Different signals control laminar specificity of commissural and entorhinal fibers to the dentate gyrus

Increased Callosal Connectivity in Reeler Mice Revealed by Brain-Wide Input Mapping of VIP Neurons in Barrel Cortex

The neocortex is composed of layers. Whether layers constitute an essential framework for the formation of functional circuits is not well understood. We investigated the brain-wide input connectivity of vasoactive intestinal polypeptide (VIP) expressing neurons in the reeler mouse. This mutant is characterized by a migration deficit of cortical neurons so that no layers are formed. Still, neurons retain their properties and reeler mice show little cognitive impairment. We focused on VIP neurons because they are known to receive strong long-range inputs and have a typical laminar bias toward upper layers. In reeler, these neurons are more dispersed across the cortex. We mapped the brain-wide inputs of VIP neurons in barrel cortex of wild-type and reeler mice with rabies virus tracing. Innervation by subcortical inputs was not altered in reeler, in contrast to the cortical circuitry. Numbers of long-range ipsilateral cortical inputs were reduced in reeler, while contralateral inputs were strongly increased. Reeler mice had more callosal projection neurons. Hence, the corpus callosum was larger in reeler as shown by structural imaging. We argue that, in the absence of cortical layers, circuits with subcortical structures are maintained but cortical neurons establish a different network that largely preserves cognitive functions.

Progesterone receptor expression in cajal-retzius cells of the developing rat dentate gyrus: Potential role in hippocampus-dependent memory

The development of medial temporal lobe circuits is critical for subsequent learning and memory functions later in life. The present study reports the expression of progesterone receptor (PR), a powerful transcription factor of the nuclear steroid receptor superfamily, in Cajal-Retzius cells of the molecular layer of the dentate gyrus of rats. PR was transiently expressed from the day of birth through postnatal day 21, but was absent thereafter. Although PR immunoreactive (PR-ir) cells did not clearly express typical markers of mature neurons, they possessed an ultrastructural morphology consistent with neurons. PRir cells did not express markers for GABAergic neurons, neuronal precursor cells, nor radial glia. However, virtually all PR cells co-expressed the calcium binding protein, calretinin, and the glycoprotein, reelin, both reliable markers for Cajal-Retzius neurons, a transient population of developmentally critical pioneer neurons that guide synaptogenesis of perforant path afferents and histogenesis of the dentate gyrus. Indeed, inhibition of PR activity during the first two weeks of life impaired adult performance on both the novel object recognition and object placement memory tasks, two behavioral tasks hypothesized to describe facets of episodic-like memory in rodents. These findings suggest that PR plays an unexplored and important role in the development of hippocampal circuitry and adult memory function.

P2Y(1) receptor mediated neuronal fibre outgrowth in organotypic brain slice co-cultures

Extracellular purines have multiple functional roles in development, plastic remodelling, and regeneration of the CNS by stimulating certain P2X/Y receptor (R) subtypes. In the present study we elucidated the involvement of P2YRs in neuronal fibre outgrowth in the developing nervous system. We particularly focused on the P2Y1R subtype and the dopaminergic system, respectively. For this purpose, we used organotypic slice co-cultures consisting of the ventral tegmental area/substantia nigra (VTA/SN) and the prefrontal cortex (PFC). After detecting the presence of the P2Y1R in VTA/SN, PFC, and on outgrowing fibres in the border region (e.g. on glial processes) connecting both brain slices, we could show that pharmacological modulation of the receptor influenced neuronal fibre outgrowth. Biocytin-tracing and tyrosine hydroxylase-immunolabelling together with quantitative image analysis revealed a significant increase in fibre growth in the border region of the co-cultures after treatment with ADPβS (P2Y1,12,13R agonist). The observed stimulatory potential of ADPβS was inhibited by pre-treatment with the P2X/YR antagonist PPADS. In P2Y1R knockout (P2Y1R(-/-)) mice, the ADPβS-induced stimulatory effect was absent, while growth was significantly enhanced in the co-cultures of the respective wild-type. This observation was confirmed in entorhino-hippocampal co-cultures, an example of a different projection system, expressing the P2Y1R. Using wortmannin and PD98059 we further showed that PI3K/Akt and MAPK/ERK cascades are involved in the mechanism underlying ADPβS-induced fibre growth. In conclusion, the data of this study clearly indicate that activation of the P2Y1R stimulates fibre growth and thereby emphasises the general role of this particular receptor subtype during development and regeneration.

