SciCombinator

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Concept: Dendritic spine

162

Distinct isoforms of the PI3K catalytic subunit have specialized functions in the brain, but their role in cognition is unknown. Here, we show that the catalytic subunit p110β plays an important role in prefrontal cortex (PFC)-dependent cognitive defects in mouse models of Fragile X syndrome (FXS), an inherited intellectual disability. FXS is caused by loss of function of the fragile X mental retardation protein (FMRP), which binds and translationally represses mRNAs. PFC-selective knockdown of p110β, an FMRP target that is translationally upregulated in FXS, reverses deficits in higher cognition in Fmr1 knockout mice. Genetic full-body reduction of p110β in Fmr1 knockout mice normalizes excessive PI3K activity, restores stimulus-induced protein synthesis, and corrects increased dendritic spine density and behavior. Notably, adult-onset PFC-selective Fmr1 knockdown mice show impaired cognition, which is rescued by simultaneous p110β knockdown. Our results suggest that FMRP-mediated control of p110β is crucial for neuronal protein synthesis and cognition.

Concepts: Brain, Autism, Down syndrome, Mental retardation, Disability, Fragile X syndrome, FMR1, Dendritic spine

109

As human lifespan increases, a greater fraction of the population is suffering from age-related cognitive impairments, making it important to elucidate a means to combat the effects of aging. Here we report that exposure of an aged animal to young blood can counteract and reverse pre-existing effects of brain aging at the molecular, structural, functional and cognitive level. Genome-wide microarray analysis of heterochronic parabionts-in which circulatory systems of young and aged animals are connected-identified synaptic plasticity-related transcriptional changes in the hippocampus of aged mice. Dendritic spine density of mature neurons increased and synaptic plasticity improved in the hippocampus of aged heterochronic parabionts. At the cognitive level, systemic administration of young blood plasma into aged mice improved age-related cognitive impairments in both contextual fear conditioning and spatial learning and memory. Structural and cognitive enhancements elicited by exposure to young blood are mediated, in part, by activation of the cyclic AMP response element binding protein (Creb) in the aged hippocampus. Our data indicate that exposure of aged mice to young blood late in life is capable of rejuvenating synaptic plasticity and improving cognitive function.

Concepts: Psychology, Brain, Blood, Cognition, Memory, Hippocampus, Aging, Dendritic spine

81

Brief periods of sleep loss have long-lasting consequences such as impaired memory consolidation. Structural changes in synaptic connectivity have been proposed as a substrate of memory storage. Here, we examine the impact of brief periods of sleep deprivation on dendritic structure. In mice, we find that five hours of sleep deprivation decreases dendritic spine numbers selectively in hippocampal area CA1 and increased activity of the filamentous actin severing protein cofilin. Recovery sleep normalizes these structural alterations. Suppression of cofilin function prevents spine loss, deficits in hippocampal synaptic plasticity, and impairments in long-term memory caused by sleep deprivation. The elevated cofilin activity is caused by cAMP-degrading phosphodiesterase-4A5 (PDE4A5), which hampers cAMP-PKA-LIMK signaling. Attenuating PDE4A5 function prevents changes in cAMP-PKA-LIMK-cofilin signaling and cognitive deficits associated with sleep deprivation. Our work demonstrates the necessity of an intact cAMP-PDE4-PKA-LIMK-cofilin activation-signaling pathway for sleep deprivation-induced memory disruption and reduction in hippocampal spine density.

Concepts: Amygdala, Sleep, Sleep deprivation, Memory, Hippocampus, Dendrite, Long-term potentiation, Dendritic spine

57

Temporal lobe epilepsy or limbic epilepsy lacks effective therapies due to a void in understanding the cellular and molecular mechanisms that set in motion aberrant neuronal network formations during the course of limbic epileptogenesis (LE). Here we show in in vivo rodent models of LE that the phospholipid mediator platelet-activating factor (PAF) increases in LE and that PAF receptor (PAF-r) ablation mitigates its progression. Synthetic PAF-r antagonists, when administered intraperitoneally in LE, re-establish hippocampal dendritic spine density and prevent formation of dysmorphic dendritic spines. Concomitantly, hippocampal interictal spikes, aberrant oscillations, and neuronal hyper-excitability, evaluated 15-16 weeks after LE using multi-array silicon probe electrodes implanted in the dorsal hippocampus, are reduced in PAF-r antagonist-treated mice. We suggest that over-activation of PAF-r signaling induces aberrant neuronal plasticity in LE and leads to chronic dysfunctional neuronal circuitry that mediates epilepsy.

