Angiogenesis and the blood-brain barrier in solid and dissociated cell grafts within the CNS

Donor-derived vasculature is required to support neocortical cell grafts after stroke

Neural precursor cells (NPCs) transplanted into the adult neocortex generate neurons that synaptically integrate with host neurons, supporting the possibility of achieving functional tissue repair. However, poor survival and functional neuronal recovery of transplanted NPCs greatly limits engraftment. Here, we test the hypothesis that combining blood vessel-forming vascular cells with neuronal precursors improves engraftment. By transplanting mixed embryonic neocortical cells into adult mice with neocortical strokes, we show that transplant-derived neurons synapse with appropriate targets while donor vascular cells form vessels that fuse with the host vasculature to perfuse blood within the graft. Although all grafts became vascularized, larger grafts had greater contributions of donor-derived vessels that increased as a function of their distance from the host-graft border. Moreover, excluding vascular cells from the donor cell population strictly limited graft size. Thus, inclusion of vessel-forming vascular cells with NPCs is required for more efficient engraftment and ultimately for tissue repair.

Single cell transcriptomics identifies stem cell-derived graft composition in a model of Parkinson's disease

Cell replacement is a long-standing and realistic goal for the treatment of Parkinsonʼs disease (PD). Cells for transplantation can be obtained from fetal brain tissue or from stem cells. However, after transplantation, dopamine (DA) neurons are seen to be a minor component of grafts, and it has remained difficult to determine the identity of other cell types. Here, we report analysis by single-cell RNA sequencing (scRNA-seq) combined with comprehensive histological analyses to characterize intracerebral grafts from human embryonic stem cells (hESCs) and fetal tissue after functional maturation in a pre-clinical rat PD model. We show that neurons and astrocytes are major components in both fetal and stem cell-derived grafts. Additionally, we identify a cell type closely resembling a class of recently identified perivascular-like cells in stem cell-derived grafts. Thus, this study uncovers previously unknown cellular diversity in a clinically relevant cell replacement PD model.

What happens to cells on engrafting into the brain in animal models to treat Parkinson’s disease is unclear. Here, the authors use scRNA-seq to examine ventral midbrain (VM)-patterned human embryonic stem cells after functional maturation in a pre-clinical rat model for Parkinson’s disease and identify perivascular-like cells.

Cell Replacement to Reverse Brain Aging: Challenges, Pitfalls, and Opportunities

Current antiaging strategies focusing on druggable targets have met with relatively limited success to date. Replacement of cells, tissues, and organs could provide an alternative means for targeting age-induced damage and potentially eliminating some of it. However, before this is a viable option, numerous challenges need to be addressed. Most notably, whether the brain, which defines our self-identity, is amenable to replacement therapies is unclear. Here, we consider whether progressive cell replacement is a potential approach to reverse brain aging without grossly altering function. We focus mainly on the neocortex, seat of our highest cognitive functions, because of abundant knowledge on neocortical development, plasticity, and how the neocortex can functionally incorporate new neurons. We outline the primary challenges for brain cell replacement, and key areas that require further investigation.

Neurovascular dysfunction in dementia - human cellular models and molecular mechanisms

From the earliest stages of development, when cerebral angiogenesis and neurogenesis are entwined, to the end of life, the interplay between vascular and neural systems of the brain is critical in health and disease. Cerebral microvascular endothelial cells constitute the blood-brain barrier and in concert with pericytes or smooth muscle cells, glia and neurons, integrate into a functional neurovascular unit (NVU). This multicellular NVU maintains homoeostasis of the brain's microenvironment by restricting the entry of systemic pathogens and neurotoxins as well as meeting the metabolic demands of neural activity. Recent evidence of cerebral microvascular pathologies in vascular diseases and dementia, including Alzheimer's disease, has challenged the notion that vascular events are merely the consequence of neuronal pathology. This review focuses on molecular mechanisms of neurovascular dysfunction in dementia and outlines currently employed in vitro models to decode such mechanisms. Deciphering neurovascular crosstalk is likely to be more important in understanding the molecular mechanisms of disease than previously anticipated and may offer novel therapeutic opportunities for dementia and related conditions.

The Development of Intracerebral Cell-Suspension Implants is Influenced by the Grafting Medium

The effect of preparing and grafting embryonic striatal and nigral tissue in four different media was evaluated in vitro and in vivo. The proportion of TH-positive and DARPP-32–positive neurons was determined after 2 days in vitro in standard culture medium following preparation in the different media. The effects were more marked for striatal neurons where DARPP-32 expression in tissue prepared in HBSS was poor compared to other media. TH expression was unaffected by the preparation medium. Striatal grafts derived from tissue prepared and grafted in HBSS were smaller, with fewer DARPP-32 cells, compared to other media. Survival of grafts in combined HBSS and DMEM was very poor. Graft volume and TH cell content was enhanced in tissue prepared in DMEM. These results suggest that preparation protocols optimized for one type of embryonic neuronal population do not necessarily transfer to other neuronal populations. © 1998 Elsevier Science Inc.

