Heterotopic neocortical transplants. An anatomical and electrophysiological analysis of host projections to occipital cortical grafts placed into sensorimotor cortical lesions made in newborn rats

Embryonic amygdalar transplants in adult rats with motor cortex lesions: a molecular and electrophysiological analysis

Transplants of embryonic nervous tissue ameliorate motor deficits induced by motor cortex lesions in adult animals. Restoration of lost brain functions has been recently shown in grafts of homotopic cortical origin, to be associated with a functional integration of the transplant after development of reciprocal host–graft connections. Nevertheless little is known about physiological properties or gene expression profiles of cortical implants with functional restorative capacity but no cortical origin. In this study, we show molecular and electrophysiological evidence supporting the functional development and integration of heterotopic transplants of embryonic amygdalar tissue placed into pre-lesioned motor cortex of adult rats. Grafts were analyzed 3 months post-transplantation. Using reverse transcriptase quantitative polymerase chain reaction, we found that key glutamatergic, GABAergic, and muscarinic receptors transcripts were expressed at different quantitative levels both in grafted and host tissues, but were all continuously present in the graft. Parallel sharp electrode recordings of grafted neurons in brain slices showed a regular firing pattern of transplanted neurons similar to host amygdalar pyramidal neurons. Synaptic connections from the adjacent host cortex on grafted neurons were electrophysiologically investigated and confirmed our molecular results. Taken together, our findings indicate that grafted neurons from a non-cortical, non-motor-related, but ontogenetical similar source, not only received functionally effective contacts from the adjacent motor cortex, but also developed electrophysiological and gene expression patterns comparable to host pyramidal neurons; suggesting an interesting tool for the field of neural repair and donor tissue in adults.

Guidance of thalamocortical innervation

We propose that a sequence of individually simple mechanisms influences the pattern of thalamocortical innervation, which itself contributes to the determination of regional differentiation of the neocortex. In co-culture, the cortex appears to exert a remote growth-promoting influence on thalamic axons from E15, becomes growth-permissive to axon invasion at about E20 and expresses a stop signal, causing termination in layer IV, from P2-3. This cascade of cortical signals may determine the timing of events in vivo. However, any part of the thalamus will innervate any region of the developing cortex in culture, without obvious preference, suggesting that the topographic distribution of thalamic fibres in vivo does not depend on regional chemospecificity. The initial extension of axons from the cortical preplate and the thalamus starts at about E14, and the topography of both may be influenced by their temporal sequences of outgrowth (chronotopy). The axon arrays meet in the basal telencephalon, after which the preplate scaffold may guide thalamic axons and ensure both their 'capture' within the subplate layer and the establishment of the waiting period. The unusual pattern of innervation in the Reeler mutant mouse supports the hypothesis that thalamic axons grow over preplate fibres to find the waiting compartment.

Transplanted neuroblasts differentiate appropriately into projection neurons with correct neurotransmitter and receptor phenotype in neocortex undergoing targeted projection neuron degeneration

Reconstruction of complex neocortical and other CNS circuitry may be possible via transplantation of appropriate neural precursors, guided by cellular and molecular controls. Although cellular repopulation and complex circuitry repair may make possible new avenues of treatment for degenerative, developmental, or acquired CNS diseases, functional integration may depend critically on specificity of neuronal synaptic integration and appropriate neurotransmitter/receptor phenotype. The current study investigated neurotransmitter and receptor phenotypes of newly incorporated neurons after transplantation in regions of targeted neuronal degeneration of cortical callosal projection neurons (CPNs). Donor neuroblasts were compared to the population of normal endogenous CPNs in their expression of appropriate neurotransmitters (glutamate, aspartate, and GABA) and receptors (kainate-R, AMPA-R, NMDA-R. and GABA-R), and the time course over which this phenotype developed after transplantation. Transplanted immature neuroblasts from embryonic day 17 (E17) primary somatosensory (S1) cortex migrated to cortical layers undergoing degeneration, differentiated to a mature CPN phenotype, and received synaptic input from other neurons. In addition, 23.1 +/- 13.6% of the donor-derived neurons extended appropriate long-distance callosal projections to the contralateral S1 cortex. The percentage of donor-derived neurons expressing appropriate neurotransmitters and receptors showed a steady increase with time, reaching numbers equivalent to adult endogenous CPNs by 4-16 weeks after transplantation. These results suggest that previously demonstrated changes in gene expression induced by synchronous apoptotic degeneration of adult CPNs create a cellular and molecular environment that is both permissive and instructive for the specific and appropriate maturation of transplanted neuroblasts. These experiments demonstrate, for the first time, that newly repopulating neurons can undergo directed differentiation with high fidelity of their neurotransmitter and receptor phenotype, toward reconstruction of complex CNS circuitry.

