The AMPA receptor subunit GluR1 regulates dendritic architecture of motor neurons

Properties of layer V pyramidal neurons in the primary motor cortex that represent acquired motor skills

Layer V neurons in primary motor cortex (M1) are required for motor skill learning. We analyzed training-induced plasticity using a whole-cell slice patch-clamp technique with a rotor rod task, and found that training induces diverse changes in intrinsic properties and synaptic plasticity in M1 layer V neurons. Although the causal relationship between specific cellular changes and motor performance is unclear, by linking individual motor performance to cellular/synaptic functions, we identified several cellular and synaptic parameters that represent acquired motor skills. With respect to cellular properties, motor performance was positively correlated with resting membrane potential and fast afterhyperpolarization, but not with the membrane resistance, capacitance, or threshold. With respect to synaptic function, the performance was positively correlated with AMPA receptor-mediated postsynaptic currents, but not with GABAA receptor-mediated postsynaptic currents. With respect to live imaging analysis in Thy1-YFP mice, we further demonstrated a cross-correlation between motor performance, spine head volume, and self-entropy per spine. In the present study, we identified several changes in M1 layer V pyramidal neurons after motor training that represent acquired motor skills. Furthermore, training increased extracellular acetylcholine levels known to promote synaptic plasticity, which is correlated with individual motor performance. These results suggest that systematic control of specific intracellular parameters and enhancement of synaptic plasticity in M1 layer V neurons may be useful for improving motor skills.

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

The long noncoding RNA nHOTAIRM1 is necessary for differentiation and activity of iPSC-derived spinal motor neurons

The mammalian nervous system is made up of an extraordinary array of diverse cells that form intricate functional connections. The programs underlying cell lineage specification, identity and function of the neuronal subtypes are managed by regulatory proteins and RNAs, which coordinate the succession of steps in a stereotyped temporal order. In the central nervous system (CNS), motor neurons (MNs) are responsible for controlling essential functions such as movement, breathing, and swallowing by integrating signal transmission from the cortex, brainstem, and spinal cord (SC) towards peripheral muscles. A prime role in guiding the progression of progenitor cells towards the MN fate has been largely attributed to protein factors. More recently, the relevance of a class of regulatory RNAs abundantly expressed in the CNS - the long noncoding RNAs (lncRNAs) - has emerged overwhelmingly. LncRNA-driven gene expression control is key to regulating any step of MN differentiation and function, and its derangement profoundly impacts neuronal pathophysiology. Here, we uncover a novel function for the neuronal isoform of HOTAIRM1 (nHOTAIRM1), a lncRNA specifically expressed in the SC. Using a model system that recapitulates spinal MN (spMN) differentiation, we show that nHOTAIRM1 intervenes in the binary cell fate decision between MNs and interneurons, acting as a pro-MN factor. Furthermore, human iPSC-derived spMNs without nHOTAIRM1 display altered neurite outgrowth, with a significant reduction of both branch and junction numbers. Finally, the expression of genes essential for synaptic connectivity and neurotransmission is also profoundly impaired when nHOTAIRM1 is absent in spMNs. Mechanistically, nHOTAIRM1 establishes both direct and indirect interactions with a number of target genes in the cytoplasm, being a novel post-transcriptional regulator of MN biology. Overall, our results indicate that the lncRNA nHOTAIRM1 is essential for the specification of MN identity and the acquisition of proper morphology and synaptic activity of post-mitotic MNs.

Motor training promotes both synaptic and intrinsic plasticity of layer V pyramidal neurons in the primary motor cortex

Layer V neurons in the primary motor cortex (M1) are important for motor skill learning. Since pretreatment of either CNQX or APV in rat M1 layer V impaired rotor rod learning, we analysed training-induced synaptic plasticity by whole-cell patch-clamp technique in acute brain slices. Rats trained for 1 day showed a decrease in small inhibitory postsynaptic current (mIPSC) frequency and an increase in the paired-pulse ratio of evoked IPSCs, suggesting a transient decrease in presynaptic GABA release in the early phase. Rats trained for 2 days showed an increase in miniature excitatory postsynaptic current (mEPSC) amplitudes/frequency and elevated AMPA/NMDA ratios, suggesting a long-term strengthening of AMPA receptor-mediated excitatory synapses. Importantly, rotor rod performance in trained rats was correlated with the mean mEPSC amplitude and the frequency obtained from that animal. In current-clamp analysis, 1-day-trained rats transiently decreased the current-induced firing rate, while 2-day-trained rats returned to pre-training levels, suggesting dynamic changes in intrinsic properties. Furthermore, western blot analysis of layer V detected decreased phosphorylation of Ser^408-409 in GABA_A receptor β₃ subunits in 1-day-trained rats, and increased phosphorylation of Ser⁸³¹ in AMPA receptor GluA1 subunits in 2-day-trained rats. Finally, live-imaging analysis of Thy1-YFP transgenic mice showed that the training rapidly recruited a substantial number of spines for long-term plasticity in M1 layer V neurons. Taken together, these results indicate that motor training induces complex and diverse plasticity in M1 layer V pyramidal neurons. KEY POINTS: Here we examined motor training-induced synaptic and intrinsic plasticity of layer V pyramidal neurons in the primary motor cortex. The training reduced presynaptic GABA release in the early phase, but strengthened AMPA receptor-mediated excitatory synapses in the later phase: acquired motor performance after training correlated with the strength of excitatory synapses rather than inhibitory synapses. As to the intrinsic property, the training transiently decreased the firing rate in the early phase, but returned to pre-training levels in the later phase. Western blot analysis detected decreased phosphorylation of Ser^408-409 in GABA_A receptor β₃ subunits in the acute phase, and increased phosphorylation of Ser⁸³¹ in AMPA receptor GluA1 subunits in the later phase. Live-imaging analysis of Thy1-YFP transgenic mice showed rapid and long-term spine plasticity in M1 layer V neurons, suggesting training-induced increases in self-entropy per spine.

Synaptic dysfunction in ALS and FTD: anatomical and molecular changes provide insights into mechanisms of disease

Synaptic loss is a pathological feature of all neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). ALS is a disease of the cortical and spinal motor neurons resulting in fatal paralysis due to denervation of muscles. FTD is a form of dementia that primarily affects brain regions controlling cognition, language and behavior. Once classified as two distinct diseases, ALS and FTD are now considered as part of a common disease spectrum based on overlapping clinical, pathological and genetic evidence. At the cellular level, aggregation of common proteins and overlapping gene susceptibilities are shared in both ALS and FTD. Despite the convergence of these two fields of research, the underlying disease mechanisms remain elusive. However, recent discovers from ALS and FTD patient studies and models of ALS/FTD strongly suggests that synaptic dysfunction is an early event in the disease process and a unifying hallmark of these diseases. This review provides a summary of the reported anatomical and cellular changes that occur in cortical and spinal motor neurons in ALS and FTD tissues and models of disease. We also highlight studies that identify changes in the proteome and transcriptome of ALS and FTD models and provide a conceptual overview of the processes that contribute to synaptic dysfunction in these diseases. Due to space limitations and the vast number of publications in the ALS and FTD fields, many articles have not been discussed in this review. As such, this review focuses on the three most common shared mutations in ALS and FTD, the hexanucleuotide repeat expansion within intron 1 of chromosome 9 open reading frame 72 (C9ORF72), transactive response DNA binding protein 43 (TARDBP or TDP-43) and fused in sarcoma (FUS), with the intention of highlighting common pathways that promote synaptic dysfunction in the ALS-FTD disease spectrum.