Concepts: Cerebral cortex, Temporal lobe, Cerebrum, Hippocampus, Epilepsy, Temporal lobe epilepsy, Hippocampal sclerosis, Dendritic spine

47

Developmental alterations of excitatory synapses are implicated in autism spectrum disorders (ASDs). Here, we report increased dendritic spine density with reduced developmental spine pruning in layer V pyramidal neurons in postmortem ASD temporal lobe. These spine deficits correlate with hyperactivated mTOR and impaired autophagy. In Tsc2+/- ASD mice where mTOR is constitutively overactive, we observed postnatal spine pruning defects, blockade of autophagy, and ASD-like social behaviors. The mTOR inhibitor rapamycin corrected ASD-like behaviors and spine pruning defects in Tsc2+/ mice, but not in Atg7(CKO) neuronal autophagy-deficient mice or Tsc2+/-:Atg7(CKO) double mutants. Neuronal autophagy furthermore enabled spine elimination with no effects on spine formation. Our findings suggest that mTOR-regulated autophagy is required for developmental spine pruning, and activation of neuronal autophagy corrects synaptic pathology and social behavior deficits in ASD models with hyperactivated mTOR.

Concepts: Neuron, Action potential, Cerebral cortex, Temporal lobe, Hippocampus, Autism, Pyramidal cell, Dendritic spine

43

The Golgi apparatus lies at the heart of the secretory pathway where it is required for secretory trafficking and cargo modification. Disruption of Golgi architecture and function has been widely observed in neurodegenerative disease, but whether Golgi dysfunction is causal with regard to the neurodegenerative process, or is simply a manifestation of neuronal death, remains unclear. Here we report that targeted loss of the golgin GM130 leads to a profound neurological phenotype in mice. Global KO of mouse GM130 results in developmental delay, severe ataxia, and postnatal death. We further show that selective deletion of GM130 in neurons causes fragmentation and defective positioning of the Golgi apparatus, impaired secretory trafficking, and dendritic atrophy in Purkinje cells. These cellular defects manifest as reduced cerebellar size and Purkinje cell number, leading to ataxia. Purkinje cell loss and ataxia first appear during postnatal development but progressively worsen with age. Our data therefore indicate that targeted disruption of the mammalian Golgi apparatus and secretory traffic results in neuronal degeneration in vivo, supporting the view that Golgi dysfunction can play a causative role in neurodegeneration.

Concepts: Neuron, Golgi apparatus, Cerebellum, Dendrite, Neurons, Purkinje cell, Parallel fiber, Dendritic spine

27

Adolescent synaptic pruning is thought to enable optimal cognition because it is disrupted in certain neuropathologies, yet the initiator of this process is unknown. One factor not yet considered is the α4βδ GABAA receptor (GABAR), an extrasynaptic inhibitory receptor which first emerges on dendritic spines at puberty in female mice. Here we show that α4βδ GABARs trigger adolescent pruning. Spine density of CA1 hippocampal pyramidal cells decreased by half post-pubertally in female wild-type but not α4 KO mice. This effect was associated with decreased expression of kalirin-7 (Kal7), a spine protein which controls actin cytoskeleton remodeling. Kal7 decreased at puberty as a result of reduced NMDAR activation due to α4βδ-mediated inhibition. In the absence of this inhibition, Kal7 expression was unchanged at puberty. In the unpruned condition, spatial re-learning was impaired. These data suggest that pubertal pruning requires α4βδ GABARs. In their absence, pruning is prevented and cognition is not optimal.

