Aequorin entrapment in mammalian cells

Intracellular Delivery by Membrane Disruption: Mechanisms, Strategies, and Concepts

Intracellular delivery is a key step in biological research and has enabled decades of biomedical discoveries. It is also becoming increasingly important in industrial and medical applications ranging from biomanufacture to cell-based therapies. Here, we review techniques for membrane disruption-based intracellular delivery from 1911 until the present. These methods achieve rapid, direct, and universal delivery of almost any cargo molecule or material that can be dispersed in solution. We start by covering the motivations for intracellular delivery and the challenges associated with the different cargo types-small molecules, proteins/peptides, nucleic acids, synthetic nanomaterials, and large cargo. The review then presents a broad comparison of delivery strategies followed by an analysis of membrane disruption mechanisms and the biology of the cell response. We cover mechanical, electrical, thermal, optical, and chemical strategies of membrane disruption with a particular emphasis on their applications and challenges to implementation. Throughout, we highlight specific mechanisms of membrane disruption and suggest areas in need of further experimentation. We hope the concepts discussed in our review inspire scientists and engineers with further ideas to improve intracellular delivery.

Power-frequency electromagnetic fields and the capacitative calcium entry system in SV40-transformed Swiss 3T3 cells

The present study was designed to test the hypothesis that a 60 Hz electromagnetic field could affect the influx of calcium ions across the plasma membrane through the so-called capacitative calcium entry system. Recordings of cytosolic calcium-ion concentrations in SV40-transformed Swiss 3T3 cells were obtained in real time during exposure to magnetic fields ranging from 0.3-50 mT or to sham conditions using the calcium-sensitive photoprotein aequorin. This was done for cell populations whose capacitative entry system was activated by either bradykinin or thapsigargin under a variety of experimental conditions. No effects of the magnetic field were observed on bradykinin-induced calcium transients and, with the exception of a small but statistically significant increase observed in experiments performed at 50 mT, no effects of the fields were observed on baseline calcium levels prior to or after such transients. The magnetic fields also had no effects on the size or kinetics of any of the thapsigargin-induced calcium transients. Overall, the data fail to support the hypothesis tested in this work.

Vesicle accumulation and exocytosis at sites of plasma membrane disruption

Plasma membrane disruptions are resealed by an active molecular mechanism thought to be composed, in part, of kinesin, CaM kinase, snap-25, and synaptobrevin. We have used HRP to mark the cytoplasmic site of a mechanically induced plasma membrane disruption. Transmission electron microscopy revealed that vesicles of a variety of sizes rapidly (s) accumulate in large numbers within the cytoplasm surrounding the disruption site and that microvilli-like surface projections overlie this region. Scanning electron microscopy confirmed that tufts of microvilli rapidly appear on wounded cells. Three assays, employing the membrane specific dye FM1-43, provide quantitative evidence that disruption induces Ca(2+)-dependent exocytosis involving one or more of the endosomal/lysosomal compartments. Confocal microscopy revealed the presence in wounded cells of cortical domains that were strikingly depleted of FM dye fluorescence, suggesting that a local bolus of exocytosis is induced by wounding rather than global exocytosis. Finally, flow cytometry recorded a disruption-induced increase in cell forward scatter, suggesting that cell size increases after injury. These results provide the first direct support for the hypothesis that one or more internal membrane compartments accumulate at the disruption site and fuse there with the plasma membrane, resulting in the local addition of membrane to the surface of the mechanically wounded cell.

Serum- and bradykinin-induced calcium transients in familial Alzheimer's fibroblasts

The calcium-sensitive photoprotein, aequorin, was used to examine serum- and bradykinin-induced transient increases in free cytosolic calcium ions in skin fibroblasts from 10 individuals with early onset familial AD (FAD), including four who were biopsied before their clinical symptoms would allow a diagnosis of AD, 2 individuals with late onset FAD, 8 at-risk but nonsymptomatic individuals, and 13 controls. The data show that (a) among controls, the peaks of the calcium transients increase in height as a function of donor age; (b) transients induced by 10% serum, 10 nM bradykinin (BK) or 100 nM BK were generally lower in FAD fibroblasts, including those from donors in the early stages of the disease, than in age-matched control cells; (c) such transients are reduced in cells from a proportion of the nonsymptomatic, at-risk individuals. Thus, serum- and BK-induced calcium transients are reduced in fibroblasts from both early and more advanced stage FAD donors and perhaps even from donors who are presymptomatic carriers of the defective gene. The data also suggest that changes in calcium transients in FAD fibroblasts neither mimic nor exaggerate the effects of normal aging.

