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

Layer-specific changes of KCC2 and NKCC1 in the mouse dentate gyrus after entorhinal denervation

The cation-chloride cotransporters KCC2 and NKCC1 regulate the intracellular Cl^- concentration and cell volume of neurons and/or glia. The Cl^- extruder KCC2 is expressed at higher levels than the Cl^- transporter NKCC1 in mature compared to immature neurons, accounting for the developmental shift from high to low Cl^- concentration and from depolarizing to hyperpolarizing currents through GABA-A receptors. Previous studies have shown that KCC2 expression is downregulated following central nervous system injury, returning neurons to a more excitable state, which can be pathological or adaptive. Here, we show that deafferentation of the dendritic segments of granule cells in the outer (oml) and middle (mml) molecular layer of the dentate gyrus via entorhinal denervation in vivo leads to cell-type- and layer-specific changes in the expression of KCC2 and NKCC1. Microarray analysis validated by reverse transcription-quantitative polymerase chain reaction revealed a significant decrease in Kcc2 mRNA in the granule cell layer 7 days post-lesion. In contrast, Nkcc1 mRNA was upregulated in the oml/mml at this time point. Immunostaining revealed a selective reduction in KCC2 protein expression in the denervated dendrites of granule cells and an increase in NKCC1 expression in reactive astrocytes in the oml/mml. The NKCC1 upregulation is likely related to the increased activity of astrocytes and/or microglia in the deafferented region, while the transient KCC2 downregulation in granule cells may be associated with denervation-induced spine loss, potentially also serving a homeostatic role via boosting GABAergic depolarization. Furthermore, the delayed KCC2 recovery might be involved in the subsequent compensatory spinogenesis.

Precise measurement of gene expression changes in mouse brain areas denervated by injury

Quantitative PCR (qPCR) is a widely used method to study gene expression changes following brain injury. The accuracy of this method depends on the tissue harvested, the time course analyzed and, in particular on the choice of appropriate internal controls, i.e., reference genes (RGs). In the present study we have developed and validated an algorithm for the accurate normalization of qPCR data using laser microdissected tissue from the mouse dentate gyrus after entorhinal denervation at 0, 1, 3, 7, 14 and 28 days postlesion. The expression stabilities of ten candidate RGs were evaluated in the denervated granule cell layer (gcl) and outer molecular layer (oml) of the dentate gyrus. Advanced software algorithms demonstrated differences in stability for single RGs in the two layers at several time points postlesion. In comparison, a normalization index of several stable RGs covered the entire post-lesional time course and showed high stability. Using these RGs, we validated our findings and quantified glial fibrillary acidic protein (Gfap) mRNA and allograft inflammatory factor 1 (Aif1/Iba1) mRNA in the denervated oml. We compared the use of single RGs for normalization with the normalization index and found that single RGs yield variable results. In contrast, the normalization index gave stable results. In sum, our study shows that qPCR can yield precise, reliable, and reproducible datasets even under such complex conditions as brain injury or denervation, provided appropriate RGs for the model are used. The algorithm reported here can easily be adapted and transferred to any other brain injury model.

Maturation-Dependent Differences in the Re-innervation of the Denervated Dentate Gyrus by Sprouting Associational and Commissural Mossy Cell Axons in Organotypic Tissue Cultures of Entorhinal Cortex and Hippocampus

Sprouting of surviving axons is one of the major reorganization mechanisms of the injured brain contributing to a partial restoration of function. Of note, sprouting is maturation as well as age-dependent and strong in juvenile brains, moderate in adult and weak in aged brains. We have established a model system of complex organotypic tissue cultures to study sprouting in the dentate gyrus following entorhinal denervation. Entorhinal denervation performed after 2 weeks postnatally resulted in a robust, rapid, and very extensive sprouting response of commissural/associational fibers, which could be visualized using calretinin as an axonal marker. In the present study, we analyzed the effect of maturation on this form of sprouting and compared cultures denervated at 2 weeks postnatally with cultures denervated at 4 weeks postnatally. Calretinin immunofluorescence labeling as well as time-lapse imaging of virally-labeled (AAV2-hSyn1-GFP) commissural axons was employed to study the sprouting response in aged cultures. Compared to the young cultures commissural/associational sprouting was attenuated and showed a pattern similar to the one following entorhinal denervation in adult animals in vivo. We conclude that a maturation-dependent attenuation of sprouting occurs also in vitro, which now offers the chance to study, understand and influence maturation-dependent differences in brain repair in these culture preparations.