Cervical motoneuron topography reflects the proximodistal organization of muscles and movements of the rat forelimb: a retrograde carbocyanine dye analysis

Behavioral evidence reveals that the laboratory rat and other rodent species display skilled paw and digit use in handling food during eating and skilled limb use in reaching for food in formal laboratory skilled reaching tests that is comparable to that described in carnivores and primates. Because less is known about the central control of skilled movements in rodents than in carnivores or primates, the purpose of the current study was to examine the relation between the rat's spinal motoneurons and the individual forelimb muscles that they innervate. In two experiments, 14 forelimb muscles (in the shoulder and the upper and lower arm segments) were injected with carbocyanine dye tracers. The topography of spinal motoneurons was reconstructed by using fluorescence microscopy. Motor neurons were found to be organized in columns throughout the length of the cervical and upper thoracic area, with 1) extensor motoneurons located more laterally than flexor motoneurons, 2) rostral motoneurons innervating more proximal muscles than caudal motoneurons, and 3) more dorsally located motoneurons innervating more distal muscles. These results reveal that the topography of rodent cervical spinal cord motoneurons is very similar to that of carnivores and of primates, which also are characterized by well-developed, skilled movements. In addition, the proximal-distal organization of motoneuron columns parallels the proximal-to-distal pattern of forelimb movement used by the rat when reaching. The data from this study enable the development of predictions about the specific movements that would be compromised by experimental transections or other injuries at different levels of the spinal cord in rat models of spinal injury.

Mature astrocytes transform into transitional radial glia within adult mouse neocortex that supports directed migration of transplanted immature neurons

Neuronal migration is an essential step in normal mammalian neocortical development, and the expression of defined cellular and molecular signals within the developing cortical microenvironment is likely crucial to this process. Therapy via transplanted or manipulated endogenous precursors for diseases which involve neuronal loss may depend critically on whether newly incorporated cells can actively migrate to repopulate areas of neuronal loss within the adult brain. Previous studies demonstrated that embryonic neurons and multipotent precursors transplanted into the neocortex of adult mice undergoing targeted apoptosis of pyramidal neurons migrate long distances into neuron-deficient regions, undergo directed differentiation, accept afferent synaptic input, and make appropriate long-distance projections. The experiments presented here: (1) use time-lapse digital confocal imaging of neuronal migration in living slice cultures to assess cellular mechanisms utilized by immature neurons during such long distance migration, and (2) identify changes within the host cortical astroglial population that may contribute to this migration. Prelabeled embryonic day 17 mouse neocortical neurons were transplanted into adult mouse primary somatosensory cortex undergoing targeted apoptotic degeneration of callosal projection neurons. Four to 7 days following transplantation, living slice cultures containing the region of transplanted cells were prepared and observed. Sequential time-lapse images were recorded using a video-based digital confocal microscope. Transplanted cells displayed bipolar morphologies characteristic of migrating neuroblasts and moved in a saltatory manner with mean rates of up to 14 microm/h. To investigate whether a permissive glial phenotype may provide a potential substrate for this directed form of neuronal migration, slice cultures were immunostained with the RC2 monoclonal antibody, which identifies radial glia that act as a substrate for neuronal migration during corticogenesis. RC2 does not label mature stellate astrocytes, which express glial fibrillary acidic protein (GFAP). RC2 expression was observed in glial cells closely apposed to migrating donor neurons within the slice cultures. The timing and specificity of RC2 expression was examined immunocytochemically at various times following transplantation. RC2 immunostaining within regions of neuronal degeneration was transient, with peak staining between 3 and 7 days following transplantation. Strongly RC2-immunoreactive cells that did not express GFAP were found within these regions, but not in distant cortical regions or within control brains. RC2-positive cells were identified in recipient transgenic mice which express beta-galactosidase under a glial specific promoter. Coexpression of RC2 and beta-galactosidase identified these cells as host astroglia. These results demonstrate that adult cortical astrocytes retain the capacity to reexpress an earlier developmental phenotype that may partially underlie the observed active migration of transplanted neurons and neural precursors. Further understanding of these processes could allow directed migration of transplanted or endogenous precursors toward therapeutic cellular repopulation and complex circuit reconstruction in neocortex and other CNS regions.

