Extracellular matrix molecules and synaptic plasticity

Matrix metalloproteinase-7 disrupts dendritic spines in hippocampal neurons through NMDA receptor activation

Dendritic spines are protrusions from the dendritic shaft that host most excitatory synapses in the brain. Although they first emerge during neuronal maturation, dendritic spines remain plastic through adulthood, and recent advances in the molecular mechanisms governing spine morphology have shown them to be exquisitely sensitive to changes in the micro-environment. Among the many factors affecting spine morphology are components and regulators of the extracellular matrix (ECM). Modification of the ECM is critical to the repair of injuries throughout the body, including the CNS. Matrix metalloproteinase (MMP)-7/matrilysin is a key regulator of the ECM during pathogen infection, after nerve crush and in encephalitogenic disorders. We have investigated the effects of MMP-7 on dendritic spines in hippocampal neuron cultures and found that it induces the transformation of mature, short mushroom-shaped spines into long, thin filopodia reminiscent of immature spines. These changes were accompanied by a dramatic redistribution of F-actin from spine heads into thick, rope-like structures in the dendritic shaft. Strikingly, MMP-7 effects on dendritic spines were similar to those of NMDA treatment, and both could be blocked by channel-specific antagonists. These findings are the first direct evidence that MMPs can influence the morphology of mature dendritic spines, and hence synaptic stability.

Integrin α3β1 suppresses long-term potentiation at inhibitory synapses on the cerebellar Purkinje neuron

At the GABAergic synapses between inhibitory interneurons and a Purkinje neuron (PN) in the cerebellum, the postsynaptic depolarization induces the long-term potentiation (called rebound potentiation; RP) of GABAA receptor responsiveness. Here, we show that integrins, a type of cell-adhesion molecules, are involved in the regulation of RP. Integrin activation by Mn2+ impaired the RP induction of GABA responsiveness and mIPSCs in PNs, which was abolished by the function blocking antibody against either integrin alpha3 or beta1 subunit, but not by that against alpha5 or alphaV subunit. Furthermore, overexpression of integrin alpha3 subunit in a PN by itself impaired the RP induction. We also show that Src-family of protein tyrosine kinases mediated the suppressive effect of integrin activity on the RP induction. Thus, the integrin/Src pathway negatively regulates the induction of long-term plasticity at inhibitory synapses on a cerebellar PN.

Reelin, very-low-density lipoprotein receptor, and apolipoprotein E receptor 2 control somatic NMDA receptor composition during hippocampal maturation in vitro

Reelin is a secreted protein that regulates brain layer formation during embryonic development. Reelin binds several receptors, including two members of the low-density lipoprotein (LDL) receptor family, the apolipoprotein E receptor 2 (ApoER2) and the very-low-density lipoprotein receptor (VLDLR). Despite the high level of expression of Reelin and ApoER2 in the postnatal brain, their functions in the adult CNS remain elusive. Here, using electrophysiological, immunocytochemical, and biochemical approaches in cultured postnatal hippocampal neurons, we show that Reelin controls the change in subunit composition of somatic NMDA glutamate receptors (NMDARs) during maturation. We found that maturation is characterized by the gradual decrease of the participation of NR1/2B receptors to whole-cell NMDAR-mediated currents. This maturational change was mirrored by a timely correlated increase of both Reelin immunoreactivity in neuronal somata and the amount of secreted Reelin. Chronic blockade of the function of Reelin with antisense oligonucleotides or the function-blocking antibody CR-50 prevented the decrease of NR1/2B-mediated whole-cell currents. Conversely, exogenously added recombinant Reelin accelerated the maturational changes in NMDA-evoked currents. The maturation-induced change in NMDAR subunits also was blocked by chronic treatment with an inhibitor of the Src kinase signaling pathway or an antagonist of the LDL receptors, but not by inhibitors of another class of Reelin receptor belonging to the integrin family. Consistent with these results, immunocytochemistry revealed that NR1-expressing neurons also expressed ApoER2 and VLDLR. These data reveal a new role for Reelin and LDL receptors and reinforce the idea of a prominent role of extracellular matrix proteins in postnatal maturation.

Enhanced hippocampal GABAergic inhibition in mice overexpressing heparin-binding growth-associated molecule

Heparin-binding growth-associated molecule is a developmentally regulated extracellular matrix protein promoting neurite outgrowth, axonal guidance and synaptogenesis. In the hippocampus, heparin-binding growth-associated molecule is expressed in an activity-dependent manner, and has been shown to suppress long-term potentiation of glutamatergic synapses in the area CA1, but the mechanisms underlying this action are unknown. One of the mechanisms by which extracellular matrix proteins might modulate fast synaptic transmission is by altering GABAergic function. Therefore, we have studied the properties of GABAA receptor-mediated inhibition in hippocampus of mutant mice overexpressing heparin-binding growth-associated molecule (heparin-binding growth-associated molecule transgenics). Under control conditions the wild-type mice have much higher level of long-term potentiation than the transgenics. However, in the absence of the GABAA receptor-mediated-inhibition a similar level of long-term potentiation is seen in both strains. In field potential recordings blockade of GABAA receptors by picrotoxin resulted in more accentuated increase in the CA1 population spike in the transgenics than in the wild-type animals. Whole-cell patch-clamp recordings revealed that when compared with the wild-type animals the transgenic mice had higher frequency of spontaneous inhibitory postsynaptic currents in CA1 pyramidal neurons. However, the frequency of action potential-independent miniature inhibitory postsynaptic currents was similar in both strains. Further, the transgenics had reduced paired-pulse depression of inhibitory postsynaptic currents, which was insensitive to the blockade of GABAB receptors in contrast to wild-type mice. The results demonstrate that the mice overexpressing heparin-binding growth-associated molecule have accentuated hippocampal GABAA receptor-mediated inhibition, which in turn may explain the lowered predisposition of glutamatergic synapses to undergo plastic changes in these animals. Thus, our findings suggest a mechanism by which heparin-binding growth-associated molecule can regulate synaptic plasticity.

