Constitutive μ-opioid receptor activity leads to long-term endogenous analgesia and dependence

Opioid receptor antagonists increase hyperalgesia in humans and animals, which indicates that endogenous activation of opioid receptors provides relief from acute pain; however, the mechanisms of long-term opioid inhibition of pathological pain have remained elusive. We found that tissue injury produced μ-opioid receptor (MOR) constitutive activity (MOR(CA)) that repressed spinal nociceptive signaling for months. Pharmacological blockade during the posthyperalgesia state with MOR inverse agonists reinstated central pain sensitization and precipitated hallmarks of opioid withdrawal (including adenosine 3',5'-monophosphate overshoot and hyperalgesia) that required N-methyl-D-aspartate receptor activation of adenylyl cyclase type 1. Thus, MOR(CA) initiates both analgesic signaling and a compensatory opponent process that generates endogenous opioid dependence. Tonic MOR(CA) suppression of withdrawal hyperalgesia may prevent the transition from acute to chronic pain.

Inactivation of Pde8b enhances memory, motor performance, and protects against age-induced motor coordination decay

Phosphodiesterases (PDEs) are critical regulatory enzymes in cyclic nucleotide signaling. PDEs have diverse expression patterns within the central nervous system (CNS), show differing affinities for cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), and regulate a vast array of behaviors. Here, we investigated the expression profile of the PDE8 gene family members Pde8a and Pde8b in the mouse brain. We find that Pde8a expression is largely absent in the CNS; by contrast, Pde8b is expressed in select regions of the hippocampus, ventral striatum, and cerebellum. Behavioral analysis of mice with Pde8b gene inactivation (PDE8B KO) demonstrate an enhancement in contextual fear, spatial memory, performance in an appetitive instrumental conditioning task, motor-coordination, and have an attenuation of age-induced motor coordination decline. In addition to improvements observed in select behaviors, we find basal anxiety levels to be increased in PDE8B KO mice. These findings indicate that selective antagonism of PDE8B may be an attractive target for enhancement of cognitive and motor functions; however, possible alterations in affective state will need to be weighed against potential therapeutic value.

A source for the special catalytic power of enzymes: orbital steering

The velocities of acid catalyzed esterification and gamma-lactonizations were studied to test the sensitivity of a chemical reaction to the orientation of the reacting atoms. Variation in orientation of the attacking oxygen atom relative to the carbon atom of the carboxylic acid was achieved by using bicyclic ring systems to limit the conformational mobility of the gamma-hydroxy acids. Factors as high as 2 x 10(4) were observed for the acceleration of a reaction due to this "orientation factor" even after corrections for proximity and torsional strain have been made. The orientation factor is related to the shape of the electron orbitals and must have an angular preference far greater than previously estimated. Such sensitivity to orientation would provide factors large enough to explain the gap in our understanding of enzyme catalysis. It is suggested that the catalytic efficiency of enzymes depends on their ability not only to juxtapose the reacting atoms but also to "steer" their orbitals along a path which takes advantage of this strong directional preference.

High precision stereotaxic surgery in mice

This unit provides protocols for cannulation and site-specific central microinjection of mice using a recently developed high-precision stereotaxic frame. The construction of cannulae, wire plugs and injection needles are also described.

Inactivation of the mouse adenylyl cyclase 3 gene disrupts male fertility and spermatozoon function

Mammalian spermatids and spermatozoa express functional G protein-coupled receptors. However, bicarbonate-regulated soluble adenylyl cyclase (AC), the major AC present in these cells, is not directly coupled to G proteins. To understand how G protein-coupled receptors signal in spermatozoa, we investigated whether a conventional transmembrane cyclase is present and biologically active in these cells. Here, we provide evidence for expression of type 3 AC (AC3) in male germ cells and describe the effects of disruption of the AC3 gene on fertility and function of mouse spermatozoa. As previously reported in rat, AC3 mRNA is expressed in mouse testes and localized, together with soluble AC mRNA, mainly in postmeiotic germ cells. AC3 protein was detected by immunolocalization in round and elongating spermatids in a region corresponding to the developing acrosome and was retained in the mature spermatozoa of the epididymis. Forskolin caused a small increase in cAMP production in mouse spermatozoa, but this increase could not be detected in the AC3(-/-) mice. Inactivation of the AC3 gene did not have overt effects on spermatogenesis; however, AC3(-/-) males were subfertile with only three litters generated by 11 males over a period of 6 months. When used in in vitro fertilization, spermatozoa from these AC3(-/-) mice produced few embryos, but their fertilizing ability was restored after removal of the zona pellucida. Despite an apparently normal structure, these spermatozoa had decreased motility and showed an increase in spontaneous acrosome reactions. These data support the hypothesis that AC3 is required for normal spermatid or spermatozoa function and male fertility.

Adenylyl cyclase 3 mediates prostaglandin E(2)-induced growth inhibition in arterial smooth muscle cells

Arterial smooth muscle cell (SMC) proliferation contributes to a number of vascular pathologies. Prostaglandin E(2) (PGE(2)), produced by the endothelium and by SMCs themselves, acts as a potent SMC growth inhibitor. The growth-inhibitory effects of PGE(2) are mediated through activation of G-protein-coupled membrane receptors, activation of adenylyl cyclases (ACs), formation of cAMP, and subsequent inhibition of mitogenic signal transduction pathways in SMCs. Of the 10 different mammalian AC isoforms known today, seven isoforms (AC2-7 and AC9) are expressed in SMCs from various species. We show that, despite the presence of several different AC isoforms, the principal AC isoform activated by PGE(2) in human arterial SMCs is a calmodulin kinase II-inhibited AC with characteristics similar to those of AC3. AC3 is expressed in isolated human arterial SMCs and in intact aorta. We further show that arterial SMCs isolated from AC3-deficient mice are resistant to PGE(2)-induced growth inhibition. In summary, AC3 is the principal AC isoform activated by PGE(2) in arterial SMCs, and AC3 mediates the growth-inhibitory effects of PGE(2). Because AC3 activity is inhibited by intracellular calcium through calmodulin kinase II, AC3 may serve as an important integrator of growth-inhibitory signals that stimulate cAMP formation and growth factors that increase intracellular calcium.

The CRE/CREB pathway is transiently expressed in thalamic circuit development and contributes to refinement of retinogeniculate axons

The development of precise connections in the mammalian brain proceeds through refinement of initially diffuse patterns, a process that occurs largely within critical developmental windows. To elucidate the molecular pathways that orchestrate these early periods of circuit remodeling, we have examined the role of a calcium- and cAMP-regulated transcriptional pathway. We show that there is a window of CRE/CREB-mediated gene expression in the developing thalamus, which precedes neocortical expression. In the LGN, this wave of gene expression occurs prior to visual experience, but requires retinal function. Mutant mice with reduced CREB expression show loss of refinement of retinogeniculate projections. These results suggest an important role of the CRE/CREB transcriptional pathway in the coordination of experience-independent circuit remodeling during forebrain development.