Development of an Intact Blood-Brain Barrier in Brain Tissue Transplants is Dependent on the Site of Transplantation

Transplantation of fetal septal forebrain tissue was performed to the anterior chamber of the eye, or intracranially to the rostral hippocampal formation in rats, to evaluate the impact of transplantation site on the development of an intact blood–brain barrier (BBB). The tissue was studied at 1, 2, 3, and 4 wk following transplantation by means of intravenous injection of Trypan blue, which is a vital stain not normally penetrating the BBB, as well as with an antibody specifically directed against the rat BBB, SMI71. In the intraocular septal transplants, there was a significant leakage of Trypan blue 1 wk postgrafting, associated with a few laminin-immunoreactive blood vessels that did not contain any SMI71-immunoreactivity. However, at 2 wk postgrafting, the intraocular grats exhibited an extensive plexus of thin-walled blood vessels expressing SMI71 immunoreactivity and no Trypan blue leakage. Thus, it appeared that a BBB had developed to some degree by 2 wk postgrafting in oculo. In the intracranial grafts, on the other hand, Trypan blue leakage could be seen as long as 3 wk postgrafting, and a dense plexus of blood vessels with SMI71 immunoreactivity was first seen at 4 wk postgrafting. Thus, the development of Trypan blue impermeability was delayed with 1 to 2 wk in the intracranial versus the intraocular grafts. Control experiments using psychological stress in adult rats as a means to transiently disrupt the BBB revealed that an increase in Trypan blue leakage correlated well with the disappearance of SMI71 immunoreactivity. Taken together, these studies demonstrate that the site of transplantation can influence the development of an intact BBB in neural tissue grafts.

Targeting the vasculature to improve neural progenitor transplant survival

Neural progenitor transplantation is a promising therapeutic option for several neurological diseases and injuries. In nearly all human clinical trials and animal models that have tested this strategy, the low survival rate of progenitors after engraftment remains a significant challenge to overcome. Developing methods to improve the survival rate will reduce the number of cells required for transplant and will likely enhance functional improvements produced by the procedure. Here we briefly review the close relationship between the blood vasculature and neural progenitors in both the embryo and adult nervous system. We also discuss previous studies that have explored the role of the vasculature and hypoxic pre-conditioning in neural transplants. From these studies, we suggest that hypoxic pre-conditioning of a progenitor pool containing both neural and endothelial cells will improve engrafted transplanted neuronal survival rates.

Survival, differentiation, and neuroprotective mechanisms of human stem cells complexed with neurotrophin-3-releasing pharmacologically active microcarriers in an ex vivo model of Parkinson's disease

Stem cell-based regenerative therapies hold great potential for the treatment of degenerative disorders such as Parkinson's disease (PD). We recently reported the repair and functional recovery after treatment with human marrow-isolated adult multilineage inducible (MIAMI) cells adhered to neurotrophin-3 (NT3) releasing pharmacologically active microcarriers (PAMs) in hemiparkinsonian rats. In order to comprehend this effect, the goal of the present work was to elucidate the survival, differentiation, and neuroprotective mechanisms of MIAMI cells and human neural stem cells (NSCs), both adhering to NT3-releasing PAMs in an ex vivo organotypic model of nigrostriatal degeneration made from brain sagittal slices. It was shown that PAMs led to a marked increase in MIAMI cell survival and neuronal differentiation when releasing NT3. A significant neuroprotective effect of MIAMI cells adhering to PAMs was also demonstrated. NSCs barely had a neuroprotective effect and differentiated mostly into dopaminergic neuronal cells when adhering to PAM-NT3. Moreover, those cells were able to release dopamine in a sufficient amount to induce a return to baseline levels. Reverse transcription-quantitative polymerase chain reaction and enzyme-linked immunosorbent assay analyses identified vascular endothelial growth factor (VEGF) and stanniocalcin-1 as potential mediators of the neuroprotective effect of MIAMI cells and NSCs, respectively. It was also shown that VEGF locally stimulated tissue vascularization, which might improve graft survival, without excluding a direct neuroprotective effect of VEGF on dopaminergic neurons. These results indicate a prospective interest of human NSC/PAM and MIAMI cell/PAM complexes in tissue engineering for PD.

SIGNIFICANCE

Stem cell-based regenerative therapies hold great potential for the treatment of degenerative disorders such as Parkinson's disease (PD). The present work elucidates and compares the survival, differentiation, and neuroprotective mechanisms of marrow-isolated adult multilineage inducible cells and human neural stem cells both adhered to neurotrophin-3-releasing pharmacologically active microcarriers in an ex vivo organotypic model of PD made from brain sagittal slices.