Molecular Identification of Pro-Excitogenic Receptor and Channel Phenotypes of the Deafferented Lumbar Motoneurons in the Early Phase after SCT in Rats

Spasticity impacts the quality of life of patients suffering spinal cord injury and impedes the recovery of locomotion. At the cellular level, spasticity is considered to be primarily caused by the hyperexcitability of spinal α-motoneurons (MNs) within the spinal stretch reflex circuit. Here, we hypothesized that after a complete spinal cord transection in rats, fast adaptive molecular responses of lumbar MNs develop in return for the loss of inputs. We assumed that early loss of glutamatergic afferents changes the expression of glutamatergic AMPA and NMDA receptor subunits, which may be the forerunners of the developing spasticity of hindlimb muscles. To better understand its molecular underpinnings, concomitant expression of GABA and Glycinergic receptors and serotoninergic and noradrenergic receptors, which regulate the persistent inward currents crucial for sustained discharges in MNs, were examined together with voltage-gated ion channels and cation-chloride cotransporters. Using quantitative real-time PCR, we showed in the tracer-identified MNs innervating extensor and flexor muscles of the ankle joint multiple increases in transcripts coding for AMPAR and 5-HTR subunits, along with a profound decrease in GABAAR, GlyR subunits, and KCC2. Our study demonstrated that both MNs groups similarly adapt to a more excitable state, which may increase the occurrence of extensor and flexor muscle spasms.

Reelin restricts dendritic growth of interneurons in the neocortex

Reelin is a large secreted glycoprotein that regulates neuronal migration, lamination and establishment of dendritic architecture in the embryonic brain. Reelin expression switches postnatally from Cajal-Retzius cells to interneurons. However, reelin function in interneuron development is still poorly understood. Here, we have investigated the role of reelin in interneuron development in the postnatal neocortex. To preclude early cortical migration defects caused by reelin deficiency, we employed a conditional reelin knockout (RelncKO) mouse to induce postnatal reelin deficiency. Induced reelin deficiency caused dendritic hypertrophy in distal dendritic segments of neuropeptide Y-positive (NPY+) and calretinin-positive (Calr+) interneurons, and in proximal dendritic segments of parvalbumin-positive (Parv+) interneurons. Chronic recombinant Reelin treatment rescued dendritic hypertrophy in Relncko interneurons. Moreover, we provide evidence that RelncKO interneuron hypertrophy is due to presynaptic GABABR dysfunction. Thus, GABABRs in RelncKO interneurons were unable to block N-type (Cav2.2) Ca2+ channels that control neurotransmitter release. Consequently, the excessive Ca2+ influx through AMPA receptors, but not NMDA receptors, caused interneuron dendritic hypertrophy. These findings suggest that reelin acts as a 'stop-growth-signal' for postnatal interneuron maturation.

Gjd2b-mediated gap junctions promote glutamatergic synapse formation and dendritic elaboration in Purkinje neurons

Gap junctions between neurons serve as electrical synapses, in addition to conducting metabolites and signaling molecules. During development, early-appearing gap junctions are thought to prefigure chemical synapses, which appear much later. We present evidence for this idea at a central, glutamatergic synapse and provide some mechanistic insights. Loss or reduction in the levels of the gap junction protein Gjd2b decreased the frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in cerebellar Purkinje neurons (PNs) in larval zebrafish. Ultrastructural analysis in the molecular layer showed decreased synapse density. Further, mEPSCs had faster kinetics and larger amplitudes in mutant PNs, consistent with their stunted dendritic arbors. Time-lapse microscopy in wild-type and mutant PNs reveals that Gjd2b puncta promote the elongation of branches and that CaMKII may be a critical mediator of this process. These results demonstrate that Gjd2b-mediated gap junctions regulate glutamatergic synapse formation and dendritic elaboration in PNs.

Size-Dependent Vulnerability of Lumbar Motor Neuron Dendritic Degeneration in SOD1G93A Mice

The motor neuron (MN) soma surface area is correlated with motor unit type. Larger MNs innervate fast fatigue-intermediate (FInt) or fast-fatiguable (FF) muscle fibers in type FInt and FF motor units, respectively. Smaller MNs innervate slow-twitch fatigue-resistant (S) or fast fatigue-resistant (FR) muscle fibers in type S and FR motor units, respectively. In amyotrophic lateral sclerosis (ALS), FInt and FF motor units are more vulnerable, with denervation and MN death occurring for these units before the more resilient S and FR units. Abnormal MN dendritic arbors have been observed in ALS in humans and rodent models. We used a Golgi-Cox impregnation protocol to examine soma size-dependent changes in the dendritic morphology of lumbar MNs in SOD1^G93A mice, a model of ALS, at pre-symptomatic, onset and mid-disease stages. In wildtype control mice, the relationship between MN soma surface area and dendritic length or dendritic spine number was highly linear (i.e., increased MN soma size correlated with increased dendritic length and spines). By contrast, in SOD1^G93A mice, this linear relationship was lost and dendritic length reduction and spine loss were observed in larger MNs, from pre-symptomatic stages onward. These changes correlated with the neuromotor symptoms of ALS in rodent models. At presymptomatic ages, changes were restricted to the larger MNs, likely to comprise vulnerable FInt and FF motor units. Our results suggest morphological changes of MN dendrites and dendritic spines are likely to contribute ALS pathogenesis, not compensate for it. Anat Rec, 303:1455-1471, 2020. © 2019 American Association for Anatomy.

A Method to Investigate Astrocyte and Microglial Morphological Changes in the Aging Brain of the Rhesus Macaque

With a rapidly aging population, studies of neuroinflammation and degeneration associated with eugeric aging are becoming critical. Using the unique archive at the Tulane National Primate Research Center as a resource, we have developed tools to quantify morphological changes in astrocytes and microglia across the life span of monkeys. This method can be used for morphometric studies of multiple parameters simultaneously in an unbiased manner.

Cyclophilin D regulates neuronal activity-induced filopodiagenesis by fine-tuning dendritic mitochondrial calcium dynamics

Recent studies have highlighted the role of mitochondria in dendritic protrusion growth and plasticity. However, the detailed mechanisms that mitochondria regulate dendritic filopodia morphogenesis remain elusive. Cyclophilin D (CypD, gene name: Ppif) controls the opening of mitochondrial permeability transition pore. Although the pathological relevance of CypD has been intensively investigated, little is known about its physiological function in neurons. Here, we have found that genetic depletion of or pharmaceutical inhibition of CypD blunts the outgrowth of dendritic filopodia in response to KCl-stimulated neuronal depolarization. Further cell biological studies suggest that such inhibitory effect of CypD loss-of-function is closely associated with compromised flexibility of dendritic mitochondrial calcium regulation during neuronal depolarization, as well as the resultant changes in intradendritic calcium homeostasis, calcium signaling activation, dendritic mitochondrial motility and redistribution. Interestingly, loss of CypD attenuates oxidative stress-induced mitochondrial calcium perturbations and dendritic protrusion injury. Therefore, our study has revealed the physiological function of CypD in dendritic plasticity by acting as a fine-tuner of mitochondrial calcium homeostasis. Moreover, CypD plays distinct roles in neuronal physiology and pathology. Cover Image for this issue: doi: 10.1111/jnc.14189.

