Familial Alzheimer's disease-linked presenilin-1 mutation M146V affects store-operated calcium entry: does gain look like loss?

Alzheimer's disease (AD) is a neurodegenerative disorder that leads to neuron death and synapse loss in the hippocampus and cortex, with consequent cognitive disability and dementia. Mutations in the presenilin-1 (PS1) gene lead to familial Alzheimer's disease (FAD). Here, we report that the expression of FAD-linked PS1 M146V mutant affects store-operated calcium channel activity (Isoc) in human neuroblastoma SK-N-SH cells. Electrophysiological measurements and calcium imaging experiments have revealed the emergent role of calcium sensor STIM2 in the inhibition of calcium release-activated calcium channel activity (Icrac) and enhancement of intracellular Ca(2+) stores content due to PS1 M146V mutant expression. In general, the results of this study suggest that the pathological inhibition of one type of store-operated calcium channels caused by FAD PS1 mutant expression may be accounted for by preceding gain of spontaneous activity of store-operated calcium channels driven by STIM2.

Activation of calcium entry in human carcinoma A431 cells by store depletion and phospholipase C- dependent mechanisms converge on ICRAC-like calcium channels

Activation of phospholipase C in nonexcitable cells causes the release of calcium (Ca2+) from intracellular stores and activation of Ca2+ influx by means of Ca2+ release-activated channels (ICRAC) in the plasma membrane. The molecular identity and the mechanism of ICRAC channel activation are poorly understood. Using the patch-clamp technique, here we describe the plasma membrane Ca2+ channels in human carcinoma A431 cells, which can be activated by extracellular UTP, by depletion of intracellular Ca2+ stores after exposure to the Ca2+-pump inhibitor thapsigargin, or by loading the cells with Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate. The observed channels display the same conductance and gating properties as previously described I(min) channels, but have significantly lower conductance for monovalent cations than the ICRAC channels. Thus, we concluded that the depletion-activated Ca2+ current in A431 cells is supported by I(CRAC)-like (ICRACL) channels, identical to I(min). We further demonstrated synergism in activation of ICRACL Ca2+ channels by extracellular UTP and intracellular inositol (1,4,5)-triphosphate (IP3), apparently because of reduction in phosphatidylinositol 4,5-bisphosphate (PIP2) levels in the patch. Prolonged exposure of patches to thapsigargin renders ICRACL Ca2+ channels unresponsive to IP3 but still available to activation by the combined action of IP3 and anti-PIP2 antibody. Based on these data, we concluded that phospholipase C-mediated and store-operated Ca2+ influx pathways in A431 cells converge on the same I(CRACL) Ca2+ channel, which can be modulated by PIP2.

Plasma membrane calcium channels in human carcinoma A431 cells are functionally coupled to inositol 1,4,5-trisphosphate receptor-phosphatidylinositol 4,5-bisphosphate complexes

In most nonexcitable cells, calcium (Ca(2+)) release from inositol 1,4,5-trisphosphate (InsP(3))-sensitive intracellular Ca(2+) stores is coupled to Ca(2+) influx (calcium release-activated channels (I(CRAC))) pathway. Despite intense investigation, the molecular identity of I(CRAC) and the mechanism of its activation remain poorly understood. InsP(3)-dependent miniature calcium channels (I(min)) display functional properties characteristic for I(CRAC). Here we used patch clamp recordings of I(min) channels in human carcinoma A431 cells to demonstrate that I(min) activity was greatly enchanced in the presence of anti-phosphatidylinositol 4, 5-bisphosphate antibody (PIP(2)Ab) and diminished in the presence of PIP(2). Anti-PIP(2) antibody induced a greater than 6-fold increase in I(min) sensitivity for InsP(3) activation and an almost 4-fold change in I(min) maximal open probability. The addition of exogenous PIP(2) vesicles to the cytosolic surface of inside-out patches inhibited I(min) activity. These results lead us to propose an existence of a Ca(2+) influx pathway in nonexcitable cells activated via direct conformational coupling with a selected population of InsP(3) receptors, located just underneath the plasma membrane and coupled to PIP(2). The described pathway provides for a highly compartmentalized Ca(2+) influx and intracellular Ca(2+) store refilling mechanism.

Functional coupling of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate receptor (InsP3R) plays a key role in intracellular Ca2+ signaling. InsP3R is activated by InsP3 produced from phosphatidylinositol 4,5-bisphosphate (PIP2) by phospholipase C cleavage. Using planar lipid bilayer reconstitution technique, we demonstrate here that rat cerebellar InsP3R forms a stable inhibitory complex with endogenous PIP2. Disruption of InsP3R-PIP2 interaction by specific anti-PIP2 monoclonal antibody resulted in 3-4-fold increase in InsP3R activity and 10-fold shift in apparent affinity for InsP3. Exogenously added PIP2 blocks InsP3 binding to InsP3R and inhibits InsP3R activity. Similar results were obtained with a newly synthesized water soluble analog of PIP2, dioctanoyl-(4,5)PIP2, indicating that insertion of PIP2 into membrane is not required to exert its inhibitory effects on the InsP3R. We hypothesize that the functional link between InsP3R and PIP2 described in the present report provides a basis for a local, rapid, and efficient coupling between phospholipase C activation, PIP2 hydrolysis, and intracellular Ca2+ wave initiation in neuronal and non-neuronal cells.

Single-channel properties of inositol (1,4,5)-trisphosphate receptor heterologously expressed in HEK-293 cells

The inositol (1,4,5)-trisphosphate receptor (InsP3R) mediates Ca2+ release from intracellular stores in response to generation of second messenger InsP3. InsP3R was biochemically purified and cloned, and functional properties of native InsP3-gated Ca2+ channels were extensively studied. However, further studies of InsP3R are obstructed by the lack of a convenient functional assay of expressed InsP3R activity. To establish a functional assay of recombinant InsP3R activity, transient heterologous expression of neuronal rat InsP3R cDNA (InsP3R-I, SI- SII+ splice variant) in HEK-293 cells was combined with the planar lipid bilayer reconstitution experiments. Recombinant InsP3R retained specific InsP3 binding properties (Kd = 60 nM InsP3) and were specifically recognized by anti-InsP3R-I rabbit polyclonal antibody. Density of expressed InsP3R-I was at least 20-fold above endogenous InsP3R background and only 2-3-fold lower than InsP3R density in rat cerebellar microsomes. When incorporated into planar lipid bilayers, the recombinant InsP3R formed a functional InsP3-gated Ca2+ channel with 80 pS conductance using 50 mM Ba2+ as a current carrier. Mean open time of recombinant InsP3-gated channels was 3.0 ms; closed dwell time distribution was double exponential and characterized by short (18 ms) and long (130 ms) time constants. Overall, gating and conductance properties of recombinant neuronal rat InsP3R-I were very similar to properties of native rat cerebellar InsP3R recorded in identical experimental conditions. Recombinant InsP3R also retained bell-shaped dependence on cytosolic Ca2+ concentration and allosteric modulation by ATP, similar to native cerebellar InsP3R. The following conclusions are drawn from these results. (a) Rat neuronal InsP3R-I cDNA encodes a protein that is either sufficient to produce InsP3-gated channel with functional properties identical to the properties of native rat cerebellar InsP3R, or it is able to form a functional InsP3-gated channel by forming a complex with proteins endogenously expressed in HEK-293 cells. (b) Successful functional expression of InsP3R in a heterologous expression system provides an opportunity for future detailed structure-function characterization of this vital protein.