Concepts: Protein, Hormone, Hippocampus, Pyramidal cell, GABAA receptor, Ligand-gated ion channel, GABA receptor, Dendritic spine

27

Actin-based remodelling underlies spine structural changes occurring during synaptic plasticity, the process that constantly reshapes the circuitry of the adult brain in response to external stimuli, leading to learning and memory formation. A positive correlation exists between spine shape and synaptic strength and, consistently, abnormalities in spine number and morphology have been described in a number of neurological disorders. In the present study, we demonstrate that the actin-regulating protein, Eps8, is recruited to the spine head during chemically induced long-term potentiation in culture and that inhibition of its actin-capping activity impairs spine enlargement and plasticity. Accordingly, mice lacking Eps8 display immature spines, which are unable to undergo potentiation, and are impaired in cognitive functions. Additionally, we found that reduction in the levels of Eps8 occurs in brains of patients affected by autism compared to controls. Our data reveal the key role of Eps8 actin-capping activity in spine morphogenesis and plasticity and indicate that reductions in actin-capping proteins may characterize forms of intellectual disabilities associated with spine defects.

Concepts: Brain, Neurology, Synaptic plasticity, Memory, Autism, Dendrite, Long-term potentiation, Dendritic spine

26

The majority of excitatory synapses in the brain exist on dendritic spines. Accordingly, the regulation of dendritic spine density in the hippocampus is thought to play a central role in learning and memory. The development of novel methods to control spine density could, therefore, have important implications for treatment of a host of neurodegenerative and developmental cognitive disorders. Herein, we report the design and evaluation of a new class of benzothiazole amphiphiles that exhibit a dose-dependent response leading to an increase in dendritic spine density in primary hippocampal neurons. Cell exposure studies reveal that the increase in spine density can persist for days in the presence of these compounds, but returns to normal spine density levels within 24 hours when the compounds are removed, demonstrating the capability to reversibly control spinogenic activity. Time-lapse imaging of dissociated hippocampal neuronal cultures shows that these compounds promote a net increase in spine density through the formation of new spines. Biochemical studies support that promotion of spine formation by these compounds correlates with Ras activation. These spinogenic molecules were also capable of inhibiting a suspected mechanism of the dendritic spine loss induced by Alzheimer’s-related aggregated amyloid-β; peptides in primary neurons. Evaluation of this new group of spinogenic agents reveals that they also exhibit relatively low toxicity at concentrations displaying activity. Collectively, these results suggest that small molecules that promote spine formation could be potentially useful for ameliorating cognitive deficiencies associated with spine loss in neurodegenerative diseases such as Alzheimer’s disease, and may also find use as general cognitive enhancers.

Concepts: Neuron, Brain, Neuroscience, Hippocampus, Dendrite, Long-term potentiation, Purkinje cell, Dendritic spine

26

The membrane protein Nogo-A is known as an inhibitor of axonal outgrowth and regeneration in the CNS. However, its physiological functions in the normal adult CNS remain incompletely understood. Here, we investigated the role of Nogo-A in cortical synaptic plasticity and motor learning in the uninjured adult rodent motor cortex. Nogo-A and its receptor NgR1 are present at cortical synapses. Acute treatment of slices with function-blocking antibodies (Abs) against Nogo-A or against NgR1 increased long-term potentiation (LTP) induced by stimulation of layer 2/3 horizontal fibers. Furthermore, anti-Nogo-A Ab treatment increased LTP saturation levels, whereas long-term depression remained unchanged, thus leading to an enlarged synaptic modification range. In vivo, intrathecal application of Nogo-A-blocking Abs resulted in a higher dendritic spine density at cortical pyramidal neurons due to an increase in spine formation as revealed by in vivo two-photon microscopy. To investigate whether these changes in synaptic plasticity correlate with motor learning, we trained rats to learn a skilled forelimb-reaching task while receiving anti-Nogo-A Abs. Learning of this cortically controlled precision movement was improved upon anti-Nogo-A Ab treatment. Our results identify Nogo-A as an influential molecular modulator of synaptic plasticity and as a regulator for learning of skilled movements in the motor cortex.

Concepts: Neuron, Action potential, Hippocampus, Cerebellum, Dendrite, Long-term potentiation, Pyramidal cell, Dendritic spine