SPARC and GluA1-Containing AMPA Receptors Promote Neuronal Health Following CNS Injury

The proper formation and maintenance of functional synapses in the central nervous system (CNS) requires communication between neurons and astrocytes and the ability of astrocytes to release neuromodulatory molecules. Previously, we described a novel role for the astrocyte-secreted matricellular protein SPARC (Secreted Protein, Acidic and Rich in Cysteine) in regulating α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and plasticity at developing synapses. SPARC is highly expressed by astrocytes and microglia during CNS development but its level is reduced in adulthood. Interestingly, SPARC has been shown to be upregulated in CNS injury and disease. However, the role of SPARC upregulation in these contexts is not fully understood. In this study, we investigated the effect of chronic SPARC administration on glutamate receptors on mature hippocampal neuron cultures and following CNS injury. We found that SPARC treatment increased the number of GluA1-containing AMPARs at synapses and enhanced synaptic function. Furthermore, we determined that the increase in synaptic strength induced by SPARC could be inhibited by Philanthotoxin-433, a blocker of homomeric GluA1-containing AMPARs. We then investigated the effect of SPARC treatment on neuronal health in an injury context where SPARC expression is upregulated. We found that SPARC levels are increased in astrocytes and microglia following middle cerebral artery occlusion (MCAO) in vivo and oxygen-glucose deprivation (OGD) in vitro. Remarkably, chronic pre-treatment with SPARC prevented OGD-induced loss of synaptic GluA1. Furthermore, SPARC treatment reduced neuronal death through Philanthotoxin-433 sensitive GluA1 receptors. Taken together, this study suggests a novel role for SPARC and GluA1 in promoting neuronal health and recovery following CNS damage.

Alterations in hypoglossal motor neurons due to GAD67 and VGAT deficiency in mice

There is an emerging body of evidence that glycinergic and GABAergic synaptic inputs onto motor neurons (MNs) help regulate the final number of MNs and axonal muscle innervation patterns. Using mutant glutamate decarboxylase 67 (GAD67) and vesicular inhibitory amino acid transporter (VGAT) deficient mice, we describe the effect that deficiencies of presynaptic GABAergic and/or glycinergic release have on the post-synaptic somato-dendritic structure of motor neurons, and the development of excitatory and inhibitory synaptic inputs to MNs. We use whole-cell patch clamp recording of synaptic currents in E18.5 hypoglossal MNs from brainstem slices, combined with dye-filling of these recorded cells with Neurobiotin™, high-resolution confocal imaging and 3-dimensional reconstructions. Hypoglossal MNs from GAD67- and VGAT-deficient mice display decreased inhibitory neurotransmission and increased excitatory synaptic inputs. These changes are associated with increased dendritic arbor length, increased complexity of dendritic branching, and increased density of spiny processes. Our results show that presynaptic release of inhibitory amino acid neurotransmitters are potent regulators of hypoglossal MN morphology and key regulators of synaptic inputs during this critical developmental time point.

Impairments in dendrite morphogenesis as etiology for neurodevelopmental disorders and implications for therapeutic treatments

Dendrite morphology is pivotal for neural circuitry functioning. While the causative relationship between small-scale dendrite morphological abnormalities (shape, density of dendritic spines) and neurodevelopmental disorders is well established, such relationship remains elusive for larger-scale dendrite morphological impairments (size, shape, branching pattern of dendritic trees). Here, we summarize published data on dendrite morphological irregularities in human patients and animal models for neurodevelopmental disorders, with focus on autism and schizophrenia. We next discuss high-risk genes for these disorders and their role in dendrite morphogenesis. We finally overview recent developments in therapeutic attempts and we discuss how they relate to dendrite morphology. We find that both autism and schizophrenia are accompanied by dendritic arbor morphological irregularities, and that majority of their high-risk genes regulate dendrite morphogenesis. Thus, we present a compelling argument that, along with smaller-scale morphological impairments in dendrites (spines and synapse), irregularities in larger-scale dendrite morphology (arbor shape, size) may be an important part of neurodevelopmental disorders' etiology. We suggest that this should not be ignored when developing future therapeutic treatments.

Emerging Roles of Filopodia and Dendritic Spines in Motoneuron Plasticity during Development and Disease

Motoneurons develop extensive dendritic trees for receiving excitatory and inhibitory synaptic inputs to perform a variety of complex motor tasks. At birth, the somatodendritic domains of mouse hypoglossal and lumbar motoneurons have dense filopodia and spines. Consistent with Vaughn's synaptotropic hypothesis, we propose a developmental unified-hybrid model implicating filopodia in motoneuron spinogenesis/synaptogenesis and dendritic growth and branching critical for circuit formation and synaptic plasticity at embryonic/prenatal/neonatal period. Filopodia density decreases and spine density initially increases until postnatal day 15 (P15) and then decreases by P30. Spine distribution shifts towards the distal dendrites, and spines become shorter (stubby), coinciding with decreases in frequency and increases in amplitude of excitatory postsynaptic currents with maturation. In transgenic mice, either overexpressing the mutated human Cu/Zn-superoxide dismutase (hSOD1^G93A) gene or deficient in GABAergic/glycinergic synaptic transmission (gephyrin, GAD-67, or VGAT gene knockout), hypoglossal motoneurons develop excitatory glutamatergic synaptic hyperactivity. Functional synaptic hyperactivity is associated with increased dendritic growth, branching, and increased spine and filopodia density, involving actin-based cytoskeletal and structural remodelling. Energy-dependent ionic pumps that maintain intracellular sodium/calcium homeostasis are chronically challenged by activity and selectively overwhelmed by hyperactivity which eventually causes sustained membrane depolarization leading to excitotoxicity, activating microglia to phagocytose degenerating neurons under neuropathological conditions.

Structure-function analysis of SAP97, a modular scaffolding protein that drives dendrite growth

Activation of AMPA receptors assembled with the GluA1 subunit can promote dendrite growth in a manner that depends on its direct binding partner, SAP97. SAP97 is a modular scaffolding protein that has at least seven recognizable protein-protein interaction domains. Several complementary approaches were employed to show that the dendrite branching promoting action of full length SAP97 depends on ligand(s) that bind to the PDZ3 domain. Ligand(s) to PDZ1, PDZ2 and I3 domains also contribute to dendrite growth. The ability of PDZ3 ligand(s) to promote dendrite growth depends on localization at the plasma membrane along with GluA1 and SAP97. These results suggest that the assembly of a multi-protein complex at or near synapses is vital for the translation of AMPA-R activity into dendrite growth.

Neuromodulation in developing motor microcircuits

Neuromodulation confers operational flexibility on motor network output and resulting behaviour. Furthermore, neuromodulators play crucial long-term roles in the assembly and maturational shaping of the same networks as they develop. Although previous studies have identified such modulator-dependent contributions to microcircuit ontogeny, some of the underlying mechanisms are only now being elucidated. Deciphering the role of neuromodulatory systems in motor network development has potentially important implications for post-lesional regenerative strategies in adults.

