Dendritic localization of type II calcium calmodulin-dependent protein kinase mRNA in normal and reinnervated rat hippocampus

Of Molecules and Mechanisms

Without question, molecular biology drives modern neuroscience. The past 50 years has been nothing short of revolutionary as key findings have moved the field from correlation toward causation. Most obvious are the discoveries and strategies that have been used to build tools for visualizing circuits, measuring activity, and regulating behavior. Less flashy, but arguably as important are the myriad investigations uncovering the actions of single molecules, macromolecular structures, and integrated machines that serve as the basis for constructing cellular and signaling pathways identified in wide-scale gene or RNA studies and for feeding data into informational networks used in systems biology. This review follows the pathways that were opened in neuroscience by major discoveries and set the stage for the next 50 years.

Stimulation-induced changes in diffusion and structure of calmodulin and calmodulin-dependent protein kinase II proteins in neurons

Calcium/calmodulin-dependent protein kinase II (CaMKII) and calmodulin (CaM) play essential roles in synaptic plasticity, which is an elementary process of learning and memory. In this study, fluorescence correlation spectroscopy (FCS) revealed diffusion properties of CaM, CaMKIIα and CaMKIIβ proteins in human embryonic kidney 293 (HEK293) cells and hippocampal neurons. A simultaneous multiple-point FCS recording system was developed on a random-access two-photon microscope, which facilitated efficient analysis of molecular dynamics in neuronal compartments. The diffusion of CaM in neurons was slower than that in HEK293 cells at rest, while the diffusion in stimulated neurons was accelerated and indistinguishable from that in HEK293 cells. This implied that activity-dependent binding partners of CaM exist in neurons, which slow down the diffusion at rest. Diffusion properties of CaMKIIα and β proteins implied that major populations of these proteins exist as holoenzymatic forms. Upon stimulation of neurons, the diffusion of CaMKIIα and β proteins became faster with reduced particle brightness, indicating drastic structural changes of the proteins such as dismissal from holoenzyme structure and further fragmentation.

Competitive tuning: Competition's role in setting the frequency-dependence of Ca2+-dependent proteins

A number of neurological disorders arise from perturbations in biochemical signaling and protein complex formation within neurons. Normally, proteins form networks that when activated produce persistent changes in a synapse’s molecular composition. In hippocampal neurons, calcium ion (Ca²⁺) flux through N-methyl-D-aspartate (NMDA) receptors activates Ca²⁺/calmodulin signal transduction networks that either increase or decrease the strength of the neuronal synapse, phenomena known as long-term potentiation (LTP) or long-term depression (LTD), respectively. The calcium-sensor calmodulin (CaM) acts as a common activator of the networks responsible for both LTP and LTD. This is possible, in part, because CaM binding proteins are “tuned” to different Ca²⁺ flux signals by their unique binding and activation dynamics. Computational modeling is used to describe the binding and activation dynamics of Ca²⁺/CaM signal transduction and can be used to guide focused experimental studies. Although CaM binds over 100 proteins, practical limitations cause many models to include only one or two CaM-activated proteins. In this work, we view Ca²⁺/CaM as a limiting resource in the signal transduction pathway owing to its low abundance relative to its binding partners. With this view, we investigate the effect of competitive binding on the dynamics of CaM binding partner activation. Using an explicit model of Ca²⁺, CaM, and seven highly-expressed hippocampal CaM binding proteins, we find that competition for CaM binding serves as a tuning mechanism: the presence of competitors shifts and sharpens the Ca²⁺ frequency-dependence of CaM binding proteins. Notably, we find that simulated competition may be sufficient to recreate the in vivo frequency dependence of the CaM-dependent phosphatase calcineurin. Additionally, competition alone (without feedback mechanisms or spatial parameters) could replicate counter-intuitive experimental observations of decreased activation of Ca²⁺/CaM-dependent protein kinase II in knockout models of neurogranin. We conclude that competitive tuning could be an important dynamic process underlying synaptic plasticity.

Learning and memory formation are likely associated with dynamic fluctuations in the connective strength of neuronal synapses. These fluctuations, called synaptic plasticity, are regulated by calcium ion (Ca²⁺) influx through ion channels localized to the post-synaptic membrane. Within the post-synapse, the dominant Ca²⁺ sensor protein, calmodulin (CaM), may activate a variety of downstream binding partners, each contributing to synaptic plasticity outcomes. The conditions at which certain binding partners most strongly activate are increasingly studied using computational models. Nearly all computational studies describe these binding partners in combinations of only one or two CaM binding proteins. In contrast, we combine seven well-studied CaM binding partners into a single model wherein they simultaneously compete for access to CaM. Our dynamic model suggests that competition narrows the window of conditions for optimal activation of some binding partners, mimicking the Ca²⁺-frequency dependence of some proteins in vivo. Further characterization of CaM-dependent signaling dynamics in neuronal synapses may benefit our understanding of learning and memory formation. Furthermore, we propose that competitive binding may be another framework, alongside feedback and feed-forward loops, signaling motifs, and spatial localization, that can be applied to other signal transduction networks, particularly second messenger cascades, to explain the dynamical behavior of protein activation.

Emerging role of CaMKII in neuropsychiatric disease

Although it has been known for decades that hippocampal calcium/calmodulin (CaM)-dependent protein kinase II (CaMKII) plays an essential role in learning and memory consolidation, the roles of CaMKII in other brain regions are only recently being explored in depth. A series of recent studies suggest that CaMKII dysfunction throughout the brain may underlie myriad neuropsychiatric disorders, including drug addiction, schizophrenia, depression, epilepsy, and multiple neurodevelopmental disorders, perhaps through maladaptations in glutamate signaling and neuroplasticity. I review here the structure, function, subcellular localization, and expression patterns of CaMKII isoforms, as well as recent advances demonstrating that disturbances in these properties may contribute to psychiatric disorders.

GluA2 mRNA distribution and regulation by miR-124 in hippocampal neurons

AMPA-type glutamate receptors mediate fast, excitatory neurotransmission in the brain, and their concentrations at synapses are important determinants of synaptic strength. We investigated the post-transcriptional regulation of GluA2, the calcium-impermeable AMPA receptor subunit, by examining the subcellular distribution of its mRNA and evaluating its translational regulation by microRNA in cultured mouse hippocampal neurons. Using computational approaches, we identified a conserved microRNA-124 (miR-124) binding site in the 3'UTR of GluA2 and demonstrated that miR-124 regulated the translation of GluA2 mRNA reporters in a sequence-specific manner in luciferase assays. While we hypothesized that this regulation might occur in dendrites, our biochemical and fluorescent in situ hybridization (FISH) data indicate that GluA2 mRNA does not localize to dendrites or synapses of mouse hippocampal neurons. In contrast, we detected significant concentrations of miR-124 in dendrites. Overexpression of miR-124 in dissociated neurons results in a 30% knockdown of GluA2 protein, as measured by immunoblot and quantitative immunocytochemistry, without producing any changes in GluA2 mRNA concentrations. While total GluA2 concentrations are reduced, we did not detect any changes in the concentration of synaptic GluA2. We conclude from these results that miR-124 interacts with GluA2 mRNA in the cell body to downregulate translation. Our data support a model in which GluA2 is translated in the cell body and subsequently transported to neuronal dendrites and synapses, and suggest that synaptic GluA2 concentrations are modified primarily by regulated protein trafficking rather than by regulated local translation.

