Movement of Ca(2+)-ATPase molecules within the sarcoplasmic/endoplasmic reticulum in skeletal muscle

Seipin regulates ER-lipid droplet contacts and cargo delivery

Seipin is an endoplasmic reticulum (ER) membrane protein implicated in lipid droplet (LD) biogenesis and mutated in severe congenital lipodystrophy (BSCL2). Here, we show that seipin is stably associated with nascent ER–LD contacts in human cells, typically via one mobile focal point per LD. Seipin appears critical for such contacts since ER–LD contacts were completely missing or morphologically aberrant in seipin knockout and BSCL2 patient cells. In parallel, LD mobility was increased and protein delivery from the ER to LDs to promote LD growth was decreased. Moreover, while growing LDs normally acquire lipid and protein constituents from the ER, this process was compromised in seipin‐deficient cells. In the absence of seipin, the initial synthesis of neutral lipids from exogenous fatty acid was normal, but fatty acid incorporation into neutral lipids in cells with pre‐existing LDs was impaired. Together, our data suggest that seipin helps to connect newly formed LDs to the ER and that by stabilizing ER–LD contacts seipin facilitates the incorporation of protein and lipid cargo into growing LDs in human cells.

Ca2+ transport ATPase isoforms SERCA2a and SERCA2b are targeted to the same sites in the murine heart

Adult SERCA2(b/b) mice expressing the non-muscle Ca2+ transport ATPase isoform SERCA2b in the heart instead of the normally predominant sarcomeric SERCA2a isoform, develop mild concentric ventricular hypertrophy with impaired cardiac contractility and relaxation [Circ. Res. 89 (2001) 838]. Results from a separate study on transgenic mice overexpressing SERCA2b in the normal SERCA2a context were interpreted to show that SERCA2b and SERCA2a are differentially targeted within the cardiac sarcoplasmic reticulum (SR) [J. Biol. Chem. 275 (2000) 24722]. Since a different subcellular distribution of SERCA2b could underlie alterations in Ca2+ handling observed in SERCA2(b/b), we wanted to compare SERCA2b distribution in SERCA2(b/b) with that of SERCA2a in wild-type (WT). Using confocal microscopy on immunostained fixed myocytes and BODIPY-thapsigargin-stained living cells, we found that in SERCA2(b/b) mice SERCA2b is correctly targeted to cardiac SR and is present in the same SR regions as SERCA2a and SERCA2b in WT. We conclude that there is no differential targeting of SERCA2a and SERCA2b since both are found in the longitudinal SR and in the SR proximal to the Z-bands. Therefore, alterations in Ca2+ handling and the development of hypertrophy in adult SERCA2(b/b) mice do not result from different SERCA2b targeting.

Manipulation of cellular excitability by cell fusion: effects of rapid introduction of transient outward K+ current on the guinea pig action potential

To investigate the still-undetermined role of the Ca2+-independent transient outward current (Ito1) on repolarization of the cardiac action potential, we used cell fusion to introduce Ito1 into guinea pig cardiomyocytes, which normally lack this current. This technique enables the rapid delivery of premade functional ion channels to cardiomyocytes within hours of isolation, thus eliminating the action potential alterations that complicate prolonged cell culture. Chinese hamster ovary (CHO) cells stably expressing Kv4.3 (CHO-Kv4. 3) were loaded with a fluorescent dye and fused to guinea pig cardiomyocytes using polyethylene glycol. As controls, nontransfected CHO cells were fused using the same protocol. Myocytes fused with CHO-Kv4.3 cells exhibited a robust Ito1 (16. 5+/-2.6 pA/pF at +40 mV; 37 degrees C; n=19), whereas controls had none. Ito1 accelerated the early repolarization velocity (r=-0.68; 3 ms after the overshoot) and progressively suppressed the voltage of the plateau phase (r=-0.90) with increasing Ito1 density. Reduction of the action potential duration to 50% repolarization (r=-0.76) and to 90% repolarization (r=-0.65) also correlated well with Ito1 density. Thus, Ito1 exerted a significant effect on the early repolarization phase and abbreviated action potential duration. Cell fusion is a valuable and generalizable technique to introduce preformed membrane proteins into native cells.