Enhanced learning after genetic overexpression of a brain growth protein

Genetic enhancement of visual learning by activation of protein kinase C pathways in small groups of rat cortical neurons

Although learning and memory theories hypothesize that memories are encoded by specific circuits, it has proven difficult to localize learning within a cortical area. Neural network theories predict that activation of a small fraction of the neurons in a circuit can activate that circuit. Consequently, altering the physiology of a small group of neurons might potentiate a specific circuit and enhance learning, thereby localizing learning to that circuit. In this study, we activated protein kinase C (PKC) pathways in small groups of neurons in rat postrhinal (POR) cortex. We microinjected helper virus-free herpes simplex virus vectors that expressed a constitutively active PKC into POR cortex. This PKC was expressed predominantly in glutamatergic and GABAergic neurons in POR cortex. This intervention increased phosphorylation of five PKC substrates that play critical roles in neurotransmitter release (GAP-43 and dynamin) or glutamatergic neurotransmission (specific subunits of AMPA or NMDA receptors and myristoylated alanine-rich C kinase substrate). Additionally, activation of PKC pathways in cultured cortical neurons supported activation-dependent increases in release of glutamate and GABA. This intervention enhanced the learning rate and accuracy of visual object discriminations. In individual rats, the numbers of transfected neurons positively correlated with this learning. During learning, neuronal activity was increased in neurons proximal to the transfected neurons. These results demonstrate that potentiating small groups of glutamatergic and GABAergic neurons in POR cortex enhances visual object learning. More generally, these results suggest that learning can be mediated by specific cortical circuits.

Specific Proteolysis of Neuronal Protein GAP-43 by Calpain: Characterization, Regulation, and Physiological Role

The mechanism of specific proteolysis of the neuronal protein GAP-43 in axonal terminals has been investigated. In synaptic terminals in vivo and in synaptosomes in vitro GAP-43 is cleaved only at the single peptide bond formed by Ser41; this is within the main effector domain of GAP-43. Proteolysis at this site involves the cysteine calcium-dependent neutral protease calpain. The following experimental evidences support this conclusion: 1) calcium-dependent proteolysis of GAP-43 in synaptosomes is insensitive to selective inhibitor of micro-calpain (PD151746), but it is completely blocked by micro- and m-calpain inhibitor PD150606; 2) GAP-43 proteolysis in the calcium ionophore A23187-treated synaptosomes is activated by millimolar concentration of calcium ions; 3) the pattern of fragmentation of purified GAP-43 by m-calpain (but not by micro-calpain) is identical to that observed in synaptic terminals in vivo. GAP-43 phosphorylated at Ser41 by protein kinase C (PKC) is resistant to the cleavage by calpain. In addition, calmodulin binding to GAP-43 decreases the rate of calpain-mediated GAP-43 proteolysis. Our results indicate that m-calpain-mediated GAP-43 proteolysis regulated by PKC and calmodulin is of physiological relevance, particularly in axonal growth cone guidance. We suggest that the function of the N-terminal fragment of GAP-43 (residues 1-40) formed during cleavage by m-calpain consists in activation of neuronal heterotrimeric GTP-binding protein G(o); this results in growth cone turning in response to repulsive signals.

Enhanced learning of normal adult rodents by repeated oral administration of soybean transphosphatidylated phosphatidylserine

Soybean lecithin transphosphatidylated phosphatidylserine (SB-tPS) is already known to improve the learning ability of aged or drug-induced amnesic rodents. In this study, its effect on normal adult rodents was evaluated using several learning tasks. Firstly, three behavioral tests (open-field, Y-maze, and active avoidance test) were consecutively carried out after the daily oral administration of SB-tPS (50 mg/kg per day, for 34 days). Repeated oral administration of SB-tPS did not affect either exploratory behavior in the open-field test or spontaneous alternation behavior in the Y-maze test, while mice pretreated with SB-tPS showed significant enhancement of conditioned avoidance response. Secondly, the brightness discrimination test was used to evaluate the effect of SB-tPS on learning ability. The daily oral administration of SB-tPS (50 mg/kg per day, for 27 days) to normal rats significantly increased the correct response ratio in the brightness discrimination test. Finally, to elucidate the necessity of SB-tPS pretreatment, another active avoidance test was carried out, and no enhancement of conditioned avoidance response was observed in non-pretreated mice. These results suggest that repeated administration of SB-tPS could enhance the learning ability of normal adult rodents as those of aged ones.

Exercise restores levels of neurotrophins and synaptic plasticity following spinal cord injury

We have conducted studies to determine the potential of exercise to benefit the injured spinal cord using neurotrophins. Adult rats were randomly assigned to one of three groups: (1) intact control (Con); (2) sedentary, hemisected at a mid-thoracic level (Sed-Hx), or (3) exercised, hemisected (Ex-Hx). One week after surgery, the Ex-Hx rats were exposed to voluntary running wheels for 3, 7, or 28 days. BDNF mRNA levels on the lesioned side of the spinal cord lumbar region of Sed-Hx rats were approximately 80% of Con values at all time points and BDNF protein levels were approximately 40% of Con at 28 days. Exercise compensated for the reductions in BDNF after hemisection, such that BDNF mRNA levels in the Ex-Hx rats were similar to Con after 3 days and higher than Con after 7 (17%) and 28 (27%) days of exercise. After 28 days of exercise, BDNF protein levels were 33% higher in Ex-Hx than Con rats and were highly correlated (r=0.86) to running distance. The levels of the downstream effectors for the action of BDNF on synaptic plasticity synapsin I and CREB were lower in Sed-Hx than Con rats at all time points. Synapsin I mRNA and protein levels were higher in Ex-Hx rats than Sed-Hx rats and similar to Con rats at 28 days. CREB mRNA values were higher in Ex-Hx than Sed-Hx rats at all time points. Hemisection had no significant effects on the levels of NT-3 mRNA or protein; however, voluntary exercise resulted in an increase in NT-3 mRNA levels after 28 days (145%). These results are consistent with the concept that synaptic pathways under the regulatory role of BDNF induced by exercise can play a role in facilitating recovery of locomotion following spinal cord injury.

Hippocampal-dependent memory is impaired in heterozygous GAP-43 knockout mice

Cajal proposed that the rearrangement and growth of neurites and synaptic terminals is a substrate for the formation and storage of long-term memories. Proteins that regulate this learning-dependent growth are therefore likely to be "core determinants" (Sanes and Lichtman, Nat Neurosci 1999; 2:597-604) of such information storage processes. Although the growth-associated, protein kinase C (PKC) substrate GAP-43 has been oft-implicated in synaptic plasticity and memory, it has never been demonstrated that a reduction in the level of this protein has a deleterious effect on memory, because most homozygotes die perinatally. In this report, we observe significant memory impairments in heterozygous GAP-43 knockout mice with GAP-43 levels reduced by one-half. Impaired memory for a context was demonstrated in contextual fear conditioning. Importantly, no significant impairments in cued conditioning or on tests of nociceptive or auditory perception were observed in the heterozygous knockout, indicating that the observed impairments were unlikely related to performance or acquisition factors and are the result of reduced GAP-43 levels in the hippocampus. The present results, taken together with the prior demonstration of enhanced memory in transgenic mice overexpressing GAP-43, provide strong evidence for a pivotal role of hippocampal GAP-43 in the bidirectional regulation of mnemonic processing.