Type I TARPs promote dendritic growth of early postnatal neocortical pyramidal cells in organotypic cultures

The ionotropic α-amino-3-hydroxy-5-methyl-4-isoxazole propionate glutamate receptors (AMPARs) have been implicated in the establishment of dendritic architecture. The transmembrane AMPA receptor regulatory proteins (TARPs) regulate AMPAR function and trafficking into synaptic membranes. In the current study, we employ type I and type II TARPs to modulate expression levels and function of endogenous AMPARs and investigate in organotypic cultures (OTCs) of rat occipital cortex whether this influences neuronal differentiation. Our results show that in early development [5-10 days in vitro (DIV)] only the type I TARP γ-8 promotes pyramidal cell dendritic growth by increasing spontaneous calcium amplitude and GluA2/3 expression in soma and dendrites. Later in development (10-15 DIV), the type I TARPs γ-2, γ-3 and γ-8 promote dendritic growth, whereas γ-4 reduced dendritic growth. The type II TARPs failed to alter dendritic morphology. The TARP-induced dendritic growth was restricted to the apical dendrites of pyramidal cells and it did not affect interneurons. Moreover, we studied the effects of short hairpin RNA-induced knockdown of endogenous γ-8 and showed a reduction of dendritic complexity and amplitudes of spontaneous calcium transients. In addition, the cytoplasmic tail (CT) of γ-8 was required for dendritic growth. Single-cell calcium imaging showed that the γ-8 CT domain increases amplitude but not frequency of calcium transients, suggesting a regulatory mechanism involving the γ-8 CT domain in the postsynaptic compartment. Indeed, the effect of γ-8 overexpression was reversed by APV, indicating a contribution of NMDA receptors. Our results suggest that selected type I TARPs influence activity-dependent dendritogenesis of immature pyramidal neurons.

GluA1 promotes the activity-dependent development of motor circuitry in the developing segmental spinal cord

The neuronal dendritic tree is a key determinant of how neurons receive, compute, and transmit information. During early postnatal life, synaptic activity promotes dendrite elaboration. Spinal motor neurons utilize GluA1-containing AMPA (2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl) propanoic acid) receptors (AMPA-R) to control this process. This form of developmental dendrite growth can occur independently of N-methyl-d-aspartate receptors (NMDA-R). This review focuses on the mechanism by which the GluA1 subunit of AMPA-R transforms synaptic activity into dendrite growth, and describes the essential role of the GluA1 binding partner SAP97 (synapse-associated protein of 97 kDa molecular weight) in this process. This work defines a new mechanism of activity-dependent development, which might be harnessed to stimulate the recovery of function following insult to the central nervous system.

Perinatal exposure to bisphenol-A inhibits synaptogenesis and affects the synaptic morphological development in offspring male mice

Our previous study indicated that perinatal exposure to low-dose BPA, one of the most common environmental endocrine disrupters, alters behavioral development in offspring mice. Given that synaptic structure of the hippocampus is closely related to behaviors, in the present study, we examined the effects of perinatal exposure to BPA (0.04, 0.4, and 4.0 mg kg(-1) day(-1)) on the synaptic density and the synaptic structural modification of pyramidal cells in hippocampus region CA1 and the expressions of synaptic proteins such as synapsin I and PSD-95 and glutamate NMDA and AMPA receptors in male offspring mice on postnatal day (PND) 14, 21, and 56. The results of electron microscope measurement showed that BPA significantly reduced the numeric synaptic density and altered the structural modification of synaptic interface of pyramidal cells with the enlarged synaptic cleft, the shortened active zone, and the thinned postsynaptic density (PSD) on PND 14, 21, and 56 and the increased curvature of synaptic interface on PND 14 and 21. Further analyses of Western blot indicated that BPA markedly reduced the levels of synapsin I and PSD-95 on PND 14, 21, and 56 and down-regulated NMDA receptor subunit NR1 and AMPA receptor subunit GluR1 during development and young adulthood. These results suggest that perinatal exposure to low level of BPA inhibits synaptogenesis and affects synaptic structural modification after birth. The reduced expressions of synaptic proteins synapsin I and PSD-95 and glutamate NMDA and AMPA receptors may be involved in the negative changes in the synaptic plasticity.

The molecular basis of experience-dependent motor system development

Neurons in the vertebrate nervous system acquire their mature features over an extended period in pre-natal and early post-natal life. The interaction of the organism with its environment (“experience”) has been shown to profoundly influence sensory neuron development. Over the past ~2 decades, it has become increasingly clear that motor system development is also experience-dependent. Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype have been implicated in both sensory and motor system experience-dependent development. An additional molecular mechanism involves the GluA1 subunit of the 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) subtype glutamate receptors. GluA1-dependent development operates in an NMDA-R independent manner and uses a distinct set of signaling molecules. The synapse associated protein of 97 kDa molecular weight (SAP97) is key. A deeper understanding of how experiences guides motor system development may lead to new ways to improve function after central nervous system insult.

Homeostatic regulation of dendritic dynamics in a motor map in vivo

Neurons and circuits are remarkably dynamic. Their gross structure can change within minutes as neurons sprout and retract processes to form new synapses. Homeostatic processes acting to regulate neuronal activity contribute to these dynamics and predict that the dendritic dynamics within pools of neurons should vary systematically in accord with the activity levels of individual neurons in the pool during behaviour. Here we test this by taking advantage of a topographic map of recruitment of spinal motoneurons in zebrafish. In vivo imaging reveals that the dendritic filopodial dynamics of motoneurons map onto their recruitment pattern, with the most electrically active cells having the lowest dynamics. Genetic reduction of activity inverts this map of dynamics. We conclude that homeostatic mechanisms driven by a gradient of activity levels in a pool of neurons can drive an associated gradation in neuronal dendritic dynamics, potentially shaping connectivity within a functionally heterogenous pool of neurons.

Electrical activity as a developmental regulator in the formation of spinal cord circuits

Spinal cord development is a complex process involving generation of the appropriate number of cells, acquisition of distinctive phenotypes and establishment of functional connections that enable execution of critical functions such as sensation and locomotion. Here we review the basic cellular events occurring during spinal cord development, highlighting studies that demonstrate the roles of electrical activity in this process. We conclude that the participation of different forms of electrical activity is evident from the beginning of spinal cord development and intermingles with other developmental cues and programs to implement dynamic and integrated control of spinal cord function.

Cell class-specific regulation of neocortical dendrite and spine growth by AMPA receptor splice and editing variants

Glutamatergic transmission converging on calcium signaling plays a key role in dendritic differentiation. In early development, AMPA receptor (AMPAR) transcripts are extensively spliced and edited to generate subunits that differ in their biophysical properties. Whether these subunits have specific roles in the context of structural differentiation is unclear. We have investigated the role of nine GluA variants and revealed a correlation between the expression of flip variants and the period of major dendritic growth. In interneurons, only GluA1(Q)-flip increased dendritic length and branching. In pyramidal cells, GluA2(Q)-flop, GluA2(Q)-flip, GluA3(Q)-flip and calcium-impermeable GluA2(R)-flip promoted dendritic growth, suggesting that flip variants with slower desensitization kinetics are more important than receptors with elevated calcium permeability. Imaging revealed significantly higher calcium signals in pyramidal cells transfected with GluA2(R)-flip as compared with GluA2(R)-flop, suggesting a contribution of voltage-activated calcium channels. Indeed, dendritic growth induced by GluA2(R)-flip in pyramidal cells was prevented by blocking NMDA receptors (NMDARs) or voltage-gated calcium channels (VGCCs), suggesting that they act downstream of AMPARs. Intriguingly, the action of GluA1(Q)-flip in interneurons was also dependent on NMDARs and VGCCs. Cell class-specific effects were not observed for spine formation, as GluA2(Q)-flip and GluA2(Q)-flop increased spine density in pyramidal cells as well as in interneurons. The results suggest that AMPAR variants expressed early in development are important determinants for activity-dependent dendritic growth in a cell type-specific and cell compartment-specific manner.

