Laminar distribution of synaptopodin in normal and reeler mouse brain depends on the position of spine-bearing neurons

Maternal separation in mice leads to anxiety-like/aggressive behavior and increases immunoreactivity for glutamic acid decarboxylase and parvalbumin in the adolescence ventral hippocampus

It has been reported that stressful events in early life influence behavior in adulthood and are associated with different psychiatric disorders, such as major depression, post-traumatic stress disorder, bipolar disorder, and anxiety disorder. Maternal separation (MS) is a representative animal model for reproducing childhood stress. It is used as an animal model for depression, and has well-known effects, such as increasing anxiety behavior and causing abnormalities in the hypothalamic-pituitary-adrenal (HPA) axis. This study investigated the effect of MS on anxiety or aggression-like behavior and the number of GABAergic neurons in the hippocampus. Mice were separated from their dams for four hours per day for 19 d from postnatal day two. Elevated plus maze (EPM) test, resident-intruder (RI) test, and counted glutamic acid decarboxylase 67 (GAD67) or parvalbumin (PV) positive cells in the hippocampus were executed using immunohistochemistry. The maternal segregation group exhibited increased anxiety and aggression in the EPM test and the RI test. GAD67-positive neurons were increased in the hippocampal regions we observed: dentate gyrus (DG), CA3, CA1, subiculum, presubiculum, and parasubiculum. PV-positive neurons were increased in the DG, CA3, presubiculum, and parasubiculum. Consistent with behavioral changes, corticosterone was increased in the MS group, suggesting that the behavioral changes induced by MS were expressed through the effect on the HPA axis. Altogether, MS alters anxiety and aggression levels, possibly through alteration of cytoarchitecture and output of the ventral hippocampus that induces the dysfunction of the HPA axis.

Ultrastructure of geniculocortical synaptic connections in the tree shrew striate cortex

To determine whether thalamocortical synaptic circuits differ across cortical areas, we examined the ultrastructure of geniculocortical terminals in the tree shrew striate cortex to compare directly the characteristics of these terminals with those of pulvinocortical terminals (examined previously in the temporal cortex of the same species; Chomsung et al. [] Cereb Cortex 20:997-1011). Tree shrews are considered to represent a prototype of early prosimian primates but are unique in that sublaminae of striate cortex layer IV respond preferentially to light onset (IVa) or offset (IVb). We examined geniculocortical inputs to these two sublayers labeled by tracer or virus injections or an antibody against the type 2 vesicular glutamate antibody (vGLUT2). We found that layer IV geniculocortical terminals, as well as their postsynaptic targets, were significantly larger than pulvinocortical terminals and their postsynaptic targets. In addition, we found that 9-10% of geniculocortical terminals in each sublamina contacted GABAergic interneurons, whereas pulvinocortical terminals were not found to contact any interneurons. Moreover, we found that the majority of geniculocortical terminals in both IVa and IVb contained dendritic protrusions, whereas pulvinocortical terminals do not contain these structures. Finally, we found that synaptopodin, a protein uniquely associated with the spine apparatus, and telencephalin (TLCN, or intercellular adhesion molecule type 5), a protein associated with maturation of dendritic spines, are largely excluded from geniculocortical recipient layers of the striate cortex. Together our results suggest major differences in the synaptic organization of thalamocortical pathways in striate and extrastriate areas.

Synaptopodin and the spine apparatus organelle-regulators of different forms of synaptic plasticity?

Synaptopodin (SP) is an actin-binding molecule, which is closely linked with the spine apparatus organelle (SA). Recent experimental evidence suggests that SP containing spines differ in their functional and structural properties from neighboring spines, which do not contain SP. These studies revealed for the first time that SP clusters colocalize with a functional internal source of calcium, which affects synaptic plasticity. Strikingly, SP-cluster associated calcium surges were shown to control synaptic strength in two ways: a ryanodine receptor (RyR) dependent potentiation of synaptic strength was reported, as well as inositol-triphosphate-receptor (IP3R) dependent depression. These results suggested that the SA is an important component of the molecular machinery controlling the calcium-dependent accumulation of AMPA-receptors (AMPA-R) at excitatory synapses. They raise the intriguing possibility that SP/SA could play a role in different forms of synaptic plasticity.