Differential effect of retinoic acid and triiodothyronine on the age-related hypo-expression of neurogranin in rat

Given the important role of retinoids and thyroid hormone for optimal brain functioning and the tenuous relationship between retinoic acid (RA) and triiodothyronine (T3) signalings, we compared the effects of RA or T3 administrations on RA and T3 nuclear receptors (RAR, RXR and TR) and on their target genes, neuromodulin (GAP43) and neurogranin (RC3) in 24-month-old rats. Quantitative real time PCR and western blot analysis allowed us to verify that retinoid and thyroid signalings and GAP43 and RC3 expression are affected by age. By in situ hybridization we observed a decreased expression of RC3 in hippocampus, striatum and cerebral cortex. RARbeta, RXRbeta/gamma and GAP43 were up-regulated by RA as well as T3 treatment. The abundance of TRalpha/beta mRNA and RC3 expression were only increased by T3 administration in the whole brain. This up-regulator effect of T3 on RC3 was only observed in the striatum. During aging, T3 become a limiting factor alone able to correct the age-related concomitant hypo-activation of retinoid and thyroid signalings and alterations of synaptic plasticity.

Behavioral characterization in a comprehensive mouse test battery reveals motor and sensory impairments in growth-associated protein-43 null mutant mice

The growth-associated protein (GAP)-43 is a major neuronal protein associated with axonal growth, neuronal plasticity and learning. The observation that only 5-10% of mice with a full GAP-43 gene deletion survive weaning suggests that basic neural functions are disturbed. Here we used a comprehensive test battery to characterise and quantify the motor and sensory function of surviving adult homozygous GAP-43 (-/-) mice as compared with GAP-43 (+/-) and wild-type animals. The test battery was comprised of motor, sensory, and reflex tests producing 25 measures of locomotion, as well as epicritic, auditory, olfactory and visual function. The analysis revealed significant impairments in muscle strength, limb coordination and balance in GAP-43 (-/-) mice. Furthermore, GAP-43 (-/-) animals were hyperactive and showed reduced anxiety as measured by open field and light dark tests. In sensory tests, GAP-43 (-/-) mice were tested for impaired tactile and labyrinthine function. Abnormal reflexes were found in the contact and vibrissa placing responses, and in the crossed extensor reflex. GAP-43 (+/-) animals showed only moderate abnormalities as compared with wild-type animals. We conclude that GAP-43 is necessary for the development and function of a variety of neuronal systems. The results also show that the comprehensive test battery used in the present study represents a sensitive approach to assess the functional integrity of ascending and descending pathways in genetically manipulated mice.

Growth-associated protein 43 (GAP-43) and synaptophysin alterations in the dentate gyrus of patients with schizophrenia

Growth-associated protein 43 (GAP-43) expression is critical for the proper establishment of neural circuitry, a process thought to be disrupted in schizophrenia. Previous work from our laboratory demonstrated decreased GAP-43 levels in post-mortem tissue from the entire hippocampal formation of affected individuals. In the present study, we used immunocytochemical techniques to localize alterations in GAP-43 protein to specific synapses. GAP-43 distribution was compared to that of synaptophysin, another synaptic protein known to be altered in schizophrenia. The levels and distribution of GAP-43 and synaptophysin proteins were measured in the dentate gyrus of subjects with schizophrenia and sex-, age-, and postmortem interval-matched normal controls and subjects with bipolar disorder. Tissue from subjects was provided by the Harvard Brain Tissue Resource Center. In control subjects, GAP-43 immunostaining was prominent in synaptic terminals in the inner molecular layer and hilar region. Subjects with schizophrenia had significant decreases in GAP-43 immunoreactivity in the hilus (p<0.05, paired t-test) and inner molecular layer (p<0.05, paired t-test) but not in the outer molecular layer. In the same tissues, synaptophysin immunoreactivity was significantly reduced in both the inner and outer molecular layers of the dentate gyrus (both p<0.01 by paired t-test), but not in the hilus. In contrast to patients with schizophrenia, GAP-43 and synaptophysin levels in subjects with bipolar disorder did not differ from controls. Given the relationship of GAP-43 and synaptophysin with the development and plasticity of synaptic connections, the observed alterations in the hippocampus of patients with schizophrenia may be related to cognitive deficits associated with this illness.

A behavioral role for dendritic integration: HCN1 channels constrain spatial memory and plasticity at inputs to distal dendrites of CA1 pyramidal neurons

The importance of long-term synaptic plasticity as a cellular substrate for learning and memory is well established. By contrast, little is known about how learning and memory are regulated by voltage-gated ion channels that integrate synaptic information. We investigated this question using mice with general or forebrain-restricted knockout of the HCN1 gene, which we find encodes a major component of the hyperpolarization-activated inward current (Ih) and is an important determinant of dendritic integration in hippocampal CA1 pyramidal cells. Deletion of HCN1 from forebrain neurons enhances hippocampal-dependent learning and memory, augments the power of theta oscillations, and enhances long-term potentiation (LTP) at the direct perforant path input to the distal dendrites of CA1 pyramidal neurons, but has little effect on LTP at the more proximal Schaffer collateral inputs. We suggest that HCN1 channels constrain learning and memory by regulating dendritic integration of distal synaptic inputs to pyramidal cells.

Prenatal ethanol exposure reduces mGluR5 receptor number and function in the dentate gyrus of adult offspring

BACKGROUND

Previous studies in our laboratory indicated that metabotropic glutamate receptor (mGluR)-stimulated phosphoinositide hydrolysis is markedly reduced in the hippocampal formation of adult rat offspring whose mothers drank moderate amounts of ethanol during pregnancy. In the present study, we extended these observations by measuring the impact of prenatal ethanol exposure on proteins associated with the mGluR5 receptor-effector system along with two mGluR5 agonist-mediated responses in dentate gyrus of adult offspring.

METHODS

Sprague-Dawley rat dams consumed one of three diets throughout gestation: (1) a BioServ liquid diet that contained 5% ethanol (v/v), (2) pair-fed an isocalorically equivalent amount of 0% ethanol liquid diet, or (3) lab chow ad libitum. Microdissected slices of dentate gyrus were prepared from adult female offspring from each diet group and used for (1) Western blot analyses of mGluR5, the G-proteins Galphaq and Galpha11, and phospholipase C-beta1; (2) 2-chloro-5-hydroxyphenylglycine (CHPG)-stimulated growth associated protein 43 (GAP-43) phosphorylation; or (3) CHPG potentiation of electrically evoked [H]-D-aspartate (D-ASP) release from dentate gyrus slices.

RESULTS

In tissue prepared from untreated control rats, CHPG produced a dose-dependent increase in phosphate incorporation into GAP-43, with maximal agonist stimulation occurring at 20 microM of CHPG. CHPG produced a quantitatively similar dose-dependent increase in the potentiation of electrically evoked D-ASP release from dentate gyrus slices from untreated controls. Fetal ethanol exposure reduced the amount of dentate gyrus mGluR5 receptor protein by 36% compared with the diet control groups. There were no significant differences between diet groups in the two G-proteins or phospholipase C-beta1 protein. Fetal ethanol exposure reduced CHPG-stimulated GAP-43 phosphorylation to approximately one half the amount of CHPG stimulation observed in the control diet groups. Prenatal ethanol exposure also reduced CHPG potentiation of D-ASP release to a similar degree compared with control.