Effects of extracellular matrix-degrading proteases matrix metalloproteinases 3 and 9 on spatial learning and synaptic plasticity

Rats learning the Morris water maze exhibit hippocampal changes in synaptic morphology and physiology that manifest as altered synaptic efficacy. Learning requires structural changes in the synapse, and multiple cell adhesion molecules appear to participate. The activity of these cell adhesion molecules is, in large part, dependent on their interaction with the extracellular matrix (ECM). Given that matrix metalloproteinases (MMPs) are responsible for transient alterations in the ECM, we predicted that MMP function is critical for hippocampal-dependent learning. In support of this, it was observed that hippocampal MMP-3 and -9 increased transiently during water maze acquisition as assessed by western blotting and mRNA analysis. The ability of the NMDA receptor channel blocker MK801 to attenuate these changes indicated that the transient MMP changes were in large part dependent upon NMDA receptor activation. Furthermore, inhibition of MMP activity with MMP-3 and -9 antisense oligonucleotides and/or MMP inhibitor FN-439 altered long-term potentiation and prevented acquisition in the Morris water maze. The learning-dependent MMP alterations were shown to modify the stability of the actin-binding protein cortactin, which plays an essential role in regulating the dendritic cytoskeleton and synaptic efficiency. Together these results indicate that changes in MMP function are critical to synaptic plasticity and hippocampal-dependent learning.

An integrated, temporal study of the behavioural, electrophysiological and neuropathological consequences of murine prion disease

We have conducted an integrated study of ME7 prion disease by examining the electrophysiological and neuropathological features of hippocampal slices from behaviourally characterised C57Bl/6J mice 12, 14, 16, 18, 20 and 24 weeks after intracerebral micro-injection of ME7 or normal brain homogenate. We describe the pathogenesis of ME7 as a three-stage process. STAGE ONE: PrPSc deposition, synaptic pathology and abnormal synaptic plasticity. STAGE TWO: Onset of behavioural changes, exemplified by an increase in open-field activity, enhancement of the slow AHP and development of vacuolation. Membrane depolarisation is also an early feature, but its exact timing remains to be confirmed. STAGE THREE: Clinical disease, substantial neurodegeneration and further disruption of the action potential profile. We suggest that the mechanisms underlying the electrophysiological changes of Stages one and two may provide novel approaches to treatment of prion disease, and that those seen in Stage three may be relevant to neurodegenerative diseases more generally.

Axon initial segment ensheathed by extracellular matrix in perineuronal nets

Perineuronal nets of extracellular matrix are associated with distinct types of neurons in the cerebral cortex and many subcortical regions. Large complexes of aggregating proteoglycans form a chemically specified microenvironment around the somata, proximal dendrites and the axon initial segment, including the presynaptic boutons attached to these domains. The subcellular distribution and the temporal course of postnatal formation suggest that perineuronal nets may be involved in the regulation of synaptic plasticity. Here we investigate structural and cytochemical characteristics of the extracellular matrix around axon initial segments virtually devoid of synaptic contacts. Wisteria floribunda agglutinin staining, the immunocytochemical detection of aggrecan and tenascin-R, as well as affinity-labeling of hyaluronan were used to analyze perineuronal nets associated with large motoneurons in the mouse superior colliculus. The molecular composition of perineuronal nets was divergent between neurons but was identical around the different cellular domains of the individual neurons. The axon initial segments largely devoid of synapses were covered by a continuous matrix sheath infiltrating the adjacent neuropil. The periaxonal zone penetrated by matrix components often increased in diameter along the initial segment from the axon hillock toward the myelinated part of the axon. The axonal and somatodendritic domains of perineuronal nets were concomitantly formed during the first three weeks of postnatal development. The common molecular properties and major structural features of subcellular perineuronal net domains were retained in organotypic midbrain slice cultures. The results support the hypothesis that the aggrecan-related extracellular matrix of perineuronal nets provides a continuous micromilieu for different subcellular domains performing integration and generation of the electrical activity of neurons.

Opposite impacts of tenascin-C and tenascin-R deficiency in mice on the functional outcome of facial nerve repair

The glycoproteins tenascin-C (TNC) and tenascin-R (TNR) are extracellular matrix proteins involved in the development, plasticity and repair of the nervous system. Altered expression patterns after nerve lesions in adult animals have suggested that these molecules influence axonal regeneration. To test this hypothesis, we investigated adult mice constitutively deficient in the expression of TNC, TNR or both, using the facial nerve injury paradigm. Quantitative analysis of vibrissal movements prior to nerve transection and repair (facial-facial anastomosis) did not reveal genotype-specific differences, and thus impacts of the mutations on motor function in intact animals. Two months after nerve repair, recovery of vibrissal whisking was poor in wild-type mice, a typical finding after facial-facial anastomosis in rodents. Differential effects of the mutations on whisking were found: recovery of function was worse in TNC-deficient and better in TNR null mice compared with wild-type littermates. In double-knockout animals, vibrissal performance was insufficient, but to a lesser extent compared with TNC null mutant mice. Retrograde labelling of motoneurons in the same animals showed that similar numbers of motoneurons had reinnervated the whisker pads in all experimental groups precluding varying extents of motoneuron death and/or axon regeneration failures as causes for the different outcomes of nerve repair. Our results provide strong evidence that TNC promotes and TNR impedes recovery after nerve lesion. These findings are of particular interest with regard to the scanty knowledge about factors determining success of regeneration in the peripheral nervous system of mammals.

Olfactory mucosa autografts in human spinal cord injury: a pilot clinical study

BACKGROUND/OBJECTIVE

Olfactory mucosa is a readily accessible source of olfactory ensheathing and stem-like progenitor cells for neural repair. To determine the safety and feasibility of transplanting olfactory mucosa autografts into patients with traumatically injured spinal cords, a human pilot clinical study was conducted.

METHODS

Seven patients ranging from 18 to 32 years of age (American Spinal Injury Association [ASIA] class A) were treated at 6 months to 6.5 years after injury. Olfactory mucosa autografts were transplanted into lesions ranging from 1 to 6 cm that were present at C4-T6 neurological levels. Operations were performed from July 2001 through March 2003. Magnetic resonance imaging (MRI), electromyography (EMG), and ASIA neurological and otolaryngological evaluations were performed before and after surgery.