Complete compensation in skilled reaching success with associated impairments in limb synergies, after dorsal column lesion in the rat

Each of the dorsal columns of the rat spinal cord conveys primary sensory information, by way of the medullary dorsal column nucleus, to the ventrobasal thalamus on the contralateral side; thus the dorsal columns are an important source of neural input to the sensorimotor cortex. Damage to the dorsal columns causes impairments in synergistic proximal or whole-body movements in cats and distal limb impairments in primates, particularly in multiarticulated finger movements and tactile foviation while handling objects, but the behavioral effects of afferent fiber lesions in the dorsal columns of rodents have not been described. Female Long-Evans rats were trained to reach with a forelimb for food pellets and subsequently received lesions of the dorsomedial spinal cord at the C2 level, ipsilateral to their preferred limb. Reaching success completely recovered within a few days of dorsal column lesion. Nevertheless, a detailed analysis of high-speed video recordings revealed that rotatory limb movements (aiming, pronation, supination, etc.) were irreversibly impaired. Compensation was achieved with whole-body and alternate limb movements. These results indicate the following: (1) in the absence of the dorsal columns, other sensorimotor pathways support endpoint success in reaching; (2) sensory input conveyed by the dorsal columns is important for both proximal and distal limb movements used for skilled reaching; and (3) detailed behavioral analyses in addition to endpoint measures are necessary to completely describe the effects of dorsal column lesions.

Transplants of fetal frontal cortex grafted into the occipital cortex of newborn rats receive a substantial thalamic input from nuclei normally projecting to the frontal cortex

A number of molecular and hodological experiments have provided evidence that there is an early commitment of neocortical neurons to express features unique to a certain cortical area. However, the findings of several transplantation experiments have indicated that late embryonic cortical tissue heterotopically grafted into the neocortex of newborn rats receives a set of thalamic projections appropriate for the host cortical locus within which it develops. To provide further information on the extent to which neocortical neurons are predetermined to develop area-specific systems of connections, in this study we have compared the pattern of thalamic afferents to grafts of embryonic day 16 occipital or frontal neocortex transplanted into the occipital cortex of newborn rats. Two months after grafting, a retrograde neurotracer (cholera toxin, subunit b) was injected into the grafts to precisely assess the number of cells in the visual- and/or motor-related nuclei of the host thalamus projecting to each category of transplants (occipital-to-occipital or frontal-to-occipital). Transplants of embryonic occipital cortex received significant input from several visual-related thalamic nuclei, i.e. the lateral posterior and lateral dorsal nuclei, and no input from motor-related thalamic nuclei. However, only few labeled cells were found in the dorsal lateral geniculate nucleus, which was systematically affected by a severe atrophy, probably in response to the lesion of the occipital cortex performed prior to the transplantation. By comparison, transplants of frontal origin received a substantial input from the ventrolateral and ventromedial thalamic nuclei, which normally project to the frontal cortex, but received a weak input from the lateral posterior and lateral dorsal nuclei. Neocortical neurons grafted heterotopically into the neocortex of newborn hosts are not only able to contact cortical and subcortical targets appropriate for their embryonic site of origin, but are also susceptible to derive thalamic inputs closely related to their embryonic origin.

