Subcellular distribution in cerebral cortex of two proteins phosphorylated by a cAMP-dependent protein kinase

Uptake of β-N-methylamino-L-alanine (BMAA) into glutamate-specific synaptic vesicles: Exploring the validity of the excitotoxicity mechanism of BMAA

The first mechanism of toxicity proposed for the cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) was excitotoxicity, and this was supported by numerous in vitro studies in which overactivation of both ionotropic and metabotropic glutamate receptors was reported. However, the excitotoxicity of BMAA is weak in comparison with other known excitotoxins and on par with that of glutamate, implying that to achieve sufficient synaptic concentrations of BMAA to cause classical in vivo excitotoxicity, BMAA must either accumulate in synapses to allow persistent glutamate receptor activation or it must be released in sufficiently high concentrations into synapses to cause the overexcitation. Since it has been shown that BMAA can be readily removed from synapses, release of high concentrations of BMAA into synapses must be shown to confirm its role as an excitotoxin in in vivo systems. This study therefore sought to evaluate the uptake of BMAA into synaptic vesicles and to determine if BMAA affects the uptake of glutamate into synaptic vesicles. There was no evidence to support uptake of BMAA into glutamate-specific synaptic vesicles but there was some indication that BMAA may affect the uptake of glutamate into synaptic vesicles. The uptake of BMAA into synaptic vesicles isolated from areas other than the cerebral cortex should be investigated before definite conclusions can be drawn about the role of BMAA as an excitotoxin.

cAMP-regulated phosphoproteins DARPP-32, ARPP16/19, and RCS modulate striatal signal transduction through protein kinases and phosphatases

Decades of research led by Paul Greengard identified protein phosphorylation as a ubiquitous and vital post-translational modification involved in many neuronal signaling pathways. In particular, his discovery that second messenger-regulated protein phosphorylation plays a central role in the propagation and transduction of signals in the nervous system has been essential in understanding the molecular mechanisms of neuronal communication. The establishment of dopamine (DA) as an essential neurotransmitter in the central nervous system, combined with observations that DA activates G-protein-coupled receptors to control the production of cyclic adenosine monophosphate (cAMP) in postsynaptic neurons, has provided fundamental insight into the regulation of neurotransmission. Notably, DA signaling in the striatum is involved in many neurological functions such as control of locomotion, reward, addiction, and learning, among others. This review focuses on the history, characterization, and function of cAMP-mediated regulation of serine/threonine protein phosphatases and their role in DA-mediated signaling in striatal neurons. Several small, heat- and acid-stable proteins, including DARPP-32, RCS, and ARPP-16/19, were discovered by the Greengard laboratory to be regulated by DA- and cAMP signaling, and found to undergo a complex but coordinated sequence of phosphorylation and dephosphorylation events. These studies have contributed significantly to the establishment of protein phosphorylation as a ubiquitous and vital process in signal propagation in neurons, paradigm shifting discoveries at the time. Understanding DA-mediated signaling in the context of signal propagation has led to numerous insights into human conditions and the development of treatments and therapies.

JIP3 localises to exocytic vesicles and focal adhesions in the growth cones of differentiated PC12 cells

The JNK-interacting protein 3 (JIP3) is a molecular scaffold, expressed predominantly in neurons, that serves to coordinate the activation of the c-Jun N-terminal kinase (JNK) by binding to JNK and the upstream kinases involved in its activation. The JNK pathway is involved in the regulation of many cellular processes including the control of cell survival, cell death and differentiation. JIP3 also associates with microtubule motor proteins such as kinesin and dynein and is likely an adapter protein involved in the tethering of vesicular cargoes to the motors involved in axonal transport in neurons. We have used immunofluorescence microscopy and biochemical fractionation to investigate the subcellular distribution of JIP3 in relation to JNK and to vesicular and organelle markers in rat pheochromocytoma cells (PC12) differentiating in response to nerve growth factor. In differentiated PC12 cells, JIP3 was seen to accumulate in growth cones at the tips of developing neurites where it co-localised with both JNK and the JNK substrate paxillin. Cellular fractionation of PC12 cells showed that JIP3 was associated with a subpopulation of vesicles in the microsomal fraction, distinct from synaptic vesicles, likely to be an anterograde-directed exocytic vesicle pool. In differentiated PC12 cells, JIP3 did not appear to associate with retrograde endosomal vesicles thought to be involved in signalling axonal injury. Together, these observations indicate that JIP3 may be involved in transporting vesicular cargoes to the growth cones of PC12 cells, possibly targeting JNK to its substrate paxillin, and thus facilitating neurite outgrowth.

NDE1 and GSK3β Associate with TRAK1 and Regulate Axonal Mitochondrial Motility: Identification of Cyclic AMP as a Novel Modulator of Axonal Mitochondrial Trafficking

Mitochondria are essential for neuronal function, providing the energy required to power neurotransmission, and fulfilling many important additional roles. In neurons, mitochondria must be efficiently transported to sites, including synapses, where their functions are required. Neurons, with their highly elongated morphology, are consequently extremely sensitive to defective mitochondrial trafficking which can lead to neuronal ill-health/death. We recently demonstrated that DISC1 associates with mitochondrial trafficking complexes where it associates with the core kinesin and dynein adaptor molecule TRAK1. We now show that the DISC1 interactors NDE1 and GSK3β also associate robustly with TRAK1 and demonstrate that NDE1 promotes retrograde axonal mitochondrial movement. GSK3β is known to modulate axonal mitochondrial motility, although reports of its actual effect are conflicting. We show that, in our system, GSK3β promotes anterograde mitochondrial transport. Finally, we investigated the influence of cAMP elevation upon mitochondrial motility, and found a striking increase in mitochondrial motility and retrograde movement. DISC1, NDE1, and GSK3β are implicated as risk factors for major mental illness. Our demonstration that they function together within mitochondrial trafficking complexes suggests that defective mitochondrial transport may be a contributory disease mechanism in some cases of psychiatric disorder.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

Lewy body extracts from Parkinson disease brains trigger α-synuclein pathology and neurodegeneration in mice and monkeys

OBJECTIVE

Mounting evidence suggests that α-synuclein, a major protein component of Lewy bodies (LB), may be responsible for initiating and spreading the pathological process in Parkinson disease (PD). Supporting this concept, intracerebral inoculation of synthetic recombinant α-synuclein fibrils can trigger α-synuclein pathology in mice. However, it remains uncertain whether the pathogenic effects of recombinant synthetic α-synuclein may apply to PD-linked pathological α-synuclein and occur in species closer to humans.

METHODS

Nigral LB-enriched fractions containing pathological α-synuclein were purified from postmortem PD brains by sucrose gradient fractionation and subsequently inoculated into the substantia nigra or striatum of wild-type mice and macaque monkeys. Control animals received non-LB fractions containing soluble α-synuclein derived from the same nigral PD tissue.

RESULTS

In both mice and monkeys, intranigral or intrastriatal inoculations of PD-derived LB extracts resulted in progressive nigrostriatal neurodegeneration starting at striatal dopaminergic terminals. No neurodegeneration was observed in animals receiving non-LB fractions from the same patients. In LB-injected animals, exogenous human α-synuclein was quickly internalized within host neurons and triggered the pathological conversion of endogenous α-synuclein. At the onset of LB-induced degeneration, host pathological α-synuclein diffusely accumulated within nigral neurons and anatomically interconnected regions, both anterogradely and retrogradely. LB-induced pathogenic effects required both human α-synuclein present in LB extracts and host expression of α-synuclein.

INTERPRETATION

α-Synuclein species contained in PD-derived LB are pathogenic and have the capacity to initiate a PD-like pathological process, including intracellular and presynaptic accumulations of pathological α-synuclein in different brain areas and slowly progressive axon-initiated dopaminergic nigrostriatal neurodegeneration.