CONCLUSIONS

These results indicate that prenatal exposure to moderate quantities of ethanol reduces mGluR5 expression in the dentate gyrus of adult offspring. Although the subcellular site(s) for reduced mGluR5 expression cannot be discerned from Western blot data, the quantitatively similar effects of prenatal ethanol exposure on mGluR5 agonist stimulation of presynaptically localized GAP-43 phosphorylation and CHPG potentiation of evoked D-ASP release suggest that the presynaptic nerve terminal is one site where prenatal ethanol exposure has reduced mGluR5 receptor number and function. Furthermore, these data implicate these neurochemical alterations as one factor contributing to the hippocampal synaptic plasticity and behavioral deficits that we have observed previously in prenatal ethanol-exposed offspring.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Afferent Input Modulates Neurotrophins and Synaptic Plasticity in the Spinal Cord

The effects of eliminating or decreasing neuromuscular activity on the expression of neurotrophins and associated molecules in the spinal cord and subsequent effects on spinal cord plasticity were determined. Spinal cord isolation (SI), which eliminates any supraspinal and peripheral monosynaptic input to the lumbar region but maintains the motoneuron-muscle connectivity, decreased the levels of brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3) mRNA and protein in the isolated segments. Synapsin I, an important mediator for the effects of BDNF on synaptic plasticity, also was lower in the lumbar region of SI rats. In contrast, the levels of BDNF, synapsin, and growth-associated protein (GAP-43) were increased in the cervical spinal cord enlargement rostral to the isolated region, most likely reflecting an increased use of the forelimbs in the SI rats. GAP-43 levels were also increased in the lumbar spinal cord region, probably associated with compensatory mechanisms related to the deafferentation. In a separate set of experiments, the soleus muscle was paralyzed unilaterally via intramuscular botulinum toxin type A (BTX-A) injection to determine the effects of reducing the propioceptive input, of this normally highly active muscle on neurotrophin expression in the spinal cord. BDNF and synapsin I mRNAs were lower and NT-3 levels were higher in the lumbar hemicord ipsilateral to the BTX-A injection. Combined, these results indicate that the level of supraspinal and muscle afferent input plays an important role in modulating the levels of BDNF and NT-3 in the spinal cord.

Overexpression of the full-length neurotrophin receptor trkB regulates the expression of plasticity-related genes in mouse brain

Significant body of evidence indicates an important role for brain-derived neurotrophic factor (BDNF) in the hippocampal synaptic plasticity; however, the exact mechanisms how the BDNF signal is converted to plastic changes during memory processes are under an intense investigation. To specifically address the role of the trkB receptor, we have previously generated transgenic mice overexpressing the full-length trkB receptor and observed a continuous activation of the trkB.TK+ receptor, improved learning and memory but an attenuated LTP in these mice. In this study, we describe the trkB.TK+ mRNA and protein distribution in the transgenic mice, showing the most prominent increase in the full-length trkB expression in the cortical layer V pyramidal neurons and dentate gyrus of the hippocampus. In addition, we have analyzed the mRNA expression patterns of a group of genes associated with both plastic changes in the nervous system and BDNF signaling. Regulated expression of immediate early genes c-fos, fra-2 and junB was observed in the transgenic mice. Furthermore, the mRNA expression of alpha-Ca2+/calmodulin-dependent kinase II (alpha-CaMKII) was reduced in both the hippocampus and parietal cortex, whereas growth-associated protein 43 (GAP-43) mRNA expressions were induced in the corresponding regions. Conversely, the mRNA expression of the transcription factor cAMP response element binding protein (CREB) was not altered in the trkB.TK+mice. Finally, the density of neuropeptide Y (NPY)-expressing cells was increased in the trkB.TK+ mice dentate hilus. Altogether, these results demonstrate in vivo that the increased trkB.TK+ signaling regulates several important plasticity-related genes.

Subfield-specific increase in brain growth protein in postmortem hippocampus of Alzheimer's patients

The neuropathology of Alzheimer's disease (AD) reflects a precarious balance between neurodegenerative phenomena and reactive events of neuroplasticity. This latter aspect of AD neuropathology has received less attention than it deserves and its contribution to memory loss is therefore not well understood. To monitor neuroplastic-related events we studied the distribution of the plasticity-associated, brain growth protein GAP-43 in AD subjects and age-matched controls. In tissue from AD patients, we observed a consistent elevation of GAP-43 in a subfield of the hippocampus, stratum lacunosum moleculare. This subfield contains inputs from multiple brain regions and is known to regulate declarative memory function. Levels of potentially aberrant sprouting, as marked by elevated growth protein, were positively correlated with the severity of AD suggesting that increased expression of GAP-43 leads to a miswiring of circuits critical for memory function. Our findings suggest a mechanism, aberrant neuroplasticity, that in concert with neurodegeneration may importantly contribute to the memory loss in AD.

Induction of memory-associated immediate early genes by nerve growth factor in rat primary cortical neurons and differentiated mouse Neuro2A cells

Activation of several immediate early genes (IEGs) is crucial for long-term memory formation in vivo. In vitro methods of inducing these genes have not been investigated extensively. Here we present data demonstrating that application of the neurotrophin, nerve growth factor (NGF), to both rat primary neuronal cultures and differentiated mouse neuroblastoma 2A (N2A) cultures reliably induces expression of several IEGs, including Zif268, Nur77 and Arc, each of which have been linked to memory consolidation. These findings provide an in vitro model in which to test other agents that might modulate the induction of memory-associated genes.

Sites of neocortical reorganization critical for remote spatial memory

The hippocampus is crucial for spatial memory formation, yet it does not store long-lasting memories. By combining functional brain imaging and region-specific neuronal inactivation in mice, we identified prefrontal and anterior cingulate cortices as critical for storage and retrieval of remote spatial memories [correction]. Imaging of activity-dependent genes also revealed an involvement of parietal and retrosplenial cortices during consolidation of remote memory. Long-term memory storage within some of these neocortical regions was accompanied by structural changes including synaptogenesis and laminar reorganization, concomitant with a functional disengagement of the hippocampus and posterior cingulate cortex [correction]. Thus, consolidation of spatial memory requires a time-dependent hippocampal-cortical dialogue, ultimately enabling widespread cortical networks to mediate effortful recall and use of cortically stored remote memories independently.

Designing the future: NBIC technologies and human performance enhancement

Never before has any civilization had the unique opportunity to enhance human performance on the scale that we will face in the near future. The convergence of nanotechnology, biotechnology, information technology, and cognitive science (NBIC) is creating a set of powerful tools that have the potential to significantly enhance human performance as well as transform society, science, economics, and human evolution. As the NBIC convergence becomes more understood, the possibility that we may be able to enhance human performance in the three domains of therapy, augmentation, and designed evolution will become anticipated and even expected. In addition, NBIC convergence represents entirely new challenges for scientists, policymakers, and business leaders who will have, for the first time, vast new and powerful tools to shape markets, societies, and lifestyles. The emergence of NBIC convergence will challenge us in new ways to balance risk and return, threat and opportunity, and social responsibility and competitive advantage as we step into the 21st century.

