AKAP150-anchored calcineurin regulates synaptic plasticity by limiting synaptic incorporation of Ca2+-permeable AMPA receptors.

AMPA receptors (AMPARs) are tetrameric ion channels assembled from GluA1-GluA4 subunits that mediate the majority of fast excitatory synaptic transmission in the brain. In the hippocampus, most synaptic AMPARs are composed of GluA1/2 or GluA2/3 with the GluA2 subunit preventing Ca(2+) influx. However, a small number of Ca(2+)-permeable GluA1 homomeric receptors reside in extrasynaptic locations where they can be rapidly recruited to synapses during synaptic plasticity. Phosphorylation of GluA1 S845 by the cAMP-dependent protein kinase (PKA) primes extrasynaptic receptors for synaptic insertion in response to NMDA receptor Ca(2+) signaling during long-term potentiation (LTP), while phosphatases dephosphorylate S845 and remove synaptic and extrasynaptic GluA1 during long-term depression (LTD). PKA and the Ca(2+)-activated phosphatase calcineurin (CaN) are targeted to GluA1 through binding to A-kinase anchoring protein 150 (AKAP150) in a complex with PSD-95, but we do not understand how the opposing activities of these enzymes are balanced to control plasticity. Here, we generated AKAP150ΔPIX knock-in mice to selectively disrupt CaN anchoring in vivo. We found that AKAP150ΔPIX mice lack LTD but express enhanced LTP at CA1 synapses. Accordingly, basal GluA1 S845 phosphorylation is elevated in AKAP150ΔPIX hippocampus, and LTD-induced dephosphorylation and removal of GluA1, AKAP150, and PSD-95 from synapses are impaired. In addition, basal synaptic activity of GluA2-lacking AMPARs is increased in AKAP150ΔPIX mice and pharmacologic antagonism of these receptors restores normal LTD and inhibits the enhanced LTP. Thus, AKAP150-anchored CaN opposes PKA phosphorylation of GluA1 to restrict synaptic incorporation of Ca(2+)-permeable AMPARs both basally and during LTP and LTD.

Embryonic cortical neural stem cells migrate ventrally and persist as postnatal striatal stem cells.

Embryonic cortical neural stem cells apparently have a transient existence, as they do not persist in the adult cortex. We sought to determine the fate of embryonic cortical stem cells by following Emx1(IREScre); LacZ/EGFP double-transgenic murine cells from midgestation into adulthood. Lineage tracing in combination with direct cell labeling and time-lapse video microscopy demonstrated that Emx1-lineage embryonic cortical stem cells migrate ventrally into the striatal germinal zone (GZ) perinatally and intermingle with striatal stem cells. Upon integration into the striatal GZ, cortical stem cells down-regulate Emx1 and up-regulate Dlx2, which is a homeobox gene characteristic of the developing striatum and striatal neural stem cells. This demonstrates the existence of a novel dorsal-to-ventral migration of neural stem cells in the perinatal forebrain.

Ephrin signalling controls brain size by regulating apoptosis of neural progenitors.

Mechanisms controlling brain size include the regulation of neural progenitor cell proliferation, differentiation, survival and migration. Here we show that ephrin-A/EphA receptor signalling plays a key role in controlling the size of the mouse cerebral cortex by regulating cortical progenitor cell apoptosis. In vivo gain of EphA receptor function, achieved through ectopic expression of ephrin-A5 in early cortical progenitors expressing EphA7, caused a transient wave of neural progenitor cell apoptosis, resulting in premature depletion of progenitors and a subsequent dramatic decrease in cortical size. In vitro treatment with soluble ephrin-A ligands similarly induced the rapid death of cultured dissociated cortical progenitors in a caspase-3-dependent manner, thereby confirming a direct effect of ephrin/Eph signalling on apoptotic cascades. Conversely, in vivo loss of EphA function, achieved through EphA7 gene disruption, caused a reduction in apoptosis occurring normally in forebrain neural progenitors, resulting in an increase in cortical size and, in extreme cases, exencephalic forebrain overgrowth. Together, these results identify ephrin/Eph signalling as a physiological trigger for apoptosis that can alter brain size and shape by regulating the number of neural progenitors.

ß-Amyloid disruption of LTP/LTD balance is mediated by AKAP150-anchored PKA and Calcineurin regulation of Ca2+-permeable AMPA receptors.

