Neuroprotective properties of anti-apoptotic BCL-2 proteins in 5xFAD mouse model of Alzheimer's disease.

Alzheimer's disease (AD) is the most common cause of dementia. An early feature of the AD pathology is the dysregulation of intracellular Ca2+ signaling in neurons. In particular, increased Ca2+ release from endoplasmic reticulum-located Ca2+ channels, including inositol-1,4,5-trisphosphate type 1 receptors (IP3R1) and ryanodine receptors type 2 (RyR2), have been extensively reported. Known for its anti-apoptotic properties, Bcl-2 also has the ability to bind to and inhibit the Ca2+-flux properties of IP3Rs and RyRs. In this study, the hypothesis that the expression of Bcl-2 proteins can normalize dysregulated Ca2+ signaling in a mouse model of AD (5xFAD) and thereby prevent or slow the progression of AD was examined. Therefore, stereotactic injections of adeno-associated viral vectors expressing Bcl-2 proteins were performed in the CA1 region of the 5xFAD mouse hippocampus. In order to assess the importance of the association with IP3R1, the Bcl-2K17D mutant was also included in these experiments. This K17D mutation has been previously shown to decrease the association of Bcl-2 with IP3R1, thereby impairing its ability to inhibit IP3R1 while not affecting Bcl-2's ability to inhibit RyRs. Here, we demonstrate that Bcl-2 protein expression leads to synaptoprotective and amyloid-protective effects in the 5xFAD animal model. Several of these neuroprotective features are also observed by Bcl-2K17D protein expression, suggesting that these effects are not associated with Bcl-2-mediated inhibition of IP3R1. Potential mechanisms for this Bcl-2 synaptoprotective action may be related to its ability to inhibit RyR2 activity as Bcl-2 and Bcl-2K17D are equally potent in inhibiting RyR2-mediated Ca2+ fluxes. This work indicates that Bcl-2-based strategies hold neuroprotective potential in AD models, though the underlying mechanisms requires further investigation.

Dynamic Microtubules in Alzheimer's Disease: Association with Dendritic Spine Pathology.

Alzheimer's disease (AD) is the most common incurable neurodegenerative disorder that affects the processes of memory formation and storage. The loss of dendritic spines and alteration in their morphology in AD correlate with the extent of patient's cognitive decline. Tubulin had been believed to be restricted to dendritic shafts, until recent studies demonstrated that dynamically growing tubulin microtubules enter dendritic spines and promote their maturation. Abnormalities of tubulin cytoskeleton may contribute to the process of dendritic spine shape alteration and their subsequent loss in AD. In this review, association between tubulin cytoskeleton dynamics and dendritic spine morphology is discussed in the context of dendritic spine alterations in AD. Potential implications of these findings for the development of AD therapy are proposed.

Neuroprotective Effect of σ1-Receptors on the Cell Model of Huntington's Disease.

Huntington's disease is a hereditary neurodegenerative disease that primarily affects striatal neurons. Recent studies demonstrated abnormalities in calcium regulation in striatal neurons in Huntington's disease, which leads to elimination of synaptic connections between cortical and striatal neurons. In the present study, we focused on the neuroprotective properties of σ1-receptor, because one of its main functions is associated with modulation of calcium homeostasis in cells. The application of selective σ1-receptor agonists to the corticostriatal cell culture restores synaptic connections between the cortical and striatal neurons. Based on the obtained data, we assume that σ1-receptor is a promising target for the development of drugs for the therapy of Huntington's disease.

Calcium signaling and neurodegeneration.

Neurodegenerative disorders, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and spinocerebellar ataxias (SCA) are very important both for fundamental science and for practical medicine. Despite extensive research into the causes of these diseases, clinical researchers have had very limited progress and, as of now, there is still no cure for any of these diseases. One of the main obstacles in the way of creating treatments for these disorders is the fact that their etiology and pathophysiology still remain unclear. This paper reviews results that support the so-called "calcium hypothesis of neurodegenerative diseases." The calcium hypothesis states that the atrophic and degenerative processes in the neurons of AD, PD, ALS, HD, and SCA patients are accompanied by alterations in calcium homeostasis. Moreover, the calcium hypothesis states that this deregulation of calcium signaling is one of the early-stage and key processes in the pathogenesis of these diseases. Based on the results we reviewed, we conclude that the calcium channels and other proteins involved in the neuronal calcium signaling system are potential drug targets for AD, PD, ALS, HD, and SCA therapy.

Regulation of store-operated channels by scaffold proteins in a431 cells.

Store-operated channels are major calcium influx pathways in nonexitable cells. Homer scaffold proteins are well known for their role in regulating calcium signaling. Here we report on a detailed single-channel level characterization of native store-operated channels regulated by Homer scaffold proteins in A431 carcinoma cells. By applying the single-channel patch-clamp technique, we found that different types of store-operated calcium channels have different sensitivities to Homer proteins.

