High-throughput measurement of the Ca2+-dependent ATPase activity in COS microsomes

The alkalinizing, lysosomotropic agent ML-9 induces a pH-dependent depletion of ER Ca2+ stores in cellulo

ML-9 elicits a broad spectrum of effects in cells, including inhibition of myosin light chain kinase, inhibition of store-operated Ca²⁺ entry and lysosomotropic actions that result in prostate cancer cell death. Moreover, the compound also affects endoplasmic reticulum (ER) Ca²⁺ homeostasis, although the underlying mechanisms remain unclear. We found that ML-9 provokes a rapid mobilization of Ca²⁺ from ER independently of IP₃Rs or TMBIM6/Bax Inhibitor-1, two ER Ca²⁺-leak channels. Moreover, in unidirectional ⁴⁵Ca²⁺ fluxes in permeabilized cells, ML-9 was able to reduce ER Ca²⁺-store content. Although the ER Ca²⁺ store content was decreased, ML-9 did not directly inhibit SERCA's ATPase activity in vitro using microsomal preparations. Consistent with its chemical properties as a cell-permeable weak alkalinizing agent (calculated pKa of 8.04), ML-9 provoked a rapid increase in cytosolic pH preceding the Ca²⁺ efflux from the ER. Pre-treatment with the weak acid 3NPA blunted the ML-9-evoked increase in intracellular pH and subsequent ML-9-induced Ca²⁺ mobilization from the ER. This experiment underpins a causal link between ML-9's impact on the pH and Ca²⁺ dynamics. Overall, our work indicates that the lysosomotropic drug ML-9 may not only impact lysosomal compartments but also have severe impacts on ER Ca²⁺ handling in cellulo.

A rare missense variant in the ATP2C2 gene is associated with language impairment and related measures

At least 5% of children present unexpected difficulties in expressing and understanding spoken language. This condition is highly heritable and often co-occurs with other neurodevelopmental disorders such as dyslexia and ADHD. Through an exome sequencing analysis, we identified a rare missense variant (chr16:84405221, GRCh38.p12) in the ATP2C2 gene. ATP2C2 was implicated in language disorders by linkage and association studies, and exactly the same variant was reported previously in a different exome sequencing study for language impairment (LI). We followed up this finding by genotyping the mutation in cohorts selected for LI and comorbid disorders. We found that the variant had a higher frequency in LI cases (1.8%, N = 360) compared with cohorts selected for dyslexia (0.8%, N = 520) and ADHD (0.7%, N = 150), which presented frequencies comparable to reference databases (0.9%, N = 24 046 gnomAD controls). Additionally, we observed that carriers of the rare variant identified from a general population cohort (N = 42, ALSPAC cohort) presented, as a group, lower scores on a range of reading and language-related measures compared to controls (N = 1825; minimum P = 0.002 for non-word reading). ATP2C2 encodes for an ATPase (SPCA2) that transports calcium and manganese ions into the Golgi lumen. Our functional characterization suggested that the rare variant influences the ATPase activity of SPCA2. Thus, our results further support the role of ATP2C2 locus in language-related phenotypes and pinpoint the possible effects of a specific rare variant at molecular level.

An N-terminal Ca2+-binding motif regulates the secretory pathway Ca2+/Mn2+-transport ATPase SPCA1

The Ca²⁺/Mn²⁺ transport ATPases 1a and 2 (SPCA1a/2) are closely related to the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) and are implicated in breast cancer and Hailey-Hailey skin disease. Here, we purified the human SPCA1a/2 isoforms from a yeast recombinant expression system and compared their biochemical properties after reconstitution. We observed that the purified SPCA1a displays a lower Ca²⁺ affinity and slightly lower Mn²⁺ affinity than SPCA2. Remarkably, the turnover rates of SPCA1a in the presence of Mn²⁺ and SPCA2 incubated with Ca²⁺ and Mn²⁺ were comparable, whereas the turnover rate of SPCA1a in Ca²⁺ was 2-fold higher. Moreover, we noted an unusual biphasic activation curve for the SPCA1a ATPase and autophosphorylation activity, not observed with SPCA2. We also found that the biphasic pattern and low apparent ion affinity of SPCA1a critically depends on ATP concentration. We further show that the specific properties of SPCA1a at least partially depend on an N-terminal EF-hand-like motif, which is present only in the SPCA1a isoform and absent in SPCA2. This motif binds Ca²⁺, and its mutation lowered the Ca²⁺ turnover rate relative to that of Mn²⁺, increased substrate affinity, and reduced the level of biphasic activation of SPCA1a. A biochemical analysis indicated that Ca²⁺ binding to the N-terminal EF-hand-like motif promotes the activity of SPCA1a by facilitating autophosphorylation. We propose that this regulation may be physiologically relevant in cells with a high Ca²⁺ load, such as mammary gland cells during lactation, or in cells with a low ATP content, such as keratinocytes.