Altered calcium regulation in SV40-transformed Swiss 3T3 fibroblasts

Calcium homeostasis has long been thought to be altered in transformed cells but mechanisms have not been established. In this study, the photoprotein, aequorin, was used to examine calcium regulation in 3T3 and SV40-transformed 3T3 cells. It was found that calcium transients induced by bradykinin or serum in serum-starved cells are lower and delayed in the transformed cells and decay kinetics are altered. These changes are not related to differences in cell cycle distribution. Though the serum transient is insensitive to nifedipine, verapamil, or lanthanum, removal of extracellular calcium accelerates transient decay in both cell types. Treatment of unstimulated cells with the ER Ca(2+)-ATPase inhibitor, thapsigargin, causes a 4-5-fold greater increase in [Ca2+]i in the transformed than in the nontransformed cells. Following serum stimulation, transformed cells still exhibit a large thapsigargin-induced increase in [Ca2+]i whereas the response in nontransformed cells is nearly abolished. When the 3T3 or SV3T3 cells are exposed to serum or thapsigargin in the absence of extracellular calcium and subsequently exposed to 11.8 mM Ca2+, a much greater influx of calcium again occurs in the SV3T3 cells. The observed changes in SV3T3 cells are most likely due to an alteration in a capacitative mechanism which regulates influx of calcium through the plasma membrane.

Cytoplasmic pH influences cytoplasmic calcium in MC3T3-E1 osteoblast cells

We found that the cytoplasmic concentration of calcium (Cai) of MC3T3-E1 osteoblasts was influenced by the type of pH buffer we used in the perfusing medium, suggesting that intracellular pH (pHi) might influence Cai. To study this effect, the Cai and pHi were monitored as we applied various experimental conditions known to change pHi. Exposure to NH4Cl caused a transient increase in both pHi and Cai without a change in extracellular pH (pHo). Decreasing pHo and pHi by lowering the bicarbonate concentration of the medium decreased Cai, and increasing pHi by the removal of 5% CO2 increased Cai. Clamping pHi to known values with 10 microM nigericin, a potassium proton ionophore, also influenced Cai: acid pHi lowered Cai, whereas alkaline pHi increased it. The rise in Cai appears to be very sensitive to the extracellular concentration of calcium, suggesting the existence of a pH-sensitive calcium influx mechanism. We conclude that physiologic changes in pH could modulate Cai by controlling the influx of calcium ions and could change the time course of the Cai transient associated with hormonal activation.

Galactose-dependent expression of the recombinant Ca2(+)-binding photoprotein aequorin in yeast

Aequorin is a Ca2(+)-binding protein that emits light upon reacting with Ca2+ and has been used as a probe for monitoring changes in the intracellular free Ca2+ concentration, [Ca2+]i. The protein consists of three components: apoaequorin (apoprotein), molecular oxygen and a chromophore. The present study was designed to conditionally express the apoaequorin cDNA of the jellyfish Aequorea victoria under the control of the GAL1 promoter in the yeast Saccharomyces cerevisiae and to investigate whether apoaequorin can be accumulated in high enough concentration in the cells to detect a Ca2+ signal in vitro. The results showed that the cells accumulated sufficient amounts of recombinant apoaequorin when incubated in the galactose-based medium and that the protein was active and not toxic to the cells, suggesting that the recombinant apoaequorin may be applicable to monitoring changes in [Ca2+]i in intact yeast cells.

