The OM series of terminal field-specific monoclonal antibodies demonstrate reinnervation of the adult rat dentate gyrus by embryonic entorhinal transplants

Functions of Plexins/Neuropilins and Their Ligands during Hippocampal Development and Neurodegeneration

There is emerging evidence that molecules, receptors, and signaling mechanisms involved in vascular development also play crucial roles during the development of the nervous system. Among others, specific semaphorins and their receptors (neuropilins and plexins) have, in recent years, attracted the attention of researchers due to their pleiotropy of functions. Their functions, mainly associated with control of the cellular cytoskeleton, include control of cell migration, cell morphology, and synapse remodeling. Here, we will focus on their roles in the hippocampal formation that plays a crucial role in memory and learning as it is a prime target during neurodegeneration.

New functions of Semaphorin 3E and its receptor PlexinD1 during developing and adult hippocampal formation

The development and maturation of cortical circuits relies on the coordinated actions of long and short range axonal guidance cues. In this regard, the class 3 semaphorins and their receptors have been seen to be involved in the development and maturation of the hippocampal connections. However, although the role of most of their family members have been described, very few data about the participation of Semaphorin 3E (Sema3E) and its receptor PlexinD1 during the development and maturation of the entorhino-hippocampal (EH) connection are available. In the present study, we focused on determining their roles both during development and in adulthood. We determined a relevant role for Sema3E/PlexinD1 in the layer-specific development of the EH connection. Indeed, mice lacking Sema3E/PlexinD1 signalling showed aberrant layering of entorhinal axons in the hippocampus during embryonic and perinatal stages. In addition, absence of Sema3E/PlexinD1 signalling results in further changes in postnatal and adult hippocampal formation, such as numerous misrouted ectopic mossy fibers. More relevantly, we describe how subgranular cells express PlexinD1 and how the absence of Sema3E induces a dysregulation of the proliferation of dentate gyrus progenitors leading to the presence of ectopic cells in the molecular layer. Lastly, Sema3E mutant mice displayed increased network excitability both in the dentate gyrus and the hippocampus proper.

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.

Monoclonal Antibodies to Late-differentiating Epitopes Identify Mossy Fibre Terminals Innervating Normal and Transplanted Hippocampal CA3 Pyramidal Cells

We have derived two monoclonal antibodies, MF-1 and MF-2, which both recognize the same 58-kD antigen. Light and electron microscopic immunocytochemistry showed that this antigen is highly expressed in the large mossy fibre terminals innervating the proximal portion of the apical dendrites of pyramidal neurons in hippocampal field CA3. Staining was seen in the adult hippocampus in rats and mice, and in a post mortem human sample. Comparison with the Timm stain showed that the antibodies recognize mossy fibres from all parts of the adult dendate gyrus except for the tip of the infrapyramidal blade (the latest part of the dentate gyrus to develop). The MF antigen is expressed by mature terminals, and is not detected immunohistochemically in developing hippocampal mossy terminals until the end of the first postnatal week (i.e. later than the Timm-positive material). It was also found in host mossy fibre terminals innervating embryonic CA3 pyramids transplanted into adult hosts, but not in areas of the graft containing transplanted CA1 pyramids. These results indicate that this previously undescribed, late-developing antigen provides a useful specific marker for the mossy fibre projection in both the normal hippocampus and in situations of experimentally manipulated connectivity.

Laminar boundaries persist in the hippocampal dentate molecular layer of the mutant Shaking Rat Kawasaki despite aberrant granule cell migration

The present report provides the first detailed description of the hippocampus in the Shaking Rat Kawasaki (SRK) mutant by using a panel of antibody markers to delineate its laminar organization. The mutant was characterised at postnatal day 21 by severe malformations of both neuronal position and orientation, the most striking of which was the presence of a rounded central granule cell mass in the dentate gyrus rather than the normal V-shaped granule cell layer. Despite this finding, the SRK dentate gyrus not only retained a cell-sparse molecular layer (thinner but similar in gross appearance to that of control littermates), but the sharp laminar boundary between its inner and outer parts was as clearly marked by IM1 and OM4 antibody staining as it was in the normal dentate gyrus. These immunocytochemical data suggest that the entorhinal terminal field of the dentate gyrus may be relatively normal in the mutant, despite entorhinal afferents appearing to take an abnormal trajectory after they fail to cross the hippocampal fissure. Laminar malformations included disruption of the SRK pyramidal cell layer, with spreading of the CA3 mossy fibre projection to an ectopic infrapyramidal position, radial displacement of CA1 pyramids, and transposition of a hitherto unremarked longitudinal fibre bundle immunoreactive for calretinin from its normal position in the stratum lacunosum-moleculare of field CA2 to an alvear position in SRK. The SRK malformations were very like but not identical to those seen in the mouse reeler mutant, suggesting similar underlying developmental mechanisms.

