In fight-or-flight circumstances, you’d do well to relieve yourself of any encumbrance. That’s what calcium channels do in your heart cells. As part of a cascade of adrenalin-instigated molecular events, the calcium channel sheds a protein called Rad, triggering an influx of calcium and boosting the heartbeat.
This mechanistic detail, which emerged from an investigation led by Harvard and Columbia University scientists, is the solution to a 50-year-old mystery, a cold case that turned hot only after investigators began poring over proteomic evidence.
“It’s exciting to finally solve how the cardiac calcium channel is stimulated in the fight-or-flight response,” said Columbia’s Steven O. Marx. “In the end, the underlying mechanism turned out to be simple and elegant. With this information, we can potentially design novel therapeutics targeting this pathway for the treatment of cardiac diseases.”
Marx, along with Harvard’s Marian Kalocsay, led a study that used proximity proteomics to identify nearly every protein located near voltage-gated calcium channels. In this study, “near” is around 20 nanometers, or 10 times the width of a strand of DNA.
Marx, Kalocsay, and colleagues profiled proteins in both mouse cardiomyocytes and intact, functional mouse hearts, before and after exposure to an adrenaline-like drug. Ultimately, only one protein, Rad, consistently exhibited a major change in levels after adrenaline exposure, decreasing by around 30–50% in the neighborhood of the channels.
Intrigued by this clue, the scientists dug deeper, as detailed in “Mechanism of adrenergic CaV1.2 stimulation revealed by proximity proteomics,” a report that appeared January 22 in Nature. According to the report, the scientists confirmed their suspicions about Rad by using human embryonic kidney cells to recreate the adrenalin-stimulated boost in calcium flow that occurs in heart cells. This work involved the transfection and expression of both Rad and voltage-gated calcium channel subunits, which normally don’t occur in kidney cells.
“We observe that the calcium-channel inhibitor Rad, a monomeric G protein, is enriched in the CaV1.2 microenvironment but is depleted during β-adrenergic stimulation,” the article’s authors wrote. “Phosphorylation by protein kinase A of specific serine residues on Rad decreases its affinity for β subunits and relieves constitutive inhibition of CaV1.2, observed as an increase in channel open probability. Expression of Rad or its homologue Rem in HEK293T cells also imparts stimulation of CaV1.3 and CaV2.2 by protein kinase A, revealing an evolutionarily conserved mechanism that confers adrenergic modulation upon voltage-gated calcium channels.”
The findings could open new paths for the development of drugs as effective as, but potentially safer than, beta-blockers—a widely prescribed class of medications that block the effects of adrenaline to address cardiovascular issues such as high blood pressure.
Adrenaline stimulates voltage-gated calcium channels by activating a protein known as protein kinase A (PKA), which in turn activates the channel. It was thought for decades that PKA does this by altering specific regions on the channel known as PKA phosphorylation sites, but a growing body of evidence indicated that this hypothesis was incorrect.
In the current study, the team reevaluated PKA’s role by genetically engineered mice with cardiomyocytes lacking PKA phosphorylation sites. They found that modified cells continued to respond when stimulated by an adrenaline-like drug, suggesting that PKA had to interact with another factor, which was ultimately identified as Rad.
“Under normal circumstances, calcium channels in the heart work efficiently, but they have a handbrake on in the form of the protein Rad,” said Kalocsay. “When we need full power, adrenaline releases this handbrake so that these channels open faster and give the boost needed to fight or flee from danger.”
The current study was accompanied by a “news and views” article (“Suspect that modulates the heartbeat is ensnared”) that contextualized the PKA-Rad interaction as follows: The hormone adrenaline activates the β-adrenergic receptor. This leads to the production of cyclic AMP (cAMP) molecules, which activate PKA. Activated PKA adds a phosphate group (P) to Rad, causing Rad to dissociate from the channel and enabling channel activity to increase. This elevation of Ca2+ in the cytoplasm boosts the heartbeat.”
In addition, the original paper included this detail: “The cAMP–PKA-mediated regulation of CaV1.2 requires both phosphorylation by PKA on the C terminus of Rad and the interaction of Rad with the β subunit.
The findings can inform new therapeutic approaches, the authors said. For example, disrupting the interaction between Rad and the calcium channel could enhance heart function by increasing calcium flow into cells. Conversely, blocking the modification of Rad by PKA may represent an alternative, more specific, strategy than beta-blockers to reduce calcium influx into the heart.
In addition, the findings could yield insights of interest to researchers in other fields, especially neuroscience, as voltage-gated calcium channels play a central role in neuronal excitation.