Transplants of embryonic cortical tissue placed in the previously damaged frontal cortex of adult rats: local cerebral glucose utilization following execution of forelimb movements

[Restoration of damaged cortical pathways by neural grafting]

The motor cortex plays a central role in the control, planning, and execution of voluntary motor commands in mammals. The loss of cortical neurons is a common feature of many neuropathological conditions such as traumatic and ischemic lesions or several neurodegenerative diseases. Cell transplantation presents a promising therapeutic strategy to overcome the limited abilities of axonal regrowth and spontaneous regeneration of the adult central nervous system. In this review, we will present a historical review of brain transplantation and the current state of research in the field of cortical transplantation.

A Delay between Motor Cortex Lesions and Neuronal Transplantation Enhances Graft Integration and Improves Repair and Recovery

We previously reported that embryonic motor cortical neurons transplanted immediately after lesions in the adult mouse motor cortex restored damaged motor cortical pathways. A critical barrier hindering the application of transplantation strategies for a wide range of traumatic injuries is the determination of a suitable time window for therapeutic intervention. Here, we report that a 1 week delay between the lesion and transplantation significantly enhances graft vascularization, survival, and proliferation of grafted cells. More importantly, the delay dramatically increases the density of projections developed by grafted neurons and improves functional repair and recovery as assessed by intravital dynamic imaging and behavioral tests. These findings open new avenues in cell transplantation strategies as they indicate successful brain repair may occur following delayed transplantation.SIGNIFICANCE STATEMENT Cell transplantation represents a promising therapy for cortical trauma. We previously reported that embryonic motor cortical neurons transplanted immediately after lesions in the adult mouse motor cortex restored damaged cortical pathways. A critical barrier hindering the application of transplantation strategies for a wide range of traumatic injuries is the determination of a suitable time window for therapeutic intervention. We demonstrate that a 1 week delay between the lesion and transplantation significantly enhances graft vascularization, survival, proliferation, and the density of the projections developed by grafted neurons. More importantly, the delay has a beneficial impact on functional repair and recovery. These results impact the effectiveness of transplantation strategies in a wide range of traumatic injuries for which therapeutic intervention is not immediately feasible.

Development and Maturation of Embryonic Cortical Neurons Grafted into the Damaged Adult Motor Cortex

Injury to the human central nervous system can lead to devastating consequences due to its poor ability to self-repair. Neural transplantation aimed at replacing lost neurons and restore functional circuitry has proven to be a promising therapeutical avenue. We previously reported in adult rodent animal models with cortical lesions that grafted fetal cortical neurons could effectively re-establish specific patterns of projections and synapses. The current study was designed to provide a detailed characterization of the spatio-temporal in vivo development of fetal cortical transplanted cells within the lesioned adult motor cortex and their corresponding axonal projections. We show here that as early as 2 weeks after grafting, cortical neuroblasts transplanted into damaged adult motor cortex developed appropriate projections to cortical and subcortical targets. Grafted cells initially exhibited characteristics of immature neurons, which then differentiated into mature neurons with appropriate cortical phenotypes where most were glutamatergic and few were GABAergic. All cortical subtypes identified with the specific markers CTIP2, Cux1, FOXP2, and Tbr1 were generated after grafting as evidenced with BrdU co-labeling. The set of data provided here is of interest as it sets biological standards for future studies aimed at replacing fetal cells with embryonic stem cells as a source of cortical neurons.

Area-specific reestablishment of damaged circuits in the adult cerebral cortex by cortical neurons derived from mouse embryonic stem cells

Pluripotent stem-cell-derived neurons constitute an attractive source for replacement therapies, but their utility remains unclear for cortical diseases. Here, we show that neurons of visual cortex identity, differentiated in vitro from mouse embryonic stem cells (ESCs), can be transplanted successfully following a lesion of the adult mouse visual cortex. Reestablishment of the damaged pathways included long-range and reciprocal axonal projections and synaptic connections with targets of the damaged cortex. Electrophysiological recordings revealed that some grafted neurons were functional and responsive to visual stimuli. No significant integration was observed following grafting of the same neurons in motor cortex, or transplantation of embryonic motor cortex in visual cortex, indicating that successful transplantation required a match in the areal identity of grafted and lesioned neurons. These findings demonstrate that transplantation of mouse ESC-derived neurons of appropriate cortical areal identity can contribute to the reconstruction of an adult damaged cortical circuit.