Receptor protein tyrosine phosphatase sigma regulates synapse structure, function and plasticity

The mechanisms that regulate synapse formation and maintenance are incompletely understood. In particular, relatively few inhibitors of synapse formation have been identified. Receptor protein tyrosine phosphatase σ (RPTPσ), a transmembrane tyrosine phosphatase, is widely expressed by neurons in developing and mature mammalian brain, and functions as a receptor for chondroitin sulfate proteoglycans that inhibits axon regeneration following injury. In this study, we address RPTPσ function in the mature brain. We demonstrate increased axon collateral branching in the hippocampus of RPTPσ null mice during normal aging or following chemically induced seizure, indicating that RPTPσ maintains neural circuitry by inhibiting axonal branching. Previous studies demonstrated a role for pre-synaptic RPTPσ promoting synaptic differentiation during development; however, subcellular fractionation revealed enrichment of RPTPσ in post-synaptic densities. We report that neurons lacking RPTPσ have an increased density of pre-synaptic varicosities in vitro and increased dendritic spine density and length in vivo. RPTPσ knockouts exhibit an increased frequency of miniature excitatory post-synaptic currents, and greater paired-pulse facilitation, consistent with increased synapse density but reduced synaptic efficiency. Furthermore, RPTPσ nulls exhibit reduced long-term potentiation and enhanced novel object recognition memory. We conclude that RPTPσ limits synapse number and regulates synapse structure and function in the mature CNS.

Synaptic vesicles are capable of synthesizing the VGLUT substrate glutamate from α-ketoglutarate for vesicular loading

Synaptic vesicle loading of glutamate is a pivotal step in glutamate synaptic transmission. The molecular machinery responsible for this step is comprised of v-type proton-pump ATPase and a vesicular glutamate transporter. Recent evidence indicates that synaptic vesicles are endowed with glycolytic ATP-synthesizing enzymes, providing energy for immediate use by vesicle-bound proton-pump ATPase. In this study, we provide evidence that synaptic vesicles are also capable of synthesizing the vesicular glutamate transporter substrate glutamate, from α-ketoglutarate and l-aspartate (as the amino group donor); glutamate thus produced is taken up into vesicles. We also report a finding that α-ketoglutarate-derived glutamate uptake into synaptic vesicles and aspartate aminotransferase are inhibited by 2,3-pyrazinedicarboxylate. Evidence is given that this is a selective inhibitor for aspartate aminotransferase. These observations provide insight into understanding the nerve endings' mechanism for high efficiency in glutamate transmission. Finding this inhibitor may have implications for further experimentation on the role of α-ketoglutarate-derived glutamate in glutamate transmission.

Identification of NIPSNAP1 as a nocistatin-interacting protein involving pain transmission

4-Nitrophenylphosphatase domain and non-neuronal SNAP25-like protein homolog 1 (NIPSNAP1) is a molecule of physiologically unknown function, although it is predominantly expressed in the brain, spinal cord, liver, and kidney. We identified NIPSNAP1 as a protein that interacts with the neuropeptide nocistatin (NST) from synaptosomal membranes of mouse spinal cord using high-performance affinity latex beads. NST, which is produced from the same precursor protein as an opioid-like neuropeptide nociceptin/orphanin FQ (N/OFQ), has opposite effects on pain transmission evoked by N/OFQ. The calculated full-length pre-protein of NIPSNAP1 was 33 kDa, whereas the N-terminal truncated form of NIPSNAP1 (29 kDa) was ubiquitously expressed in the neuronal tissues, especially in synaptic membrane and mitochondria of brain. The 29-kDa NIPSNAP1 was distributed on the cell surface, and NST interacted with the 29-kDa but not the 33-kDa NIPSNAP1. Although intrathecal injection of N/OFQ induced tactile allodynia in both wild-type and NIPSNAP1-deficient mice, the inhibition of N/OFQ-evoked tactile allodynia by NST seen in wild-type mice was completely lacking in the deficient mice. These results suggest that NIPSNAP1 is an interacting molecule of NST and plays a crucial role in pain transmission.

Synapse adhesion: a dynamic equilibrium conferring stability and flexibility

Cell adhesion molecules (CAMs) linked to cytoskeleton generate stable cell-cell junctions. Cadherins provide a canonical example, but paradoxically, they participate in a multitude of transient and regulatable interactions. Their extracellular binding generates weak adhesion that is modified by clustering; interactions with F-actin are regulated, can be transient, and can alter F-actin dynamics. Additionally, cadherin recycling from the cell surface can modify the size and location of junctions and strength of adhesion. In epithelial cells, this ongoing dynamic behavior is important for maintaining stable junctions. Recent work supports that cadherins act similarly at synapses where their actions are likely to be shared by integrins and other actin-linked CAMs. Together the collaborative activities of such CAMs provide a stable, but flexible structure that can promote and support changes in synapse shape and size while maintaining stable junctions to permit information flow.

Identification of an endogenous substrate of zebrafish doublecortin-like protein kinase using a highly active truncation mutant

Doublecortin-like protein kinase (DCLK), a Ser/Thr protein kinase predominantly expressed in brain and eyes, is believed to play crucial roles in neuronal functions. However, the regulatory mechanisms for DCLK activation and its physiological targets are still unknown. In the present study, we found that a deletion mutant consisting of the catalytic domain of zebrafish DCLK, zDCLK(377-677), exhibited the highest activity among various mutants. Since fully active zDCLK(377-677) showed essentially the same substrate specificity as wild-type zDCLK, we used it to search for physiological substrates of zDCLK. When a zebrafish brain extract was resolved by isoelectric focusing and then phosphorylated by zDCLK(377-677), a highly basic protein with a molecular mass of approximately 90 kDa was detected. This protein was identified as synapsin II by mass spectrometric analysis. Synapsin II was found to interact with the catalytic domain of zDCLK and was phosphorylated at Ser-9 and Ser-58. When synaptosomes were isolated from zebrafish brain, both synapsin II and zDCLK were found to coexist in this preparation. Furthermore, synapsin II in the synaptosomes was efficiently phosphorylated by zDCLK. These results suggest that zDCLK mediates its neuronal functions through phosphorylation of physiological substrates such as synapsin II.

Contactin-associated protein (Caspr) 2 interacts with carboxypeptidase E in the CNS

To identify proteins interacting with the intracellular domain of the neural cell adhesion molecule contactin-associated protein 2 (Caspr2), yeast two-hybrid screening was performed. We identified carboxypeptidase E (CPE) as a Caspr2-interacting candidate protein. Glutathione S-transferase pull-down and immunoprecipitation analyses indicated that Caspr2 was associated with CPE in vitro and in vivo. Both Caspr2 and CPE were expressed predominantly in the CNS. Immunohistochemical analyses revealed that both Caspr2- and CPE-like immunoreactivities were found to co-localize in the apical dendrites and cell bodies of rat cortical neurons. In subcellular localization analysis, Caspr2- and CPE-like immunoreactivities were co-migrated in the fractions of Golgi/ER. Additionally, in COS-7 cells co-transfected with CPE and Caspr2 cDNAs, Caspr2- and CPE-immunoreactivities were co-localized in both Golgi and membrane, whereas it was only observed in Golgi of either COS-7 cell transfected with CPE or Caspr2 cDNA alone. It is known that the membrane-bound form of CPE functions as a sorting receptor of prohormones in the trans-Golgi network. Taken together, our data suggest that CPE may be a key molecule to regulate Caspr2 trafficking to the cell membrane.