Specific invasion of occipital-to-frontal neocortical grafts by axons from the lateral posterior thalamic nucleus consecutive to neonatal lesion of the rat occipital cortex

Previous work found that transplants of embryonic (E) day 16 occipital cortex placed into the frontal cortex of newborn hosts failed to receive input from visual-related nuclei of the host thalamus. The present study is aimed at determining the possible causes of the lack of visual-related thalamic input to these transplants. For that purpose, a retrograde neurotracer was injected into transplants of embryonic (E16) occipital origin which were placed into the frontal cortex of newborn rats with either intact or damaged occipital cortex. In rats with intact occipital cortex, occipital-to-frontal transplants were indeed not contacted by axons from the dorsal lateral geniculate (DLG) nucleus and received only sparse to negligible input from, respectively, the lateral posterior (LP) and laterodorsal (LD) thalamic nuclei. Yet, following neonatal lesion of the host occipital cortex, the occipital-to-frontal transplants received a significant input from the LP and to a much lesser degree from the LD but practically none from the DLG. Additional control cases with frontal-to-frontal transplants and prior lesion of the occipital cortex did not receive significant input from any of these thalamic nuclei. Thus, following neonatal deprivation of cortical target cells in their main terminal field, LP and to a lesser extent LD axons have the capacity to recognize and significantly innervate appropriate targets even those at some distance from their normal terminal site. DLG neurons degenerate or are not able to contact and invade available terminal space that is provided at some distance from the occipital cortex.

Transplantation of fetal neocortex ameliorates sensorimotor and locomotor deficits following neonatal ischemic-hypoxic brain injury in rats

Ischemic brain injury in neonates can result in the degeneration of cortical and subcortical areas of brain and is associated with neurologic deficits. One approach to restoring function in conditions of ischemic brain injury is the use of neural transplants to repair damaged connections. This approach has been shown to reestablish neural circuitry and to ameliorate associated motor deficits in models of neonatal sensorimotor cortex damage. In this study, we utilized the Rice et al. rodent model of neonatal ischemic-hypoxic (IH) brain injury to assess whether transplantation of fetal neocortical tissue can promote functional recovery in tests of sensorimotor and locomotor ability throughout development and as adults. We show that animals that received neocortical grafts 3 days following the IH injury performed significantly better as adults on two measures of motor ability, the Rota-Rod treadmill and apomorphine-induced rotations, than did control animals that received sham transplants after the IH injury. Transplants were identifiable in 72% of the animals 10-12 weeks after implantation. Histochemical studies revealed that while the transplanted tissue did not establish normal cortical cytoarchitecture, cells and fibers within the grafts stained for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), choline acetyl transferase (ChAT), cholecystokinin (CCK), and glial fibrillary acidic protein (GFAP). These results suggest that transplantation of fetal neocortical tissue following IH injury in the neonatal period is associated with amelioration of motor deficits and that the grafted tissue demonstrated a neurochemical phenotype that resembled normal neocortex. This approach warrants continued investigation in light of potential therapeutic uses.

Abnormalities in the development of the tectal projection from transplants of embryonic occipital cortex placed in the damaged occipital cortex of newborn rats