Repurposing Reelin: the new role of radial glia, Reelin and Notch in motor neuron migration

The role of Reelin during cerebral cortical neuron migration has long been studied, but the Reelin signaling pathway and its possible interactions are just beginning to be unraveled. Reelin is not only important in cerebral cortical migration, but has recently been shown to interact with the Notch signaling pathway and to be critical for radial glial cell number and morphology. Lee and Song (2013) show a new Notch- and Reelin-dependent role for radial glia in the mouse spinal cord: to act as a fine filter that allows somatic motor neuron axons but not cell bodies to traverse out of the CNS. Here, the types of neuronal migration are discussed, focusing on motor neurons and cues for proper localization. The interaction of Reelin signaling with the Notch pathway is reviewed, which dictates the proper formation of radial glia in the spinal cord in order to prevent ectopic motor neuron migration (Lee and Song, 2013). Future studies may reveal novel interactions and further insights as to how Reelin functions throughout the developing nervous system.

Hippocampal development - old and new findings

The hippocampus, derived from medial regions of the telencephalon, constitutes a remarkable brain structure. It is part of the limbic system, and it plays important roles in information encoding, related to short-term and long-term memory, and spatial navigation. It has also attracted the attention of many clinicians and neuroscientists for its involvement in a wide spectrum of pathological conditions, including epilepsy, intellectual disability, Alzheimer disease and others. Here we address the topic of hippocampal development. As well as original landmark findings, modern techniques such as large-scale in situ hybridizations, in utero electroporation and the study of mouse mutants with hippocampal phenotypes, add further detail to our knowledge of the finely regulated processes which form this intricate structure. Molecular signatures are being revealed related to field, intra-field and laminar cell identity, as well as, cell compartments expressing surface proteins instrumental for connectivity. We summarize here old and new findings, and highlight elegant tools used to fine-study hippocampal development.

The immune inhibitory complex CD200/CD200R is developmentally regulated in the mouse brain

The CD200/CD200R inhibitory immune ligand-receptor system regulates microglial activation/quiescence in adult brain. Here, we investigated CD200/CD200R at different stages of postnatal development, when microglial maturation takes place. We characterized the spatiotemporal, cellular, and quantitative expression pattern of CD200 and CD200R in the developing and adult C57/BL6 mice brain by immunofluorescent labeling and Western blotting. CD200 expression increased from postnatal day 1 (P1) to P5-P7, when maximum levels were found, and decreased to adulthood. CD200 was located surrounding neuronal bodies, and very prominently in cortical layer I, where CD200(+) structures included glial fibrillary acidic protein (GFAP)(+) astrocytes until P7. In the hippocampus, CD200 was mainly observed in the hippocampal fissure, where GFAP(+) /CD200(+) astrocytes were also found until P7. CD200(+) endothelium was seen in the hippocampal fissure and cortical blood vessels, notably from P14, showing maximum vascular CD200 in adults. CD200R(+) cells were a population of ameboid/pseudopodic Iba1(+) microglia/macrophages observed at all ages, but significantly decreasing with increasing age. CD200R(+) /Iba1(+) macrophages were prominent in the pial meninges and ventricle lining, mainly at P1-P5. CD200R(+) /Iba1(+) perivascular macrophages were observed in cortical and hippocampal fissure blood vessels, showing maximum density at P7, but being prominent until adulthood. CD200R(+) /Iba1(+) ameboid microglia in the cingulum at P1-P5 were the only CD200R(+) cells in the nervous tissue. In conclusion, the main sites of CD200/CD200R interaction seem to include the molecular layer and pial surface in neonates and blood vessels from P7 until adulthood, highlighting the possible role of the CD200/CD200R system in microglial development and renewal.

The synaptic remodeling between regenerated perforant pathway and granule cells in slice culture

In order to understand the synaptic remodeling in the course of axonal regeneration, the synaptic remodeling of the perforant path in hippocampus was investigated in the present study with entorhino-hippocampal coculture, DiI DiOlistic assay and transmission electron microscopy. The results showed that the regeneration of the perforant pathway occurred in entorhino-hippocampal slice coculture, and putative synaptic contacts formed between the regenerated fibers and dendritic spines of granule cells. Ultrastructural analysis confirmed the formation of new synaptic contacts. In conclusion, the synaptic formation implicated in the neuroregeneration could integrate into the network in CNS.