Extracellular and ER-stored Ca2+ contribute to BIRD-2-induced cell death in diffuse large B-cell lymphoma cells

The anti-apoptotic protein Bcl-2 is upregulated in several cancers, including diffuse large B-cell lymphoma (DLBCL) and chronic lymphocytic leukemia (CLL). In a subset of these cancer cells, Bcl-2 blocks Ca²⁺-mediated apoptosis by suppressing the function of inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) located at the endoplasmic reticulum (ER). A peptide tool, called Bcl-2/IP₃ receptor disruptor-2 (BIRD-2), was developed to disrupt Bcl-2/IP₃R complexes, triggering pro-apoptotic Ca²⁺ signals and killing Bcl-2-dependent cancer cells. In DLBCL cells, BIRD-2 sensitivity depended on the expression level of IP₃R2 channels and constitutive IP₃ signaling downstream of the B-cell receptor. However, other cellular pathways probably also contribute to BIRD-2-provoked cell death. Here, we examined whether BIRD-2-induced apoptosis depended on extracellular Ca²⁺ and more particularly on store-operated Ca²⁺ entry (SOCE), a Ca²⁺-influx pathway activated upon ER-store depletion. Excitingly, DPB162-AE, a SOCE inhibitor, suppressed BIRD-2-induced cell death in DLBCL cells. However, DPB162-AE not only inhibits SOCE but also depletes the ER Ca²⁺ store. Treatment of the cells with YM-58483 and GSK-7975A, two selective SOCE inhibitors, did not protect against BIRD-2-induced apoptosis. Similar data were obtained by knocking down STIM1 using small interfering RNA. Yet, extracellular Ca²⁺ contributed to BIRD-2 sensitivity in DLBCL, since the extracellular Ca²⁺ buffer ethylene glycol tetraacetic acid (EGTA) blunted BIRD-2-triggered apoptosis. The protective effects observed with DPB162-AE are likely due to ER Ca²⁺-store depletion, since a similar protective effect could be obtained using the sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin. Thus, both the ER Ca²⁺-store content and extracellular Ca²⁺, but not SOCE, are critical factors underlying BIRD-2-provoked cell death.

Measuring Ca2+ pump activity in overexpression systems and cardiac muscle preparations

Sarco-/endoplasmic reticulum (SR/ER) Ca(2+) pumps (SERCAs) build up vital Ca(2+) gradients across the intracellular SR/ER membrane, helping to control cell function, proliferation, growth, differentiation, and death. We describe two techniques to measure the SERCA activity either in mammalian culture cells overexpressing SERCAs or in muscle tissue containing high levels of endogenous SERCAs. As Ca(2+) transport is tightly coupled to ATP hydrolysis, it is possible to determine the rate of Ca(2+)-dependent ATP hydrolysis and use it as a measure for SERCA activity or, in a second approach, to quantify ATP-stimulated uptake of radioactive (45)Ca(2+). Here, we first provide an overview of the mechanism of Ca(2+)-transport ATPases and show how this can be taken advantage of in protocols for measuring Ca(2+) pump activity.

Measuring Ca2+-dependent Ca2+-uptake activity in the mouse heart

The apparent Ca(2+) affinity of the isoforms of the sarco/endoplasmic reticulum Ca(2+) ATPase SERCA2 is controlled primarily by two proteins, phospholamban (PLB) and sarcolipin (SLN). The rate of ATP-driven Ca(2+) uptake into sarcoplasmic reticulum (SR)-derived vesicles can be monitored by a technique in which the net uptake of (45)Ca(2+) in the form of an intravesicular calcium oxalate precipitate is recorded. Here, we present details of a modification of such a protocol for determining the apparent Ca(2+) affinity of the Ca(2+) pump, and its control by various regulators, in crude homogenates of mouse heart.