Role of Ca2+-stimulated adenylyl cyclases in LTP and memory formation

Studies with invertebrates and vertebrates have strongly implicated the CREB/CRE transcriptional pathway in long-term memory (LTM) and transcriptionally-dependent L-LTP. It is hypothesized that LTM and L-LTP are both dependent upon a Ca2+ signal generated through activation of NMDA receptors. This review discusses evidence that Ca2+ signals generated through activation of NMDA receptors coactivate the Erk/MAP kinase and cAMP signal transduction pathways. It is hypothesized that activation of these two regulatory pathways increases the transcription of a family of genes through the CREB/CRE transcriptional pathway. Gene disruption studies have shown that Ca2+ activated adenylyl cyclases play a critical role in generating the cAMP signal required for LTM and L-LTP. Although cAMP may be required for several events in this complex signal transduction cascade, one of the major roles of cAMP may be to support nuclear translocation of Erk/MAP kinase in hippocampal neurons.

DNA elements of the type 1 adenylyl cyclase gene locus enhance reporter gene expression in neurons and pinealocytes

The Ca2+-stimulated type 1 adenylyl cyclase (AC1) contributes to several forms of synaptic plasticity and is the only known neurospecific adenylyl cyclase. Furthermore, the protein and mRNA levels of AC1 undergo a circadian oscillation in the pineal gland, and AC1 may play a pivotal role in regulating nocturnal melatonin synthesis. To better understand the expression of AC1, we isolated mouse genomic DNA clones of AC1. The transcription and translation start regions of mouse AC1 share extensive homologies with the bovine counterpart. The upstream proximal region has potential binding sites for transcription factors, including the steroid receptor family, the E-box factors, and Sp1. A 280-bp fragment that contains the transcription start site directed reporter gene expression in cultured cortical neurons and pinealocytes functioning as a basal neuro- and pineal-directed promoter. Interestingly, pinealocyte expression of the reporter gene was inhibited by increases in cAMP. This cAMP sensitivity may explain why AC1 mRNA in the pineal is low at night when cAMP is elevated and high during the day when cAMP signals drop. An adjacent 330-bp fragment interacted specifically with nuclear factor(s) that we designate binary E-box factor (BEF). Methylation interference and DNase I footprinting identified the BEF-binding site sequence as 5'-CCAAGGTCACGTGGC-3'. When linked to the basal tissue-directed promoter, this 15-bp sequence further enhanced reporter expression in neurons and pinealocytes. We propose that this 15-bp sequence may contribute to increased expression of AC1 in neurons and pinealocytes relative to other cells.

Expression of type I adenylyl cyclase in intrinsic pathways of the hippocampal formation of the macaque (Macaca nemestrina)

The mossy fiber pathway of the hippocampal formation and type 1 adenylyl cyclase (AC1) have been implicated in long-term potentiation and memory function. Using immunohistochemical labeling and light microscopy we demonstrated intense labeling of AC1 in the mossy fibers and less intense labeling in the molecular layers of both the dentate gyrus and fields CA1, CA2 and CA3 of the hippocampus, i.e. in terminal fields of the perforant pathway. These findings indicate that, in the non-human primate, AC1 is found in the mossy fibers and in terminal fields of the perforant pathway where it may play a role in long term potentiation similar to that demonstrated in the rodent.

Odorants stimulate the ERK/mitogen-activated protein kinase pathway and activate cAMP-response element-mediated transcription in olfactory sensory neurons

Olfactory sensory neurons (OSNs) respond acutely to volatile molecules and exhibit adaptive responses including desensitization to odorant exposure. Although mechanisms for short term adaptation have been described, there is little evidence that odorants cause long lasting, transcription-dependent changes in OSNs. Here we report that odorants stimulate cAMP-response element (CRE)-mediated transcription in OSNs through Ca2+ activation of the ERK/MAPK/p90rsk pathway. Odorant stimulation of ERK phosphorylation was ablated by inhibition of calmodulin-dependent protein kinase II suggesting that odorant activation of ERK is mediated through this kinase. Moreover, a brief exposure in vivo to an odorant in vapor phase stimulated CRE-mediated gene transcription in discrete populations of OSNs. These data suggest that like central nervous system neurons, OSNs may undergo long term adaptive changes mediated through CRE-mediated transcription.

SRF-dependent gene expression is required for PI3-kinase-regulated cell proliferation

Recent evidence indicates that phosphatidylinositol 3-kinase (PI3K) is a central regulator of mitosis, apoptosis and oncogenesis. Nevertheless, the mechanisms by which PI3K regulates proliferation are not well characterized. Mitogens stimulate entry into the cell cycle by inducing the expression of immediate early genes (IEGs) that in turn trigger the expression of G(1) cyclins. Here we describe a novel PI3K- regulated transcriptional cascade that is critical for mitogen regulation of the IEG, c-fos. We show that PI3K activates gene expression by transactivating SRF-dependent transcription independently of the previously described Rho and ETS TCF pathways. PI3K-stimulated cell cycle progression requires transactivation of SRF and expression of dominant- negative PI3K blocks mitogen-stimulated cell cycle progression. Furthermore, dominant-interfering SRF mutants attenuate mitogen-stimulated cell cycle progression, but are without effect on MEK-stimulated cell cycle entry. Moreover, expression of constitutively active SRF is sufficient for cell cycle entry. Thus, we delineate a novel SRF-dependent mitogenic cascade that is critical for PI3K- and growth factor-mediated cell cycle progression.

Disruption of the type III adenylyl cyclase gene leads to peripheral and behavioral anosmia in transgenic mice

Cyclic nucleotide-gated ion channels in olfactory sensory neurons (OSNs) are hypothesized to play a critical role in olfaction. However, it has not been demonstrated that the cAMP signaling is required for olfactory-based behavioral responses, and the contributions of specific adenylyl cyclases to olfaction have not been defined. Here, we report the presence of adenylyl cyclases 2, 3, and 4 in olfactory cilia. To evaluate the role of AC3 in olfactory responses, we disrupted the gene for AC3 in mice. Interestingly, electroolfactogram (EOG) responses stimulated by either cAMP- or inositol 1,4,5-triphosphate- (IP3-) inducing odorants were completely ablated in AC3 mutants, despite the presence of AC2 and AC4 in olfactory cilia. Furthermore, AC3 mutants failed several olfaction-based behavioral tests, indicating that AC3 and cAMP signaling are critical for olfactory-dependent behavior.

Altered stress-induced anxiety in adenylyl cyclase type VIII-deficient mice

Stress results in alterations in behavior and physiology that can be either adaptive or maladaptive. To define the molecular pathways involved in the response to stress further, we generated mice deficient (KO) in the calcium-stimulated adenylyl cyclase type VIII (AC8) by homologous recombination in embryonic stem cells. AC8 KO mice demonstrate a compromise in calcium-stimulated AC activity in the hippocampus, hypothalamus, thalamus, and brainstem. Hippocampal slices derived from AC8 KO mice fail to demonstrate CA1-region long-term depression after low-frequency stimulation, and AC8 KO mice also fail to activate CRE-binding protein in the CA1 region after restraint stress. To define the behavioral consequences of AC8 deficiency, we evaluated AC8 KO mice in the elevated plus-maze and open field. Although naive AC8 KO mice exhibit indices of anxiety comparable with that of wild-type mice, AC8 KO mice do not show normal increases in behavioral markers of anxiety when subjected to repeated stress such as repetitive testing in the plus-maze or restraint preceding plus-maze testing. These results demonstrate a novel role for AC8 in the modulation of anxiety.