The mitogenic activities of phosphatidate are acyl-chain-length dependent and calcium independent in C3H/10T1/2 cells

Phosphatidates (PA or phosphatidic acid) were shown to have mitogenic properties, including the stimulation of DNA synthesis and calcium mobilization in C3H/10T1/2 cells. Their continuous presence for a minimum of 7 h induced DNA synthesis with kinetics similar to that observed when 10% fetal bovine serum was used as a mitogen. PAs with long chain saturated fatty acid moieties were more mitogenic, in a dose-dependent fashion, than PAs with short saturated or unsaturated fatty acid moieties. When compared with lysostearoyl-PA (LSPA), distearoyl-PA (DSPA) was as potent with respect to the induction of DNA synthesis. Lysooleoyl-PA (LOPA) was slightly more potent than dioleoyl-PA (DOPA), but much weaker than DSPA and LSPA. Preincubation with dilauroyl-PA (DLPA) reduces the mitogenic effect of DSPA by 85%. The pattern of mitogenic inhibition suggests that a chain-length-independent, yet PA-specific, mechanism is involved. Both DSPA and DLPA are equally taken up by the cells after 30 min. LOPA, but not LSPA, produced a large calcium transient (1.3 microM), which we found to be derived from intracellular sources. DSPA, the most mitogenic PA tested, produced a weaker transient (0.6 microM). Interestingly, LSPA did not produce any detectable calcium transient. These results suggest that the chain-length-specific step in the signaling mechanism of PA occurs after the initial chain-length-independent partitioning and/or binding to the membrane and that the induction of DNA synthesis is not related to the observed calcium transients.

GTP gamma S inhibits early c-myc protein accumulation but not DNA synthesis in Swiss 3T3 fibroblasts

Quiescent Swiss 3T3 fibroblasts stimulated with epidermal growth factor and insulin showed large transient increases in c-myc mRNA and c-myc protein accumulation which were maximal at about 2 h after addition of the co-mitogens. When the cells were loaded with 0.1 mM of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) by transient permeabilisation immediately before mitogenic stimulation, the increase in c-myc mRNA was similar to that observed in unloaded cells but the corresponding c-myc protein peak was reduced by at least 95%. The GTP gamma S completely blocked incorporation of [35S]methionine into cell proteins for 3-4 h after addition of the mitogens, but not thereafter, and caused a delay in the subsequent onset of DNA synthesis by the same period. The data show that less than 5% of the early increase in c-myc protein normally observed after mitogenic stimulation is required for its obligatory role in the progression of cells to S phase implied by other evidence.

Assessment of Fura-2 for measurements of cytosolic free calcium

Fura-2 has become the most popular fluorescent probe with which to monitor dynamic changes in cytosolic free calcium in intact living cells. In this paper, we describe many of the currently recognized limitations to the use of Fura-2 in living cells and certain approaches which can circumvent some of these problems. Many of these problems are cell type specific, and include: (a) incomplete hydrolysis of Fura-2 acetoxymethyl ester bonds by cytosolic esterases, and the potential presence of either esterase resistant methyl ester complexes on the Fura-2/AM molecule or other as yet unidentified contaminants in commercial preparations of Fura-2/AM; (b) sequestration of Fura-2 in non-cytoplasmic compartments (i.e. cytoplasmic organelles); (c) dye loss (either active or passive) from labeled cells; (d) quenching of Fura-2 fluorescence by heavy metals; (e) photobleaching and photochemical formation of fluorescent non-Ca2+ sensitive Fura-2 species; (f) shifts in the absorption and emission spectra, as well as the Kd for Ca2+ of Fura-2 as a function of either polarity, viscosity, ionic strength or temperature of the probe environment; and (g) accurate calibration of the Fura-2 signal inside cells. Solutions to these problems include: (a) labeling of cells with Fura-2 pentapotassium salt (by scrape loading, microinjection or ATP permeabilization) to circumvent the problems of ester hydrolysis; (b) labeling of cells at low temperatures or after a 4 degrees C pre-chill to prevent intracellular organelle sequestration; (c) performance of experiments at lower than physiological temperatures (i.e. 15-33 degrees C) and use of ratio quantitation to remedy inaccuracies caused by dye leakage; (d) addition of N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) to chelate heavy metals; (e) use of low levels of excitation energy and high sensitivity detectors to minimize photobleaching or formation of fluorescent non-Ca2+ sensitive forms of Fura-2; and (f) the use of 340 nm and 365 nm (instead of 340 nm and 380 nm) for ratio imaging, which diminishes the potential contributions of artifacts of polarity, viscosity and ionic strength on calculated calcium concentrations, provides a measure of dye leakage from the cells, rate of Fura-2 photobleaching, and can be used to perform in situ calibration of Fura-2 fluorescence in intact cells; however, use of this wavelength pair diminishes the dynamic range of the ratio and thus makes it more sensitive to noise involved in photon detection. Failure to consider these potential problems may result in erroneous estimates of cytosolic free calcium.(ABSTRACT TRUNCATED AT 400 WORDS)