Regulated insertion and removal of postsynaptic AMPA glutamate receptors (AMPARs) mediates hippocampal long-term potentiation (LTP) and long-term depression (LTD) synaptic plasticity underlying learning and memory. In Alzheimer's disease ß-amyloid (Aß) oligomers may impair learning and memory by altering AMPAR trafficking and LTP/LTD balance. Importantly, Ca2+-permeable AMPARs (CP-AMPARs) assembled from GluA1 subunits are excluded from hippocampal synapses basally but can be recruited rapidly during LTP and LTD to modify synaptic strength and signaling. By employing mouse knockin mutations that disrupt anchoring of the kinase PKA or phosphatase Calcineurin (CaN) to the postsynaptic scaffold protein AKAP150, we find that local AKAP-PKA signaling is required for CP-AMPAR recruitment, which can facilitate LTP but also, paradoxically, prime synapses for Aß impairment of LTP mediated by local AKAP-CaN LTD signaling that promotes subsequent CP-AMPAR removal. These findings highlight the importance of PKA/CaN signaling balance and CP-AMPARs in normal plasticity and aberrant plasticity linked to disease.

FAK deficiency in cells contributing to the basal lamina results in cortical abnormalities resembling congenital muscular dystrophies.

Targeted deletion of focal adhesion kinase (fak) in the developing dorsal forebrain resulted in local disruptions of the cortical basement membrane located between the neuroepithelium and pia-meninges. At disruption sites, clusters of neurons invaded the marginal zone. Retraction of radial glial endfeet, midline fusion of brain hemispheres, and gliosis also occurred, similar to type II cobblestone lissencephaly as seen in congenital muscular dystrophy. Interestingly, targeted deletion of fak in neurons alone did not result in cortical ectopias, indicating that fak deletion from glia is required for neuronal mislocalization. Unexpectedly, fak deletion specifically from meningeal fibroblasts elicited similar cortical ectopias in vivo and altered laminin organization in vitro. These observations provide compelling evidence that FAK plays a key signaling role in cortical basement membrane assembly and/or remodeling. In addition, FAK is required within neurons during development because neuron-specific fak deletion alters dendritic morphology in the absence of lamination defects.

Inactivation of Numb and Numblike in embryonic dorsal forebrain impairs neurogenesis and disrupts cortical morphogenesis.

Numb and Numblike, conserved homologs of Drosophila Numb, have been implicated in cortical neurogenesis; however, analysis of their involvement in later stages of cortical development has been hampered by early lethality of double mutants in previous studies. Using Emx1(IREScre) to induce more restricted inactivation of Numb in the dorsal forebrain of numblike null mice beginning at E9.5, we have generated viable double mutants that displayed striking brain defects. It was thus possible to examine neurogenesis during the later peak phase (E12.5-E16.5). Loss of Numb and Numblike in dorsal forebrain resulted in neural progenitor hyperproliferation, delayed cell cycle exit, impaired neuronal differentiation, and concomitant defects in cortical morphogenesis. These findings reveal novel and essential function of Numb and Numblike during the peak period of cortical neurogenesis. Further, these double mutant mice provide an unprecedented viable animal model for severe brain malformations due to defects in neural progenitor cells.

Association of an A-kinase-anchoring protein signaling scaffold with cadherin adhesion molecules in neurons and epithelial cells.

A-kinase-anchoring protein (AKAP) 79/150 organizes a scaffold of cAMP-dependent protein kinase (PKA), protein kinase C (PKC), and protein phosphatase 2B/calcineurin that regulates phosphorylation pathways underlying neuronal long-term potentiation and long-term depression (LTD) synaptic plasticity. AKAP79/150 postsynaptic targeting requires three N-terminal basic domains that bind F-actin and acidic phospholipids. Here, we report a novel interaction of these domains with cadherin adhesion molecules that are linked to actin through beta-catenin (beta-cat) at neuronal synapses and epithelial adherens junctions. Mapping the AKAP binding site in cadherins identified overlap with beta-cat binding; however, no competition between AKAP and beta-cat binding to cadherins was detected in vitro. Accordingly, AKAP79/150 exhibited polarized localization with beta-cat and cadherins in epithelial cell lateral membranes, and beta-cat was present in AKAP-cadherin complexes isolated from epithelial cells, cultured neurons, and rat brain synaptic membranes. Inhibition of epithelial cell cadherin adhesion and actin polymerization redistributed intact AKAP-cadherin complexes from lateral membranes to intracellular compartments. In contrast, stimulation of neuronal pathways implicated in LTD that depolymerize postsynaptic F-actin disrupted AKAP-cadherin interactions and resulted in loss of the AKAP, but not cadherins, from synapses. This neuronal regulation of AKAP79/150 targeting to cadherins may be important in functional and structural synaptic modifications underlying plasticity.

