Myelin does not influence the choice behaviour of entorhinal axons but strongly inhibits their outgrowth length in vitro

Applications of stripe assay in the study of CXCL12-mediated neural progenitor cell migration and polarization

The polarization and migration of neural progenitor cells (NPCs) are critical for embryonic brain development and neurogenesis after brain injury. Although stromal-derived factor-1α (SDF-1α, CXCL12) and its receptor CXCR4 are well-known to mediate the migration of NPCs in the developing brain, the dynamic cellular processes and structure-related molecular events remain elusive. Transwell and microfluidic-based assays are classical assays to effectively study cellular migration. However, both of them have limitations in the analysis of a single cell. In this study, we modified the stripe assay and extended its applications in the study of NPC polarization and intracellular molecular events associated with CXCL12-mediated migration. In response to localized CXCL12, NPCs formed lamellipodia in the stripe assay. Furthermore, CXCR4 and Rac1 quickly re-distributed to the area of lamellipodia, indicating their roles in NPC polarization upon CXCL12 stimulation. Although the chemokine stripes in the assay provided concentration gradients that can be best used to study cellular polarization and migration through immunocytochemistry, they can also generate live imaging data with comparable quality. In conclusion, stripe assay is a visual, dynamic and economical tool to study cellular mobility and its related molecule mechanisms.

Investigating axonal guidance with microdevice-based approaches

The precise wiring of the nervous system relies on processes by which axons navigate in a complex environment and are guided by a concerted action of attractive and repulsive factors to reach their target. Investigating these guidance processes depends critically on our ability to control in space and time the microenvironment of neurons. The implementation of microfabrication techniques in cell biology now enables a precise control of the extracellular physical and chemical environment of cultured cells. However, microtechnology is only beginning to be applied in the field of axon guidance due to specific requirements of neuronal cultures. Here we review microdevices specifically designed to study axonal guidance and compare them with the conventional assays used to probe gradient sensing in cell biology. We also discuss how innovative microdevice-based approaches will enable the investigation of important systems-level questions on the gradient sensing properties of nerve cells, such as the sensitivity and robustness in the detection of directional signals or the combinatorial response to multiple cues.

Engineered cell culture substrates for axon guidance studies: moving beyond proof of concept

Promoting axon regeneration following injury is one of the ultimate challenges of neuroscience, and understanding the mechanisms that regulate axon growth and guidance is essential to achieve this goal. During development axons are directed over relatively long distances by a precise extracellular distribution of chemical signals in the embryonic nervous system. Multiple guidance proteins, including netrins, slits, semaphorins, ephrins and neurotrophins have been identified as key players in this process. During the last decade, engineered cell culture substrates have been developed to investigate the cellular and molecular mechanisms underlying axon guidance. This review is focused on the biological insights that have been achieved using new techniques that attempt to mimic in vitro the spatial patterns of proteins that growth cones encounter in vivo.

Neonatal lesions of the entorhinal cortex induce long-term changes of limbic brain regions and maze learning deficits in adult rats

We here investigated the effects of neonatal lesions of the entorhinal cortex (EC) in rats on maze learning and on structural alterations of its main projection region, the hippocampus, as well as other regions with anatomical connections to the EC that are involved in maze learning. Since early brain damage is considered to be involved in certain neuropsychiatric diseases, this approach sought to model certain aspects of this etiopathogenesis. Bilateral neonatal lesions were induced on postnatal day 7 by microinjection of ibotenic acid (1.3 microg/0.2 microl phosphate-buffered saline (PBS)) into the EC. Naive and sham-lesioned rats served as controls. Rats were trained and tested on an eight-arm radial maze for allocentric and egocentric learning. Subsequently, gold-chloride staining and immunohistochemical staining for the microtubule-associated protein MAP-2 was used to assess myelination and dendritic density in the hippocampus, striatum and medial prefrontal cortex (mPFC) of these rats. Additionally, parvalbumin-expressing, presumably GABAergic interneurons, were evaluated in these regions. Performance in both the allocentric and the egocentric strategy was disturbed after neonatal EC lesion as shown by an increase of repeated arm entries, which indicates disturbed working memory. Histological evaluation revealed that the density of parvalbumin-immunopositive neurons and myelin sheaths was reduced in the hippocampus but not in the striatum and mPFC in neonatally lesioned rats. Density of MAP-2 staining did not differ between groups in all regions tested. Since structural alterations were only found in the EC and hippocampus our findings support their eminent role in working memory and show that no functional restoration occurs after neonatal lesions.

