Stable expression of heterologous multisubunit protein complexes established by calcium phosphate- or lipid-mediated cotransfection

Endogenous ion channels expressed in human embryonic kidney (HEK-293) cells

Mammalian expression systems, particularly the human embryonic kidney (HEK-293) cells, combined with electrophysiological studies, have greatly benefited our understanding of the function, characteristic, and regulation of various ion channels. It was previously assumed that the existence of endogenous ion channels in native HEK-293 cells could be negligible. Still, more and more ion channels are gradually reported in native HEK-293 cells, which should draw our attention. In this regard, we summarize the different ion channels that are endogenously expressed in HEK-293 cells, including voltage-gated Na⁺ channels, Ca²⁺ channels, K⁺ channels, Cl^- channels, nonselective cation channels, TRP channels, acid-sensitive ion channels, and Piezo channels, which may complicate the recording of the heterogeneously expressed ion channels to a certain degree. We noted that the expression patterns and channel profiles varied with different studies, which may be due to the distinct originality of the cells, cell culture conditions, passage numbers, and different recording protocols. Therefore, a better knowledge of endogenous ion channels may help minimize potential problems in characterizing heterologously expressed ion channels. Based on this, it is recommended that HEK-293 cells from unknown sources should be examined before transfection for the characterization of their functional profile, especially when the expression level of exogenous ion channels does not overwhelm the endogenous ion channels largely, or the current amplitude is not significantly higher than the native currents.

Palmitoylation regulates glutamate receptor distributions in postsynaptic densities through control of PSD95 conformation and orientation

Postsynaptic density protein 95 (PSD95) and synapse-associated protein 97 (SAP97) are homologous scaffold proteins with different N-terminal domains, possessing either a palmitoylation site (PSD95) or an L27 domain (SAP97). Here, we measured PSD95 and SAP97 conformation in vitro and in postsynaptic densities (PSDs) using FRET and EM, and examined how conformation regulated interactions with AMPA-type and NMDA-type glutamate receptors (AMPARs/NMDARs). Palmitoylation of PSD95 changed its conformation from a compact to an extended configuration. PSD95 associated with AMPARs (via transmembrane AMPAR regulatory protein subunits) or NMDARs [via glutamate ionotropic receptor NMDA-type subunit 2B (GluN2B) subunits] only in its palmitoylated and extended conformation. In contrast, in its extended conformation, SAP97 associates with NMDARs, but not with AMPARs. Within PSDs, PSD95 and SAP97 were largely in the extended conformation, but had different orientations. PSD95 oriented perpendicular to the PSD membrane, with its palmitoylated, N-terminal domain at the membrane. SAP97 oriented parallel to the PSD membrane, likely as a dimer through interactions of its N-terminal L27 domain. Changing PSD95 palmitoylation in PSDs altered PSD95 and AMPAR levels but did not affect NMDAR levels. These results indicate that in PSDs, PSD95 palmitoylation, conformation, and its interactions are dynamic when associated with AMPARs and more stable when associated with NMDARs. Altogether, our results are consistent with differential regulation of PSD95 palmitoylation in PSDs resulting from the clustering of palmitoylating and depalmitoylating enzymes into AMPAR nanodomains segregated away from NMDAR nanodomains.

CASK regulates SAP97 conformation and its interactions with AMPA and NMDA receptors

SAP97 interacts with AMPA receptors (AMPARs) and NMDA receptors (NMDARs) during sorting and trafficking to synapses. Here we addressed how SAP97 distinguishes between AMPARs and NMDARs and what role the adaptor/scaffold protein, CASK, plays in the process. Using intramolecular SAP97 Förster resonance energy transfer sensors, we demonstrated that SAP97 is in "extended" or "compact" conformations in vivo. SAP97 conformation was regulated by a direct interaction between SAP97 and CASK through L27 protein-interaction domains on each protein. Unbound SAP97 was mostly in the compact conformation, while CASK binding stabilized it in an extended conformation. In HEK cells and rat hippocampal neurons, SAP97 in the compact conformation preferentially associated and colocalized with GluA1-containing AMPARs, and in the extended conformation colocalized with GluN2B-containing NMDARs. Altogether, our findings suggest a molecular mechanism by which CASK binding regulates SAP97 conformation and its subsequent sorting and synaptic targeting of AMPARs and NMDARs during trafficking to synapses.

