Shira Knafo

PIP3 controls synaptic function by maintaining AMPA receptor clustering at the postsynaptic membrane

Despite their low abundance, phosphoinositides are critical regulators of intracellular signaling and membrane compartmentalization. However, little is known of phosphoinositide function at the postsynaptic membrane. Here we show that continuous synthesis and availability of phosphatidylinositol-(3,4,5)-trisphosphate (PIP3) at the postsynaptic terminal is necessary for sustaining synaptic function in rat hippocampal neurons. This requirement was specific for synaptic, but not extrasynaptic, AMPA receptors, nor for NMDA receptors. PIP3 downregulation impaired PSD-95 accumulation in spines. Concomitantly, AMPA receptors became more mobile and migrated from the postsynaptic density toward the perisynaptic membrane within the spine, leading to synaptic depression. Notably, these effects were only revealed after prolonged inhibition of PIP3 synthesis or by direct quenching of this phosphoinositide at the postsynaptic cell. Therefore, we conclude that a slow, but constant, turnover of PIP3 at synapses is required for maintaining AMPA receptor clustering and synaptic strength under basal conditions.

Widespread Changes in Dendritic Spines in a Model of Alzheimer's Disease

The mechanism by which dementia occurs in patients with Alzheimers disease (AD) is not known. We assessed changes in hippocampal dendritic spines of APP/PS1 transgenic mice that accumulate amyloid beta throughout the brain. Three-dimensional analysis of 21,507 dendritic spines in the dentate gyrus, a region crucial for learning and memory, revealed a substantial decrease in the frequency of large spines in plaque-free regions of APP/PS1 mice. Plaque-related dendrites also show striking alterations in spine density and morphology. However, plaques occupy only 3.9% of the molecular layer volume. Because large spines are considered to be the physical traces of long-term memory, widespread decrease in the frequency of large spines likely contributes to the cognitive impairments observed in this AD model.

PTEN is recruited to the postsynaptic terminal for NMDA receptor-dependent long-term depression

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) is an important regulator of phosphatidylinositol‐(3,4,5,)‐trisphosphate signalling, which controls cell growth and differentiation. However, PTEN is also highly expressed in the adult brain, in which it can be found in dendritic spines in hippocampus and other brain regions. Here, we have investigated specific functions of PTEN in the regulation of synaptic function in excitatory hippocampal synapses. We found that NMDA receptor activation triggers a PDZ‐dependent association between PTEN and the synaptic scaffolding molecule PSD‐95. This association is accompanied by PTEN localization at the postsynaptic density and anchoring within the spine. On the other hand, enhancement of PTEN lipid phosphatase activity is able to drive depression of AMPA receptor‐mediated synaptic responses. This activity is specifically required for NMDA receptor‐dependent long‐term depression (LTD), but not for LTP or metabotropic glutamate receptor‐dependent LTD. Therefore, these results reveal PTEN as a regulated signalling molecule at the synapse, which is recruited to the postsynaptic membrane upon NMDA receptor activation, and is required for the modulation of synaptic activity during plasticity.

PTEN recruitment controls synaptic and cognitive function in Alzheimer's models

Dyshomeostasis of amyloid-β peptide (Aβ) is responsible for synaptic malfunctions leading to cognitive deficits ranging from mild impairment to full-blown dementia in Alzheimers disease. Aβ appears to skew synaptic plasticity events toward depression. We found that inhibition of PTEN, a lipid phosphatase that is essential to long-term depression, rescued normal synaptic function and cognition in cellular and animal models of Alzheimers disease. Conversely, transgenic mice that overexpressed PTEN displayed synaptic depression that mimicked and occluded Aβ-induced depression. Mechanistically, Aβ triggers a PDZ-dependent recruitment of PTEN into the postsynaptic compartment. Using a PTEN knock-in mouse lacking the PDZ motif, and a cell-permeable interfering peptide, we found that this mechanism is crucial for Aβ-induced synaptic toxicity and cognitive dysfunction. Our results provide fundamental information on the molecular mechanisms of Aβ-induced synaptic malfunction and may offer new mechanism-based therapeutic targets to counteract downstream Aβ signaling.