RESULTS

MRI studies revealed moderate to complete filling of the lesion sites. Two patients reported return of sensation in their bladders, and one of these patients regained voluntary contraction of anal sphincter. Two of the 7 ASIA A patients became ASIA C. Every patient had improvement in ASIA motor scores. The mean increase for the 3 subjects with tetraplegia in the upper extremities was 6.3 +/- 1.2 (SEM), and the mean increase for the 4 subjects with paraplegia in the lower extremities was 3.9 +/- 1.0. Among the patients who improved in their ASIA sensory neurological scores (all except one patient), the mean increase was 20.3 +/- 5.0 for light touch and 19.7 +/- 4.6 for pinprick. Most of the recovered sensation below the initial level of injury was impaired. Adverse events included sensory decrease in one patient that was most likely caused by difficulty in locating the lesion, and there were a few instances of transient pain that was relieved by medication. EMG revealed motor unit potential when the patient was asked to perform movement.

CONCLUSION

This study shows that olfactory mucosa autograft transplantation into the human injured spinal cord is feasible, relatively safe, and potentially beneficial. The procedure involves risks generally associated with any surgical procedure. Long-term patient monitoring is necessary to rule out any delayed side effects and assess any further improvements.

Gene expression pathways induced by axotomy and decentralization of rat superior cervical ganglion neurons

To identify genes potentially involved in remodelling synaptic connections, we induced the temporary detachment of pre- and post-synaptic elements by axotomy or denervation of rat superior cervical ganglion neurons. cDNA microarray analysis followed by stringent selection criteria allowed the identification of a panel of genes whose expression was modulated by axotomy at various time points after injury. Among these genes, 11 were validated by real-time reverse transcriptase-polymerase chain reaction on independently prepared samples after superior cervical ganglion neuron axotomy (1, 3 and 6 days) and compared with the effect of decentralization (8 h, 1 and 3 days). These genes code for extracellular matrix/space [apolipoprotein D (apoD), decorin, collagen alpha1 type I, collagen alpha1 type III] and intermediate filament (vimentin) proteins, for modulators of neurite outgrowth (thrombin receptor, plasminogen activator inhibitor-1, bone morphogenetic protein 4, annexin II and S-100-related protein, clone 42C) and for a nerve cell transcription factor (brain finger protein). Eight of these 11 genes showed significant and persistent modulations after both types of injury. Finally, protein levels of apoD were shown to increase in superior cervical ganglion after axotomy. Our results identify hitherto unrecorded genes responsive to axotomy and decentralization of superior cervical ganglion neurons, and probably involved in synapse formation, remodelling and elimination.

Expression and identification of a new splice variant of neuroglycan C, a transmembrane chondroitin sulfate proteoglycan, in the human brain

Neuroglycan C (NGC) is a transmembrane chondroitin sulfate proteoglycan with an EGF module. We studied the expression of NGC in the human brain, mainly in the hippocampus, and confirmed some observations by conducting experiments using rat brain. In humans, NGC mRNA was expressed exclusively in the brain, especially in the immature brain. The telencephalon, including the hippocampus and neocortex, showed strong mRNA expression. NGC was immunolocalized to neuropils in the hippocampus and neocortex of the adult rat. RT-PCR experiments showed that four splice variants (NGC-I, -II, -III, and -IV) were expressed in the adult human hippocampus. By Western blotting, the expression as proteins of all splice variants except NGC-II was confirmed in the adult rat hippocampus. NGC-IV, which was first found in the present study, had the shortest cytoplasmic domain among the four variants. NGC-IV mRNA was expressed by neurons, but not by astrocytes, in culture prepared from the fetal rat hippocampus, suggesting that NGC-IV plays a role specific to neurons. In addition, the human NGC gene, which is registered as CSPG5, comprised six exons and was approximately 19 kb in size. In exon 2, a single nucleotide polymorphism resulting in Val188Gly in the NGC ectodomain was observed.

Reduced extracellular space in the brain of tenascin-R- and HNK-1-sulphotransferase deficient mice

Tenascin-R (TN-R), a large extracellular glycoprotein, is an important component of the adult brain's extracellular matrix (ECM); tenascin-C (TN-C) is expressed mainly during early development, while human natural killer 1 (HNK-1) is a sulphated carbohydrate epitope that attaches to these molecules, modifying their adhesive properties. To assess their influence on extracellular space (ECS) volume and geometry, we used the real-time iontophoretic method to measure ECS volume fraction alpha and tortuosity lambda, and diffusion-weighted magnetic resonance imaging (MRI) to measure the apparent diffusion coefficient of water (ADC(W)). Measurements were performed in vivo in the cortex and CA1 hippocampal region of TN-R-, TN-C- and HNK-1 sulphotransferase (ST)-deficient adult mice and their wild-type littermate controls. In both cortex and hippocampus, the lack of TN-R or HNK-1 sulphotransferase resulted in a significant decrease in alpha and lambda. Compared with controls, alpha in TN-R-/- and ST-/- mice decreased by 22-26% and 9-15%, respectively. MRI measurements revealed a decreased ADC(W) in the cortex, hippocampus and thalamus. ADC(W) reflected the changes in alpha; the decrease in lambda indicated fewer diffusion obstacles in the ECS, presumably due to a decreased macromolecular content. No significant changes were found in TN-C-/- animals. We conclude that in TN-R-/- and ST-/- mice, which show morphological, electrophysiological and behavioural abnormalities, the ECS is reduced and its geometry altered. TN-R, as an important component of the ECM, appears to maintain an optimal distance between cells. The altered diffusion of neuroactive substances in the brain will inevitably affect extrasynaptic transmission, neuron-glia interactions and synaptic efficacy.

siRNA targeted against amyloid precursor protein impairs synaptic activity in vivo

The amyloid precursor protein (APP) plays a central role in Alzheimer's disease (AD) pathogenesis through its cleavage leading to the accumulation of the peptide betaA4. Diffusible oligomeric assemblies of amyloid beta peptide are thought to induce synaptic dysfunction, an early change in AD. We tested the hypothesis that a reduction in presynaptic APP could itself lead to a decrease in synaptic efficacy in vivo. Twenty-four hours after intraocular injection, siRNA targeted against APP accumulated in retinal cells and the APP in retinal terminals in the superior colliculus was significantly reduced. Surprisingly, the amyloid precursor-like protein 2 (APLP2) was reduced as well. Functional imaging experiments in rats during visual stimulation showed that knockdown of presynaptic APP/APLP2 significantly reduced the stimulation-induced glucose utilization in the superior colliculus. Our results suggest that perturbations in the amount of APP/APLP2 axonally transported to, and/or in their turnover in the nerve terminal alter synaptic function and could be a pathogenic mechanism in AD.