Calcium/calmodulin-dependent protein kinase IV mediates distinct features of basal and activity-dependent dendrite complexity

Intracellular signaling mechanisms translate extracellular signals, such as neuronal activity, into effects on dendrite complexity. Deciphering these mechanisms has considerable impact on understanding how the brain develops and what can go wrong in developmental disorders. How neurons regulate intracellular signaling to control their dendrite morphology remains poorly understood and is likely to be determined at the level of individual neuronal types. Calcium/calmodulin-dependent protein kinase IV (CaMKIV) is a signaling mechanism involved in the regulation of gene expression and dendrite growth. Expression of CaMKIV is developmentally regulated in the cerebral cortex, with highest expression occurring concomitant with the period of extensive dendrite growth and elaboration. Interestingly, cortical neurons heterogeneously expressed CaMKIV in postnatal rat cortices and cortical neurons in vitro. We tested if this differential CaMKIV expression mediated distinct arborization patterns in the dendrites of pyramidal and non-pyramidal neurons. In fact, CaMKIV mediated dendrite complexity via regulation of specific morphological features of the dendrite arbor: branching and elongation, but not primary dendrite formation. We found that small interfering RNA (siRNA) knockdown of CaMKIV decreased basal dendrite complexity indicating that endogenously expressed CaMKIV mediated dendrite complexity. CaMKIV was also required for activity-induced dendrite elaboration. Active CaMKIV expression in cortical neurons increased dendrite elaboration indicating that enzymatic activity was involved. These data indicated neuronal CaMKIV expression was required for basal and activity-induced dendrite complexity. Further, the data presented in this study indicate CaMKIV contributes to the diversity of dendrite arbors via restricted expression and regulation of distinct modes of dendrite elaboration.

Distinct AMPA-type glutamatergic synapses in developing rat CA1 hippocampus

We assessed synaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor (AMPAR) properties during synaptogenesis to describe the development of individual glutamatergic synapses on rat hippocampal CA1 principal neurons. Pharmacologically isolated AMPAR-mediated glutamatergic synaptic currents [evoked by stimulation of the Schaffer Collateral pathway, excitatory postsynaptic currents (EPSCs)], had significantly greater inward-rectification at ages P5-7 compared with P8-18. These inward rectifying EPSCs demonstrated paired-pulse dependent unblocking at positive holding potentials, consistent with voltage-dependent internal polyamine block. Measurements of paired-pulse facilitation did not support altered presynaptic properties associated with inward rectification. Using asynchronous EPSCs (aEPSCs) to analyze populations of individual synapses, we found that quantal amplitudes (Q) increased across early postnatal development (P5-P18) and were directly modulated by increases in the number of activated receptors. Quantal AMPAR decay kinetics (aEPSC τ(decay)s) exhibited the highest coefficient of variation (CV) from P5 to 7 and became markedly less variable at P8-18. At P5-7, faster quantal kinetics coexisted with much slower kinetics; only slower quantal kinetics were found at P8-18. This supports diverse quantal synaptic properties limited to P5-7. Multivariate cluster analysis of Q, CV(τ decay), and median τ(decay) supported a segregation of neurons into two distinct age groups of P5-7 and P8-18, similar to the age-related segregation suggested by inward rectification. Taken together, these findings support synaptic, calcium permeable AMPARs at a subset of synapses onto CA1 pyramidal neurons exclusively at P5-7. These distinct synapses coexist with those sharing the properties of more mature synapses. These synapses disappear after P7 as activated receptor numbers increase with age.

SK2 potassium channel overexpression in basolateral amygdala reduces anxiety, stress-induced corticosterone secretion and dendritic arborization

The basolateral amygdala is critical for generation of anxiety. In addition, exposure to both stress and glucocorticoids induces anxiety. Demonstrated ability of the amygdala to change in response to stress and glucocorticoids could thus be important therapeutic target for anxiety management. Several studies have reported a relationship between anxiety and dendritic arborization of the amygdaloid neurons. In this study we employed a gene therapeutic approach to reduce anxiety and dendritic arborization of the amygdala neurons. Specifically, we overexpressed SK2 potassium channel in the basolateral amygdala using a herpes simplex viral system. Our choice of therapeutic cargo was guided by the indications that activation of the amygdala might underlie anxiety and that SK2 could reduce neuronal activation by exerting inhibitory influence on action potentials. We report that SK2 overexpression reduced anxiety and stress-induced corticosterone secretion at a systemic level. SK2 overexpression also reduced dendritic arborization of the amygdala neurons. Hence, SK2 is a potential gene therapy candidate molecule that can be used against stress-related neuropsychiatric disorders such as anxiety.

AMPA glutamate receptor subunits 1 and 2 regulate dendrite complexity and spine motility in neurons of the developing neocortex

Within neurons of several regions of the CNS, mature dendrite architecture is attained via extensive reorganization of arbor during the developmental period. Since dendrite morphology determines the firing patterns of the neuron, morphological refinement of dendritic arbor may have important implications for mature network activity. In the neocortex, a region of brain that is sensitive to activity-dependent structural rearrangement of dendritic arbor, the proportion of AMPA receptors increases over the developmental period. However, it is unclear whether changes in AMPA receptor expression contribute to maturation of dendritic architecture. To determine the effects of increasing AMPA receptor expression on dendrite morphology and connectivity within the neocortex, and to determine whether these effects are dependent on specific AMPA receptor subunits, we overexpressed the AMPA glutamate receptor subunit 1 (GluR1) and glutamate receptor subunit 2 (GluR2) in cultured rat neocortical neurons at the time that AMPA receptors would normally be incorporated into synapses. Following expression of GluR1 or GluR2 we observed increases in the length and complexity of dendritic arbor of cortical neurons, and a concurrent reduction in motility of spines. In addition, expression of either subunit was associated with an increased density of excitatory postsynaptic puncta. These results suggest that AMPA receptor expression is an important determinant of dendrite morphology and connectivity in neocortical neurons, and further, that contrary to other regions of the CNS, the effects of AMPA receptors on dendrite morphology are not subunit-specific.

Modest alterations in patterns of motor neuron dendrite morphology in the Fmr1 knockout mouse model for fragile X

Fragile X, an inheritable form of mental retardation, is caused by the inactivation of a gene on the X chromosome, FMR1 which codes for an RNA binding protein, fragile X mental retardation protein. Loss of this protein is associated with reduced complexities of neuronal dendrites and alterations in spine morphology in a number of cortical brain regions, and these deficits may underlie the cognitive impairment observed in fragile X patients. Among the many symptoms of fragile X are altered motor functions, although the neuronal basis for these remains unclear. In this study we investigated whether knockout of Fmr1 in the mouse model of fragile X altered dendrite morphology in developing spinal cord motor neurons. We find that Fmr1 knockout leads to modest alterations in the distribution of dendritic arbor across the span of the motor neuron dendritic tree in 2- and 4-week-old mice, compared to wild-type controls, consistent with slower rates of extension and abnormal pruning of intermediate dendritic segments. These studies suggest that some motor deficits in fragile X patients may be due to abnormal maturation of dendritic patterning within spinal motor neurons, and suggest that strategies aimed at preventing motor impairment in fragile X patients may be targeted at motor functions during early development.