Axonal cap-dependent translation regulates presynaptic p35

Axonal growth cones synthesize proteins during development and in response to injury in adult animals. Proteins locally translated in axons are used to generate appropriate responses to guidance cues, contribute to axon growth, and can serve as retrograde messengers. In addition to growth cones, mRNAs and translational machinery are also found along the lengths of axons where synapses form en passant, but contributions of intra-axonal translation to developing synapses are poorly understood. Here, we engineered a subcellular-targeting translational repressor to inhibit mRNA translation in axons, and we used this strategy to investigate presynaptic contributions of cap-dependent protein translation to developing CNS synapses. Our data show that intra-axonal mRNA translation restrains synaptic vesicle recycling pool size and that one target of this regulation is p35, a Cdk5 activating protein. Cdk5/p35 signaling regulates the size of vesicle recycling pools, p35 levels diminish when cap-dependent translation is repressed, and restoring p35 levels rescues vesicle recycling pools from the effects of spatially targeted translation repression. Together our findings show that intra-axonal synthesis of p35 is required for normal vesicle recycling in developing neurons, and that targeted translational repression provides a novel strategy to investigate extrasomal protein synthesis in neurons.

Age-dependent targeting of protein phosphatase 1 to Ca2+/calmodulin-dependent protein kinase II by spinophilin in mouse striatum

Mechanisms underlying age-dependent changes of dendritic spines on striatal medium spiny neurons are poorly understood. Spinophilin is an F-actin- and protein phosphatase 1 (PP1)-binding protein that targets PP1 to multiple downstream effectors to modulate dendritic spine morphology and function. We found that calcium/calmodulin-dependent protein kinase II (CaMKII) directly and indirectly associates with N- and C-terminal domains of spinophilin, but F-actin can displace CaMKII from the N-terminal domain. Spinophilin co-localizes PP1 with CaMKII on the F-actin cytoskeleton in heterologous cells, and spinophilin co-localizes with synaptic CaMKII in neuronal cultures. Thr286 autophosphorylation enhances the binding of CaMKII to spinophilin in vitro and in vivo. Although there is no change in total levels of Thr286 autophosphorylation, maturation from postnatal day 21 into adulthood robustly enhances the levels of CaMKII that co-immunoprecipitate with spinophilin from mouse striatal extracts. Moreover, N- and C-terminal domain fragments of spinophilin bind more CaMKII from adult vs. postnatal day 21 striatal lysates. Total levels of other proteins that interact with C-terminal domains of spinophilin decrease during maturation, perhaps reducing competition for CaMKII binding to the C-terminal domain. In contrast, total levels of α-internexin and binding of α-internexin to the spinophilin N-terminal domain increases with maturation, perhaps bridging an indirect interaction with CaMKII. Moreover, there is an increase in the levels of myosin Va, α-internexin, spinophilin, and PP1 in striatal CaMKII immune complexes isolated from adult and aged mice compared to those from postnatal day 21. These changes in spinophilin/CaMKII interactomes may contribute to changes in striatal dendritic spine density, morphology, and function during normal postnatal maturation and aging.

A quick and simple FISH protocol with hybridization-sensitive fluorescent linear oligodeoxynucleotide probes

Fluorescence in situ hybridization (FISH) is a powerful tool used in karyotyping, cytogenotyping, cancer diagnosis, species specification, and gene-expression analysis. Although widely used, conventional FISH protocols are cumbersome and time consuming. We have now developed a FISH method using exciton-controlled hybridization-sensitive fluorescent oligodeoxynucleotide (ECHO) probes. ECHO-FISH uses a 25-min protocol from fixation to mounting that includes no stringency washing steps. We use ECHO-FISH to detect both specific DNA and RNA sequences with multicolor probes. ECHO-FISH is highly reproducible, stringent, and compatible with other fluorescent cellular labeling techniques. The resolution allows detection of intranuclear speckles of poly(A) RNA in HeLa cells and dissociated hippocampal primary cultures, and mRNAs in the distal dendrites of hippocampal neurons. We also demonstrate detection of telomeric and centromeric DNA on metaphase mouse chromosomes. The simplicity of the ECHO-FISH method will likely accelerate cytogenetic and gene-expression analysis with high resolution.

Multiplexed dendritic targeting of alpha calcium calmodulin-dependent protein kinase II, neurogranin, and activity-regulated cytoskeleton-associated protein RNAs by the A2 pathway

In neurons, many different RNAs are targeted to dendrites where local expression of the encoded proteins mediates synaptic plasticity during learning and memory. It is not known whether each RNA follows a separate trafficking pathway or whether multiple RNAs are targeted to dendrites by the same pathway. Here, we show that RNAs encoding alpha calcium calmodulin-dependent protein kinase II, neurogranin, and activity-regulated cytoskeleton-associated protein are coassembled into the same RNA granules and targeted to dendrites by the same cis/trans-determinants (heterogeneous nuclear ribonucleoprotein [hnRNP] A2 response element and hnRNP A2) that mediate dendritic targeting of myelin basic protein RNA by the A2 pathway in oligodendrocytes. Multiplexed dendritic targeting of different RNAs by the same pathway represents a new organizing principle for coordinating gene expression at the synapse.

The transport of Staufen2-containing ribonucleoprotein complexes involves kinesin motor protein and is modulated by mitogen-activated protein kinase pathway

There is increasing evidence showing that mRNA is transported to the neuronal dendrites in ribonucleoprotein (RNP) complexes or RNA granules, which are aggregates of mRNA, rRNA, ribosomal proteins, and RNA-binding proteins. In these RNP complexes, Staufen, a double-stranded RNA-binding protein, is believed to be a core component that plays a key role in the dendritic mRNA transport. This study investigated the molecular mechanisms of the dendritic mRNA transport using green fluorescent protein-tagged Staufen2 produced employing a Sindbis viral expression system. The kinesin heavy chain was found to be associated with Staufen2. The inhibition of kinesin resulted in a significant decrease in the level of dendritic transport of the Staufen2-containing RNP complexes in neurons under non-stimulating or stimulating conditions. This suggests that the dendritic transport of the Staufen2-containing RNP complexes use kinesin as a motor protein. A mitogen-activated protein kinase inhibitor, PD98059, inhibited the activity-induced increase in the amount of both the Staufen2-containing RNP complexes and Ca(2+)/calmodulin-dependent protein kinase II alpha-subunit mRNA in the distal dendrites of cultured hippocampal neurons. Overall, these results suggest that dendritic mRNA transport is mediated via the Staufen2 and kinesin motor proteins and might be modulated by the neuronal activity and mitogen-activated protein kinase pathway.

Neonatal isolation accelerates the developmental switch in the signalling cascades for long-term potentiation induction

The molecular mechanisms underlying long-term potentiation (LTP) in the CA1 region of the hippocampus are known to vary with developmental age. The physiological factors regulating this developmental change, however, have not yet been elucidated. Here we show that mild neonatal isolation accelerates the developmental switch in the signalling cascades for hippocampal CA1 LTP induction from a cyclic AMP-dependent protein kinase (PKA)- to a Ca2(+)/calmodulin-dependent protein kinase II (CaMKII)-dependent pattern via the activation of the corticotrophin-releasing factor (CRF) system. Furthermore, this action appears to be mediated through an increased transcription of the alpha isoform of the CaMKII (CaMKIIalpha) gene. We also demonstrate that application of CRF to cultured hippocampal neurones significantly increases the expression of CaMKIIalpha, which is blocked by the non-specific CRF receptor antagonist astressin, the specific CRF receptor 1 antagonist NBI 27911, and the PKA inhibitor KT5720, but not by the CRF receptor 2 antagonist K 41498, or the protein kinase C inhibitor, bisindolylmaleimide I. CRF signalling also mediates the normal maturation of LTP. These results suggest a novel role for CRF in regulating early developmental events in the hippocampus, and indicate that, although maternal deprivation is stressful for the neonate, appropriate neonatal isolation can serve to promote an endocrine state that fosters the rate of maturation of the signalling cascades underlying the induction of LTP in the developing hippocampus.