Synaptic vesicle-bound pyruvate kinase can support vesicular glutamate uptake

Glucose metabolism is essential for normal brain function and plays a vital role in synaptic transmission. Recent evidence suggests that ATP synthesized locally by glycolysis, particularly via glyceraldehyde 3-phosphate dehydrogenase/3-phosphoglycerate kinase, is critical for synaptic transmission. We present evidence that ATP generated by synaptic vesicle-associated pyruvate kinase is harnessed to transport glutamate into synaptic vesicles. Isolated synaptic vesicles incorporated [(3)H]glutamate in the presence of phosphoenolpyruvate (PEP) and ADP. Pyruvate kinase activators and inhibitors stimulated and reduced PEP/ADP-dependent glutamate uptake, respectively. Membrane potential was also formed in the presence of pyruvate kinase activators. "ATP-trapping" experiments using hexokinase and glucose suggest that ATP produced by vesicle-associated pyruvate kinase is more readily used than exogenously added ATP. Other neurotransmitters such as GABA, dopamine, and serotonin were also taken up into crude synaptic vesicles in a PEP/ADP-dependent manner. The possibility that ATP locally generated by glycolysis supports vesicular accumulation of neurotransmitters is discussed.

Behavioral phenotypes of Disc1 missense mutations in mice

To support the role of DISC1 in human psychiatric disorders, we identified and analyzed two independently derived ENU-induced mutations in Exon 2 of mouse Disc1. Mice with mutation Q31L showed depressive-like behavior with deficits in the forced swim test and other measures that were reversed by the antidepressant bupropion, but not by rolipram, a phosphodiesterase-4 (PDE4) inhibitor. In contrast, L100P mutant mice exhibited schizophrenic-like behavior, with profound deficits in prepulse inhibition and latent inhibition that were reversed by antipsychotic treatment. Both mutant DISC1 proteins exhibited reduced binding to the known DISC1 binding partner PDE4B. Q31L mutants had lower PDE4B activity, consistent with their resistance to rolipram, suggesting decreased PDE4 activity as a contributory factor in depression. This study demonstrates that Disc1 missense mutations in mice give rise to phenotypes related to depression and schizophrenia, thus supporting the role of DISC1 in major mental illness.

Presynaptic alpha-synuclein aggregates, not Lewy bodies, cause neurodegeneration in dementia with Lewy bodies

Lewy bodies, the pathological hallmark of dementia with Lewy bodies (DLB), are large juxtanuclear inclusions of aggregated alpha-synuclein. However, the small number of cortical Lewy bodies relative to the total neuron count does not correlate with the extent of cognitive impairment. In contrast to dopaminergic neurons in Parkinson's disease, nerve cell loss is usually less prevalent in the cortex of DLB, suggesting a different mechanism of neurodegeneration. Because antibodies used for immunodetection per se do not generally differentiate the aggregated from the physiological and monomeric isoform of alpha-synuclein, we developed the paraffin-embedded tissue (PET) blot and the protein aggregate filtration (PAF) assay for the sensitive and selective detection of alpha-synuclein aggregates in tissue slides and brain homogenates, respectively. In contrast to common immunohistochemistry, the PET blot detected an enormous number of small alpha-synuclein aggregates, which, in contrast to the few Lewy bodies, may explain the cognitive impairment in DLB. Using the PAF assay, we demonstrate that the absolute majority of alpha-synuclein aggregates are located at presynaptic terminals, suggesting a severe pathological impact on synaptic function. Indeed, parallel to the massive presynaptic accumulation of alpha-synuclein aggregates, we observed significant synaptic pathology with almost complete loss of dendritic spines at the postsynaptic area. Our results provide strong evidence for a novel concept of neurodegeneration for DLB in which synaptic dysfunction is caused by presynaptic accumulation of alpha-synuclein aggregates. This concept may also be valid for Parkinson's disease.

Phosphorylation of adult type Sept5 (CDCrel-1) by cyclin-dependent kinase 5 inhibits interaction with syntaxin-1

Increasing evidence implicates cyclin-dependent kinase 5 (Cdk5) in neuronal synaptic function. We searched for Cdk5 substrates in synaptosomal fractions prepared from mouse brains. Mass spectrometric analysis after two-dimensional SDS-PAGE identified several synaptic proteins phosphorylated by Cdk5-p35; one protein identified was Sept5 (CDCrel-1). Although septins were isolated originally as cell division-related proteins in yeast, Sept5 is expressed predominantly in neurons and is implicated in exocytosis. We confirmed that Sept5 is phosphorylated by Cdk5-p35 in vitro and identified Ser17 of adult type Sept5 (Sept5_v1) as a major phosphorylation site. We found that Ser17 of Sept5_v1 is phosphorylated in mouse brains. Coimmunoprecipitation from synaptosomal fractions and glutathione S-transferase-syntaxin-1A pulldown assays of Sept5_v1 expressed in COS-7 cells showed that phosphorylation of Sept5_v1 by Cdk5-p35 decreases the binding to syntaxin-1. These results indicate that the interaction of Sept5 with syntaxin-1 is regulated by the phosphorylation of Sept5_v1 at Ser17 by Cdk5-p35.

Subcellular localization of neural-specific NPDC-1 protein

NPDC-1 is a gene specifically expressed in neural cells when they stop to divide and begin to differentiate. Immunocytochemical study analysis of differentiated PC12 cells transfected with NPDC-tag vectors showed that NPDC-1 is transported in vesicles from the Golgi apparatus to the cell membrane and is then likely internalized into endosomes. The protein colocalized, at least partially, with synaptic vesicle proteins: synaptophysin, synaptobrevin 2, and Rab3 GEP (Rab3 GTP/GDP exchange protein). Moreover, subcellular fractionation of rat brain showed that crude synaptic membrane and crude synaptic vesicle fractions were enriched in NPDC-1. Although NPDC-1 bound Rab3 GEP in vitro, it seems unlikely to be involved in Ca2+-dependent exocytosis and, thus, in synaptic vesicle trafficking.

Interaction with Protocadherin-γ Regulates the Cell Surface Expression of Protocadherin-α

The protocadherin-alpha (CNR/Pcdhalpha) and protocadherin-gamma (Pcdhgamma) proteins, members of the cadherin superfamily, are putative cell recognition/adhesion molecules in the brain. Overexpressed cadherins are generally expressed on the cell surface and elicit cell adhesion activity in several cell lines, although hardly any overexpressed CNR/Pcdhalpha proteins are expressed on the cell surface, except on HEK293T cells, which show low expression. We analyzed the expression of CNR/Pcdhalpha and Pcdhgamma in HEK293T cells and found that they formed a protein complex and that Pcdhgamma enhanced the surface expression of CNR/Pcdhalpha. This enhanced surface expression was confirmed by flow cytometry analysis and by marking cell surface proteins with biotin. The enhancement was observed using different combinations of CNR/Pcdhalpha and Pcdhgamma proteins. The surface expression activity was enhanced by the extracellular domains of the proteins, which could bind each other. Their cytoplasmic domains also had binding activity and influenced their localization. Their protein-protein interaction was also detected in extracts of mouse brain and two neuroblastoma cell lines. Thus, interactions between CNR/Pcdhalpha and Pcdhgamma regulate their surface expression and contribute to the combinatorial diversity of cell recognition proteins in the brain.

Identification of a nerve ending-enriched 29-kDa protein, labeled with [3-32P]1,3-bisphosphoglycerate, as monophosphoglycerate mutase: inhibition by fructose-2,6-bisphosphate via enhancement of dephosphorylation

Glucose metabolism is of vital importance in normal brain function. Evidence indicates that glycolysis, in addition to production of ATP, plays an important role in maintaining normal synaptic function. In an effort to understand the potential involvement of a glycolytic intermediate(s) in synaptic function, we have prepared [3-32P]1,3-bisphosphoglycerate and [32P]3-phosphoglycerate and sought their interaction with a specific nerve-ending protein. We have found that a 29-kDa protein is the major component labeled with either [3-32P]1,3-bisphosphoglycerate or [32P]3-phosphoglycerate. The protein was identified as monophosphoglycerate mutase (PGAM). This labeling was remarkably high in the brain and synaptosomal cytosol fraction, consistent with the importance of glycolysis in synaptic function. Of interest, fructose-2,6-bisphosphate (Fru-2,6-P2) inhibited PGAM phosphorylation and enzyme activity. Moreover, Fru-2,6-P2 potently stimulated release of [32P]phosphate from the 32P-labeled PGAM (EC50 = 1 microM), suggesting that apparent reduction of PGAM phosphorylation and enzyme activity by Fru-2,6-P2 may be due to stimulation of dephosphorylation of PGAM. The significance of these findings is discussed.

