Structure-function relations in dendritic spines: is size important?

Dynamic Regulation of Mitochondrial [Ca2+] in Hippocampal Neurons

While neuronal mitochondria have been studied extensively in their role in health and disease, the rules that govern calcium regulation in mitochondria remain somewhat vague. In the present study using cultured rat hippocampal neurons transfected with the mtRCaMP mitochondrial calcium sensor, we investigated the effects of cytosolic calcium surges on the dynamics of mitochondrial calcium ([Ca²⁺]m). Cytosolic calcium ([Ca²⁺]c) was measured using the high affinity sensor Fluo-2. We recorded two types of calcium events: local and global ones. Local events were limited to a small, 2–5 µm section of the dendrite, presumably caused by local synaptic activity, while global events were associated with network bursts and extended throughout the imaged dendrite. In both cases, cytosolic surges were followed by a delayed rise in [Ca²⁺]m. In global events, the rise lasted longer and was observed in all mitochondrial clusters. At the end of the descending part of the global event, [Ca²⁺]m was still high. Global events were accompanied by short and rather high [Ca²⁺]m surges which we called spikelets, and were present until the complete decay of the cytosolic event. In the case of local events, selective short-term responses were limited to the part of the mitochondrial cluster that was located directly in the center of [Ca²⁺]c activity, and faded quickly, while responses in the neighboring regions were rarely observed. Caffeine (which recruits ryanodine receptors to supply calcium to the mitochondria), and carbonyl cyanide m-chlorophenyl hydrazine (CCCP, a mitochondrial uncoupler) could affect [Ca²⁺]m in both global and local events. We constructed a computational model to simulate the fundamental role of mitochondria in restricting calcium signals within a narrow range under synapses, preventing diffusion into adjacent regions of the dendrite. Our results indicate that local cytoplasmic and mitochondrial calcium concentrations are highly correlated. This reflects a key role of signaling pathways that connect the postsynaptic membrane to local mitochondrial clusters.

Beta-Site Amyloid Precursor Protein-Cleaving Enzyme Inhibition Partly Restores Sevoflurane-Induced Deficits on Synaptic Plasticity and Spine Loss

Evidence indicates that inhalative anesthetics enhance the β-site amyloid precursor protein (APP)-cleaving enzyme (BACE) activity, increase amyloid beta 1-42 (Aβ_1–42) aggregation, and modulate dendritic spine dynamics. However, the mechanisms of inhalative anesthetics on hippocampal dendritic spine plasticity and BACE-dependent APP processing remain unclear. In this study, hippocampal slices were incubated with equipotent isoflurane (iso), sevoflurane (sevo), or xenon (Xe) with/without pretreatment of the BACE inhibitor LY2886721 (LY). Thereafter, CA1 dendritic spine density, APP processing-related molecule expressions, nectin-3 levels, and long-term potentiation (LTP) were tested. The nectin-3 downregulation on LTP and dendritic spines were evaluated. Sevo treatment increased hippocampal mouse Aβ_1–42 (mAβ_1–42), abolished CA1-LTP, and decreased spine density and nectin-3 expressions in the CA1 region. Furthermore, CA1-nectin-3 knockdown blocked LTP and reduced spine density. Iso treatment decreased spine density and attenuated LTP. Although Xe blocked LTP, it did not affect spine density, mAβ_1–42, or nectin-3. Finally, antagonizing BACE activity partly restored sevo-induced deficits. Taken together, our study suggests that sevo partly elevates BACE activity and interferes with synaptic remodeling, whereas iso mildly modulates synaptic changes in the CA1 region of the hippocampus. On the other hand, Xe does not alternate dendritic spine remodeling.