Brain-derived neurotrophic factor induces mammalian target of rapamycin-dependent local activation of translation machinery and protein synthesis in neuronal dendrites

In neurons, perisynaptic or dendritic translation is implicated in synapse-wide alterations of function and morphology triggered by neural activity. The molecular mechanisms controlling local translation activation, however, have yet to be elucidated. Here, we show that local protein synthesis and translational activation in neuronal dendrites are upregulated by brain-derived neurotrophic factor (BDNF) in a rapamycin and small interfering RNA specific for mammalian target of rapamycin (mTOR)-sensitive manner. In parallel, BDNF induced the phosphorylation of tuberin and the activation of mTOR in dendrites and the synaptoneurosome fraction. mTOR activation stimulated translation initiation processes involving both eIF4E/4E-binding protein (4EBP) and p70S6 kinase/ribosomal S6 protein. BDNF induced phosphorylation of 4EBP in isolated dendrites. Moreover, local puff application of BDNF to dendrites triggered S6 phosphorylation in a restricted area. Taken together, these data indicate that mTOR-dependent translation activation is essential for the upregulation of local protein synthesis in neuronal dendrites.

Immunohistochemical study of Ca2+/calmodulin-dependent protein kinase II in the Drosophila brain using a specific monoclonal antibody

To analyze the distribution of Drosophila calcium/calmodulin-dependent protein kinase II (dCaMKII) in the adult brain, we generated monoclonal antibodies against the bacterially expressed 490-amino acid (a.a.) form of dCaMKII. One of those, named #18 antibody, was used for this study. Western blot analysis of the adult head extracts showed that the antibody specifically detects multiple bands between 55 and 60 kDa corresponding to the molecular weights of the splicing isoforms of dCaMKII. Epitope mapping revealed that it was in the region between 199 and 283 a.a. of dCaMKII. Preferential dCaMKII immunoreactivity in the embryonic nervous system, adult thoracic ganglion and gut, and larval neuro-muscular junction (NMJ) was consistent with previous observations by in situ hybridization and immunostaining with a polyclonal antibody at the NMJ, indicating that the antibody is applicable to immunohistochemistry. Although dCaMKII immunoreactive signal was low in the retina, it was found at regular intervals in the outer margin of the compound eye. These signals were most likely to be interommatidial bristle mechanosensory neurons. dCaMKII immunoreactivity in the brain was observed in almost all regions and relatively higher staining was found in the neuropilar region than in the cortex. Higher dCaMKII immunoreactivity in the mushroom body (MB) was found in the entire gamma lobe including the heel, and dorsal tips of the alpha and alpha' lobes, while cores of alpha and beta lobes were stained light. Finding abundant dCaMKII accumulation in the gamma lobe suggested that this lobe might especially require high levels of dCaMKII expression to function properly among MB lobes.

Developmentally regulated expression of CaMKII and iGluRs in the rat retina

Calcium/calmodulin-dependent protein kinase II (CaMKII) and the ionotropic glutamate receptors (iGluRs) have been shown to be pivotal in the maturation of synapses during development of the central nervous system. The purpose of the current study was to assay the expression profiles of these molecules during the development of the rat retina. The mRNA levels of CaMKII were determined by the semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) method. The protein levels of CaMKII were assayed in slot blots. The CaMKII enzyme activity was also measured. In addition, the protein levels of iGluRs in a retinal membrane-enriched fraction were evaluated in Western blots. The results show that the levels of CaMKII (mRNA, protein, and activity) and distinct subunits of iGluR proteins increased during the first 2 weeks after birth. The highest level of CaMKII was reached during the second postnatal week, coincident with the peak of synaptogenesis in the inner plexiform layer of the rat retina. The expressions of NMDAR-NR1 and -NR2A were relatively low in the first postnatal week but rose quickly thereafter. However, NMDAR-NR2B was relatively high at postnatal day 5 (P5) and increased steadily during the postnatal period. Thus, the subunit compositional profile of the retinal NMDARs was altered during retinal maturation. The developmental pattern of AMPAR-GluR1 was similar to that of NMDAR-NR2B, with high expression at P5, and modest increases thereafter. The patterns of CaMKII and NR1/NR2A were better correlated than were CaMKII and NR2B, or CaMKII and GluR1. The temporal differences in subunit expression of these synaptically relevant molecules suggest that they play distinct roles during the development of the retina.

Dendritic targeting of mRNAs for plasticity genes in experimental models of temporal lobe epilepsy

PURPOSE

To analyze whether the subcellular localization of the messenger RNAs (mRNAs) coding for the neurotrophin brain-derived neurotrophic factor (BDNF), its receptor TrkB, and the alpha and beta subunits of calcium-calmodulin-dependent kinase II (CaMKII) are modified after pilocarpine and kindled seizures.

METHODS

Epilepsy models: pilocarpine and kindling. Analysis of mRNA levels in the dendrites: high-resolution, nonradioactive in situ hybridization.

RESULTS

Nonstimulated rats: BDNF, TrkB, and CaMKII-beta mRNAs localized in the soma and in the proximal dendrites of hippocampal pyramidal cells, and in the soma only of dentate gyrus (DG) granule cells; CaMKII-alpha mRNA localized throughout the dendritic length in neurons of all hippocampal subfields. Pilocarpine seizures: increased staining levels of CaMKII-alpha mRNA throughout the whole dendritic length in all hippocampal subfields; induction of CaMKII-beta, BDNF, and TrkB mRNAs dendritic targeting in CA1, CA3, and DG neurons. Class 2 kindled seizures: increase in dendritic staining intensity for CaMKII-alpha in CA1, CA3, and DG neurons; induction of dendritic localization of CaMKII-beta, BDNF, and TrkB mRNAs in CA3 neurons. Fully kindled seizures: no change in the subcellular distribution of BDNF, TrkB and CaMKII-beta mRNAs; reduction of CaMKII-alpha mRNA dendritic staining, as compared with unstimulated kindled animals.

CONCLUSIONS

Data provide evidence that BDNF, TrkB, and CaMKII-alpha and -beta mRNAs are accumulated in the dendrites of specific hippocampal neurons during pilocarpine seizures and kindling development. The dendritic targeting of these genes may be causally involved in epileptogenesis and thus may represent a new therapeutic target for some forms of partial epilepsy.

Invited review: Estrogens effects on the brain: multiple sites and molecular mechanisms

Besides their well-established actions on reproductive functions, estrogens exert a variety of actions on many regions of the nervous system that influence higher cognitive function, pain mechanisms, fine motor skills, mood, and susceptibility to seizures; they also appear to have neuroprotective actions in relation to stroke damage and Alzheimer's disease. Estrogen actions are now recognized to occur via two different intracellular estrogen receptors, ER-alpha and ER-beta, that reside in the cell nuclei of some nerve cells, as well as by some less well-characterized mechanisms. In the hippocampus, such nerve cells are sparse in number and yet appear to exert a powerful influence on synapse formation by neurons that do not have high levels of nuclear estrogen receptors. However, we also find nonnuclear estrogen receptors outside of the cell nuclei in dendrites, presynaptic terminals, and glial cells, where estrogen receptors may couple to second messenger systems to regulate a variety of cellular events and signal to the nuclear via transcriptional regulators such as CREB. Sex differences exist in many of the actions of estrogens in the brain, and the process of sexual differentiation appears to affect many brain regions outside of the traditional brain areas involved in reproductive functions. Finally, the aging brain is responsive to actions of estrogens, which have neuroprotective effects both in vivo and in vitro. However, in an animal model, the actions of estrogens on the hippocampus appear to be somewhat attenuated with age. In the future, estrogen actions over puberty and in pregnancy and lactation should be further explored and should be studied in both the hypothalamus and the extrahypothalamic regions.