Morphological features of the entorhinal-hippocampal connection

The goal of this review in an overview of the structural elements of the entorhinal-hippocampal connection. The development of the dendrites of hippocampal neurons will be outlined in relation to afferent pathway specificity and the mature dendritic structure compared. Interneurons will be contrasted to pyramidal cells in terms of processing of physiological signals and convergence and divergence in control of hippocampal circuits. Mechanisms of axonal guidance and target recognition, target structures, the involvement of receptor distribution on hippocampal dendrites and the involvement of non-neuronal cellular elements in the establishment of specific connections will be presented. Mechanisms relevant for the maintenance of shape and morphological specializations of hippocampal dendrites will be reviewed. One of the significant contexts in which to view these structural elements is the degree of plasticity in which they participate, during development and origination of dendrites, mature synaptic plasticity and after lesions, when the cells must continue to maintain and reconstitute function, to remain part of the circuitry in the hippocampus. This review will be presented in four main sections: (1) interneurons-development, role in synchronizing influence and hippocampal network functioning; (2) principal cells in CA1, CA3 and dentate gyrus regions-their development, function in terms of synaptic integration, differentiating structure and alterations with lesions; (3) glia and glia/neuronal interactions-response to lesions and developmental guidance mechanisms; and (4) network and circuit aspects of hippocampal morphology and functioning. Finally, the interwoven role of these various elements participating in hippocampal network function will be discussed.

Density of choline acetyltransferase-immunoreactive terminals in the rat dentate gyrus after entorhinal cortex lesions: a quantitative light microscope study

Lesion of the entorhinal cortex in the adult rat is a model for Alzheimer's disease and produces a marked increase in acetylcholinesterase (AChE) activity in the outer molecular layer (OML) of the dentate gyrus. This has been attributed to the sprouting of cholinergic axons terminals in response to denervation of the OML. The aim of this study was to investigate the density changes of cholinergic terminals in the OML at the light microscope level by using choline acetyltransferase (ChAT) immunohistochemistry and quantitative analysis. The results showed that between days 10 and 33 after an entorhinal cortex lesion, there was a measurable increase in the density of ChAT-positive boutons in the OML of the ipsilateral dentate gyrus (x1.2-1.6 of contralateral). However, when shrinkage of the ipsilateral OML (x0.5-0.75 of contralateral) was taken into account, the apparent increase in ChAT terminal density was entirely accounted for by shrinkage of the OML. Thus ChAT immunohistochemistry at the light microscope level provides no positive evidence for a proliferation of cholinergic terminals in the entorhinal cortex lesion model. This is in agreement with previous biochemical assays that have shown no change of total ChAT activity in the dentate gyrus after entorhinal cortex lesions.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Contrasting effects of fetal CA1 and CA3 hippocampal grafts on deficits in spatial learning and working memory induced by global cerebral ischaemia in rats

