Jason D. Shepherd

Arc/Arg3.1 mediates homeostatic synaptic scaling of AMPA receptors.

Homeostatic plasticity may compensate for Hebbian forms of synaptic plasticity, such as long-term potentiation (LTP) and depression (LTD), by scaling neuronal output without changing the relative strength of individual synapses. This delicate balance between neuronal output and distributed synaptic weight may be necessary for maintaining efficient encoding of information across neuronal networks. Here, we demonstrate that Arc/Arg3.1, an immediate-early gene (IEG) that is rapidly induced by neuronal activity associated with information encoding in the brain, mediates homeostatic synaptic scaling of AMPA type glutamate receptors (AMPARs) via its ability to activate a novel and selective AMPAR endocytic pathway. High levels of Arc/Arg3.1 block the homeostatic increases in AMPAR function induced by chronic neuronal inactivity. Conversely, loss of Arc/Arg3.1 results in increased AMPAR function and abolishes homeostatic scaling of AMPARs. These observations, together with evidence that Arc/Arg3.1 is required for memory consolidation, reveal the importance of Arc/Arg3.1s dynamic expression as it exerts continuous and precise control over synaptic strength and cellular excitability.

Arc/Arg3.1 Interacts with the Endocytic Machinery to Regulate AMPA Receptor Trafficking

Arc/Arg3.1 is an immediate-early gene whose mRNA is rapidly transcribed and targeted to dendrites of neurons as they engage in information processing and storage. Moreover, Arc/Arg3.1 is known to be required for durable forms of synaptic plasticity and learning. Despite these intriguing links to plasticity, Arc/Arg3.1s molecular function remains enigmatic. Here, we demonstrate that Arc/Arg3.1 protein interacts with dynamin and specific isoforms of endophilin to enhance receptor endocytosis. Arc/Arg3.1 selectively modulates trafficking of AMPA-type glutamate receptors (AMPARs) in neurons by accelerating endocytosis and reducing surface expression. The Arc/Arg3.1-endocytosis pathway appears to regulate basal AMPAR levels since Arc/Arg3.1 KO neurons exhibit markedly reduced endocytosis and increased steady-state surface levels. These findings reveal a novel molecular pathway that is regulated by Arc/Arg3.1 and likely contributes to late-phase synaptic plasticity and memory consolidation.

Elongation Factor 2 and Fragile X Mental Retardation Protein Control the Dynamic Translation of Arc/Arg3.1 Essential for mGluR-LTD

Group I metabotropic glutamate receptors (mGluR) induce long-term depression (LTD) that requires protein synthesis. Here, we demonstrate that Arc/Arg3.1 is translationally induced within 5 min of mGluR activation, and this response is essential for mGluR-dependent LTD. The increase in Arc/Arg3.1 translation requires eEF2K, a Ca(2+)/calmodulin-dependent kinase that binds mGluR and dissociates upon mGluR activation, whereupon it phosphorylates eEF2. Phospho-eEF2 acts to slow the elongation step of translation and inhibits general protein synthesis but simultaneously increases Arc/Arg3.1 translation. Genetic deletion of eEF2K results in a selective deficit of rapid mGluR-dependent Arc/Arg3.1 translation and mGluR-LTD. This rapid translational mechanism is disrupted in the fragile X disease mouse (Fmr1 KO) in which mGluR-LTD does not require de novo protein synthesis but does require Arc/Arg3.1. We propose a model in which eEF2K-eEF2 and FMRP coordinately control the dynamic translation of Arc/Arg3.1 mRNA in dendrites that is critical for synapse-specific LTD.

New views of Arc, a master regulator of synaptic plasticity

Many proteins have been implicated in synaptic and experience-dependent plasticity. However, few demonstrate the exquisite regulation of expression and breadth of functional importance as the immediate early gene product Arc. Here we review and attempt to synthesize the disparate views of Arc in neuronal function. The main conclusion garnered from this body of work is that Arc is a critical effector molecule downstream of many molecular signaling pathways and that dysregulation of Arc expression can have dire consequences for normal brain function.

Loss of Arc renders the visual cortex impervious to the effects of sensory experience or deprivation

A myriad of mechanisms have been suggested to account for the full richness of visual cortical plasticity. We found that visual cortex lacking Arc is impervious to the effects of deprivation or experience. Using intrinsic signal imaging and chronic visually evoked potential recordings, we found that Arc−/− mice did not exhibit depression of deprived-eye responses or a shift in ocular dominance after brief monocular deprivation. Extended deprivation also failed to elicit a shift in ocular dominance or open-eye potentiation. Moreover, Arc−/− mice lacked stimulus-selective response potentiation. Although Arc−/− mice exhibited normal visual acuity, baseline ocular dominance was abnormal and resembled that observed after dark-rearing. These data suggest that Arc is required for the experience-dependent processes that normally establish and modify synaptic connections in visual cortex.

