An important step for cholinergic transmission involves the vesicular storage of acetylcholine (ACh), a process mediated by the vesicular acetylcholine transporter (VAChT). In order to understand the physiological roles of the VAChT, we developed a genetically altered strain of mice with reduced expression of this transporter. Heterozygous and homozygous VAChT knockdown mice have a 45% and 65% decrease in VAChT protein expression, respectively. VAChT deficiency alters synaptic vesicle filling and affects ACh release. Whereas VAChT homozygous mutant mice demonstrate major neuromuscular deficits, VAChT heterozygous mice appear normal in that respect and could be used for analysis of central cholinergic function. Behavioral analyses revealed that aversive learning and memory are not altered in mutant mice; however, performance in cognitive tasks involving object and social recognition is severely impaired. These observations suggest a critical role of VAChT in the regulation of ACh release and physiological functions in the peripheral and central nervous system.

There are three isoforms of the inositol 1,4,5-trisphosphate receptor (InsP 3 R), each of which has a distinct effect on Ca 2؉ signaling. However, it is not known whether each isoform similarly plays a distinct role in the activation of Ca 2؉ -mediated events. To investigate this question, we examined the effects of each InsP 3 R isoform on transmission of Ca 2؉ signals to mitochondria and induction of apoptosis. Each isoform was selectively silenced using isoform-specific small interfering RNA in Chinese hamster ovary cells, which express all three InsP 3 R isoforms. ATP-induced cytosolic Ca 2؉ signaling patterns were altered, regardless of which isoform was silenced, but in a different fashion depending on the isoform. ATP also induced Ca 2؉ signals in mitochondria, which were inhibited more effectively by silencing the type III InsP 3 R than by silencing either the type I or type II isoform. The type III isoform also co-localized most strongly with mitochondria. When apoptosis was induced by activation of either the extrinsic or intrinsic apoptotic pathway, induction was reduced most effectively by silencing the type III InsP 3 R. These findings provide evidence that the type III isoform of the InsP 3 R plays a special role in induction of apoptosis by preferentially transmitting Ca 2؉ signals into mitochondria.Cytosolic Ca 2ϩ (Ca i 2ϩ ) 3 is a versatile second messenger that can simultaneously regulate multiple processes within an individual cell (1). This complex regulatory action of Ca 2ϩ is thought to result in part from the varied spatial and temporal patterns of Ca 2ϩ signals that can occur (1). These signaling patterns in turn are thought to result from special properties of the inositol-1,4,5-trisphosphate (InsP 3 ) receptor (InsP 3 R), which is the principal intracellular Ca 2ϩ release channel in most types of cells (2). The InsP 3 R is gated by InsP 3 and is localized to the endoplasmic reticulum (3) and to a lesser extent the nucleus (4, 5). There are three isoforms of the InsP 3 R, each of which has a different affinity for InsP 3 (6) and distinct functional properties at the single channel level (7,8). Each InsP 3 R isoform also has distinct effects on Ca 2ϩ signaling patterns in intact cells (9, 10). Although some cells and tissues express a single or predominant isoform of the receptor, most cells instead express two or all three InsP 3 R isoforms (9, 11-13). The presence of multiple isoforms within an individual cell suggests that Ca 2ϩ released from each isoform might mediate distinct cellular events. Apoptosis is one Ca 2ϩ -mediated event that may be influenced differently by each InsP 3 R isoform. Morphologically, apoptosis is characterized by membrane blebbing, chromatin condensation, DNA fragmentation, and eventually the formation of apoptotic bodies, which are phagocytosed by neighboring cells (14). Studies at the molecular level suggest that the InsP 3 R plays an important role in the development of apoptosis (15-17). Initial evidence suggested that Ca 2ϩ -dependent apoptotic death was ...