Voluntary exercise increases axonal regeneration from sensory neurons

Recent advances in understanding the role of neurotrophins on activity-dependent plasticity have provided insight into how behavior can affect specific aspects of neuronal biology. We present evidence that voluntary exercise can prime adult dorsal root ganglion neurons for increased axonal regeneration through a neurotrophin-dependent mechanism. Dorsal root ganglion neurons showed an increase in neurite outgrowth when cultured from animals that had undergone 3 or 7 days of exercise compared with sedentary animals. Neurite length over 18-22 h in culture correlated directly with the distance that animals ran. The exercise-conditioned animals also showed enhanced regrowth of axons after an in vivo nerve crush injury. Sensory ganglia from the 3- and 7-day-exercised animals contained higher brain-derived neurotrophic factor, neurotrophin 3, synapsin I, and GAP43 mRNA levels than those from sedentary animals. Consistent with the rise in brain-derived neurotrophic factor and neurotrophin 3 during exercise, the increased growth potential of the exercise-conditioned animals required activation of the neurotrophin signaling in vivo during the exercise period but did not require new mRNA synthesis in culture.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.

Presynaptic protein kinase C controls maturation and branch dynamics of developing retinotectal arbors: possible role in activity-driven sharpening

Visual activity refines developing retinotectal maps and shapes individual retinal arbors via an NMDA receptor-dependent mechanism. As retinal axons grow into tectum, they slow markedly and emit many transient side branches behind the tip, assuming a "bottlebrush" morphology. Some branches are stabilized and branch further, giving rise to a compact arbor. The dynamic rate of branch addition and deletion is increased twofold when MK801 is used to block NMDA receptors, as if this prevents release of a stabilizing signal such as arachidonic acid (AA) from the postsynaptic neuron. In optic tract, AA mediates NCAM and L1 stimulation of axon growth by activating presynaptic protein kinase C (PKC) to phosphorylate GAP-43 and stabilize F-actin, and, if present in tectum, this growth control pathway could be modulated by postsynaptic activation. To test for the effects on arbor morphology of blocking PKC or AA release, we examined DiO-labeled retinal axons of larval zebrafish with time-lapse videomicroscopy. Bath application of the selective PKC inhibitor bisindolylmaleimide from 2 or 3 days onward doubled the rate at which side branches were added and deleted, as seen with MK801, and also prevented maturation of the arbor so that it retained a "bottlebrush" morphology. In order to selectively block the PKC being transported to retinal terminals, we injected the irreversible inhibitor calphostin C into the eye from which the ganglion cells were labeled, and this produced both effects seen with bath application. In contrast, there were no effects of control injections, which included Ringers into the same eye and the same dose into the opposite eye (actually much closer to the tectum of interest), to rule out the possibility that the inhibitor leaked from the eye to act on tectal cells. For comparison, we examined arbors treated with the NMDA blocker MK801 at half-hour time-lapse intervals, and detected the twofold rise in rates of branch addition and deletion previously reported in Xenopus larvae, but not the structural effect seen with the PKC inhibitors. In addition, we could produce both effects seen with PKC inhibitors by using RHC80267 to block AA release from DAG lipase, indicating that AA is the main drive for PKC activation. Thus, the results show a distinct role of AA and presynaptic PKC in both maturation of arbor structure and in the dynamic control of branching. The effects on branch dynamics were present regardless of the level of maturity of arbor structure. The fact that they mimicked those of MK801 suggests that presynaptic PKC may be involved in the NMDA receptor-driven stabilization of developing retinal arbors.

Expression of neurogranin and neuromodulin is affected in the striatum of vitamin A-deprived rats

Our previous data showed that vitamin A deficiency (VAD) induces, in whole brain, a reduced amount of mRNA for brain retinoic acid (RA) and triiodothyronine (T3) nuclear receptors (i.e., RAR, RXR, and TR, respectively), which is accompanied by reduced amounts of mRNA and protein of neurogranin (RC3, a neuronal protein involved in synaptic plasticity) as well as selective behavioral impairment. Given the important role of retinoids for optimal brain functioning, the effects of vitamin A depletion and subsequent administration of RA or T3 on the mRNA levels of RA and T3 nuclear receptors and on two target genes' (RC3 and neuromodulin or GAP43) mRNA and protein levels were examined in the hippocampus, striatum, and cerebral cortex. A quantitative real-time polymerase chain reaction (PCR), in situ hybridization, and Western blot analysis demonstrated that the striatal region is the brain site where both RA and T3 signaling pathways are most affected by VAD. Indeed, rats fed a vitamin A-free diet for 10 weeks exhibited decreased expression of RAR, RXR, TR, RC3, and GAP43 in the striatum. The administration of T3 to these vitamin A-deprived rats reversed the reduction in mRNA levels of RA and T3 nuclear receptors and in mRNA and protein levels of target genes in this region. These data suggest that modifications that appear preferentially in the striatum, a region highly sensitive to vitamin A bioavailability, may contribute to neurobiological alterations and the spatial learning impairment that occurs in vitamin A-deprived animals.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

Can fat explain the human brain's big bang evolution?-Horrobin's leads for comparative and functional genomics

When David Horrobin suggested that phospholipid and fatty acid metabolism played a major role in human evolution, his 'fat utilization hypothesis' unified intriguing work from paleoanthropology, evolutionary biology, genetic and nervous system research in a novel and coherent lipid-related context. Interestingly, unlike most other evolutionary concepts, the hypothesis allows specific predictions which can be empirically tested in the near future. This paper summarizes some of Horrobin's intriguing propositions and suggests as to how approaches of comparative genomics published in Cell, Nature, Science and elsewhere since 1997 may be used to examine his evolutionary hypothesis. Indeed, systematic investigations of the genomic clock in the species' mitochondrial DNA, the Y and autosomal chromosomes as evidence of evolutionary relationships and distinctions can help to scrutinize associated predictions for their validity, namely that key mutations which differentiate us from Neanderthals and from great apes are in the genes coding for proteins which regulate fat metabolism, and particularly the phospholipid metabolism of the synapses of the brain. It is concluded that beyond clues to humans' relationships with living primates and to the Neanderthals' cognitive performance and their disappearance, the suggested molecular clock analyses may provide crucial insights into the biochemical evolution-and means of possible manipulation-of our brain.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Increase of the RNA-binding protein HuD and posttranscriptional up-regulation of the GAP-43 gene during spatial memory

Neuronal ELAV-like proteins (HuB, HuC, and HuD) are highly conserved RNA-binding proteins able to selectively associate with the 3' UTR of a subset of target mRNAs and increase their cytoplasmic stability and rate of translation. We previously demonstrated the involvement of these proteins in learning, reporting that they undergo a sustained up-regulation in the hippocampus of mice trained in a spatial discrimination task. Here, we extend this finding, showing that a similar up-regulation occurs in the hippocampus of rats trained in another spatial learning paradigm, the Morris water maze. HuD, a strictly neuron-specific ELAV-like protein, is shown to increase after learning, with a preferential binding to the cytoskeletal fraction. HuD up-regulation is associated with an enhancement of GAP-43 mRNA and protein levels, with an apparently increased HuD colocalization with the GAP-43 mRNA and an increased association of neuronal ELAV-like proteins with the GAP-43 mRNA. These learning-dependent biochemical events appear to be spatiotemporally controlled, because they do not occur in another brain region involved in learning, the retrosplenial cortex, and at the level of protein expression they show extinction 1 month after training despite memory retention. By contrast, HuD mRNA levels still remain increased after 1 month in the CA1 region. This persistence may have implications for long-term memory recall.