Proper layering is important for precisely timed activation of hippocampal mossy cells

The mammalian cortex exhibits a laminated structure that may underlie optimal synaptic connectivity and support temporally precise activation of neurons. In 'reeler' mice, the lack of the extracellular matrix protein Reelin leads to abnormal positioning of cortical neurons and disrupted layering. To address how these structural changes impact neuronal function, we combined electrophysiological and neuroanatomical techniques to investigate the synaptic activation of hippocampal mossy cells (MCs), the cell type that integrates the output of dentate gyrus granule cells (GCs). While somatodendritic domains of wild-type (WT) MCs were confined to the hilus, the somata and dendrites of reeler MCs were often found in the molecular layer, where the perforant path (PP) terminates. Most reeler MCs received aberrant monosynaptic excitatory input from the PP, whereas the disynaptic input to MCs via GCs was decreased and inhibition was increased. In contrast to the uniform disynaptic discharge of WT MCs, many reeler cells discharged with short, monosynaptic latencies, while others fired with long latencies over a broad temporal window in response to PP activation. Thus, disturbed lamination results in aberrant synaptic connectivity and altered timing of action potential generation. These results highlight the importance of a layered cortical structure for information processing.

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.

Many paths to synaptic specificity

The most impressive structural feature of the nervous system is the specificity of its synaptic connections. Even after axons have navigated long distances to reach target areas, they must still choose appropriate synaptic partners from the many potential partners within easy reach. In many cases, axons also select a particular domain of the postsynaptic cell on which to form a synapse. Thus, synapse formation is selective at both cellular and subcellular levels. Unsurprisingly, the nervous system uses multiple mechanisms to ensure proper connectivity; these include complementary labels, coordinated growth of synaptic partners, sorting of afferents, prohibition or elimination of inappropriate synapses, respecification of targets, and use of short-range guidance mechanisms or intermediate targets. Specification of any circuit is likely to involve integration of multiple mechanisms. Recent studies of vertebrate and invertebrate systems have led to the identification of molecules that mediate a few of these interactions.

The crosstalk of hyaluronan-based extracellular matrix and synapses

Many neurons and their synapses are enwrapped in a brain-specific form of the extracellular matrix (ECM), the so-called perineuronal net (PNN). It forms late in the postnatal development around the time when synaptic contacts are stabilized. It is made of glycoproteins and proteoglycans of glial as well as neuronal origin. The major organizing polysaccharide of brain extracellular space is the polymeric carbohydrate hyaluronic acid (HA). It forms the backbone of a meshwork consisting of CNS proteoglycans such as the lectican family of chondroitin sulphate proteoglycans (CSPG). This family comprises four abundant components of brain ECM: aggrecan and versican as broadly expressed CSPGs and neurocan and brevican as nervous-system-specific family members. In this review, we intend to focus on the specific role of the HA-based ECM in synapse development and function.

Aberrant dentate gyrus cytoarchitecture and fiber lamination in Lis1 mutant mice

Mutant mice with a heterozygous deletion of LIS1, show varying degrees of hippocampal abnormality and enhanced excitability. To examine how LIS1 affects cytoarchitecture and fiber lamination in dentate gyrus (DG), we performed a series of immunohistochemistry studies. By using different neuronal- and glial-specific antibodies, we found that the majority of hippocampal cell populations were affected by heterozygous mutation of LIS1; some reelin-positive Cajal-Retzius cells were left undisturbed. Granule cell dispersion was significant in hippocampal sections from Lis1-deficient mice. However, the fiber termination of commissural/associational fibers and mossy fibers appeared relatively compact despite obvious granule cell dispersion and CA1-CA3 pyramidal cell disorganization. vGlut1-immunoreactive axon terminals were found aberrantly traversing the dispersed granule cell layer. Consistent with previous observations, we also found that immature granule cells in Lis1 mutants, here stained with antibodies to doublecortin (DCX) and Mash-1, are aberrantly located and bear an abnormal cellular morphology. Our findings suggest that LIS1 mutants exhibit abnormal cell positioning and aberrant hippocampal neurogenesis, but maintain relatively normal fiber termination patterns. The functional consequences of hippocampal granule cell dispersion could offer critical insight to the epileptic and cognitive disorder associated with LIS1 haploinsufficiency.