Hepatocyte growth factor (HGF) is important for cell proliferation, differentiation, and related activities. HGF acts through its receptor c-Met, which activates downstream signaling pathways. HGF binds to c-Met at the plasma membrane, where it is generally believed that c-Met signaling is initiated. Here we report that c-Met rapidly translocates to the nucleus upon stimulation with HGF. Ca 2؉ signals that are induced by HGF result from phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol 1,4,5-trisphosphate formation within the nucleus rather than within the cytoplasm. Translocation of c-Met to the nucleus depends upon the adaptor protein Gab1 and importin ␤1, and formation of Ca 2؉ signals in turn depends upon this translocation. HGF may exert its particular effects on cells because it bypasses signaling pathways in the cytoplasm to directly activate signaling pathways in the nucleus. Hepatocyte growth factor (HGF)2 is secreted by stromal cells and binds to its receptor c-Met, which is a prototypic receptor tyrosine kinase (RTK) (1). Activation of c-Met is responsible for cell proliferation under normal conditions such as tissue regeneration (2), as well as under abnormal conditions such as neoplasia (3). For example, in the liver, HGF is secreted by stellate cells and then binds to c-Met on hepatocytes (1) to mediate processes such as liver regeneration following liver resection (2) and development of hepatocellular carcinoma (4). The mechanisms of action of c-Met are not entirely understood, but it is generally believed that this and other RTKs act at the plasma membrane (5-7). However, several RTKs have been found in the nucleus, including receptors for insulin, epidermal growth factor, and fibroblast growth factor (8 -10). The function of these receptors in the nucleus is controversial (11, 12).C-met and other RTKs act in part through Ca 2ϩ signaling, which is initiated by the phospholipase C-␥ (PLC␥)/inositol 1,4,5-trisphosphate (InsP 3 )/Ca 2ϩ signaling cascade (13). A number of peptide hormones also increase Ca 2ϩ via InsP 3 but typically do so via G protein-coupled receptors that activate PLC␤ (13). Ca 2ϩ signals induced by growth factors typically have separate effects from hormone-induced Ca 2ϩ signals, although both are mediated by PLC and InsP 3 (13). In hepatocytes, for example, vasopressin activates the V 1a receptor to modulate secretion (14), glucose release (15), and apical contractility (16), whereas HGF activates c-Met to regulate cell proliferation (17). The versatility of Ca 2ϩ as a second messenger in part reflects that Ca 2ϩ signals have distinct effects in different parts of the cell (13). Moreover, cell proliferation depends upon Ca 2ϩ signals in the nucleus rather than in the cytoplasm (18). Therefore, we examined the subcellular distribution of c-Met and the mechanism by which HGF induces c-Met to form Ca 2ϩ signals. EXPERIMENTAL PROCEDURESCells and Cell Culture-SkHep1 cells were cultured at 37°C in 5% CO 2 in Dulbecco's modified Eagle's medium (Invitrogen) containing 10% fe...

The vesicular acetylcholine (ACh) transporter (VAChT) mediates ACh storage by synaptic vesicles. However, the VAChT-independent release of ACh is believed to be important during development. Here we generated VAChT knockout mice and tested the physiological relevance of the VAChT-independent release of ACh. Homozygous VAChT knockout mice died shortly after birth, indicating that VAChT-mediated storage of ACh is essential for life. Indeed, synaptosomes obtained from brains of homozygous knockouts were incapable of releasing ACh in response to depolarization. Surprisingly, electrophysiological recordings at the skeletalneuromuscular junction show that VAChT knockout mice present spontaneous miniature end-plate potentials with reduced amplitude and frequency, which are likely the result of a passive transport of ACh into synaptic vesicles. Interestingly, VAChT knockouts exhibit substantial increases in amounts of choline acetyltransferase, high-affinity choline transporter, and ACh. However, the development of the neuromuscular junction in these mice is severely affected. Mutant VAChT mice show increases in motoneuron and nerve terminal numbers. End plates are large, nerves exhibit abnormal sprouting, and muscle is necrotic. The abnormalities are similar to those of mice that cannot synthesize ACh due to a lack of choline acetyltransferase. Our results indicate that VAChT is essential to the normal development of motor neurons and the release of ACh.

We have investigated the intracellular traffic of PrP c , a glycosylphosphatidylinositol (GPI)-anchored protein implicated in spongiform encephalopathies. A fluorescent functional green fluorescent protein (GFP)-tagged version of PrP c is found at the cell surface and in intracellular compartments in SN56 cells. Confocal microscopy and organelle-specific markers suggest that the protein is found in both the Golgi and the recycling endosomal compartment. Perturbation of endocytosis with a dynamin I-K44A dominant-negative mutant altered the steady-state distribution of the GFP-PrP c , leading to the accumulation of fluorescence in unfissioned endocytic intermediates. These pre-endocytic intermediates did not seem to accumulate GFP-GPI, a minimum GPI-anchored protein, suggesting that PrP c trafficking does not depend solely on the GPI anchor. We found that internalized GFP-PrP c accumulates in Rab5-positive endosomes and that a Rab5 mutant alters the steady-state distribution of GFP-PrP c but not that of GFP-GPI between the plasma membrane and early endosomes. Therefore, we conclude that PrP c internalizes via a dynamin-dependent endocytic pathway and that the protein is targeted to the recycling endosomal compartment via Rab5-positive early endosomes. These observations indicate that traffic of GFP-PrP c is not determined predominantly by the GPI anchor and that, different from other GPI-anchored proteins, PrP c is delivered to classic endosomes after internalization.

