A protein kinase A-dependent molecular switch in synapsins regulates neurite outgrowth

Inhibition of neurotransmitter release by a nonphysiological target requires protein synthesis and involves cAMP-dependent and mitogen-activated protein kinases

During the development of neuronal circuits, axonal growth cones can contact many inappropriate targets before they reach an appropriate postsynaptic partner. Although it is well known that the contact with synaptic partners upregulates the secretory machinery of the presynaptic neuron, little is known about the signaling mechanisms involved in preventing the formation of connections with inappropriate target cells. Here, we show that the contact with a nonphysiological postsynaptic target inhibits neurotransmitter release from axonal terminals of the Helix serotonergic neuron C1 by means of an active mechanism requiring ongoing protein synthesis and leading to the inhibition of cAMP-dependent protein kinase (PKA) and mitogen-activated protein kinase (MAPK)-extracellular signal-related kinase (Erk) pathways. The reversal of the inhibitory effect of the nonphysiological target by blockade of protein synthesis was prevented by cAMP-PKA or MAPK-Erk inhibitors, whereas disinhibition of neurotransmitter release promoted by cAMP-PKA activation was not affected by MAPK-Erk inhibitors. The data indicate that the inhibitory effect of the nonphysiological target on neurotransmitter release is an active process that requires protein synthesis and involves the downregulation of the MAPK-Erk and cAMP-PKA pathways, the same protein kinases that are activated after contact with a physiological target neuron. These mechanisms could play a relevant role in the prevention of synapse formation between inappropriate partners by modulating the neurotransmitter release capability of growing nerve terminals according to the nature of the targets contacted during their development.

Involvement of cAMP in neuronal survival and axonal regeneration

In vitro, cAMP elevation alters neuronal responsiveness to diffusible growth factors and overcomes myelin-associated inhibitory molecules. Significant advances have been made recently in understanding the role of increases in cAMP in promoting axonal growth. Importantly, it has now been shown that cAMP elevation can promote axonal regeneration and functional recovery after central nervous system injury. Elevation of cAMP can be achieved via either direct application of cAMP analogs or an inhibitor of the enzyme phosphodiesterase that degrades cAMP in vivo. Current information points to a number of protein kinase A-mediated pathways (mitogen-activated protein kinase/extracellular signal-regulated kinase and phosphatidylinositol 3-kinase/akt pathway activation and Rho inactivation) underlying cAMP elevation-induced neuronal survival and axonal regeneration.

Inhibition of Calcium/Calmodulin-dependent Protein Kinase Kinase by Protein 14-3-3

Intracellular calcium concentrations regulate diverse cellular events including cytoskeletal dynamics, gene transcription, and synaptic plasticity. The calcium signal is transduced in part by the calcium/calmodulin-dependent protein kinase (CaMK) cascade that is comprised of CaMK kinase (CaMKK) and its primary downstream substrates, CaMKI and CaMKIV. The CaMK cascade also participates in cross-talk with other signaling pathways: CaMKK/CaMKI can activate the mitogen-activated protein kinase pathway and cAMP-dependent protein kinase (PKA) can directly phosphorylate two inhibitory sites (Thr108 and Ser458) in CaMKK. Here we report an additional PKA-dependent regulation of CaMKK through its interaction with protein 14-3-3. CaMKK and 14-3-3 co-immunoprecipitated from co-transfected heterologous cells as well as from rat brain homogenate, and site-directed mutagenesis studies identified phospho-Ser74 in CaMKK as the primary 14-3-3 binding site. In cultured rat hippocampal neurons and acute hippocampal slices this interaction was robustly stimulated by activation of PKA through forskolin treatment and was blocked by inhibition of PKA. Interaction of 14-3-3 with CaMKK had two regulatory consequences in vitro. It directly inhibited CaMKK activity, and it also blocked dephosphorylation of Thr108, an inhibitory PKA phosphorylation site. In human embryonic kidney 293 cells transfected with CaMKK and stimulated with forskolin, co-transfection with 14-3-3 prevented dephosphorylation of Thr108 to the same extent as did inhibition of protein phosphatases with okadaic acid. We conclude that binding of 14-3-3 to CaMKK stabilizes its inhibition by PKA-mediated phosphorylation, which may have important consequences in the regulation of CaMKI, CaMKIV, protein kinase B, and ERK signaling pathways.