Stripe assay to examine axonal guidance and cell migration

Stripe assays have been widely employed as in vitro test systems to study the responses of growing axons, as well as migrating cells, to established or novel guidance molecules. We provide detailed protocols for both the original and the modified version of this assay, as they allow the analysis of the 'guidance properties' of active components present in crude membrane fractions or as purified molecules. Silicon matrices are used to produce striped patterns of active molecules on a surface (referred to as 'carpet'), followed by culturing of neurons, or any other cell type, on these carpets. After 1-2 days in culture, striped outgrowth of extending neurites--indicative of guided migration of cell processes--can be observed. We also discuss potential other applications (e.g., in neuronal regeneration and development) and modifications of the assay. The preparation of 10-12 carpets takes approximately 4-5 h.

Perinatal iron deficiency predisposes the developing rat hippocampus to greater injury from mild to moderate hypoxia-ischemia

The hippocampus is injured in both hypoxia-ischemia (HI) and perinatal iron deficiency that are co-morbidities in infants of diabetic mothers and intrauterine growth restricted infants. We hypothesized that preexisting perinatal iron deficiency predisposes the hippocampus to greater injury when exposed to a relatively mild HI injury. Iron-sufficient and iron-deficient rats (hematocrit 40% lower and brain iron concentration 55% lower) were subjected to unilateral HI injury of 15, 30, or 45 mins (n=12 to 13/HI duration) on postnatal day 14. Sixteen metabolite concentrations were measured from an 11 microL volume on the ipsilateral (HI) and contralateral (control) hippocampi 1 week later using in vivo 1H NMR spectroscopy. The concentrations of creatine, glutamate, myo-inositol, and N-acetylaspartate were lower on the control side in the iron-deficient group (P<0.02, each). Magnetic resonance imaging showed hippocampal injury in the majority of the iron-deficient rats (58% versus 11%, P<0.0001) with worsening severity with increasing durations of HI (P=0.0001). Glucose, glutamate, N-acetylaspartate, and taurine concentrations were decreased and glutamine, lactate and myo-inositol concentrations, and glutamine/glutamate ratio were increased on the HI side in the iron-deficient group (P<0.01, each), mainly in the 30 and 45 mins HI subgroups (P<0.02, each). These neurochemical changes likely reflect the histochemically detected neuronal injury and reactive astrocytosis in the iron-deficient group and suggest that perinatal iron deficiency predisposes the hippocampus to greater injury from exposure to a relatively mild HI insult.

Targeting myelin to optimize plasticity of spared spinal axons

Functional re-innervation of target neurons following neurological damage such as spinal cord injury is an essential requirement of potential therapies. There are at least two avenues by which this can be achieved: (a) through the regeneration of injured axons and (b) through promoting plasticity of those spared by the initial insult. There are several reasons why the latter approach may be more feasible, not the least of which are the inhibitory character of the glial scar, the often long distances over which injured axons must regrow, and the fact that spared axons are often already in the vicinity of denervated targets. The challenge is to unveil the well-recognized intrinsic plasticity of spared axons in a way that avoids complications, such as pain or autonomic dysfunction. One approach that we as well as others have taken is to target growth-suppressing signaling pathways initiated in spared axons by myelin-derived proteins. This article reviews models used for the study of spinal axon plasticity and describes the anatomical and behavioral effects of interfering with myelinderived proteins, their receptors, and components of their intracellular signaling cascades.

Lesion-induced gelsolin upregulation in the hippocampus following entorhinal deafferentation