Differential Regulation of Wnt Signaling Components During Hippocampal Reorganization After Entorhinal Cortex Lesion

Entorhinal cortex lesions have been established as a model for hippocampal deafferentation and have provided valuable information about the mechanisms of synapse reorganization and plasticity. Although several molecules have been proposed to contribute to these processes, the role of Wnt signaling components has not been explored, despite the critical roles that Wnt molecules play in the formation and maintenance of neuronal and synaptic structure and function in the adult brain. In this work, we assessed the reorganization process of the dentate gyrus (DG) at 1, 3, 7, and 30 days after an excitotoxic lesion in layer II of the entorhinal cortex. We found that cholinergic fibers sprouted into the outer molecular layer of the DG and revealed an increase of the developmental regulated MAP2C isoform 7 days after lesion. These structural changes were accompanied by the differential regulation of the Wnt signaling components Wnt7a, Wnt5a, Dkk1, and Sfrp1 over time. The progressive increase in the downstream Wnt-regulated elements, active-β-catenin, and cyclin D1 suggested the activation of the canonical Wnt pathway beginning on day 7 after lesion, which correlates with the structural adaptations observed in the DG. These findings suggest the important role of Wnt signaling in the reorganization processes after brain lesion and indicate the modulation of this pathway as an interesting target for neuronal tissue regeneration.

Crossed Entorhino-Dentate Projections Form and Terminate With Correct Layer-Specificity in Organotypic Slice Cultures of the Mouse Hippocampus

The entorhino-dentate projection, i.e., the perforant pathway, terminates in a highly ordered and laminated fashion in the rodent dentate gyrus (DG): fibers arising from the medial entorhinal cortex (MEC) terminate in the middle molecular layer, whereas fibers arising from the lateral entorhinal cortex (LEC) terminate in the outer molecular layer of the DG. In rats and rabbits, a crossed entorhino-dentate projection exists, which originates from the entorhinal cortex (EC) and terminates in the contralateral DG. In contrast, in mice, such a crossed projection is reportedly absent. Using single and double mouse organotypic entorhino-hippocampal slice cultures, we studied the ipsi- and crossed entorhino-dentate projections. Viral tracing revealed that entorhino-dentate projections terminate with a high degree of lamina-specificity in single as well as in double cultures. Furthermore, in double cultures, entorhinal axons arising from one slice freely intermingled with entorhinal axons originating from the other slice. In single as well as in double cultures, entorhinal axons exhibited a correct topographical projection to the DG: medial entorhinal axons terminated in the middle and lateral entorhinal axons terminated in the outer molecular layer. Finally, entorhinal neurons were virally transduced with Channelrhodopsin2-YFP and stimulated with light, revealing functional connections between the EC and dentate granule cells. We conclude from our findings that entorhino-dentate projections form bilaterally in the mouse hippocampus in vitro and that the mouse DG provides a permissive environment for crossed entorhinal fibers.

Chronic exposure to IL-6 induces a desensitized phenotype of the microglia

Background

When the homeostasis of the central nervous system (CNS) is altered, microglial cells become activated displaying a wide range of phenotypes that depend on the specific site, the nature of the activator, and particularly the microenvironment generated by the lesion. Cytokines are important signals involved in the modulation of the molecular microenvironment and hence play a pivotal role in orchestrating microglial activation. Among them, interleukin-6 (IL-6) is a pleiotropic cytokine described in a wide range of pathological conditions as a potent inducer and modulator of microglial activation, but with contradictory results regarding its detrimental or beneficial functions. The objective of the present study was to evaluate the effects of chronic IL-6 production on the immune response associated with CNS-axonal anterograde degeneration.

Methods

The perforant pathway transection (PPT) paradigm was used in transgenic mice with astrocyte-targeted IL6-production (GFAP-IL6Tg). At 2, 3, 7, 14, and 21 days post-lesion, the hippocampal areas were processed for immunohistochemistry, flow cytometry, and protein microarray.

Results

An increase in the microglia/macrophage density was observed in GFAP-IL6Tg animals in non-lesion conditions and at later time-points after PPT, associated with higher microglial proliferation and a major monocyte/macrophage cell infiltration. Besides, in homeostasis, GFAP-IL6Tg showed an environment usually linked with an innate immune response, with more perivascular CD11b⁺/CD45^high/MHCII⁺/CD86⁺ macrophages, higher T cell infiltration, and higher IL-10, IL-13, IL-17, and IL-6 production. After PPT, WT animals show a change in microglia phenotype expressing MHCII and co-stimulatory molecules, whereas transgenic mice lack this shift. This lack of response in the GFAP-IL6Tg was associated with lower axonal sprouting.

Conclusions

Chronic exposure to IL-6 induces a desensitized phenotype of the microglia.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-020-02063-1.

Differential Roles of TREM2+ Microglia in Anterograde and Retrograde Axonal Injury Models