Glycolysis and glutamate accumulation into synaptic vesicles. Role of glyceraldehyde phosphate dehydrogenase and 3-phosphoglycerate kinase

Glucose is the major source of brain energy and is essential for maintaining normal brain and neuronal function. Hypoglycemia causes impaired synaptic transmission. This occurs even before significant reduction in global cellular ATP concentration, and relationships among glycolysis, ATP supply, and synaptic transmission are not well understood. We demonstrate that the glycolytic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH) and 3-phosphoglycerate kinase (3-PGK) are enriched in synaptic vesicles, forming a functional complex, and that synaptic vesicles are capable of accumulating the excitatory neurotransmitter glutamate by harnessing ATP produced by vesicle-bound GAPDH/3-PGK at the expense of their substrates. The GAPDH inhibitor iodoacetate suppressed GAPDH/3-PGK-dependent, but not exogenous ATP-dependent, [(3)H]glutamate uptake into isolated synaptic vesicles. It also decreased vesicular [(3)H]glutamate content in the nerve ending preparation synaptosome; this decrease was reflected in reduction of depolarization-induced [(3)H]glutamate release. In contrast, oligomycin, a mitochondrial ATP synthase inhibitor, had minimal effect on any of these parameters. ADP at concentrations above 0.1 mm inhibited vesicular glutamate and dissipated membrane potential. This suggests that the coupled GAPDH/3-PGK system, which converts ADP to ATP, ensures maximal glutamate accumulation into presynaptic vesicles. Together, these observations provide insight into the essential nature of glycolysis in sustaining normal synaptic transmission.

The formation of synapses in the central nervous system

Intemeuronal synapses are specialized contact zones formed between the transmitting pole of one neuron, usually an axon, and the receptive pole of another nerve cell, usually a dendritic process or the soma. The formation of these synaptic contacts is the result of cellular events related to neurite elongation, the establishment of polarity, axon guidance, and target recognition. A series of morphological rearrangements takes place once synaptic targets establish their initial contact. These changes include the clustering of synaptic vesicles in the presynaptic element and the formation of a specialized area capable of signal transduction at the postsynaptic target. The present review discusses the role of different synaptic proteins in the cellular events leading to the formation of synapses among neurons in the central nervous system.

Guanine nucleotide exchange factors CalDAG-GEFI and CalDAG-GEFII are colocalized in striatal projection neurons

CalDAG-GEFI and CalDAG-GEFII (identical to RasGRP) are novel, brain-enriched guanine nucleotide exchange factors (GEFs) that can be stimulated by calcium and diacylglycerol and that can activate small GTPases, including Ras and Rap1, molecules increasingly recognized as having signaling functions in neurons. Here, we show that CalDAG-GEFI and CalDAG-GEFII mRNAs, detected by in situ hybridization analysis, have sharply contrasting forebrain-predominant distributions in the mature brain: CalDAG-GEFI is expressed mainly in the striatum and olfactory structures and deep cortical layers, whereas CalDAG-GEFII is expressed widely in the forebrain. Within the striatum, however, the two CalDAG-GEF mRNAs have nearly identical distributions: they are coexpressed in striatal projection neurons that give rise to the direct and indirect pathways of the basal ganglia. Subcellular fractionation analysis of the substantia nigra with monoclonal antibodies against CalDAG-GEFI suggests that CalDAG-GEFI protein is present not only in the cell bodies of striatal projection neurons but also in their axons and axon terminals. These results suggest that the CalDAG-GEFs may be key intracellular regulators whereby calcium and diacylglycerol signals can regulate cellular functions through small GTPases in the basal ganglia circuits.

IPF, a vesicular uptake inhibitory protein factor, can reduce the Ca(2+)-dependent, evoked release of glutamate, GABA and serotonin

Synaptic vesicles in the nerve terminal play a pivotal role in neurotransmission. Neurotransmitter accumulation into synaptic vesicles is catalyzed by distinct vesicular transporters, harnessing an electrochemical proton gradient generated by V-type proton-pump ATPase. However, little is known about regulation of the transmitter pool size, particularly in regard to amino acid neurotransmitters. We previously provided evidence for the existence of a potent endogenous inhibitory protein factor (IPF), which causes reduction of glutamate and GABA accumulation into isolated, purified synaptic vesicles. In this study we demonstrate that IPF is concentrated most in the synaptosomal cytosol fraction and that, when introduced into the synaptosome, it leads to a decrease in calcium-dependent exocytotic (but not calcium-independent) release of glutamate in a concentration-dependent manner. In contrast, alpha-fodrin (non-erythroid spectrin), which is structurally related to IPF and thought to serve as the precursor for IPF, is devoid of such inhibitory activity. Intrasynaptosomal IPF also caused reduction in exocytotic release of GABA and the monoamine neurotransmitter serotonin. Whether IPF affects vesicular storage of multiple neurotransmitters in vivo would depend upon the localization of IPF. These results raise the possibility that IPF may modulate synaptic transmission by acting as a quantal size regulator of one or more neurotransmitters.

Pike. A nuclear gtpase that enhances PI3kinase activity and is regulated by protein 4.1N

While cytoplasmic PI3Kinase (PI3K) is well characterized, regulation of nuclear PI3K has been obscure. A novel protein, PIKE (PI3Kinase Enhancer), interacts with nuclear PI3K to stimulate its lipid kinase activity. PIKE encodes a 753 amino acid nuclear GTPase. Dominant-negative PIKE prevents the NGF enhancement of PI3K and upregulation of cyclin D1. NGF treatment also leads to PIKE interactions with 4.1N, which has translocated to the nucleus, fitting with the initial identification of PIKE based on its binding 4.1N in a yeast two-hybrid screen. Overexpression of 4.1N abolishes PIKE effects on PI3K. Activation of nuclear PI3K by PIKE is inhibited by the NGF-stimulated 4.1N translocation to the nucleus. Thus, PIKE physiologically modulates the activation by NGF of nuclear PI3K.

The subcellular and cellular distribution of G protein-coupled receptor kinase 2 in rat brain

G protein-coupled receptor kinase 2 has been found to phosphorylate and thus regulate the activity of several G protein-coupled receptors implicated in neuronal signalling pathways. Although this kinase was initially described as a soluble protein, our laboratory has recently found that a significant amount of G protein-coupled receptor kinase 2 is associated with microsomal membranes in liver and different types of cultured cells. In the present report we show that high G protein-coupled receptor kinase 2 specific activity and protein levels are present in microsomal fractions of rat brain homogenates. On the other hand, immunochemical detection using a new antibody raised against the N-terminus of the kinase revealed a specific and widely distributed staining in different areas of the central nervous system, and the association of G protein-coupled receptor kinase 2 with intracellular structures in nervous cells. Our results further suggest that this receptor kinase may be involved in the modulation of G protein-coupled receptor-mediated neurotransmission and that association with microsomal membranes may play a role in G protein-coupled receptor kinase 2 functions in the brain.