The type 8 adenylyl cyclase is critical for Ca2+ stimulation of cAMP accumulation in mouse parotid acini

Capacitative Ca(2+) entry stimulates cAMP synthesis in mouse parotid acini, suggesting that one of the Ca(2+)-sensitive adenylyl cyclases (AC1 or AC8) may play an important role in the regulation of parotid function (Watson, E. L., Wu, Z., Jacobson, K. L., Storm, D. R., Singh, J. C., and Ott, S. M. (1998) Am. J. Physiol. 274, C557-C565). To evaluate the role of AC1 and AC8 in Ca(2+) stimulation of cAMP synthesis in parotid cells, acini were isolated from AC1 mutant (AC1-KO) and AC8 mutant (AC8-KO) mice and analyzed for Ca(2+) stimulation of intracellular cAMP levels. Although Ca(2+) stimulation of intracellular cAMP levels in acini from AC1-KO mice was indistinguishable from wild type mice, acini from AC8-KO mice showed no Ca(2+)-stimulated cAMP accumulation. This indicates that AC8, but not AC1, plays a major role in coupling Ca(2+) signals to cAMP synthesis in parotid acini. Interestingly, treatment of acini from AC8-KO mice with agents, i.e. carbachol and thapsigargin that increase intracellular Ca(2+), lowered cAMP levels. This decrease was dependent upon Ca(2+) influx and independent of phosphodiesterase activation. Immunoblot analysis revealed that AC5/6 and AC3 are expressed in parotid glands. Inhibition of calmodulin (CaM) kinase II with KN-62, or inclusion of the CaM inhibitor, calmidazolium, did not prevent agonist-induced inhibition of stimulated cAMP accumulation. In vitro studies revealed that Ca(2+), independently of CaM, inhibited isoproterenol-stimulated AC. Data suggest that agonist augmentation of stimulated cAMP levels is due to activation of AC8 in mouse parotid acini, and strongly support a role for AC5/6 in the inhibition of stimulated cAMP levels.

Calcium-stimulated adenylyl cyclase activity is critical for hippocampus-dependent long-term memory and late phase LTP

It is hypothesized that Ca2+ stimulation of calmodulin (CaM)-activated adenylyl cyclases (AC1 or AC8) generates cAMP signals critical for late phase LTP (L-LTP) and long-term memory (LTM). However, mice lacking either AC1 or AC8 exhibit normal L-LTP and LTM. Here, we report that mice lacking both enzymes (DKO) do not exhibit L-LTP or LTM. To determine if these defects are due to a loss of cAMP increases in the hippocampus, DKO mice were unilaterally cannulated to deliver forskolin. Administration of forskolin to area CA1 before training restored normal LTM. We conclude that Ca2+-stimulated adenylyl cyclase activity is essential for L-LTP and LTM and that AC1 or AC8 can produce the necessary cAMP signal.

Circadian regulation of cAMP response element-mediated gene expression in the suprachiasmatic nuclei

A program of stringently-regulated gene expression is thought to be a fundamental component of the circadian clock. Although recent work has implicated a role for E-box-dependent transcription in circadian rhythmicity, the contribution of other enhancer elements has yet to be assessed. Here, we report that cells of the suprachiasmatic nuclei (SCN) exhibit a prominent circadian oscillation in cAMP response element (CRE)-mediated gene expression. Maximal reporter gene expression occurred from late-subjective night to mid-subjective day. Cycling of CRE-dependent transcription was not observed in other brain regions, including the supraoptic nucleus and piriform cortex. Levels of the phospho-active form of the transcription factor CREB (P-CREB) varied as a function of circadian time. Peak P-CREB levels occurred during the mid- to late-subjective night. Furthermore, photic stimulation during the subjective night, but not during the subjective day, triggered a marked increase in CRE-mediated gene expression in the SCN. Reporter gene experiments showed that activation of the p44/42 mitogen-activated protein kinase signaling cascade is required for Ca2+-dependent stimulation of CRE-mediated transcription in the SCN. These findings reveal the CREB/CRE transcriptional pathway to be circadian-regulated within the SCN, and raise the possibility that this pathway provides signaling information essential for normal clock function.

Ca2+/calmodulin binds to and modulates P/Q-type calcium channels

Neurotransmitter release at many central synapses is initiated by an influx of calcium ions through P/Q-type calcium channels, which are densely localized in nerve terminals. Because neurotransmitter release is proportional to the fourth power of calcium concentration, regulation of its entry can profoundly influence neurotransmission. N- and P/Q-type calcium channels are inhibited by G proteins, and recent evidence indicates feedback regulation of P/Q-type channels by calcium. Although calcium-dependent inactivation of L-type channels is well documented, little is known about how calcium modulates P/Q-type channels. Here we report a calcium-dependent interaction between calmodulin and a novel site in the carboxy-terminal domain of the alpha1A subunit of P/Q-type channels. In the presence of low concentrations of intracellular calcium chelators, calcium influx through P/Q-type channels enhances channel inactivation, increases recovery from inactivation and produces a long-lasting facilitation of the calcium current. These effects are prevented by overexpression of a calmodulin-binding inhibitor peptide and by deletion of the calmodulin-binding domain. Our results reveal an unexpected association of Ca2+/calmodulin with P/Q-type calcium channels that may contribute to calcium-dependent synaptic plasticity.

Interactions between neurogranin and calmodulin in vivo

Neurogranin is a neural-specific, calmodulin (CaM)-binding protein that is phosphorylated by protein kinase C (PKC) within its IQ domain at serine 36. Since CaM binds to neurogranin through the IQ domain, PKC phosphorylation and CaM binding are mutually exclusive. Consequently, we hypothesize that neurogranin may function to concentrate CaM at specific sites in neurons and release free CaM in response to increased Ca2+ and PKC activation. However, it has not been established that neurogranin interacts with CaM in vivo. In this study, we examined this question using yeast two-hybrid methodology. We also searched for additional proteins that might interact with neurogranin by screening brain cDNA libraries. Our data illustrate that CaM binds to neurogranin in vivo and that CaM is the only neurogranin-interacting protein isolated from brain cDNA libraries. Single amino acid mutagenesis indicated that residues within the IQ domain are important for CaM binding to neurogranin in vivo. The Ile-33 --> Gln point mutant completely inhibited and Arg-38 --> Gln and Ser-36 --> Asp point mutants reduced neurogranin/CaM interactions. These data demonstrate that CaM is the major protein that interacts with neurogranin in vivo and support the hypothesis that phosphorylation of neurogranin at Ser-36 regulates its binding to CaM.

CRE-mediated gene transcription in neocortical neuronal plasticity during the developmental critical period

Neuronal activity-dependent processes are believed to mediate the formation of synaptic connections during neocortical development, but the underlying intracellular mechanisms are not known. In the visual system, altering the pattern of visually driven neuronal activity by monocular deprivation induces cortical synaptic rearrangement during a postnatal developmental window, the critical period. Here, using transgenic mice carrying a CRE-lacZ reporter, we demonstrate that a calcium- and cAMP-regulated signaling pathway is activated following monocular deprivation. We find that monocular deprivation leads to an induction of CRE-mediated lacZ expression in the visual cortex preceding the onset of physiologic plasticity, and this induction is dramatically downregulated following the end of the critical period. These results suggest that CRE-dependent coordinate regulation of a network of genes may control physiologic plasticity during postnatal neocortical development.