Critical period plasticity in local cortical circuits

Neuronal circuits in the brain are shaped by experience during 'critical periods' in early postnatal life. In the primary visual cortex, this activity-dependent development is triggered by the functional maturation of local inhibitory connections and driven by a specific, late-developing subset of interneurons. Ultimately, the structural consolidation of competing sensory inputs is mediated by a proteolytic reorganization of the extracellular matrix that occurs only during the critical period. The reactivation of this process, and subsequent recovery of function in conditions such as amblyopia, can now be studied with realistic circuit models that might generalize across systems.

Prepulse inhibition and fear-potentiated startle are altered in tissue inhibitor of metalloproteinase-2 (TIMP-2) knockout mice

The ability to discriminate between potential dangers and recall those stimuli is essential for survival. This emotional learning requires the involvement of higher brain structures, including the amygdala, hippocampus and related cortical structures. Long-term changes in synaptic transmission and structure are important for the establishment and consolidation of fear memory. The structural changes associated with this synaptic plasticity likely require alterations in the composition of the extracellular matrix (ECM). ECM integrity is maintained by the opposing action of matrix metalloproteinases (MMPs) and their specific inhibitors, tissue inhibitors of metalloproteinases (TIMPs). To date, no studies have examined the role of MMPs or TIMPs in conditioned fear. Here, we show that neither male nor female mice deficient in TIMP-2 (knockout) exhibit prepulse inhibition of the startle reflex, suggesting deficits in pre-attentional sensorimotor gating. In addition, knockout mice and mice expressing a mutant truncated TIMP-2 (knock-down) show deficits in fear-potentiated startle. This is the first report of a phenotype for the TIMP-2(-/-) mice and suggests that TIMP-2 may play a role in the synaptic plasticity underlying learning and memory.

Expression of neuroserpin is linked to neuroendocrine cell activation

Inhibitors of serine proteases (serpins) are important regulators of intracellular and extracellular proteolytic pathways, and they function by forming an irreversible complex with their substrate. Neuroserpin represents a neuroendocrine-specific serpin family member that is expressed in brain regions displaying synaptic plasticity. In this study, we explored the biosynthesis of endogenous neuroserpin in a neuroendocrine model system, namely the melanotrope cells of Xenopus intermediate pituitary. The biosynthetic activity of these cells can be physiologically manipulated (high and low production of the prohormone proopiomelanocortin in black and white animals, respectively), resulting from a synaptic plasticity in innervating hypothalamic neurons. We found that neuroserpin was also differentially expressed in the Xenopus intermediate, but not anterior, pituitary with a 3-fold higher mRNA and more than 30-fold higher protein expression in the active vs. the inactive melanotrope cells. Two newly synthesized glycosylated forms of the neuroserpin protein (47 and 50 kDa) were produced and secreted by the active cells. Intriguingly, neuroserpin was found in an approximately 130-kDa sodium dodecyl sulfate-stable complex in the active, but not in the inactive, melanotrope cells, which correlated with the high and low proopiomelanocortin expression levels, respectively. In conclusion, we report on the biosynthesis of neuroserpin in a physiological context, and we find that the induction of neuroserpin expression and the formation of the 130-kDa neuroserpin-containing complex are linked to neuroendocrine cell activation.

Decomposition and long-lasting downregulation of extracellular matrix in perineuronal nets induced by focal cerebral ischemia in rats

The upregulation of extracellular matrix components, especially chondroitin sulfate proteoglycans, after brain injury and stroke is known to accompany the glial reaction, forming repellent scars that hinder axonal growth and the reorganization of the injured neuronal networks. The extracellular matrix associated with perineuronal nets (PNs) in the primarily injured and remote regions has not yet been systematically analyzed. We use the model of permanent middle cerebral artery occlusion (MCAO) to investigate the acute and long-lasting consequences of ischemia for PNs, related to the damage of neurons and reactions of glial cells, in spontaneously hypertensive rats. Extracellular matrix components associated with PNs around cortical interneurons and neurons in thalamic nuclei were characterized 1, 7, 14, and 35 days after MCAO, using Wisteria floribunda agglutinin (WFA) staining and immunocytochemistry. The degradation of PNs in the infarct core was initiated by loss of WFA-binding matrix components, indicating the cleavage of glycosaminoglycan chains of chondroitin sulfate proteoglycans. Immunostaining showed the subsequent removal of proteoglycan core proteins within the extending microglia/macrophage invasion zone lasting for 2 weeks after MCAO. In the cortical periinfarct region, delineated by an astrocytic scar against the infarct core, the number of WFA-stained and proteoglycan core protein-immunoreactive PNs was permanently reduced. In the homolateral ventroposterior thalamus, the delayed decrease in perineuronal matrix was related to the distribution pattern of activated microglia and massive neuronal degeneration. It can be concluded from these results that complementary to the known upregulation of matrix components in the glial scar, deficits in the expression of the neuron-associated extracellular matrix develop in the periinfarct and remote regions. These deficits may contribute to the long-lasting functional impairments after stroke.

Proteoglycans in brain development

Proteoglycans, as part of the extracellular or cell-surface milieu of most tissues and organ systems, play important roles in morphogenesis by modulating cell-matrix or cell-cell interactions, cell adhesiveness, or by binding and presenting growth and differentiation factors. Chondroitin sulfate proteoglycans which constitute the major population of proteoglycans in the central nervous system may influence formation of neuronal nuclei, establishment of boundaries for axonal growth and act as modulators of neuronal outgrowth during brain development, as well as during regeneration after injury. There is a paucity of information on the role of chondroitin sulfate proteoglycans in central nervous system organogenesis. In the chick embryo, aggrecan has a regionally specific and developmentally regulated expression profile during brain development. By Northern and Western blot analysis, aggrecan expression is first detected in chick brain on embryonic day 7 (E7), increases from E7 to E13, declines markedly after E16, and is not evident in hatchling brains. The time course and pattern of aggrecan expression observed in ventricular zone cells suggested that it might play a role in gliogenesis. We have analyzed the role of aggrecan during brain development using a aggrecan-deficient model, nanomelia. In nanomelic chicks, expression and levels of neurocan and brevican is not affected, indicating a non-redundant role for these members of the aggrecan gene family. Our analysis of the aggrecan-deficient model found a severely altered phenotype which affects cell behavior in a neuronal culture paradigm and expression of astrocytic markers in vivo . Taken together our results suggest a function for aggrecan in the specification of a sub-set of glia precursors that might give rise to astrocytes in vivo.

