Peroxynitrite donor SIN-1 alters high-affinity choline transporter activity by modifying its intracellular trafficking.

Sodium-coupled, high-affinity choline transporters (CHTs) are inhibited by 3-morpholinosydnonimine (SIN-1) [peroxynitrite (ONOO⁻) donor]; ONOO⁻ can be produced from nitric oxide and reactive oxygen species during neurodegeneration. SIN-1 rapidly increases CHT internalization from the cell surface, and this correlates with decreased choline uptake. This study addresses mechanisms by which SIN-1 inhibits CHT function in human neuronal SH-SY5Y cells. Thus, mutant L531A-CHT, which does not constitutively internalize into cells by a clathrin-mediated process, is resistant to SIN-1 effects. This suggests that CHT inhibition is not due to oxidative-nitrosative inactivation of the protein and that decreased levels of cell surface CHT in SIN-1-treated cells is related to alterations in its trafficking and subcellular disposition. Dominant-negative proteins AP180C and dynamin-K44A, which interfere with clathrin-mediated and dynamin-dependent endocytosis, respectively, attenuate CHT inhibition by SIN-1. CHT in both vehicle- and SIN-1-treated cells colocalizes with Rab7, Rab9, and Lamp-1 in late endosomes and lysosomes to a similar extent. Lysosome inhibitors increase choline uptake, suggesting that CHT proteins are normally degraded by lysosomes, and this is not altered by oxidative stress. Unexpectedly, inhibitors of proteasomes, but not lysosomes, attenuate SIN-1-mediated inhibition of choline uptake, indicating that proteasomal degradation plays a role in regulating CHT disposition in SIN-1-treated cells. SIN-1 treatment also enhances CHT ubiquitination. Thus, CHT inhibition in SIN-1-treated cells is mediated by proteasomal degradation, which differs from inhibitory mechanisms for some neurotransmitter transporters under similar conditions. Increased oxidative-nitrosative stress in the microenvironment of cholinergic nerve terminals would diminish cholinergic transmission by reducing choline availability for ACh synthesis.

Surface expression and function of Cav3.2 T-type calcium channels are controlled by asparagine-linked glycosylation.

Low-voltage-activated T-type calcium channels play important roles in neuronal physiology where they control cellular excitability and synaptic transmission. Alteration in T-type channel expression has been linked to various pathophysiological conditions such as pain arising from diabetic neuropathy. In the present study, we looked at the role of asparagine (N)-linked glycosylation on human Cav3.2 T-type channel expression and function. Manipulation of N-glycans on cells expressing a recombinant Cav3.2 channel revealed that N-linked glycosylation is critical for proper functional expression of the channel. Using site-directed mutagenesis to disrupt the canonical N-linked glycosylation sites of Cav3.2 channel, we show that glycosylation at asparagine N192 is critical for channel expression at the surface, whereas glycosylation at asparagine N1466 controls channel activity. Moreover, we demonstrate that N-linked glycosylation of Cav3.2 not only controls surface expression and activity of the channel but also underlies glucose-dependent potentiation of T-type Ca(2+) current. Our data suggest that N-linked glycosylation of T-type channels may play an important role in aberrant upregulation of T-type channel activity in response to glucose elevations.

Diagnosing Alzheimer's Disease from Circulating Blood Leukocytes Using a Fluorescent Amyloid Probe.

