Helmut W. Kessels

Synaptic AMPA Receptor Plasticity and Behavior

The ability to change behavior likely depends on the selective strengthening and weakening of brain synapses. The cellular models of synaptic plasticity, long-term potentiation (LTP) and depression (LTD) of synaptic strength, can be expressed by the synaptic insertion or removal of AMPA receptors (AMPARs), respectively. We here present an overview of studies that have used animal models to show that such AMPAR trafficking underlies several experience-driven phenomena-from neuronal circuit formation to the modification of behavior. We argue that monitoring and manipulating synaptic AMPAR trafficking represents an attractive means to study cognitive function and dysfunction in animal models.

Amyloid beta from axons and dendrites reduces local spine number and plasticity

Excessive synaptic loss is thought to be one of the earliest events in Alzheimers disease. Amyloid beta (Aβ), a peptide secreted in an activity-modulated manner by neurons, has been implicated in the pathogenesis of Alzheimers disease by removing dendritic spines, sites of excitatory synaptic transmission. However, issues regarding the subcellular source of Aβ, as well as the mechanisms of its production and actions that lead to synaptic loss, remain poorly understood. In rat organotypic slices, we found that acute overproduction of either axonal or dendritic Aβ reduced spine density and plasticity at nearby (∼5–10 μm) dendrites. The production of Aβ and its effects on spines were sensitive to blockade of action potentials or nicotinic receptors; the effects of Aβ (but not its production) were sensitive to NMDA receptor blockade. Notably, only 30–60 min blockade of Aβ overproduction permitted induction of plasticity. Our results indicate that continuous overproduction of Aβ at dendrites or axons acts locally to reduce the number and plasticity of synapses.

The prion protein as a receptor for amyloid-beta.

Arising from: J. Laurén et al. 457, 1128–1132 (2009)10.1038/nature07761; Laurén et al. replyIncreased levels of brain amyloid-β, a secreted peptide cleavage product of amyloid precursor protein (APP), is believed to be critical in the aetiology of Alzheimer’s disease. Increased amyloid-β can cause synaptic depression, reduce the number of spine protrusions (that is, sites of synaptic contacts) and block long-term synaptic potentiation (LTP), a form of synaptic plasticity; however, the receptor through which amyloid-β produces these synaptic perturbations has remained elusive. Laurén et al. suggested that binding between oligomeric amyloid-β (a form of amyloid-β thought to be most active) and the cellular prion protein (PrPC) is necessary for synaptic perturbations. Here we show that PrPC is not required for amyloid-β-induced synaptic depression, reduction in spine density, or blockade of LTP; our results indicate that amyloid-β-mediated synaptic defects do not require PrPc.

GluR1 Links Structural and Functional Plasticity at Excitatory Synapses

Long-term potentiation (LTP), a cellular model of learning and memory, produces both an enhancement of synaptic function and an increase in the size of the associated dendritic spine. Synaptic insertion of AMPA receptors is known to play an important role in mediating the increase in synaptic strength during LTP, whereas the role of AMPA receptor trafficking in structural changes remains unexplored. Here, we examine how the cell maintains the correlation between spine size and synapse strength during LTP. We found that cells exploit an elegant solution by linking both processes to a single molecule: the AMPA-type glutamate receptor subunit 1 (GluR1). Synaptic insertion of GluR1 is required to permit a stable increase in spine size, both in hippocampal slice cultures and in vivo. Synaptic insertion of GluR1 is not sufficient to drive structural plasticity. Although crucial to the expression of LTP, the ion channel function of GluR1 is not required for the LTP-driven spine size enhancement. Remarkably, a recombinant cytosolic C-terminal fragment (C-tail) of GluR1 is driven to the postsynaptic density after an LTP stimulus, and the synaptic incorporation of this isolated GluR1 C-tail is sufficient to permit spine enlargement even when postsynaptic exocytosis of endogenous GluR1 is blocked. We conclude that during plasticity, synaptic insertion of GluR1 has two functions: the established role of increasing synaptic strength via its ligand-gated ion channel, and a novel role through the structurally stabilizing effect of its C terminus that permits an increase in spine size.

Metabotropic NMDA receptor function is required for NMDA receptor-dependent long-term depression

NMDA receptor (NMDAR) activation controls long-term potentiation (LTP) as well as long-term depression (LTD) of synaptic transmission, cellular models of learning and memory. A long-standing view proposes that a high level of Ca2+ entry through NMDARs triggers LTP; lower Ca2+ entry triggers LTD. Here we show that ligand binding to NMDARs is sufficient to induce LTD; neither ion flow through NMDARs nor Ca2+ rise is required. However, basal levels of Ca2+ are permissively required. Lowering, but not maintaining, basal Ca2+ levels with Ca2+ chelators blocks LTD and drives strong synaptic potentiation, indicating that basal Ca2+ levels control NMDAR-dependent LTD and basal synaptic transmission. Our findings indicate that metabotropic actions of NMDARs can weaken active synapses without raising postsynaptic calcium, thereby revising and expanding the mechanisms controlling synaptic plasticity.