Nanoscale molecular architecture controls calcium diffusion and ER replenishment in dendritic spines

Dendritic spines are critical components of neuronal synapses as they receive and transform synaptic inputs into a succession of calcium-regulated biochemical events. The spine apparatus (SA), an extension of smooth endoplasmic reticulum, regulates slow and fast calcium dynamics in spines. Calcium release events deplete SA calcium ion reservoir rapidly, yet the next cycle of signaling requires its replenishment. How spines achieve this replenishment without triggering calcium release remains unclear. Using computational modeling, calcium and STED superresolution imaging, we show that the SA replenishment involves the store-operated calcium entry pathway during spontaneous calcium transients. We identified two main conditions for SA replenishment without depletion: a small amplitude and a slow timescale for calcium influx, and a close proximity between SA and plasma membranes. Thereby, spine’s nanoscale organization separates SA replenishment from depletion. We further conclude that spine’s receptor organization also determines the calcium dynamics during the induction of long-term synaptic changes.

Chronic Fluoxetine Induces the Enlargement of Perforant Path-Granule Cell Synapses in the Mouse Dentate Gyrus

A selective serotonin reuptake inhibitor is the most commonly prescribed antidepressant for the treatment of major depression. However, the mechanisms underlying the actions of selective serotonin reuptake inhibitors are not fully understood. In the dentate gyrus, chronic fluoxetine treatment induces increased excitability of mature granule cells (GCs) as well as neurogenesis. The major input to the dentate gyrus is the perforant path axons (boutons) from the entorhinal cortex (layer II). Through voltage-sensitive dye imaging, we found that the excitatory neurotransmission of the perforant path synapse onto the GCs in the middle molecular layer of the mouse dentate gyrus (perforant path-GC synapse) is enhanced after chronic fluoxetine treatment (15 mg/kg/day, 14 days). Therefore, we further examined whether chronic fluoxetine treatment affects the morphology of the perforant path-GC synapse, using FIB/SEM (focused ion beam/scanning electron microscopy). A three-dimensional reconstruction of dendritic spines revealed the appearance of extremely large-sized spines after chronic fluoxetine treatment. The large-sized spines had a postsynaptic density with a large volume. However, chronic fluoxetine treatment did not affect spine density. The presynaptic boutons that were in contact with the large-sized spines were large in volume, and the volumes of the mitochondria and synaptic vesicles inside the boutons were correlated with the size of the boutons. Thus, the large-sized perforant path-GC synapse induced by chronic fluoxetine treatment contains synaptic components that correlate with the synapse size and that may be involved in enhanced glutamatergic neurotransmission.

Effect of tibolone on dendritic spine density in the rat hippocampus

INTRODUCTION

Oestrogen deficiency produces oxidative stress (OS) and changes in hippocampal neurons and also reduces the density of dendritic spines (DS). These alterations affect the plastic response of the hippocampus. Oestrogen replacement therapy reverses these effects, but it remains to be seen whether the same changes are produced by tibolone (TB). The aim of this study was to test the neuroprotective effects of long-term oral TB treatment and its ability to reverse DS pruning in pyramidal neurons (PN) of hippocampal area CA1.

METHODS

Young Sprague Dawley rats were distributed in 3 groups: a control group in proestrus (Pro) and two ovariectomised groups (Ovx), of which one was provided with a daily TB dose (1mg/kg), OvxTB and the other with vehicle (OvxV), for 40 days in both cases. We analysed lipid peroxidation and DS density in 3 segments of apical dendrites from PNs in hippocampal area CA1.

RESULTS

TB did not reduce lipid peroxidation but it did reverse the spine pruning in CA1 pyramidal neurons of the hippocampus which had been caused by ovariectomy.

CONCLUSIONS

Oestrogen replacement therapy for ovariectomy-induced oestrogen deficiency has a protective effect on synaptic plasticity in the hippocampus.