Microglia are the main immune cells of the central nervous system (CNS), and they are devoted to the active surveillance of the CNS during homeostasis and disease. In the last years, the microglial receptor Triggering Receptor Expressed on Myeloid cells-2 (TREM2) has been defined to mediate several microglial functions, including phagocytosis, survival, proliferation, and migration, and to be a key regulator of a new common microglial signature induced under neurodegenerative conditions and aging, also known as disease-associated microglia (DAM). Although microglial TREM2 has been mainly studied in chronic neurodegenerative diseases, few studies address its regulation and functions in acute inflammatory injuries. In this context, the present work aims to study the regulation of TREM2 and its functions after reparative axonal injuries, using two-well established animal models of anterograde and retrograde neuronal degeneration: the perforant pathway transection (PPT) and the facial nerve axotomy (FNA). Our results indicate the appearance of a subpopulation of microglia expressing TREM2 after both anterograde and retrograde axonal injury. TREM2+ microglia were not directly related to proliferation, instead, they were associated with specific recognition and/or phagocytosis of myelin and degenerating neurons, as assessed by immunohistochemistry and flow cytometry. Characterization of TREM2+ microglia showed expression of CD16/32, CD68, and occasional Galectin-3. However, specific singularities within each model were observed in P2RY12 expression, which was only downregulated after PPT, and in ApoE, where de novo expression was detected only in TREM2+ microglia after FNA. Finally, we report that the pro-inflammatory or anti-inflammatory cytokine microenvironment, which may affect phagocytosis, did not directly modify the induction of TREM2+ subpopulation in any injury model, although it changed TREM2 levels due to modification of the microglial activation pattern. In conclusion, we describe a unique TREM2+ microglial subpopulation induced after axonal injury, which is directly associated with phagocytosis of specific cell remnants and show different phenotypes, depending on the microglial activation status and the degree of tissue injury.

Re-innervation of the Denervated Dentate Gyrus by Sprouting Associational and Commissural Mossy Cell Axons in Organotypic Tissue Cultures of Entorhinal Cortex and Hippocampus

Collateral sprouting of surviving axons contributes to the synaptic reorganization after brain injury. To study this clinically relevant phenomenon, we used complex organotypic tissue cultures of mouse entorhinal cortex (EC) and hippocampus (H). Single EC-H cultures were generated to analyze associational sprouting, and double EC-H cultures were used to evaluate commissural sprouting of mossy cells in the dentate gyrus (DG) following entorhinal denervation. Entorhinal denervation (transection of the perforant path) was performed at 14 days in vitro (DIV) and associational/commissural sprouting was assessed at 28 DIV. First, associational sprouting was studied in genetically hybrid EC-H cultures of beta-actin-GFPtg and wild-type mice. Using calretinin as a marker, associational axons were found to re-innervate almost the entire entorhinal target zone. Denervation experiments performed with EC-H cultures of Thy1-YFPtg mice, in which mossy cells are YFP-positive, confirmed that the overwhelming majority of sprouting associational calretinin-positive axons are mossy cell axons. Second, we analyzed associational/commissural sprouting by combining wild-type EC-H cultures with calretinin-deficient EC-H cultures. In these cultures, only wild-type mossy cells contain calretinin, and associational and commissural mossy cell collaterals can be distinguished using calretinin as a marker. Nearly the entire DG entorhinal target zone was re-innervated by sprouting of associational and commissural mossy cell axons. Finally, viral labeling of newly formed associational/commissural axons revealed a rapid post-lesional sprouting response. These findings demonstrate extensive and rapid re-innervation of the denervated DG outer molecular layer by associational and commissural mossy cell axons, similar to what has been reported to occur in juvenile rodent DG in vivo.

Injury Leads to the Appearance of Cells with Characteristics of Both Microglia and Astrocytes in Mouse and Human Brain

Microglia and astrocytes have been considered until now as cells with very distinct identities. Here, we assessed the heterogeneity within microglia/monocyte cell population in mouse hippocampus and determined their response to injury, by using single-cell gene expression profiling of cells isolated from uninjured and deafferented hippocampus. We found that in individual cells, microglial markers Cx3cr1, Aif1, Itgam, and Cd68 were co-expressed. Interestingly, injury led to the co-expression of the astrocyte marker Gfap in a subpopulation of Cx3cr1-expressing cells from both the injured and contralesional hippocampus. Cells co-expressing astrocyte and microglia markers were also detected in the in vitro LPS activation/injury model and in sections from human brain affected by stroke, Alzheimer's disease, and Lewy body dementia. Our findings indicate that injury and chronic neurodegeneration lead to the appearance of cells that share molecular characteristics of both microglia and astrocytes, 2 cell types with distinct embryologic origin and function.

Comparative Examination of Temporal Glyoxalase 1 Variations Following Perforant Pathway Transection, Excitotoxicity, and Controlled Cortical Impact Injury

Following acute neuronal lesions, metabolic imbalance occurs, the rate of glycolysis increases, and methylglyoxal (MGO) forms, finally leading to metabolic dysfunction and inflammation. The glyoxalase system is the main detoxification system for MGO and is impaired following excitotoxicity and stroke. However, it is not known yet whether alterations of the glyoxalase system are also characteristic for other neuronal damage models. Neuronal damage was induced in organotypic hippocampal slice cultures by transection of perforant pathway (PPT; 5 min to 72 h) and N-methyl-D-aspartate (NMDA; 50 μM for 4 h) or in vivo after controlled cortical impact (CCI) injury (2 h to 14 days). Temporal and spatial changes of glyoxalase I (GLO1) were investigated by Western blot analyses and immunohistochemistry. In immunoblot, the GLO1 protein content was not significantly affected by PPT at all investigated time points. As described previously, NMDA treatment led to a GLO1 increase 24 and 48 h after the lesion, whereas PPT increased GLO1 immunoreactivity within neurons only at 48 h postinjury. Immunohistochemistry of brain tissue subjected to CCI unveiled positive GLO1 immunoreactivity in neurons and astrocytes at 1 and 3 days after injury. Two hours and 14 days after CCI, no GLO1 immunoreactivity was observed. GLO1 protein content changes are associated with excitotoxicity but seemingly not to fiber transection. Cell-specific changes in GLO1 immunoreactivity after different in vitro and in vivo lesion types might be a common phenomenon in the aftermath of neuronal lesions.