Metabotropic NMDA receptor function is required for β-amyloid–induced synaptic depression

The mechanisms by which β-amyloid (Aβ), a peptide fragment believed to contribute to Alzheimer’s disease, leads to synaptic deficits are not known. Here we find that elevated oligomeric Aβ requires ion flux-independent function of NMDA receptors (NMDARs) to produce synaptic depression. Aβ activates this metabotropic NMDAR function on GluN2B-containing NMDARs but not on those containing GluN2A. Furthermore, oligomeric Aβ leads to a selective loss of synaptic GluN2B responses, effecting a switch in subunit composition from GluN2B to GluN2A, a process normally observed during development. Our results suggest that conformational changes of the NMDAR, and not ion flow through its channel, are required for Aβ to produce synaptic depression and a switch in NMDAR composition. This Aβ-induced signaling mediated by alterations in GluN2B conformation may be a target for therapeutic intervention of Alzheimer’s disease.

Serotonin mediates cross-modal reorganization of cortical circuits

Loss of one type of sensory input can cause improved functionality of other sensory systems. Whereas this form of plasticity, cross-modal plasticity, is well established, the molecular and cellular mechanisms underlying it are still unclear. Here, we show that visual deprivation (VD) increases extracellular serotonin in the juvenile rat barrel cortex. This increase in serotonin levels facilitates synaptic strengthening at layer 4 to layer 2/3 synapses within the barrel cortex. Upon VD, whisker experience leads to trafficking of the AMPA-type glutamate receptors (AMPARs) into these synapses through the activation of ERK and increased phosphorylation of AMPAR subunit GluR1 at the juvenile age when natural whisker experience no longer induces synaptic GluR1 delivery. VD thereby leads to sharpening of the functional whisker-barrel map at layer 2/3. Thus, sensory deprivation of one modality leads to serotonin release in remaining modalities, facilitates GluR1-dependent synaptic strengthening, and refines cortical organization.

Roles of stargazin and phosphorylation in the control of AMPA receptor subcellular distribution

Understanding how the subcellular fate of newly synthesized AMPA receptors (AMPARs) is controlled is important for elucidating the mechanisms of neuronal function. We examined the effect of increased synthesis of AMPAR subunits on their subcellular distribution in rat hippocampal neurons. Virally expressed AMPAR subunits (GluR1 or GluR2) accumulated in cell bodies and replaced endogenous dendritic AMPAR with little effect on total dendritic amounts and caused no change in synaptic transmission. Coexpressing stargazin (STG) or mimicking GluR1 phosphorylation enhanced dendritic GluR1 levels by protecting GluR1 from lysosomal degradation. However, STG interaction or GluR1 phosphorylation did not increase surface or synaptic GluR1 levels. Unlike GluR1, STG did not protect GluR2 from lysosomal degradation or increase dendritic GluR2 levels. In general, AMPAR surface levels, and not intracellular amounts, correlated strongly with synaptic levels. Our results suggest that AMPAR surface expression, but not its intracellular production or accumulation, is critical for regulating synaptic transmission.

A robust automated method to analyze rodent motion during fear conditioning

A central question in the study of LTP has been to determine what role it plays in memory formation and storage. One valuable form of learning for addressing this issue is associative fear conditioning. In this paradigm an animal learns to associate a tone and shock, such that subsequent presentation of a tone evokes a fear response (freezing behavior). Recent studies indicate that overlapping cellular processes underlie fear conditioning and LTP. The fear response has generally been scored manually which is both labor-intensive and subject to potential artifacts such as inconsistent or biased results. Here we describe a simple automated method that provides unbiased and rapid analysis of animal motion. We show that measured motion, in units termed significant motion pixels (SMPs), is both linear and robust over a wide range of animal speeds and detection thresholds and scores freezing in a quantitatively similar manner to trained human observers. By comparing the frequency distribution of motion during baseline periods and to the response to fox urine (which causes unconditioned fear), we suggest that freezing and non-freezing are distinct behaviors. Finally, we show how this algorithm can be applied to a fear conditioning paradigm yielding information on long and short-term associative memory as well as habituation. This automated analysis of fear conditioning will permit a more rapid and accurate assessment of the role of LTP in memory.

Synapto-depressive effects of amyloid beta require PICK1

Amyloid beta (Aβ), a key component in the pathophysiology of Alzheimers disease, is thought to target excitatory synapses early in the disease. However, the mechanism by which Aβ weakens synapses is not well understood. Here we showed that the PDZ domain protein, protein interacting with C kinase 1 (PICK1), was required for Aβ to weaken synapses. In mice lacking PICK1, elevations of Aβ failed to depress synaptic transmission in cultured brain slices. In dissociated cultured neurons, Aβ failed to reduce surface α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor subunit 2, a subunit of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptors that binds with PICK1 through a PDZ ligand–domain interaction. Lastly, a novel small molecule (BIO922) discovered through structure‐based drug design that targets the specific interactions between GluA2 and PICK1 blocked the effects of Aβ on synapses and surface receptors. We concluded that GluA2–PICK1 interactions are a key component of the effects of Aβ on synapses.