Selective estrogen receptor modulators regulate dendritic spine plasticity in the hippocampus of male rats

Some selective estrogen receptor modulators, such as raloxifene and tamoxifen, are neuroprotective and reduce brain inflammation in several experimental models of neurodegeneration. In addition, raloxifene and tamoxifen counteract cognitive deficits caused by gonadal hormone deprivation in male rats. In this study, we have explored whether raloxifene and tamoxifen may regulate the number and geometry of dendritic spines in CA1 pyramidal neurons of the rat hippocampus. Young adult male rats were injected with raloxifene (1 mg/kg), tamoxifen (1 mg/kg), or vehicle and killed 24 h after the injection. Animals treated with raloxifene or tamoxifen showed an increased numerical density of dendritic spines in CA1 pyramidal neurons compared to animals treated with vehicle. Raloxifene and tamoxifen had also specific effects in the morphology of spines. These findings suggest that raloxifene and tamoxifen may influence the processing of information by hippocampal pyramidal neurons by affecting the number and shape of dendritic spines.

Increase of mushroom spine density in CA1 apical dendrites produced by water maze training is prevented by ovariectomy

Dendritic spine density increases after spatial learning in hippocampal CA1 pyramidal neurons. Gonadal activity also regulates spine density, and abnormally low levels of circulating estrogens are associated with deficits in hippocampus-dependent tasks. To determine if gonadal activity influences behaviorally induced structural changes in CA1, we performed a morphometric analysis on rapid Golgi-stained tissue from ovariectomized (Ovx) and sham-operated (Sham) female rats 7 days after they were given a single water maze (WM) training session (hidden platform procedure) or a swimming session in the tank containing no platform (SC). We evaluated the density of different dendritic spine types (stubby, thin, and mushroom) in three segments (distal, medial, and proximal) of the principal apical dendrite from hippocampal CA1 pyramidal neurons. Performance in the WM task was impaired in Ovx animals compared to Sham controls. Total spine density increased after WM in Sham animals in the proximal and distal CA1 apical dendrite segments but not in the medial. Interestingly, mushroom spine density consistently increased in all CA1 segments after WM. As compared to the Sham group, SC-Ovx rats showed spine pruning in all the segments, but mushroom spine density did not change significantly. In Ovx rats, WM training increased the density of stubby and thin, but not mushroom spines. Thus, ovariectomy alone produces spine pruning, while spatial learning increases spine density in spite of ovariectomy. Finally, the results suggest that mushroom spine production in CA1 after spatial learning requires gonadal activity, whereas this activity is not required for mushroom spine maintenance.

Automated quantification of dendritic spine density and spine head diameter in medium spiny neurons of the nucleus accumbens

Dendritic spines are postsynaptic specializations thought to regulate the strength of synaptic transmission and play a critical role in neuronal plasticity. While changes in dendritic spine density can be pharmacologically- or environmentally-induced, the widespread utility of this important measure of synaptic plasticity in vivo has been hampered by the labor-intensive nature, and potential for bias and inconsistency inherent in manual spine counting. Here we report a method for obtaining high-resolution, three-dimensional confocal images of accumbens spiny neurons labeled with a diolistically delivered lipophilic fluorescence dye (DiI) that permits automated analysis of spine density and spine head diameter. The automated quantification was verified by manual counts of spine density and electron microscopic measures of spine head diameter. The density of spines was relatively constant over 2nd to 4th order dendrites within a neuron, and spine density was normally distributed. The mean spine density (2.68 spines/microm; N = 45 neurons) was higher than previous reports, due in part to analysis in three rather than two dimensions and the capacity of lipophilic dyes to fill very thin spines. The distribution of spine head diameters was continuous and skewed to the right (mean = 0.43 microm; N = 8,891), and approximately 25% of all spines were thin and filopodia-like (< or = 0.20 microm diameter). The density of spines was not correlated with average spine head diameter or with the number of filopodia-like spines. The capacity to rapidly assess spine density and spine head diameter will facilitate quantifying spine plasticity induced by pharmacological and environmental manipulations.