Region-Specific Differences in Amyloid Precursor Protein Expression in the Mouse Hippocampus

The physiological role of amyloid precursor protein (APP) has been extensively investigated in the rodent hippocampus. Evidence suggests that APP plays a role in synaptic plasticity, dendritic and spine morphogenesis, neuroprotection and—at the behavioral level—hippocampus-dependent forms of learning and memory. Intriguingly, however, studies focusing on the role of APP in synaptic plasticity have reported diverging results and considerable differences in effect size between the dentate gyrus (DG) and area CA1 of the mouse hippocampus. We speculated that regional differences in APP expression could underlie these discrepancies and studied the expression of APP in both regions using immunostaining, in situ hybridization (ISH), and laser microdissection (LMD) in combination with quantitative reverse transcription polymerase chain reaction (RT-qPCR) and western blotting. In sum, our results show that APP is approximately 1.7-fold higher expressed in pyramidal cells of Ammon’s horn than in granule cells of the DG. This regional difference in APP expression may explain why loss-of-function approaches using APP-deficient mice revealed a role for APP in Hebbian plasticity in area CA1, whereas this could not be shown in the DG of the same APP mutants.

Sensorimotor Intervention Recovers Noradrenaline Content in the Dentate Gyrus of Cortical Injured Rats

Nowadays, a consensus has been reached that designates the functional and structural reorganization of synapses as the primary mechanisms underlying the process of recovery from brain injury. We have reported that pontine noradrenaline (NA) is increased in animals after cortical ablation (CA). The aim of the present study was to explore the noradrenergic and morphological response after sensorimotor intervention (SMI) in rats injured in the motor cortex. We used male Wistar adult rats allocated in four conditions: sham-operated, injured by cortical ablation, sham-operated with SMI and injured by cortical ablation with SMI. Motor and somatosensory performance was evaluated prior to and 20 days after surgery. During the intervening period, a 15-session, SMI program was implemented. Subsequently, total NA analysis in the pons and dentate gyrus (DG) was performed. All groups underwent histological analysis. Our results showed that NA content in the DG was reduced in the injured group versus control, and this reduction was reverted in the injured group that underwent SMI. Moreover, injured rats showed reduction in the number of granule cells in the DG and decreased dentate granule cell layer thickness. Notably, after SMI, the loss of granule cells was reverted. Locus coeruleus showed turgid cells in the injured rats. These results suggest that SMI elicits biochemical and structural modifications in the hippocampus that could reorganize the system and lead the recovery process, modulating structural and functional plasticity.

A general homeostatic principle following lesion induced dendritic remodeling

INTRODUCTION

Neuronal death and subsequent denervation of target areas are hallmarks of many neurological disorders. Denervated neurons lose part of their dendritic tree, and are considered "atrophic", i.e. pathologically altered and damaged. The functional consequences of this phenomenon are poorly understood.

RESULTS

Using computational modelling of 3D-reconstructed granule cells we show that denervation-induced dendritic atrophy also subserves homeostatic functions: By shortening their dendritic tree, granule cells compensate for the loss of inputs by a precise adjustment of excitability. As a consequence, surviving afferents are able to activate the cells, thereby allowing information to flow again through the denervated area. In addition, action potentials backpropagating from the soma to the synapses are enhanced specifically in reorganized portions of the dendritic arbor, resulting in their increased synaptic plasticity. These two observations generalize to any given dendritic tree undergoing structural changes.

CONCLUSIONS

Structural homeostatic plasticity, i.e. homeostatic dendritic remodeling, is operating in long-term denervated neurons to achieve functional homeostasis.

Motor System Reorganization After Stroke: Stimulating and Training Toward Perfection

Stroke instigates regenerative responses that reorganize connectivity patterns among surviving neurons. The new connectivity patterns can be suboptimal for behavioral function. This review summarizes current knowledge on post-stroke motor system reorganization and emerging strategies for shaping it with manipulations of behavior and cortical activity to improve functional outcome.

The connectomics of brain disorders

Pathological perturbations of the brain are rarely confined to a single locus; instead, they often spread via axonal pathways to influence other regions. Patterns of such disease propagation are constrained by the extraordinarily complex, yet highly organized, topology of the underlying neural architecture; the so-called connectome. Thus, network organization fundamentally influences brain disease, and a connectomic approach grounded in network science is integral to understanding neuropathology. Here, we consider how brain-network topology shapes neural responses to damage, highlighting key maladaptive processes (such as diaschisis, transneuronal degeneration and dedifferentiation), and the resources (including degeneracy and reserve) and processes (such as compensation) that enable adaptation. We then show how knowledge of network topology allows us not only to describe pathological processes but also to generate predictive models of the spread and functional consequences of brain disease.