Functional effects of fetal hippocampal field grafts were assessed in rats with spatial learning and memory impairments following global cerebral ischaemia. Experiment 1 examined effects of grafts dissected from fields CA1 and CA3 at embryonic day 19 and from the dentate gyrus at postnatal day 1. Cell suspensions (15,000 cells/site) were implanted bilaterally at two points above the dorsal CA1 area two weeks after four-vessel occlusion (electrocoagulation of the vertebral arteries followed the 24 h later by occlusion of the carotid arteries for 15 min). Histological examination showed that CA1 neuronal loss (60-70%) was equivalent in all ischaemic groups and that 80% of CA1 and 60% of CA3 grafts survived and were sited appropriately in the alveus or corpus callosum above the area of ischaemic CA1 damage in the host, but there was no survival of dentate grafts. Results from rats with poor pyramidal cell graft survival were excluded, but those from rats with non-surviving dentate grafts were retained as an additional control group. Acquisition in the water maze was examined nine and 25 weeks after transplantation, and spatial working memory was assessed in three-door runway and water maze matching-to-position tasks 19 and 28 weeks after grafting, respectively. For water maze acquisition rats were trained with two trails/day and a 10 min inter-trial interval for 10-12 days to locate a submerged platform. Ischaemic rats with CA1 grafts learned the platform position as rapidly as non-ischaemic controls, searched appropriately in the training quadrant and were accurate in heading towards the platform, but were initially impaired on recall of the precise platform position on probe trials with the platform removed. Performance of ischaemic controls and groups with CA3 and non-surviving dentate graft groups was significantly impaired relative to controls and to the CA1 grafted group. The CA1 grafted group was also as successful as controls in matching-to-position in the water maze and substantially superior to the other ischaemic groups, assessed using three trials/day, with a 30-s inter-trial interval and a different platform position on each day. In a more complex matching-to-position task in the three-door runway, the performance of the CA1 grafted group was significantly impaired relative to controls, although superior to that of the other ischaemic control and graft groups. Functional recovery with CA1, but not CA3, grafts in ischaemic rats was replicated in a second experiment which assessed water maze acquisition and working memory at 10 and 14 weeks after transplantation, in rats with 90% graft survival. These results indicate that long-lasting, task-dependent improvements can be seen in ischaemic rats with CA1 fetal grafts in both aversively and appetitively motivated spatial learning tasks. The findings suggest that functional recovery requires homotypic replacement of CA1 cells damaged by ischaemia, rather than provision of structurally similar glutamate-releasing CA3 pyramidal cells.

Layer-specific sprouting of commissural fibres to the rat fascia dentata after unilateral entorhinal cortex lesion: a Phaseolus vulgaris leucoagglutinin tracing study

After unilateral entorhinal cortex lesion commissural fibres to the inner molecular layer of the rat fascia dentata are said to sprout into the former termination zone of entorhinal afferents. This sprouting process has not yet been demonstrated at the level of individual fibres. In the present study, Phaseolus vulgaris leucoagglutinin tracing was used to analyse the commissural projection to the inner molecular layer in rats with longstanding entorhinal cortex lesions. In comparison with controls, the commissural fibre plexus in the inner molecular layer had expanded by 20-45 microns outwards on the side of the entorhinal lesion. Unexpectedly, only a small number of axons arising from the bulk of commissural fibres in the inner molecular layer left the main fibre plexus and entered the outer molecular layer. Thus, there was still a clearly recognizable border between the Phaseolus vulgaris leucoagglutinin-labelled commissural fibre plexus in the inner molecular layer and the unstained outer molecular layer. The few commissural axons invading the outer molecular layer rarely branched but formed multiple en passant boutons, and occasionally exhibited growth cones. The data indicate that only few commissural fibres appear to be able to sprout beyond the border of their appropriate layer suggesting that the characteristic laminar specificity of hippocampal afferents is largely retained following deafferentation.

Sprouting of crossed entorhinodentate fibers after a unilateral entorhinal lesion: anterograde tracing of fiber reorganization with Phaseolus vulgaris-leucoagglutinin (PHAL)

Fibers from the contralateral entorhinal cortex (EC) to the dentate gyrus partially replace the input lost after an ipsilateral EC lesion. To study the morphology and course of single sprouted crossed entorhinodentate fibers, the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHAL) was used. Rats that survived for 4 to 8 weeks after a unilateral entorhinal lesion received PHAL deposits into the entorhinal cortex contralateral to the lesion. Control animals received a similar PHAL deposit. Single PHAL-labeled fibers in the molecular layer of the contralateral (EC lesion) fascia dentata were drawn with a camera lucida, and an axon-branching index (branch points/100 microns axon length) was calculated for these crossed entorhinodentate fibers in controls and operated animals. In animals with EC lesions, the density of PHAL-labeled crossed entorhinodentate fibers had increased remarkably. Single crossed entorhinodentate axons showed significantly more axon branch points in experimental than in control animals. In addition, some axon segments displayed high densities of small axonal extensions. Frequently, tanglelike structures were observed in the denervated outer molecular layer. These tangles consisted of one or more PHAL-labeled axons that intertwined and formed an axon tangle filled completely with branches, extensions, and boutons. Our data indicate that crossed EC fibers sprout by forming additional collaterals, axonal extensions, and tangles. Abnormal neurite formations are a characteristic feature of plaques in Alzheimer's disease. Future studies must be done to show whether or not there is a close relationship between axonal tangles and plaques in Alzheimer's disease, which, like the present lesion paradigm, severely affects entorhinal projection neurons.