Tracking the estrogen receptor in neurons: implications for estrogen-induced synapse formation

Estrogens (E) and progestins regulate synaptogenesis in the CA1 region of the dorsal hippocampus during the estrous cycle of the female rat, and the functional consequences include changes in neurotransmission and memory. Synapse formation has been demonstrated by using the Golgi technique, dye filling of cells, electron microscopy, and radioimmunocytochemistry. N-methyl-d-aspartate (NMDA) receptor activation is required, and inhibitory interneurons play a pivotal role as they express nuclear estrogen receptor alpha (ERalpha) and show E-induced decreases of GABAergic activity. Although global decreases in inhibitory tone may be important, a more local role for E in CA1 neurons seems likely. The rat hippocampus expresses both ERalpha and ERbeta mRNA. At the light microscopic level, autoradiography shows cell nuclear [3H]estrogen and [125I]estrogen uptake according to a distribution that primarily reflects the localization of ERalpha-immunoreactive interneurons in the hippocampus. However, recent ultrastructural studies have revealed extranuclear ERalpha immunoreactivity (IR) within select dendritic spines on hippocampal principal cells, axon terminals, and glial processes, localizations that would not be detectable by using standard light microscopic methods. Based on recent studies showing that both types of ER are expressed in a form that activates second messenger systems, these findings support a testable model in which local, non-genomic regulation by estrogen participates along with genomic actions of estrogens in the regulation of synapse formation.

Colocalization of the mu-opioid receptor and calcium/calmodulin-dependent kinase II in distinct pain-processing brain regions

The mu-opioid receptor (MOR1) mediates the main analgesic effects of morphine and several other opioids. However, the clinical benefit of these drugs is limited by the development of tolerance and dependence. In vitro the mu-opioid receptor undergoes a rapid homologous desensitization during prolonged agonist exposure. We have recently identified the serine residues, Ser(261) and Ser(266), within the third intracellular loop as two consensus calcium/calmodulin-dependent protein kinase II (CaMKII) sites required for agonist-induced phosphorylation and desensitization of the mu-opioid receptor in HEK 293 cells. Since the specific pattern of mu-opioid receptor regulation in vivo is thought to depend on the cell- and tissue-specific complement of protein kinases, we examined the spatial relation between MOR1 and CaMKII in rat brain using specific antibodies. We found that MOR1 and CaMKII alpha which is a major CaMKII isoform expressed in the central nervous system co-exist in distinct pain-processing brain regions including the superficial layers of the spinal cord dorsal horn and dorsal root ganglia. At high power magnification it was evident that virtually all MOR1-expressing nociceptive spinal cord neurons also co-contain CaMKII. In naive or saline-treated animals the mu-opioid receptor was almost exclusively confined to the plasma membrane, while CaMKII was localized to vesicle-like structures throughout the cytoplasm. After subcutaneous administration of the mu-opioid receptor agonist, etorphine, a large proportion of the mu-opioid receptor proteins redistributed from the plasma membrane into the cytosol where it was frequently co-localized with CaMKII. Together, we identify CaMKII as a potential protein kinase, which by virtue of its colocalization with MOR1 may be in a position to phosphorylate the mu-opioid receptor and may thus contribute to the development of tolerance to opioid analgesics.

Two cis-acting elements in the 3' untranslated region of alpha-CaMKII regulate its dendritic targeting

Dendritic localization of the alpha subunit of Ca2+/calmodulin-dependent protein kinase II (alphaCaMKII) mRNA in CNS neurons requires its 3' untranslated region (3'UTR). We investigated this targeting mechanism by identifying two cis-acting elements in the 3'UTR. One is a 30-nucleotide element that mediated dendritic translocation. A homologous sequence in the 3'UTR of neurogranin, transcripts of which also reside in dendrites, also funtioned in cis to promote its dendritic transport. Other putative elements in the alphaCaMKII mRNA inhibit its transport in a resting state. This inhibition was removed in depolarized neurons, and such activity-dependent derepression was a primary requirement for their dendritic targeting.

Somatodendritic localization of Translin, a component of the Translin/Trax RNA binding complex

Recent studies implicating dendritic protein synthesis in synaptic plasticity have focused attention on identifying components of the molecular machinery involved in processing dendritic RNA. Although Translin was originally identified as a protein capable of binding single-stranded DNA, subsequent studies have demonstrated that it also binds RNA in vitro. Because previous studies indicated that Translin-containing RNA/single-stranded DNA binding complexes are highly enriched in brain, we and others have proposed that it may be involved in dendritic RNA processing. To assess this possibility, we have conducted studies aimed at defining the localization of Translin and its partner protein, Trax, in brain. In situ hybridization studies demonstrated that both Translin and Trax are expressed in neurons with prominent staining apparent in cerebellar Purkinje cells and neuronal layers of the hippocampus. Subcellular fractionation studies demonstrated that both Translin and Trax are highly enriched in the cytoplasmic fraction compared with nuclear extracts. Furthermore, immunohistochemical studies with Translin antibodies revealed prominent staining in Purkinje neuron cell bodies that extends into proximal and distal dendrites. A similar pattern of somatodendritic localization was observed in hippocampal and neocortical pyramidal neurons. These findings demonstrate that Translin is expressed in neuronal dendrites and therefore support the hypothesis that the Translin/Trax complex may be involved in dendritic RNA processing.

A working model of CaM kinase II activity in hippocampal long-term potentiation and memory

Recent advances in molecular genetics provide strong evidence for a relationship between hippocampal long-term potentiation (LTP) and hippocampus-dependent memory. The alpha-CaM kinase II knock-out mouse and transgenic mice expressing a mutant form of CaM kinase II clearly demonstrate that CaM kinase II plays a prominent role in hippocampal LTP and hippocampus-dependent memory. Furthermore, the observation that there is a diversity of silent as well as functional synapses has shed light on the molecular basis of learning and memory during development as well as in adult brain. Here we present a working model of CaM kinase II activity as a memory molecule in hippocampal LTP and describe molecular targets of CaM kinase II involved in the establishment of functional synapses following LTP induction.

Differential regulation of Ca(2+)-calmodulin stimulated and Ca(2+)-insensitive adenylyl cyclase messenger RNA in intact and denervated mouse hippocampus

The Ca(2+)-calmodulin stimulated AC1 and Ca(2+)-insensitive AC2 are major isoforms of adenylyl cyclase, playing an important role in synaptic plasticity in the mammalian brain. We studied the pattern of expression of AC1 and AC2 genes in the hippocampus of C57BL/6 mice. We found that there were differences in their patterns of distribution in the dentate gyrus. AC1 messenger RNA was detected both in the dentate granule cell bodies and the corresponding molecular field whereas AC2 messenger RNA was preferentially distributed in the dentate granule cell layer, suggesting that AC1 and AC2 messenger RNA are differentially regulated in the dentate gyrus. In order to examine the regulation of AC1 and AC2 expression in response to synaptic deafferentation and reinnervation, the distribution patterns of the two AC messenger RNA in the hippocampal fields and the parietal cortex were analysed 2, 5, 9 and 30 days following an unilateral entorhinal cortex lesion. Interestingly, we found significantly reduced levels of AC1 hybridization signal following the lesion whereas the level of AC2 messenger RNA remained unaffected in all lesioned groups. The changes in AC1 messenger RNA were transient, with a maximal reduction at five days postlesion, and were restricted to the granule cell bodies and stratum moleculare of the deafferented dentate gyrus. No significant change in AC1 messenger RNA levels was detected in other hippocampal fields nor for any other postlesion times studied. These findings suggest that, at least in the dentate gyrus, messenger RNA for AC1 and AC2 might be differentially compartmentalized in cell bodies and dendritic fields. The activity-dependent regulation of AC1 messenger RNA levels by afferent synapses may provide an elegant mechanism for achieving a selective local regulation of AC1 protein, close to its site of action.