We have examined the degree of precision in the topographic arrangement of the tectal projection developed by homotopic transplants of embryonic occipital cortex and tried to determine whether the development of the corticotectal projection is exclusively dependent on environmental cues or is also controlled by intrinsic factors. Transplants of embryonic (E16) occipital cortex were grafted into various areas of the occipital cortex (Oc1 or Oc2) of newborn rats and the organization of the tectal projection arising from the transplants was subsequently examined by injecting different neurotracers into the transplants. Our results indicate that in most cases the laminar and tangential distributions of the tectal projections from the transplants were abnormal. Indeed, whatever the location of the transplant in the host occipital cortex and whatever the placement of the injection into the transplant, a hybrid distribution of the tectal labeling was found, reminiscent of the pattern observed following tracer deposits in both Oc1 and Oc2 in intact animals. Since the grafts were composed of cells of both Oc1 and Oc2 embryonic origin, it is likely that the hybrid pattern of efferents reflects the heterogeneity of the embryonic origin of the cells composing the graft. These findings provide evidence that the development of the topographic distribution of neocortical efferents is not only dependent on factors extrinsic to the cortex and further indicate that even within one single cortical region, the occipital cortex, different areas (Oc1 vs Oc2) are not totally interchangeable. These findings might have important implications in transplantation experiments aiming at the reconstruction of damaged neocortical circuitry where a precise "point-to-point" reconstruction of the circuitry is expected.

Patterning and specification of the cerebral cortex

Regionalization of the cerebral cortex occurs during development by the formation of anatomically and functionally discrete areas of the brain. Descriptive evidence based on expression of molecules and structural features suggests that an early parcelation of the cerebral wall may occur during fetal development. Experimental strategies using tissue transplants and cell culture models have explored the nature of the timing of areal specification. New signaling systems displaying the sensitivity of precursor cells to environmental cues that define the fate of neurons destined for specific areas of the cortex have been discovered. Studies in the field now suggest mechanisms of regulating cell phenotype in the cortex that are common to all parts of the neuraxis.

Development of spinal cord projections from neocortical transplants heterotopically placed in the neocortex of newborn hosts is highly dependent on the embryonic locus of origin of the graft

Previous experiments based on heterotopic transplantation paradigms have indicated that the distribution of efferents developed by layer V pyramidal cells seems to be related to where in the neocortex the cells develop and not to where they were generated. The present study was undertaken in an attempt to obtain a quantitative estimation of the weight of extrinsic factors in the development of neocortical efferents. Fragments of embryonic (E15-E19) frontal or occipital cortex were grafted homotopically or heterotopically into the frontal or occipital cortex of newborn rats. As adults, the hosts received an injection of a retrograde tracer into the pyramidal tract decussation, and the distribution of the subsequent cell labeling was examined in each category of transplant. The mean numbers of labeled cells were 725 in frontal-to-frontal transplants and 250 in frontal-to-occipital transplants. In occipital-to-frontal transplants, the numbers of labeled cells were extremely low, ranging from 0 to 14. Finally, as expected, practically no cell labeling was found in occipital-to-occipital transplants. Thus, transplants of presumptive frontal origin systematically develop and maintain in adulthood a spinal cord projection even though they are placed in the host occipital cortex. Conversely, transplants of presumptive occipital origin are practically incapable of maintaining a spinal cord projection in adulthood even though they are placed in the host frontal cortex. It seems, therefore, that the generation of regional differences in efferent connectivity found in the mature cortex depends on early regional specification within the neocortical neuroepithelium.

Embryonic cortical neurons differentiate into various types of interneurons when heterotopically transplanted into the adult rat brain

The adult cortex represents a heterogeneous mixture of different classes of pyramidal neurons and non-pyramidal interneurons. After grafting embryonic cortical anlage into the adult striatum, the present study investigated whether the development of different populations of interneurons in heterotopic cortical grafts is similar to the adult cortex. The presence of specific subpopulations of interneurons in grafts was assessed by immunocytochemistry using various antibodies against marker molecules for interneurons such as neuropeptides and calcium-binding proteins. These molecules are expressed to a different extent in specific subpopulations of cortical interneurons. Cortical primordia obtained on embryonic day 14 (E14) were stereotactically grafted into the center of the left striatum of adult recipient rats. After an 8-week differentiation period, host rats were perfusion fixed and immunocytochemistry was performed using antibodies against neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), somatostatin, parvalbumin and calbindin D-28k. Within the grafts, the number of immunopositive interneurons as well as the intensity of immunostaining for different marker molecules corresponded well with those of the adult cortex. In contrast, the expression pattern in the graft demonstrated clear differences when compared with the surrounding host striatum. The present study demonstrates, that at E14 at least some cells of the cortical anlage are primed to develop into different classes of interneurons independent of their normal environment and their regular synaptic connections. Thus, different interneuron progenitor cells survive transplantation and develop cell-specific morphological and cytochemical characteristics. Differentiation into various subpopulations of neurons may be a prerequisite for potential therapeutic approaches in humans.