Cognitive deficits induced by global cerebral ischaemia: relationship to brain damage and reversal by transplants

The CA1 and hilar fields of the hippocampus are highly vulnerable to lack of oxygen after interruption of blood flow to the brain. Severe anterograde memory loss, seen in a significant proportion of heart attack survivors, has been attributed to selective bilateral ischaemic damage to the hippocampus. Animal models of global ischaemia, induced by extracranial occlusion of the major ascending arteries, enable assessment of the neuropathological and functional consequences of transient interruption of cerebral blood flow, and can inform strategies to reduce or alleviate ischaemic brain damage. This review focuses firstly on the nature of cognitive deficits induced by global ischaemia, how far they are consistent with lesion-based accounts of hippocampal function, and the extent to which these deficits can be correlated with CA1 cell loss. The second focus of the review is to examine the limited evidence for graft-induced recovery of cognitive function in animals subjected to global ischaemia. Recent findings that grafted foetal cells from discrete hippocampal fields follow appropriate laminar routes to form functional connections with host neurons, and that growth factors protect cells from ischaemic damage, have suggested that CA1 or trophic grafts placed in the region of ischaemic CA1 cell loss might restore or protect this vulnerable sector, and reduce cognitive deficits.

Rapid decline in the ability of entorhinal axons to innervate the dentate gyrus with increasing time in organotypic co-culture

We have used the species-specific monoclonal antibodies OM1 and OM4 to identify the histiotypic pattern of projection from late embryonic rat entorhinal explants to the outer molecular layer of the dentate gyrus in organotypic cultures of 6-day postnatal mouse hippocampal slices. The presence of this entorhinal projection was detectable with the rat-specific OM1 and OM4 markers after 3-7 days in co-culture, and confirmed by use of the later-forming rat neuron-specific marker THy-1.1, which appeared during the second week. Hippocampal slices confronted with control explants of superior colliculus for 4 weeks in culture showed only sparse, non-specific growth of axons with no histiotypic pattern in the dentate gyrus. In order to assess whether the formation of specific entorhino-dentate projections in vitro is age-dependent, embryonic rat entorhinal cortical explants were cultured alone for periods of 1-5 weeks before cutting across the halo of axons radiating into the collagen matrix and presenting each with 6-day-old mouse hippocampal slices as targets to innervate. After allowing a 2 week period for fibre growth to take place, the density of immunostained axonal outgrowth was scored on a five-point scale for each weekly interval. The amount of new axon growth when the cuts were made after 1 week was slightly reduced compared to undamaged control cultures. However, outgrowth was greatly diminished when the cuts were made after 2 or 3 weeks, and essentially abolished if the interval was extended to > or = 4 weeks. Thus we demonstrate that, although hippocampal slices can survive in organotypic co-culture with entorhinal explants and maintain previously formed connections, the explants show an age-related failure in the ability to form new connections. Such a system provides a possible in vitro model for study of the factors influencing the failure of regeneration in the adult central nervous system.

Schwann cells transplanted into the CNS

A small volume of purified Schwann cells, cultured from early postnatal rat sciatic nerve, was injected into the hippocampus or fimbria of syngeneic adult hosts. The procedure caused minimal structural disturbance at the transplantation site, with close graft-host contact and maximal opportunity for integration. The donor Schwann cells were identified by a combination of light and electron microscopic features (which include characteristic deep and complex infoldings of a well marked nuclear envelope), antigenic profile (especially low affinity nerve growth factor receptor immunoreactivity), uptake of fluorescent latex microspheres and autoradiography of [3H]thymidine-labelled dividing cells. The donor Schwann cells adopted a distinctive elongated form, with a central, ovoid nucleus flanked by processes which were up to 300 microns long, and which ranged from swollen segments with a diameter as large as 12 microns down to thread-like fibres of 1 microns or less with growth cone-like expansions. Transplanted cells migrated from the graft, particularly along blood vessels and could permeate all cytoarchitectonic regions of the adjacent host hippocampal neuropil. Donor Schwann cells also migrated along the longitudinal axis of the fimbria, where they were interspersed in parallel with the interfascicular glial rows and axons. The grafted cells induced a transient but marked host astrocytic hypertrophy, which did not appear to impede the migration of the donor Schwann cells. The transplanted Schwann cells did not form peripheral myelin (as detected by P0 immunoreactivity), and it is not clear whether they survive beyond the period at which we detect them.