Single-cell suspension methodology favors survival and vascularization of fetal striatal grafts in the YAC128 mouse model of Huntington's disease

Cell replacement therapies have yielded variable and short-lived benefits in Huntington's disease (HD) patients. This suboptimal outcome is likely due to the fact that graft survival is compromised long term because grafts are subjected to a host's microglial inflammatory response, to a lack of adequate trophic support, and possibly to cortical excitotoxicity. However, graft demise may also relate to more straightforward issues such as cell preparation methodology (solid grafts vs. cell suspension). Indeed, we recently reported that solid grafts are poorly revascularized in HD patients transplanted 9 and 12 years previously. To evaluate whether methodological issues relating to cell preparation may have an impact on graft viability, we implanted green fluorescent protein (GFP(+)) single-cell suspensions of fetal striatal neuronal cells into the striatum of YAC128 HD mice. Postmortem evaluation yielded comparable graft survival in YAC128 mice and their wild-type littermates (noncarrier) at 1 and 3 months posttransplantation. Additionally, the degrees of graft revascularization in the YAC128 and noncarrier mice were similar, with both capillaries and large-caliber vessels observable within the grafted tissue. Furthermore, GFP(+) cells interacted well with host blood vessels, indicating integration of the donor cells within the recipient brain. These observations, combined with our recent report of poor revascularization of solid grafts in the HD-transplanted patients, suggest that the success of cell transplantation can be improved by optimizing methodological aspects relating to cell preparation.

Cellular therapy and induced neuronal replacement for Huntington's disease

Huntington's disease (HD) is an inherited, relentlessly progressive neurodegenerative disease with an invariably fatal outcome. HD is inherited in an autosomal dominant fashion, and is characterized pathologically by the loss of cortical and striatal neurons, and clinically by involuntary choreiform movements accompanied by progressive cognitive impairment and emotional lability. The disorder is caused by an expanded cystosine adenine guanine (CAG) tri-nucleotide repeat encoding polyglutamine (polyQ) in the first exon of the Huntingtin gene. There is a correlation between the number of CAG repeats and disease onset, such that in patients with CAG repeat lengths of 36 to 60, disease symptoms typically manifest after 35 years of age, whereas CAG repeat lengths >60 yield the more severe juvenile form of the disease. Even though mutant huntingtin is expressed throughout the brain, it is characterized by the selective degeneration of medium spiny neurons of the caudate and putamen, which heralds more widespread neuronal degeneration with disease progression. The mechanisms of cell dysfunction and death in HD have been the subjects of a number of studies, which have led to therapeutic strategies largely based on the amelioration of mutant huntingtin-related metabolic impairment and cellular toxicity. Each of these approaches has aimed to delay or stop the preferential degeneration of medium spiny neurons early in the disease course. Yet, in later stages of the disease, after cell death has become prominent, cell replacement therapy (whether by direct cell transplantation or by the mobilization of endogenous progenitors) may comprise a stronger potential avenue for therapy. In this review, we will consider recent progress in the transplantation of fetal striatal cells to the HD brain, as well as emerging alternative sources for human striatal progenitor cells. We will then consider the potential application of gene therapy toward the induction of striatal neurogenesis and neuronal recruitment, with an eye toward its potential therapeutic use in HD.

LDL receptor-related protein 1: unique tissue-specific functions revealed by selective gene knockout studies

The LDL receptor-related protein (originally called LRP, but now referred to as LRP1) is a large endocytic receptor that is widely expressed in several tissues. LRP1 is a member of the LDL receptor family that plays diverse roles in various biological processes including lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the LRP1 gene leads to lethality in mice, revealing a critical, but as of yet, undefined role in development. Tissue-specific gene deletion studies reveal an important contribution of LRP1 in the vasculature, central nervous system, macrophages, and adipocytes. Three important properties of LRP1 dictate its diverse role in physiology: 1) its ability to recognize more than 30 distinct ligands, 2) its ability to bind a large number of cytoplasmic adaptor proteins via determinants located on its cytoplasmic domain in a phosphorylation-specific manner, and 3) its ability to associate with and modulate the activity of other transmembrane receptors such as integrins and receptor tyrosine kinases.

Cell transplantation for Huntington's disease

Cell transplantation for Huntington's disease has developed over the last decade to clinical application in pilot trials in the USA, France and the UK. Although the procedures are feasible, and under appropriate conditions safe, evidence for efficacy is still limited, which has led to some calls that further development should be discontinued. We review the background of striatal cell transplantation in experimental animal models of Huntington's disease and the rationale for applying similar strategies in the human disease, and we survey the present status of the preliminary studies that have so far been undertaken in patients. When we consider the variety of parameters and principles that remain poorly defined -- such as the optimal source, age, dissection, preparation, implantation, immunoprotection and assessment protocols -- it is not surprising that clinical efficacy is still unreliable. However, since these protocols are all tractable to experimental refinement, we consider that the potential for cell transplantation in Huntington's disease is greater than has yet been realised, and remains a therapeutic strategy worthy of investigation and pursuit.