Expression of the N-methyl-D-aspartate receptor subunit NR3B regulates dendrite morphogenesis in spinal motor neurons

During postnatal development, the dendrites of spinal motor neurons are refined in an activity-dependent manner that can be influenced by blocking activation of N-methyl-D-aspartate (NMDA) receptors. In late postnatal life, dendritic refinement ceases, and dendrite architecture is unaffected by NMDA antagonists; however the molecular substrate for limiting dendritic plasticity is not understood. During late postnatal development, expression of the NR3B NMDA receptor subunit, a putative dominant-negative subunit that reduces glutamate-induced ionic currents, is upregulated within motor neurons. To investigate whether increasing NR3B expression may contribute to the loss in late development of activity-dependent dendritic reorganization in the spinal cord, we over-expressed NR3B in cultured rat spinal motor neurons, and compared its effects on dendrite morphology with the effects of pharmacological blockade of NMDA receptors. We found that over-expression of the NR3B receptor subunit increased the length and complexity of dendritic arbor, and increased numbers of dendritic filopodia, suggesting that NR3B promotes the addition of branch segments in developing motor neurons. In contrast, blockade of NMDA receptor activity by the NMDA antagonist DL-2-amino-5-phosphonovalerate (AP5) had little effect on the overall length or complexity of dendritic arbor. Instead, treatment with AP5 resulted in significant reorganization of dendritic arbor in a manner that favored addition of dendritic segments of high branch orders, at the expense of those closer to the cell body. These results suggest that expression of the NR3B subunit may participate in activity-dependent reorganization of dendritic architecture, but via a mechanism that may be inconsistent with loss of NMDA receptor activity.

Differential regulation of dendrite complexity by AMPA receptor subunits GluR1 and GluR2 in motor neurons

Activity-dependent developmental mechanisms in many regions of the central nervous system are thought to be responsible for shaping dendritic architecture and connectivity, although the molecular mechanisms underlying these events remain obscure. Since AMPA glutamate receptors are developmentally regulated in spinal motor neurons, we have investigated the role of activation of AMPA receptors in dendritic outgrowth of spinal motor neurons by overexpression of two subunits, GluR1 and GluR2, and find that dendrite outgrowth is differentially controlled by expression of these subunits. Overexpression of GluR1 was associated with greater numbers of filopodia, and an increase in the length and complexity of dendritic arbor. In contrast, GluR2 expression did not alter dendritic complexity, but was associated with a moderate increase in length of arbor, and decreased numbers of filopodia. Neither GluR1 nor GluR2 had any effect on the motility of filopodia. In addition, GluR1 but not GluR2 expression increased the density of dendritic puncta incorporating a GFP-labeled PSD95, suggesting that GluR1 may mediate its effect in part by augmenting the number of excitatory synapses within motor neuron dendrites. Together these results suggest that in spinal motor neurons, AMPA receptors composed of GluR1 subunits may facilitate neurotrophic mechanisms in these neurons, permitting sustained dendrite outgrowth and synaptogenesis, whereas expression of AMPA receptors containing GluR2 acts to preserve existing dendritic arbor. Thus, the observed downregulation of GluR1 in motor neurons during postnatal development may limit the formation of new dendrite segments and synapses, promoting stabilized synaptic connectivity.

Acute corticosterone treatment is sufficient to induce anxiety and amygdaloid dendritic hypertrophy

Stress is known to induce dendritic hypertrophy in the basolateral amygdala (BLA) and to enhance anxiety. Stress also leads to secretion of glucocorticoids (GC), and the BLA has a high concentration of glucocorticoid receptors. This raises the possibility that stress-induced elevation in GC secretion might directly affect amygdaloid neurons. To address the possible effects of GC on neurons of amygdala and on anxiety, we used rats treated either acutely with a single dose or chronically with 10 daily doses of high physiological levels of corticosterone (the rat-specific glucocorticoid). Behavior and morphological changes in neurons of BLA were measured 12 days after the initiation of treatment in both groups. A single acute dose of corticosterone was sufficient to induce dendritic hypertrophy in the BLA and heightened anxiety, as measured on an elevated plus maze. Moreover, this form of dendritic hypertrophy after acute treatment was of a magnitude similar to that caused by chronic treatment. Thus, plasticity of BLA neurons is sufficiently sensitive so as to be saturated by a single day of stress. The effects of corticosterone were specific to anxiety, as neither acute nor chronic treatment caused any change in conditioned fear or in general locomotor activity in these animals.

Translating neuronal activity into dendrite elaboration: signaling to the nucleus

Growth and elaboration of neuronal processes is key to establishing neuronal connectivity critical for an optimally functioning nervous system. Neuronal activity clearly influences neuronal connectivity and does so via intracellular calcium signaling. A number of CaMKs and MAPKs convey the calcium signal initiated by neuronal activity. Several of these kinases interact with substrates in close proximity to the plasma membrane and alter dendrite structure locally via these local interactions. However, many calcium-activated kinases, such as Ras-MAPK and CaMKIV, target proteins in the nucleus, either by activating a downstream substrate that is a component of a signaling cascade or by directly acting within the nucleus. It is the activation of nuclear signaling and gene transcription that is thought to mediate global changes in dendrite complexity. The identification of calcium-sensitive transcription factors and transcriptional coactivators provides substantial evidence that gene transcription is a prevalent mechanism by which neuronal activity is translated into changes in dendrite complexity. The present review presents an overview of the role of neuronal activity in the development of neuronal dendrites, the signaling mechanisms that translate neuronal activity into gene transcription, and the transcribed effectors that regulate dendrite complexity.

Embryonic motor neuron dendrite growth is stunted by inhibition of nitric oxide-dependent activation of soluble guanylyl cyclase and protein kinase G

We have examined the participation of a neuronal nitric oxide synthase (nNOS) signaling pathway in the elaboration of motor neuron dendrites during embryonic life. During chick embryogenesis, nNOS is expressed by interneurons that surround the motor neuron pools in the ventral horn. Pseudorabies virus tracing suggests that these cells, while juxtaposed to motor neurons are not synaptically connected to them. The downstream effectors, soluble guanylyl cyclase (sGC) and protein kinase G (PKG), are found in motor neurons as well as several other populations of spinal cord cells. To determine the functional significance of the nNOS/sGC/PKG signaling pathway, pharmacological inhibitors were applied to chick embryos and the effects on motor neuron dendrites monitored. Inhibition of nNOS activity led to a lasting reduction in the overall size and degree of branching of the dendritic tree. These alterations in dendritic architecture were also seen when the activity of sGC or PKG was blocked. Our results suggest that normal motor neuron dendrite elaboration depends, in part, on the activity-dependent generation of NO by ventral horn interneurons, which then activates sGC and PKG in motor neurons.

Transcription factor Sp4 regulates dendritic patterning during cerebellar maturation

Integration of inputs by a neuron depends on dendritic arborization patterns. In mammals, the genetic programs that regulate dynamic remodeling of dendrites during development and in response to activity are incompletely understood. Here we report that knockdown of the transcription factor Sp4 led to an increased number of highly branched dendrites during maturation of cerebellar granule neurons in dissociated cultures and in cerebellar cortex. Time-course analysis revealed that depletion of Sp4 led to persistent generation of dendritic branches and a failure in resorption of transient dendrites. Depolarization induced a reduction in the number of dendrites, and knockdown of Sp4 blocked depolarization-induced remodeling. Furthermore, overexpression of Sp4 wild type, but not a mutant lacking the DNA-binding domain, was sufficient to promote dendritic pruning in nondepolarizing conditions. These findings indicate that the transcription factor Sp4 controls dendritic patterning during cerebellar development by limiting branch formation and promoting activity-dependent pruning.