Upregulation of APP, ADAM10 and ADAM17 in the denervated mouse dentate gyrus

The disintegrin and metalloproteinases ADAM10 and ADAM17 are regarded as the most important α-secretases involved in the physiological processing of amyloid precursor protein (APP) in brain. Since it has been suggested that processing of APP by α-secretases could be involved in the reorganization of the brain following injury, we studied mRNA expression of the two α-secretases Adam10 and Adam17, the ß-secretase Bace1, and the App-gene family (App, Aplp1, Aplp2) in the dentate gyrus of the mouse following entorhinal denervation. Using laser microdissection, tissue was harvested from the outer molecular layer and the granule cell layer of the denervated dentate gyrus. Expression levels of candidate genes were assessed using Affymetrix GeneChip Mouse Gene 1.0 ST arrays and reverse transcription-quantitative PCR, revealing an upregulation of Adam10 mRNA and Adam17 mRNA in the denervated outer molecular layer and an upregulation of Adam10 mRNA and App mRNA in the dentate granule cell layer. Immunolabeling for ADAM10 or ADAM17 in combination with markers for astro- and microglia revealed an increased labeling of ADAM10 and ADAM17 in the denervated outer molecular layer that was associated with reactive astrocytes but not with microglia. Collectively, these data show that denervation affects the expression level of APP and its two most important α-secretases. This suggests that APP-processing could be shifted towards the non-amyloidogenic pathway in denervated areas of the brain and, thus, towards the formation of neuroprotective APP cleavage products, such as APPsα.

Axonal tract tracing for delineating interacting brain regions: implications for Alzheimer's disease-associated memory

We are studying the projections from the entorhinal cortex to the hippocampal formation in the mouse. The dentate gyrus is innervated by the lateral entorhinal cortex (lateral perforant path) and medial entorhinal cortex (medial perforant path). The entorhinal cortex also projects to hippocampal areas CA3 and CA1, and to the subiculum. In young transgenic Alzheimer's disease mouse models (before amyloid-β pathology), the connections are not different from normal mice. In Alzheimer's disease mice with pathology, two changes occur: first, dystrophic axon endings appear near amyloid-β plaques, and second, there are sparse aberrant axon terminations not in the appropriate area or lamina of the hippocampus. Furthermore, MRI-diffusion tensor imaging analysis indicates a decrease in the quality of the white matter tracts connecting the hippocampus to the brain; in other words, the fimbria/fornix and perforant path. Similar changes in white matter integrity have been found in Alzheimer's disease patients and could potentially be used as early indicators of disease onset.

Plasticity response in the contralesional hemisphere after subtle neurotrauma: gene expression profiling after partial deafferentation of the hippocampus

Neurotrauma or focal brain ischemia are known to trigger molecular and structural responses in the uninjured hemisphere. These responses may have implications for tissue repair processes as well as for the recovery of function. To determine whether the plasticity response in the uninjured hemisphere occurs even after a subtle trauma, we subjected mice to a partial unilateral deafferentation of the hippocampus induced by stereotactically performed entorhinal cortex lesion (ECL). The expression of selected genes was assessed by quantitative real-time PCR in the hippocampal tissue at the injured side and the contralesional side at day 4 and 14 after injury. We observed that expression of genes coding for synaptotagmin 1, ezrin, thrombospondin 4, and C1q proteins, that have all been implicated in the synapse formation, re-arrangement and plasticity, were upregulated both in the injured and the contralesional hippocampus, implying a plasticity response in the uninjured hemisphere. Several of the genes, the expression of which was altered in response to ECL, are known to be expressed in astrocytes. To test whether astrocyte activation plays a role in the observed plasticity response to ECL, we took advantage of mice deficient in two intermediate filament (nanofilament) proteins glial fibrillary acidic protein (GFAP) and vimentin (GFAP(-/-)Vim(-/-) ) and exhibiting attenuated astrocyte activation and reactive gliosis. The absence of GFAP and vimentin reduced the ECL-induced upregulation of thrombospondin 4, indicating that this response to ECL depends on astrocyte activation and reactive gliosis. We conclude that even a very limited focal neurotrauma triggers a distinct response at the contralesional side, which at least to some extent depends on astrocyte activation.

Neural injury alters proliferation and integration of adult-generated neurons in the dentate gyrus

Neural plasticity following brain injury illustrates the potential for regeneration in the central nervous system. Lesioning of the perforant path, which innervates the outer two-thirds of the molecular layer of the dentate gyrus, was one of the first models to demonstrate structural plasticity of mature granule cells (Parnavelas et al., 1974; Caceres and Steward, 1983; Diekmann et al., 1996). The dentate gyrus also harbors a continuously proliferating population of neuronal precursors that can integrate into functional circuits and show enhanced short-term plasticity (Schmidt-Hieber et al., 2004; Abrous et al., 2005). To examine the response of adult-generated granule cells to unilateral complete transection of the perforant path in vivo, we tracked these cells using transgenic POMC-EGFP mice or by retroviral expression of GFP. Lesioning triggered a marked proliferation of newborn neurons. Subsequently, the dendrites of newborn neurons showed reduced complexity within the denervated zone, but dendritic spines still formed in the absence of glutamatergic nerve terminals. Electron micrographs confirmed the lack of intact presynaptic terminals apposing spines on mature cells and on newborn neurons. Newborn neurons, but not mature granule cells, had a higher density of dendritic spines in the inner molecular layer postlesion accompanied by an increase in miniature EPSC amplitudes and rise times. Our results indicate that injury causes an increase in newborn neurons and lamina-specific synaptic reorganization indicative of enhanced plasticity. The presence of de novo dendritic spines in the denervated zone suggests that the postlesion environment provides the necessary signals for spine formation.