Intracerebral co-transplantation of liposomal tacrolimus improves xenograft survival and reduces graft rejection in the hemiparkinsonian rat

Immunosuppression remains a key issue in neural transplantation. Systemic administration of cyclosporin-A is currently widely used but has many severe adverse side effects. Newer immunosuppressive agents, such as tacrolimus (TAC) and rapamycin (RAPA), have been investigated for their neuroprotective properties on dopaminergic neurons. These drugs have been formulated into liposomal preparations [liposomal tacrolimus (LTAC) and liposomal rapamycin (LRAPA)] which retain these neuroprotective properties. Due to the slower release of the drugs from the liposomes, we hypothesized that co-transplantation of either LTAC or LRAPA within a xenogeneic cell suspension would increase cell survival and decrease graft rejection in the hemiparkinsonian rat, and that a combination of the two drugs may have a synergistic effect. 6-hydroxydopamine-lesioned rats were divided to four groups which received intra-striatal transplants of the following: 1) a cell suspension containing 400,000 fetal mouse ventral mesencephalic cells; 2) the cell suspension containing 0.63 microM LRAPA; 3) the cell suspension containing a dose of 2.0 microM LTAC; 4) the cell suspension containing 2.0 microM LTAC and 0.63 microM LRAPA. Functional recovery was assessed by amphetamine-induced rotational behavior. Animals were killed at 4 days or 6 weeks post-transplantation, and immunohistochemistry was performed to look at the expression of tyrosine hydroxylase and major histocompatibility complex classes I and II. Only the group receiving LTAC had a decrease in rotational behavior. This observation correlated well with significantly more surviving tyrosine hydroxylase immunoreactive cells compared with the other groups and significantly lower levels of immunorejection as assessed by major histocompatibility complex class I and II staining. This study has shown the feasibility of using local immunosuppression in xenotransplantation. These findings may be useful in optimizing immunosuppression in experimental neural transplantation in the laboratory and its translation into the clinical setting.

Model of acute injury to study neuroprotection

A major causative factor in the paralysis that often follows an acute injury to the central nervous system (CNS) is the paradoxical inability of the CNS to tolerate its own mechanism of self-repair. The dismal result is often a wider spread of damage (part of the inevitable "secondary" or "delayed" degeneration) rather than contribution toward a cure. Ever since the phenomenon of posttraumatic damage spread in the CNS was first recognized, neuroscientists have attempted to identify the players in this destructive process and have sought ways to neutralize or bypass them with the object of rescuing any neurons that are still viable. This approach is collectively termed neuroprotection. In this chapter, we present a view of experimental paradigms used to study neuroprotection.

From angiogenesis to neuropathology

Angiogenesis--the growth of new blood vessels--is a crucial force for shaping the nervous system and protecting it from disease. Recent advances have improved our understanding of how the brain and other tissues grow new blood vessels under normal and pathological conditions. Angiogenesis factors, especially vascular endothelial growth factor, are now known to have roles in the birth of new neurons (neurogenesis), the prevention or mitigation of neuronal injury (neuroprotection), and the pathogenesis of stroke, Alzheimer's disease and motor neuron disease. As our understanding of pathophysiology grows, these developments may point the way towards new molecular and cell-based therapies.

Beyond endocytosis: LRP function in cell migration, proliferation and vascular permeability

The low-density lipoprotein (LDL) receptor related protein (LRP1 or LRP) is a large endocytic receptor widely expressed in several tissues and known to play roles in areas as diverse as lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes and cellular entry of bacterial toxins and viruses. This member of the LDL receptor superfamily is constitutively endocytosed from the membrane and recycled back to the cell surface. Its many functions were long thought to involve its ability to bind over 30 different ligands and deliver them to lysosomes for degradation. However, LRP has since been shown to interact with scaffolding and signaling proteins via its intracellular domain in a phosphorylation-dependent manner and to function as a co-receptor partnering with other cell surface or integral membrane proteins. This multi-talented receptor has been implicated in regulation of platelet derived growth factor receptor activity, integrin maturation and recycling, and focal adhesion disassembly. These functions may account for recent studies identifying LRP's role in protection of the vasculature, regulation of cell migration, and modulation of the integrity of the blood-brain barrier.

Cell transplantation in Parkinson's disease: how can we make it work?

Previous open-label clinical trials have provided proof of principle that intrastriatal transplants of fetal dopaminergic neurons can induce substantial and long-lasting functional benefits in patients with Parkinson's disease. However, in two recent NIH-sponsored double-blind trials, functional improvements were only marginal and the primary endpoints were not met. Severe off-phase dyskinesias were observed in a significant proportion of the transplanted patients, raising doubts about the viability of the cell-transplantation approach. Here, we discuss the problems raised by the NIH-sponsored trials and point to several shortcomings that might explain the overall poor outcome, and we identify several crucial issues that remain to be resolved to develop cell replacement into an effective and safe therapy.