Phosphorylation by cAMP-dependent protein kinase is essential for synapsin-induced enhancement of neurotransmitter release in invertebrate neurons

Synapsins are synaptic vesicle-associated phosphoproteins involved in the regulation of neurotransmitter release and synapse formation; they are substrates for multiple protein kinases that phosphorylate them on distinct sites. We have previously found that injection of synapsin into Helix snail neurons cultured under low-release conditions increases the efficiency of neurotransmitter release. In order to investigate the role of phosphorylation in this modulatory action of synapsins, we examined the substrate properties of the snail synapsin orthologue recently cloned in Aplysia (apSyn) for various protein kinases and compared the effects of the intracellular injection of wild-type apSyn with those of its phosphorylation site mutants. ApSyn was found to be an excellent in vitro substrate for cAMP-dependent protein kinase, which phosphorylated it at high stoichiometry on a single site (Ser-9) in the highly conserved domain A, unlike the other kinases reported to phosphorylate mammalian synapsins, which phosphorylated apSyn to a much lesser extent. The functional effect of apSyn phosphorylation by cAMP-dependent protein kinase on neurotransmitter release was studied by injecting wild-type or Ser-9 mutated apSyn into the soma of Helix serotonergic C1 neurons cultured under low-release conditions, i.e. in contact with the non-physiological target neuron C3. In this model of impaired neurotransmitter release, the injection of wild-type apSyn induced a significant enhancement of release. This enhancement was virtually absent after injection of the non-phosphorylatable mutant (Ser-9→Ala), but it was maintained after injection of the pseudophosphorylated mutant (Ser-9→Asp). These functional effects of apSyn injection were paralleled by marked ultrastructural changes in the C1 neuron, with the formation of extensive interdigitations of neurite-like processes containing an increased complement of C1 dense core vesicles at the sites of cell-to-cell contact. This structural rearrangement was virtually absent in mock-injected C1 neurons or after injection of the non-phosphorylatable apSyn mutant. These data indicate that phosphorylation of synapsin domain A is essential for the synapsin-induced enhancement of neurotransmitter release and suggest that endogenous kinases phosphorylating this domain play a central role in the regulation of the efficiency of the exocytotic machinery.

Molecular determinants of synapsin targeting to presynaptic terminals

Although synapsins are abundant synaptic vesicle proteins that are widely used as markers of presynaptic terminals, the mechanisms that target synapsins to presynaptic terminals have not been elucidated. We have addressed this question by imaging the targeting of green fluorescent protein-tagged synapsins in cultured hippocampal neurons. Whereas all synapsin isoforms targeted robustly to presynaptic terminals in wild-type neurons, synapsin Ib scarcely targeted in neurons in which all synapsins were knocked-out. Coexpression of other synapsin isoforms significantly strengthened the targeting of synapsin Ib in knock-out neurons, indicating that heterodimerization is required for synapsin Ib to target. Truncation mutagenesis revealed that synapsin Ia targets via distributed binding sites that include domains B, C, and E. Although domain A was not necessary for targeting, its presence enhanced targeting. Domain D inhibited targeting, but this inhibition was overcome by domain E. Thus, multiple intermolecular and intramolecular interactions are required for synapsins to target to presynaptic terminals.

Cyclic AMP-induced repair of zebrafish spinal circuits

Neurons in the human central nervous system (CNS) are unable to regenerate, as a result of both an inhibitory environment and their inherent inability to regrow. In contrast, the CNS environment in fish is permissive for growth, yet some neurons still cannot regenerate. Fish thus offer an opportunity to study molecules that might surmount the intrinsic limitations they share with mammals, without the complication of an inhibitory environment. We show by in vivo imaging in zebrafish that post-injury application of cyclic adenosine monophosphate can transform severed CNS neurons into ones that regenerate and restore function, thus overcoming intrinsic limitations to regeneration in a vertebrate.