The ontogeny of estrogen receptors in heterochronic hippocampal and neocortical transplants demonstrates an intrinsic developmental program

We investigated the intrinsic vs. environmental regulation of estrogen receptor (ER) ontogeny in the neocortex, hippocampus and hypothalamus by employing a heterochronic transplantation paradigm. These studies were based on previous reports demonstrating that neural ER develop asynchronously with quantitatively distinct ontogenetic profiles in various brain regions. Fetal (E14-15) hippocampal, frontal cortical or hypothalamic preoptic area (HPOA) primordial tissue was grafted into frontal cortical lesion cavities made in newborn (PND-0) rats. Thus, the grafted tissue was 1 week younger than the host. Two and 4 weeks following transplantation surgery, which corresponds to a theoretical donor age of PND-7 and PND-21, the grafts, a region of the host neocortex surrounding the transplant, and the host hippocampus, frontal cortex or HPOA (depending on graft type) were assayed for ER content using in vitro binding assays. ER concentration in hippocampal grafts at theoretical age PND-7 were significantly higher than those found in the host (PND-14) hippocampus and in the host neocortex adjacent to the transplant. By theoretical graft age PND-21, ER concentration in hippocampal transplants had decreased to levels comparable to those found in the host. This developmental pattern is analogous to that previously reported for the in situ hippocampus. A similar profile of ER concentration corresponding to the donor age developmental timetable was observed in neocortical grafts. ER levels in HPOA grafts did not change from theoretical donor age PND-7 to PND-21, which also corresponds to the normal ontogenetic profile. These data suggest that region-specific developmental patterns of ER expression in the rat brain are specified by embryonic day 14.

Retinal afferents innervate functionally tectal but not neocortical grafts placed in lesioned superior colliculus of adult rats

Solid pieces of tectum or occipital neocortex derived from 17-day rat fetuses were placed over the lesioned right superior colliculus (SC) in adult rats as sheets retaining the internal structure of the embryonal tissue. The upper laminae of the recipient's SC (approximately up to stratum opticum) were removed by aspiration after the neocortex overlying the SC was aspirated out. Two to 5 months after the operation a microelectrode study of the neuronal electrical activity in the grafts was performed. Recordings from the tectal transplants revealed normal patterns of the spontaneous neuronal activity in all grafts and clear neuronal reactions to visual stimuli in a large portion of them (6 out of 11). Visual reactions in these grafts were recorded from the majority of studied neurons (185/226). The properties of the receptive fields as well as the range of latencies of the reactions corresponded to those characteristic of the normal SC. Topographic representation of the visual field upon the transplants was found. Recordings from the cortical grafts showed an abnormal character of the spontaneous neuronal activity and the absence of reactions to any sensory stimulation of the recipients. The data obtained suggest that regenerating optic axons in adult hosts retain specificity in functional innervation of only appropriate target neurons and can re-establish the topographic representation of the retina upon tectal grafts. Retinal afferents innervate functionally tectal but not neocortical grafts placed in lesioned superior colliculus of adult rats.

Olfactory bulb transplantation into the olfactory bulb of neonatal rats: a WGA-HRP study

After unilateral bulbectomy in neonatal (P1-P5) rats, autoradiographically prelabeled presumptive olfactory bulbs from E15 and E17 embryos were transplanted in place of the removed tissue. After 2-7 months, the animals received injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the piriform cortex. Nine of the twenty animals revealed WGA-HRP-positive neurons among neurons autoradiographically labeled, providing thus evidence that the axons of the output neurons from the homotopically transplanted olfactory bulb reconnect with the host piriform cortex.