Identification of T lymphocytes in simian immunodeficiency virus encephalitis: distribution of CD8+ T cells in association with central nervous system vessels and virus

T lymphocytes are found within brains infected with human immunodeficiency virus (HIV) or simian immunodeficiency virus (SIV) where they are a minor, but consistently identified, population. However, little analysis of their phenotypes has been done, and questions concerning whether or not they are viral antigen specific has not been thoroughly examined. We investigated the central nervous system (CNS) of SIV-infected rhesus macaques to identify T-lymphocyte subsets in relation to virus-infected cells and brain microvessels. We have found that a sensitive antigen-retrieval technique greatly enhanced immunohistochemical detection of CD4+ and CD8+ T lymphocytes in control studies. In encephalitic brains of SIV-infected monkeys with acquired immunodeficiency syndrome (AIDS), we found a significant accumulation of CD8+ T lymphocytes but little-to-no accumulation of CD4+ T lymphocytes. CD4+ cells, when detected, were mostly monocyte/macrophages closely associated with CNS vessels. Using a combination of in situ hybridization for SIV RNA, and immunohistochemistry for CD8+ T lymphocytes and/or Glut-1 for endothelial cells on brain microvessels, we found CD8+ T lymphocytes with an angiocentric distribution often adjacent to virus-infected cells. In the CNS of animals with SIV encephalitis, there was a trend of CD8+ T lymphocytes that were not directly juxtaposed with CNS vessels. These data suggest that in brains of SIV-infected monkeys and HIV-infected humans, CD8+ T lymphocytes traffic to and are retained in the CNS in an angiocentric and possibly antigen-specific manner.

The blood-brain barrier: an overview: structure, regulation, and clinical implications

The blood-brain barrier (BBB) is a diffusion barrier, which impedes influx of most compounds from blood to brain. Three cellular elements of the brain microvasculature compose the BBB-endothelial cells, astrocyte end-feet, and pericytes (PCs). Tight junctions (TJs), present between the cerebral endothelial cells, form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the TJ barrier, but astrocytes are not believed to have a barrier function in the mammalian brain. Dysfunction of the BBB, for example, impairment of the TJ seal, complicates a number of neurologic diseases including stroke and neuroinflammatory disorders. We review here the recent developments in our understanding of the BBB and the role of the BBB dysfunction in CNS disease. We have focused on intraventricular hemorrhage (IVH) in premature infants, which may involve dysfunction of the TJ seal as well as immaturity of the BBB in the germinal matrix (GM). A paucity of TJs or PCs, coupled with incomplete coverage of blood vessels by astrocyte end-feet, may account for the fragility of blood vessels in the GM of premature infants. Finally, this review describes the pathogenesis of increased BBB permeability in hypoxia-ischemia and inflammatory mechanisms involving the BBB in septic encephalopathy, HIV-induced dementia, multiple sclerosis, and Alzheimer disease.

Distribution of adoptively transferred, tumor-sensitized lymphocytes in the glioma-bearing rat

For adoptively transferred lymphocytes to exert anti-tumor effects in vivo, they must traffic or initiate the migration of endogenous immune cells to the site of tumor. Using a rat model, we examined the trafficking of tumor-sensitized lymphocytes to an intracerebral glioma. By labeling the cells with 111Indium oxine (111In) prior to intravenous injection, we were able to quantify the relative number of lymphocytes that traveled to the tumor site. There was no difference in lymphocytic influx between the tumor-bearing and non-tumor-bearing cerebral hemispheres in 3-day rat glioma models. However, in 7-day models, significantly greater numbers of 111In-labeled lymphocytes resided in the tumor-bearing hemisphere at 12 h post-administration. This number increased more than two-fold by 24 h post-adoptive transfer. Using fluorescent-labeled lymphocytes and microscopy, we confirmed that the detection of radioactivity within the brain was truly due to tumor infiltrating 111In-labeled lymphocytes. Adoptively transferred cells were found in perivascular and peritumoral locations. These data demonstrate that tumor-sensitized lymphocytes traffic to an intracerebral target site where they can exert an effect, further supporting adoptive immunotherapy as a treatment for glioma.

Selection of viable cardiomyocytes for cell transplantation using three-dimensional tissue culture

BACKGROUND

We developed a three-dimensional cardiac tissue culture system to select viable and contractile cells for the purpose of cardiomyocyte transplantation. In this study we will show that reaggregation of cardiomyocytes in culture is an active process indicative of cardiomyocyte viability and functionality.

METHODS

Myocardial tissue from newborn mice has been enzymatically digested, incubated in culture inserts, and studied by phase contrast microscopy, conventional histology, immunohistochemistry, electron microscopy, and TUNEL assay.

RESULTS

Cells that are plated on nonadhesive surfaces, reaggregate to spontaneously contracting cell aggregates. The capacity to reaggregate was significantly dependent from the age of the tissue donor (P<0.0001) and on the method of enzymatic dissociation (P<0.0001). The majority of cells within the aggregates consisted of cardiomyocytes. After 24 hr incubation, significant amounts of laminin and fibronectin had been deposited between the cells. Ultrastructural analysis revealed viable cardiomyocytes attached to each other by tight junctions. The apoptotic rate within the aggregates was 11.4+/-4.6 vs. 44.5+/-10.5% immediately after dissociation (P<0.05).

CONCLUSIONS

The capacity to form spontaneously contracting aggregates is an inherent characteristic of viable cardiomyocytes in 3-dimensional cultures, which could be successfully exploited for cellular cardiomyocyte transplantation.

