Ca(2+) channel, while in GATA4 overexpression myocytes, the T-type Ca(2+) channel was unchanged. On the other hand, the L-type Ca(2+) channel was down-regulated by both Csx/Nkx2.5 and GATA4 overexpression; the level of Ca(v)1.3 mRNA was dramatically decreased by Csx/Nkx2.5 overexpression. These results indicate that Csx/Nkx2.5 and GATA4 play important roles on the generation of pacemaker potentials modulating voltage-dependent Ca(2+) channels in the neonatal cardiomyocyte.

Calcium Influx and Mitochondrial Alterations at Synapses Exposed to Snake Neurotoxins or Their Phospholipid Hydrolysis Products

Snake presynaptic phospholipase A2 neurotoxins (SPANs) bind to the presynaptic membrane and hydrolyze phosphatidylcholine with generation of lysophosphatidylcholine (LysoPC) and fatty acid (FA). The LysoPC+FA mixture promotes membrane fusion, inducing the exocytosis of the ready-to-release synaptic vesicles. However, also the reserve pool of synaptic vesicles disappears from nerve terminals intoxicated with SPAN or LysoPC+FA. Here, we show that LysoPC+FA and SPANs cause a large influx of extracellular calcium into swollen nerve terminals, which accounts for the extensive synaptic vesicle release. This is paralleled by the change of morphology and the collapse of membrane potential of mitochondria within nerve bulges. These results complete the picture of events occurring at nerve terminals intoxicated by SPANs and define the LysoPC+FA lipid mixture as a novel and effective agonist of synaptic vesicle release.