Activation of alpha 2A adrenoceptors alters dendritic spine development and the expression of spinophilin in cultured cortical neurones

alpha2 adrenoceptors have been shown to regulate the development of dendrites in mammalian cortical neurones. In this study we have investigated how agonists of alpha2 adrenoceptors affect length and density of dendritic spines in cultured cortical neurones from C57/B6 mice. A twenty-four hour incubation of 14 day old cultured neurones with UK 14304, an alpha2-adrenoceptor agonist, resulted in a significant increase in the average length and density of dendritic spines. Furthermore, incubation of neurones with the selective alpha 2A agonist guanfacine resulted in 1.2-fold increase in spine length and 1.8-fold increase in spine density. These effects were blocked by RX 821002 and BRL 44408, alpha2- and alpha 2A-adrenoceptor antagonists, respectively. The observed changes in the density and length of dendritic spines were correlated with increased expression of spinophilin, a key cytoskeletal protein in the formation and maintenance of dendritic spines, and a decrease in the phosphorylation of spinophilin on serine residues. The latter finding points to a possible mechanism by which adrenoceptors may regulate spinophilin function in dendritic spine development and structure in cortical neurones in vitro.

Long-term study of dendritic spines from hippocampal CA1 pyramidal cells, after neuroprotective melatonin treatment following global cerebral ischemia in rats

Melatonin reduces pyramidal neuronal death in the hippocampus and prevents the impairment of place learning and memory in the Morris water maze, otherwise occurring following global cerebral ischemia. The cytoarchitectonic characteristics of the hippocampal CA1 remaining pyramidal neurons in brains of rats submitted 120 days earlier to acute global cerebral ischemia (15-min four vessel occlusion, and melatonin 10mg/(kg h 6h), i.v. or vehicle administration) were compared to those of intact control rats in order to gain information concerning the neural substrate underlying preservation of hippocampal functioning. Hippocampi were processed according to a modification of the Golgi method. Dendritic bifurcations from pyramidal neurons in both the oriens-alveus and the striatum radiatum; as well as spine density and proportions of thin, stubby, mushroom-shaped, wide, ramified, and double spines in a 50 microm length segment of an oblique dendrite branching from the apical dendrite of the hippocampal CA1 remaining pyramidal neurons were evaluated. No impregnated CA1 pyramidal neurons were found in the ischemic-vehicle-treated rats. CA1 pyramidal neurons from ischemic-melatonin-treated rats showed stick-like and less ramified dendrites than those seen in intact control neurons. In addition, lesser density of spines, lower proportional density of thin spines, and higher proportional density of mushroom spines were counted in ischemic-melatonin-treated animals than those in the sinuously branched dendrites of the intact control group. These cytoarchitectural arrangements seem to be compatible with place learning and memory functions long after ischemia and melatonin neuroprotection.

Adaptive plasticity of NMDA receptors and dendritic spines: implications for enhanced vulnerability of the adolescent brain to alcohol addiction

It is now known that brain development continues into adolescence and early adulthood and is highly influenced by experience-dependent adaptive plasticity during this time. Behaviorally, this period is also characterized by increased novelty seeking and risk-taking. This heightened plasticity appears to be important in shaping behaviors and cognitive processes that contribute to proper development of an adult phenotype. However, increasing evidence has linked these same experience-dependent learning mechanisms with processes that underlie drug addiction. As such, the adolescent brain appears to be particularly susceptible to experience-dependent learning processes associated with consumption of alcohol and addictive drugs. At the level of the synapse, homeostatic changes during ethanol consumption are invoked to counter the destabilizing effects of ethanol on neural networks. This homeostatic response may be especially pronounced in the adolescent and young adult brain due to its heightened capacity to undergo experience-dependent changes, and appears to involve increased synaptic targeting of NMDA receptors. Interestingly, recent work from our lab also indicates that the enhanced synaptic localization of NMDA receptors promotes increases in the size of dendritic spines. This increase may represent a structural-based mechanism that supports the formation and stabilization of maladapted synaptic connections that, in a sense, "fix" the addictive behavior in the adolescent and young adult brain.