Gelsolin is an actin-binding protein that regulates actin filament-severing and capping activity in the various processes of cell motilities. Here, we report the expression of gelsolin mRNA and protein in the hippocampus following transections of the entorhinal afferents. Northern blot analysis showed that transcript of gelsolin was upregulated in a transient manner in the deafferented hippocampus by 1.3-, 2.1-, 1.7-, and 1.1- folds of controls, respectively, at 1, 3, 7, and 15 days postlesion (dpl). In situ hybridization and immunohistochemistry confirmed the temporal expression of gelsolin specifically in the entorhinally denervated zones: the stratum lacunosum-molecular (SLM) of the hippocampus and the outer molecular layer (OML) of the dentate gyrus (DG), which initiated as early as at 1 dpl, reached the maximum at 3 dpl, remained prominently elevated by 7 dpl, and discernibly higher at 15 dpl than that of controls. Double labeling of either gelsolin mRNA or protein with markers of glial cells (Griffonia simplicifolia IB4 and CD11b for microglial cells, GFAP for astroglial cells) revealed that gelsolin was highly expressed by both activated microglia and astrocytes. The results suggest that the spatiotemporal upregulation of gelsolin in the hippocampus is induced by entorhinal deafferentation, and that gelsolin would participate in the activation processes of both microglial and astroglial cells and thereby, indirectly play important roles in the subsequent lesion-induced neural reorganization in the hippocampus following entorhinal deafferentation.

Overexpression of myelin-associated glycoprotein after axotomy of the perforant pathway

Myelin-associated glycoprotein (MAG) contributes to the prevention of axonal regeneration in the adult central nervous system (CNS). However, changes in MAG expression following lesions and the involvement of MAG in the failure of cortical connections to regenerate are still poorly understood. Here, we show that MAG expression is differently regulated in the entorhinal cortex (EC) and the hippocampus in response to axotomy of the perforant pathway. In the EC, MAG mRNA is transiently overexpressed by mature oligodendrocytes after lesion. In the hippocampus, MAG overexpression is accompanied by an increase in the number of MAG-expressing cells. Lastly, the participation of MAG in preventing axonal regeneration was tested in vitro, where neuraminidase treatment of axotomized entorhino-hippocampal cultures potentiates axonal regeneration. These results demonstrate that MAG expression is regulated in response to cortical axotomy, and indicate that it may limit axonal regeneration after CNS injury.

Induction of ephrin-B1 and EphB receptors during denervation-induced plasticity in the adult mouse hippocampus

Abstract It has been widely demonstrated that Eph receptors and their ephrin ligands play multiple pivotal roles in the development of the nervous system. However, less is known about their roles in the adult brain. Here we reported the expression of ephrin-B1 and its cognate EphB receptors in the adult mouse hippocampus at 3, 7, 15, 30 and 60 days after transections of the entorhinal afferents. In situ hybridization and immunohistochemistry showed the time-dependent up-regulation of ephrin-B1 in the denervated areas of the hippocampus, which initiated at 3 days postlesion (dpl), reached maximal levels at 7-15 dpl, remained slightly elevated at 30 dpl and recovered to normal levels by 60 dpl. Double labeling of ephrin-B1 and glial fibrillary acidic protein revealed that ephrin-B1-expressing cells in the denervated areas were reactive astrocytes. Furthermore, a ligand-binding assay using ephrin-B1/Fc chimera protein also displayed the up-regulation of EphB receptors in the denervated areas of the hippocampus in a similar manner to that of ephrin-B1. Within the first week postlesion, the EphB receptors were expressed by reactive astrocytes. After 7 dpl, however, EphB receptors were expressed not only by reactive astrocytes but also first by sprouting axons and later by regrowing dendrites. These results suggest that the ephrin-B1/EphB system may participate in the lesion-induced plasticity processes in the adult mouse hippocampus.

Entorhinal deafferentation induces the expression of profilin mRNA in the reactive microglial cells in the hippocampus

Profilin has been identified as an actin monomer sequestering protein and is thought to be a key regulator of actin polymerization in many fundamental cellular processes. We report the expression of profilin mRNA in the murine hippocampus following transections of the entorhinal afferents. Northern blot analysis showed that transcript of profilin was upregulated in a transient manner in the deafferented rat hippocampus by 1.5-, 1.9-, 1.4-, and 1.1-fold of controls, respectively, at 1, 3, 7, and 15 days post-lesion. In situ hybridization confirmed the temporal upregulation of profilin mRNA in the deafferented zones of the mouse hippocampus, which showed a remarkable increase as early as at 1 day post-lesion, reached maximal level at 3 days post-lesion, and returned to the control level at 15 days post-lesion. The expression modulation of profilin mRNA was observed to occur specifically in the entorhinally denervated zones: the stratum lacunosum-moleculare of the hippocampus and the outer molecular layer of the dentate gyrus. The combination of in situ hybridization for profilin mRNA with lectin cytochemistry for Griffonia simplicifolia IB4 showed that the cells expressing profilin transcript in the denervated zones are activated microglial cells. The results suggest that the spatial and temporal upregulation of profilin mRNA in the hippocampus is induced by entorhinal deafferentation and profilin is involved in microglial activation associated with morphological change, migration, and phagocytic behavior of microglial cells.