Structural plasticity in the dentate gyrus- revisiting a classic injury model

The adult brain is in a continuous state of remodeling. This is nowhere more true than in the dentate gyrus, where competing forces such as neurodegeneration and neurogenesis dynamically modify neuronal connectivity, and can occur simultaneously. This plasticity of the adult nervous system is particularly important in the context of traumatic brain injury or deafferentation. In this review, we summarize a classic injury model, lesioning of the perforant path, which removes the main extrahippocampal input to the dentate gyrus. Early studies revealed that in response to deafferentation, axons of remaining fiber systems and dendrites of mature granule cells undergo lamina-specific changes, providing one of the first examples of structural plasticity in the adult brain. Given the increasing role of adult-generated new neurons in the function of the dentate gyrus, we also compare the response of newborn and mature granule cells following lesioning of the perforant path. These studies provide insights not only to plasticity in the dentate gyrus, but also to the response of neural circuits to brain injury.

Time-lapse imaging of granule cells in mouse entorhino-hippocampal slice cultures reveals changes in spine stability after entorhinal denervation

Principal neurons that are partially denervated after brain injury remodel their synaptic connections and show biphasic changes in their dendritic spine density: during an early phase after denervation spine density decreases and during a late phase spine density recovers again. It has been hypothesized that these changes in spine density are caused by a period of increased spine loss followed by a period of increased spine formation. We have tested this hypothesis, which is based on data from fixed tissues, by using time-lapse imaging of denervated dentate granule cells in organotypic entorhino-hippocampal slice cultures of Thy1-GFP mice. Our data show that nondenervated granule cells turn over spines spontaneously while keeping their spine density constant. Denervation influenced this equilibrium and induced biphasic changes in the spine loss rate but not in the rate of spine formation: during the early phase after denervation the spine loss rate was increased and during the late phase after denervation the spine loss rate was decreased compared with nondenervated control cultures. In line with these observations, time-lapse imaging of identified spines formed after the lesion revealed that the stability of these spines was decreased during the early phase and increased during the late phase after the lesion. We conclude that biphasic changes in spine loss rate and spine stability but not in the rate of spine formation play a central role in the reorganization of dentate granule cells after entorhinal denervation in vitro.

ERK activation and expression of neuronal cell cycle markers in the hippocampus after entorhinal cortex lesion

Current findings suggest that neuronal cell death is frequently associated with the aberrant expression of cell cycle-regulatory proteins in postmitotic neurons. Aberrant cell cycle reentry has been implicated in diverse neurodegenerative conditions, including Alzheimer's disease (AD). Previously we reported that the appearance of cell cycle markers in postmitotic neurons of the entorhinal cortex (EC) after excitotoxic hippocampal damage is associated with the expression of phospho-tau and amyloid precursor protein (APP). However, the question of the signaling pathway involved in this cell cycle reentry remains unresolved. Differentiated neurons use the molecular mechanisms initially acquired to direct cell proliferation, such as the Ras-extracellular signal-regulated kinase (ERK1/2) pathway, to regulate synaptic plasticity. In this work we explored whether ERK1/2-related signaling might contribute to the cell cycle reentry in hippocampal neurons after a unilateral EC lesion. We showed that, within the first 24 hr after hippocampal deafferentation, numerous neurons expressed phospho-ERK1/2, concomitantly with the gradual increases in cyclin D1 and cyclin B immunoreactivity in the dentate gyrus and hilus. Several of these immunopositive cells to phospho-ERK1/2 and cyclin B in hippocampus are postmitotic neurons, insofar as they are positive to NeuN. The intracisternal administration of U0126 (an MEK inhibitor), previous to the excitotoxic lesion, decreased the activation of ERK1/2 and the expression of cyclin D1 and cyclin B in the hippocampus. The present findings support the notion that ERK1/2 plays a role in cell cycle reactivation in mature neurons efferently connected to the lesion site.

Site-specific and time-dependent activation of the endocannabinoid system after transection of long-range projections

Background

After focal neuronal injury the endocannabinioid system becomes activated and protects or harms neurons depending on cannabinoid derivates and receptor subtypes. Endocannabinoids (eCBs) play a central role in controlling local responses and influencing neural plasticity and survival. However, little is known about the functional relevance of eCBs in long-range projection damage as observed in stroke or spinal cord injury (SCI).