Altered mRNA expression for brain-derived neurotrophic factor and type II calcium/calmodulin-dependent protein kinase in the hippocampus of patients with intractable temporal lobe epilepsy

The expression of brain-derived neurotrophic factor and the alpha subunit of calcium/calmodulin-dependent protein kinase II mRNA in hippocampi obtained during surgical resections for intractable temporal lobe epilepsy were examined. Both calcium/calmodulin-dependent protein kinase II and brain-derived neurotrophic factor are localized heavily within the hippocampus and have been implicated in regulating hippocampal activity (Kang and Schuman [1995] Science 267:1658-1662; Suzuki [1994] Intl J Biochem 26:735-744). Also, the autocrine and paracrine actions of brain-derived neurotrophic factor within the central nervous system make it a likely candidate for mediating morphologic changes typically seen in the epileptic hippocampus. Quantitative assessments of mRNA levels in epileptic hippocampi relative to autopsy controls were made by using normalized densitometric analysis of in situ hybridization. In addition, correlations between clinical data and mRNA levels were studied. Relative to autopsy control tissue, decreased hybridization to mRNA of the alpha subunit of calcium/calmodulin-dependent protein kinase II and increased hybridization to brain-derived neurotrophic factor mRNA were found throughout the granule cells of the epileptic hippocampus. There also was a significant negative correlation between the duration of epilepsy and the expression of mRNA for brain-derived neurotrophic factor. These results are similar qualitatively to those found in animal models of epilepsy and suggest that chronic seizure activity in humans leads to persistent alterations in gene expression. Furthermore, these alterations in gene expression may play a role in the etiology of the epileptic condition.

The suppression of testis-brain RNA binding protein and kinesin heavy chain disrupts mRNA sorting in dendrites

Ribonucleoprotein particles (RNPs) are thought to be key players in somato-dendritic sorting of mRNAs in CNS neurons and are implicated in activity-directed neuronal remodeling. Here, we use reporter constructs and gel mobility shift assays to show that the testis brain RNA-binding protein (TB-RBP) associates with mRNPs in a sequence (Y element) dependent manner. Using antisense oligonucleotides (anti-ODN), we demonstrate that blocking the TB-RBP Y element binding site disrupts and mis-localizes mRNPs containing (alpha)-calmodulin dependent kinase II (alpha)-CAMKII) and ligatin mRNAs. In addition, we show that suppression of kinesin heavy chain motor protein alters only the localization of (alpha)-CAMKII mRNA. Thus, differential sorting of mRNAs involves multiple mRNPs and selective motor proteins permitting localized mRNAs to utilize common mechanisms for shared steps.

Functional implications of the subunit composition of neuronal CaM kinase II

The assembly of 6-12 subunits of Ca(2+)/calmodulin-dependent kinase II (CaM kinase II) into holoenzymes is an important structural feature of the enzyme and its postulated role as a molecular detector of Ca(2+) oscillations. Using single cell reverse transcriptase-polymerase chain reaction, we show that alpha- and beta-CaM kinase II mRNAs are simultaneously present in the majority of hippocampal neurons examined and that co-assembly of their protein products into heteromers is therefore possible. The subunit composition of CaM kinase II holoenzymes was analyzed by immunoprecipitation with subunit-specific monoclonal antibodies. Rat forebrain CaM kinase II consists of heteromers composed of alpha and beta subunits at a ratio of 2:1 and homomers composed of only alpha subunits. We examined the functional effect of the heteromeric assembly by analyzing the calmodulin dependence of autophosphorylation. Recombinant homomers of alpha- or beta-CaM kinase II, as well as of alternatively spliced beta isoforms, have distinct calmodulin dependences for autophosphorylation based on differences in their calmodulin affinities. Half-maximal autophosphorylation of alpha is achieved at 130 nM calmodulin, while that for beta occurs at 15 nM calmodulin. In CaM kinase II isolated from rat forebrain, however, the calmodulin dependence for autophosphorylation of the beta subunits is shifted toward that of alpha homomers. This suggests that Thr(287) in beta subunits is phosphorylated by alpha subunits present in the same holoenzyme. Once autophosphorylated, beta-CaM kinase II traps calmodulin by reducing the rate of calmodulin dissociation.

Dendritic localization of mRNAs

The dendritic localization of mRNAs has been proposed to underlie the structural and functional polarity of neurons, as well as certain aspects of synaptic plasticity. Even though there is no conclusive evidence that such a localization is a physiological requirement, studies of mRNA localization in relation to function in other cell types and recent experiments on synaptic plasticity suggest that this proposal may be correct.

Signal transduction molecules at the glutamatergic postsynaptic membrane

We have applied techniques from modern molecular biology and biochemistry to unravel the complex molecular structure of the postsynaptic membrane at glutamatergic synapses in the central nervous system. We have characterized a set of new proteins that are constituents of the postsynaptic density, including PSD-95, densin-180, citron (a rho/rac effector protein), and synaptic gp130 Ras GAP (a new Ras GTPase-activating protein). The structure of PSD-95 revealed a new protein motif, the PDZ domain, that plays an important role in the assembly of signal transduction complexes at intercellular junctions. More recently, we have used new imaging tools to observe the dynamics of autophosphorylation of CaM kinase II in intact hippocampal tissue. We have been able to detect changes in the amount of autophosphorylated CaM kinase II in dendrites, individual synapses, and somas of hippocampal neurons following induction of long-term potentiation by tetanic stimulation. In addition, we have observed a specific increase in the concentration of CaM kinase II in dendrites of neurons receiving tetanic stimulation. This increase appears to be the result of dendritic synthesis of new protein. Over the next several years we will apply similar methods to study regulatory changes that occur at the molecular level in glutamatergic synapses in the CNS as the brain processes and stores new information.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Multiple subcellular mRNA distribution patterns in neurons: a nonisotopic in situ hybridization analysis

Previous studies have established that most of the mRNAs that neurons express are localized in the cell body and very proximal dendrites, whereas a small subset of mRNAs is present at relatively high levels in dendrites. It is not clear, however, whether particular mRNAs have the same subcellular distribution in different types of neurons or whether different types of neurons sort mRNAs in different ways. The present study was undertaken to address these questions. Nonisotopic in situ hybridization techniques were used to define the subcellular localization of representative mRNAs including beta-tubulin, low-molecular-weight neurofilament protein (NF-68), high-molecular-weight microtubule-associated protein (MAP2), growth-associated protein 43 (F1/GAP43), the alpha subunit of calcium/calmodulin-dependent protein kinase II (alpha CaMII kinase), and poly (A+) mRNA. The mRNAs for beta-tubulin, neurofilament 68, and F1/GAP43 were restricted to the region of the cell body and very proximal dendrites in most neurons. In some neuron types, however, labeling for NF-68 extended for considerable distances into dendrites. In some neurons that express MAP2, the mRNA was present at the highest levels in the proximal third to half of the dendritic arbor, whereas in other neurons the highest levels of labeling were in the cell body. In most neurons that express alpha CaMII kinase, the highest levels of the mRNA were in the cell body, but labeling was also present throughout dendrites. However, in a few types of neurons, alpha CaMII kinase mRNA was largely restricted to the cell body. The fact that there are no general rules for mRNA localization that apply to all neuron types implies the existence of neuron type-specific mechanisms that regulate mRNA distribution.