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.

Elevation of hippocampal MMP-3 expression and activity during trauma-induced synaptogenesis

The matrix metalloproteinase (MMP) enzyme family contributes to the regulation of a variety of brain extracellular matrix molecules. In order to assess their role in synaptic plasticity following traumatic brain injury (TBI), we compared expression of stromelysin-1 (MMP-3) protein and mRNA in two rodent models of TBI exhibiting different levels of recovery: adaptive synaptic plasticity following central fluid percussion injury and maladaptive synaptic plasticity generated by combined TBI and bilateral entorhinal cortical lesion (TBI + BEC). We sampled the hippocampus at 7 days postinjury, targeting a selectively vulnerable brain region and a survival interval exhibiting rapid synaptogenesis. We report elevated expression of hippocampal MMP-3 mRNA and protein after TBI. MMP-3 immunohistochemical staining showed increased protein levels relative to sham-injured controls, primarily localized to cell bodies within the deafferented dendritic laminae. Injury-related differences in MMP-3 protein were also observed. TBI alone elevated MMP-3 immunobinding over the stratum lacunosum moleculare (SLM), inner molecular layer and hilus, while TBI + BEC generated more robust increases in MMP-3 reactivity within the deafferented SLM and dentate molecular layer (DML). Double labeling with GFAP confirmed the presence of MMP-3 within reactive astrocytes induced by each injury model. Semi-quantitative RT-PCR revealed that MMP-3 mRNA also increased after each injury, however, the combined insult induced a much greater elevation than fluid percussion alone: 1.9-fold vs. 79%, respectively. In the TBI + BEC model, MMP-3 up-regulation was spatio-temporally correlated with increased enzyme activity, an effect which was attenuated with the neuroprotective compound MK-801. These results show that distinct pathological conditions elicited by TBI can differentially affect MMP-3 expression during reactive synaptic plasticity. Notably, these effects are both transcriptional and translational and are correlated with functionally active enzyme.

Experience-dependent pruning of dendritic spines in visual cortex by tissue plasminogen activator

Sensory experience physically rewires the brain in early postnatal life through unknown processes. Here, we identify a robust anatomical consequence of monocular deprivation (MD) in layer II/III of visual cortex that corresponds to the rapid, functional loss of responsiveness preceding any changes in axonal input. Protrusions on pyramidal cell apical dendrites increased steadily after eye opening, but were transiently lost through competitive mechanisms after brief MD only during the physiological critical period. Proteolysis by tissue-type plasminogen activator (tPA) conversely declined with age and increased with MD only in young mice. Targeted disruption of tPA release or its upstream regulation by glutamic acid decarboxylase (GAD65) prevented MD-induced spine loss that was pharmacologically rescued concomitant with critical period plasticity. An extracellular mechanism for structural remodeling that is limited to the binocular zone upon proper detection of competing inputs thus links early sensory experience to visual function.

Dendritic spine dynamics are regulated by monocular deprivation and extracellular matrix degradation

The mammalian primary visual cortex (V1) is especially susceptible to changes in visual input over a well-defined critical period, during which closing one eye leads to a loss of responsiveness of neurons to the deprived eye and a shift in response toward the open eye. This functional plasticity can occur rapidly, following even a single day of eye closure, although the structural bases of these changes are unknown. Here, we show that rapid structural changes at the level of dendritic spines occur following brief monocular deprivation. These changes are evident in the supra- and infragranular layers of the binocular zone and can be mimicked by degradation of the extracellular matrix with the tPA/plasmin proteolytic cascade. Further, monocular deprivation occludes a subsequent effect of matrix degradation, suggesting that this mechanism is active in vivo to permit structural remodeling during ocular dominance plasticity.

Extracellular matrix and visual cortical plasticity: freeing the synapse

The effects of monocular deprivation (MD) on the ocular dominance of visual cortical neurons are a paradigmatic example of experience-dependent plasticity. Here we review recent data showing that extracellular matrix (ECM) plays an important role in the control of experience-dependent plasticity both in the developing and adult visual cortex.

Critical Period Mechanisms in Developing Visual Cortex

Binocular vision is shaped by experience during a critical period of early postnatal life. Loss of visual acuity following monocular deprivation is mediated by a shift of spiking output from the primary visual cortex. Both synaptic and network explanations have been offered for this heightened brain plasticity. Direct experimental control over its timing, duration, and closure has now been achieved through a consideration of balanced local circuit excitation-inhibition. Notably, canonical models of homosynaptic plasticity at excitatory synapses alone (LTP/LTD) fail to produce predictable manipulations of the critical period in vivo. Instead, a late functional maturation of intracortical inhibition is the driving force, with one subtype in particular standing out. Parvalbumin-positive large basket cells that innervate target cell bodies with synapses containing the alpha1-subunit of GABA(A) receptors appear to be critical. With age, these cells are preferentially enwrapped in peri-neuronal nets of extracellular matrix molecules, whose disruption by chondroitinase treatment reactivates ocular dominance plasticity in adulthood. In fact, critical period plasticity is best viewed as a continuum of local circuit computations ending in structural consolidation of inputs. Monocular deprivation induces an increase of endogenous proteolytic (tPA-plasmin) activity and consequently motility of spines followed by their pruning, then re-growth. These early morphological events faithfully reflect competition only during the critical period and lie downstream of excitatory-inhibitory balance on a timescale (of days) consistent with the physiological loss of deprived-eye responses in vivo. Ultimately, thalamic afferents retract or expand accordingly to hardwire the rapid functional changes in connectivity. Competition detected by local inhibitory circuits then implemented at an extracellular locus by proteases represents a novel, cellular understanding of the critical period mechanism. It is hoped that this paradigm shift will lead to novel therapies and training strategies for rehabilitation, recovery from injury, and lifelong learning in adulthood.