Methods

In rat organotypic entorhino-hippocampal slice cultures (OHSC) as a relevant and suitable model for investigating projection fibers in the CNS we performed perforant pathway transection (PPT) and subsequently analyzed the spatial and temporal dynamics of eCB levels. This approach allows proper distinction of responses in originating neurons (entorhinal cortex), areas of deafferentiation/anterograde axonal degeneration (dentate gyrus) and putative changes in more distant but synaptically connected subfields (cornu ammonis (CA) 1 region).

Results

Using LC-MS/MS, we measured a strong increase in arachidonoylethanolamide (AEA), oleoylethanolamide (OEA) and palmitoylethanolamide (PEA) levels in the denervation zone (dentate gyrus) 24 hours post lesion (hpl), whereas entorhinal cortex and CA1 region exhibited little if any changes. NAPE-PLD, responsible for biosynthesis of eCBs, was increased early, whereas FAAH, a catabolizing enzyme, was up-regulated 48hpl.

Conclusion

Neuronal damage as assessed by transection of long-range projections apparently provides a strong time-dependent and area-confined signal for de novo synthesis of eCB, presumably to restrict neuronal damage. The present data underlines the importance of activation of the eCB system in CNS pathologies and identifies a novel site-specific intrinsic regulation of eCBs after long-range projection damage.

Entorhinal denervation induces homeostatic synaptic scaling of excitatory postsynapses of dentate granule cells in mouse organotypic slice cultures

Denervation-induced changes in excitatory synaptic strength were studied following entorhinal deafferentation of hippocampal granule cells in mature (≥ 3 weeks old) mouse organotypic entorhino-hippocampal slice cultures. Whole-cell patch-clamp recordings revealed an increase in excitatory synaptic strength in response to denervation during the first week after denervation. By the end of the second week synaptic strength had returned to baseline. Because these adaptations occurred in response to the loss of excitatory afferents, they appeared to be in line with a homeostatic adjustment of excitatory synaptic strength. To test whether denervation-induced changes in synaptic strength exploit similar mechanisms as homeostatic synaptic scaling following pharmacological activity blockade, we treated denervated cultures at 2 days post lesion for 2 days with tetrodotoxin. In these cultures, the effects of denervation and activity blockade were not additive, suggesting that similar mechanisms are involved. Finally, we investigated whether entorhinal denervation, which removes afferents from the distal dendrites of granule cells while leaving the associational afferents to the proximal dendrites of granule cells intact, results in a global or a local up-scaling of granule cell synapses. By using computational modeling and local electrical stimulations in Strontium (Sr(2+))-containing bath solution, we found evidence for a lamina-specific increase in excitatory synaptic strength in the denervated outer molecular layer at 3-4 days post lesion. Taken together, our data show that entorhinal denervation results in homeostatic functional changes of excitatory postsynapses of denervated dentate granule cells in vitro.

Unilateral entorhinal denervation leads to long-lasting dendritic alterations of mouse hippocampal granule cells

Following brain injury, neurons efferently connected from the lesion site are denervated and remodel their dendritic tree. Denervation-induced dendritic reorganization of granule cells was investigated in the dentate gyrus of the Thy1-GFP mouse. After mechanical transection of the perforant path, single granule cells were 3D-reconstructed at different time points post-lesion (3d, 7d, 10d, 30 d, 90 d and 180 d) and their soma size, total dendritic length, number of dendritic segments and dendritic branch orders were studied. Changes in spine densities were determined using 3D-analysis of individual dendritic segments. Following entorhinal denervation the granule cell arbor progressively atrophied until 90 d post-lesion (reduction of total dendritic length to ~50% of control). Dendritic alterations occurred selectively in the denervated outer molecular layer, where a loss of distal dendritic segments and a reduction of mean segment length were seen. At 180 d post-lesion total dendritic length partially recovered (~70% of control). This recovery appeared to be the result of a re-elongation of surviving dendrites rather than dendritic re-branching, since the number of dendritic segments did not recover. In contrast to the protracted dendritic changes, spine density changes followed a faster time course. In the denervated layer spine densities dropped to ~65% of control values and fully recovered by 30 d post-lesion. We conclude that entorhinal denervation in mouse causes protracted and long-term structural alterations of the granule cell dendritic tree. Spontaneously occurring reinnervation processes, such as the sprouting of surviving afferent fibers, are insufficient to maintain the granule cell dendritic arbor.

Calcium homeostasis of acutely denervated and lesioned dentate gyrus in organotypic entorhino-hippocampal co-cultures

Denervation of neurons, e.g. upon traumatic injury or neuronal degeneration, induces transneuronal degenerative events, such as spine loss, dendritic pruning, and even cell loss. We studied one possible mechanism proposed to trigger such events, i.e. excess glutamate release from severed axons conveyed transsynaptically via postsynaptic calcium influx. Using 2-photon microscopical calcium imaging in organotypic entorhino-hippocampal co-cultures, we show that acute transection of the perforant path elicits two independent effects on calcium homeostasis in the dentate gyrus: a brief, short-latency elevation of postsynaptic calcium levels in denervated granule cells, which can be blocked by preincubation with tetrodotoxin, and a long-latency astroglial calcium wave, not blocked by tetrodotoxin and propagating slowly through the hippocampus. While neuronal calcium elevations upon axonal transection placed remote from the target area were similar to those elicited by brief trains of electrical stimulation of the perforant path, large-scale calcium signals were observed upon lesions placed close to or within the dendritic field of granule cells. Concordantly, induction of c-fos in denervated neurons coincided spatially with cell populations showing prolonged calcium elevations upon concomitant dendritic damage. Since denervation of dentate granule cells by remote transection of the perforant path induces transsynaptic dendritic reorganization in the utilized organotypic cultures, a generalized breakdown of the cellular calcium homeostasis is unlikely to underlie these transneuronal changes.