Regulation and immunohistochemical localization of betagamma-stimulated adenylyl cyclases in mouse hippocampus

Specific forms of synaptic plasticity such as long-term potentiation (LTP) are modulated by or require increases in cAMP. The various adenylyl cyclase isoforms possess unique regulatory properties, and thus cAMP increases in a given cell type or tissue in response to converging signals are subject to the properties of the adenylyl cyclase isoforms expressed. In most tissues, adenylyl cyclase activity is stimulated by neurotransmitters or hormones via stimulatory G-protein (Gs)-coupled receptors and is inhibited via inhibitory G-protein (Gi)-linked receptors. However, in the hippocampus, stimulation of Gi-coupled receptors potentiates Gs-stimulated cAMP levels. This effect may be associated with the regulatory properties of adenylyl cyclase types 2 and 4 (AC2 and AC4), isoforms that are potentiated by the betagamma subunit of Gi in vitro. Although AC2 has been shown to be stimulated by betagamma in whole cells, reports describing the sensitivity of AC4 to betagamma in vivo have yet to emerge. Our results demonstrate that Gs-mediated stimulation of AC4 is potentiated by betagamma released from activated Gi-coupled receptors in intact human embryonic kidney (HEK) 293 cells. Furthermore, we show that the AC2 and AC4 proteins are expressed in the mouse hippocampal formation and that they colocalize with MAP2, a dendritic and/or postsynaptic marker. The presence of AC2 and AC4 in the hippocampus and the ability of each of these enzymes to detect coincident activation of Gs- and Gi-coupled receptors suggest that they may play a crucial role in certain forms of synaptic plasticity by coordinating such overlapping synaptic inputs.

Light and circadian rhythmicity regulate MAP kinase activation in the suprachiasmatic nuclei

Although the circadian time-keeping properties of the suprachiasmatic nuclei (SCN) require gene expression, little is known about the signal transduction pathways that initiate transcription. Here we report that a brief exposure to light during the subjective night, but not during the subjective day, activates the p44/42 mitogen-activated protein kinase (MAPK) signaling cascade in the SCN. In addition, MAPK stimulation activates CREB (cAMP response element binding protein), indicating that potential downstream transcription factors are stimulated by the MAPK pathway in the SCN. We also observed striking circadian variations in MAPK activity within the SCN, suggesting that the MAPK cascade is involved in clock rhythmicity.

Stimulation of cAMP response element (CRE)-mediated transcription during contextual learning

Recent studies suggest that the CREB-CRE transcriptional pathway is pivotal in the formation of some types of long-term memory. However, it has not been demonstrated that stimuli that induce learning and memory activate CRE-mediated gene expression. To address this issue, we used a mouse strain transgenic for a CRE-lac Z reporter to examine the effects of hippocampus-dependent learning on CRE-mediated gene expression in the brain. Training for contextual conditioning or passive avoidance led to significant increases in CRE-dependent gene expression in areas CA1 and CA3 of the hippocampus. Auditory cue fear-conditioning, which is amygdala dependent, was associated with increased CRE-mediated gene expression in the amygdala, but not the hippocampus. These data demonstrate that learning in response to behavioral conditioning activates the CRE transcriptional pathway in specific areas of brain.

Stepwise transplantation of an active site loop between heat-labile enterotoxins LT-II and LT-I and characterization of the obtained hybrid toxins

Members of the cholera toxin family, including Escherichia coli heat-labile enterotoxins LT-I and LT-II, catalyze the covalent modification of intracellular proteins by transfer of ADP-ribose from NAD to a specific arginine of the target protein. The ADP-ribosylating activity of these toxins is located in the A-subunit, for which LT-I and LT-II share a 63% sequence identity. The flexible loop in LT-I, ranging from residue 47 to 56, closes over the active site cleft. Previous studies have shown that point mutations in this loop have dramatic effects on the activity of LT-I. Yet, in LT-II the sequence of the equivalent loop differs at four positions from LT-I. Therefore five mutants of the active site loop were created by a stepwise replacement of the loop sequence in LT-I with virtually all the corresponding residues in LT-II. Since we discovered that LT-II had no activity versus the artificial substrate diethylamino-benzylidine-aminoguanidine (DEABAG) while LT-I does, our active site mutants most likely probe the NAD binding, not the arginine binding region of the active site. The five hybrid toxins obtained (Q49A, F52N, V53T, Q49V/F52N and Q49V/F52N/V53T) show (i) great differences in holotoxin assembly efficiency; (ii) decreased cytotoxicity in Chinese hamster ovary cells; and (iii) increased in vitro enzymatic activity compared with wild type LT-I. Specifically, the three mutants containing the F52N substitution display a greater Vmax for NAD than wild type LT-I. The enzymatic activity of the V53T mutant is significantly higher than that of wild type LT-I. Apparently this subtle variation at position 53 is beneficial, in contrast to several other substitutions at position 53 which previously had been shown to be deleterious for activity. The most striking result of this study is that the active site loop of LT-I, despite great sensitivity for point mutations, can essentially be replaced by the active site loop of LT-II, yielding an active 'hybrid enzyme' as well as 'hybrid toxin'.

Cross talk between ERK and PKA is required for Ca2+ stimulation of CREB-dependent transcription and ERK nuclear translocation

Although Ca2+-stimulated cAMP response element binding protein- (CREB-) dependent transcription has been implicated in growth, differentiation, and neuroplasticity, mechanisms for Ca2+-activated transcription have not been defined. Here, we report that extracellular signal-related protein kinase (ERK) signaling is obligatory for Ca2+-stimulated transcription in PC12 cells and hippocampal neurons. The sequential activation of ERK and Rsk2 by Ca2+ leads to the phosphorylation and transactivation of CREB. Interestingly, the Ca2+-induced nuclear translocation of ERK and Rsk2 to the nucleus requires protein kinase A (PKA) activation. This may explain why PKA activity is required for Ca2+-stimulated CREB-dependent transcription. Furthermore, the full expression of the late phase of long-term potentiation (L-LTP) and L-LTP-associated CRE-mediated transcription requires ERK activation, suggesting that the activation of CREB by ERK plays a critical role in the formation of long lasting neuronal plasticity.