Expression of Disabled 1 suppresses astroglial differentiation in neural stem cells

Disabled 1 (Dab1), a cytoplasmic adaptor protein expressed predominantly in the CNS, transduces a Reelin-initiated signaling that controls neuronal migration and positioning during brain development. To determine the role of Dab1 in neural stem cell (NSC) differentiation, we established a culture of neurospheres derived from the embryonic forebrain of the Dab1(-/-) mice, yotari. Differentiating Dab1(-/-) neurospheres exhibited a higher expression of GFAP, an astrocytic marker, at the expense of neuronal markers. Under Dab1-deficient condition, the expression of NeuroD, a transcription factor for neuronal differentiation, was decreased and the JAK-STAT pathway was evidently increased during differentiation of NSC, suggesting the possible involvement of Dab1 in astrocyte differentiation via JAK-STAT pathway. Notably, expression of neural and glial markers and the level of JAK-STAT signaling molecules were not changed in differentiating NSC by Reelin treatment, indicating that differentiation of NSC is Reelin-independent. Immunohistochemical analyses showed a decrease in the number of neurons and an increase in the number of GFAP-positive cells in developing yotari brains. Our results suggest that Dab1 participates in the differentiation of NSCs into a specific cell lineage, thereby maintaining a balance between neurogenesis and gliogenesis.

Organotypic entorhino-hippocampal slice cultures--a tool to study the molecular and cellular regulation of axonal regeneration and collateral sprouting in vitro

Organotypic slice cultures of the brain are widely used as a tool to study fundamental questions in neuroscience. In this chapter, we focus on a protocol based on organotypic slice cultures of mouse entorhinal cortex and hippocampus that can be employed to study axonal regeneration and collateral sprouting in the central nervous system in vitro. Using pharmacological as well as genetic approaches, axonal regeneration and sprouting can be influenced, and some of the molecular and cellular mechanisms involved in these processes can be identified. The protocol describes in detail (1) the generation of organotypic entorhino-hippocampal slice cultures, (2) the conditions needed for the analysis of axonal regeneration and collateral sprouting, respectively, (3) the lesioning technique, (4) tracing techniques to visualize regenerating entorhinal axons, and (5) an immunohistochemical technique to visualize sprouting fibers.

Reelin, a guidance signal for the regeneration of the entorhino-hippocampal path

The importance of reelin for cortical lamination in the developing CNS is well established, but its role in nerve fiber growth is not well understood. In this study, hippocampal slices were co-cultured with entorhinal slices of GFP mice, in order to compare the growth of the entorhino-hippocampal path in wild type and reeler mice. On day 6, regenerated fibers were seen to navigate from the entorhinal cortex into the hippocampus. The results showed that in wild type mice, regenerated fibers grew along the molecular layer of the dentate gyrus, and only a few fibers were found to penetrate through the granular layer into the hilus. This specific orientation was similar to the perforant path in vivo. Compared with perforant path regeneration in wild type mice, reeler mice seemed to have lost their specific orientation and proper termination in the hippocampus. Without the guidance signal from reelin, the regenerated fibers left the molecular layers and continued to grow aberrantly, i.e., in the granular layer, hilus, pyramidal layer and even stratum oriens. Particularly in the dentate gyrus, the fibers meandered around the cells in the hilus and resembled a network. The study concludes that reelin also serves as an important guidance signal for neuroregeneration of the perforant path.

A 'GAG' reflex prevents repair of the damaged CNS

The extracellular matrix of the central nervous system (CNS) serves as both a supporting structure for cells and a rich source of signaling molecules that can influence cell proliferation, survival, migration and differentiation. A large proportion of this matrix is composed of proteoglycans--proteins with long chains of polysaccharides, called glycosaminoglycans (GAGs), covalently attached. Although many of the activities of proteoglycans depend on their core proteins, GAGs themselves can influence cell signaling. Here we review accumulating evidence that two GAGs, chondroitin sulfate and hyaluronan, play essential roles during nervous system development but also accumulate in chronic CNS lesions and inhibit axonal regeneration and remyelination, making them significant hindrances to CNS repair. We propose that the balance between the synthesis and degradation of these molecules dictates, in part, how regeneration and recovery from CNS damage occurs.