Electrophysiological and synaptic characterization of transplanted neurons in adult rat motor cortex

Lesions in specific areas of the rat motor cortex generate deficits related to fine movement performance affecting the forelimb. We have previously shown that transplants of embryonic frontal cortex ameliorate these motor deficits. Amelioration has been associated with a functional integration of the transplant due to the connections established between the host brain and the graft. In the current investigation, the electrophysiological properties of the transplanted cells and the connections both intra-transplant and with the adjacent host cortex are analyzed. For this purpose, adult rats with a motor cortical lesion plus a fetal cortical graft were used. Neurons in the transplant were recorded using sharp electrodes or whole-cell recordings in brain slices. Application of intracellular depolarizing pulses showed two patterns of cell firing: regular and burst spiking. Postsynaptic responses evoked by both, intra-transplant and adjacent host cortex stimulation were mediated by glutamic acid acting on non-NMDA and NMDA receptors, and were modulated by both cholinergic and GABAergic drugs. In some cells, supra-threshold intra-transplant stimulation generated an epileptiform-like discharge, suggesting an imbalance between excitatory and inhibitory synapses. As expected, immunohistochemistry for cholinergic and GABAergic markers confirmed the electrophysiological results. Thus we show electrophysiological and immunohistochemical evidence supporting the functional development and integration of grafted cells into the host neocortex of adult animals.

Reestablishment of damaged adult motor pathways by grafted embryonic cortical neurons

Damage to the adult motor cortex leads to severe and frequently irreversible deficits in motor function. Transplantation of embryonic cortical neurons into the damaged adult motor cortex was previously shown to induce partial recovery, but reports on graft efferents have varied from no efferent projections to sparse innervation. Here, we grafted embryonic cortical tissue from transgenic mice overexpressing a green fluorescent protein into the damaged motor cortex of adult mice. Grafted neurons developed efferent projections to appropriate cortical and subcortical host targets, including the thalamus and spinal cord. These projections were not a result of cell fusion between the transplant and the host neurons. Host and transplanted neurons formed synaptic contacts and numerous graft efferents were myelinated. These findings demonstrate that there is substantial anatomical reestablishment of cortical circuitry following embryonic cortex grafting into the adult brain. They suggest that there is an unsuspected potential for neural cell transplantation to promote reconstruction after brain injury.

Axonal regrowth of layer II-III visual-projecting cortical neurons in rats fails beyond eye opening

Fetal neurons (embryonic age E16) of occipital origin grafted in the visual cortex of albino rats at increasing postnatal stages (P0, P7, P15, P30, P60, P120) can be activated by photic stimulation. Inputs originate from five major areas of the brain ipsilateral to the graft, namely, the claustrum, the periallocortex/proisocortex, the isocortex, the visual thalamus, and some unspecific subthalamic and hypothalamic nuclei. All inputs decrease in number with the age at which grafting was performed. Isocortical afferents exhibit furthermore a progressive laminar shaping. In neonates, layer II-III and layer V-VI neurons contribute equally to the graft input. In adults, grafts receive prominent input (approximately 70-80%) from layer VI neurons whereas layer II-III neurons account for less than 10%. Proportions of layer IV (approximately 2-4%) and layer V (approximately 15-20%) neurons innervating the graft remain stable, irrespective of the age of the recipient. The adult pattern of connectivity between the host brain and the graft establishes in frontal and temporal areas 1 week earlier than in occipital areas. It is nearly completed in postnatal day 15 (P15) grafted recipients. Supragranular neurons would be thus unable to innervate and to make stable synapses at the graft level beyond P15, i.e., when eyes open. Some infragranular neurons (supposedly remnants of the earliest generated cortical cell population) still have this capacity in adults.

Effects of DSP4 and dizolcipine on connectivity of solid E19 cortical grafts to ablated SmI region of adult rats; an in vivo electrophysiological study

The functional connectivity of an embryonic graft implanted into the lesioned somatosensory cortex and the effect of DSP4 (a selective noradrenergic neurotoxin to noradrenergic terminals) and dizolcipine (a non-competitive NMDA receptor antagonist), was studied electrophysiologically. The forepaw representational area of the rat primary somatosensory cortex was lesioned unilaterally and, 3-4 weeks later, tissue from the same region of E19 rat embryos was implanted into the cavity. At 7-9 months later, the rats were anaesthetized and single unit activity was recorded from the grafts in response to contralateral forepaw, ipsilateral hindpaw and contralateral hindpaw stimulation and compared with that obtained in control rats, in rats pretreated with dizolcipine immediately after lesioning and in rats given DSP4 24 h before transplantation. Neurones within the graft were integrated into the host brain and developed a pattern of representation similar to that of intact rats, but with a reduced proportion of neurones exhibiting short-latency response to contralateral forepaw stimulation and an increased proportion responding to stimulation of more than one paw. Pretreatment with dizolcipine did not increase short-latency responses to stimulation of contralateral forepaw stimulation however pretreatment with DSP4 reduced such responses and increased proportion of inhibitory responses. It was concluded that the noradrenergic system plays an important role in establishing host-graft connectivity. The importance of further pharmacological studies on host-graft connectivity and the relation of such connections to neural plasticity were discussed.