Oxytocin neuron activation in NCAM-deficient mice: anatomical and functional consequences

During stimulated neurosecretion in the rat, oxytocin neurons display a reduced glial coverage and receive an increased number of synapses, changes that are reversed on arrest of stimulation. We identified polysialic acid on the neural cell adhesion molecule (NCAM) as an important mediator of such plasticity. To investigate further the role of this cell surface glycoprotein, we examined the oxytocin system in mice genetically deficient in NCAM. First, ultrastructural analyses revealed that in wild-type mice, the supraoptic nucleus (SON) underwent the same remodelling as in the rat because oxytocin neurons had a diminished astrocytic coverage and increased synaptic input during lactation or chronic salt loading. Surprisingly, the SON displayed this morphology in NCAM-deficient mice as well, whether they were nongestating and hydrated, lactating or dehydrated. The oxytocin system in NCAM-deficient mice was abnormally hyperactive, as illustrated by enhanced plasma and intranuclear concentrations of oxytocin and reduced anxiety-related behaviour. Plasma oxytocin concentrations were also high in lactating NCAM-deficient dams but certain parameters of lactation and maternal behaviour were impaired. NCAM-deficient mice survived ingestion of 2% saline for 7 days and had increased plasma oxytocin but they did not cope with more severe osmotic challenges. Our observations highlight further the remarkable capacity of the adult oxytocin system to undergo neuronal and glial remodelling whenever it is activated. That lack of NCAM did not prevent remodelling indicates that NCAM can be substituted by other molecular mechanisms. Finally, while NCAM deficiency greatly enhanced oxytocin release, it led to impaired oxytocin-dependent physiological and behavioural responses.

Neuronal, glial and synaptic remodeling in the adult hypothalamus: functional consequences and role of cell surface and extracellular matrix adhesion molecules

The adult hypothalamo-neurohypophysial system (HNS) undergoes activity-dependent morphological plasticity which modifies astrocytic coverage of its oxytocinergic neurons and their synaptic inputs. Thus, during physiological conditions that enhance central and peripheral release of oxytocin (OT), adjacent somata and dendrites of OT neurons become extensively juxtaposed, without intervening astrocytic processes and receive an increased number of synapses. The morphological changes occur within a few hours and are reversible with termination of stimulation. The reduced astrocytic coverage has direct functional consequences since it modifies extracellular ionic homeostasis, synaptic transmission, and the size and geometry of the extracellular space. It also contributes indirectly to neuronal function by permitting formation of synapses on neuronal surfaces freed of astrocytic processes. Overall, such remodeling is expected to potentiate activated neuronal firing, especially in clusters of tightly packed neurons, an anatomical arrangement characterizing OT neurons. This plasticity connotes dynamic cell interactions that must bring into play cell surface and extracellular matrix adhesive proteins like those intervening in developing neuronal systems undergoing neuronal-glial and synaptogenic transformations. It is worth noting, therefore, that adult HNS neurons and glia continue to express such molecules, including polysialic acid (PSA)-enriched neural cell adhesion molecule (PSA-NCAM) and the glycoprotein, tenascin-C. PSA is a large, complex sugar on the extracellular domain of NCAM considered a negative regulator of adhesion; it occurs in large amounts on the surfaces of HNS neurons and astrocytes. Tenascin-C, on the other hand, possesses adhesive and repulsive properties; it is secreted by HNS astrocytes and occurs in extracellular spaces and on cell surfaces after interaction with appropriate ligands. These molecules have been considered permissive factors for morphological plasticity. However, because of their localization and inherent properties, they may also serve to modulate the extracellular environment and in consequence, synaptic and volume transmission in a system in which the extracellular compartment is constantly being modified.

Perineuronal nets potentially protect against oxidative stress

A specialized form of extracellular matrix (ECM) termed perineuronal nets (PNs) consisting of large aggregating chondroitin sulfate proteoglycans (CSPGs), with hyaluronan and tenascin as main components, surrounds subpopulations of neurons. The glycosaminoglycan components of perineuronal nets form highly charged structures in the direct microenvironment of neurons and thus might be involved in local ion homeostasis. The polyanionic character suggests that perineuronal nets also potentially contribute to reduce the local oxidative potential in the neuronal microenvironment by scavenging and binding redox-active iron, thus providing some neuroprotection to net-associated neurons. Here, we show that neurons ensheathed by a perineuronal net in the human cerebral cortex are less frequently affected by lipofuscin accumulation than neurons without a net both in normal-aged brain and Alzheimer's disease (AD). As lipofuscin is an intralysosomal pigment composed of cross-linked proteins and lipids generated by iron-catalyzed oxidative processes, the present results suggest a neuroprotective function of perineuronal nets against oxidative stress, potentially involved in neurodegeneration.

Enhancement of excitatory postsynaptic potentials by preceding application of acetylcholine in mesencephalic dopamine neurons

Previously, we reported that Ca(2+) influx through nicotinic acetylcholine (ACh) receptors (nAChRs) activates a fulfenamic acid (FFA)-sensitive inward current, presumably a Ca(2+)-activated nonselective cation current (I(CAN)), in mesencephalic dopamine (DA) neurons. This current exhibited a negative slope conductance in the voltage range between -80 and -40mV and its activation led to a dramatic change in the responses to a transient application of glutamate, from single spikes to burst discharges. In this study, to examine the effect of activation of the FFA-sensitive current on EPSPs, we applied ACh (1mM) by transient air pressure shortly before electrical stimulation to evoke EPSPs in DA neurons. Application of ACh enhanced the amplitude of EPSPs when it preceded the electrical stimulation by less than 2 s, but not when the interval was longer than 3 s. In addition, this enhancement was critically dependent on intracellular Ca(2+) and the membrane potentials of the postsynaptic cell. Furthermore, the enhancing effect of ACh on EPSPs was sensitive to FFA and phenytoin. These results suggest that Ca(2+) influx caused by cholinergic inputs enhances EPSPs via activation of the FFA- and phenytoin-sensitive current.