Homeostatic plasticity during alcohol exposure promotes enlargement of dendritic spines

Modifications of the size, shape and number of dendritic spines is thought to be an important component of activity-dependent changes of neuronal circuits, and may play an important role in the plasticity of drug addiction. The present study examined whether homeostatic increases in synaptic N-methyl-d-aspartate (NMDA) receptors in response to chronic ethanol exposure is associated with corresponding morphological changes in dendritic spines. Prolonged exposure of rat hippocampal cultures to either the NMDA receptor antagonist d(-)-2-amino-5-phosphono-pentanoic acid or to ethanol increased punctate staining of F-actin and the postsynaptic density protein-95 (PSD-95). The increase in dendritic F-actin occurred only with clusters that co-localized with PSD-95 clusters, indicating that these actin structures likely represent dendritic spines. The ethanol-induced increases in PSD-95 and F-actin clusters were activity-dependent and reversible. Finally, inhibition of protein palmitoylation prevented ethanol-induced increases in synaptic NMDA receptor clustering and F-actin without altering the basal clustering of either F-actin or PSD-95. These observations support a model in which chronic ethanol exposure induces homeostatic increases of NR2B-containing NMDA receptors and PSD-95 to the postsynaptic density. This in turn may provide a scaffolding platform for the subsequent recruitment of actin signaling cascades that alter actin cycling and promote spine enlargement.

BDNF enhances dendritic Ca2+ signals evoked by coincident EPSPs and back-propagating action potentials in CA1 pyramidal neurons

BDNF, a member of the neurotrophin family, is emerging as a key modulator of synaptic structure and function in the CNS. Due to the critical role of postsynaptic Ca(2+) signals in dendritic development and synaptic plasticity, we tested whether long-term exposure to BDNF affects Ca(2+) elevations evoked by coincident excitatory postsynaptic potentials (EPSPs) and back-propagating action potentials (bAPs) in spiny dendrites of CA1 pyramidal neurons within hippocampal slice cultures. In control neurons, a train of 5 coincident EPSPs and bAPs evoked Ca(2+) elevations in oblique radial branches of the main apical dendrite that were of similar amplitude than those evoked by a train of 5 bAPs alone. On the other hand, dendritic Ca(2+) signals evoked by coincident EPSPs and bAPs were always larger than those triggered by bAPs in CA1 neurons exposed to BDNF for 48 h. This difference was not observed after blockade of NMDA receptors (NMDARs) with D,L-APV, but only in BDNF-treated neurons, suggesting that Ca(2+) signals in oblique radial dendrites include a synaptic NMDAR-dependent component. Co-treatment with the receptor tyrosine kinase inhibitor k-252a prevented the effect of BDNF on coincident dendritic Ca(2+) signals, suggesting the involvement of neurotrophin Trk receptors. These results indicate that long-term exposure to BDNF enhances Ca(2+) signaling during coincident pre- and postsynaptic activity in small spiny dendrites of CA1 pyramidal neurons, representing a potential functional consequence of neurotrophin-mediated dendritic remodeling in developing neurons.

Ultrastructural features of neurons and synaptic contacts in the posterodorsal medial amygdala of adult male rats