Phosphorylation and inhibition of olfactory adenylyl cyclase by CaM kinase II in Neurons: a mechanism for attenuation of olfactory signals

Acute desensitization of olfactory signaling is a critical property of the olfactory system that allows animals to detect and respond to odorants. Correspondingly, an important feature of odorant-stimulated cAMP increases is their transient nature, a phenomenon that may be attributable to the unique regulatory properties of the olfactory adenylyl cyclase (AC3). AC3 is stimulated by receptor activation and inhibited by Ca2+ through Ca2+/calmodulin kinase II (CaMKII) phosphorylation at Ser-1076. Since odorant-stimulated cAMP increases are accompanied by elevated intracellular Ca2+, CaMKII inhibition of AC3 may contribute to termination of olfactory signaling. To test this hypothesis, we generated a polyclonal antibody specific for AC3 phosphorylated at Ser-1076. A brief exposure of mouse olfactory cilia or primary olfactory neurons to odorants stimulated phosphorylation of AC3 at Ser-1076. This phosphorylation was blocked by inhibitors of CaMKII, which also ablated cAMP decreases associated with odorant-stimulated cAMP transients. These data define a novel mechanism for termination of olfactory signaling that may be important in olfactory responses.

Enhanced hippocampal CA1 LTP but normal spatial learning in inositol 1,4,5-trisphosphate 3-kinase(A)-deficient mice

To define the physiological role of IP(3)3-kinase(A) in vivo, we have generated a mouse strain with a null mutation of the IP(3)3-kinase(A) locus by gene targeting. Homozygous mutant mice were fully viable, fertile, apparently normal, and did not show any morphological anomaly in brain sections. In the mutant brain, the IP4 level was significantly decreased whereas the IP3 level did not change, demonstrating a major role of IP(3)3-kinase(A) in the generation of IP4. Nevertheless, no significant difference was detected in the hippocampal neuronal cells of the wild-type and the mutant mice in the kinetics of Ca2+ regulation after glutamate stimulation. Electrophysiological analyses carried out in hippocampal slices showed that the mutation significantly enhanced the LTP in the hippocampal CA1 region, but had no effect on the LTP in dentate gyrus (DG). No difference was noted, however, between the mutant and the wild-type mice in the Morris water maze task. Our results indicate that IP(3)3-kinase(A) may play an important role in the regulation of LTP in hippocampal CA1 region through the generation of IP4, but the enhanced LTP in the hippocampal CA1 does not affect spatial learning and memory.

Hippocampal neurotoxicity of Delta9-tetrahydrocannabinol

Marijuana consumption elicits diverse physiological and psychological effects in humans, including memory loss. Here we report that Delta9-tetrahydrocannabinol (THC), the major psychoactive component of marijuana, is toxic for hippocampal neurons. Treatment of cultured neurons or hippocampal slices with THC caused shrinkage of neuronal cell bodies and nuclei as well as genomic DNA strand breaks, hallmarks of neuronal apoptosis. Neuron death induced by THC was inhibited by nonsteroidal anti-inflammatory drugs, including indomethacin and aspirin, as well as vitamin E and other antioxidants. Furthermore, treatment of neurons with THC stimulated a significant increase in the release of arachidonic acid. We hypothesize that THC neurotoxicity is attributable to activation of the prostanoid synthesis pathway and generation of free radicals by cyclooxygenase. These data suggest that some of the memory deficits caused by cannabinoids may be caused by THC neurotoxicity.

Stimulation of type 1 and type 8 Ca2+/calmodulin-sensitive adenylyl cyclases by the Gs-coupled 5-hydroxytryptamine subtype 5-HT7A receptor

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) plays an important regulatory role in developing and adult nervous systems. With the exception of the 5-HT3 receptor, all of the cloned serotonin receptors belong to the G protein-coupled receptor superfamily. Subtypes 5-HT6 and 5-HT7 couple to stimulation of adenylyl cyclases through Gs and display high affinities for antipsychotic and antidepressant drugs. In the brain, mRNA for 5-HT6 is found at high levels in the hippocampus, striatum, and nucleus accumbens. 5-HT7 mRNA is most abundant in the hippocampus, neocortex, and hypothalamus. To better understand how serotonin might control cAMP levels in the brain, we coexpressed 5-HT6 or 5-HT7A receptors with specific isoforms of adenylyl cyclase in HEK 293 cells. The 5-HT6 receptor functioned as a typical Gs-coupled receptor in that it stimulated AC5, a Gs-sensitive adenylyl cyclase, but not AC1 or AC8, calmodulin (CaM)-stimulated adenylyl cyclases that are not activated by Gs-coupled receptors in vivo. Surprisingly, serotonin activation of 5-HT7A stimulated AC1 and AC8 by increasing intracellular Ca2+. 5-HT also increased intracellular Ca2+ in primary neuron cultures. These data define a novel mechanism for the regulation of intracellular cAMP by serotonin.

Loss of adenylyl cyclase I activity disrupts patterning of mouse somatosensory cortex

The somatosensory (SI) cortex of mice displays a patterned, nonuniform distribution of neurons in layer IV called the 'barrelfield' (ref. 1). Thalamocortical afferents (TCAs) that terminate in layer IV are segregated such that each barrel, a readily visible cylindrical array of neurons surrounding a cell-sparse center, represents a distinct receptive field. TCA arbors are confined to the barrel hollow and synapse on barrel-wall neurons whose dendrites are oriented toward the center of the barrel. Mice homozygous for the barrelless (brl) mutation, which occurred spontaneously in ICR stock at Université de Lausanne (Switzerland), fail to develop this patterned distribution of neurons, but still display normal topological organization of the SI cortex. Despite the absence of barrels and the overlapping zones of TCA arborization, the size of individual whisker representations, as judged by 2-deoxyglucose uptake, is similar to that of wild-type mice. We identified adenylyl cyclase type I (Adcy1) as the gene disrupted in brl mutant mice by fine mapping of proximal chromosome 11, enzyme assay, mutation analysis and examination of mice homozygous for a targeted disruption of Adcy1. These results provide the first evidence for involvement of cAMP signalling pathways in pattern formation of the brain.

Impaired cerebellar long-term potentiation in type I adenylyl cyclase mutant mice

Activation of adenylyl cyclase and the consequent production of cAMP is a process that has been shown to be central to invertebrate model systems of information storage. In the vertebrate brain, it has been suggested that a presynaptic cascade involving Ca influx, cAMP production, and subsequent activation of cAMP-dependent protein kinase is necessary for induction of long-term potentiation (LTP) at the cerebellar parallel fiber-Purkinje cell synapse. We have used mutant mice in which the major Ca-sensitive adenylyl cyclase isoform of cerebellar cortex (type I) is deleted to show that this results in an approximately 65% reduction in cerebellar Ca-sensitive cyclase activity and a nearly complete blockade of cerebellar LTP assessed using granule cell-Purkinje cell pairs in culture. This blockade is not accompanied by alterations in a number of basal electrophysiological parameters and may be bypassed by application of an exogenous cAMP analog, suggesting that it results specifically from deletion of the type I adenylyl cyclase.

Cloning, chromosomal mapping, and regulatory properties of the human type 9 adenylyl cyclase (ADCY9)

The type 9 adenylyl cyclase (AC9) is a widely distributed adenylyl cyclase that was originally cloned from a mouse cDNA library. Here we report the cloning, chromosomal mapping, and regulatory properties of human AC9 (HGMW-approved symbol ADCY9). Although the human AC9 sequence shows 86% homology with mouse AC9, divergence at the C2a/C2b junction results in an alternative C2b amino acid sequence. In situ hybridization localized the human AC9 gene to both human and mouse chromosomes 16. AC9 mRNA is present in all tissues examined, with the highest levels found in skeletal muscle, heart, and brain. To characterize the regulatory properties of human AC9 in vivo, the enzyme was expressed in HEK-293 cells. Human AC9 is stimulated by beta-adrenergic receptor activation but is insensitive to forskolin, Ca2+ and somatostatin. In contrast to mouse AC9, the activity of human AC9 is unaffected by inhibitors of calcineurin. These data emphasize the importance of determining the regulatory properties of human adenylyl cyclases.