Regulation of axonal extension and growth cone motility by calmodulin-dependent protein kinase I

Calcium and calmodulin (CaM) are important signaling molecules that regulate axonal or dendritic extension and branching. The Ca2+-dependent stimulation of neurite elongation has generally been assumed to be mediated by CaM-kinase II (CaMKII), although other members of the CaMK family are highly expressed in developing neurons. We have examined this assumption using a combination of dominant-negative CaMKs (dnCaMKs) and other specific CaMK inhibitors. Here we report that inhibition of cytosolic CaMKI, but not CaMKII or nuclear CaMKIV, dramatically decreases axonal outgrowth and branching in cultured neonatal hippocampal and postnatal cerebellar granule neurons. CaMKI is found throughout the cell cytosol, including the growth cone. Growth cones of neurons expressing dnCaMI or dnCaMKK, the upstream activator of CaMKI, exhibit collapsed morphology with a prominent reduction in lamellipodia. Live-cell imaging confirms that these morphological changes are associated with a dramatic decrease in growth cone motility. Treatment of neurons with 1,8-naphthoylene benzimidazole-3-carboxylic acid (STO-609), an inhibitor of CaMKK, causes a similar change in morphology and reduction in growth cone motility, and this inhibition can be rescued by transfection with an STO-609-insensitive mutant of CaMKK or by transfection with constitutively active CaMKI. These results identify CaMKI as a positive transducer of growth cone motility and axon outgrowth and provide a new physiological role for the CaMKK-CaMKI pathway.

Regulation of axonal growth and guidance by the neurotrophin family of neurotrophic factors

1. The neurotrophins play an important role during development to stimulate and guide axonal growth for the establishment of a correctly wired and functional neural system. Neurotrophins can also regulate adult nervous system plasticity by promoting neuronal survival and stimulating nerve regrowth following injury. 2. Therefore, the potential exists for these neurotrophic factors to be used as therapeutic agents for the treatment of neurodegenerative disorders. However, in order to realize the full capacity of neurotrophic factors as therapeutic agents, it is important to understand the mechanisms by which they elicit their survival and regenerative effects. 3. The present paper reviews some of the ways in which neurotrophins regulate axonal growth and guidance.

The two regulatory subunits of aplysia cAMP-dependent protein kinase mediate distinct functions in producing synaptic plasticity

Activation of the cAMP-dependent protein kinase (PKA) is critical for both short- and long-term facilitation in Aplysia sensory neurons. There are two types of the kinase, I and II, differing in their regulatory (R) subunits. We cloned Aplysia RII; RI was cloned previously. Type I PKA is mostly soluble in the cell body whereas type II is enriched at nerve endings where it is bound to two prominent A kinase-anchoring-proteins (AKAPs). Disruption of the binding of RII to AKAPs by Ht31, an inhibitory peptide derived from a human thyroid AKAP, prevents both the short- and the long-term facilitation produced by serotonin (5-HT). During long-term facilitation, RII is transcriptionally upregulated; in contrast, the amount of RI subunits decreases, and previous studies have indicated that the decrease is through ubiquitin-proteosome-mediated proteolysis. Experiments with antisense oligonucleotides injected into the sensory neuron cell body show that the increase in RII protein is essential for the production of long-term facilitation. Using synaptosomes, we found that 5-HT treatment causes RII protein to increase at nerve endings. In addition, using reverse transcription-PCR, we found that RII mRNA is transported from the cell body to nerve terminals. Our results suggest that type I operates in the nucleus to maintain cAMP response element-binding protein-dependent gene expression, and type II PKA acts at sensory neuron synapses phosphorylating proteins to enhance release of neurotransmitter. Thus, the two types of the kinase have distinct but complementary functions in the production of facilitation at synapses of an identified neuron.