Regulation of Nogo and Nogo receptor during the development of the entorhino-hippocampal pathway and after adult hippocampal lesions

Axonal regeneration in the adult CNS is limited by the presence of several inhibitory proteins associated with myelin. Nogo-A, a myelin-associated inhibitor, is responsible for axonal outgrowth inhibition in vivo and in vitro. Here we study the onset and maturation of Nogo-A and Nogo receptor in the entorhino-hippocampal formation of developing and adult mice. We also provide evidence that Nogo-A does not inhibit embryonic hippocampal neurons, in contrast to other cell types such as cerebellar granule cells. Our results also show that Nogo and Nogo receptor mRNA are expressed in the adult by both principal and local-circuit hippocampal neurons, and that after lesion, Nogo-A is also transiently expressed by a subset of reactive astrocytes. Furthermore, we analyzed their regulation after kainic acid (KA) treatment and in response to the transection of the entorhino-hippocampal connection. We found that Nogo-A and Nogo receptor are differentially regulated after kainic acid or perforant pathway lesions. Lastly, we show that the regenerative potential of lesioned entorhino-hippocampal organotypic slice co-cultures is increased after blockage of Nogo-A with two IN-1 blocking antibodies. In conclusion, our results show that Nogo and its receptor might play key roles during development of hippocampal connections and that they are implicated in neuronal plasticity in the adult.

A semi-automated image analysis method to quantify neurite preference/axon guidance on a patterned substratum

Axon outgrowth and guidance are differentially promoted or inhibited by specific extracellular matrix (ECM) molecules. The effects of these molecules can be examined by culturing neuronal explants on patterned substrata consisting of alternating stripes adsorbed with the molecules of interest. While outgrowth on substrata adsorbed with homogenous molecules can be reliably quantified, current methods of quantifying neurite preference on patterned substrata are subjective, labor intensive, and overall less reliable. Here, we present a quick, semi-automated, lowly subjective macro-based method to quantify the effects of a change in substratum on axon extension and guidance. We plated chick dorsal root ganglion explants on a substratum consisting of alternating stripes of laminin-1 (outgrowth supportive) and chondroitin sulfate proteoglycans (CSPGs, outgrowth inhibitory). We evaluated neurite preference for laminin or CSPG-coated regions by measuring total neurite area, and produced an inhibition index. The quantitative data confirmed previous qualitative data showing that increasing concentrations of CSPGs induced increases in inhibition. The methods presented here: (1) require less stringent image capture criteria; (2) are quicker; (3) are less subjective compared to previously described methods; and (4) are versatile in that they can be used to assay neurite preference for any substratum-bound molecules in living or fixed cultures.

Regeneration of entorhinal fibers in mouse slice cultures is age dependent and can be stimulated by NT-4, GDNF, and modulators of G-proteins and protein kinase C

Axonal regeneration after lesions is normally not possible in the mature central nervous system, but occurs in the embryonic and neonatal nervous system. Slice cultures offer a convenient experimental system to study the decline of axonal regeneration with increasing maturation of central nervous system tissue. We have used mouse entorhinohippocampal slice cultures to assess regeneration of entorhinal fibers after mechanical lesions in vitro. We found that entorhinal axons regenerate well in cultures derived from postnatal days 5-7 mouse pups when the lesion is made at the second and fourth days in vitro (DIV 2 and DIV 4). Only little regenerative outgrowth is seen after lesions made at DIV 6 and DIV 10. This indicates that a maturation of the cultures occurs within a short time period in vitro resulting in a loss of the regenerative potential. We have used this system to screen for neurotrophic factors and pharmacological compounds that may promote axonal regeneration. Treatments were added to the cultures 1 day before the lesion was made. We found that most added factors did not promote regeneration. Only treatment with the neurotrophic factors NT-4 and GDNF stimulated regeneration in cultures where normally little regeneration is found. A similar improvement of regeneration was found after treatment with pertussis toxin, an inhibitor of G(i)-proteins, and with GF109203X, an inhibitor of protein kinase C. These substances may promote regeneration by interfering with intracellular signaling pathways activated by outgrowth inhibitors. Our findings indicate that the application of neurotrophic factors and the modulation of intracellular signal transduction pathways could be useful strategies to enhance axonal regeneration in a complex microenvironment.