Netrin requires focal adhesion kinase and Src family kinases for axon outgrowth and attraction.

Although netrins are an important family of neuronal guidance proteins, intracellular mechanisms that mediate netrin function are not well understood. Here we show that netrin-1 induces tyrosine phosphorylation of proteins including focal adhesion kinase (FAK) and the Src family kinase Fyn. Blockers of Src family kinases inhibited FAK phosphorylation and axon outgrowth and attraction by netrin. Dominant-negative FAK and Fyn mutants inhibited the attractive turning response to netrin. Axon outgrowth and attraction induced by netrin-1 were significantly reduced in neurons lacking the FAK gene. Our results show the biochemical and functional links between netrin, a prototypical neuronal guidance cue, and FAK, a central player in intracellular signaling that is crucial for cell migration.

Regulation of neuronal PKA signaling through AKAP targeting dynamics.

Central to organization of signaling pathways are scaffolding, anchoring and adaptor proteins that mediate localized assembly of multi-protein complexes containing receptors, second messenger-generating enzymes, kinases, phosphatases, and substrates. At the postsynaptic density (PSD) of excitatory synapses, AMPA (AMPAR) and NMDA (NMDAR) glutamate receptors are linked to signaling proteins, the actin cytoskeleton, and synaptic adhesion molecules on dendritic spines through a network of scaffolding proteins that may play important roles regulating synaptic structure and receptor functions in synaptic plasticity underlying learning and memory. AMPARs are rapidly recruited to dendritic spines through NMDAR activation during induction of long-term potentiation (LTP) through pathways that also increase the size and F-actin content of spines. Phosphorylation of AMPAR-GluR1 subunits by the cAMP-dependent protein kinase (PKA) helps stabilize AMPARs recruited during LTP. In contrast, induction of long-term depression (LTD) leads to rapid calcineurin-protein phosphatase 2B (CaN) mediated dephosphorylation of PKA-phosphorylated GluR1 receptors, endocytic removal of AMPAR from synapses, and a reduction in spine size. However, mechanisms for coordinately regulating AMPAR localization, phosphorylation, and synaptic structure by PKA and CaN are not well understood. A kinase-anchoring protein (AKAP) 79/150 is a PKA- and CaN-anchoring protein that is linked to NMDARs and AMPARs through PSD-95 and SAP97 membrane-associated guanylate kinase (MAGUK) scaffolds. Importantly, disruption of PKA-anchoring in neurons and functional analysis of GluR1-MAGUK-AKAP79 complexes in heterologous cells suggests that AKAP79/150-anchored PKA and CaN may regulate AMPARs in LTD. In the work presented at the "First International Meeting on Anchored cAMP Signaling Pathways" (Berlin-Buch, Germany, October 15-16, 2005), we demonstrate that AKAP79/150 is targeted to dendritic spines by an N-terminal basic region that binds phosphatidylinositol-4,5-bisphosphate (PIP(2)), F-actin, and actin-linked cadherin adhesion molecules. Thus, anchoring of PKA and CaN as well as physical linkage of the AKAP to both cadherin-cytoskeletal and MAGUK-receptor complexes could play roles in coordinating changes in synaptic structure and receptor signaling functions underlying plasticity. Importantly, we provide evidence showing that NMDAR-CaN signaling pathways implicated in AMPAR regulation during LTD lead to a disruption of AKAP79/150 interactions with actin, MAGUKs, and cadherins and lead to a loss of the AKAP and anchored PKA from postsynapses. Our studies thus far indicate that this AKAP79/150 translocation depends on activation of CaN, F-actin reorganization, and possibly Ca(2+)-CaM binding to the N-terminal basic regions. Importantly, this tranlocation of the AKAP79/150-PKA complex from spines may shift the balance of PKA kinase and CaN/PP1 phosphatase activity at the postsynapse in favor of the phosphatases. This loss of PKA could then promote actions of CaN and PP1 during induction of LTD including maintaining AMPAR dephosphorylation, promoting AMPAR endocytosis, and preventing AMPAR recycling. Overall, these findings challenge the accepted notion that AKAPs are static anchors that position signaling proteins near fixed target substrates and instead suggest that AKAPs can function in more dynamic manners to regulate local signaling events.