A beta (25-35) peptide displays H2O2-like reactivity towards aqueous Fe2+, nitroxide spin probes, and synaptosomal membrane proteins

Amyloid beta peptides (A beta s) are found in abnormally high accumulations in brains of persons with Alzheimer's disease, and are believed to contribute to cognitive decline in this disorder. Synthetic A beta and its peptide fragment 25-35 [A beta (25-35)] are toxic to cells in culture; however, the exact mechanism of amyloid peptide toxicity is not known. An emerging hypothesis contends that A beta toxicity results from peptide-mediated free radical reactions and generation of reactive oxygen species. Recently, we reported that reactivity of A beta toward the oxidation-sensitive enzyme glutamine synthetase is related to the peptide's reactivity toward the spin trap phenyl-tert-butyl nitrone (PBN). Neuronal damage may be due, in part, to oxidative processes initiated by amyloid-derived free radicals species. This work presents evidence from electron paramagnetic resonance (EPR) spin labeling techniques and spectrophotometric assays that a portion of synthetic A beta (25-35) demonstrates hydrogen peroxide-like reactivity toward Fe2+, nitroxide spin probes, and neocortical synaptasomal membrane proteins. These results are discussed with reference to free radical membrane damage and neurotoxicity in Alzheimer's disease.

Purification, characterization, and localization of an ADP-ribosylactin hydrolase that uses ADP-ribosylated actin from rat brains as a substrate

Mammalian ADP-ribosylation is poorly understood. An ADP-ribosylprotein hydrolase that acted on ADP-ribosylated actin was purified from rat brain. The molecular weight of this enzyme was 62, 000 as determined by SDS-polyacrylamide gel electrophoresis and gel filtration. Enzyme activity with ADP-ribosylated actin as a substrate was inhibited by NAD, ATP, ADP, and ADP-ribose, but not by AMP. Mg2+ increased Vmax. Purified ADP-ribosylactin hydrolase catalyzed the hydrolysis of ADP-ribosylated subunits Gsalpha, Gialpha, and Goalpha and elongation factor-2. After de-ADP-ribosylation by the purified ADP-ribosylactin hydrolase, the proteins were re-ADP-ribosylated by brain mono-ADP-ribosyltransferases and bacterial toxins. The actin that was de-modified by ADP-ribosylactin hydrolase could form actin filaments. Two kinds of monoclonal antibodies against ADP-ribosylactin hydrolase were prepared and characterized. In an immunohistochemical study, the plasma membranes and cytoplasmic regions of the nerve cells in the rat brain were immunoreactive. In subcellular fractionation of the brains, most of the ADP-ribosylactin hydrolase activity was found in the cytosol and synaptosome fractions. When the synaptosomes were treated with a hypotonic solution, ADP-ribosylactin hydrolase activity was found in the supernatant. Our findings suggest that brain ADP-ribosylactin hydrolase has the important function of polymerizing actin for signal transduction in the cytosol of nerve cells and synaptosomes.

Suppression of KIF2 in PC12 cells alters the distribution of a growth cone nonsynaptic membrane receptor and inhibits neurite extension

In the present study, we present evidence about the cellular functions of KIF2, a kinesin-like superfamily member having a unique structure in that its motor domain is localized at the center of the molecule (Noda Y., Y. Sato-Yoshitake, S. Kondo, M. Nangaku, and N. Hirokawa. 1995. J. Cell Biol. 129:157-167.). Using subcellular fractionation techniques, isopicnic sucrose density centrifugation of microsomal fractions from developing rat cerebral cortex, and immunoisolation with KIF2 antibodies, we have now identified a type of nonsynaptic vesicle that associates with KIF2. This type of organelle lacks synaptic vesicle markers (synapsin, synaptophysin), amyloid precursor protein, GAP-43, or N-cadherin. On the other hand, it contains betagc, which is a novel variant of the beta subunit of the IGF-1 receptor, which is highly enriched in growth cone membranes. Both betagc and KIF2 are upregulated by NGF in PC12 cells and highly concentrated in growth cones of developing neurons. We have also analyzed the consequences of KIF2 suppression by antisense oligonucleotide treatment on nerve cell morphogenesis and the distribution of synaptic and nonsynaptic vesicle markers. KIF2 suppression results in a dramatic accumulation of betagc within the cell body and in its complete disappearance from growth cones; no alterations in the distribution of synapsin, synaptophysin, GAP-43, or amyloid percursor protein are detected in KIF2-suppressed neurons. Instead, all of them remained highly enriched at nerve terminals. KIF2 suppression also produces a dramatic inhibition of neurite outgrowth; this phenomenon occurs after betagc has disappeared from growth cones. Taken collectively, our results suggest an important role for KIF2 in neurite extension, a phenomenon that may be related with the anterograde transport of a type of nonsynaptic vesicle that contains as one of its components a growth cone membrane receptor for IGF-1, a growth factor implicated in nerve cell development.

Effect of 2-cyclohexene-1-one-induced glutathione diminution on ischemia/reperfusion-induced alterations in the physical state of brain synaptosomal membrane proteins and lipids

Glutathione is able to protect membrane proteins from oxidative stress. In ischemia/reperfusion injury, free radicals cause synaptosomal membrane protein and lipid oxidation that is prevented by the free radical scavenger N-tert-butyl-alpha-phenylnitrone (Hall N. C. et al. (1995) Neuroscience 64, 81-89; 69, 591-600). We wondered if diminution of glutathione would lead to further membrane alterations. Accordingly, the effects of glutathione depletion, by intraperitoneal administration of 2-cyclohexene-1-one, on the physical state of cortical synaptosomal membrane proteins and lipids, with and without global ischemia/reperfusion, were studied in vivo and in vitro in adult and aged gerbils utilizing electron paramagnetic resonance spectrometry. 2-Cyclohexene-1-one (100 mg/kg, i.p.) was administered 30 min prior to 10-min ischemia followed by 1 or 14 h reperfusion. This glutathione reduction agent was also administered to gerbils under the same temporal schedule in the absence of ischemia and compared to untreated controls. Synaptosomal membranes were labeled with a protein-specific spin label, 2,2,6,6-tetramethyl-4-maleimidopiperidine-1-oxyl, or a lipid-specific spin probe, 5-doxylstearic acid. There were no significant changes in the physical state of the lipid portion of synaptosomal membranes when comparing ischemia reperfusion and 2-cyclohexene-1-one-treated ischemia reperfusion in either the adult or aged gerbils. However, glutathione depletion without ischemia/reperfusion caused significant changes in the physical state of the protein portion of cortical synaptosomal membranes in both the adult and aged models. Glutathione depletion, without ischemia/reperfusion, in the adult model showed a maximum change at 3 h that returned to control values by 14 h. In contrast, the aged model showed significant changes at 1 h reperfusion, which did not return to control values by 14 h reperfusion. Glutathione depletion combined with ischemia/reperfusion caused initial protein change in both adult and aged models at 1 h reperfusion, which did not return toward control values by 14 h reperfusion. The results of this study suggest that glutathione depletion increases the severity of membrane protein damage associated with ischemia/reperfusion injury.