Bi-directional control of motor neuron dendrite remodeling by the calcium permeability of AMPA receptors

Motor neurons express particularly high levels of the AMPA receptor subunit GluR1(Q)flip (GluR1(Q)i) during the period in early postnatal life when their dendritic tree grows and becomes more branched. To investigate how GluR1-containing AMPA receptors contribute to dendrite morphogenesis, we characterized a mutant form of GluR1 (containing a histidine in the Q/R editing site) with unique electrophysiological properties. Most notably, AMPA receptors assembled from GluR1(H)i display less calcium permeability than AMPA receptors assembled from GluR1(Q)i. Expression of GluR1(Q)i in vivo or in vitro led to an increase in dendrite branching with no net change in the overall tree size while GluR1(H)i led to a loss of branches and a net reduction in overall tree size. GluR1(H)i-dependent dendrite atrophy is mediated by protein phosphatase 2B. The results suggest that the electrophysiological properties of cell surface AMPA receptors, specifically their permeability to calcium, can be a central determinant of whether the dendrites undergo activity-dependent branching or atrophy.

Effect of N-methyl-d-aspartate receptor blockade on plasticity of frontal cortex after cholinergic deafferentation in rat

Cholinergic projections from the nucleus basalis play a critical role in cortical plasticity. For instance, cholinergic deafferentation increases dendritic spine density and expression of the GluR1 subunit of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor in frontal cortex. Acetylcholine modulates glutamatergic activity in cortex, and the N-methyl-d-aspartate subtype of glutamate receptor plays a role in many forms of synaptic plasticity. To assess whether N-methyl-d-aspartate receptors mediate the increase in GluR1 and spine density resulting from cholinergic deafferentation, we examined the effect of N-methyl-d-aspartate receptor blockade on nucleus basalis lesion-induced upregulation of GluR1 and dendritic spines. Rats received unilateral sham or 192 IgG saporin lesions of the nucleus basalis. Half of the rats in each group were treated with the N-methyl-d-aspartate antagonist MK-801 or phosphate-buffered saline. Two weeks later, brains were processed for either immunohistochemical staining of the GluR1 subunit or Golgi histology. In layer II-III of frontal cortex, neuronal GluR1 expression was assessed using an unbiased stereological technique, and spine density was assessed on basilar branches of pyramidal neurons. GluR1 expression was increased after nucleus basalis lesion, but this increase was prevented with MK-801. Similarly, nucleus basalis-lesioned animals had significantly higher spine densities, and this effect was also prevented by treatment with MK-801. Thus, N-methyl-d-aspartate receptor blockade prevented both GluR1 and spine density upregulation following cholinergic deafferentation, suggesting that these effects are N-methyl-d-aspartate receptor-mediated.

Excitotoxicity and ALS: what is unique about the AMPA receptors expressed on spinal motor neurons?

It has been repeatedly reported that spinal motor neurons are selectively vulnerable to AMPA receptor-mediated excitotoxicity. Therefore, identifying the uniqueness of AMPA receptors that are expressed on motor neurons, especially in individuals affected with sporadic amyotrophic lateral sclerosis (ALS) is essential for elucidating the etiology of this disorder. The mechanism that initiates motor neuronal death appears to be an exaggerated influx of Ca(2+) through AMPA receptors. The determinants that affect this Ca(2+) influx are Ca(2+) permeability, which is regulated by the presence of the GluR2 subunit and by RNA editing at the Q/R site of GluR2; channel desensitization, which is regulated by alternative splicing at the flip/flop site and by RNA editing at the R/G site of GluR subunits; and receptor density on the cell surface, which is controlled by many factors including regulatory proteins, direct phosphorylation and RNA editing at the Q/R site. This review focuses on recent progress on the molecular dynamics of AMPA receptors and discusses the pathophysiology of selective motor neuron death mediated by AMPA receptors in individuals affected with sporadic ALS.

Enriched environment promotes behavioral and morphological recovery in a mouse model for the fragile X syndrome

Fragile X syndrome, the most frequent form of hereditary mental retardation, is due to a mutation of the fragile X mental retardation 1 (FMR1) gene on the X chromosome. Like fragile X patients, FMR1-knockout (FMR1-KO) mice lack the normal fragile X mental retardation protein (FMRP) and show both cognitive alterations and an immature neuronal morphology. We reared FMR1-KO mice in a C57BL/6 background in enriched environmental conditions to examine the possibility that experience-dependent stimulation alleviates their behavioral and neuronal abnormalities. FMR1-KO mice kept in standard cages were hyperactive, displayed an altered pattern of open field exploration, and did not show habituation. Quantitative morphological analyses revealed a reduction in basal dendrite length and branching together with more immature-appearing spines along apical dendrites of layer five pyramidal neurons in the visual cortex. Enrichment largely rescued these behavioral and neuronal abnormalities while increasing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit 1 (GluR1) levels in both genotypes. Enrichment did not, however, affect FMRP levels in the WT mice. These data suggest that FMRP-independent pathways activating glutamatergic signaling are preserved in FMR1-KO mice and that they can be elicited by environmental stimulation.

Aging and cholinergic deafferentation alter GluR1 expression in rat frontal cortex

Previously, we demonstrated that plasticity of frontal cortex is altered in aging rats: lesions of the nucleus basalis magnocellularis (NBM) produce larger declines in dendritic morphology in frontal cortex of aged rats compared to young adults. Cholinergic afferents from the NBM modulate glutamatergic transmission in neocortex, and glutamate is known to be involved in dendritic plasticity. To begin to identify possible mechanisms underlying age-related differences in plasticity after NBM lesion, we assessed the effect of cholinergic deafferentation on expression of the AMPA receptor subunit GluR1 in frontal cortex of young adult and aging rats. Young adult, middle-aged, and aged rats received sham or 192 IgG-saporin lesions of the NBM, and an unbiased stereological technique was used to estimate the total number of intensely GluR1-immunopositive neurons in layer II-III of frontal cortex. While the number of GluR1-positive neurons was increased in both middle-aged and aged rats, lesions markedly increased the number of intensely GluR1-immunopositive neurons in frontal cortex of young adult rats only. This age-related difference in lesion-induced expression of AMPA receptor subunit protein could underlie the age-related differences in dendritic plasticity after NBM lesions.

Perinatal development of respiratory motoneurons

Breathing movements require the coordinated recruitment of cranial and spinal motoneurons innervating muscles of the upper airway and ribcage. A significant part of respiratory motoneuron development and maturation occurs prenatally to support the generation of fetal breathing movements in utero and sustained breathing at birth. Postnatally, motoneuron properties are further refined and match changes in the maturing respiratory musculoskeletal system. In this review, we outline developmental changes in key respiratory motoneuronal populations occurring from the time of motoneuron birth in the embryo through the postnatal period. We will also bring attention to major deficiencies in the current knowledge of perinatal respiratory motoneuron development. To date, our understanding of processes occurring during the prenatal period comes primarily from analysis of phrenic motoneurons (PMNs), whereas information about postnatal development derives largely from studies of PMN and hypoglossal motoneuron properties.