Complement C1q inhibits cellular spreading and stimulates adenylyl cyclase activity of fibroblasts

C1q selectively localizes at injured tissues, where it may function as a regulator of cell-matrix interactions. We show here that purified C1q, added to the culture medium of human gingival fibroblasts (HF) spread onto fibronectin substrates, elicited a round morphology that was accompanied by altered F-actin and correlated with inhibition of cellular spreading. Shape modification required integrity of the molecule and was specific, dose dependent, nontoxic, and reversible. Antispreading activity was mediated, at least in part, by specific cell-surface C1q receptors. We hypothesized that ligand occupancy of C1q receptors could influence shape by affecting intracellular levels of cyclic AMP (cAMP). Within 20 min of exposure of adhering HF to C1q, we detected an increase in adenylyl cyclase activity (six- to ninefold) in cAMP accumulation (by 20%) and in cAMP-dependent protein kinase activity (by 20%). These changes suggested that the rounding effect of C1q may be associated with activation of the adenylyl cyclase pathway.

Type I adenylyl cyclase mutant mice have impaired mossy fiber long-term potentiation

Long-term potentiation (LTP) at the mossy fiber-->CA3 pyramidal cell synapse in the hippocampus is an NMDA-independent form of LTP that requires cAMP-dependent protein kinase (PKA) activity and can be induced by forskolin, a general activator of adenylyl cyclases. Presynaptic Ca2+ influx and elevated cAMP may be obligatory for mossy fiber LTP. Because the Ca2+-stimulated type 1 adenylyl cyclase (AC1) is expressed in the dentate gyrus and CA3 pyramidal cells, it is hypothesized that AC1 may be critical for mossy fiber LTP. To test this hypothesis, we examined several forms of hippocampal LTP in wild-type and AC1 mutant mice. Wild-type and AC1 mutant mice exhibited comparable perforant path LTP recorded in the dentate gyrus as well as decremental LTP at the Schaffer collateral-->CA1 pyramidal cell synapse. Although the mutant mice exhibited normal paired pulse facilitation, mossy fiber LTP was impaired significantly in AC1 mutants. High concentrations of forskolin induced mossy fiber LTP to comparable levels in wild-type and AC1 mutant mice, indicating that signaling components downstream from the adenylyl cyclase, including PKA, ion channels, and secretory machinery, were not affected by disruption of the AC1 gene. These data indicate that coupling of Ca2+ to activation of AC1 is crucial for mossy fiber LTP, most likely via activation of PKA and enhancement of excitatory amino acid secretion.

Capacitative Ca2+ entry is involved in cAMP synthesis in mouse parotid acini

Muscarinic receptor interaction leading to augmentation of isoproterenol-stimulated cAMP accumulation in mouse parotid acini involves Ca2+ (28). The effectiveness of capacitative Ca2+ entry and intracellular Ca2+ release on this response was determined in time course studies by using three independent tools to manipulate the free intracellular Ca2+ concentration: the muscarinic agonist carbachol, thapsigargin, and ionomycin. Time course studies revealed that Ca2+ release from intracellular stores by carbachol produced an early rapid increase (0.25-0.5 min) in stimulated cAMP levels, whereas capacitative Ca2+ entry resulted in a sustained increase in stimulated cAMP levels that was blocked by La3+. Capacitative Ca2+ entry, alone, was involved in thapsigargin and ionomycin augmentation of stimulated cAMP accumulation. The inability of phosphodiesterase inhibitors, 3-isobutyl-1-methylxanthine and milrinone, to prevent agonist augmentation of cAMP levels, as well as the finding that the type VIII adenylyl cyclase (ACVIII) is expressed in parotid acini, suggests that capacitative Ca2+ entry augments stimulated cAMP accumulation, at least in part, via activation of this adenylyl cyclase isoenzyme.

The prooncoprotein EWS binds calmodulin and is phosphorylated by protein kinase C through an IQ domain

A growing family of proteins is regulated by protein kinase C and calmodulin through IQ domains, a regulatory motif originally identified in neuromodulin (Alexander, K. A., Wakim, B. T., Doyle, G. S., Walsh, K. A., and Storm, D. R. (1988) J. Biol. Chem. 263, 7544-7549). Here we report that EWS, a nuclear RNA-binding prooncoprotein, contains an IQ domain, is phosphorylated by protein kinase C, and interacts with calmodulin. Interestingly, PKC phosphorylation of EWS inhibits its binding to RNA homopolymers, and conversely, RNA binding to EWS interferes with PKC phosphorylation. Several other RNA-binding proteins, including TLS/FUS and PSF, co-purify with EWS. PKC phosphorylation of these proteins also inhibits their binding to RNA in vitro. These data suggest that PKC may regulate interactions of EWS and other RNA-binding proteins with their RNA targets and that IQ domains may provide a regulatory link between Ca2+ signal transduction pathways and RNA processing.

Dynamic palmitoylation of neuromodulin (GAP-43) in cultured rat cerebellar neurons and mouse N1E-115 cells

We conducted pulse-chase and metabolic labeling experiments to determine directly whether palmitoylation of neuromodulin in neurons is dynamic, and if acylation is regulated. The rates of turnover of neuromodulin protein and associated palmitoyl groups were quantified using cultured cerebellar granule neurons and the neuronal cell line N1E-115. The half-life of [3H]palmitate bound to neuromodulin was approximately 5 h, whereas the half-life of the [35S]methionine-labeled neuromodulin was greater than 50 h. Metabolic and pulse-chase labeling experiments were carried out in the presence of various activators of cellular signaling pathways. Our data indicate that dynamic acylation and deacylation of neuromodulin in neurons are constitutive and are not regulated by G protein activation or other signals that control growth cone dynamics.

Sensitization of olfactory guanylyl cyclase to a specific imprinted odorant in coho salmon

The role of cGMP in olfactory signaling is not fully understood, but it is believed to play a modulatory role in intracellular signaling in vertebrate olfactory receptor neurons (ORNs). Here, we present evidence that cGMP in ORNs may play an important role in recognition of biologically relevant odors and olfactory learning. Specifically, we investigated the cellular mechanisms underlying olfactory imprinting in salmon. Salmon learn odors associated with their natal site as juveniles and later use these odors to guide their homing migration. This imprinting is believed to involve sensitization of the peripheral olfactory system to specific homestream odorants. We imprinted juvenile salmon to the odorant beta-phenylethyl alcohol (PEA) and examined the sensitivity of olfactory adenylyl and guanylyl cyclases to PEA during development. Stimulation of guanylyl cyclase activity by PEA was significantly greater in olfactory cilia isolated from PEA-imprinted salmon compared with PEA-naive fish only at the time of the homing migration, 2 years after PEA exposure. These results suggest that sensitization of olfactory guanylyl cyclase may play an important role in olfactory imprinting by salmon.