Imaging the rat brain on a 1.5 T clinical MR-scanner

Magnetic resonance imaging (MRI) offers a noninvasive technique for studying neurodegenerative events in the rat brain, however, most of the studies are performed on small bore purpose dedicated MR scanners of limited availability and at high cost. The present study explored the feasibility of using a clinical whole body MR-scanner to perform imaging in rat brain and specifically in models of Parkinson's (PD) and Huntington's disease (HD). For that purpose rats were placed into a specially designed PVC device equipped with a flexible surface coil-and T2-weighted spin echo sequences were acquired on a Siemens Magnetom Vision at 1.5 T. In the experimental protocols of PD and HD, animals underwent 6-hydroxydopamine (6-OHDA) and quinolinic acid (QA) injections, respectively and were subsequently grafted with fetal tissue. T2-weighted images showed a small hyperintense area at the 6-OHDA lesion site and a diffuse hyperintensity in the striata with QA lesions. Transplants were seen as a hypointense area surrounded by a hyperintense rim on T1-weighted images. Moreover, disturbances of the blood-brain-barrier and its time of restoration could be monitored. In conclusion, high-resolution in vivo imaging of small animals is feasible with clinical MR-scanners and hence allows the study of various experimental protocols.

Volume and differentiation of striatal grafts in rats: relationship to the number of cells implanted

A growing body of evidence suggests that graft-mediated functional recovery in animal models of Huntington's disease is influenced by the morphology of the striatal grafts. Various parameters, including embryonic dissection, tissue preparation, and surgical delivery into the brain, have been investigated with the aim of increasing the proportion of the grafts comprising striatum-like tissue. While growing evidence suggests that implants derived from the selective dissection of the lateral ganglionic eminence (LGE) contain more striatal tissue, the relationship between the quantity of LGE tissue implanted and the striatum-like proportion of the resultant grafts has not been formally investigated. In this study the volume of striatum-like tissue within the grafts did not increase in a linear manner with increasing numbers of cells implanted. The proportion of the grafts that comprised the striatum-like patch compartment or P-zone remained constant after an initial rapid increase as the number of LGE cells implanted was increased. These results have important practical implications in determining the optimum number of LGE cells to implant and hence in the design of any surgical protocol for the clinical application of this technique.

Striatal grafts in a rat model of Huntington's disease: time course comparison of MRI and histology

Survival and integration into the host brain of grafted tissue are crucial factors in neurotransplantation approaches. The present study explored the feasibility of using a clinical MR scanner to study striatal graft development in a rat model of Huntington's disease. Rat fetal lateral ganglionic eminences grown as free-floating roller-tube cultures were grafted into the quinolinic acid-lesioned striatum, and T1- and T2-weighted sequences were acquired at 2, 7, 21, and 99 days posttransplantation. MR images were then compared with images of corresponding histological sections. The lesion-induced striatal degeneration caused a progressive ventricle enlargement, which was significantly different from controls at 21 days posttransplantation. Seven days posttransplantation, T1-weighted images revealed a defined liquid-isointense signal surrounded by a hyperintense rim at the site of graft placement, which was found unaltered for the first 21 days posttransplantation, whereas a hypointense graft signal was detected at 99 days posttransplantation. At 2 days posttransplantation, T2-weighted images showed the graft region as a hyperintense area surrounded by a rim of low signal intensity but at later time-points graft location could not be further verified. Measures for graft size and ventricle size obtained from MR images highly correlated with measures obtained from histologically processed sections (R = 0.8, P < 0.001). In conclusion, the present study shows that fetal rat lateral ganglionic eminences grown as free-floating roller-tube cultures can be successfully grafted in a rat Huntington model and that a clinical MR scanner offers a useful noninvasive tool for studying striatal graft development.

Neural transplantation for Parkinson's disease

1. Neural transplantation is one promising approach for the treatment of Parkinson's disease. Fetal substantia nigra cells are a good source of dopamine, but in order to avoid ethical and immunological problems, adrenal medullary chromaffin cells have been investigated as an alternative source. 2. Grafted adrenal medullary chromaffin cells can provide dopamine as well as several neurotrophic factors that affect dopaminergic neurons in the brain. 3. We review experimental studies for application of neural transplantation techniques in Parkinson's disease, including immunological studies, cryopreservation, microvasculature, donor tissue, and direct gene delivery studies performed in our laboratory. Our clinical experience and new approach involving a polymer-encapsulated cell grafting procedure are also described.