Inositol 1,4,5-tripshosphate receptor, calcium signalling and Huntington's disease.

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder that has no cure. HD primarily affects medium spiny striatal neurons (MSN). HD is caused by polyglutamine (polyQ) expansion (exp) in the amino-terminal region of a protein huntingtin (Htt). The connection between polyQ expansion in Htt(exp) and MSN neurodegeneration remains elusive. My laboratory discovered that mutant Htt(exp) protein specifically binds to the carboxy-terminal region of the type 1 inositol 1,4,5-trisphosphate receptor (InsP3R1), an intracellular Ca2+ release channel. Moreover, we found that Htt(exp) association with InsP3R1 causes sensitization of InsP3R1 to activation by InsP3 in planar lipid bilayers and in primary MSN. Mutant Htt(exp) has also been shown to activate Ca2(+)-permeable NR2B-containing NMDA receptors. All these results suggested that deranged neuronal Ca2+ signaling may play an important role in pathogenesis of HD. In support of this idea, we demonstrated a connection between abnormal Ca2+ signaling and apoptosis of MSN cultured from YAC128 HD mouse model. These results indicate that InsP3R and other Ca2+ signaling proteins should be considered as potential therapeutic targets for treatment of HD.

Classification of PDZ domains.

A diverse family of PDZ domains has been identified, but the rules that govern their ligand specificity are not clear. Here we propose a novel classification of PDZ domains based on the nature of amino acids in the two critical positions in the PDZ domain fold. Using these principles, we classified PDZ domains present in the SMART database. Using yeast two-hybrid, in vitro pull-down and plasmon surface resonance assays, we demonstrated that in agreement with their position in the proposed classification the Mint1-1, hINADL-5, and PAR6 PDZ domains display similar dual ligand specificity. The proposed classification helps to organize PDZ domain containing proteins.

Ca(2+)-sensor region of IP(3) receptor controls intracellular Ca(2+) signaling.

Many important cell functions are controlled by Ca(2+) release from intracellular stores via the inositol 1,4,5-trisphosphate receptor (IP(3)R), which requires both IP(3) and Ca(2+) for its activity. Due to the Ca(2+) requirement, the IP(3)R and the cytoplasmic Ca(2+) concentration form a positive feedback loop, which has been assumed to confer regenerativity on the IP(3)-induced Ca(2+) release and to play an important role in the generation of spatiotemporal patterns of Ca(2+) signals such as Ca(2+) waves and oscillations. Here we show that glutamate 2100 of rat type 1 IP(3)R (IP(3)R1) is a key residue for the Ca(2+) requirement. Substitution of this residue by aspartate (E2100D) results in a 10-fold decrease in the Ca(2+) sensitivity without other effects on the properties of the IP(3)R1. Agonist-induced Ca(2+) responses are greatly diminished in cells expressing the E2100D mutant IP(3)R1, particularly the rate of rise of initial Ca(2+) spike is markedly reduced and the subsequent Ca(2+) oscillations are abolished. These results demonstrate that the Ca(2+) sensitivity of the IP(3)R is functionally indispensable for the determination of Ca(2+) signaling patterns.

Maintaining the stability of neural function: a homeostatic hypothesis.

The precise regulation of neural excitability is essential for proper nerve cell, neural circuit, and nervous system function. During postembryonic development and throughout life, neurons are challenged with perturbations that can alter excitability, including changes in cell size, innervation, and synaptic input. Numerous experiments demonstrate that neurons are able to compensate for these types of perturbation and maintain appropriate levels of excitation. The mechanisms of compensation are diverse, including regulated changes to synaptic size, synaptic strength, and ion channel function in the plasma membrane. These data are evidence for homeostatic regulatory systems that control neural excitability. A model of neural homeostasis suggests that information about cell activity, cell size, and innervation is fed into a system of cellular monitors. Intracellular- and intercellular-signaling systems transduce this information into regulated changes in synaptic and ion channel function. This review discusses evidence for such a model of homeostatic regulation in the nervous system.

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.

Molecular determinants of the functional interaction between syntaxin and N-type Ca2+ channel gating.

Syntaxin is a key presynaptic protein that binds to N- and P/Q-type Ca(2+) channels in biochemical studies and affects gating of these Ca(2+) channels in expression systems and in synaptosomes. The present study was aimed at understanding the molecular basis of syntaxin modulation of N-type channel gating. Mutagenesis of either syntaxin 1A or the pore-forming alpha(1B) subunit of N-type Ca(2+) channels was combined with functional assays of N-type channel gating in a Xenopus oocyte coexpression system and in biochemical binding experiments in vitro. Our analysis showed that the transmembrane region of syntaxin and a short region within the H3 helical cytoplasmic domain of syntaxin, containing residues Ala-240 and Val-244, appeared critical for the channel modulation but not for biochemical association with the "synprint site" in the II/III loop of alpha(1B). These results suggest that syntaxin and the alpha(1B) subunit engage in two kinds of interactions: an anchoring interaction via the II/III loop synprint site and a modulatory interaction via another site located elsewhere in the channel sequence. The segment of syntaxin H3 found to be involved in the modulatory interaction would lie hidden within the four-helix structure of the SNARE complex, supporting the hypothesis that syntaxin's ability to regulate N-type Ca(2+) channels would be enabled after SNARE complex disassembly after synaptic vesicle exocytosis.