New molecules for hippocampal development

Pathfinding by developing axons towards their proper targets is an essential step in establishing appropriate neuronal connections. Recent work involving cell culture assays and molecular biology strategies, including knockout animals, strongly indicates that a complex network of guidance signals regulates the formation of hippocampal connections during development. Outgrowing axons are routed towards the hippocampal formation by specific expression of long-range cues, which include secreted class 3 semaphorins, netrin 1 and Slit proteins. Local membrane- or substrate-anchored molecules, such as ligands of the ephrin A subclass, provide layer-specific positional information. Understanding the molecular mechanisms that underlie axonal guidance during hippocampal development might be of importance in making therapeutic use of sprouting fibers, which are produced following the loss of afferents in CNS lesion.

Involvement of non-neuronal cells in entorhinal-hippocampal reorganization following lesions

Entorhinal lesion leads to anterograde degeneration of perforant path fibers in their main hippocampal termination zones. Subsequently, remaining fibers sprout and form new synapses on the denervated dendrites. This degeneration and reorganization is accompanied by sequential changes in glial morphology and function. Within a few hours following the lesion, amoeboid microglia migrate into the zone of denervation. Some hours later, signs of activation can be seen on astrocytes in the zone of denervation, where both cell types proliferate and remain in an activated state for more than two weeks. These activated glial cells might be involved in lesion-induced plasticity in at least two ways: (1) by releasing cytokines and growth factors which regulate layer-specific sprouting and (2) by phagocytosis of axonal debris, because myelin sheaths act as obstacles for sprouting fibers in the central nervous system. Whereas direct evidence for the former is still missing, the latter was investigated using phagocytosis-dependent labeling techniques. Both microglial cells and astrocytes incorporate axonal debris. Phagocytosing microglial cells develop the immune phenotype of antigen-presenting cells, whereas astrocytes strongly express FasL (CD95L), which induces apoptosis of activated lymphocytes. Thus, the interaction of glial cells with immune cells might be another, previously underestimated, aspect of reorganization following entorhinal lesion.

Outgrowth-promoting molecules in the adult hippocampus after perforant path lesion

Lesion-induced neuronal plasticity in the adult central nervous system of higher vertebrates appears to be controlled by region- and layer-specific molecules. In this study we demonstrate that membrane-bound hippocampal outgrowth-promoting molecules, as present during the development of the entorhino-hippocampal system and absent or masked in the adult hippocampus, appear 10 days after transection of the perforant pathway. We used an outgrowth preference assay to analyse the outgrowth preference of axons from postnatal entorhinal explants on alternating membrane lanes obtained from hippocampus deafferented from its entorhinal input taken 4, 10, 20, 30 and 80 days post-lesion and from adult control hippocampus. Neurites from the entorhinal cortex preferred to extend axons on hippocampal membranes disconnected from their entorhinal input for 10 days in comparison with membranes obtained from unlesioned adult animals. Membranes obtained from hippocampi disconnected from their entorhinal input for 10 days were equally as attractive for growing entorhinal cortex (EC) axons as membranes from early postnatal hippocampi. Further analysis of membrane properties in an outgrowth length assay showed that entorhinal axons extended significantly longer on stripes of lesioned hippocampal membranes in comparison with unlesioned hippocampal membranes. This effect was most prominent 10 days after lesion, a time point at which axonal sprouting and reactive synaptogenesis are at their peak. Phospholipase treatment of membranes obtained from unlesioned hippocampi of adult animals strongly promoted the outgrowth length of entorhinal axons on these membranes but did not affect their outgrowth preference for deafferented hippocampal membranes. Our results indicate that membrane-bound outgrowth-promoting molecules are reactivated in the adult hippocampus following transection of the perforant pathway, and that neonatal entorhinal axons are able to respond to these molecules. These findings support the hypothesis of a temporal accessibility of membrane-bound factors governing the layer-specific sprouting of remaining axons following perforant path lesion in vivo.