Transneuronal changes in dendrites of GABAergic parvalbumin-containing neurons of the rat fascia dentata following entorhinal lesion

The perforant path fibers from the entorhinal cortex form synapses with both granule cells and GABAergic, parvalbumin-containing (PARV) nongranule cells. The authors recently reported a persistent reduction of PARV-positive dendrites in the termination zones of entorhinal fibers in the hippocampus proper and fascia dentata after lesion of the entorhinal cortex. In the present study the authors analyzed the effects of de-entorhination on the ultrastructure of postsynaptic PARV-positive dendrites in the molecular layer of the fascia dentata. PARV immunocytochemistry was performed 2, 8, 55, and 360 days after an ipsilateral entorhinal lesion and, for comparison, 10 days after an ipsilateral fimbria-fornix transection that disconnects the hippocampus from its septal and commissural afferents. Two days after entorhinal lesion, the authors observed swelling of the tissue close to the hippocampal fissure. Adjacent distal dendritic tips of PARV-positive dentate neurons appeared bloated and reduced in number. Reduction of PARV-positive dendrites in the former perforant path termination zone persisted 55 days after entorhinal lesion and could still observed after postlesional survival times for 1 year. Degenerating axon terminals were still present 55 days following lesion and PARV-positive dendrites exhibited abnormal invaginations. Fimbria transection did not result in similar dendritic changes in PARV-positive neurons. The results indicate a long-lasting process of reorganization in the molecular layer of the fascia dentata following entorhinal lesion and persisting changes in the morphology of PARV-immunoreactive dendrites. Entorhinal fibers seem to play a specific role for the maintenance of these dendrites, since similar changes did not occur following removal of septal and commissural fibers.

Entorhinal axons project to dentate gyrus in organotypic slice co-culture

We have demonstrated the formation of entorhinodentate projections by axons arising from explants of embryonic mouse entorhinal cortex or slices of postnatal rat entorhinal area co-cultured in contact with slices of postnatal rat hippocampus in roller tube and static culture. Species-specific markers (Thy-1 alleles and M6) showed that the most dense part of the projection was to the outer part of the molecular layer of the dentate gyrus (i.e. excluding the commissural-association zone). Retrograde axonal transport of fluorescent tracers placed in the dentate gyrus labelled a densely packed superficial layer of stellate cells in the entorhinal cortex. Anterograde axonal transport of biocytin placed in the entorhinal cortex showed that the entorhinodentate fibres formed typical parallel bundles oriented at right angles to the dentate granule cell dendrites and had short-stalked boutons. The formation of entorhinodentate synapses was confirmed in the electron microscope by electron-dense degeneration after cutting the previously formed connection between the co-cultures. Synaptic transmission was demonstrated by extracellular recording of postsynaptic field potentials after entorhinal stimulation. The entorhinal fibres also projected to the hippocampal stratum lacunosum-moleculare of fields CA1 and CA3, and were present in the outer part of the stratum oriens of the subiculum; in some cases they perforated the pyramidal cell layer of the subiculum. We conclude that the necessary molecular and tissue organizational signals for the formation of an entorhinodentate projection are present in tissues maintained in organotypic slice co-culture, and remain effective in the cross-species mouse-to-rat situation.

Monoclonal antibodies reveal molecular differences between terminal fields in the rat dentate gyrus

We have derived a number of monoclonal antibodies which detect molecular differences correlating with the afferent inputs to the molecular layer of the adult rat hippocampal dentate gyrus. One group, dubbed OM-1 to OM-4, strongly stain the outer zone of the molecular layer, which receives its major innervation from the ipsilateral entorhinal cortex. A second group, IM-1 and IM-2, show a complementary pattern and preferentially stain the inner molecular layer, which receives inputs from the ipsilateral and contralateral hippocampus. These antigens are not, however, restricted to these layers, being found outside the hippocampus in several other areas of neuropil in the adult brain. In the developing brain the IM-1 antigen appears ubiquitously from the earliest age studied, embryonic day 12. Within the dentate gyrus, its restriction to the inner terminal field of the molecular layer only occurs during the second postnatal week. In contrast, OM staining appears only sparsely and late in the prenatal brain, appearing in developing cortical white matter between embryonic days 18 and 20. The outer dentate molecular layer becomes OM-positive from birth onwards, corresponding to the time of arrival of entorhinal axons during the first postnatal week. These two groups of monoclonal antibodies recognize a number of different glycoproteins. Ultrastructural immunohistochemistry shows they are cell surface molecules, and as such may be involved in the recognition events required for the establishment of specific patterns of neuronal connectivity.