Fetal Alcohol Exposure Alters GAP-43 Phosphorylation and Protein Kinase C Responses to Contextual Fear Conditioning in the Hippocampus of Adult Rat Offspring

BACKGROUND

The growth- and plasticity-associated protein GAP-43 plays a significant role in the establishment and remodeling of neuronal connections. We have previously shown that GAP-43 levels, protein kinase C (PKC) activity, and GAP-43 phosphorylation increase during contextual fear conditioning and that fetal alcohol exposure (FAE) decreases PKC activity and GAP-43 phosphorylation in the hippocampus of adult offspring. Drawing on these observations, we hypothesized that FAE manifests its cognitive impairment by disrupting PKC activation and membrane translocation, thereby decreasing GAP-43 phosphorylation and function.

METHODS

Three groups of pregnant rat dams (FAE and two control diet groups) were placed on different diet regimens. Offspring from each of these groups were placed into each of four test groups, a contextual fear conditioned (CFC) group, a naïve unhandled group, and two nonlearning stress control groups. Hippocampi were dissected, homogenized, and used to prepare a cytosolic and a membrane fraction. These fractions were probed for total GAP-43, PKC-phosphorylated GAP-43, and several PKC subtypes. PKC activity also was measured in total homogenates.

RESULTS

Compared with both control diet groups, FAE animals showed a deficit in the activation of PKC in the hippocampus at 24 hr but not at 1.5 hr after CFC. Likewise, we found that the amount of GAP-43 and its phosphorylation were decreased 24 hr after CFC in FAE rats but not at early times after training. Analysis of the translocation of various PKC isoforms revealed that FAE animals had decreased levels of membrane-bound PKC beta2 and PKC epsilon 24 hr after CFC.

CONCLUSIONS

Considering the role of PKC activation and GAP-43 phosphorylation in synaptic plasticity, our results suggest that deficient translocation of PKC beta2 and PKC epsilon in the hippocampus may mediate the electrophysiological and behavioral deficits observed in fetal alcohol exposed animals.

Amyloid suppresses induction of genes critical for memory consolidation in APP + PS1 transgenic mice

Mice transgenic for mutated forms of the amyloid precursor protein (APP) plus presenilin-1 (PS1) genes (APP + PS1 mice) gradually develop memory deficits which correlate with the extent of amyloid deposition. The expression of several immediate-early genes (IEGs: Arc, Nur77 and Zif268) and several other plasticity-related genes (GluR1, CaMKIIalpha and Na-K- ATPase alphaIII) critical for learning and memory was normal in young APP + PS1 mice preceding amyloid deposition, but declined as mice grew older and amyloid deposits accumulated. Gene repression was less in APP + PS1 mouse brain regions that contain less Abeta and in APP mice compared with APP + PS1 mice, further linking the extent of amyloid deposition and the extent of gene repression. Critically, we demonstrated that amyloid deposition led specifically to impaired induction of the IEGs with no effects on basal expression using exposure to a novel environment 30 min prior to being killed to induce IEGs. These data imply that Abeta deposition can selectively reduce expression of multiple genes linked to synaptic plasticity, and provide a molecular basis for memory deficiencies found in transgenic APP mice and, most likely, in early stage Alzheimer's disease (AD). Presumably, pharmacological agents blocking the Abeta-related inhibition of gene expression will have benefit in AD.

Plasticity following Injury to the Adult Central Nervous System: Is Recapitulation of a Developmental State Worth Promoting?

The adult central nervous system (CNS) appears to initiate a transient increase in plasticity following injury, including increases in growth-related proteins and generation of new cells. Recent evidence is reviewed that the injured adult CNS exhibits events and patterns of gene expression that are also observed during development and during regeneration following damage to the mature peripheral nervous system (PNS). The growth of neurons during development or regeneration is correlated, in part, with a coordinated expression of growth-related proteins, such as growth-associated-protein-43 (GAP-43), microtubule-associated-protein-1B (MAP1B), and polysialylated-neural-cell-adhesion-molecule (PSA-NCAM). For each of these proteins, evidence is discussed regarding its specific role in neuronal development, signals that modify its expression, and reappearance following injury. The rate of adult hippocampal neurogenesis is also affected by numerous endogenous and exogenous factors including injury. The continuing study of developmental neurobiology will likely provide further gene and protein targets for increasing plasticity and regeneration in the mature adult CNS.

Cell type- and region-specific expression of protein kinase C-substrate mRNAs in the cerebellum of the macaque monkey

We performed nonradioactive in situ hybridization histochemistry in the monkey cerebellum to investigate the localization of protein kinase C-substrate (growth-associated protein-43 [GAP-43], myristoylated alanine-rich C-kinase substrate [MARCKS], and neurogranin) mRNAs. Hybridization signals for GAP-43 mRNA were observed in the molecular and granule cell layers of both infant and adult cerebellar cortices. Signals for MARCKS mRNA were observed in the molecular, Purkinje cell, and granule cell layers of both infant and adult cortices. Moreover, both GAP-43 and MARCKS mRNAs were expressed in the external granule cell layer of the infant cortex. In the adult cerebellar vermis, signals for both GAP-43 and MARCKS mRNAs were more intense in lobules I, IX, and X than in the remaining lobules. In the adult hemisphere, both mRNAs were more intense in the flocculus and the dorsal paraflocculus than in other lobules. Such lobule-specific expressions were not prominent in the infant cerebellar cortex. Signals for neurogranin, a postsynaptic substrate for protein kinase C, were weak or not detectable in any regions of either the infant or adult cerebellar cortex. The prominent signals for MARCKS mRNA were observed in the deep cerebellar nuclei, but signals for both GAP-43 and neurogranin mRNAs were weak or not detectable. The prominent signals for both GAP-43 and MARCKS mRNAs were observed in the inferior olive, but signals for neurogranin were weak or not detectable. The cell type- and region-specific expression of GAP-43 and MARCKS mRNAs in the cerebellum may be related to functional specialization regarding plasticity in each type of cell and each region of the cerebellum.

Identification of upregulated SCG10 mRNA expression associated with late-phase long-term potentiation in the rat hippocampal Schaffer-CA1 pathway in vivo

The maintenance of long-term potentiation (LTP) depends on alteration of gene transcription. By screening a subtracted cDNA library that is enriched in upregulated transcripts in rat hippocampus 3 hr after Schaffer-CA1 LTP induction in vivo, we identified a neural growth-associated protein SCG10 (superior cervical ganglia clone 10) gene. The semiquantitative reverse transcription-PCR and Northern blot experiments confirmed that SCG10 mRNA levels were elevated in tetanized rat hippocampi compared with those of sham controls that received only low-frequency stimulation. Both 1 and 2 kb forms of SCG10 mRNAs contributed to the increased expression. Using a riboprobe with a sequence specific to the 3'-untranslated region of rat SCG10 mRNA, in situ hybridization further revealed a significant increase of the SCG10 mRNA 2 kb form in the ipsilateral CA3 and CA1 regions of LTP animals. In addition, we systemically injected the competitive NMDA receptor antagonist d,l-3[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid (CPP) to determine whether the alteration of SCG10 expression depends on NMDA receptor activation or tetanus alone. Administration of CPP 1 hr before tetanus completely blocked LTP induction and the increase of SCG10 mRNA levels. Thus, these results suggest that the transcription of SCG10 in vivo is regulated by long-lasting synaptic activity and may contribute to the maintenance of long-term synaptic plasticity via a presynaptic remodeling mechanism.