The great escape; phosphorylation of Ena/VASP by PKA promotes filopodial formation

The Ena/VASP family of proteins consists of adaptor molecules that localize to subcellular sites of actin polymerization. The role of Ena/VASP proteins in the regulation of cell motility and axon outgrowth has been controversial. Recently, these proteins have been proposed to function as "anticapping" factors, which may have differential effects on filopodial versus lammelipodial actin-based protrusions. A study by Lebrand et al. in this issue of Neuron supports this model and identifies PKA as a key regulator of Ena/VASP function downstream of the chemoattractant Netrin.

Induction of Cβ splice variants and formation of novel forms of protein kinase A type II holoenzymes during retinoic acid-induced differentiation of human NT2 cells

Cyclic AMP (cAMP) and cAMP-dependent protein kinase (PKA) are critical regulators of neuronal differentiation. The expression, levels and activities of PKA subunits were studied prior to and during differentiation of the human neuronal precursor cell line NTera 2 (NT2). Undifferentiated NT2 cells expressed mainly cytoplasmic PKA type I, consisting of the regulatory subunit RIalpha and the catalytic subunit Calpha. Low levels of PKA type II consisting of RIIalpha or RIIbeta associated with Calpha were also detected, mainly in the cytoplasm and in the Golgi-centrosomal area. During retinoic acid-induced differentiation, the RIalpha and RIIalpha expressions remained in the cytoplasm, while we observed a strong upregulation of RIIbeta, located to the whole cytoplasm including neurite extensions. This upregulation coincided with increased PKA-specific activity accompanied by a strong induction of a number of neuronal-specific Cbeta splice variants that together with RIIbeta form novel PKAII holoenzymes. Formation of novel PKAII holoenzymes may imply specific PKA features which may have consequences for the process of neuronal differentiation and nerve cell function.

A rare polymorphism affects a mitogen-activated protein kinase site in synapsin III: possible relationship to schizophrenia

BACKGROUND

Synapsin III plays a role in neuronal plasticity and maps to chromosome 22q12-13, a region suggested to be linked to schizophrenia. To determine if synapsin III plays a role in this disease, we searched for polymorphisms in this gene in patients with schizophrenia and controls.

METHODS

The synapsin III gene was initially sequenced from 10 individuals with schizophrenia to identify polymorphisms. Association analysis was then performed using 118 individuals with schizophrenia and 330 population controls. Synapsin III expression was studied by immunoblot analyses, and phosphorylation sites were mapped by sequencing trypsin-digested synapsin III fragments phosphorylated with phosphorus-32.

RESULTS

A rare, missense polymorphism, S470N, was identified in the synapsin III gene and appeared more frequently in individuals with schizophrenia than in controls (p =.0048). The site affected by the polymorphism, Ser470, was determined to be a substrate for mitogen-activated protein kinase, a downstream effector of neurotrophin action. Phosphorylation at Ser470 was increased during neonatal development and in response to neurotrophin-3 in cultured hippocampal neurons.

CONCLUSIONS

Our observations suggest an association of a rare polymorphism in synapsin III with schizophrenia, but further studies will be required to clarify its role in this disease.

Cyclic phosphatidic acid elicits neurotrophin-like actions in embryonic hippocampal neurons

Cyclic phosphatidic acid (cPA; 1-acyl-sn-glycerol-2,3-cyclic phosphate) is an analog of the growth factor-like phospholipid mediator lysophosphatidic acid (LPA). As brain tissue is the richest source of cPA we tested its effects on hippocampal neurons from day 16/17 embryonic rat cultured in a serum-free medium. Nanomolar concentrations of cPA elicited a neurotrophic effect and promoted neurite outgrowth that exceeded that of 50 ng/mL nerve growth factor (NGF). Pertussis toxin, the LPA1/LPA3 receptor-selective antagonist dioctylglycerol pyrophosphate, the myristoylated inhibitory pseudosubstrate peptide of protein kinase A (PKI), Wortmannin and PD98059 abolished the neurite-promoting effect. cPA elicited a sustained activation of extracellular signal-related kinases (ERK) 1/2 and Akt. Clostridium difficile toxin B, an inhibitor of the Rho family of GTPases, reduced cPA-induced enhancement of neurite outgrowth. In B5P cells, a clonal cell line of PC12 cells overexpressing tyrosine kinase NGF receptor (TrkA), cPA elicited transphosphorylation of TrkA. cPA-elicited ERK activation was blocked by K252a and PKI. These results suggest that cPA mimics the effects of, and activates signaling pathways similar to, the neurotrophin NGF in cultured embryonic hippocampal neurons and B5P cells.