Morphological correlates of emotional and cognitive behaviour: insights from studies on inbred and outbred rodent strains and their crosses

Every study in rodents is also a behavioural genetic study even if only a single strain is used. Outbred strains are genetically heterogeneous populations with a high intrastrain variation, whereas inbred strains are based on the multiplication of a unique individual. The aim of the present review is to summarize findings on brain regions involved in three major components of rodent behaviour, locomotion, anxiety-related behaviour and cognition, by paying particular attention to the genetic context, genetic models used and interstrain comparisons. Recent trends correlating gene expression in inbred strains with behavioural data in databases, morpho-behavioural-haplotype analyses and problems arising from large-scale multivariate analyses are discussed. Morpho-behavioural correlations in multiple strains are presented, including correlations with projection neurons, interneurons and fibre systems in the striatum, midbrain, amygdala, medial septum and hippocampus, by relating them to relevant transmitter systems. In addition, brain areas differentially activated in different strains are described (hippocampus, prefrontal cortex, nucleus accumbens, locus ceruleus). Direct interstrain comparisons indicate that strain differences in behavioural variables and neuronal markers are much more common than usually thought. The choice of the appropriate genetic model can therefore contribute to an interpretation of positive results in a wider context, and help to avoid misleading interpretations of negative results.

3D-reconstruction and functional properties of GFP-positive and GFP-negative granule cells in the fascia dentata of the Thy1-GFP mouse

Granule cells of the mouse fascia dentata are widely used in studies on neuronal development and plasticity. In contrast to the rat, however, high-resolution morphometric data on these cells are scarce. Thus, we have analyzed granule cells in the fascia dentata of the adult Thy1-GFP mouse (C57BL/6 background). In this mouse line, single neurons in the granule cell layer are GFP-labeled, making them amenable to high-resolution 3D-reconstruction. First, calbindin or parvalbumin-immunofluorescence was used to identify GFP-positive cells as granule cells. Second, the dorsal-ventral distribution of GFP-positive granule cells was studied: In the dorsal part of the fascia dentata 11% and in the ventral part 15% of all granule cells were GFP-positive. Third, GFP-positive and GFP-negative granule cells were compared using intracellular dye-filling (fixed slice technique) and patch-clamp recordings; no differences were observed between the cells. Finally, GFP-positive granule cells (dorsal and ventral fascia dentata) were imaged at high resolution with a confocal microscope, 3D-reconstructed in their entirety and analyzed for soma size, total dendritic length, number of segments, total number of spines and spine density. Sholl analysis revealed that dendritic complexity of granule cells is maximal 150-200 mum from the soma. Granule cells located in the ventral part of the hippocampus revealed a greater degree of dendritic complexity compared to cells in the dorsal part. Taken together, this study provides morphometric data on granule cells of mice bred on a C57BL/6 background and establishes the Thy1-GFP mouse as a tool to study granule cell neurobiology.

Differential regulation of chondroitin sulfate proteoglycan mRNAs in the denervated rat fascia dentata after unilateral entorhinal cortex lesion

Following brain trauma, chondroitin sulphate proteoglycans (CSPGs) are enriched at injury sites and in denervated areas. At injury sites, CSPGs are regarded as inhibitors of axonal regeneration because of their growth inhibitory properties. In areas of denervation their role is less clear, since they are enriched in zones of sprouting, i.e. zones of axonal growth. To identify CSPGs expressed in a denervated brain area and to quantify changes in their mRNA expression, neurocan, brevican, NG2, phosphacan and aggrecan mRNA were analyzed in the rat fascia dentata following entorhinal denervation. Laser microdissection was combined with quantitative RT-PCR to measure mRNA changes specifically within the denervated portion of the molecular layer (1h, 6h, 10h, 12h, 1d, 2d, 3d, 4d, 7d and 14d post-lesion). Changes in glial fibrillary protein mRNA were measured at the same time points and used as lesion control. This approach revealed a differential regulation of CSPG mRNAs in the denervated zone: neurocan, brevican and NG2 mRNA were upregulated with a maximum around 2 days post-lesion. In contrast, aggrecan mRNA levels reached a maximum 7 days post-lesion and phosphacan mRNA levels were not significantly altered. Taken together, our data reveal a temporal pattern in CSPG mRNA expression in the denervated fascia dentata. This suggests specific biological functions for CSPGs during the denervation-induced reorganization process: whereas the early increase in CSPGs in the denervated zone could influence the pattern of sprouting, the late increase of aggrecan mRNA suggests a different role during the late phase of reorganization.