Selectively reduced expression of synaptic plasticity-related genes in amyloid precursor protein + presenilin-1 transgenic mice

A critical question in Alzheimer's disease (AD) research is the cause of memory loss that leads to dementia. The amyloid precursor protein + presenilin-1 (APP+PS1) transgenic mouse is a model for amyloid deposition, and like AD, the mice develop memory deficits as amyloid deposits accumulate. We profiled gene expression in these transgenic mice by microarray and quantitative RT-PCR (qRT-PCR). At the age when these animals developed cognitive dysfunction, they had reduced mRNA expression of several genes essential for long-term potentiation and memory formation (Arc, Zif268, NR2B, GluR1, Homer-1a, Nur77/TR3). These changes appeared to be related to amyloid deposition, because mRNA expression was unchanged in the regions that did not accumulate amyloid. Transgene expression was similar in both amyloid-containing and amyloid-free regions of the brain. Interestingly, these changes occurred without apparent changes in synaptic structure, because a number of presynaptic marker mRNAs (growth-associated protein-43, synapsin, synaptophysin, synaptopodin, synaptotagmin, syntaxin) remained stable. Additionally, a number of genes related to inflammation were elevated in transgenic mice, primarily in the regions containing amyloid. In AD cortical tissue, the same memory-associated genes were downregulated. However, all synaptic and neuronal transcripts were reduced, implying that the loss of neurons and synapses contributed to these changes. We conclude that reduced expression of selected genes associated with memory consolidation are linked to memory loss in both circumstances. This suggests that the memory loss in APP+PS1 transgenic mice may model the early memory dysfunction in AD before the degeneration of synapses and neurons.

Identification of upregulated SCG10 mRNA expression associated with late-phase long-term potentiation in the rat hippocampal Schaffer-CA1 pathway in vivo

The maintenance of long-term potentiation (LTP) depends on alteration of gene transcription. By screening a subtracted cDNA library that is enriched in upregulated transcripts in rat hippocampus 3 hr after Schaffer-CA1 LTP induction in vivo, we identified a neural growth-associated protein SCG10 (superior cervical ganglia clone 10) gene. The semiquantitative reverse transcription-PCR and Northern blot experiments confirmed that SCG10 mRNA levels were elevated in tetanized rat hippocampi compared with those of sham controls that received only low-frequency stimulation. Both 1 and 2 kb forms of SCG10 mRNAs contributed to the increased expression. Using a riboprobe with a sequence specific to the 3'-untranslated region of rat SCG10 mRNA, in situ hybridization further revealed a significant increase of the SCG10 mRNA 2 kb form in the ipsilateral CA3 and CA1 regions of LTP animals. In addition, we systemically injected the competitive NMDA receptor antagonist d,l-3[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid (CPP) to determine whether the alteration of SCG10 expression depends on NMDA receptor activation or tetanus alone. Administration of CPP 1 hr before tetanus completely blocked LTP induction and the increase of SCG10 mRNA levels. Thus, these results suggest that the transcription of SCG10 in vivo is regulated by long-lasting synaptic activity and may contribute to the maintenance of long-term synaptic plasticity via a presynaptic remodeling mechanism.

Selectively reduced expression of synaptic plasticity-related genes in amyloid precursor protein + presenilin-1 transgenic mice

A critical question in Alzheimer's disease (AD) research is the cause of memory loss that leads to dementia. The amyloid precursor protein + presenilin-1 (APP+PS1) transgenic mouse is a model for amyloid deposition, and like AD, the mice develop memory deficits as amyloid deposits accumulate. We profiled gene expression in these transgenic mice by microarray and quantitative RT-PCR (qRT-PCR). At the age when these animals developed cognitive dysfunction, they had reduced mRNA expression of several genes essential for long-term potentiation and memory formation (Arc, Zif268, NR2B, GluR1, Homer-1a, Nur77/TR3). These changes appeared to be related to amyloid deposition, because mRNA expression was unchanged in the regions that did not accumulate amyloid. Transgene expression was similar in both amyloid-containing and amyloid-free regions of the brain. Interestingly, these changes occurred without apparent changes in synaptic structure, because a number of presynaptic marker mRNAs (growth-associated protein-43, synapsin, synaptophysin, synaptopodin, synaptotagmin, syntaxin) remained stable. Additionally, a number of genes related to inflammation were elevated in transgenic mice, primarily in the regions containing amyloid. In AD cortical tissue, the same memory-associated genes were downregulated. However, all synaptic and neuronal transcripts were reduced, implying that the loss of neurons and synapses contributed to these changes. We conclude that reduced expression of selected genes associated with memory consolidation are linked to memory loss in both circumstances. This suggests that the memory loss in APP+PS1 transgenic mice may model the early memory dysfunction in AD before the degeneration of synapses and neurons.

Long-term potentiation recruits a trisynaptic excitatory associative network within the mouse dentate gyrus

Granule cells of the hippocampal dentate gyrus receive two powerful excitatory inputs: the perforant path, originating from the entorhinal cortex, and the associational pathway, originating from mossy cells, the principal neurons of the dentate gyrus hilus. We examined the electrophysiological properties of the less well-studied associational pathway and its interaction with the perforant path in the intact mouse hippocampus and then tested homosynaptic, trans-synaptic and associative long-term potentiation of these pathways. The associational pathway was either monosynaptically activated by stimulation within the inner molecular layer or trisynaptically activated after stimulation of the perforant path. Laminar profiles of extracellularly recorded associational pathway field potentials demonstrated a bell-shaped curve with a peak in the inner molecular layer. Tetanization of the perforant path induced not only homosynaptic potentiation of the perforant path (162.4 +/- 6.7% at 0.5-1.5 h after tetanus) but also heterosynaptic potentiation of the associational pathway (115.7 +/- 4.9%). Direct tetanization of the associational pathway within the inner molecular layer was ineffective in either the septo-temporal (97.2 +/- 4.5%) or temporal-septal (104.4 +/- 4.6%) direction. In contrast, conjoint tetanization of the associational pathway with the perforant path potentiated the associational pathway responses in both the septo-temporal (123.4 +/- 5.8%) and the temporal-septal (124.8 +/- 7.3%) directions. Paired-pulse facilitation was attenuated by long-term potentiation in the perforant path and the associational pathway, suggesting pre-synaptic involvement. These results demonstrate that long-term potentiation of the associational pathway and the perforant path is a product of the network properties of the dentate gyrus rather than of each monosynaptic input alone. The architecture of this neural network may be designed for flexible dynamic associations of the afferent perforant path inputs to configure encoded information within hippocampal neuronal ensembles.

Changes in protein kinase C (PKC) activity, isozyme translocation, and GAP-43 phosphorylation in the rat hippocampal formation after a single-trial contextual fear conditioning paradigm

The hippocampus plays an important role in spatial learning and memory. However, the biochemical alterations that subserve this function remain to be fully elucidated. In this study, rats were subjected to a single-trial contextual fear conditioning (CFC) paradigm; the activation of different protein kinase C (PKC) subtypes and the levels and phosphorylation of the plasticity-associated protein GAP-43 were assayed in the hippocampus at varying times after training. We observed a rapid activation of hippocampal PKC (15 min through 24 h), with differential translocation of the PKC isotypes studied. At early times after CFC (15-90 min), PKCalpha and PKCgamma translocated to the membrane, while PKCbetaII and PKCepsilon moved more transiently (15 to 30 min) to the cytosol. These PKC isotypes returned to the membrane at later time points after CFC. Correlating with these changes in PKC translocation and activity, there was an early decrease in GAP-43 phosphorylation followed by a more sustained increase from 1.5-72 h. GAP-43 protein levels were also increased after 3 h, and these levels remained elevated for at least 72 h. These changes in PKC and GAP-43 were specific to the CFC trained animals and no changes were seen in animals exposed to the same stimuli in a non-associative fashion. Comparison of translocation of different PKC isotypes with the changes in GAP-43 phosphorylation suggested that PKCbetaII and PKCepsilon may mediate both the early changes in the phosphorylation of this protein and the increases in GAP-43 expression at later times after CFC.