Antiproliferative activity of KCl contributes to EGF-induced neurite outgrowth in PC12 cells

Combinations of epidermal growth factor (EGF) and KCl stimulate differentiation in PC12 cells, independent of extracellular calcium [Mark et al., Stimulation of neurite outgrowth in PC12 cells by EGF and KCl depolarization: a Ca2+-independent phenomenon, J. Cell Biol., 130 (1995) 701-710]. Since EGF is a proliferative agent that normally does not stimulate differentiation of PC12 cells, we hypothesize that KCl plus EGF may cause differentiation because of the anti-proliferative activity of KCl. Here we report that treatment of PC12 cells with KCl plus EGF resulted in a significant decrease in proliferation and DNA synthesis compared with cells treated with EGF alone. In addition, KCl significantly reduced the EGF-induced expression of cell cycle progression factors cdk2, cdk4, cyclin B1 and PCNA. These data suggest that the anti-proliferative activity of KCl may convert EGF from a proliferative factor to a progression factor.

Coupling of epidermal growth factor (EGF) with the antiproliferative activity of cAMP induces neuronal differentiation

Nerve growth factor (NGF) functions as a progression factor with both mitogenic and antimitogenic activities. When PC12 cells are treated with NGF, they advance to the G1 stage of the cell cycle before they differentiate. The correlation between cessation of proliferation and differentiation suggests that the antimitotic activity of NGF may be obligatory for differentiation. Although epidermal growth factor- (EGF) and NGF-treated PC12 cells share several common properties, including activation of the mitogen-activated protein (MAP) kinase pathway and induction of immediate early genes, EGF is mitogenic for PC12 cells and does not normally stimulate differentiation. However, combinations of EGF and low levels of cAMP stimulate differentiation even though neither agent alone does (Mark, M. D., Liu, Y., Wong, S. T., Hinds, T. R., and Storm, D.R. (1995) J. Cell Biol. 130, 701-710). Since EGF is mitogenic for PC12 cells and differentiation may not occur until proliferation is inhibited, differentiation caused by cAMP and EGF may be due to the antiproliferative activity of cAMP. To test this hypothesis, we examined the effect of EGF or combinations of EGF and cAMP on PC12 cell proliferation. EGF alone stimulated proliferation of PC12 cells and increased the levels of several cell cycle progression factors including cdk2, cdk4, and cyclin B1. Cyclic AMP inhibited the EGF-stimulated increases in cell cycle progression factors as well as proliferation. Other antiproliferative agents including rapamycin, mimosine, and nitric oxide agonists also synergized with EGF to stimulate differentiation. These data indicate that the coupling of antiproliferative signals with EGF modifies the biological properties of EGF and converts it to a differentiating growth factor.

Calmodulin-regulated adenylyl cyclases and neuromodulation

Coincidence detection and crosstalk between signal transduction systems play very important regulatory roles in the nervous system, particularly in the regulation of transcription. Coupling of the Ca2+ and cAMP regulatory systems by calmodulin-regulated adenylyl cyclases is hypothesized to be important for some forms of synaptic plasticity, neuroendocrine function, and olfactory detection. Recent studies of a mutant mouse deficient in type I calmodulin-sensitive adenylyl cyclase have provided the first evidence that adenylyl cyclases are important for synaptic plasticity, as well as for learning and memory in vertebrates.

Differential regulation of type I and type VIII Ca2+-stimulated adenylyl cyclases by Gi-coupled receptors in vivo

Coupling of intracellular Ca2+ to cAMP increases may be important for some forms of synaptic plasticity. The type I adenylyl cyclase (I-AC) is a neural-specific, Ca2+-stimulated enzyme that couples intracellular Ca2+ to cAMP increases. Since optimal cAMP levels may be crucial for some types of synaptic plasticity, mechanisms for inhibition of Ca2+-stimulated adenylyl cyclases may also be important for neuroplasticity. Here we report that Ca2+ stimulation of I-AC is inhibited by activation of Gi-coupled somatostatin and dopamine D2L receptors. This inhibition is due primarily to Gialpha and not betagamma subunits since coexpression of betagamma-binding proteins with I-AC did not affect somatostatin inhibition. However, betagamma released from Gs did inhibit I-AC, indicating that the enzyme can be inhibited by betagamma in vivo. Interestingly, type VIII adenylyl cyclase (VIII-AC), another Ca2+-stimulated adenylyl cyclase, was not inhibited by Gi-coupled receptors. These data indicate that I-AC and VIII-AC are differentially regulated by Gi-coupled receptors and provide distinct mechanisms for interactions between the Ca2+ and cAMP signal transduction systems. We propose that I-AC may be particularly important for synaptic plasticity that depends upon rapid and transient cAMP increases, whereas VIII-AC may contribute to transcriptional-dependent synaptic plasticity that is dependent upon prolonged, Ca2+-stimulated cAMP increases.

Regulation of type I adenylyl cyclase by calmodulin kinase IV in vivo

Type I adenylyl cyclase is a neurospecific enzyme that is stimulated by Ca2+ and calmodulin (CaM). This enzyme couples the Ca2+ and cyclic AMP (cAMP) regulatory systems in neurons, and it may play an important role for some forms of synaptic plasticity. Mutant mice lacking type I adenylyl cyclase show deficiencies in spatial memory and altered long-term potentiation (Z. Wu, S. A. Thomas, Z. Xia, E. C. Villacres, R. D. Palmiter, and D. R. Storm, Proc. Natl. Acad. Sci. USA 92:220-224, 1995). Although type I adenylyl cyclase is synergistically stimulated by Ca2+ and G-protein-coupled receptors in vivo, very little is known about mechanisms for inhibition of the enzyme. Here, we report that type I adenylyl cyclase is inhibited by CaM kinase IV in vivo. Expression of constitutively active or wild-type CaM kinase IV inhibited Ca2+ stimulation of adenylyl cyclase activity without affecting basal or forskolin-stimulated activity. Type I adenylyl cyclase has two CaM kinase IV consensus phosphorylation sequences near its CaM binding domain at Ser-545 and Ser-552. Conversion of either serine to alanine by mutagenesis abolished CaM kinase IV inhibition of adenylyl cyclase. This suggests that the activity of this enzyme may be directly inhibited by CaM kinase IV phosphorylation. Type VIII adenylyl cyclase, another enzyme stimulated by CaM, was not inhibited by CaM kinase II or IV. We propose that CaM kinase IV may function as a negative feedback regulator of type I adenylyl cyclase and that CaM kinases may regulate cAMP levels in some cells.