Anatomical localization of protease-activated receptor-1 and protease-mediated neuroglial crosstalk on peri-synaptic astrocytic endfeet

We studied the localization, activation and function of protease-activated receptor 1 (PAR-1) at the CNS synapse utilizing rat brain synaptosomes and slices. Confocal immunofluoresence and transmission electron microscopy in brain slices with pre-embedding diaminobenzidine (DAB) immunostaining found PAR-1 predominantly localized to the peri-synaptic astrocytic endfeet. Structural confocal immunofluorescence microscopy studies of isolated synaptosomes revealed spherical structures stained with anti-PAR-1 antibody which co-stained mainly for glial-filament acidic protein compared with the neuronal markers synaptophysin and PSD-95. Immunoblot studies of synaptosomes demonstrated an appropriate major band corresponding to PAR-1 and activation of the receptor by a specific agonist peptide (SFLLRN) significantly modulated phosphorylated extracellular signal-regulated kinase. A significant membrane potential depolarization was produced by thrombin (1 U/mL) and the PAR-1 agonist (100 μM) and depolarization by high K(+) elevated extracellular thrombin-like activity in the synaptosomes preparation. The results indicate PAR-1 localized to the peri-synaptic astrocytic endfeet is most likely activated by synaptic proteases and induces cellular signaling and modulation of synaptic electrophysiology. A protease mediated neuron-glia pathway may be important in both physiological and pathological regulation of the synapse.

Synaptopodin regulates plasticity of dendritic spines in hippocampal neurons

The spine apparatus is an essential component of dendritic spines of cortical and hippocampal neurons, yet its functions are still enigmatic. Synaptopodin (SP), an actin-binding protein, is tightly associated with the spine apparatus and it may play a role in synaptic plasticity, but it has not yet been linked mechanistically to synaptic functions. We studied endogenous and transfected SP in dendritic spines of cultured hippocampal neurons and found that spines containing SP generate larger responses to flash photolysis of caged glutamate than SP-negative ones. An NMDA-receptor-mediated chemical long-term potentiation caused the accumulation of GFP-GluR1 in spine heads of control but not of shRNA-transfected, SP-deficient neurons. SP is linked to calcium stores, because their pharmacological blockade eliminated SP-related enhancement of glutamate responses, and release of calcium from stores produced an SP-dependent increase of GluR1 in spines. Thus, SP plays a crucial role in the calcium store-associated ability of neurons to undergo long-term plasticity.

Structural reorganization of the dentate gyrus following entorhinal denervation: species differences between rat and mouse

Deafferentation of the dentate gyrus by unilateral entorhinal cortex lesion or unilateral perforant pathway transection is a classical model to study the response of the central nervous system (CNS) to denervation. This model has been extensively characterized in the rat to clarify mechanisms underlying denervation-induced gliosis, transneuronal degeneration of denervated neurons, and collateral sprouting of surviving axons. As a result, candidate molecules have been identified which could regulate these changes, but a causal link between these molecules and the postlesional changes has not yet been demonstrated. To this end, mutant mice are currently studied by many groups. A tacit assumption is that data from the rat can be generalized to the mouse, and fundamental species differences in hippocampal architecture and the fiber systems involved in sprouting are often ignored. In this review, we will (1) provide an overview of some of the basics and technical aspects of the entorhinal denervation model, (2) identify anatomical species differences between rats and mice and will point out their relevance for the axonal reorganization process, (3) describe glial and local inflammatory changes, (4) consider transneuronal changes of denervated dentate neurons and the potential role of reactive glia in this context, and (5) summarize the differences in the reorganization of the dentate gyrus between the two species. Finally, we will discuss the use of the entorhinal denervation model in mutant mice.

A role for the spine apparatus in LTP and spatial learning

Long-term potentiation (LTP) of synaptic strength is a long-lasting form of synaptic plasticity that has been linked to information storage. Although the molecular and cellular events underlying LTP are not yet fully understood, it is generally accepted that changes in dendritic spine calcium levels as well as local protein synthesis play a central role. These two processes may be influenced by the presence of a spine apparatus, a distinct neuronal organelle found in a subpopulation of telencephalic spines. Mice lacking spine apparatuses (synaptopodin-deficient mice) show deficits in LTP and impaired spatial learning supporting the involvement of the spine apparatus in synaptic plasticity. In our review, we consider the possible roles of the spine apparatus in LTP1 (protein synthesis-independent), LTP2 (translation-dependent and transcription-independent) and LTP3 (translation- and transcription-dependent) and discuss the effects of the spine apparatus on learning and memory.