Intracerebral fetal raphé implants normalize hippocampal function but not cerebrovascular control in serotonin-depleted adult rat brain

The effects of hypercapnia upon local cerebral blood flow and local cerebral glucose utilization were measured by quantitative autoradiography in parallel groups of rats (six per group) which 14-16 weeks previously had been treated with the serotonergic neurotoxin, methylenedioxymethamphetamine, followed by implantation of fetal raphé or basal forebrain tissues. Following the experiments, transplants were visualized by acetylcholinesterase histochemistry, and serotonergic reinnervation assessed using [3H]paroxetine binding to serotonin reuptake sites. In methylenedioxymethamphetamine-treated rats, contralateral to the implants, [3H]paroxetine binding was reduced by between 50 and 90% in the neocortex and hippocampus. Hippocampal glucose utilization was significantly increased in these rats, and the normal increase in flow which accompanies hypercapnia was also significantly enhanced. High levels of [3H]paroxetine binding were found within the raphé transplants (308 +/- 13 fmol/mg tissue). In host brain adjacent to the implant, binding levels were normalized, and in these same areas glucose utilization was also normalized. Basal forebrain implants had no effect upon either [3H]paroxetine binding or glucose utilization. Raphé transplants did not, however, alter the enhanced cerebrovascular response to hypercapnia induced by methylenedioxymethamphetamine, even in those areas where there was evidence of serotonergic reinnervation. The transplants also showed the same enhanced response. In conclusion, intracerebral fetal raphé implants normalize hippocampal function but not cerebrovascular control in serotonin-depleted adult rat brain, and despite not sharing the serotonergic deficit, blood flow in the implants follows that of the dysfunctional host.

Remodeling of lesioned kitten visual cortex after xenotransplantation of fetal mouse neopallium

Remodeling of the mechanically injured cerebral cortex of kittens was studied in the presence of a neural xenograft taken from mouse fetuses. Solid neural tissue from the neopallium of a 14-day-old fetus was transferred into a cavity prepared in visual cortical area 18 of 33-day-old kittens. Injections of bromodeoxyuridine (BrdU) were used to monitor postoperative cell proliferation. Two months after transplantation, the presence of graft tissue in the recipient brain was assessed by Thy-1 immunohistochemistry. Antibodies specific for neurons, astrocytes, and oligodendrocytes and hematoxylin staining for endothelial cells were used for the characterization of proliferating (BrdU+) cells. The following were the major observations: 1) Of ten transplanted kittens, four had the cavity completely filled with neural tissue that resembled the intact cerebral cortex in its cytoarchitecture, whereas, in four other kittens, the cavity was partially closed. In two kittens, the cavity remained or became larger, which was also the case with all four sham-operated (lesioned, without graft) animals. 2) A substantial part of the remodeled tissue was of host origin. Only a few donor cells survived and dispersed widely in the host parenchyme. 3) In the remodeled region of transplanted animals, the densities of nerve, glial, and endothelial cells were similar to those in intact animals. 4) Cell proliferation increased after transplantation but only within a limited time, because, 2 months after the operation, the number of mitotic cells in the grafted cerebral cortex did not differ from that in intact controls. Our data suggest that the xenograft evokes repair processes in the kitten visual cortex that lead to structural recovery from a mechanical insult. The regeneration seems to rely on a complex interplay of many different mechanisms, including attenuation of necrosis, cell proliferation, and immigration of host cells into the wounded area.

In vivo survival and function of transplanted rat cardiomyocytes

Recent studies have demonstrated the feasibility of transplanting fetal mouse cardiomyocytes into the hearts of adult syngeneic mice. However, the function of the transplanted cardiomyocytes and their capacity to survive in fibrous connective tissue were not assessed. In the present study, we evaluated the viability and contractility of transplanted fetal and neonatal rat cardiomyocytes in the connective tissue of the adult rat hindlimb. Purified fetal or neonatal rat cardiomyocytes were cultured. These cells contained sarcomeres, formed junctions composed of desmosomes and fascia adherens, and contracted regularly and spontaneously. A fetal or neonatal cardiomyocyte suspension was injected into the subcutaneous tissue of adult rat hindlimbs. Cyclosporin A (5 mg/kg) was administered subcutaneously daily for the 3-month duration of the study, at which time the animals were killed. The transplanted cardiomyocytes formed 'tissue' in vivo that increased in size for the first 2 weeks and remained the same size at the third week. The tissue derived from the transplanted fetal cardiomyocytes contracted spontaneously at a rate of 73 +/- 12 bpm, and that from the neonatal cardiomyocytes contracted at a rate of 43 +/- 21 bpm. The electrocardiogram was similar to that seen in myocardium with an idioventricular rhythm. Histologically, the tissue appeared to be cardiac muscle with sarcomeres. Angiogenesis occurred in the cardiomyocyte graft. In summary, a cell suspension of cultured fetal and neonatal rat cardiomyocytes transplanted into the adult rat hindlimb formed contractile cardiac tissue in the subcutaneous connective tissue.

Quantitative analysis of vascularization and cytochrome oxidase following fetal transplantation in the contused rat spinal cord