Regulation of dendritic growth and plasticity by local and global calcium dynamics

The dendritic arbors of neurons are organized into unique spatial patterns that are well suited for their specific functions. Although the intracellular signaling pathways that determine dendritic arbor size and branching patterns during development are not completely understood, it is evident that neurotransmission-mediated elevation in intracellular calcium levels ([Ca2+]i) plays a major role. Recent advances in calcium imaging and genetic approaches have provided new insight into how calcium acts to regulate dendritic growth and remodeling. Global increases in [Ca2+]i that occur upon neuronal depolarization control dendritic arbor growth by regulating transcription. However, the detailed branching patterns of dendritic arbors are regulated by local calcium signaling. Calcium-induced calcium release (CICR) from intracellular stores acts to locally stabilize dendritic branches, presumably triggered by neurotransmission upon contact with newly established inputs. The effects of global and local increases in [Ca2+]i on dendritic structure are cell type- and age-dependent. Unraveling the signaling pathways evoked by local and global rises in [Ca2+]i that shape the developing dendritic arbor at each developmental stage remains challenging but highly important.

Differential environmental exposure alters NMDA but not AMPA receptor subunit expression in nucleus accumbens core and shell

Environmentally enriched (EE) rats show neurobehavioral differences relative to less stimulated, socially isolated (SI) littermates. Although experience-dependent cortical changes are presumed to underlie learning differences in these differentially housed animals, EE rats show reduced reward-seeking behavior and altered cytoarchitecture and dopaminergic function in the nucleus accumbens (NAcc), a brain area involved in adaptive, goal-directed activity. Given that glutamate and its interaction with dopamine regulate motivational and associative processing in this brain region, we assessed expression of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor and the GluR1 subunit of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor in the NAcc core and shell of EE and SI rats. Our results indicate fewer intensely stained NR1 immunopositive neurons in both core and shell of EE relative to SI rats. No such differences were observed in GluR1 staining. These results suggest that environmental experience alters NMDA but not AMPA receptor expression in NAcc. Increased expression of the NR1 subunit in the NAcc of SI rats may augment impulsivity and reward-seeking behavior relative to EE rats.

Developmental changes in spinal motoneuron dendrites in neonatal mice

We examined the age-dependent morphological changes of lumbar spinal motoneurons (MNs) in neonatal Swiss-Webster mice during the first 2 weeks of postnatal life. Neurons labeled by intracellular injection of biocytin in hemisected lumbosacral spinal cords in vitro were reconstructed from serial sections. Digitized data were compared for young (P3; postnatal days 2-4; n = 9) and older animals (P11; postnatal days 10-13; n = 8). As expected, measures of dendritic size (e.g., stem branch diameter, total surface area, maximum distance to tips, and lateral tree spread) were all significantly greater for P11 than for P3 mice. In contrast, the number of dendrites per MN and parameters related to tree topology (e.g., terminations per tree and maximum branch order), although slightly greater for P11 animals, were not significantly different between the two ages. Dendrite growth appeared to be proportional throughout the tree because the ratios between average terminal and internal branch lengths were similar for the two groups. Furthermore, this elongation was proportional to enlargement of overall spinal cord dimensions. A variety of other morphometric measures showed no significant difference between age groups. The relative constancy of MN dendritic topology up to P13 was surprising, given the striking maturation in motor function during this time period.

Serum/glucocorticoid-inducible kinase can phosphorylate the cyclic AMP response element binding protein, CREB

To maintain homeostasis, cells often respond to stressful extra-cellular stimuli by new gene expression. Serum/glucocorticoid-induced kinase (SGK) is an immediate early gene whose expression is induced by a variety of extra-cellular stimuli. Here, we examine the possibility that SGK can directly phosphorylate the transcription factor cyclic AMP response element binding protein (CREB). In a cell-free context, SGK physically associates with CREB and SGK phosphorylates it on serine 133. Phospho-serine 133 is essential for stimulating the transcriptional activity of CREB. Further, we show that in a variety of cellular contexts, SGK phosphorylates CREB. Activation of receptor tyrosine kinase pathways or the phosphoinositide-dependent kinase 1 (PDK1) lead to SGK-dependent CREB phosphorylation. Hormonal stimulation of epithelial cells leads to the induction of endogenous SGK and CREB phosphorylation. A dominant-negative form of SGK blocks dexamethasone-induced CREB phosphorylation. Our studies indicate that stimulation of SGK can lead to CREB phosphorylation, suggesting that CREB-dependent gene transcription is an important link between stressful extra-cellular signals and cellular responses.

Transient Ca2+-permeable AMPA receptors in postnatal rat primary auditory neurons

Fast excitatory transmission in the nervous system is mostly mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors whose subunit composition governs physiological characteristics such as ligand affinity and ion conductance properties. Here, we report that AMPA receptors at inner hair cell (IHC) synapses lack the GluR2 subunit and are transiently Ca2+-permeable before hearing onset as evidenced using agonist-induced Co2+ accumulation, Western blots and GluR2 confocal microscopy in the rat cochlea. AMPA (100 microM) induced Co2+ accumulation in primary auditory neurons until postnatal day (PND) 10. This accumulation was concentration-dependent, strengthened by cyclothiazide (50 microM) and blocked by GYKI 52466 (80 microM) and Joro spider toxin (1 microM). It was unaffected by D-AP5 (50 microM), and it could not be elicited by 56 mM K+ or 1 mM NMDA + 10 microM glycine. Western blots showed that GluR1 immunoreactivity, present in homogenates of immature cochleas, had disappeared by PND12. GluR2 immunoreactivity was not detected until PND10 and GluR3 and GluR4 immunoreactivities were detected at all the ages examined. Confocal microscopy confirmed that the GluR2 immunofluorescence was not located postsynaptically to IHCs before PND10. In conclusion, AMPA receptors on maturing primary auditory neurons differ from those on adult neurons. They are probably composed of GluR1, GluR3 and GluR4 subunits and have a high Ca2+ permeability. The postsynaptic expression of GluR2 subunits may be continuously regulated by the presynaptic activity allowing for variations in the Ca2+ permeability and physiological properties of the receptor.

Neural Cell Adhesion Molecule-associated Polysialic Acid Potentiates α-Amino-3-hydroxy-5-methylisoxazole-4-propionic Acid Receptor Currents

The highly negatively charged polysialic acid (PSA) is a carbohydrate predominantly carried by the neural cell adhesion molecule (NCAM) in mammals. NCAM and, in particular, PSA play important roles in cellular and synaptic plasticity. Here we investigated whether PSA modulates the activity of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of glutamate receptors (AMPA-Rs). Single channel recordings of affinity-purified AMPA-Rs reconstituted in lipid bilayers revealed that bacterially derived PSA, called colominic acid, prolonged the open channel time of AMPA-R-mediated currents by severalfold and altered the bursting pattern of the receptor channels but did not modify AMPA-R single channel conductance. This effect was reversible, concentration-dependent, and specific, since monomers of sialic acid and another negatively charged carbohydrate, chondroitin sulfate, did not potentiate single channel AMPA-R currents. Recombinant PSA-NCAM also potentiated currents mediated by reconstituted AMPA-Rs. In pyramidal neurons acutely isolated from the CA1 region of the early postnatal hippocampus, l-glutamate or AMPA (applied in the presence of antagonists blocking voltage-gated Na(+) and K(+) currents and N-methyl-d-aspartate and metabotropic glutamate receptors) induced inward currents, which were significantly increased by co-application of colominic acid. Chondroitin sulfate did not affect AMPA-R-mediated currents in CA1 neurons. The effect of colominic acid was age-dependent, since in pyramidal neurons from adult hippocampus, colominic acid failed to potentiate glutamate responses. Thus, our study demonstrates age-dependent potentiation of AMPA receptors by PSA via a mechanism probably involving direct PSA-AMPA-R interactions. This mechanism might amplify AMPA-R-mediated signaling in immature cells, thereby affecting their development.