Effects of exogenous hyaluronan on midline crossing and axon divergence in the optic chiasm of mouse embryos

Perturbation of the transmembrane glycoprotein, CD44, has been shown to cause multiple errors in axon routing in the mouse optic chiasm. In a recent report we have shown that the major CD44 ligand, hyaluronan (HA), is colocalized with CD44 at the midline of the chiasm, suggesting a possible contribution to the control of axon routing in the chiasm. We examined this issue by investigating the effects of exogenous HA on routing of axons in the chiasm in slice preparations of the optic pathway. In preparations of the E13 optic pathway, administration of exogenous HA produced a dose-dependent failure in midline crossing of the first generated optic axons. In E15 slices, when the adult pattern of axon divergence develops in the chiasm, anterograde filling of the optic axons showed an obvious reduction in the uncrossed pathway after HA treatment. This reduction was confirmed by retrograde filling of the ganglion cells in E15 slices, and later in E16 pathways where the uncrossed projection is better developed. Furthermore, we have demonstrated in explant cultures of the retina that HA, when presented in soluble or substrate-bound form, does not affect outgrowth and extension of retinal neurites. These findings together indicate the crucial functions of this matrix molecule in regulating midline crossing and axon divergence, probably through interactions with guidance molecules including CD44, at the midline of the chiasm.

The tracing study of developing entorhino-hippocampal pathway

The entorhino-hippocampal pathway is the major excitatory input from neurons of the entorhinal cortex on both ipsilateral and contralateral hippocampus/dentate gyrus. This fiber tract consists of the alvear path, the perforant path and a crossed commissural projection. In this study, the histogenesis and development of the various subsets of the entorhino-hippocampal projection have been investigated. DiI, DiO, Fast Blue tracing and calretinin immunocytochemistry as well as were carried out with pre and postnatal rats at different developmental stages. The alvear path and the commissural pathway start to develop as early as embryonic day E16, while the first perforant afferents reach the stratum lacunosum-moleculare of the hippocampus at E17 and at outer molecular layer of the denate gyrus at postnatal day 2. Retrograde tracing with DiI identifies entorhinal neurons in layer II-IV as the developmental origin of the entorhino-hippocampal pathway. Furthermore, calretinin immunocytochemistry revealed transitory Cajal-Retzius cells in the stratum lacunosum-moleculare of the hippocampus from E16. DiI labeling of entorhinal cortex fibers and combined calretinin-immunocytochemistry reveal a close relationship between Cajal-Retzius cells and entorhinal afferents. This temporal and spatial relationship suggests that Cajal-Retzius cell serves as a guiding cue for entorhinal afferents at early cortical development.

Postnatal development of entorhinodentate projection of the Reeler mutant mouse

We anterogradely labeled entorhinodentate axons by the injection of biotin dextran amine into the entorhinal cortex of adult wildtype and reeler mice to clarify whether the course and terminal endings of the reeler entorhinal projection are normal or not. We found that in the reeler mouse, biotin dextran amine-labeled entorhinodentate fibers arising from the entorhinal cortex curved around the hippocampal fissure instead of crossing it, whereas in the wildtype mouse, they crossed the fissure as a perforant pathway. Next, we examined carbocyanine dye (DiI) labeling of the immature entorhinodentate projection and the developmental changes of the hippocampal fissure during early postnatal days based on the laminin and glial fibrillary acidic protein (GFAP) immunohistochemistry. Injection of DiI into the entorhinal area of the wildtype and reeler mice at postnatal day 1 resulted in anterograde labeling of pioneer axons passing through the hippocampal fissure. However, follower axons could not penetrate through the hippocampal fissure in reeler mice, whereas in the normal controls, many DiI-labeled axons continued to pass through the fissure. GFAP immunohistochemistry demonstrated that GFAP-immunopositive astrocytes were abundant along the hippocampal fissure both in the wildtype and reeler mice at birth. In the wildtype mouse, GFAP-positive neurons nearby the fissure were decreasing in number during the early postnatal days, whereas in the reeler mouse, many GFAP-positive astrocytes were continuing to accumulate there. This barrier made of astrocytes in the reeler mouse may obstruct the ingrowth of the follower axons arising from the entorhinal cortex through the hippocampal fissure, resulting in the abnormal course of the entorhinodentate axons in this mutant.

Interactions between Plexin-A2, Plexin-A4, and Semaphorin 6A Control Lamina-Restricted Projection of Hippocampal Mossy Fibers

Hippocampal mossy fibers project preferentially to the stratum lucidum, the proximal-most lamina of the suprapyramidal region of CA3. The molecular mechanisms that govern this lamina-restricted projection are still unknown. We examined the projection pattern of mossy fibers in mutant mice for semaphorin receptors plexin-A2 and plexin-A4, and their ligand, the transmembrane semaphorin Sema6A. We found that plexin-A2 deficiency causes a shift of mossy fibers from the suprapyramidal region to the infra- and intrapyramidal regions, while plexin-A4 deficiency induces inappropriate spreading of mossy fibers within CA3. We also report that the plexin-A2 loss-of-function phenotype is genetically suppressed by Sema6A loss of function. Based on these results, we propose a model for the lamina-restricted projection of mossy fibers: the expression of plexin-A4 on mossy fibers prevents them from entering the Sema6A-expressing suprapyramidal region of CA3 and restricts them to the proximal-most part, where Sema6A repulsive activity is attenuated by plexin-A2.