Morphological alterations to neurons of the amygdala and impaired fear conditioning in a transgenic mouse model of Alzheimer's disease

Patients with Alzheimers disease (AD) suffer from impaired memory and emotional disturbances, the pathogenesis of which is not entirely clear. In APP/PS1 transgenic mice, a model of AD in which amyloid β (Aβ) accumulates in the brain, we have examined neurons in the lateral nucleus of the amygdala (LA), a brain region crucial to establish cued fear conditioning. We found that although there was no neuronal loss in this region and Aβ plaques only occupy less than 1% of its volume, these mice froze for shorter times after auditory fear conditioning when compared to their non‐transgenic littermates. We performed a three‐dimensional analysis of projection neurons and of thousands of dendritic spines in the LA. We found changes in dendritic tree morphology and a substantial decrease in the frequency of large spines in plaque‐free neurons of APP/PS1 mice. We suggest that these morphological changes in the neurons of the LA may contribute to the impaired auditory fear conditioning seen in this AD model. Copyright

Facilitation of AMPA receptor synaptic delivery as a molecular mechanism for cognitive enhancement

A small peptide from a neuronal cell adhesion molecule enhances synaptic plasticity in the hippocampus and results in improved cognitive performance in rats.

Dynamics of learning‐induced spine redistribution along dendrites of pyramidal neurons in rats

We have previously shown that olfactory‐discrimination (OD) learning is accompanied by enhanced spine density along proximal apical dendrites of layer II pyramidal neurons in the piriform (olfactory) cortex. Here we studied the temporal dynamics of learning‐induced modifications in dendritic spine density throughout the dendritic trees of these neurons. We observed a transient increase in proximal apical spine density after OD learning, suggesting a strengthening of intrinsic excitatory inputs interconnecting neurons within the olfactory cortex. By contrast, the afferent pathway receiving direct input from the olfactory bulb shows spine pruning, suggesting that the connectivity is weakened. The changes in spine density can be attributed to a net change in number of spines, as the morphometric parameters of the dendrites are unaffected by learning. We suggest that spine density changes may represent a mechanism of selective synaptic reorganization required for olfactory learning consolidation.

Layer‐specific alterations to CA1 dendritic spines in a mouse model of Alzheimer's disease

Why memory is a particular target for the pathological changes in Alzheimers Disease (AD) has long been a fundamental question when considering the mechanisms underlying this disease. It has been established from numerous biochemical and morphological studies that AD is, at least initially, a consequence of synaptic malfunction provoked by Amyloid β (Aβ) peptide. APP/PS1 transgenic mice accumulate Aβ throughout the brain, and they have therefore been employed to investigate the effects of Aβ overproduction on brain circuitry and cognition. Previous studies show that Aβ overproduction affects spine morphology in the hippocampus and amygdala, both within and outside plaques (Knafo et al., (2009) Cereb Cortex 19:586‐592; Knafo et al., (in press) J Pathol). Hence, we conducted a detailed analysis of dendritic spines located in the stratum oriens and stratum radiatum of the CA1 hippocampal subfield of APP/PS1 mice. Three‐dimensional analysis of 18,313 individual dendritic spines revealed a substantial layer‐specific decrease in spine neck length and an increase in the frequency of spines with a small head volume. Since dendritic spines bear most of the excitatory synapses in the brain, changes in spine morphology may be one of the factors contributing to the cognitive impairments observed in this AD model.

Olfactory learning-induced morphological modifications in single dendritic spines of young rats.

Learning‐related morphological modifications in single dendritic spines were studied quantitatively in the brains of young Sprague–Dawley rats. We have previously shown that olfactory discrimination rule‐learning results in transient physiological and morphological modifications in piriform cortex pyramidal neurons. In particular, spine density along the apical dendrites of neurons from trained rats is increased after learning. The aim of the present study was to identify and describe olfactory learning‐induced modifications in the morphology of single spines along apical dendrites of the same type of neurons. By using laser‐scanning confocal microscopy, we show that 3 days after training completion spines on neurons from olfactory discrimination trained rats are shorter as compared to spines on neurons from control rats. Further analysis revealed that spine shortening attributed to olfactory discrimination learning derives from shortening of spine head and not from shortening of spine neck. In addition, detailed analysis of spine head volume suggests that spines with large heads are absent after learning. As spine head size may be related to the efficacy of the synapse it bears, we suggest that modifications in spine head dimensions following olfactory rule‐learning enhance the cortical network ability to enter into a ‘learning mode’, in which memories of new odours can be acquired rapidly and efficiently.

Common pathways for growth and for plasticity

Cell growth and differentiation in developing tissues are, at first impression, quite different endeavors from readjusting synaptic strength during activity-dependent synaptic plasticity in mature neurons. Nevertheless, it is becoming increasingly clear that these two distinct processes share multiple intracellular signaling events. How these common pathways result in cell division (during proliferation), large-scale cellular remodeling (during differentiation) or synapse-specific changes (during synaptic plasticity) is only starting to be elucidated. Here we review the latest findings on two prototypical examples of these shared mechanisms: the Ras-PI3K pathway and the intracellular signaling elicited by neural cell adhesion molecules interacting with growth factor receptors.