SRF binding to SRE 6.9 in the Arc promoter is essential for LTD in cultured Purkinje cells

It has been suggested that gene expression and protein synthesis are required for both long-term memory consolidation and late phases of long-term potentiation and long-term depression (LTD). The necessary genes and the specific transcription factor binding sites in their promoters remain unknown. We found that inhibition of the transcription factor SRF or its cofactor MAL blocked the late phase of LTD in mouse cultured cerebellar Purkinje cells, as did deletion of the immediate early gene Arc. Using neuronal bacterial artificial chromosome (BAC) transfection, we found that, in Arc−/− cells transfected with a wild-type Arc BAC, late-phase LTD was rescued. However, mutation of one SRF-binding site in the Arc promoter (SRE 6.9) blocked this rescue. Co-transfection of wild-type Arc and SRF engineered to bind mutated SRE 6.9 restored late-phase LTD in Arc−/−, SRE 6.9 mutant BAC cells. Thus, SRF binding to SRE 6.9 in the Arc promoter is required for the late phase of cerebellar LTD.

The Neuronal Gene Arc Encodes a Repurposed Retrotransposon Gag Protein that Mediates Intercellular RNA Transfer

The neuronal gene Arc is essential for long-lasting information storage in the mammalian brain, mediates various forms of synaptic plasticity, and has been implicated in neurodevelopmental disorders. However, little is known about Arcs molecular function and evolutionary origins. Here, we show that Arc self-assembles into virus-like capsids that encapsulate RNA. Endogenous Arc protein is released from neurons in extracellular vesicles that mediate the transfer of Arc mRNA into new target cells, where it can undergo activity-dependent translation. Purified Arc capsids are endocytosed and are able to transfer Arc mRNA into the cytoplasm of neurons. These results show that Arc exhibits similar molecular properties to retroviral Gag proteins. Evolutionary analysis indicates that Arc is derived from a vertebrate lineage of Ty3/gypsy retrotransposons, which are also ancestors to retroviruses. These findings suggest that Gag retroelements have been repurposed during evolution to mediate intercellular communication in the nervous system.

Memory, Plasticity and Sleep - A role for calcium permeable AMPA receptors?

Experience shapes and molds the brain throughout life.These changes in neuronal circuits are produced by a myriad of molecular and cellular processes. Simplistically, circuits are modified through changes in neurotransmitter release or through neurotransmitter detection at synapses. The predominant neurotransmitter receptor in excitatory transmission, the AMPA-type glutamate receptor (AMPAR), is exquisitely sensitive to changes in experience and synaptic activity. These ion channels are usually impermeable to calcium, a property conferred by the GluA2 subunit. However, GluA2-lacking AMPARs are permeable to calcium and have recently been shown to play a unique role in synaptic function. In this review, I will describe new findings on the role of calcium permeable AMPARs (CP-AMPARs) in experience-dependent and synaptic plasticity.These studies suggest that CP-AMPARs play a prominent role in maintaining circuits in a labile state where further plasticity can occur, thus promoting metaplasticity. Moreover, the abnormal expression of CP-AMPARs has been implicated in drug addiction and memory disorders and thus may be a novel therapeutic target.

Simplification of unstructured tetrahedral meshes by point sampling

Tetrahedral meshes are widely used in scientific computing for representing 3D scalar, vector, and tensor fields. The size and complexity of some of these meshes can limit the performance of many visualization algorithms, making it hard to achieve interactive visualization. The use of simplified models is one way to enable the real-time exploration of these datasets. In this paper, we propose a novel technique for simplifying large unstructured meshes. Most current techniques simplify the geometry of the mesh using edge collapses. Our technique simplifies an underlying scalar field directly by segmenting the original scalar field into two pieces: the boundary of the original domain and the interior samples of the scalar field. We then simplify each piece separately, taking into account proper error bounds. Finally, we combine the simplified domain boundary and scalar field into a complete, simplified mesh that can be visualized with standard unstructured-data visualization tools. Our technique is much faster than edge-collapse-based simplification approaches. Furthermore, it is particularly suitable for aggressive simplification. Experiments show that isosurfaces and volume renderings of meshes produced by our technique have few noticeable visual artifacts.

Activity-dependent arc expression and homeostatic synaptic plasticity are altered in neurons from a mouse model of angelman syndrome

Angelman syndrome (AS) is a neurodevelopmental disorder that results from deletions or mutations in chromosome 15, which usually includes the UBE3A gene. Ube3A protein is an E3 ubiquitin ligase that ubiquitinates proteins and targets them for degradation. The immediate-early gene Arc, a master regulator of synaptic plasticity, was identified as a putative substrate of Ube3A, but there have been conflicting reports on whether Arc is a bona fide E3 ligase substrate. Using multiple approaches, we found no evidence for a physical interaction between Arc and Ube3A in vivo. Nonetheless, activity-induced subcellular distribution of Arc is altered in brains from Ube3am−/p+ mice, with abnormal concentration of Arc at synapses. Furthermore, although activation of Arc transcription is normal, the stability of Arc protein is enhanced in dendrites of hippocampal neurons cultured from Ube3am−/p+ mice. Finally, homeostatic synaptic scaling of surface AMPA receptors does not occur in Ube3am−/p+ hippocampal neurons, reminiscent of neurons that lack Arc protein. Although Ube3A does not seem to bind Arc in a canonical E3 ligase-substrate interaction, Arc-dependent synaptic plasticity is still altered in Ube3am−/p+ mice, which may underlie the cognitive deficits observed in AS.