NMDA Receptor-Dependent LTD Requires Transient Synaptic Incorporation of Ca²âº-Permeable AMPARs Mediated by AKAP150-Anchored PKA and Calcineurin.

Information processing in the brain requires multiple forms of synaptic plasticity that converge on regulation of NMDA and AMPA-type glutamate receptors (NMDAR, AMPAR), including long-term potentiation (LTP) and long-term depression (LTD) and homeostatic scaling. In some cases, LTP and homeostatic plasticity regulate synaptic AMPAR subunit composition to increase the contribution of Ca(2+)-permeable receptors (CP-AMPARs) containing GluA1 but lacking GluA2 subunits. Here, we show that PKA anchored to the scaffold protein AKAP150 regulates GluA1 phosphorylation and plays a novel role controlling CP-AMPAR synaptic incorporation during NMDAR-dependent LTD. Using knockin mice that are deficient in AKAP-anchoring of either PKA or the opposing phosphatase calcineurin, we found that CP-AMPARs are recruited to hippocampal synapses by anchored PKA during LTD induction but are then rapidly removed by anchored calcineurin. Importantly, blocking CP-AMPAR recruitment, removal, or activity interferes with LTD. Thus, CP-AMPAR synaptic recruitment is required to transiently augment NMDAR Ca(2+) signaling during LTD induction.

Early striatal dendrite deficits followed by neuron loss with advanced age in the absence of anterograde cortical brain-derived neurotrophic factor.

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, modulates neuronal survival, differentiation, and synaptic function. Reduced BDNF expression in the cortex caused by mutation of the huntingtin gene has been suggested to play a role in the striatal degeneration observed in Huntington's disease. BDNF expression rises dramatically in the cortex during the first few weeks of postnatal life in mice. Previously, it has been impossible to study the specific long-term effects of BDNF absence on CNS structures because of the early postnatal lethality of BDNF-/- mice. Mice harboring a floxed BDNF gene were bred with Emx1(IREScre/+) mice to generate Emx-BDNF(KO) mice that lack cortical BDNF but are viable. Adult Emx-BDNF(KO) mice display a hindlimb clasping phenotype similar to that observed in mouse models of Huntington's disease. The striatum of postnatal Emx-BDNF(KO) mice was reduced in volume compared with controls, and the most abundant neuron type of the striatum, medium spiny neurons (MSNs), had shrunken cell somas, thinner dendrites, and fewer dendritic spines at 35 d of age. Although significant striatal neuron losses were not detected at 35 or 120 d postnatal, 35% of striatal neurons were missing in Emx-BDNF(KO) mice aged beyond 1 year. Thus, cortical BDNF, although not required for the generation or near-term survival of MSN, is necessary for normal striatal neuron dendrite morphology during the period when BDNF expression rises in the cortex. Furthermore, a long-term in vivo requirement for cortical BDNF in supporting the survival of MSNs is revealed.

Brain-derived neurotrophic factor is required for the maintenance of cortical dendrites.

Brain-derived neurotrophic factor (BDNF) is thought to be involved in neuronal survival, migration, morphological and biochemical differentiation, and modulation of synaptic function in the CNS. In the rodent cortex, postnatal BDNF expression is initially low but subsequently increases to reach maximal levels around weaning. Thus, BDNF expression peaks at a time when both structural and functional maturation of cortical circuitry occurs. Although the function of BDNF has been probed using many approaches, its requirements during this phase of life have not previously been examined genetically. To test the in vivo requirements for BDNF during this important phase of development we generated early-onset forebrain-specific BDNF mutant mice. Although these mice undergo forebrain-restricted deletion of BDNF by Cre-mediated recombination during embryogenesis, they are healthy, and we did not detect the loss of specific cortical excitatory or inhibitory neurons. However, the neocortex of 5-week-old mice was thinner, attributable at least partly to neuronal shrinkage. Importantly, although visual cortical layer 2/3 neurons in the mutants initially developed normal dendrite structure, dendritic retraction became apparent by 3 weeks of age. Thus, our observations suggest that cortically expressed BDNF functions to support the maintenance of cortical neuron size and dendrite structure rather than the initial development of these features. This is consistent with a role for BDNF in stabilizing the "survival" of circuitry during the phase of activity-dependent reorganization of cortical connectivity.