SAP97 and CASK mediate sorting of NMDA receptors through a previously unknown secretory pathway

Synaptic plasticity is dependent upon the differential sorting, delivery and retention of neurotransmitter receptors, yet the mechanisms underlying these processes are poorly understood. In the present study, we have found that differential sorting of glutamate receptor subtypes begins within the endoplasmic reticulum (ER) of rat hippocampal neurons. While AMPARs are trafficked to the plasma membrane via the conventional somatic Golgi network, NMDARs are diverted from the somatic ER into a specialized ER sub-compartment that bypasses somatic Golgi, merging instead with dendritic Golgi outposts. Intriguingly, this ER sub-compartment is composed of highly mobile vesicles containing the NMDAR subunits NR1 and NR2B, the microtubule-dependent motor protein KIF17, and the postsynaptic adaptor proteins CASK and SAP97. Furthermore, our data demonstrate that the retention and trafficking of NMDARs within this ER sub-compartment requires both CASK and SAP97. These data indicate that NMDARs are sorted away from AMPARs via a non-conventional secretory pathway that utilizes dendritic Golgi outposts.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Nanotechnology for neuronal ion channels

Ion channels provide the basis for the regulation of electrical excitability in the central and peripheral nervous systems. This review deals with the techniques that make the study of structure and function of single channel molecules in living cells possible. These are the patch clamp technique, which was derived from the conventional voltage clamp method and is currently being developed for automated and high throughput measurements; and fluorescence and nano-techniques, which were originally applied to non-biological surfaces and are only recently being used to study cell membranes and their proteins, especially in combination with the patch clamp technique. The characterisation of the membrane channels by techniques that resolve their morphological and physical properties and dynamics in space and time in the nano range is termed nanoscopy.

alpha-bungarotoxin receptors contain alpha7 subunits in two different disulfide-bonded conformations

Neuronal nicotinic alpha7 subunits assemble into cell-surface complexes that neither function nor bind alpha-bungarotoxin when expressed in tsA201 cells. Functional alpha-bungarotoxin receptors are expressed if the membrane-spanning and cytoplasmic domains of the alpha7 subunit are replaced by the homologous regions of the serotonin-3 receptor subunit. Bgt-binding surface receptors assembled from chimeric alpha7/serotonin-3 subunits contain subunits in two different conformations as shown by differences in redox state and other features of the subunits. In contrast, alpha7 subunit complexes in the same cell line contain subunits in a single conformation. The appearance of a second alpha7/serotonin-3 subunit conformation coincides with the formation of alpha-bungarotoxin-binding sites and intrasubunit disulfide bonding, apparently within the alpha7 domain of the alpha7/serotonin-3 chimera. In cell lines of neuronal origin that produce functional alpha7 receptors, alpha7 subunits undergo a conformational change similar to alpha7/serotonin-3 subunits. alpha7 subunits, thus, can fold and assemble by two different pathways. Subunits in a single conformation assemble into nonfunctional receptors, or subunits expressed in specialized cells undergo additional processing to produce functional, alpha-bungarotoxin-binding receptors with two alpha7 conformations. Our results suggest that alpha7 subunit diversity can be achieved postranslationally and is required for functional homomeric receptors.

Transient expression of heteromeric ion channels

Transient transfection is an excellent method for the expression and study of cell-surface, heteromeric ion channels. The cell type, the total amount of DNA, the combination of subunits and the ratio of subunit DNA are all important parameters to consider when attempting to optimize expression. A serious drawback of this method is that the efficiency of subunit assembly is very low in comparison to the efficiency of assembly for stably expressed heteromeric ion channels. The low efficiency of assembly prevents use of transient expression methods for detailed studies of heteromeric AChR assembly, and caution should be taken in the use of these methods for the study of intracellular heteromeric ion channel subunits. After the transient expression of heteromeric AChR subunits, virtually all of the expressing cells contained all four AChR subunits. However, the subunits were heterogeneously distributed among the cells, and the low efficiency of AChR assembly appears to be due to cell-to-cell variations in the ratio of the four subunits.

Identification of endogenous outward currents in the human embryonic kidney (HEK 293) cell line