Molecular and cellular cognitive studies of the role of synaptic plasticity in memory

Synaptic plasticity has a central role in nearly all models of learning and memory. Besides experiments documenting changes in synaptic function during learning, most of the evidence supporting a role for synaptic plasticity in memory comes from manipulations that either enhance or lesion synaptic processes. In the last decade, mouse transgenetics (knock outs and transgenics) have provided compelling evidence that the molecular mechanisms responsible for the induction and stability of synaptic changes have a critical role in the acquisition and storage of information. Here, I will review this literature, with a special focus on studies of hippocampal-dependent learning and memory.

Constitutively and autonomously active protein kinase C associated with 14-3-3 zeta in the rodent brain

Persistent activation of protein kinase C (PKC) is required for the expression of synaptic plasticity in the brain. There are several mechanisms proposed that can lead to the prolonged activation of PKC. These include long lasting production of lipid activators (diacylglycerol and fatty acid) through mitogen-activated protein (MAP) kinase pathway, and a modification of PKC by reactive oxygen species. In nerve growth factor (NGF)-differentiated PC12 cells, we found that constitutive and autonomous Ca2+-independent PKC activity is associated with 14-3-3 zeta. Because PKC and 14-3-3 zeta are both involved in synaptic plasticity and learning and memory, we examined whether PKC interacts with 14-3-3 zeta in the brain and whether the PKC/14-3-3 zeta complex has autonomous activity. Here we show that three subclasses of PKC, Ca2+-dependent classical PKC, Ca2+-independent novel PKC, and Ca2+-independent and diacylglycerol-insensitive atypical PKC, all interact with 14-3-3 zeta in the rodent brain. The pool size of 14-3-3 zeta bound form of PKC is small (1-4% of each PKC isoform), but they show constitutive and autonomous activity. Our study indicates that the binding of PKC with 14-3-3 zeta is at least in part independent of phosphorylation of PKC and that the C1 domain of PKC is involved in the binding. As both molecules are enriched in synaptic locus, the constitutive PKC activity and its interaction with 14-3-3 zeta could be a mechanism for the persistent PKC activation in the brain.

Voluntary exercise induces a BDNF-mediated mechanism that promotes neuroplasticity

We have investigated potential mechanisms by which exercise can promote changes in neuronal plasticity via modulation of neurotrophins. Rodents were exposed to voluntary wheel running for 3 or 7 days, and their lumbar spinal cord and soleus muscle were assessed for changes in brain-derived neurotrophic factor (BDNF), its signal transduction receptor (trkB), and downstream effectors for the action of BDNF on synaptic plasticity. Exercise increased the expression of BDNF and its receptor, synapsin I (mRNA and phosphorylated protein), growth-associated protein (GAP-43) mRNA, and cyclic AMP response element-binding (CREB) mRNA in the lumbar spinal cord. Synapsin I, a synaptic mediator for the action of BDNF on neurotransmitter release, increased in proportion to GAP-43 and trkB mRNA levels. CREB mRNA levels increased in proportion to BDNF mRNA levels. In separate experiments, the soleus muscle was paralyzed unilaterally via intramuscular botulinum toxin type A (BTX-A) injection to determine the effects of reducing the neuromechanical output of a single muscle on the neurotrophin response to motor activity. In sedentary BTX-A-treated rats, BDNF and synapsin I mRNAs were reduced below control levels in the spinal cord and soleus muscle. Exercise did not change the BDNF mRNA levels in the spinal cord of BTX-A-treated rats but further reduced the BDNF mRNA levels in the paralyzed soleus relative to the levels in sedentary BTX-A-treated rats. Exercise also restored synapsin I to near control levels in the spinal cord. These results indicate that basal levels of neuromuscular activity are required to maintain normal levels of BDNF in the neuromuscular system and the potential for neuroplasticity.

Targeting the "species gene ensemble"

The members of a given species display a normal distribution of gene expression which is here termed the species gene ensemble. This more specific designation of the 'genetic background' leads to a consideration of why gene targeting outcomes are regulated by the species gene ensemble. One consequence is the utility of the heterozygous knockout which buffers the drastic compensatory reactions of the homozygous knockout thereby revealing subtle but yet consistent alterations in hippocampally-dependent behaviors.

Neonatal maternal separation reduces hippocampal mossy fiber density in adult Long Evans rats

Neonatal maternal separation of rat pups leads to a stable stress hyper-responsive phenotype characterized by increased basal levels of corticotropin releasing factor (CRF) mRNA in the hypothalamic and extra-hypothalamic nuclei, increased hypothalamic CRF release, and enhanced adrenocorticotrophin hormone (ACTH) and corticosterone (CORT) responses to psychological stressors. Stress and exposure to glucocorticoids either early in life or in adulthood have been associated with hippocampal atrophy and impairments in learning and memory. In this study, male Long Evans rat pups were exposed to daily 3-h (HMS180) or 15-min (HMS15) periods of maternal separation on postnatal days (PND) 2-14 or normal animal facility rearing. Maternal separation and subsequent reunion with the dam resulted in elevated plasma CORT levels versus HMS15 animals at PND7, a time when rat pups are normally hyporesponsive to stressors and show limited pituitary-adrenal responses. As adults, HMS180 rats exhibited elevated indices of anxiety, startle-induced pituitary-adrenal hyper-responsiveness, and slight, but significant impairment on acquisition in the Morris water maze task. In addition, HMS180 rats exhibited decreased mossy fiber density in the stratum oriens region of the hippocampus as measured by Timm's staining, but no change in volume of the dentate gyrus. These changes may be the result of neonatal exposure to elevated glucocorticoids and/or changes in other signaling systems in response to maternal separation. Overall the results suggest that repeated, daily, 3-h maternal separations during critical periods of hippocampal development can disrupt hippocampal cytoarchitecture in a stable manner. The resulting change in morphology may contribute to the subtle, but consistent learning deficit and overall stress hyper-responsive phenotype observed in these animals.

Nerve growth factor controls GAP-43 mRNA stability via the phosphoprotein ARPP-19

The membrane phosphoprotein GAP-43 is involved in axon growth and synaptic plasticity. In PC12 pheochromocytoma cells, induction of a neuronal phenotype by nerve growth factor (NGF) is accompanied by a marked increase in GAP-43 levels. NGF regulates GAP-43 expression by altering the half-life of its mRNA. We report here that the phosphoprotein ARPP-19 mediates this regulation. In an NGF-dependent manner, ARPP-19 bound to a region in the 3' end of GAP-43 mRNA previously found to be important for regulating the half-life of the mRNA. Overexpression of wild-type ARPP-19 in PC12 cells increased the NGF-dependent expression of a reporter construct linked to the critical 3' region of GAP-43 mRNA. Mutation of serine 104, the site of phosphorylation by protein kinase A in ARPP-19, to either alanine or aspartate abolished this regulation in PC12 cells. These findings demonstrate that ARPP-19 is an important link between NGF signaling and post-transcriptional control of neuronal gene expression.