A role for synaptopodin and the spine apparatus in hippocampal synaptic plasticity

Spines are considered sites of synaptic plasticity in the brain and are capable of remodeling their shape and size. A molecule thathas been implicated in spine plasticity is the actin-associated protein synaptopodin. This article will review a series of studies aimed at elucidating the role of synaptopodin in the rodent brain. First, the developmental expression of synaptopodin mRNA and protein were studied; secondly, the subcellular localization of synaptopodin in hippocampal principal neurons was analyzed using confocal microscopy as well as electron microscopy and immunogold labelling; and, finally, the functional role of synaptopodin was investigated using a synaptopodin-deficient mouse. The results of these studies are: (1) synaptopodin expression byhippocampal principal neurons develops during the first postnatal weeks and increases in parallel with the maturation of spines in the hippocampus. (2) Synaptopodin is sorted to the spine compartment, where it is tightly associated with the spine apparatus, an enigmatic organelle believed to be involved in calcium storage or local protein synthesis. (3) Synaptopodin-deficient mice generated by gene targeting are viable but lack the spine apparatus organelle. These mice show deficitsin synaptic plasticity as well as impaired learning and memory. Taken together, these data implicate synaptopodin and the spine apparatus in the regulation of synaptic plasticity in the hippocampus. Future studies will be aimed at finding the molecular link between synaptopodin, the spine apparatus organelle, and synaptic plasticity.

Loss of the cisternal organelle in the axon initial segment of cortical neurons in synaptopodin-deficient mice

The axon initial segment of cortical neurons contains the so-called cisternal organelle, an enigmatic formation of stacked endoplasmic reticulum and interdigitating plates of electron-dense material. This organelle shows many structural similarities to the spine apparatus, a cellular organelle found in a subpopulation of dendritic spines. Whereas roles in calcium signaling and protein trafficking have been proposed for the spine apparatus, little is yet known about the physiological function of its putative axonal counterpart. Considering the structural similarity of these two organelles, we hypothesized that synaptopodin, a protein essential for the formation of the dendritic spine apparatus, could also be a component of the cisternal organelle. By using immunofluorescence microscopy, we found that synaptopodin is indeed located within the axon initial segments of principal neurons in the mouse neocortex and hippocampus. Pre-embedding immunogold labeling demonstrated a close association of synaptopodin immunoreactivity with the dense plates of cisternal organelles. In synaptopodin-deficient mice, ultrastructural analysis of identified axon initial segments of CA1 pyramidal cells revealed a lack of cisternal organelles similar to the reported lack of spine apparatuses in these mutants. However, in vitro patch clamp recording of mutant neurons showed that the lack of cisternal organelles did not lead to any changes in basic electrophysiological parameters of action potentials. Taken together, our data demonstrate that synaptopodin is an essential component of the cisternal organelle of axons and of the dendritic spine apparatus, two organelles that are structurally and molecularly related.

Lamina-specific distribution of Synaptopodin, an actin-associated molecule essential for the spine apparatus, in identified principal cell dendrites of the mouse hippocampus

Synaptopodin is an actin-associated molecule found in a subset of telencephalic spines. It is an essential component of the spine apparatus, a Ca(2+)-storing organelle and has been implicated in synaptic plasticity (Deller et al. [2003] Proc Natl Acad Sci U S A 100:10494-10499). In the rodent hippocampus, Synaptopodin is distributed in a characteristic region- and lamina-specific manner. To learn more about the cellular basis underlying this distribution, the regional, laminar, and cellular localization of Synaptopodin and its mRNA were analyzed in mouse hippocampus. First, Synaptopodin puncta densities were quantified after immunofluorescent labeling using confocal microscopy. Second, the dendritic distribution of Synaptopodin-positive puncta was studied using three-dimensional confocal reconstructions of Synaptopodin-immunostained and enhanced green fluorescence protein (EGFP)-labeled principal neurons. Synaptopodin puncta located within dendrites of principal neurons were primarily found in spines (>95%). Analysis of dendritic segments located in different layers revealed lamina-specific differences in the percentage of Synaptopodin-positive spines. Densities ranged between 37% (outer molecular layer) and 14% (stratum oriens; CA1). Finally, synaptopodin mRNA expression was studied using in situ hybridization, laser microdissection, and quantitative reverse transcriptase-polymerase chain reaction. Expression levels were comparable between all regions. These data demonstrate a lamina-specific distribution of Synaptopodin within dendritic segments of identified neurons. Within dendrites, the majority of Synaptopodin-positive puncta were located in spines where they represent spine apparatuses. We conclude, that this organelle is distributed in a region- and layer-specific manner in the mouse hippocampus and suggest that differences in the activity of afferent fiber systems could determine its distribution.