Visualization of the distribution of autophosphorylated calcium/calmodulin-dependent protein kinase II after tetanic stimulation in the CA1 area of the hippocampus

Autophosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) at threonine-286 produces Ca2+-independent kinase activity and has been proposed to be involved in induction of long-term potentiation by tetanic stimulation in the hippocampus. We have used an immunocytochemical method to visualize and quantify the pattern of autophosphorylation of CaMKII in hippocampal slices after tetanization of the Schaffer collateral pathway. Thirty minutes after tetanic stimulation, autophosphorylated CaM kinase II (P-CaMKII) is significantly increased in area CA1 both in apical dendrites and in pyramidal cell somas. In apical dendrites, this increase is accompanied by an equally significant increase in staining for nonphosphorylated CaM kinase II. Thus, the increase in P-CaMKII appears to be secondary to an increase in the total amount of CaMKII. In neuronal somas, however, the increase in P-CaMKII is not accompanied by an increase in the total amount of CaMKII. We suggest that tetanic stimulation of the Schaffer collateral pathway may induce new synthesis of CaMKII molecules in the apical dendrites, which contain mRNA encoding its alpha-subunit. In neuronal somas, however, tetanic stimulation appears to result in long-lasting increases in P-CaMKII independent of an increase in the total amount of CaMKII. Our findings are consistent with a role for autophosphorylation of CaMKII in the induction and/or maintenance of long-term potentiation, but they indicate that the effects of tetanus on the kinase and its activity are not confined to synapses and may involve induction of new synthesis of kinase in dendrites as well as increases in the level of autophosphorylated kinase.

Differential subcellular regulation of NMDAR1 protein and mRNA in dendrites of dentate gyrus granule cells after perforant path transection

Unilateral transection of the excitatory perforant path results in the acute deafferentation of a segregated zone on the distal dendrites of hippocampal dentate gyrus granule cells (i.e., outer molecular layer), followed by sprouting, reactive synaptogenesis, and a return of physiological and behavioral function. To investigate cellular mechanisms underlying NMDA receptor plasticity in response to such extensive synaptic reorganization, we quantitatively evaluated changes in intensity levels of NMDAR1 immunofluorescence and NMDAR1 mRNA hybridization within subcellular compartments of dentate gyrus granule cells 2, 5, and 9 d after perforant path lesions. There were no significant changes in either measure at 2 d postlesion. However, at 5 and 9 d postlesion, during the period of axonal sprouting and synaptogenesis, there was an increase in NMDAR1 immunolabeling that was restricted to the dendritic segments of the denervated outer molecular layer and the granule cell somata. In contrast, NMDAR1 mRNA levels at 5 and 9 d postlesion increased throughout the full extent of the molecular layer, including both denervated and nondenervated segments of granule cell dendrites. These findings reveal that NMDAR1 mRNA is one of a limited population of mRNAs that is transported into dendrites and further suggest that in response to terminal proliferation and sprouting, increased mRNA transport occurs throughout the full dendritic extent, whereas increased local protein synthesis is restricted to denervated regions of the dendrites whose afferent activity is perturbed. These results begin to elucidate the dynamic postsynaptic subcellular regulation of receptor subunits associated with synaptic plasticity after denervation.

Dendritic compartmentation of NMDA receptor mRNA in cultured hippocampal neurons

The intracellular localization of mRNA encoding the obligatory subunit for the N-methyl-D-aspartate (NMDA) receptor complex (NMDAR1) was determined in cultured hippocampal neurons by in situ hybridization. From the time at which dendrites begin to develop, labeling for NMDAR1 mRNA extended beyond cell somata into incipient dendritic processes. By 10 days in culture, label was found throughout dendritic arbors in a distribution pattern similar to that for microtublin associated proteins 2 mRNA, but contrasted sharply with that for GluR1 or beta-actin mRNAs, both of which were restricted to cell somata. NMDAR1 mRNA was also expressed in astrocytes, where it was diffusely localized throughout the cytoplasm. These data suggest that transport of NMDAR1 mRNA may serve as a mechanism to target and manipulate local concentrations of NMDA receptors.

Single-cell analysis of gene expression in the nervous system. Measurements at the edge of chaos

The characteristic functions of tissues and organs result from the integrated activity of individual cells. Nowhere is this more evident than in the nervous system, where the activities of single neurons communicating via electrical and chemical signals mediate complex functions, such as learning and memory. The past decade has seen an explosion in the identification of genes encoding proteins, such as voltage-gated channels and neurotransmitter receptors, responsible for neuronal excitability. These studies have highlighted the fact that even within a neuroanatomically defined region, the coexistence of multiple cell types makes it difficult, if not impossible, to correlate patterns of gene expression with function. The recent development of techniques sensitive enough to study gene expression at the single-cell level promises to break this bottleneck to our further understanding. Using examples taken from our own laboratories and the work of others, we review these techniques, their application, and discuss some of the difficulties associated with the interpretation of the data.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

CaM I mRNA is localized to apical dendrites during postnatal development of neurons in the rat brain

In the rat, a single calmodulin (CaM) protein is encoded by three separate genes which produce five different transcripts. The significance of the multiple CaM genes is not known; however, individual CaM transcripts could be targeted to specific intracellular sites. In this report, the cellular distribution of CaM I mRNAs was analyzed in the postnatal rat brain. The 4.0-kb CaM I transcript was present in neuronal cell bodies and also localized to apical dendritic processes. In cerebral cortical neurons, the 4.0-kb CaM I mRNA was detected in apical dendrites at postnatal day (PD) 5 to 15. In hippocampal neurons, this CaM message was present in dendritic processes from PD S to 20, whereas in Purkinje neurons it was detected in dendrites at PD 15 and 20. The presence of the 4.0-kb CaM I mRNA in dendrites of the rat brain supports the notion of targeting transcripts derived from the CaM multigene family to discrete intracellular destinations.

Alterations of Ca2+/calmodulin-dependent protein kinase II and its messenger RNA in the rat hippocampus following normo- and hypothermic ischemia

The change in the subcellular distribution of Ca2+/calmodulin-dependent protein kinase II was studied in the rat hippocampus following normothermic and hypothermic transient cerebral ischemia of 15 min duration. A decrease in immunostaining of Ca2+/calmodulin-dependent protein kinase II was observed at 1 h of reperfusion which persisted until cell death in the CA1 region. In the CA3 and dentate gyrus areas immunostaining recovered at one to three days of reperfusion. The CA2+/calmodulin-dependent protein kinase II was translocated to synaptic junctions during ischemia and reperfusion which could be due to a persistent change in the intracellular calcium ion homeostasis. The expression of the messenger RNA of the alpha-subunit of Ca2+/calmodulin-dependent protein kinase II decreased in the entire hippocampus during reperfusion, and was most marked in the dentate gyrus at 12 h of reperfusion. This decrease could be a feedback downregulation of the mRNA due to increased Ca2+/calmodulin-dependent protein kinase II activation. Intraischemic hypothermia protected against ischemic neuronal damage and attenuated the ischemia-induced decrease of Ca2+/calmodulin-dependent protein kinase II immunostaining in all hippocampal regions. Hypothermia also reduced the translocation of Ca2+/calmodulin-dependent protein kinase II and restored Ca2+/calmodulin-dependent protein kinase II alpha messenger RNA after ischemia. The data suggest that ischemia leads to an aberrant Ca2+/calmodulin-dependent protein kinase II mediated signal transduction in the CA1 region, which is important for the development of delayed neuronal damage. Hypothermia enhances the restoration of the Ca2+/calmodulin-dependent protein kinase II mediated cell signalling.