Functional recovery of skilled forelimb use in rats obliged to use the impaired limb after grafting of the frontal cortex lesion with homotopic fetal cortex

The long-term effect of transplanting embryonic frontal cortex into a unilateral frontal cortex lesion has been studied in adult rats. Before surgery, activity in an open field, muscular strength of both forelimbs, and performance in a paw-reaching-for-food task were scored in 26 rats. In 21 animals a unilateral cortex lesion was then made in the forelimb motor area of the hemisphere contralateral to the preferred paw in the paw-reaching-for-food task, while the other 5 animals were sham-operated. On retesting, the lesion animals changed the preferred paw. A solid homotopic transplant of embryonic tissue (embryonic day 17) was then placed in the lesion cavity in 11 of the lesion rats. Three months later neither lesion alone nor lesion plus transplantation affected open field behavior and muscular strength, but the lesion permanently affected performance in the paw-reaching-for-food task, as shown by a change of preferred paw and a functional deficit in the paw contralateral to the lesion. Transplantation ameliorated the deficits caused by the lesion, but this was only evident when animals were forced to reach with the paw contralateral to the lesion plus transplant. The behavioral results were independent of the size of the lesion and graft. Connections between graft and host tissue were studied by means of the fluorescent tracer 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI). A dense array of labeled fibers was found in the host cortex adjacent to the transplant. The results suggest that functional recovery depends on grafting but is only evident when the animal is obliged to use the affected limb.

Laminar distribution of isocortical neurons projecting to occipital grafts in neonate and adult rats

Physiologically responsive grafts of embryonic (E16) occipital neurons placed into the visual cortex of adult rats were shown previously (Gaillard et al., 1998, Restor. Neurol. Neurosci. 12: 13-25) to receive a predominant (93-97%) cortical input from the infragranular layers V-VI. The present paper examines whether this specific pattern of connections is related to some process of maturation of the host cortex. Pieces of embryonic (E16) occipital cortical tissue were grafted into the visual cortex of neonate (P0), 1-week-old (P7), and adult (P120) subjects. Four months later, graft responsiveness was assessed through field potential recordings and host-to-graft afferents were labeled with a retrograde tracer (cholera toxin subunit B). The data show first that afferents to physiologically active grafts originate about equally from both supra- and infragranular cortical layers in newborn subjects and second that supragranular neurons contribute only 20 and 1.5% of these inputs in P7 and P120 recipients, respectively. This strong upside-down laminar shift of afferents may correlate with the layout of subsets of host neurons that at a given developmental stage would have the intrinsic capacity to regrow an axon. Substantial axogenesis and synaptic stabilization of host-to-graft cortical afferents appear possible only within the critical period for the supragranular neurons but may occur throughout life for the infragranular neurons.

Are neuronal markers and neocortical graft-host interface influenced by housing conditions in rats with cortical infarct cavity?

The aim was to study if exposure to an enriched environment influenced graft-host interface and neuronal markers in neocortical grafts implanted in cortical infarct cavities 3 weeks after distal ligation of the middle cerebral artery in adult hypertensive rats. Half the rats were exposed to an enriched environment for 2 h daily 5 days a week starting 1 week after the arterial ligation. The brain was fixed by perfusion 4 weeks postgrafting. The immunoreactivity to glial fibrillary acidic protein, microtubule associated protein 2, and synaptophysin was studied in coronal paraffin-embedded sections. A distinct glial border separated the infarct cavity from the surrounding brain in sham-transplanted rats. Most grafts filled the larger part of the infarct cavity. In 8 of 18 transplants, 4 in each experimental group, part of the transplants protruded through the thin glial membrane that delineated the transplant-host interface into the adjacent host brain tissue. Microtubule associated protein 2 immunostained sections indicated bridging of dendrites in the host-transplant interface. Synaptophysin immunoreactivity was significantly higher in grafts than in contralateral cortex. However, graft morphology and neuronal marker immunoreactivity did not differ between rats housed in standard and activity stimulating cages.

Neonatal lesion of the rat's frontal cortex and subsequent transplantation of embryonic frontal cortex: evidence of appropriate synaptic integration of the graft neurons within the host thalamo-fronto-striate circuit

Previous observations in intact rats have indicated that axons from the ventrolateral thalamic nucleus (VL) establish direct axo-somatic or axo-dendritic contacts onto frontal cortical neurons projecting to the striatum. The embryonic frontal cortex was grafted into the damaged frontal cortex of newborn rats to study the capacity of homotopic transplants to restore the thalamo-fronto-striate pathway. Several months later, grafted neurons projecting to the striatum were identified by injecting a retrograde neurotracer (subunit b of the cholera toxin) into the ipsilateral caudate putamen. In the same animal, axons and terminations from the VL were labeled within the transplant with an anterograde neurotracer (Phaseolus vulgaris leuco-agglutinin) injected into the ipsilateral VL. The findings show that VL axons establish direct synaptic contacts onto grafted neurons projecting to the striatum. Although the synaptic contacts were scarce in the transplants, their organization was similar to that observed in intact rats. The contacts were axo-somatic or axo-dendritic. Our observations for the first time indicate that synaptic contacts are formed in cortical grafts and that fetal frontal cortex is susceptible to develop appropriate synaptic integration within the host thalamo-fronto-striate system.