BACKGROUND: Toxic amyloid-ß (Aß) peptides aggregate into higher molecular weight assemblies and accumulate not only in the extracellular space, but also in the walls of blood vessels in the brain, increasing their permeability, and promoting immune cell migration and activation. Given the prominent role of the immune system, phagocytic blood cells may contact pathological brain materials. OBJECTIVE: To develop a novel method for early Alzheimer's disease (AD) detection, we used blood leukocytes, that could act as "sentinels" after trafficking through the brain microvasculature, to detect pathological amyloid by labelling with a conformationally-sensitive fluorescent amyloid probe and imaging with confocal spectral microscopy. METHODS: Formalin-fixed peripheral blood mononuclear cells (PBMCs) from cognitively healthy control (HC) subjects, mild cognitive impairment (MCI) and AD patients were stained with the fluorescent amyloid probe K114, and imaged. Results were validated against cerebrospinal fluid (CSF) biomarkers and clinical diagnosis. RESULTS: K114-labeled leukocytes exhibited distinctive fluorescent spectral signatures in MCI/AD subjects. Comparing subjects with single CSF biomarker-positive AD/MCI to negative controls, our technique yielded modest AUCs, which improved to the 0.90 range when only MCI subjects were included in order to measure performance in an early disease state. Combining CSF Aß42 and t-Tau metrics further improved the AUC to 0.93. CONCLUSION: Our method holds promise for sensitive detection of AD-related protein misfolding in circulating leukocytes, particularly in the early stages of disease.

Mutation of copper binding sites on cellular prion protein abolishes its inhibitory action on NMDA receptors in mouse hippocampal neurons.

We have previously reported that cellular prion protein (PrPC) can down-regulate NMDA receptor activity and in a copper dependent manner. Here, we employed AAV9 to introduce murine cellular prion protein into mouse hippocampal neurons in primary cultures from PrP null mice to determine the role of the six copper binding motifs located within the N-terminal domain of PrPC. The results demonstrate that viral expression of wild type PrPC lowers NMDAR activity in PrP null mouse hippocampal neurons by reducing the magnitude of non-desensitizing currents. Elimination of the last two copper binding sites alone, or in combination with the remaining four attenuates this protective effect. Thus our data suggest that copper ion interactions with specific binding sites on PrPC are critical for PrPC dependent modulation of NMDA receptor function.

Activity and subcellular trafficking of the sodium-coupled choline transporter CHT is regulated acutely by peroxynitrite.

Excess formation of nitric oxide and superoxide by-products (peroxynitrite, reactive oxygen, and reactive nitrogen species) attenuates cholinergic transmission potentially having a role in Alzheimer disease pathogenesis. In this study, we investigated mechanisms by which acute exposure to peroxynitrite impairs function of the sodium-dependent hemicholinium-3 (HC-3)-sensitive choline transporter (CHT) that provides substrate for acetylcholine synthesis. The peroxynitrite generator 3-morpholinosydnonimine (SIN-1) acutely inhibited choline uptake in cells stably expressing FLAG-tagged rat CHT in a dose- and time-dependent manner, with an IC(50) = 0.9 +/- 0.14 mM and t((1/2)) = 4 min. SIN-1 significantly reduced V(max) of choline uptake without altering the K(m). This correlated with a SIN-1-induced decrease in cell surface CHT protein, observed as lowered levels of HC-3 binding and biotinylated CHT at the plasma membrane. It is noteworthy that short-term exposure of cells to SIN-1 accelerated the rate of internalization of CHT from the plasma membrane, but it did not alter return of CHT back to the cell surface. SIN-1 did not disrupt cell membrane integrity or cause cell death. Thus, the inhibitory effect of SIN-1 on choline uptake activity and HC-3 binding was related to enhanced internalization of CHT proteins from the plasma membrane to subcellular organelles.

Regulated recycling and plasma membrane recruitment of the high-affinity choline transporter.

The high-affinity choline transporter (CHT1) is responsible for uptake of choline from the synaptic cleft and supplying choline for acetylcholine synthesis. CHT1 internalization by clathrin-coated vesicles is proposed to represent a mechanism by which high-affinity choline uptake can be modulated. We show here that internalized CHT1 is rapidly recycled back to the cell surface in both human embryonic kidney cells (HEK 293 cells) and SH-SY5Y neuroblastoma cells. This rapidly recycling pool of CHT1 comprises about 10% of total CHT1 protein. In the SH-SY5Y neuroblastoma cell line K(+)-depolarization promotes Ca(2+)-dependent increase in the rate of CHT1 recycling to the plasma membrane without affecting the rate of CHT1 internalization. K(+)-depolarization also increases the size of the pool of CHT1 protein that can be mobilized to the plasma membrane. Thus, the activity-dependent increase in plasma membrane CHT1 localization appears to be regulated by two mechanisms: (i) an increase in the rate of externalization of the intracellular CHT1 pool; and (ii) the recruitment of additional intracellular transporters to the recycling pool.