Decreased expression of the alpha subunit of Ca2+/ calmodulin-dependent protein kinase type II mRNA in the adult rat CNS following recurrent limbic seizures

Calcium/calmodulin-dependent protein kinase type II (CamKII) is a ubiquitous brain enzyme implicated in a wide variety of neuronal processes. Understanding CamKII has become increasingly complicated with the recent identification of multiple gene transcripts coding for separate subunits. Previous studies have shown that mRNA for the alpha subunit of CamKII can be increased by reduction of afferent input. In this study we have examined the regulation of alpha CamKII mRNA following increased activity due to seizures. Using in situ hybridization with a cRNA probe against the rat alpha CamKII sequence we found reduced levels of hybridization following limbic seizures induced by lesions of the hilus of the dentate gyrus. Hybridization was most dramatically reduced in the granule cells of the dentate gyrus and the pyramidal cells of hippocampal region CA1. There were also significant reductions in hybridization in the superficial layers of neocortex and piriform cortex. In each of these region hybridization was decreased in the molecular layers which is consistent with the reported dendritic localization of alpha CamKII mRNA. All changes in mRNA content were transient, with maximal reductions at 24 h following lesion placement and a return to control levels by 96 h. These findings demonstrate the negative regulation of alpha CamKII mRNA by seizure activity and raise the possibility that synthesis of this kinase may be regulated by normal physiological activity.

The process of reinnervation in the dentate gyrus of adult rats: gene expression by neurons during the period of lesion-induced growth

Neurons in the hippocampal dentate gyrus are extensively reinnervated following the destruction of their normal inputs from the ipsilateral entorhinal cortex (EC). The present study evaluates gene expression by dentate granule neurons and the neurons giving rise to the sprouting connections during the period of synapse growth. Adult male rats were prepared for in situ hybridization at 2, 4, 6, 8, 10, 12, 14, 20, and 30 days following unilateral EC lesions. Sections were hybridized using 35S-labeled cRNA probes for mRNAs that encode proteins thought to be important for neuronal structure and/or synapse function, including (1) mRNAs that are normally present in dendrites--the mRNAs for the high molecular weight microtubule-associated protein 2 (MAP2) and the alpha-subunit of calcium/calmodulin-dependent protein kinase II (CAMII kinase), (2) mRNAs that are upregulated in neurons that are regenerating their axons (T alpha 1 tubulin and F1/GAP43) and (3) mRNAs for proteins that are the principal constituents of neurofilaments and microtubules (the low molecular weight neurofilament protein NF68 and beta-tubulin). Although there were small changes in the levels of labeling for the mRNAs that are normally present in dendrites, there were no dramatic increases in the levels of any of the mRNAs either in dentate granule cells or in neurons giving rise to the reinnervating fibers at any postlesion interval. These results indicate that neurons in mature animals can substantially remodel their synaptic terminals and their dendrites in the absence of large-scale changes in gene expression (at least as measured by steady-state mRNA levels at various time points).

Increased phosphorylation of Ca2+/calmodulin-dependent protein kinase II and its endogenous substrates in the induction of long-term potentiation

Induction of long-term potentiation in the CA1 region of hippocampal slices is associated with increased activity of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) (Fukunaga, K., Stoppini, L., Miyamoto, E., and Muller, D. (1993) J. Biol. Chem. 268, 7863-7867). Here we report that application of high but not low frequency stimulation to two groups of afferents in the CA1 region of 32P-labeled slices resulted in the phosphorylation of two major substrates of this enzyme, synapsin I and microtubule-associated protein 2, as well as in the autophosphorylation of CaM kinase II. Furthermore, immunoblotting analysis revealed that long term potentiation induction was associated with an increase in the amount of CaM kinase II in the same region. All these changes were prevented when high frequency stimulation was applied in the presence of the N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonopentanoate. These results indicate that activation of CaM kinase II is involved in the induction of synaptic potentiation in both the postsynaptic and presynaptic regions.

mRNA distribution within dendrites: relationship to afferent innervation

The majority of neuronal mRNAs are confined to cell bodies, but a few mRNAs are present at high levels in dendrites. Here we report an initial analysis of the relationship between afferent innervation and the distribution of mRNA within dendritic fields. In situ hybridization techniques were used to compare the subcellular distribution of dendritic mRNAs in principal neurons of the hippocampal formation in vivo. The mRNA encoding the alpha subunit of calcium/calmodulin dependent protein kinase II (CAMII kinase) was present at high levels throughout the layers that contain the dendrites of hippocampal pyramidal cells and dentate granule cells. In contrast, the mRNA encoding the high molecular weight microtubule-associated protein MAP2 had a more limited distribution. In the dentate gyrus, labeling for MAP2 was present in a discrete band in the lamina containing proximal dendrites and decreased to low levels in laminae containing distal dendrites. This laminar pattern resembles the distinct terminations of the commissural/associational projection (high MAP2 labeling) and the entorhinal projection (lower MAP2 labeling) upon dendrites of granule cells. To determine if the differential distribution of dendritic mRNAs was regulated by either the presence or activity of afferents, we evaluated mRNA distribution in the dentate molecular layer following (1) removal of the entorhinal input by lesions of the entorhinal cortex or (2) prolonged delivery of potentiating stimulation to entorhinal afferents. Denervation led to modest decreases in the levels of mRNAs for both CAMII and MAP2 but did not lead to detectable alterations in mRNA distribution. Also, prolonged stimulation did not lead to detectable alterations in MAP2 or CAMII mRNA distribution although such stimulation clearly elevated the expression of mRNA for glial fibrillary acidic protein (GFAP).

Transcripts encoding a neural membrane CD26 peptidase-like protein are stimulated by synaptic activity

We isolated a cDNA clone, named BSPL, that encodes a brain-specific dipeptidyl peptidase-like protein with 30% identity and 50% similarity to CD26, a lymphocyte membrane antigen involved in T-cell activation. BSPL lacks, however, the catalytic residue responsible for peptidase activity. The expression of BSPL is widespread throughout the CNS but restricted to neurons under normal conditions. Twenty-four hours after injection of kainic acid into the hippocampus, a dramatic increase in the concentration of BSPL mRNA was detected by in situ hybridization in the CA3 region of the injected hemisphere as compared with the contralateral hemisphere or sham-injected animals. An increase in the steady-state level of BSPL mRNA concentration was also found following tetanic stimulation of the perforant path to produce LTP in granule cells of the dentate gyrus. Hybridization signals could be detected in dendritic processes of pyramidal neurons and in some glial cells upon either type of stimulation. These data suggest that BSPL may be involved in synaptic plasticity.

Alternative splicing introduces a nuclear localization signal that targets multifunctional CaM kinase to the nucleus

Intracellular targeting may enable protein kinases with broad substrate-specificities, such as multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) to achieve a selectivity of action in vivo. We have examined the intracellular targeting of three delta-CaM kinase isoforms. The delta B-CaM kinase isoform is targeted to the nucleus in transfected cells while the delta A- and delta C-CaM kinase isoforms are cytosolic/cytoskeletal. A chimeric construct of alpha-CaM kinase containing the delta B-CaM kinase variable domain is rerouted to the nucleus while the native alpha-CaM kinase and chimeras of alpha-CaM kinase which contain the delta A- or delta C-CaM kinase variable domains are retained in the cytoplasm. Using site-directed mutagenesis, we have defined a nuclear localization signal (NLS) within an 11-amino acid sequence, likely inserted by alternative splicing, in the variable domain of delta B-CaM kinase. Isoform-specific nuclear targeting of CaM kinase is probably a key mechanism in the selective regulation of nuclear functions by CaM kinase. CaM kinase is a multimer that can be composed of several isoforms. We find that when cells express two different isoforms of CaM kinase, cellular targeting is determined by the ratio of the isoforms. When an excess of the cytoplasmic isoform of CaM kinase is coexpressed along with the nuclear isoform, both isoforms are localized in the cytoplasm. Conversely an excess of the nuclear isoform can reroute the cytoplasmic isoform to the nucleus. The nuclear isoform likely coassembles with the cytosolic isoform, to form a heteromultimeric holoenzyme which is transported into the nucleus. These experiments demonstrate isoform-specific targeting of CaM kinase and indicate that such targeting can be modified by the expression of multiple isoforms of the enzyme.