The brain angiotensin system and extracellular matrix molecules in neural plasticity, learning, and memory

The brain renin-angiotensin system (RAS) has long been known to regulate several classic physiologies including blood pressure, sodium and water balance, cyclicity of reproductive hormones and sexual behaviors, and pituitary gland hormones. These physiologies are thought to be under the control of the angiotensin II (AngII)/AT1 receptor subtype system. The AT2 receptor subtype is expressed during fetal development and is less abundant in the adult. This receptor appears to oppose growth responses facilitated by the AT1 receptor, as well as growth factor receptors. Recent evidence points to an important contribution by the brain RAS to non-classic physiologies mediated by the newly discovered angiotensin IV (AngIV)/AT4 receptor subtype system. These physiologies include the regulation of blood flow, modulation of exploratory behavior, and a facilitory role in learning and memory acquisition. This system appears to interact with brain matrix metalloproteinases in order to modify extracellular matrix molecules thus permitting the synaptic remodeling critical to the neural plasticity presumed to underlie memory consolidation, reconsolidation, and retrieval. There is support for an inhibitory influence by AngII activation of the AT1 subtype, and a facilitory role by AngIV activation of the AT4 subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning. The discovery of the AT4 receptor subtype, and its facilitory influence upon learning and memory, suggest an important role for the brain RAS in normal cognitive processing and perhaps in the treatment of dysfunctional memory disease states.

Trans-Golgi network delivery of synaptic proteins in synaptogenesis

Synapse formation, stabilization and maintenance comprise several remarkably precise and rapid stages. The initial steps involve delivery to the site of initial contact between axon and dendrite of transport carriers containing several sets of synaptic proteins necessary for proper synaptic function. This occurs both pre- and postsynaptically and is mediated by apparently distinct vesicular carriers that fuse with the synaptic plasma membrane to deliver receptors for neurotransmitters, ion channels, transporters and pumps. The presynaptic carriers in the developing axon give rise to synaptic vesicles. On the postsynaptic side, the so-called spine apparatus may represent a tubular reservoir that gives rise to the postsynaptic players in synaptic function. Recent evidence indicates that recognition molecules, particularly neural cell adhesion molecule (NCAM), are associated with trans-Golgi-network-derived structures and thus can provide a signal for accumulation of these transport carriers at nascent synapses.

Eph receptors in the adult brain

The Eph receptors are a large family of receptor tyrosine kinases with important roles in the establishment of neuronal and vascular networks during embryonic development. The functions of Eph receptors in the adult brain have only recently been investigated, and the results are forcing us to amend the conventional view that these molecules function predominantly in a developmental context. This review summarizes this rapidly expanding new area of research, which has shown that the Eph receptors regulate the structure and physiological function of excitatory synapses through multiple mechanisms, and might thus play a significant role in higher brain functions.

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.

Laser capture microdissection and analysis of amplified antisense RNA from distinct cell populations of the young and aged rat brain: effect of traumatic brain injury on hippocampal gene expression

To explore the molecular mechanisms underlying the increased vulnerability of the aged brain to traumatic brain injury (TBI), we compared the expression of several age-related genes in the CA1, CA3 and dentate gyrus subfields of the young and aged rat hippocampus before and after lateral fluid percussion TBI. Using laser capture microdissection (LCM), we obtained hippocampal neurons and glia from the neuropil adjacent to the pyramidal and granule cell layers. Subsequently, we linearly amplified and analyzed the antisense mRNA using Northern blot and ribonuclease protection assays (RPA). Our procedures, which have not been previously applied to quantitative analysis of LCM mRNA from neural tissue, included a modified reverse transcription step to enhance full-length cDNA synthesis, thus enhancing the yield of larger components of in vitro-transcribed mRNA for downstream analysis. Northern analysis showed greater expression of two aging-associated genes, p21 and brain-derived neurotrophic factor (BDNF) in the aged hippocampus. The age-related differences in p21 and BDNF expression were particularly prominent after TBI. By quantitative RPA analysis, we found that the expression of p21, known to be induced in senescent cells, was significantly greater in the CA3 region of aged rats, an area that is selectively vulnerable to TBI. However, expression of genes associated with regenerative and repair functions was significantly decreased in aged hippocampus. Our RPA results indicate that substantial age-dependent differences in the transcriptional profile of distinct regions of the hippocampal formation may account, in part, for their differential susceptibility to brain injury.

Glycans and neural cell interactions

Carbohydrate-carrying molecules in the nervous system have important roles during development, regeneration and synaptic plasticity. Carbohydrates mediate interactions between recognition molecules, thereby contributing to the formation of a complex molecular meshwork at the cell surface and in the extracellular matrix. The tremendous structural diversity of glycan chains allows for immense combinatorial possibilities that might underlie the fine-tuning of cell-cell and cell-matrix interactions.

Functional neural anatomy of talent

The terms gifted, talented, and intelligent all have meanings that suggest an individual's highly proficient or exceptional performance in one or more specific areas of strength. Other than Spearman's g, which theorizes about a general elevated level of potential or ability, more contemporary theories of intelligence are based on theoretical models that define ability or intelligence according to a priori categories of specific performance. Recent studies in cognitive neuroscience report on the neural basis of g from various perspectives such as the neural speed theory and the efficiency of prefrontal function. Exceptional talent is the result of interactions between goal-directed behavior and nonvolitional perceptual processes in the brain that have yet to be fully characterized and understood by the fields of psychology and cognitive neuroscience. Some developmental studies report differences in region-specific neural activation, recruitment patterns, and reaction times in subjects who are identified with high IQ scores according to traditional scales of assessment such as the WISC-III or Stanford-Binet. Although as cases of savants and prodigies illustrate, talent is not synonymous with high IQ. This review synthesizes information from the fields of psychometrics and gifted education, with findings from the neurosciences on the neural basis of intelligence, creativity, profiles of expert performers, cognitive function, and plasticity to suggest a paradigm for investigating talent as the maximal and productive use of either or both of one's high level of general intelligence or domain-specific ability. Anat Rec (Part B: New Anat) 277B:21-36, 2004.

Chondroitin sulphate proteoglycans: preventing plasticity or protecting the CNS?

It is well established that axonal regeneration in the adult CNS is largely unsuccessful. Numerous axon-inhibitory molecules are now known to be present in the injured CNS, and various strategies for overcoming these obstacles and enhancing CNS regeneration have been experimentally developed. Recently, the use of chondroitinase-ABC to treat models of CNS injury in vivo has proven to be highly beneficial towards regenerating axons, by degrading the axon-inhibitory chondroitin sulphate glycosaminoglycan chains found on many proteoglycans in the astroglial scar. This enzyme has now been shown to restore synaptic plasticity in the visual cortex of adult rats by disrupting perineuronal nets, which contain high levels of chondroitin sulphate proteoglycans (CS-PGs) and are expressed postnatally around groups of certain neurons in the normal CNS. The findings suggest exciting prospects for enhancing growth and plasticity in the adult CNS; however, some protective roles of CS-PGs in the CNS have also been demonstrated. Clearly many questions concerning the mechanisms regulating expression of extracellular matrix molecules in CNS pathology remain to be answered.