Release of secretory phospholipase A2 from rat neuronal cells and its possible function in the regulation of catecholamine secretion

Here we show that secretory phospholipase A2 (sPLA2) that is immunochemically indistinguishable from type II sPLA2 is (i) stored in neuroendocrine cells, (ii) released in response to neurotransmitters or depolarization, and (iii) involved in the regulation of catecholamine secretion by these cells. Rat brain synaptic vesicle fractions contained PLA2 activity, which was neutralized completely by an antibody raised against rat type II sPLA2. sPLA2 immunoreactive with anti-(type II sPLA2) antibody was released from synaptosomes in response to depolarization evoked by a high concentration of potassium in the presence of Ca2+. Rat pheochromocytoma PC12 cells, which differentiated into adherent cells similar to sympathetic neurons in response to nerve growth factor, were used for the detailed analysis of the dynamics and function of sPLA2 in neuronal cells. Antibody against rat type II sPLA2 precipitated approximately 80% of the PLA2 activity in PC12 cell lysates. Transcript for type II sPLA2 was detected in PC12 cells by reverse transcriptase-PCR. When neuronally differentiated PC12 cells were stimulated with carbamylcholine or potassium, sPLA2 was released into the medium and reached a maximal approximately 40% release by 15 min. Inhibitors specific to type II sPLA2 suppressed catecholamine secretion by PC12 cells which had been activated by carbamylcholine. Furthermore, treatment of PC12 cells with exogenous type II sPLA2 alone elicited catecholamine secretion. These observations indicate that sPLA2 released from neuronal cells may regulate the degranulation process leading to release of neurotransmitters and are compatible with our earlier finding that this enzyme is involved in the degranulation of rat mast cells.

Nitric oxide increases calcium/calmodulin-dependent phosphorylation of proteins in the postsynaptic density of adult rat cerebral cortex

Nitric oxide (NO) plays important roles in diverse processes, including neurotransmission in the peripheral and central nervous systems. Nitric oxide synthase (NOS), the enzyme that catalyzes formation of NO from L-arginine, is an intrinsic component of the postsynaptic density (PSD), a specialization of the postsynaptic membrane. This raises the possibility that NO may play a role in postsynaptic function. To begin defining postsynaptic actions of NO, we examined effects of NO on Ca2+/calmodulin-dependent phosphorylation (C/C-DP) of proteins in the cortical PSD of adult rat brain. Treatment of the PSD with sodium nitroprusside, a NO donor, caused a 4-fold increase in C/C-DP of the major PSD protein (mPSDp), relative to C/C treatment alone. Another NO donor, S,S'-dinitrosodithiol, elicited a 2-fold increase in C/C-DP of the mPSDp. Treatment of PSD fractions with L-arginine, a substrate for endogenous NOS, caused a 3-fold increase in C/C-DP activity. The competitive NOS inhibitor, N-L-arginine-methyl ester, decreased basal C/C-DP of cortical mPSDp by 50% and blocked the increase elicited by L-arginine. The inhibitor had no effect on cAMP-dependent phosphorylation, suggesting specificity of NO action on C/C-DP. Our observations indicate that NO enhances C/C-DP of PSD proteins. As C/C-DP inactivates NOS, our findings raise the possibility that NO effects on C/C-DP constitute a feedback mechanism for regulation of NOS activity.

Characterization of dual leucine zipper-bearing kinase, a mixed lineage kinase present in synaptic terminals whose phosphorylation state is regulated by membrane depolarization via calcineurin

The biochemistry and regulation of dual leucine zipper bearing kinase (DLK), a member of the mixed lineage kinase or MLK subfamily of protein kinases, was examined in the nervous system. DLK transcript expression in the nervous system was predominantly neuronal. DLK protein was present in synaptic terminals where it was associated with both plasma membrane and cytosol fractions. Within these two fractions, DLK had differing characteristics. Cytosolic DLK existed in both a phosphorylated and dephosphorylated state; DLK associated with plasma membrane existed in the dephosphorylated state only. On nonreducing SDS-polyacrylamide gel electrophoresis, cytosolic DLK migrated at 130 kDa, while membrane associated DLK migrated with an apparent Mr >/= 260,000. Similarly, DLK transiently expressed in COS 7 cells autophosphorylated in vivo and migrated at approximately 260 kDa when separated by nonreducing SDS-polyacrylamide gel electrophoresis. In cotransfection experiments, FLAG-tagged DLK or a FLAG-tagged truncated DLK mutant (F-Delta520) was coimmunoprecipitated with Myc-tagged DLK and formed complexes under nonreducing conditions consistent with the conclusion that DLK formed covalently associated homodimers in overexpressing COS 7 cells. In aggregating neuronal-glial cultures, depolarization of plasma membrane lead to dephosphorylation of DLK. Treatment of aggregates with 5 nM or 200 nM okadaic acid lead to a shift in electrophoretic mobility consistent with phosphorylation of DLK. Treatment with cyclosporin A, a specific inhibitor of the calcium/calmodulin-dependent protein phosphatase 2B (calcineurin), had no effect on DLK phosphorylation under basal conditions. However, cyclosporin A completely inhibited DLK dephosphorylation upon membrane depolarization.

Fimbrial-dependent mating inMicrobotryum violaceuminvolves a mannose–lectin interaction

Fimbriae of the anther smut fungus, Microbotryum violaceum are polymers of six 74-kDa glycoprotein isoforms. Digestion of fimbrial monomers with α-mannosidase yielded two polypeptides with masses of 70 and 48 kDa. The 70-kDa polypeptide is probably a product of incomplete digestion and the 48-kDa polypeptide is the aglycone. Thus, most of the carbohydrate component of fimbrial protein is mannose. Previous observations have suggested that fimbriae are necessary for mating in M. violaceum. Further evidence for this role was obtained in the present study by showing that mating is inhibited by an anti-fimbrial protein antiserum, by mannose and related sugars glucose and arabinose, and by the lectin concanavalin A. Since inhibition was not complete, however, two mechanisms for adhesion between compatible cells were proposed, one fimbrial dependent and one independent. Lastly, fimbrial protein from a1but not a2mating types bound to a mannose–agarose column, suggesting a lectin-like capability. The fimbrial dependent mechanism of cell-to-cell adhesion may involve binding of the mannose residues of the fimbriae of a2cells by the fimbriae of a1cells.Key words: mating, Microbotryum violaceum, lectin, fimbriae.

Subcellular distribution of phosphorylase kinase in rat brain. Association of the enzyme with mitochondria and membranes

The evaluation of glycogen phosphorylase kinase in rat brain subcellular fractions was undertaken in order to get further insight into the association of this kinase with specific neuronal cell compartments. The enzyme was found to be primarily soluble, but considerable latent specific activities were observed in particulate fractions, especially in microsomes, mitochondria and synaptosomes, which could be unmasked by treatment with Triton-X-100. The submitochondrial and subsynaptic distribution patterns of phosphorylase kinase revealed high overt activity in the mitochondrial intermembrane space and high latent activities in mitochondrial membranes, and synaptic vesicles, membranes and mitochondria. The Ca(2+)-dependency of soluble phosphorylase kinase was similar to that of microsomal enzyme but higher than that of other particulate enzyme forms. Mitochondrial phosphorylase kinase showed a higher pH 6.8:8.2 activity ratio than the soluble and the microsomal enzyme. The rate of inactivation of cytosolic phosphorylase kinase by proteinase K was higher than that of microsomal and mitochondrial enzymes. Antibodies against rabbit skeletal muscle phosphorylase kinase effectively inhibited both cytosolic and microsomal enzyme but failed to significantly affect the kinase activity present in intact mitochondria and intermembrane space. Western blotting with anti-phosphorylase kinase showed that rat brain mitochondria exhibited a significantly lower immunoreactivity compared to soluble cytosol. In conclusion, the presence of phosphorylase kinase activity in a variety of particulate fractions of rat brain suggests a multiplicity of actions of this kinase in neuronal tissues.