Synthesis of acetylcholine depends on the plasma membrane uptake of choline by a high affinity choline transporter (CHT1). Choline uptake is regulated by nerve impulses and trafficking of an intracellular pool of CHT1 to the plasma membrane may be important for this regulation. We have generated a hemagglutinin (HA) epitope tagged CHT1 to investigate the organelles involved with intracellular trafficking of this protein.Expression of CHT1-HA in HEK 293 cells establishes Na + -dependent, hemicholinium-3 sensitive high-affinity choline transport activity. Confocal microscopy reveals that CHT1-HA is found predominantly in intracellular organelles in three different cell lines. Importantly, CHT1-HA seems to be continuously cycling between the plasma membrane and endocytic organelles via a constitutive clathrin-mediated endocytic pathway. In a neuronal cell line, CHT1-HA colocalizes with the early endocytic marker green fluorescent protein (GFP)-Rab 5 and with two markers of synaptic-like vesicles, VAMP-myc and GFP-VAChT, suggesting that in cultured cells CHT1 is present mainly in organelles of endocytic origin. Subcellular fractionation and immunoisolation of organelles from rat brain indicate that CHT1 is present in synaptic vesicles. We propose that intracellular CHT1 can be recruited during stimulation to increase choline uptake in nerve terminals.

The secreted cochaperone STI1 triggers activation of protein kinase A (PKA) and ERK1/2 signaling by interacting with the cellular prion (PrP C ) at the cell surface, resulting in neuroprotection and increased neuritogenesis. Here, we investigated whether STI1 triggers PrP C trafficking and tested whether this process controls PrP C -dependent signaling. We found that STI1, but not a STI1 mutant unable to bind PrP C , induced PrP C endocytosis. STI1-induced signaling did not occur in cells devoid of endogenous PrP C ; however, heterologous expression of PrP C reconstituted both PKA and ERK1/2 activation. In contrast, a PrP C mutant lacking endocytic activity was unable to promote ERK1/2 activation induced by STI1, whereas it reconstituted PKA activity in the same condition, suggesting a key role of endocytosis in the former process. The activation of ERK1/2 by STI1 was transient and appeared to depend on the interaction of the two proteins at the cell surface or shortly after internalization. Moreover, inhibition of dynamin activity by expression of a dominant-negative mutant caused the accumulation and colocalization of these proteins at the plasma membrane, suggesting that both proteins use a dynamin-dependent internalization pathway. These results show that PrP C endocytosis is a necessary step to modulate STI1-dependent ERK1/2 signaling involved in neuritogenesis.

Calcium influx through neuronal voltage-sensitive calcium channels (VSCC S) mediates nociceptive information in the spinal dorsal horn. In fact, spinally administered VSCC S blockers, such as omega-conotoxin MVIIA, have analgesic effect apart of their low therapeutic index and many side effects. Here we study the analgesic potential of Ph alpha 1beta, a calcium channel blocker, in rodent models of acute and persistent pain. Spinally administered Ph alpha 1beta showed higher efficacy and long-lasting analgesia in a thermal model of pain, when compared with omega-conotoxin MVIIA. Moreover, Ph alpha 1beta was more effective and potent than omega-conotoxin MVIIA not only to prevent, but especially to reverse, previously installed persistent chemical and neuropathic pain. Furthermore, the analgesic action of both toxins are related with the inhibition of Ca2+-evoked release of pro-nociceptive neurotransmitter, glutamate, from rat spinal cord synaptosomes and decrease of glutamate overflow in cerebrospinal fluid. When side effects were assessed, we found that Ph alpha 1beta had a therapeutic index wider than omega- conotoxin MVIIA. Finally, recombinant Ph alpha 1beta expressed in Escherichia coli showed marked analgesic activity similar to the native toxin. Taken together, the present study demonstrates that native and recombinant Ph alpha 1beta have analgesic effects in rodent models of pain, suggesting that this toxin may have potential to be used as a drug in the control of persistent pathological pain.