Proteomics Analysis of Rat Brain Postsynaptic Density

The postsynaptic density contains multiple protein complexes that together relay the presynaptic neurotransmitter input to the activation of the postsynaptic neuron. In the present study we took two independent proteome approaches for the characterization of the protein complement of the postsynaptic density, namely 1) two-dimensional gel electrophoresis separation of proteins in conjunction with mass spectrometry to identify the tryptic peptides of the protein spots and 2) isolation of the trypsin-digested sample that was labeled with isotope-coded affinity tag, followed by liquid chromatography-tandem mass spectrometry for the partial separation and identification of the peptides, respectively. Functional grouping of the identified proteins indicates that the postsynaptic density is a structurally and functionally complex organelle that may be involved in a broad range of synaptic activities. These proteins include the receptors and ion channels for glutamate neurotransmission, proteins for maintenance and modulation of synaptic architecture, sorting and trafficking of membrane proteins, generation of anaerobic energy, scaffolding and signaling, local protein synthesis, and correct protein folding and breakdown of synaptic proteins. Together, these results imply that the postsynaptic density may have the ability to function (semi-) autonomously and may direct various cellular functions in order to integrate synaptic physiology.

The role of ECM molecules in activity-dependent synaptic development and plasticity

Growth and guidance of neurites (axons and dendrites) during development is the prerequisite for the establishment of functional neural networks in the adult organism. In the adult, mechanisms similar to those used during development may regulate plastic changes that underlie important nervous system functions, such as memory and learning. There is now ever-increasing evidence that extracellular matrix (ECM)-associated factors are critically involved in the formation of neuronal connections during development, and their plastic changes in the adult. Here, we review the current literature on the role of ECM components in activity-dependent synaptic development and plasticity, with the major focus on the thrombospondin type I repeat (TSR) domain-containing proteins. We propose that ECM components may modulate neuronal development and plasticity by: 1) regulating cellular motility and morphology, thus contributing to structural alterations that are associated with the expression of synaptic plasticity, 2) coordinating transsynaptic signaling during plasticity via their cell surface receptors, and 3) defining the physical parameters of the extracellular space, thereby regulating diffusion of soluble signaling molecules in the extracellular space (ECS).

NCAM180 and glutamate receptor subtypes in potentiated spine synapses: an immunogold electron microscopic study

Activity-dependent changes in expression and localization of the largest major isoform of the neural cell adhesion molecule NCAM180 and three subtypes of glutamate receptors predominantly expressed in the outer part of the molecular layer of the dentate gyrus of adult rats-the NMDA receptor NR2A, the AMPA receptor GluR2/3, and the metabotropic glutamate receptor mGluR1 - were investigated using postembedding immunogold labeling, and electron microscopy. In synaptic membranes of nonstimulated spine synapses, NCAM180 and NR2A accumulated in the center of the postsynaptic density, whereas GluR2/3 and mGluR1 were distributed evenly. Twenty-four hours following induction of long-term potentiation in vivo, NCAM180 and NR2A accumulated at the edges of postsynaptic densities, whereas GluR2/3 was localized more centrally. Also, the distribution of gold particles per synapse significantly changed for NCAM180, NR2A, and mGluR1. Thus, changes in synaptic strength are associated with concomitant changes in the expression and distribution of NCAM180 and glutamate receptors, particularly of the NR2A subtype.

Developmental up-regulation of KCC2 in the absence of GABAergic and glutamatergic transmission

Postsynaptic gamma-aminobutyric acid (GABA)A-mediated responses switch from depolarizing to hyperpolarizing during postnatal development of the rodent hippocampus. This is attributable to a decrease in the concentration of intracellular chloride set by the expression of the neuron-specific K+-Cl- co-transporter, KCC2. A recent in vitro study [Ganguly et al. (2001) Cell, 105, 521-532] showed that KCC2 expression may be under the trophic control of GABAA receptor-mediated transmission. Here we have studied the developmental expression of KCC2 protein in mouse hippocampal dissociated cultures as well as organotypic cultures. A low somatic expression level was found in neurons prior to the formation of the first synapses, as detected by synaptophysin immunoreactivity. Thereafter, KCC2 expression was strongly up-regulated during neuronal maturation. The developmental up-regulation of KCC2 expression was not altered by a chronic application (throughout the culturing period; 2-15 days in vitro) of the action-potential blocker TTX or the N-methyl-d-aspartate (NMDA) and non-NMDA antagonists APV and NBQX. Blockade of GABAA-mediated transmission with picrotoxin did not affect the expression levels of KCC2 protein either. These data show that neither neuronal spiking nor ionotropic glutamatergic and GABAergic transmission are required for the developmental expression of KCC2 in mouse hippocampal neurons in vitro.

Phosphatidylinositol 3-kinase interacts with the adaptor protein Dab1 in response to Reelin signaling and is required for normal cortical lamination

Reelin is a large secreted signaling protein that binds to two members of the low density lipoprotein receptor family, the apolipoprotein E receptor 2 and the very low density lipoprotein receptor, and regulates neuronal positioning during brain development. Reelin signaling requires activation of Src family kinases as well as tyrosine phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1). This results in activation of phosphatidylinositol 3-kinase (PI3K), the serine/threonine kinase Akt, and the inhibition of glycogen synthase kinase 3beta, a protein that is implicated in the regulation of axonal transport. Here we demonstrate that PI3K activation by Reelin requires Src family kinase activity and depends on the Reelin-triggered interaction of Dab1 with the PI3K regulatory subunit p85alpha. Because the Dab1 phosphotyrosine binding domain can interact simultaneously with membrane lipids and with the intracellular domains of apolipoprotein E receptor 2 and very low density lipoprotein receptor, Dab1 is preferentially recruited to the neuronal plasma membrane, where it is phosphorylated. Efficient Dab1 phosphorylation and activation of the Reelin signaling cascade is impaired by cholesterol depletion of the plasma membrane. Using a neuronal migration assay, we also show that PI3K signaling is required for the formation of a normal cortical plate, a step that is dependent upon Reelin signaling.