Association of the inositol (1,4,5)-trisphosphate receptor ligand binding site with phosphatidylinositol (4,5)-bisphosphate and adenophostin A.

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) is activated by InsP(3) binding to amino-terminal ligand binding domain (InsP(3)R-N). Recently we reported functional coupling of phosphatidylinositol (4, 5)-bisphosphate (PIP(2)) to the InsP(3)R. Specific binding of PIP(2) to InsP(3)R-N domain was postulated as a part of the InsP(3)R-PIP(2) functional coupling model. Here we utilized bacterially expressed and purified InsP(3)R-N domain to characterize its binding specificity for InsP(3), Adenophostin A (AdA) and the water-soluble PIP(2) analog dioctanoyl-(4,5)PIP(2) (ShPIP(2)). Obtained data led us to conclude that specific InsP(3), AdA, and ShPIP(2) binding sites are located within the InsP(3)R-N domain, that the extra receptor binding element responsible for enhanced binding of AdA is an integral part of the InsP(3)R-N domain, that ShPIP(2) is able to displace InsP(3) from the InsP(3)R-N, but InsP(3) or AdA is unable to completely displace ShPIP(2). These results support the InsP(3)R-PIP(2) functional coupling model and provide novel insights into InsP(3)R ligand specificity.

Functional properties of a new voltage-dependent calcium channel alpha(2)delta auxiliary subunit gene (CACNA2D2).

We have positionally cloned and characterized a new calcium channel auxiliary subunit, alpha(2)delta-2 (CACNA2D2), which shares 56% amino acid identity with the known alpha(2)delta-1 subunit. The gene maps to the critical human tumor suppressor gene region in chromosome 3p21.3, showing very frequent allele loss and occasional homozygous deletions in lung, breast, and other cancers. The tissue distribution of alpha(2)delta-2 expression is different from alpha(2)delta-1, and alpha(2)delta-2 mRNA is most abundantly expressed in lung and testis and well expressed in brain, heart, and pancreas. In contrast, alpha(2)delta-1 is expressed predominantly in brain, heart, and skeletal muscle. When co-expressed (via cRNA injections) with alpha(1B) and beta(3) subunits in Xenopus oocytes, alpha(2)delta-2 increased peak size of the N-type Ca(2+) currents 9-fold, and when co-expressed with alpha(1C) or alpha(1G) subunits in Xenopus oocytes increased peak size of L-type channels 2-fold and T-type channels 1.8-fold, respectively. Anti-peptide antibodies detect the expression of a 129-kDa alpha(2)delta-2 polypeptide in some but not all lung tumor cells. We conclude that the alpha(2)delta-2 gene encodes a functional auxiliary subunit of voltage-gated Ca(2+) channels. Because of its chromosomal location and expression patterns, CACNA2D2 needs to be explored as a potential tumor suppressor gene linking Ca(2+) signaling and lung, breast, and other cancer pathogenesis. The homologous location on mouse chromosome 9 is also the site of the mouse neurologic mutant ducky (du), and thus, CACNA2D2 is also a candidate gene for this inherited idiopathic generalized epilepsy syndrome.

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.

Association of neuronal calcium channels with modular adaptor proteins.

Presynaptic voltage-gated calcium (Ca(2+)) channels mediate Ca(2+) influx into the presynaptic terminal that triggers synaptic vesicle fusion and neurotransmitter release. The immediate proximity of Ca(2+) channels to the synaptic vesicle release apparatus is critical for rapid and efficient synaptic transmission. In a series of biochemical experiments, we demonstrate a specific association of the cytosolic carboxyl terminus of the N-type Ca(2+) channel pore-forming alpha(1B) subunit with the modular adaptor proteins Mint1 and CASK. The carboxyl termini of alpha(1B) bind to the first PDZ domain of Mint1 (Mint1-1). The proline-rich region present in the carboxyl termini of alpha(1B) binds to the SH3 domain of CASK. Mint1-1 is specific for the E/D-X-W-C/S-COOH consensus, which defines a novel class of PDZ domains (class III). The Mint1-1 PDZ domain-binding motif is present only in the "long" carboxyl-terminal splice variants of N-type (alpha(1B)) and P/Q-type (alpha(1A)) Ca(2+) channels, but not in R-type (alpha(1E)) or L-type (alpha(1C)) Ca(2+) channels. Our results directly link presynaptic Ca(2+) channels to a macromolecular complex formed by modular adaptor proteins at synaptic junction and advance our understanding of coupling between cell adhesion and synaptic vesicle exocytosis.

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.