Prevention of ischemia/reperfusion-induced alterations in synaptosomal membrane-associated proteins and lipids by N-tert-butyl-alpha-phenylnitrone and difluoromethylornithine

Previous studies in our laboratory demonstrated the alteration in the physical state of synaptosomal membrane lipids and proteins in ischemia/reperfusion injury using selective spin labels and electron paramagnetic resonance spectroscopy [Hall et al. (1995) Neuroscience 61, 84-89]. Since many investigations have provided evidence for free radical generation during ischemia/reperfusion injury, we investigated whether a free radical scavenger would prevent the membrane damage, in gerbils. Further, experiments to determine if a secondary effect of polyamine generation at 14 h reperfusion could be blocked by this free radical scavenger or by an inhibitor of ornithine decarboxylase were also carried out. The alterations in synaptosomal membrane integrity observed during ischemia/reperfusion injury were selectively neutralized by treatment with the free radical spin trap N-tert-butyl-alpha-phenylnitrone or an inhibitor of ornithine decarboxylase, difluoromethylornithine. Administration of N-tert-butyl-alpha-phenylnitrone prior to ischemia totally abrogated both lipid and protein alterations observed at 1 and 14 h reperfusion. Pretreatment with difluoromethylornithine neutralized only the 14 h change in lipid label motion. Treatment with N-tert-butyl-alpha-phenylnitrone at 6 h post ischemia showed only a slight attenuation of the 14 h lipid effect and no change in the protein effect. Difluoromethylornithine treatment at 6 h post ischemia negated the 14 h ischemia/reperfusion injury-induced lipid effect and had no effect on the protein change. These data support previous suggestions that free radicals and polyamines play a critical role in neuronal damage and cell loss following ischemia/reperfusion injury and that the polyamine effect is dependent upon free radical generation during ischemia/reperfusion injury.

Structural alterations in synaptosomal membrane-associated proteins and lipids by transient middle cerebral artery occlusion in the cat

We have previously reported that ischemia reperfusion injury results from free radical generation following transient global ischemia, and that this radical induced damage is evident in the synaptosomal membrane of the gerbil. [Hall et al, (1995) Neuroscience 64: 81-89]. In the present study we have extended these observations to transient focal ischemia in the cat. We prepared synaptosomal membranes from frontal, parietal-temporal, and occipital regions of the cat cerebral cortex with reperfusion times of 1 and 3 hours following 1 hour right middle cerebral artery occlusion. The membranes were selectively labeled with protein and lipid specific paramagnetic spin labels and analyzed using electron paramagnetic resonance spectrometry. There were significant motional changes of both the protein and lipid specific spin labels in the parietal-temporal and occipital regions with 1 hour reperfusion; but, both parameters returned to control values by 3 hours reperfusion. No significant changes were observed in the normally perfused frontal pole at either reperfusion time. These results support the argument that free radicals play a critical role in cell damage at early reperfusion times following ischemia.

KIF3A/B: a heterodimeric kinesin superfamily protein that works as a microtubule plus end-directed motor for membrane organelle transport

We cloned a new member of the murine brain kinesin superfamily, KIF3B, and found that its amino acid sequence is highly homologous but not identical to KIF3A, which we previously cloned and named KIF3 (47% identical). KIF3B is localized in various organ tissues and developing neurons of mice and accumulates with anterogradely moving membranous organelles after ligation of nerve axons. Immunoprecipitation assay of the brain revealed that KIF3B forms a complex with KIF3A and three other high molecular weight (approximately 100 kD)-associated polypeptides, called the kinesin superfamily-associated protein 3 (KAP3). In vitro reconstruction using baculovirus expression systems showed that KIF3A and KIF3B directly bind with each other in the absence of KAP3. The recombinant KIF3A/B complex (approximately 50-nm rod with two globular heads and a single globular tail) demonstrated plus end-directed microtubule sliding activity in vitro. In addition, we showed that KIF3B itself has motor activity in vitro, by making a complex of wild-type KIF3B and a chimeric motor protein (KIF3B head and KIF3A rod tail). Subcellular fractionation of mouse brain homogenates showed a considerable amount of the native KIF3 complex to be associated with membrane fractions other than synaptic vesicles. Immunoprecipitation by anti-KIF3B antibody-conjugated beads and its electron microscopic study also revealed that KIF3 is associated with membranous organelles. Moreover, we found that the composition of KAP3 is different in the brain and testis. Our findings suggest that KIF3B forms a heterodimer with KIF3A and functions as a new microtubule-based anterograde translocator for membranous organelles, and that KAP3 may determine functional diversity of the KIF3 complex in various kinds of cells in vivo.

Purification and characterization of a fatty acid-activated protein kinase (PKN) from rat testis

PKN, a novel protein kinase with a catalytic domain homologous to that of the protein kinase C (PKC) family and unique N-terminal leucine-zipper-like sequences, was identified by molecular cloning from a human hippocampus cDNA library [Mukai and Ono (1994) Biochem. Biophys. Res. Commun. 199, 897-904]. Recently we partially purified recombinant PKN from COS7 cells transfected with the cDNA construct encoding human PKN, and demonstrated that the recombinant PKN was activated by unsaturated fatty acids and limited proteolysis [Mukai, Kitagawa, Shibata et al. (1994) Biochem. Biophys. Res. Commun. 204, 348-356]. The present work has focused on the further purification and characterization of PKN from native rat tissue. Immunochemical measurement revealed that PKN was found in every tissue, and was especially abundant in testis, spleen and brain; subcellular fractionation of rat brain showed that half of the PKN was localized in the soluble cytosolic fraction. PKN was purified approx. 8000-fold to apparent homogeneity from the cytosolic fraction of rat testis by DEAE-cellulose chromatography, ammonium sulphate fractionation and chromatography on butyl-Sepharose, heparin-Sepharose, Mono Q and protamine-CH-Sepharose. The enzyme migrates as a band of apparent molecular mass 120 kDa. Using serine-containing peptides based on the pseudosubstrate sequence of PKC-delta as phosphate acceptors, the kinase activity was stimulated several-fold by 40 microM unsaturated fatty acids or by detergents such as 0.04% sodium deoxycholate and 0.004% SDS. In the absence of modifiers, protamine sulphate, myelin basic protein and synthetic peptides based on the pseudosubstrate site of PKCs or ribosomal S6 protein were good substrates for phosphorylation by the kinase. In the presence of 40 microM arachidonic acid the kinase activity of PKN for these phosphate acceptors was increased 2-18-fold. The autophosphorylation activity of purified PKN was partially inhibited by pretreatment with alkaline phosphatase. These properties appear to distinguish PKN from many protein kinases isolated previously.

Existence of a putative specific postsynaptic density protein produced during Purkinje cell spine maturation

This study identified a 140 kDa polypeptide as a putative specific component of Purkinje cell spines' postsynaptic densities and which began to appear during the critical period of cerebellar cortex synaptogenesis. Mouse cerebellar cortices at postnatal days 5, 7, 9, 11, 15 and young adult, between days 30 and 40, were used to purify subcellular fractions of synaptosomes, synaptic membranes and postsynaptic densities. The purity of the subcellular fractions was assessed by electron microscopy and the protein composition of the different fractions was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polypeptides of apparent molecular weights of 25, 26, 27, 30, 33, 37, 43, 45, 52, 64, 74, 85, 94, 110, 125, 130, 165 and 174 kDa were found in the synaptosomal fractions of all the ages studied, even before the critical period of synaptogenesis, at postnatal day 7, when the postsynaptic densities were still nonexistent, indicating that the polypeptides are nonspecific constituents of these structures. On the other hand, a 140 kDa polypeptide was detected in the postsynaptic density fractions at postnatal day 11, immediately after postsynaptic structures began to appear, suggesting the possibility that this protein is a specific component of the cerebellar cortex postsynaptic densities. The 140 kDa polypeptide was electroeluted from the gel and analysed for its amino acid composition by reverse-phase high-pressure liquid chromatography. The analysis showed that this protein has a high content of nonpolar amino acid residues, such as leucine, isoleucine, glycine, phenylalanine and valine. A hypothetical model relative to the participation of the 140 kDa protein in the molecular organization of the postsynaptic density is suggested which may contribute to the understanding of the role played by this structure in synaptic function.