A high-fat, refined sugar diet reduces hippocampal brain-derived neurotrophic factor, neuronal plasticity, and learning

We have investigated a potential mechanism by which a diet, similar in composition to the typical diet of most industrialized western societies rich in saturated fat and refined sugar (HFS), can influence brain structure and function via regulation of neurotrophins. We show that animals that learn a spatial memory task faster have more brain-derived neurotrophic factor (BDNF) mRNA and protein in the hippocampus. Two months on the HFS diet were sufficient to reduce hippocampal level of BDNF and spatial learning performance. Consequent to the action of BDNF on synaptic function, downstream effectors for the action of BDNF on synaptic plasticity were reduced proportionally to BDNF levels, in the hippocampus of rats maintained on the HFS diet between 2 and 24 months. In particular, animals maintained on the HFS diet showed a decrease in levels of: (i) synapsin I mRNA and protein (total and phosphorylated), important for neurotransmitter release; (ii) cyclic AMP-response element-binding protein (CREB) mRNA and protein (total and phosphorylated); CREB is required for various forms of memory and is under regulatory control of BDNF; (iii) growth-associated protein 43 mRNA, important for neurite outgrowth, neurotransmitter release, and learning and memory. Diet-related changes were specific for the hippocampus consequent to its role in memory formation, and did not involve neurotrophin-3, another member of the neurotrophin family. Our results indicate that a popularly consumed diet can influence crucial aspects of neuronal and behavioral plasticity associated with the function of BDNF.

Transgenic and knock-out mouse pups: the growing need for behavioral analysis

Few laboratories working with transgenic and knockout mice analyze the neurobehavioral consequences of genetic manipulation in early ontogeny. However, the study of behavioral endpoints during the early postnatal period in genetically modified mice is important not only to assess possible developmental abnormalities, but also to better understand and disentangle the effects of genetic manipulations in adulthood. We propose that the assessment of neurobehavioral development represents an appropriate strategy to identify possible compensatory and/or unexpected effects. Nowadays, a large number of experimental protocols that take into account the practical constraints imposed by the peculiar physiological and behavioral responses of an immature subject are available to assess the neurobehavioral profile of developing mice. While this knowledge should be applied to the field of transgenic and knock-out mice in general, it should be recommended, in particular, for the study of mouse models of neurodevelopmental disorders.

A point mutant of GAP-43 induces enhanced short-term and long-term hippocampal plasticity

The growth-associated protein GAP-43 (or neuromodulin or B-50) plays a critical role during development in mechanisms of axonal growth and formation of synaptic networks. At later times, GAP-43 has also been implicated in the regulation of synaptic transmission and properties of plasticity such as long-term potentiation. In a molecular approach, we have analyzed transgenic mice overexpressing different mutated forms of GAP-43 or deficient in GAP-43 to investigate the role of the molecule in short-term and long-term plasticity. We report that overexpression of a mutated form of GAP-43 that mimics constitutively phosphorylated GAP-43 results in an enhancement of long-term potentiation in CA1 hippocampal slices. This effect is specific, because LTP was affected neither in transgenic mice overexpressing mutated forms of non-phosphorylatable GAP-43 nor in GAP-43 deficient mice. The increased LTP observed in transgenic mice expressing a constitutively phosphorylated GAP-43 was associated with an increased paired-pulse facilitation as well as an increased summation of responses during high frequency bursts. These results indicate that, while GAP-43 is not necessary for LTP induction, its phosphorylation may regulate presynaptic properties, thereby affecting synaptic plasticity and the induction of LTP.

Eukaryotic mRNPs may represent posttranscriptional operons

Genomic array analysis of endogenous mammalian ribonucleoproteins has recently revealed three novel findings: (1) mRNA binding proteins are associated with unique subpopulations of messages, (2) the compositions of these mRNA subsets can vary with growth conditions, and (3) the same mRNA species can be found in multiple mRNP complexes. Based on these and other findings, we propose a model of posttranscriptional gene expression in which mRNA binding proteins regulate mRNAs as subpopulations during cell growth and development. This model predicts that functionally related genes are regulated posttranscriptionally as groups by specific mRNA binding proteins that recognize sequence elements in common among the mRNAs.

The role of PKA, CaMKII, and PKC in avoidance conditioning: permissive or instructive?

This article explores the causal and correlative relationships between kinases and learning and memory. Specifically, the contributions of three kinases-protein kinase A (PKA), calcium calmodulin-dependent kinase II (CaMKII), and protein kinase C (PKC)-are assessed during the consolidation phase of avoidance conditioning. The following sources of evidence are considered: inhibitor data, activity monitoring, and transgenic studies. An exhaustive effort is made to address several issues regarding the participation of these kinases in (a) posttraining timing and magnitude, (b) location across many brain regions, and (c) the use of multiple pharmacological agents and assays. In addition, this article attempts to integrate the behavioral data with the purported role of kinases in long-term potentiation (LTP).

Synaptic plasticity in the hippocampus of awake C57BL/6 and DBA/2 mice: interstrain differences and parallels with behavior

C57BL/6 mice consistently outperform DBA/2 mice in a range of hippocampal-dependent spatial learning behaviors. We recorded evoked responses from the dentate gyrus of awake, freely-moving mice and measured synaptic plasticity (LTP) and performance in a hippocampal-dependent task in individual animals from these two inbred strains. Spatial alternation tasks confirmed the behavioral divergence between the two strains, with C57BL/6 mice demonstrating more robust alternation than DBA/2 mice. Recording changes in field potentials in the dentate gyrus following three different high-frequency stimulation paradigms in the same groups of animals revealed differences in neural plasticity: both strains were able to support long-term potentiation (LTP) at perforant path synapses, but brief high-frequency stimulation induced larger and longer potentiation of the population spike in C57BL/6 than in DBA/2 mice. This greater propensity for population-spike potentiation in the strain that performed better in a hippocampal-dependent task is in accord with the different neurochemical profiles of C57BL/6 and DBA/2 mice.

Activity-dependent plasticity in the adult auditory brainstem

Over the past few years we have studied the plasticity of the adult auditory brainstem in the rat following unilateral changes to the pattern of sensory activation, either by intracochlear electrical stimulation or by deafening. We discovered that modifications to afferent activity induced changes in the molecular composition and cellular morphology throughout the auditory brainstem, including its major centers: the cochlear nucleus complex, the superior olivary complex, and the inferior colliculus. The time window studied ranged from 2 h to over 1 year following induction of changes to afferent activity. The molecular markers employed include the NMDA receptor subunit type 1, the cAMP response element binding protein (CREB), the immediate early gene products c-Fos, c-Jun and Egr-1, the growth and plasticity-associated protein GAP-43 and its mRNA, the calcium binding protein calbindin, the cell adhesion molecule integrin-alpha(1), the microtubule-associated protein MAP-1b, and the neurofilament light chain (NF-L). As a consequence of the specific electrical stimulation of the auditory afferents or the loss of hearing, a cascade of events is triggered that apparently modifies the integrative action and computational abilities of the central auditory system. An attempt is made to relate the diverse phenomena observed to a common molecular signaling network that is suspected to bridge sensory experience to changes in the structure and function of the brain. Eventually, a thorough understanding of these events will be essential for the specific diagnosis of patients, optimal timing for implantation, and suitable parameters for running of a cochlear implant or an auditory brainstem implant in humans. In this report an overview of the results obtained in the past years in our lab is presented, flanked by an introduction into the history of plasticity research and a model proposed for intracellular signal cascades related to activity-dependent plasticity.