usEP Effects on the Endoplasmic Reticulum (ER)

The endoplasmic reticulum (ER) is a tubular membranous labyrinth throughout the cytoplasm that is continuous with the nuclear membrane, exhibits contact sites with mitochondria and the plasma membrane, and displays domains for specific functions. It is smooth or rough with ribosomes for protein synthesis and is the home for folding proteins in their proper tertiary structures. The ER includes cellular stress response sensors that can lead to unfolded protein response (UPR), leading to regulated cell death (RCD). The ER is also a primary storage site for Ca2+ that can be used for scores of Ca2+ mediated signal transduction responses such as neurotransmitter release, muscle contraction, or contributors to RCD, among others. Since usEPs were unique for intracellular electric field effects, usEP-induced Ca2+ release was an excellent way to define these intracellular effects, albeit not without caveats. Because usEPs also induced plasma membrane permeabilization for Ca2+ influx, which occurred at lower charging conditions than ER-induced nanopore formation and Ca2+ release; because some cells expressed voltage-gated Ca2+ channels (VGCC), which could be directly activated or activated due to usEP-induced plasma membrane depolarization; because capacitative Ca2+ increases could increase intracellular Ca2+; and Ca2+ increases from Ca2+-induced Ca2+ release, significant care, and experimental manipulations were required to be sure that increases in intracellular Ca2+ were due to release from internal stores. Also, because there were other intracellular stores for Ca2+, other approaches were needed to ensure that the source of intracellular Ca2+ release was from the ER. This chapter provides details from several studies using many different experimental techniques that lead to the conclusion that usEPs could induce Ca2+ release from the ER by forming ER nanopores. Notably, another series of experimental studies supported theoretical evidence that shorter pulse durations lead to more significant increases in intracellular Ca2+ than longer pulse durations.