Electric field-directed cell shape changes, displacement, and cytoskeletal reorganization are calcium dependent

C3H/10T1/2 mouse embryo fibroblasts were stimulated by a steady electric field ranging up to 10 V/cm. Some cells elongated and aligned perpendicular to the field direction. A preferential positional shift toward the cathode was observed which was inhibited by the calcium channel blocker D-600 and the calmodulin antagonist trifluoperazine. Rhodaminephalloidin labeling of actin filaments revealed a field-induced disorganization of the stress fiber pattern, which was reduced when stimulation was conducted in calcium-depleted buffer or in buffer containing calcium antagonist CoCl2, calcium channel blocker D-600, or calmodulin antagonist trifluoperazine. Treatment with calcium ionophore A23187 had similar effects, except that the presence of D-600 did not reduce the stress fiber disruption. The calcium-sensitive photoprotein aequorin was used to monitor changes in intracellular-free calcium. Electric stimulation caused an increase of calcium to the micromolar range. This increase was inhibited by calcium-depleted buffer or by CoCl2, and was reduced by D-600. A calcium-dependent mechanism is proposed to explain the observed field-directed cell shape changes, preferential orientation, and displacement.

GTP gamma S activation of proto-oncogene expression in transiently permeabilised Swiss 3T3 fibroblasts

A technique of transient permeabilisation has been used to show that the introduction of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), a non-hydrolysable analogue of GTP, into intact Swiss 3T3 fibroblasts stimulates phosphoinositide hydrolysis, cyclic AMP accumulation and the activation of c-fos and c-myc proto-oncogenes. Of a number of nucleotide triphosphates introduced into the cells, only GTP and its non-hydrolysable analogues activated inositol phosphate release, suggesting that this response is mediated by guanine nucleotide regulatory (G) protein(s). The data demonstrate that transient permeabilisation provides a method of examining the involvement of G-proteins in nuclear activation.

Mechanisms of mitochondrial calcium transport

Mitochondria are known to possess a rapid calcium uptake mechanism or uniport and both sodium-dependent and sodium-independent efflux mechanisms. Whether sodium-independent calcium efflux is mediated and whether sodium-dependent calcium efflux can be found in liver mitochondria have been questioned. Kinetics results relevant to the answers of these questions are discussed below. A slow, mediated, sodium-independent calcium efflux mechanism is identified which shows second order kinetics. This mechanism, which shows "nonessential activation" kinetics, has a Vmax around 1.2 nmol calcium per mg protein per min and a half maximal velocity around 8.4 nmol calcium per mg protein. A slow, sodium-dependent calcium efflux mechanism is identified, which is first order in calcium and second order in sodium. This mechanism has a Vmax around 2.6 nmol of calcium per mg protein per min. The sodium dependence is half saturated at an external sodium concentration of 9.4 mM, and the calcium dependence is half saturated at an internal calcium concentration of 8.1 nmol calcium per mg protein. The cooperativity of the sodium dependence effectively permits a terreactant system to be fit by a bireactant model in which [Na] only appears as the square of [Na]. This liver system shows simultaneous, as opposed to ping-pong, kinetics. It is also found to be sensitive to inhibition by tetraphenyl phosphonium, magnesium, and ruthenium red. A model is proposed in which mitochondrial calcium transport could function to "shape the pulses" of cytosolic calcium. Simultaneously, mitochondria may mediate a "calcium memory" coupled perhaps to activation of cytosolic events through calmodulin or perhaps to activation of electron transport through the activation of specific dehydrogenases by intramitochondrial calcium.

Distinct functions of down-regulation-sensitive and -resistant types of protein kinase C in rabbit aortic smooth muscle cells

In quiescent cultures of rabbit aortic smooth muscle cells, 12-O-tetradecanoylphorbol-13-acetate (TPA) induced DNA synthesis to some extent in the presence of rabbit plasma-derived serum but inhibited the rabbit whole blood serum (WBS)-induced DNA synthesis and increase in cytoplasmic free Ca2+ concentration Ca2+]i). Prolonged treatment of the cells with phorbol-12,13-dibutyrate (PDBu) caused the partial down-regulation of protein kinase C to a level of 25-35% of that in control cells. In these PDBu-pretreated cells, TPA neither induced DNA synthesis nor inhibited the WBS-induced DNA synthesis, but still inhibited the WBS-induced increase in [Ca2+]i. These results suggest that there are down-regulation-sensitive and -resistant types of protein kinase C in rabbit aortic smooth muscle cells; that the down-regulation-sensitive type has the proliferative and antiproliferative actions whereas the down-regulation-resistant type lacks them; and that the down-regulation-resistant type has the activity to inhibit the WBS-induced increase in [Ca2+]i.