The aim of the present study was to describe the ultrastructure of neurons (from eight animals) and to analyse the synaptic terminal distribution (from two animals) in the posterodorsal subnucleus of the medial amygdala (MePD) of adult male rats. Using transmission electron microscopy, it was possible to identify many spiny and aspiny dendrites, unmyelinated axonal bundles, single axonal processes, a few myelinated axons, blood vessels and glial processes in the neuropil. Axodendritic synapses were the most frequently observed (67.5%), appearing to be of either the inhibitory or the excitatory types. The presynaptic region contained round or flattened vesicles that occurred either singly or with dense-cored vesicles (DCVs). The dendrites often received many synapses on a single shaft, and axon terminals displayed synaptic contacts with one or more postsynaptic structures. Dendritic spines showed different morphologies and the synapses on them (23.1%) formed a single and apparently excitatory synaptic contact with round, electron-lucid vesicles alone or, less frequently, with DCVs. Inhibitory and excitatory axosomatic synapses (8.2%) and excitatory axoaxonic synapses (1.2%) were also identified. The present report provides new findings relevant to the study of the MePD cellular organization and could be combined with other morphological data in order to reveal the functional activity of this area in male rats.

Influence of predator stress on the consolidation versus retrieval of long-term spatial memory and hippocampal spinogenesis

We have studied the influence of predator stress (30 min of cat exposure) on long-term (24 h) spatial memory and the density of spines in basilar dendrites of CA1 neurons. Predator stress occurred either immediately before water maze training (Stress Pre-Training) or before the 24 h memory test (Stress Pre-Retrieval). The Control (nonstress) group exhibited excellent long-term spatial memory and a robust increase in the density of stubby, but not mushroom, shaped spines. The Stress Pre-Training group had impaired long-term memory and did not exhibit any changes in spine density. The Stress Pre-Retrieval group was also impaired in long-term memory performance, but this group exhibited an increase in the density of stubby, but not mushroom, shaped spines, which was indistinguishable from the control group. These findings indicate that: (1) A single day of water maze training under control conditions produced intact long-term memory and an increase in the density of stubby spines in CA1; (2) Stress before training interfered with the consolidation of information into long-term memory and suppressed the training-induced increase in spine density; and (3) Stress immediately before the 24 h memory test trial impaired the retrieval of the stored memory, but did not reverse the training-induced increase in CA1 spine density. Overall, this work provides evidence of structural plasticity in dendrites of CA1 neurons which may be involved in the consolidation process, and how spinogenesis and memory are modulated by stress.

Actin redistribution underlies the sparing effect of mild hypothermia on dendritic spine morphology after in vitro ischemia

Brain hypothermia is at present the most effective neuroprotective treatment against brain ischemia in man. Ischemia induces a redistribution of proteins involved in synaptic functions, which is markedly diminished by therapeutic hypothermia (33 degrees C). Dendritic spines at excitatory synapses are motile and show both shape changes and rearrangement of synaptic proteins as a consequence of neuronal activity. We investigated the effect of reduced temperature (33 degrees C and 27 degrees C compared with 37 degrees C), on spine motility, length and morphology by studying the distribution of GFP-actin before, during and after induction of in vitro ischemia. Because high-concentration actin filaments are located inside spines, dissociated hippocampal neurons (7-11 DIV) from transgenic mice expressing GFP-actin were used in this study. The movement of the spines and the distribution of GFP-actin were recorded using time-lapse fluorescence microscopy. Under normal conditions rapid rearrangement of GFP-actin was seen in dendritic spines, indicating highly motile spines at 37 degrees C. Decreasing the incubation temperature to 33 degrees C or 27 degrees C, dramatically reduces actin dynamics (spine motility) by approximately 50% and 70%, respectively. In addition, the length of the spine shaft was reduced by 20%. We propose that decreasing the temperature from 37 degrees C to 33 degrees C during ischemia decreases the neuronal actin polymerization rate, which reduces spine calcium kinetics, disrupts detrimental cell signaling and protects neurons against damage.

