Calcium Dynamics Mediated by the Endoplasmic/Sarcoplasmic Reticulum and Related Diseases

Abstract

The flow of intracellular calcium (Ca²⁺) is critical for the activation and regulation of important biological events that are required in living organisms. As the major Ca²⁺ repositories inside the cell, the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of muscle cells are central in maintaining and amplifying the intracellular Ca²⁺ signal. The morphology of these organelles, along with the distribution of key calcium-binding proteins (CaBPs), regulatory proteins, pumps, and receptors fundamentally impact the local and global differences in Ca²⁺ release kinetics. In this review, we will discuss the structural and morphological differences between the ER and SR and how they influence localized Ca²⁺ release, related diseases, and the need for targeted genetically encoded calcium indicators (GECIs) to study these events.

Keywords: GECI; IP3R; JP45; RyR; SERCA pump; calcium signaling; calsequestrin; endoplasmic reticulum; sarcoplasmic reticulum.

Figure 1

A comparison of the ER and SR morphology and the arrangement of the common surface receptors and pumps. (a) The ER, found in all cell types, excitable and non-excitable but mainly non-excitable cells, have an even distribution of the receptors that release Ca²⁺ from the organelle, IP₃R and RyR, and the pump which refills this organelle, the SERCA pump. Notably, the isoforms of the receptors and pumps expressed hinge upon the tissue they are located; (b) The SR, found only in excitable cells, has a higher distribution of RyR, primarily concentrated in the TC, and the SERCA pump being on the longitudinal SR. Additionally, the RyR isoforms 1 and 2 are more predominately found in the SR than isoform 3. The black arrows indicate the direction Ca²⁺ flows through the respective receptor or pump.

Figure 2

Membrane contact sites within the ER/SR. The black arrows indicate the direction Ca²⁺ flows through the respective receptor or pump. (a) Representation of the organization of the junctional zone and the channels, receptors, pumps, and proteins involved in the E-C coupling process in skeletal muscle cells. The ΔV here represents a voltage change applied to the cell membrane; (b) Representation of the mitochondria and ER/SR microdomain found in excitable and non-excitable cells; (c) Representation of the endo/lysosome and ER/SR microdomain found in excitable and non-excitable cells. The receptor/mechanism that returns Ca²⁺ to the endo/lysosome is indicated here by a “?” symbol, as it is still unknown; (d) Legend of symbols used in (a–c). DHPR (dihydropyridine receptor), JP45 (junctional protein 45), CASQ1 (calsequestrin), Mfn1 and Mfn2 (mitofusion proteins 1 and 2), VDAC (voltage-dependent anion channel), VMP1 (vacuole membrane protein 1), TPC (two-pore channels), and TRP (transient receptor channels).

Figure 3

Capturing fast Ca²⁺ dynamics with CatchER. (a) Cartoon representation of the crystal structure of Ca²⁺ bound CatchER (PDB: 4l1i). Using site-directed mutagenesis, a Ca²⁺ binding site consisting of mutated residues S147E, S202D, Q204E, F223E and T225E was created on the surface of enhanced green fluorescent protein (EGFP); (b) k_off traces measured by stopped-flow. Fluorescence decreased when Ca²⁺ bound CatchER was mixed with 200 µM ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA). Measurements were carried out in 10 mM Tris (pH 7.4) at room temperature; (c) CatchER and D1ER kinetics in response to 100-ms pulses to 20 mV in FDB fibers under patch clamp. Time to peak, half recovery time, and response range normalized to basal fluorescence (ΔF/F) were analyzed for both indicators. (* p < 0.01).