Abstract:Junctophilins (JPHs) comprise a family of structural proteins that connect the plasma membrane to intracellular organelles such as the endo/sarcoplasmic reticulum. Tethering of these membrane structures results in the formation of highly organized subcellular junctions that play important signaling roles in all excitable cell types. There are four JPH isoforms, expressed primarily in muscle and neuronal cell types. Each JPH protein consists of 6 'membrane occupation and recognition nexus' (MORN) motifs, a join… Show more
“…The jSR is a complex structure whose architecture, function, and stability reflect the contribution of several proteins ( Dixon, 2022 ; Jones et al, 2018 ). One of these proteins, junctophilin-2 (JPH2), is anchored to the jSR via its C-terminus and contacts the sarcolemma through lipid-interacting motifs in its N-terminus ( Garbino and Wehrens, 2010 ; Lehnart and Wehrens, 2022 ; Pritchard et al, 2019 ). JPH2, which binds to Ca V 1.2 and RYR2 channels ( Feng et al, 2020 ; Gross et al, 2021 ; van Oort et al, 2011 ), is hypothesized to provide a molecular bridge between the jSR and T-tubules.…”
Each heartbeat begins with the generation of an action potential in pacemaking cells in the sinoatrial node. This signal triggers contraction of cardiac muscle through a process termed excitation–contraction (EC) coupling. EC coupling is initiated in dyadic structures of cardiac myocytes, where ryanodine receptors in the junctional sarcoplasmic reticulum come into close apposition with clusters of CaV1.2 channels in invaginations of the sarcolemma. Cooperative activation of CaV1.2 channels within these clusters causes a local increase in intracellular Ca2+ that activates the juxtaposed ryanodine receptors. A salient feature of healthy cardiac function is the reliable and precise beat-to-beat pacemaking and amplitude of Ca2+ transients during EC coupling. In this review, we discuss recent discoveries suggesting that the exquisite reproducibility of this system emerges, paradoxically, from high variability at subcellular, cellular, and network levels. This variability is attributable to stochastic fluctuations in ion channel trafficking, clustering, and gating, as well as dyadic structure, which increase intracellular Ca2+ variance during EC coupling. Although the effects of these large, local fluctuations in function and organization are sometimes negligible at the macroscopic level owing to spatial–temporal summation within and across cells in the tissue, recent work suggests that the “noisiness” of these intracellular Ca2+ events may either enhance or counterintuitively reduce variability in a context-dependent manner. Indeed, these noisy events may represent distinct regulatory features in the tuning of cardiac contractility. Collectively, these observations support the importance of incorporating experimentally determined values of Ca2+ variance in all EC coupling models. The high reproducibility of cardiac contraction is a paradoxical outcome of high Ca2+ signaling variability at subcellular, cellular, and network levels caused by stochastic fluctuations in multiple processes in time and space. This underlying stochasticity, which counterintuitively manifests as reliable, consistent Ca2+ transients during EC coupling, also allows for rapid changes in cardiac rhythmicity and contractility in health and disease.
“…The jSR is a complex structure whose architecture, function, and stability reflect the contribution of several proteins ( Dixon, 2022 ; Jones et al, 2018 ). One of these proteins, junctophilin-2 (JPH2), is anchored to the jSR via its C-terminus and contacts the sarcolemma through lipid-interacting motifs in its N-terminus ( Garbino and Wehrens, 2010 ; Lehnart and Wehrens, 2022 ; Pritchard et al, 2019 ). JPH2, which binds to Ca V 1.2 and RYR2 channels ( Feng et al, 2020 ; Gross et al, 2021 ; van Oort et al, 2011 ), is hypothesized to provide a molecular bridge between the jSR and T-tubules.…”
Each heartbeat begins with the generation of an action potential in pacemaking cells in the sinoatrial node. This signal triggers contraction of cardiac muscle through a process termed excitation–contraction (EC) coupling. EC coupling is initiated in dyadic structures of cardiac myocytes, where ryanodine receptors in the junctional sarcoplasmic reticulum come into close apposition with clusters of CaV1.2 channels in invaginations of the sarcolemma. Cooperative activation of CaV1.2 channels within these clusters causes a local increase in intracellular Ca2+ that activates the juxtaposed ryanodine receptors. A salient feature of healthy cardiac function is the reliable and precise beat-to-beat pacemaking and amplitude of Ca2+ transients during EC coupling. In this review, we discuss recent discoveries suggesting that the exquisite reproducibility of this system emerges, paradoxically, from high variability at subcellular, cellular, and network levels. This variability is attributable to stochastic fluctuations in ion channel trafficking, clustering, and gating, as well as dyadic structure, which increase intracellular Ca2+ variance during EC coupling. Although the effects of these large, local fluctuations in function and organization are sometimes negligible at the macroscopic level owing to spatial–temporal summation within and across cells in the tissue, recent work suggests that the “noisiness” of these intracellular Ca2+ events may either enhance or counterintuitively reduce variability in a context-dependent manner. Indeed, these noisy events may represent distinct regulatory features in the tuning of cardiac contractility. Collectively, these observations support the importance of incorporating experimentally determined values of Ca2+ variance in all EC coupling models. The high reproducibility of cardiac contraction is a paradoxical outcome of high Ca2+ signaling variability at subcellular, cellular, and network levels caused by stochastic fluctuations in multiple processes in time and space. This underlying stochasticity, which counterintuitively manifests as reliable, consistent Ca2+ transients during EC coupling, also allows for rapid changes in cardiac rhythmicity and contractility in health and disease.