The roles of dendritic spine shapes in Purkinje cells

Shapes of dendritic spines are changed by various physiological or pathological states. The high degree of spine shape heterogeneity suggests that they would be the morphological basis for synaptic plasticity. An increasing number of proteins and signal transduction pathways have recently been shown to be associated with structural modifications of spines. Here, we review the possible functional roles of spine shapes in cerebellar Purkinje neurons. Several studies have suggested that spine shapes in Purkinje cells are regulated by both intrinsic and environmental factors, and different spine shapes could have significantly different consequences for brain function. Clearly constricted necks observed in thin, mushroom-shaped, and branched spines serve for compartmentalization of calcium and other second messenger molecules, influencing different signaling mechanisms and synaptic plasticity. Mushroom-shaped spines frequently have perforated postsynaptic density and the area of the spine head is much larger than simple spines, implying that membrane dynamics and receptor turnover are occurring. Branched spines might form additional synapses with afferent inputs resulting in the modification of neuronal circuits. Taken together, all these studies suggest that each spine shape is likely to have a distinct role in Purkinje cell function.

Local calcium signaling in neurons

Transient rises in the cytoplasmic concentration of calcium ions serve as second messenger signals that control many neuronal functions. Selective triggering of these functions is achieved through spatial localization of calcium signals. Several qualitatively different forms of local calcium signaling can be distinguished by the location of open calcium channels as well as by the distance between these channels and the calcium binding proteins that serve as the molecular targets of calcium action. Local calcium signaling is especially prominent at presynaptic active zones and postsynaptic densities, structures that are distinguished by highly organized macromolecular arrays that yield precise spatial arrangements of calcium signaling proteins. Similar forms of local calcium signaling may be employed throughout the nervous system, though much remains to be learned about the molecular underpinnings of these events.

Control of hippocampal dendritic spine morphology through ephrin-A3/EphA4 signaling

Communication between glial cells and neurons is emerging as a critical parameter of synaptic function. However, the molecular mechanisms underlying the ability of glial cells to modify synaptic structure and physiology are poorly understood. Here we describe a repulsive interaction that regulates postsynaptic morphology through the EphA4 receptor tyrosine kinase and its ligand ephrin-A3. EphA4 is enriched on dendritic spines of pyramidal neurons in the adult mouse hippocampus, and ephrin-A3 is localized on astrocytic processes that envelop spines. Activation of EphA4 by ephrin-A3 was found to induce spine retraction, whereas inhibiting ephrin/EphA4 interactions distorted spine shape and organization in hippocampal slices. Furthermore, spine irregularities in pyramidal neurons from EphA4 knockout mice and in slices transfected with kinase-inactive EphA4 indicated that ephrin/EphA4 signaling is critical for spine morphology. Thus, our data support a model in which transient interactions between the ephrin-A3 ligand and the EphA4 receptor regulate the structure of excitatory synaptic connections through neuroglial cross-talk.

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

Synaptopodin is the first member of a novel class of proline-rich actin-associated proteins. In brain, it is present in the neck of a subset of mature telencephalic spines and is associated closely with the spine apparatus, a Ca(2+) storing organelle within the spine compartment. The characteristic region- and lamina-specific distribution of synaptopodin in rat brain suggested that the distribution pattern of synaptopodin depends on the cytoarchitectonic arrangement of spine-bearing neurons. To test this hypothesis, synaptopodin was studied in the cortex, striatum, and hippocampus of normal and reeler mice, in which developmental cell migration defects have disrupted the normal array of cells. In situ hybridization histochemistry as well as light- and electron microscopic immunocytochemistry were used. In brain of normal mice, the pattern of synaptopodin mRNA-expressing cells corresponds to that of spine-bearing neurons and synaptopodin protein is found in a region- and lamina-specific distribution pattern. It is specifically sorted to the spine neck where it is associated closely with the spine apparatus. In brain of reeler mice, the pattern of synaptopodin mRNA-expressing cells corresponds to that of the abnormally positioned spine-bearing neurons and the region- and lamina-specific distribution pattern is absent or altered. Nevertheless, synaptopodin was specifically sorted to the spine neck, as in controls. These data demonstrate that the light microscopic distribution pattern of synaptopodin protein depends on the position and orientation of the spine-bearing neurons. The intracellular sorting process, however, is independent of positional cues.