Phosphorylation influences the translation state of FMRP-associated polyribosomes

Fragile X mental retardation protein, FMRP, is absent in patients with fragile X syndrome, a common form of mental retardation. FMRP is a nucleocytoplasmic RNA binding protein that is primarily associated with polyribosomes. FMRP is believed to be a translational repressor and may regulate the translation of certain mRNAs at the base of dendritic spines in neurons. However, little is known about the regulation of FMRP. Using mass spectrometry and site-directed mutagenesis, we show that FMRP is phosphorylated between residues 483 and 521, N-terminal to the RGG box, both in murine brain and in cultured cells. Primary phosphorylation occurs on the highly conserved serine 499, which triggers hierarchical phosphorylation of nearby serines. FMRP is phosphorylated within 2-4 h of synthesis, however, phosphorylation has no effect on the half-life of the protein. In contrast to the Drosophila ortholog dFxr, the phosphorylation status of mammalian FMRP does not influence its association with specific mRNAs in vivo. However, we find unphosphorylated FMRP associated with actively translating polyribosomes while a fraction of phosphorylated FMRP is associated with apparently stalled polyribosomes. Our data suggest that the phosphorylation may regulate FMRP and that the release of FMRP-induced translational suppression may involve a dephosphorylation signal.

Phosphorylation-regulated inhibition of the Gz GTPase-activating protein activity of RGS proteins by synapsin I

Synapsins are neuronal proteins that bind and cluster synaptic vesicles in the presynaptic space, presumably by anchoring to actin filaments, but specific regulatory functions of the synapsins are unknown. We found that a sub-population of brain synapsin Ia, a splice variant of one of three synapsin isoforms, inhibits the GTPase-activating protein (GAP) activity of several RGS proteins. Inhibition is highly selective for Galphaz, a member of the Gi family that is found in neurons, platelets, adrenal chromaffin cells, and a few other neurosecretory cells. Gz has been indirectly implicated in the regulation of secretion. Synapsin Ia constitutes a major fraction of the total GAP-inhibitory activity in brain, and its inhibitory activity is absent from the brains of synapsin I(-/-)/II(-/-) mice. Inhibition depends on the cationic D/E domain of synapsin. Phosphorylation of synapsin Ia at serine 9 by either cyclic AMP-dependent protein kinase or p21-activated protein kinase (PAK1) attenuates its potency as a GAP inhibitor more than 7-fold. Synapsin can thus act as a phosphorylation-modulated mediator of feedback regulation of Gz signaling by the synaptic machinery.

Promotion of axonal regeneration in the injured CNS

Molecules that are found in the extracellular environment at a CNS lesion site, or that are associated with myelin, inhibit axon growth. In addition, neuronal changes--such as an age-dependent reduction in concentrations of cyclic AMP--render the neuron less able to respond to axotomy by a rapid, forward, actin-dependent movement. An alternative mechanism, based on the protrusive forces generated by microtubule elongation or the anterograde transport of cytoskeletal elements, may underlie a slower form of axon elongation that happens during regeneration in the mature CNS. Therapeutic approaches that restore the extracellular CNS environment or the neuron's characteristics back to a more embryonic state increase axon regeneration and improve functional recovery after injury. These advances in the understanding of regeneration in the CNS have major implications for neurorehabilitation and for the use of axonal regeneration as a therapeutic approach to disorders of the CNS such as spinal-cord injury.