Fc-receptor-mediated phagocytosis occurs in macrophages without an increase in average [Ca++]i

The calcium ion has been implicated as a cytosolic signal or regulator in phagocytosis. Using the Ca++-sensitive photoprotein aequorin we have measured intracellular free Ca++ ion concentration ([Ca++]i) in thioglycolate-elicited mouse peritoneal macrophages during phagocytosis and IgG-induced spreading. Macrophages plated on glass were loaded with aequorin and [Ca++]i was then measured from cell populations, both as previously described (McNeil, P. L., and D. L. Taylor, 1985, Cell Calcium, 6:83-92). Aequorin indicated a resting [Ca++]i in adherent macrophages of 84 nM and was responsive to changes in [Ca++]i induced by the addition of Mg-ATP (0.1 mM) or serum to medium. However, during the 15 min required for phagocytosis of seven or eight IgG-coated erythrocytes per macrophage loaded with aequorin, we measured no change in [Ca++]i. Similarly, the ligation of Fc-receptors that occurs when macrophages spread on immune complex-coated coverslips did not change macrophage [Ca++]i. In contrast, a rise in [Ca++]i of macrophages was measured during phagocytosis occurring in a serum-free saline of pH 7.85, and as a consequence of incubation with quin2 A/M. We estimate that had a change in [Ca++]i occurred during phagocytosis, aequorin would have detected a rise from 0.1 to 1.0 microM taking place in as little as 2% of the macrophage's cytoplasmic volume. We therefore suggest that either Ca++ is not involved as a cytoplasmic signal for phagocytosis or that increases in [Ca++]i during phagocytosis are confined to such small regions of cytoplasm as to be below the limits of detection by our cellular averaging method. Our data emphasizes, moreover, the need for well-defined, nonperturbing conditions in such measurements of [Ca++]i.

A transient rise in cytosolic calcium follows stimulation of quiescent cells with growth factors and is inhibitable with phorbol myristate acetate

We have used aequorin as an indicator for the intracellular free calcium ion concentration [( Ca++]i) of Swiss 3T3 fibroblasts. Estimated [Ca++]i of serum-deprived, subconfluent fibroblasts was 89 (+/-20) nM, almost twofold higher than that of subconfluent cells growing in serum, whose [Ca++]i was 50 (+/-19) nM. Serum, partially purified platelet-derived growth factor (PDGF), and fibroblast growth factor (FGF) stimulated DNA synthesis by the serum-deprived cells, whereas epidermal growth factor (EGF) did not. Serum immediately and transiently elevated the [Ca++]i of serum-deprived cells, which reached a maximal value of 5.3 microM at 18 s poststimulation but returned to near prestimulatory levels within 3 min. Moreover, no further changes in [Ca++]i were observed during 12 subsequent h of continuous recording. PDGF produced a peak rise in [Ca++]i to approximately 1.4 microM at 115 s after stimulation, and FGF to approximately 1.2 microM at 135 s after stimulation. EGF caused no change in [Ca++]i. The primary source of calcium for these transients was intracellular, since the magnitude of the serum-induced rise in [Ca++]i was reduced by only 30% in the absence of exogenous calcium. Phorbol 12-myristate 13-acetate (PMA) had no effect on resting [Ca++]i. When, however, quiescent cells were treated for 30 min with 100 nM PMA, serum-induced rises in [Ca++]i were reduced by sevenfold. PMA did not inhibit growth factor-induced DNA synthesis and was by itself partially mitogenic. We suggest that if calcium is involved as a cytoplasmic signal for mitogenic activation of quiescent fibroblasts, its action is early, transient, and can be partially substituted for by PMA. Activated protein kinase C may regulate growth factor-induced increases in [Ca++]i.