Phosphorylation and inhibition of type III adenylyl cyclase by calmodulin-dependent protein kinase II in vivo

Inhibition of type III adenylyl cyclase (III-AC) by intracellular Ca2+ in vivo provides a mechanism for attenuation of hormone-stimulated cAMP signals in olfactory epithelium, heart, and other tissues (Wayman, G. A., Impey, S., and Storm, D. R. (1995) J. Biol. Chem. 270, 21480-21486). Although the mechanism for Ca2+ inhibition of III-AC in vivo has not been defined, inhibition is not mediated by Gi, cAMP-dependent protein kinase, or protein kinase C. However, Ca2+ inhibition of III-AC is antagonized by KN-62, a CaM-dependent kinase inhibitor. In addition, constitutively activated CaM kinase II inhibits the enzyme. These data suggest that CaM kinase II regulates the activity of III-AC by direct phosphorylation or by an indirect mechanism involving phosphorylation of a protein that inhibits III-AC. Here we report that III-AC is phosphorylated in vivo when intracellular Ca2+ is increased and that phosphorylation is prevented by CaM-dependent kinase inhibitors. Site-directed mutagenesis of a CaM kinase II consensus site (Ser-1076 to Ala-1076) in III-AC greatly reduced Ca2+-stimulated phosphorylation and inhibition of III-AC in vivo. These data support the hypothesis that Ca2+ inhibition of III-AC is due to direct phosphorylation of the enzyme by CaM kinase II in vivo.

Protein engineering studies of A-chain loop 47-56 of Escherichia coli heat-labile enterotoxin point to a prominent role of this loop for cytotoxicity

Heat-labile enterotoxin (LT), produced by enterotoxigenic Escherichia coli, is a close relative of cholera toxin (CT). These two toxins share approximately 80% sequence identity, and consists of one 240-residue A chain and five 103-residue B subunits. The B pentamer is responsible for GM1 receptor recognition, whereas the A subunit carries out an ADP-ribosylation of an arginine residue in the G protein, Gs alpha, in the epithelial target cell. This paper explores the importance of specific amino acids in loop 47-56 of the A subunit. This loop was observed to be highly mobile in the inactive R7K mutant of the A subunit. The position of the loop in wild-type protein is such that it might require considerable reorganization during substrate binding and is likely to have a crucial role in substrate binding. Five single-site substitutions have been made in the LT-A subunit 47-56 loop to investigate its possible role in the enzymatic activity and toxicity of LT and CT. The wild-type residues Thr-50 and Val-53 were replaced either by a glycine or by a proline. The glycine substitutions were intended to increase the mobility of this active-site loop, and the proline substitutions were intended to decrease the mobility of this same loop by restricting the accessible conformational space. Under the hypothesis that mobility of the loop is important for catalysis, the glycine-substitution mutants T50G and V53G would be expected to exhibit activity equal to or greater than that of the wild-type A subunit, while the proline substitution mutants T50P and T53P would be less active. Cytotoxicity assays showed, however, that all four of these mutants were considerably less active than wild-type LT. These results lend support for assignment of a prominent role to loop 47-56 in catalysis by LT and CT.

Induction of CRE-mediated gene expression by stimuli that generate long-lasting LTP in area CA1 of the hippocampus

Gene expression regulated by the cAMP response element (CRE) has been implicated in synaptic plasticity and long-term memory. It has been proposed that CRE-mediated gene expression is stimulated by signals that induce long-term potentiation (LTP). To test this hypothesis, we made mice transgenic for a CRE-regulated reporter construct. We focused on long-lasting long-term potentiation (L-LTP), because it depends on cAMP-dependent protein kinase activity (PKA) and de novo gene expression. CRE-mediated gene expression was markedly increased after L-LTP, but not after decremental UP (D-LTP). Furthermore, inhibitors of PKA blocked L-LTP and associated increases in CRE-mediated gene expression. These data demonstrate that the signaling required for the generation of L-LTP but not D-LTP is sufficient to stimulate CRE-mediated transcription in the hippocampus.

Hormone stimulation of type III adenylyl cyclase induces Ca2+ oscillations in HEK-293 cells

Various forms of cross-talk between the Ca2+ and cAMP signal transduction systems can occur in animal cells depending upon the types of adenylyl cyclases present. Here, we report that Ca2+ oscillations can be generated by hormone stimulation of type III adenylyl cyclase expressed in HEK-293 cells. These Ca2+ oscillations are apparently due to the unique regulatory features of type III adenylyl cyclase, which is stimulated by hormones and inhibited by elevated Ca2+ in vivo. Ca2+ oscillations were generated by glucagon, isoproterenol, or forskolin stimulation of type III adenylyl cyclase and were dependent upon the activity of cAMP- and calmodulin-dependent protein kinases. Ca2+ oscillations were not solely dependent upon cAMP increases since dibutyryl cAMP or (Sp)-cAMP did not stimulate Ca2+ oscillations. We hypothesize that stimulation of type III adenylyl cyclase leads to increased cAMP, activation of inositol 1,4,5-trisphosphate receptors, and elevation of intracellular Ca2+. As free Ca2+ increases, type III adenylyl cyclase activity is attenuated by CaM kinase(s) and intracellular cAMP levels decrease. When cAMP levels drop below a threshold level, the inositol 1,4,5-trisphosphate receptor is dephosphorylated and Ca2+ is resequestered. This cycle is repeated if type III adenylyl cyclase is chronically exposed to an activator. This unique mechanism for generation of Ca2+ oscillations in cells is distinct from others documented in the literature.

Ca2+ inhibition of type III adenylyl cyclase in vivo

Type III adenylyl cyclase is stimulated by beta-adrenergic agonists and glucagon in vitro and in vivo, but not by Ca2+ and calmodulin. However, the enzyme is stimulated by Ca2+ and calmodulin in vitro when it is concomitantly activated by the guanyl nucleotide stimulatory protein Gs (Choi, E. J., Xia, Z., and Storm, D. R. (1992a) Biochemistry 31, 6492-6498). Here, we examined regulation of type III adenylyl cyclase by Gs-coupled receptors and intracellular Ca2+ in vivo. Surprisingly, intracellular Ca2+ inhibited hormone-stimulated type III adenylyl cyclase activity. Submicromolar concentrations of intracellular free Ca2+, which stimulated type I adenylyl cyclase, inhibited glucagon- or isoproterenol-stimulated type III adenylyl cyclase. Inhibition of type III adenylyl cyclase by intracellular Ca2+ was not mediated by Gi, cAMP-dependent protein kinase, or protein kinase C. However, an inhibitor of CaM kinases antagonized Ca2+ inhibition of the enzyme, and coexpression of constitutively activated CaM kinase II completely inhibited isoproterenol-stimulated type III adenylyl cyclase activity. We propose that Ca2+ inhibition of type III adenylyl cyclase may serve as a regulatory mechanism to attenuate hormone-stimulated cAMP levels in some tissues.

Stimulation of neurite outgrowth in PC12 cells by EGF and KCl depolarization: a Ca(2+)-independent phenomenon

MAP kinase activity is necessary for growth factor induction of neurite outgrowth in PC12 cells. Although NGF and EGF both stimulate MAP kinase activity, EGF does not stimulate neurite extension. We report that EGF, in combination with KCl, stimulates neurite outgrowth in PC12 cells. This phenomenon was independent of intracellular Ca2+ increases and not due to enhancement of MAP kinase activity over that seen with EGF alone. However, EGF plus KCl increased intracellular cAMP, and other cAMP elevating agents acted synergistically with EGF to promote neurite outgrowth. Stimulation of neurite outgrowth by cAMP and EGF was blocked by inhibitors of transcription suggesting that synergistic regulation of transcription by the cAMP and MAP kinase pathways may stimulate neurite growth.