Cortical excitatory neurons and glia, but not GABAergic neurons, are produced in the Emx1-expressing lineage.

By homologous recombination of an internal ribosome entry site and Cre recombinase coding region into the 3'-untranslated region of the mouse Emx1 gene, we have generated a strain of mice, Emx1(IRES)cre, that expresses the Cre recombinase in a spatial and temporal pattern like that observed for Emx1. When mated to reporter strains, these mice are a sensitive means to fate-map the Emx1-expressing cells of the developing forebrain. Our results demonstrate that radial glia, Cajal-Retzius cells, glutamatergic neurons, astrocytes, and oligodendrocytes of most pallial structures originate from an Emx1-expressing lineage. On the other hand, most of the pallial GABAergic neurons arise outside the Emx1-expressing lineage. Structures that are located near the basal ganglia (e.g., the amygdala and endopiriform nuclei) are not uniformly derived from Emx1-expressing cells.

AKAP-anchored PKA maintains neuronal L-type calcium channel activity and NFAT transcriptional signaling.

L-type voltage-gated Ca2+ channels (LTCC) couple neuronal excitation to gene transcription. LTCC activity is elevated by the cyclic AMP (cAMP)-dependent protein kinase (PKA) and depressed by the Ca2+-dependent phosphatase calcineurin (CaN), and both enzymes are localized to the channel by A-kinase anchoring protein 79/150 (AKAP79/150). AKAP79/150 anchoring of CaN also promotes LTCC activation of transcription through dephosphorylation of the nuclear factor of activated T cells (NFAT). We report here that the basal activity of AKAP79/150-anchored PKA maintains neuronal LTCC coupling to CaN-NFAT signaling by preserving LTCC phosphorylation in opposition to anchored CaN. Genetic disruption of AKAP-PKA anchoring promoted redistribution of the kinase out of postsynaptic dendritic spines, profound decreases in LTCC phosphorylation and Ca2+ influx, and impaired NFAT movement to the nucleus and activation of transcription. Thus, LTCC-NFAT transcriptional signaling in neurons requires precise organization and balancing of PKA and CaN activities in the channel nanoenvironment, which is only made possible by AKAP79/150 scaffolding.

Genetically encoded sensors to elucidate spatial distribution of cellular zinc.

Transition metals are essential enzyme cofactors that are required for a wide range of cellular processes. Paradoxically, whereas metal ions are essential for numerous cellular processes, they are also toxic. Therefore cells must tightly regulate metal accumulation, transport, distribution, and export. Improved tools to interrogate metal ion availability and spatial distribution within living cells would greatly advance our understanding of cellular metal homeostasis. In this work, we present genetically encoded sensors for Zn2+ based on the principle of fluorescence resonance energy transfer. We also develop methodology to calibrate the probes within the cellular environment. To identify both sources of and sinks for Zn2+, these sensors are genetically targeted to specific locations within the cell, including cytosol, plasma membrane, and mitochondria. Localized probes reveal that mitochondria contain an elevated pool of Zn2+ under resting conditions that can be released into the cytosol upon glutamate stimulation of hippocampal neurons. We also observed that Zn2+ is taken up into mitochondria following glutamate/Zn2+ treatment and that there is heterogeneity in both the magnitude and kinetics of the response. Our results suggest that mitochondria serve as a source of and a sink for Zn2+ signals under different cellular conditions.

Conditional activation of Pax6 in the developing cortex of transgenic mice causes progenitor apoptosis.

During development, Pax6 is expressed in a rostrolateral-high to caudomedial-low gradient in the majority of the cortical radial glial progenitors and endows them with neurogenic properties. Using a Cre/loxP-based approach, we studied the effect of conditional activation of two Pax6 isoforms, Pax6 and Pax6-5a, on the corticogenesis of transgenic mice. We found that activation of either Pax6 or Pax6-5a inhibits progenitor proliferation in the developing cortex. Upon activation of transgenic Pax6, specific progenitor pools with distinct endogenous Pax6 expression levels at different developmental stages show defects in cell cycle progression and in the acquisition of apoptotic or neuronal cell fate. The results provide new evidence for the complex role of Pax6 in mammalian corticogenesis.