KIF2 is a new microtubule-based anterograde motor that transports membranous organelles distinct from those carried by kinesin heavy chain or KIF3A/B

Kinesin is known as a representative cytoskeletal motor protein that is engaged in cell division and axonal transport. In addition to the mutant assay, recent advances using the PCR cloning technique have elucidated the existence of many kinds of kinesin-related proteins in yeast, Drosophila, and mice. We previously cloned five different members of kinesin superfamily proteins (KIFs) in mouse brain (Aizawa, H., Y. Sekine, R. Takemura, Z. Zhang, M. Nangaku, and N. Hirokawa. 1992. J. Cell Biol. 119:1287-1296) and demonstrated that one of them, KIF3A, is an anterograde motor (Kondo, S., R. Sato-Yashitake, Y. Noda, H. Aizawa, T. Nakata, Y. Matsuura, and N. Hirokawa. J. Cell Biol. 1994. 125:1095-1107). We have now characterized another axonal transport motor, KIF2. Different from other KIFs, KIF2 is a central type motor, since its motor domain is located in the center of the molecule. Recombinant KIF2 exists as a dimer with a bigger head and plus-end directionally moves microtubules at a velocity of 0.47 +/- 0.11 microns/s, which is two thirds that of kinesin's. Immunocytological examination showed that native KIF2 is abundant in developing axons and that it accumulates in the proximal region of the ligated nerves after a 20-h ligation. Soluble KIF2 exists without a light chain, and KIF2's associated-vesicles, immunoprecipitated by anti-KIF2 antibody, are different from those carried by existing motors such as kinesin and KIF3A. They are also distinct from synaptic vesicles, although KIF2 is accumulated in so-called synaptic vesicle fractions and embryonal growth cone particles. Our results strongly suggest that KIF2 functions as a new anterograde motor, being specialized for a particular group of membranous organelles involved in fast axonal transport.

Ischemia/reperfusion-induced changes in membrane proteins and lipids of gerbil cortical synaptosomes

The effects of transient bilateral carotid occlusion on the physical state of synaptosomal membrane proteins and lipids were studied in adult and aged gerbils employing electron paramagnetic resonance. Transient ischemia was produced in adult and aged gerbils by bilateral occlusion of the common carotid arteries with reperfusion times ranging from 0 to 24 h. Synaptosomes of the cerebral cortices were isolated and labeled with a protein-specific spin probe (2,2,6,6-tetramethyl-4-maleimido-piperidine-1-oxyl) and a lipid-specific spin probe (5-doxylstearic acid). Changes in the physical state of the protein peaked at 60 min reperfusion for both adult and aged gerbil models, with a more intense change in aged, but did not return to control values by 24 h. A biphasic change occurred with the lipid-specific label in both the aged and adult models. The onset of the first phase of change occurred at an earlier time (30 min reperfusion) for aged gerbil tissue than for adult tissue (between 3 and 6 h reperfusion), while the second phase of change occurred at 12 h reperfusion for both adult and aged. These results are consistent with the hypothesis that protein oxidation and lipid peroxidation are direct results of free radicals produced during the reperfusion following ischemia and that protein oxidation may be intensified by peroxidation of the surrounding lipids. Phospholipase A2 activation is implicated to cause changes in membrane phospholipid organization as seen in these studies.

KIF1B, a novel microtubule plus end-directed monomeric motor protein for transport of mitochondria

To further elucidate the mechanism of organelle transport, we cloned a novel member of the mouse kinesin superfamily, KIF1B. This N-terminal-type motor protein is expressed ubiquitously in various kinds of tissues. In situ hybridization revealed that KIF1B is expressed abundantly in differentiated nerve cells. Interestingly, K1F1B works as a monomer, having a microtubule plus end-directed motility. Our rotary shadowing electron microscopy revealed mostly single globular structures. Immunocytochemically, KIF1B was colocalized with mitochondria in vivo. Furthermore, a subcellular fractionation study showed that KIF1B was concentrated in the mitochondrial fraction, and purified K1F1B could transport mitochondria along microtubules in vitro. These data strongly suggested that KIF1B works as a monomeric motor for anterograde transport of mitochondria.

Electron paramagnetic resonance investigations of free radical-induced alterations in neocortical synaptosomal membrane protein infrastructure

Evidence is presented that free radical stress can directly induce physico-chemical alterations in rodent neocortical synaptosomal membrane proteins. Synaptosomes were prepared from gerbil cortical brain tissue and incubated with 3 mM ascorbate and various concentrations of exogenous Fe2+ for 30-240 min at 37 degrees C. Synaptosomes were then lysed and covalently labeled with the protein thiol-selective spin label MAL-6 (2,2,6,6-tetramethyl-4-maleimidopiperdin-1-oxyl) and subjected to electron paramagnetic resonance (EPR) spectrometry. In separate experiments, synaptosomal membranes were labeled with the thiol-specific spin label MTS ((1-oxyl-2,2,5,5-tetramethyl-pyrroline-3-methyl)-methanethiosulfonate), or the lipid-specific spin probe 5-NS (5-nitroxide stearate). Free radical stress induced by iron/ascorbate treatment has a rigidizing effect on the protein infrastructure of these membranes, as appraised by EPR analysis of membrane protein-bound spin label, but no change was detected in the lipid component of the membrane. These results are discussed with reference to potential oxidative mechanisms in aging and neurological disorders.

KIF3A is a new microtubule-based anterograde motor in the nerve axon

Neurons are highly polarized cells composed of dendrites, cell bodies, and long axons. Because of the lack of protein synthesis machinery in axons, materials required in axons and synapses have to be transported down the axons after synthesis in the cell body. Fast anterograde transport conveys different kinds of membranous organelles such as mitochondria and precursors of synaptic vesicles and axonal membranes, while organelles such as endosomes and autophagic prelysosomal organelles are conveyed retrogradely. Although kinesin and dynein have been identified as good candidates for microtubule-based anterograde and retrograde transporters, respectively, the existence of other motors for performing these complex axonal transports seems quite likely. Here we characterized a new member of the kinesin super-family, KIF3A (50-nm rod with globular head and tail), and found that it is localized in neurons, associated with membrane organelle fractions, and accumulates with anterogradely moving membrane organelles after ligation of peripheral nerves. Furthermore, native KIF3A (a complex of 80/85 KIF3A heavy chain and a 95-kD polypeptide) revealed microtubule gliding activity and baculovirus-expressed KIF3A heavy chain demonstrated microtubule plus end-directed (anterograde) motility in vitro. These findings strongly suggest that KIF3A is a new motor protein for the anterograde fast axonal transport.

Localization of dynamin: widespread distribution in mature neurons and association with membranous organelles

Tissue distribution and intracellular localization of dynamin by immunoblotting and immunocytochemistry is investigated in this study. Dynamin was widely expressed in all the neurons we examined, and was especially abundant in the central nervous system after maturation, although its expression presented regional heterogeneity. Dynamin was present most abundantly in cerebellar Purkinje cells and hippocampal pyramidal cells, and to a lesser extent in motor neurons and peripheral nerves. However, dynamin was nearly absent in cells such as anterior pituitary cells and adrenal medullary cells which secrete mainly dense cored vesicles. Dynamin was localized not only in cell bodies, axons, and synapses but also in dendrites. Subcellular fractionation indicated that dynamin existed in the membrane fraction as well as in the soluble fraction. In ligated peripheral nerves, dynamin colocalized with tubulovesicular membranous organelles transported mainly anterogradely. By transfection of dynamin cDNA into mouse fibroblast L-cells, we showed it colocalized with some membranous organelles but not with microtubules. Our results show that dynamin is associated with membranous organelles in vivo, although a certain amount of dynamin also exists in the soluble fraction and is distributed diffusely throughout mature neurons. The data suggest that dynamin's fundamental role involves membrane trafficking in neurons in the central nervous system rather than in sliding microtubules as a motor protein.