Human embryonic kidney cells (HEK 293) are widely used as an expression system in studies of ion channels. However, their endogenous ionic currents remain largely unidentified. To characterize these currents, we performed patch clamp experiments on this expression system. In whole-cell voltage clamp mode, the HEK 293 cells showed mainly outward currents using physiological concentrations of Na+ and K+ and symmetric concentrations of Cl- (150 mM) across the plasma membranes. K+ currents contributed to a small portion of these outward currents, since a shift of the reversal potentials of only approximately 20 mV was seen with a change of extracellular K+ concentration from 3 to 150 mM. In contrast, the reversal potential shifted approximately 25 mV when extracellular Cl- was reduced to 50 mM, indicating that most of the outward currents are carried by Cl-. In inside-out patches, several distinct Cl- currents were identified. They were: (1) 350 pS Cl- current, which was voltage-activated and had a moderate outward rectification; (2) 240 pS Cl- current with a weak outward rectification; and (3) 55 pS Cl- current, which was voltage-activated, sensitive to DIDS, and showed a strong outward rectification. Activation of these Cl- currents did not require an elevation of free Ca2+ level in the cytosol. Besides these three currents, we observed two other Cl- currents with much smaller conductances (25 and 16 pS, respectively). Two different K+ currents were seen in the HEK 293 cells, with one of them (125 pS) showing inward rectification and the other (70 pS) outward rectification. Moreover, a 50 pS cation channel was recorded in these cells. The presence of a variety of ion channels in the HEK 293 cells suggests that a great precaution needs to be taken when this expression system is used in studies of several similar ion channels.

Neuronal alpha-bungarotoxin receptors differ structurally from other nicotinic acetylcholine receptors

We have characterized the alpha-bungarotoxin receptors (BgtRs) found on the cell surface of undifferentiated pheochromocytoma (PC12) cells. The PC12 cells express a homogeneous population of alpha7-containing receptors that bind alpha-Bgt with high affinity (Kd = 94 pM). The BgtRs mediate most of the response elicited by nicotine, because the BgtR-specific antagonists methyllycaconitine and alpha-Bgt block approximately 90% of the whole-cell current. The binding of nicotinic agonists to cell-surface BgtRs was highly cooperative with four different agonists showing Hill coefficients in the range of 2.3-2.4. A similar agonist binding cooperativity was observed for BgtR homomers formed from chimeric alpha7/5HT3 subunits expressed in tsA 201 cells. Two classes of agonist binding sites, in the ratio of 4:1 for PC12 cell BgtRs and 3:1 for alpha7/5HT3 BgtRs, were revealed by bromoacetylcholine alkylation of the reduced sites on both PC12 BgtRs and alpha7/5HT3 BgtRs. We conclude from this data that PC12 BgtRs and alpha7/5HT3 homomers contain at least three distinguishable agonist binding sites and thus are different from other nicotinic receptors.

Spatial dynamics of GFP-tagged proteins investigated by local fluorescence enhancement

We describe a method of monitoring the spatial dynamics of proteins in intact cells by locally enhancing the blue excited fluorescence of green fluorescent protein (GFP) using a spatially focused ultraviolet-laser pulse. GFP fusion proteins were efficiently expressed by micro-electroporation of in vitro synthesized mRNA into adherent mammalian cells. We found that the diffusion coefficient of cycle 3 mutant GFP was 43 microns2/sec, compared to 4 microns2/sec for wild-type GFP, suggesting that cycle 3 GFP diffuses freely in mammalian cells and is ideally suited as a fusion tag. The local fluorescence enhancement method was used to study the membrane dissociation rate of GFP-tagged K-ras, a small GTP binding protein that localizes to plasma membranes by a farnesyl lipid group and a polybasic region. Our data suggest that K-ras exists in a dynamic equilibrium and rapidly switches between a plasma membrane bound form and a cytosolic form with a plasma membrane dissociation time constant of 1.5 sec.

Cyclic AMP-regulated AChR assembly is independent of AChR subunit phosphorylation by PKA

Forskolin treatment of cells expressing Torpedo acetylcholine receptors leads to enhanced assembly efficiency of subunits, which correlates with increased phosphorylation of the gamma subunit. To determine the role of the two potential protein kinase A sites of the gamma subunit in receptor assembly, cell lines expressing different mutant receptors were established. Mouse fibroblast cell lines stably expressing wild-type Torpedo acetylcholine receptor alpha, beta, delta subunits plus one of three gamma subunit mutations (S353A, S354A, or S353,354A) were established to identify the protein kinase A phosphorylation sites of gamma in vivo, and to determine if increased phosphorylation of the gamma subunit leads to enhanced expression of receptors. We found that both serines (353, 354) in gamma are phosphorylated in vivo by protein kinase A, however, phosphorylation of either or both of these sites does not lead to increased assembly efficiency. We established a cell line expressing alpha, beta, and gamma(S353,354A) subunits only (no delta), and found that the presence of delta (or its phosphorylation) is also not necessary for the observed stimulation by forskolin. alpha beta gamma, alpha gamma, and beta gamma associations were stimulated by forskolin but alpha beta and alpha delta interactions were not. These data imply that the presence of gamma is necessary for forskolin action. We postulate that forskolin may stimulate acetylcholine receptor expression through a cellular protein that is involved in the folding and/or assembly of protein complexes, and that forskolin may regulate the action of such a protein through phosphorylation.