Loss of input from the mossy cells blocks maturation of newly generated granule cells

The objective of this work is to check whether the input from the mossy cells to the inner molecular layer is necessary for the integration and maturation of the newly generated granule cells of the dentate gyrus (DG) in mice, and if after status epilepticus the sprouting of the mossy fibers can substitute for this projection. Newly generated cells were labeled by administration of 5-bromo-deoxyuridine either before or after pilocarpine administration. The neuronal loss in the hippocampus after administration of pilocarpine combined with scopolamine and diazepam seemed restricted to the hilar mossy cells. The maturation of the granule cells was studied using immunohistochemistry for calretinin and NeuN in combination with detection of 5-bromo-deoxyuridine. The sprouting of the mossy fibers was detected using Timm staining for zinc-rich boutons. In normal conditions, granule cells took about 2 weeks to lose the immature marker calretinin. After the loss of the mossy cells, newly generated granule cells remained expressing calretinin for more than a month, until the sprouting of the mossy fibers substituted for the projection of the mossy cells in the inner molecular layer of the DG. Therefore, a proper pattern of connectivity is necessary for the normal development and integration of newly generated granule cells in the adult brain. In a changed environment they cannot adapt transforming in other cell types; simply they are unable to mature. The sprouting of the mossy fibers, although aberrant and a probable source of epileptic activity, may be important for the correct physiology of the granule cells, restoring a likeness of normality in their connective environment. The survival of granule cells incorporated as mature neurons was increased after pilocarpine when compared with normal conditions. Thus, it is likely that the reorganization of the circuitry after the loss of the mossy cells facilitates the survival and incorporation of the newly generated granule cells.

Development of cell and fiber layers in the dentate gyrus

This chapter deals with the laminated organization of the dentate gyrus, particularly with the molecular signals controlling its development. First, sites of granule cell generation, their modes and routes of migration are described. This is followed by an analysis of the molecular determinants governing the formation of a tightly packed granule cell layer that is normal in rodents and primates. Reelin, a protein of the extracellular matrix, plays an important role for the proper migration and lamination of the granule cells during development and for the maintenance of a laminated dentate gyrus in adulthood. Granule cell positioning is crucial for the laminated termination of commissural/associational fibers to the dentate gyrus, suggesting that the granule cells carry positional signals for these fibers. In contrast, not signals of the target cells but molecules of the extracellular matrix, such as hyaluronan, underlie the layer-specific termination of fibers from the entorhinal cortex. The molecular determinants controlling axonal pathfinding and target recognition of the profusely terminating cholinergic and GABAergic subcortical afferents still need to be elucidated.

Potential roles for hyaluronan and CD44 in kainic acid-induced mossy fiber sprouting in organotypic hippocampal slice cultures

The most well-documented synaptic rearrangement associated with temporal lobe epilepsy is mossy fiber sprouting (MFS). MFS is a pronounced expansion of granule cell mossy fiber axons into the inner dentate molecular layer. The recurrent excitatory network formed by MFS is hypothesized to play a critical role in epileptogenesis, which is the transformation of the normal brain into one that is prone to recurrent spontaneous seizures. While many studies have focused on the functional consequences of MFS, relatively few have investigated the molecular mechanisms underlying the increased propensity of mossy fibers to invade the inner molecular layer. We hypothesized that changes in two components of the extracellular matrix, hyaluronan and its primary receptor, CD44, contribute to MFS. Hyaluronan contributes to laminar-specificity in the hippocampus and increases in hyaluronan and CD44 are associated with temporal lobe epilepsy. We tested our hypothesis in an in vitro model of MFS using a combination of histological and biochemical approaches. Application of kainic acid (KA) to organotypic hippocampal slice cultures induced robust MFS into the inner dentate molecular layer compared with vehicle-treated controls. Degradation of hyaluronan with hyaluronidase significantly reduced but did not eliminate KA-induced MFS, suggesting that hyaluronan played a permissive role in MFS, but that loss of hyaluronan signaling alone was not sufficient to block mossy fiber reorganization. Comparison of CD44 expression with MFS revealed that when CD44 expression in the molecular layers was high, MFS was minimal and when CD44 expression/function was reduced following KA treatment or with function blocking antibodies, MFS was increased. The time course of KA-induced reductions in CD44 expression was identical to the temporal progression of KA-induced MFS reported previously in hippocampal slice cultures, suggesting that reduced CD44 expression may help promote MFS. Understanding the molecular mechanisms underlying MFS may lead to therapeutic interventions that limit epileptogenesis.