Reduced Hyperpolarization-Activated Current Contributes to Enhanced Intrinsic Excitability in Cultured Hippocampal Neurons from PrP(-/-) Mice.

Genetic ablation of cellular prion protein (PrP(C)) has been linked to increased neuronal excitability and synaptic activity in the hippocampus. We have previously shown that synaptic activity in hippocampi of PrP-null mice is increased due to enhanced N-methyl-D-aspartate receptor (NMDAR) function. Here, we focused on the effect of PRNP gene knock-out (KO) on intrinsic neuronal excitability, and in particular, the underlying ionic mechanism in hippocampal neurons cultured from P0 mouse pups. We found that the absence of PrP(C) profoundly affected the firing properties of cultured hippocampal neurons in the presence of synaptic blockers. The membrane impedance was greater in PrP-null neurons, and this difference was abolished by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker ZD7288 (100 µM). HCN channel activity appeared to be functionally regulated by PrP(C). The amplitude of voltage sag, a characteristic of activating HCN channel current (I h), was decreased in null mice. Moreover, I h peak current was reduced, along with a hyperpolarizing shift in activation gating and slower kinetics. However, neither HCN1 nor HCN2 formed a biochemical complex with PrP(C). These results suggest that the absence of PrP downregulates the activity of HCN channels through activation of a cell signaling pathway rather than through direct interactions. This in turn contributes to an increase in membrane impedance to potentiate neuronal excitability.

Cellular prion protein and NMDA receptor modulation: protecting against excitotoxicity.

Although it is well established that misfolding of the cellular prion protein (PrP(C)) into the ß-sheet-rich, aggregated scrapie conformation (PrP(Sc)) causes a variety of transmissible spongiform encephalopathies (TSEs), the physiological roles of PrP(C) are still incompletely understood. There is accumulating evidence describing the roles of PrP(C) in neurodegeneration and neuroinflammation. Recently, we identified a functional regulation of NMDA receptors by PrP(C) that involves formation of a physical protein complex between these proteins. Excessive NMDA receptor activity during conditions such as ischemia mediates enhanced Ca(2+) entry into cells and contributes to excitotoxic neuronal death. In addition, NMDA receptors and/or PrP(C) play critical roles in neuroinflammation and glial cell toxicity. Inhibition of NMDA receptor activity protects against PrP(Sc)-induced neuronal death. Moreover, in mice lacking PrP(C), infarct size is increased after focal cerebral ischemia, and absence of PrP(C) increases susceptibility of neurons to NMDA receptor-dependent death. Recently, PrP(C) was found to be a receptor for oligomeric beta-amyloid (Aß) peptides, suggesting a role for PrP(C) in Alzheimer's disease (AD). Our recent findings suggest that Aß peptides enhance NMDA receptor current by perturbing the normal copper- and PrP(C)-dependent regulation of these receptors. Here, we review evidence highlighting a role for PrP(C) in preventing NMDA receptor-mediated excitotoxicity and inflammation. There is a need for more detailed molecular characterization of PrP(C)-mediated regulation of NMDA receptors, such as determining which NMDA receptor subunits mediate pathogenic effects upon loss of PrP(C)-mediated regulation and identifying PrP(C) binding site(s) on the receptor. This knowledge will allow development of novel therapeutic interventions for not only TSEs, but also for AD and other neurodegenerative disorders involving dysfunction of PrP(C).

Choline transporter CHT regulation and function in cholinergic neurons.