“…Full-length (FL) JP2 functions as a membrane tether, residing as a single-pass tail-anchored protein in the sarcoplasmic reticulum (SR) membrane in cardiomyocytes 11 – 13 . Bioinformatic analysis predicts eight highly conserved N-terminal MORN (membrane occupation and recognition nexus) motifs 11 , 14 , which provide the capacity for JP2 binding to the cytosolic membrane leaflet (Fig. 1 A).…”
Section: Introductionmentioning
confidence: 99%
“…Furthermore, the MORN and TM domains are connected by the α-helical and the divergent region (Fig. 1 A), together bridging the discontinuous dyadic membrane contact subspace (≈15 nm) between the junctional SR and the Transverse (T-)tubule membrane invaginations in cardiomyocytes 15 and between the SR and plasma membrane in smooth muscle cells 14 . Figure 1 Identification of Calpain-specific cleavage fragments of JP2.…”
Calpains are calcium-activated neutral proteases involved in the regulation of key signaling pathways. Junctophilin-2 (JP2) is a Calpain-specific proteolytic target and essential structural protein inside Ca2+ release units required for excitation-contraction coupling in cardiomyocytes. While downregulation of JP2 by Calpain cleavage in heart failure has been reported, the precise molecular identity of the Calpain cleavage sites and the (patho-)physiological roles of the JP2 proteolytic products remain controversial. We systematically analyzed the JP2 cleavage fragments as function of Calpain-1 versus Calpain-2 proteolytic activities, revealing that both Calpain isoforms preferentially cleave mouse JP2 at R565, but subsequently at three additional secondary Calpain cleavage sites. Moreover, we identified the Calpain-specific primary cleavage products for the first time in human iPSC-derived cardiomyocytes. Knockout of RyR2 in hiPSC-cardiomyocytes destabilized JP2 resulting in an increase of the Calpain-specific cleavage fragments. The primary N-terminal cleavage product NT1 accumulated in the nucleus of mouse and human cardiomyocytes in a Ca2+-dependent manner, closely associated with euchromatic chromosomal regions, where NT1 is proposed to function as a cardio-protective transcriptional regulator in heart failure. Taken together, our data suggest that stabilizing NT1 by preventing secondary cleavage events by Calpain and other proteases could be an important therapeutic target for future studies.
“…Four different subtypes of Jph have been identified, with Jph1 primarily expressed in skeletal muscle, Jph2 in cardiac and skeletal muscle, and Jph3 and Jph4 in neurons (see reviews in refs. 6 and 9 ). In terms of molecular architecture, the Jphs are composed of eight membrane occupation and recognition nexus (MORN) domains, a long α-helical region, thought to be a spacer between the ER and PM, a long divergent region, and a transmembrane domain.…”
mentioning
confidence: 99%
“…By comparison, dyadic and triadic junctions are preassembled. In this broader context, the junctophilin family of proteins (Jph1 to Jph4) have been shown to play an essential role in both the formation of ER–PM junctions in muscle and neurons as well as to serve as a scaffold that recruits relevant ion channels to this specialized Ca 2+ -signaling compartment ( 6 ). In PNAS, the paper by Yang et al.…”
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