Laminating the hippocampus

Lamination of neurons and fibre projections is a fundamental organizational principle of the mammalian cerebral cortex. A laminated organization is likely to be essential for cortical function, as studies in mutant mice have revealed causal relationships between lamination defects and functional deficits. Unveiling the determinants of the laminated cortical architecture will contribute to our understanding of how cortical functions have evolved in phylogenetic and ontogenetic development. Recently, the hippocampus, with its clearly segregated cell and fibre layers, has become a major subject of studies on cortical lamination.

P2 receptor-stimulation influences axonal outgrowth in the developing hippocampus in vitro

Extracellular ATP might act as a trophic factor on growing axons during development of the CNS via P2 receptors. In the present study the postnatal presence of selected P2 receptor subtypes was analyzed and their putative trophic capacity in entorhino-hippocampal slice co-cultures of mouse brain was tested. The effect of the P2 receptor ligands 2-methylthioadenosine-5'-triphosphate (P2X/Y receptor agonist) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (P2X/Y receptor antagonist) on axonal growth and fiber density of biocytin-labeled hippocampal projections was compared both with untreated cultures and with cultures treated with artificial cerebrospinal fluid. After 10 days in vitro, double immunofluorescence labeling revealed the expression of P2X(1), P2X(2), P2X(4) as well as P2Y(1) and P2Y(2) receptors in the examined regions of entorhinal fiber termination. Further, quantitative analysis of identified biocytin-traced entorhinal fibers showed a significant increase in fiber density in the dentate gyrus after incubation of the slices with the P2 receptor agonist 2-methylthioadenosine-5'-triphosphate. This neurite outgrowth promoting effect was completely abolished by the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid. Our in vitro data indicate that ATP via its P2X and P2Y receptors can shape hippocampal connectivity during development.

Patterns of afferent projections to the dentate gyrus studied in organotypic co-cultures

Cholinergic axons originating from the septum form a characteristic layer of preterminal axons and apparent termination in the molecular layer of the hippocampal dentate gyrus. The present study explored the specificity of this characteristic axonal pattern, through the use of organotypic slice co-cultures. Slices of hippocampus were co-cultured with a slice from one of a variety of other potential sources of afferents, and the afferent axons were labeled histochemically or immunocytochemically to determine which afferents distribute within the dentate molecular layer in a pattern similar to that formed by septal cholinergic projections. Acetylcholinesterase (AChE) histochemistry demonstrated that cholinergic axons from septum, substantia innominata, and striatum all consistently targeted the inner molecular layer of the dentate gyrus. AChE-labeled cholinergic axons from dorsal lateral pontine tegmentum and from spinal cord sometimes formed this pattern, while axons from the habenula failed to extend into the dentate gyrus. Immunocytochemically identified monoaminergic axons from the substantia nigra, locus coeruleus, and raphe extended into co-cultured hippocampus; each of these afferent systems displayed a prominent axonal plexus within the hilus of the dentate, but only the raphe axons projected prominently to the molecular layer. These data demonstrate that the molecular layer of the dentate gyrus provides an attractive target zone for some cholinergic and monoaminergic afferents, but not all. Commonalities between neuronal populations that preferentially project to the molecular layer in vitro may offer clues regarding the axon guidance mechanisms that normally direct cholinergic axons to target sites in the dentate gyrus molecular layer.