Choline uptake into cholinergic nerve terminals by the sodium-dependent high-affinity choline transporter CHT is essential for providing choline as substrate for synthesis of acetylcholine (ACh); ACh is used by cholinergic neurons to communicate information to a wide range of tissues in central and peripheral nervous systems. CHT is expressed almost exclusively in cholinergic neurons, and is subject to transcriptional and post-translational control by factors that promote or diminish cholinergic neurotransmission. The distribution of CHT proteins within cholinergic presynaptic terminals is dynamically regulated. Thus, choline uptake activity is determined largely by the plasma membrane CHT level, and this is finely controlled by a balance between internalization and recycling of CHT proteins in endosomal compartments. CHT proteins are also in synaptic vesicle membranes, thereby allowing cell surface CHT levels to increase rapidly in conjunction with exocytotic transmitter release to provide enhanced choline for ACh re-synthesis. Little is known about post-translational modification of CHT, although data is emerging that CHT activity and subcellular trafficking is modulated by kinase-mediated phosphorylation. Recent studies have also identified proteins with which CHT interacts, but this requires further investigation to reveal the role of other proteins in regulating CHT function and activity. Polymorphisms in CHT protein and modifications in its expression are linked to neurological and psychiatric disorders, and can alter function of peripheral systems that are regulated by cholinergic innervation, such as the cardiovascular system. The critical role of CHT in maintaining cholinergic transmission indicates that it could be a target for therapeutic intervention to promote ACh synthesis, but mechanisms by which this can be accomplished have not been adequately addressed.

The "ins" and "outs" of the high-affinity choline transporter CHT1.

Maintenance of acetylcholine (ACh) synthesis depends on the activity of the high-affinity choline transporter (CHT1), which is responsible for the reuptake of choline from the synaptic cleft into presynaptic neurons. In this review, we discuss the current understanding of mechanisms involved in the cellular trafficking of CHT1. CHT1 protein is mainly found in intracellular organelles, such as endosomal compartments and synaptic vesicles. The presence of CHT1 at the plasma membrane is limited by rapid endocytosis of the transporter in clathrin-coated pits in a mechanism dependent on a dileucine-like motif present in the carboxyl-terminal region of the transporter. The intracellular pool of CHT1 appears to constitute a reserve pool of transporters, important for maintenance of cholinergic neurotransmission. However, the physiological basis of the presence of CHT1 in intracellular organelles is not fully understood. Current knowledge about CHT1 indicates that stimulated and constitutive exocytosis, in addition to endocytosis, will have major consequences for regulating choline uptake. Future investigations of CHT1 trafficking should elucidate such regulatory mechanisms, which may aid in understanding the pathophysiology of diseases that affect cholinergic neurons, such as Alzheimer's disease.

Constitutive high-affinity choline transporter endocytosis is determined by a carboxyl-terminal tail dileucine motif.

Maintenance of acetylcholine synthesis depends on the effective functioning of a high-affinity sodium-dependent choline transporter (CHT1). Recent studies have shown that this transporter is predominantly localized inside the cell, unlike other neurotransmitter transporters, suggesting that the trafficking of CHT1 to and from the plasma membrane may play a crucial role in regulating choline uptake. Here we found that CHT1 is rapidly and constitutively internalized in clathrin-coated vesicles to Rab5-positive early endosomes. CHT1 internalization is controlled by an atypical carboxyl-terminal dileucine-like motif (L531, V532) which, upon replacement by alanine residues, blocks CHT1 internalization in both human embryonic kidney 293 cells and primary cortical neurons and results in both increased CHT1 cell surface expression and choline transport activity. Perturbation of clathrin-mediated endocytosis with dynamin-I K44A increases cell surface expression and transport activity to a similar extent as mutating the dileucine motif, suggesting that we have identified the motif responsible for constitutive CHT1 internalization. Based on the observation that the localization of CHT1 to the plasma membrane is transient, we propose that acetylcholine synthesis may be influenced by processes that lead to the attenuation of constitutive CHT1 endocytosis.