Differential regulation of brain-derived neurotrophic factor and type II calcium/calmodulin-dependent protein kinase messenger RNA expression in Alzheimer's disease

The relative levels of messenger RNA for brain-derived neurotrophic factor and the alpha subunit of calcium/calmodulin-dependent protein kinase type II were examined in hippocampal sections from Alzheimer's diseased and age matched non-diseased brains by in situ hybridization histochemistry. Consistent with previous reports in monkey and rodent, calcium/calmodulin-dependent protein kinase II messenger RNA was prevalent throughout the dentate gyrus, all the principal hippocampal subfields, and adjacent cortical regions. A distribution consistent with the dendritic localization of calcium/calmodulin-dependent protein kinase II was also observed. In contrast, brain-derived neurotrophic factor messenger RNA levels were much lower than calcium/calmodulin-dependent protein kinase II messenger RNA levels and were less widely distributed. Within the hippocampus of Alzheimer's diseased brains, levels of calcium/calmodulin-dependent protein kinase II messenger RNA were increased and levels of brain-derived neurotrophic factor messenger RNA were decreased in comparison with matched controls. These changes were consistently seen in four out of six cases processed for both messenger RNA species and ranged from 150-300% relative to non-diseased brain tissue for calcium/calmodulin-dependent protein kinase II and 20-70% for brain-derived neurotrophic factor. These results suggest that within the Alzheimer's hippocampus an altered program of gene expression is occurring leading to aberrant levels of both calcium/calmodulin-dependent protein kinase II and brain-derived neurotrophic factor messenger RNA. Previous studies of the activity-dependent regulation of these messenger RNA species suggest these results are consistent with a decrease in afferent activity within the Alzheimer's hippocampus.

Dynamics of synapsin I gene expression during the establishment and restoration of functional synapses in the rat hippocampus

Synapse development and injury-induced reorganization have been extensively characterized morphologically, yet relatively little is known about the underlying molecular and biochemical events. To examine molecular mechanisms of synaptic development and rearrangement, we looked at the developmental pattern of expression of the neuron-specific gene synapsin I in granule cell neurons of the dentate gyrus and their accompanying mossy fibers during the main period of synaptogenic differentiation in the rat hippocampus. We found a significant difference between the temporal expression of synapsin I messenger RNA in dentate granule somata and the appearance of protein in their mossy fiber terminals during the postnatal development of these neurons. Next, to investigate the regulation of neuron-specific gene expression during the restoration of synaptic contacts in the central nervous system, we examined the expression of the synapsin I gene following lesions of hippocampal circuitry. These studies show marked changes in the pattern and intensity of synapsin I immunoreactivity in the dendritic fields of dentate granule cell neurons following perforant pathway transection. In contrast, changes in synapsin I messenger RNA expression in target neurons, and in those neurons responsible for the reinnervation of this region of the hippocampus, were not found to accompany new synapse formation. On a molecular level, both developmental and lesion data suggest that the expression of the synapsin I gene is tightly regulated in the central nervous system, and that considerable changes in synapsin I protein may occur in neurons without concomitant changes in the levels of its messenger RNA. Finally, our results suggest that the appearance of detectable levels of synapsin I protein in in developing and sprouting synapses coincides with the acquisition of function by those central synapses.

Subcellular distribution of rRNA and poly(A) RNA in hippocampal neurons in culture

In situ hybridization was used to assess the subcellular distribution of rRNA and poly(A) RNA in hippocampal neurons maintained in culture. Labeling produced with 35S-labeled probes to either rRNA or poly(A) was heaviest over the cell body with lighter, patchy labeling of proximal dendrites. In contrast, 3H-labeled probes labeled dendrites throughout their length, and the ratio of dendritic to cell body labeling was higher with 3H-labeled probes. There was no detectable labeling of axons of mature neurons with either probe. The pattern of hybridization produced by 35S-labeled oligonucleotide probes to rRNA varied depending on the concentration of the oligonucleotide. These studies provide the first detailed study of the subcellular distribution of rRNA and poly(A) RNA in neurons, and highlight technical issues to consider when evaluating results of hybridizations carried out with 35S- and 3H-labeled probes on cells in culture.

Inhibition of protein synthesis alters the subcellular distribution of mRNA in neurons but does not prevent dendritic transport of RNA

This study evaluates whether protein synthesis plays a role in targeting RNA molecules to different subcellular domains within neurons. Transport of newly synthesized RNA (labeled with [3H]uridine) was examined in the presence of the protein synthesis inhibitors puromycin and cycloheximide. In situ hybridization was used to determine whether inhibition of protein synthesis altered the subcellular distribution of mRNAs. Transport of recently synthesized RNA was not disrupted after prolonged exposure to either inhibitor. However, inhibition of protein synthesis caused several mRNAs that are normally confined to the cell body to appear in dendrites. The distribution of mRNAs that are normally present in dendrites was unaffected. These findings suggest that protein synthesis is not required to translocate RNA into the dendrites but may play a role in restricting particular mRNAs to the neuronal cell body.

Localization of the multiple calmodulin messenger RNAs in differentiated PC12 cells

Calmodulin, a ubiquitous calcium-binding protein which is involved in many biological processes, including cell proliferation and differentiation, has been shown to be encoded by three genes from which five calmodulin messenger RNAs are transcribed. In our previous studies, using the PC12 pheochromocytoma cell line as a model system for neuronal differentiation, all five calmodulin messenger RNAs were found to be present, and treatment with both nerve growth factor and dibutyryl cyclic AMP, which induce neurite outgrowth in these cells, increased the level of calmodulin and differentially increased the levels of the various calmodulin messenger RNAs. In an attempt to uncover the nature of the differential increase in the calmodulin messenger RNAs during neuronal differentiation, we examined here the subcellular distribution of the individual calmodulin messenger RNAs in PC12 cells treated with nerve growth factor and dibutyryl cyclic AMP by in situ hybridization cytochemistry, using radiolabeled oligodeoxynucleotide probes. Using an oligodeoxynucleotide probe which detects all of the calmodulin transcripts, the calmodulin messenger RNAs were found to be distributed throughout the cell bodies of differentiated PC12 cells; significant amounts of calmodulin messenger RNAs were also found in most neurites (approximately 70% of the total number). Using specific probes for the calmodulin messenger RNAs derived from each calmodulin gene, distinct patterns of localization of the different calmodulin messenger RNAs were revealed. The messenger RNAs from calmodulin genes I and II were readily detected in all cell bodies and in about one-half of the neurites. In contrast, a weak signal for the messenger RNAs from calmodulin gene III was associated with cell bodies, while no significant signal was found in neurites. A population distribution analysis of the labeling of individual PC12 cell bodies, as determined by counting autoradiographic grains, revealed differences in the relative abundance of each group of messenger RNAs derived from each of the three calmodulin genes. The order of relative abundance of the messenger RNAs in cell bodies was found to be: calmodulin gene II messenger RNA > calmodulin gene I messenger RNAs >> calmodulin gene III messenger RNAs. An analysis of the labeling density along neurites indicated a similar density of neuritic messenger RNAs from calmodulin gene I and calmodulin gene II, whereas there was no significant signal for the messenger RNAs from calmodulin gene III.(ABSTRACT TRUNCATED AT 400 WORDS)