Modulation of dendritic differentiation by corticotropin-releasing factor in the developing hippocampus

The interplay of environmental and genetic factors in the developmental organization of the hippocampus has not been fully elucidated. The neuropeptide corticotropin-releasing factor (CRF) is released from hippocampal interneurons by environmental signals, including stress, to increase synaptic efficacy. In the early postnatal hippocampus, we have previously characterized a transient population of CRF-expressing Cajal-Retzius-like cells. Here we queried whether this stress-activated neuromodulator influences connectivity in the developing hippocampal network. Using mice deficient in the principal hippocampal CRF receptor [CRF(1)(-/-)] and organotypic cultures grown in the presence of synthetic CRF, or CRF receptor antagonists, we found robust effects of CRF on dendritic differentiation in hippocampal neurons. In CRF(1)(-/-) mice, the dendritic trees of hippocampal principal cells were exuberant, an effect that was induced in normal hippocampi in vitro by the presence of CRF(1) antagonists. In both cases, total dendritic length and dendritic branching were significantly increased. In contrast, exogenous synthetic CRF blunted the dendritic growth in hippocampal organotypic cultures. Taken together, these findings suggest that endogenous CRF, if released excessively by previous early postnatal stress, might influence neuronal connectivity and thus function of the immature hippocampus.

Reelin is a positional signal for the lamination of dentate granule cells

Reelin is required for the proper positioning of neurons in the cerebral cortex. In the reeler mutant lacking reelin, the granule cells of the dentate gyrus fail to form a regular, densely packed cell layer. Recent evidence suggests that this defect is due to the malformation of radial glial processes required for granule cell migration. Here, we show that recombinant reelin in the medium significantly increases the length of GFAP-positive radial glial fibers in slice cultures of reeler hippocampus, but does not rescue either radial glial fiber orientation or granule cell lamination. However, rescue of radial glial fiber orientation and granule cell lamination was achieved when reelin was present in the normotopic position provided by wild-type co-culture, an effect that is blocked by the CR-50 antibody against reelin. These results indicate a dual function of reelin in the dentate gyrus,as a differentiation factor for radial glial cells and as a positional cue for radial fiber orientation and granule cell migration.

Repair of the entorhino-hippocampal projection in vitro

The repair of axonal projections and the reconstruction of neuronal circuits after CNS lesions or during neurodegenerative disease are major challenges in restorative neuroscience. We have explored the potential of transplanted immature neurons to repair a specific axonal projection in an entorhino-hippocampal slice culture model system. When slices of immature entorhinal cortex (EC) from tau-GFP transgenic mice were cultured next to slices from postnatal hippocampus, an axonal projection from the E18 embryonic entorhinal cortex to the dentate gyrus of the postnatal hippocampus developed, which was similar to that observed in control cultures. Even more immature neuronal precursors in slices from E15 developing cerebral cortex differentiated and established an axonal projection to the hippocampal slice. This projection terminated specifically in the outer molecular layer of the dentate gyrus, the normal target area of the entorhino-hippocampal projection. When embryonic tissue from the presumptive brainstem area was used, there was still a subpopulation of fibers with a specific termination in the outer molecular layer, but few specific fibers were found in cocultures with embryonic midbrain. Our results show that very immature cortical neurons are potentially able to form an entorhino-hippocampal projection that terminates in a correct lamina-specific fashion in the dentate gyrus. These findings support the idea that immature neuronal precursor cells could be used for the reconstruction of specific neuronal circuits.

Laminar organization of the mouse dentate gyrus: Insights from BETA2/Neuro D mutant mice

The dentate gyrus of rodents is characterized by a highly laminar organization: above a compact granule cell layer, commissural/associational (C/A) fibers terminate on proximal granule cell dendrites and entorhinal fibers terminate on distal granule cell dendrites in a nonoverlapping manner. To gain insights into mechanisms that underlie the formation of this laminar structure, we studied mice deficient for BETA2/NeuroD, a basic helix-loop-helix transcription factor essential for granule cell differentiation. Anterograde tracing was used to label C/A and entorhinal fibers and combined with confocal double immunofluorescence for calbindin, calretinin, parvalbumin, and reelin to visualize putative target cells. The dentate gyrus of mutant mice contained only few granule cells, which formed a cap-like structure adjacent to area CA3. Despite the severe hypoplasia of the dentate gyrus, the remaining BETA2/NeuroD-deficient granule cells expressed mature markers, extended dendrites into the molecular layer, and extended mossy fibers into area CA3. Entorhinal and C/A fibers terminated in a nonoverlapping manner in the dendritic field overlying the rudiment. Entorhinal fibers terminated in the outermost portion of the dentate gyrus where they surrounded reelin-positive Cajal-Retzius cells, and C/A fibers terminated above and within the dentate rudiment. The laminar termination of C/A fibers was closest to normal in zones of the rudiment in which granule cells were densely packed. These data indicate that granule cells are able to differentiate in the absence of BETA2/NeuroD and suggest that the signals underlying the laminar anatomy of the dentate gyrus are present in the absence of most target cells.