In the normal adult central nervous system, a coupling between energy consumption and vascular density is well established. Likewise, the survival of fetal neural tissue grafts is highly dependent on the establishment of functional vascular integration with the host. However, to what degree graft vascularization and tissue metabolism influence the normal host response to traumatic injury has not been extensively studied. In the present report, embryonic day 14 fetal spinal cord suspension grafts were made into the lesion epicenter of subchronic (10 days) contusion-injured rats. Three months later, intraspinal transplants were analyzed using correlative cytochrome oxidase histochemistry and vascular morphometric analysis. The same approaches were applied to the host spinal cord and injured, non-transplanted animals in order to determine the ability of a graft to alter the level of post-injury vascularization and/or metabolism. In general, graft vascular density was increased over that measured in normal or injured gray matter. Vascular density in gray matter near the host/graft interface was markedly increased when compared to either gray matter of the same spinal level in injured non-grafted animals or normal control spinal gray matter. Vascular changes were not noted in gray matter 3 mm distal to the lesion epicenter (rostral or caudal) in all groups analyzed. Cytochrome oxidase was up-regulated at this time in the graft and gray matter at the host/graft interfaces when compared to either gray matter of the same spinal level in injured, non-grafted animals or that of uninjured controls. These data indicate that an intraspinal transplant placed into the contused adult rat spinal cord reaches a metabolic capacity that is likely to be associated with high levels of oxidative metabolism in the well-vascularized graft neuropil. In addition, transplantation chronically alters vascularization and metabolic patterns of adjacent spinal gray matter following contusion injury.

The exogenous control of transfected c-fos gene expression and angiogenesis in cells implanted into the rat brain

Previously, we established a stable transfectant, Nf-1, from normal rat kidney (NRK) fibroblasts transfected with a human metallothionein II A (hMT-IIA) promoter/human genomic c-fos fusion gene to produce c-Fos protein. Since the hMT-IIA promoter can be activated by heavy metals, the level of human c-fos gene expression can be increased by addition of heavy metals to the culture medium of Nf-1 cells and the anchorage-independent growth of Nf-1 in soft agar is markedly enhanced in the presence of transforming growth factor-beta (TGF-beta) and epidermal growth factor (EGF). In this study, we found that the hMT-IIA promoter can be activated by zinc, resulting in the elevation of fused c-fos gene expression in Nf-1 cells. We transplanted NRK and Nf-1 cells into the striatum of the rat brain and investigated whether expression of the human c-fos gene could be modified in the brain by exogenous zinc. After 8 weeks, we found that the Nf-1 cells could survive in the rat brain without any immunosuppression and grafts of Nf-1 induced angiogenesis when zinc was administered. Such implants enhanced the expression of c-fos mRNA by zinc. These results indicated that the transplanted cells continued expressing the c-fos transgene when the rats were given drinking water containing zinc, resulting in the promotion of cell growth and of neovascularization. This study will present a useful animal model of gene therapy by control of transgene expression in the brain.

Neovascularization of rat fetal neocortical grafts transplanted into a previously prepared cavity in the cerebral cortex: a three-dimensional morphological study using the scanning electron microscope

Neovascularization within syngeneic rat fetal neocortical grafts transplanted into a previously prepared cavity in the cerebral cortex was studied 1 to 3 months after transplantation, utilizing scanning electron microscopy of vascular corrosion casts. The grafts were easily identified and the outer surface of the grafts, especially at the host-graft interface, was surrounded by large regenerated vessels of leptomeninges and connective tissue (e.g. dura). Large vessels originating from the choroid plexus also coated the grafts in animals whose lateral ventricles had been opened at the time of cavitation. These large regenerated vessels were mainly observed on the surface of the grafts, and they ramified markedly to form capillary networks in the vicinity of the host-graft interface. Occasionally several relatively large regenerated vessels were noted to extend into the grafts, and to ramify and connect with graft capillary networks having the same features as that of the host brain. Moreover, direct vascular connections between host capillaries and those within the grafts were observed. In some animals, arteries and arterioles which fed the grafts were identified in the perimeter of the grafts with their characteristic morphology. The interior microvasculature structure of the grafts was largely composed of the capillary network of graft origin, and of several relatively large penetrating vessels originating from the regenerated leptomeningeal vessels or the vessels of the choroid plexus. The present study demonstrated that the blood supply to the solid grafts transplanted into the previously prepared cavities originated primarily from the regenerated host vessels. These host vessels perfused the intrinsic graft vessels via new anastomoses which formed predominantly at the host-graft interface.

Angiogenesis and the blood-brain barrier in intracerebral solid and cell suspension grafts

Solid and suspension grafts of fetal central nervous system (CNS) tissue rapidly reform an intact blood-brain barrier (BBB), whereas solid grafts of peripheral nervous system (PNS) tissue fail to establish a BBB as detected by horseradish peroxide (HRP) leakage, administrated intravenously. We examined the acute changes in the BBB after grafting of fetal CNS tissue in solid and suspension form and superior cervical ganglion (SCG) and PNS tissue in the same manner. Adult rats (n = 20) received fetal (day 14-15) forebrain grafts (either solid or cell suspension) to their rostral corpus callosum bilaterally. A second group (n = 20) received SCG solid and cell suspension grafts at the same coordinates with the same technique. The animals were killed on first, third, seventh, and tenth days after grafting. Intravenous HRP (Sigma, type VI, 75 mg/5-g rat) was given 1 hour before perfusion with mixed aldehydes. Fifty-micron coronal sections were examined for the presence and location of the graft by cresyl violet and AChE staining and Mesulam's TMB method to detect HRP leakage. HRP leakage was detected in the parenchyma in all groups on the first and the third days post-transplantation indicating a disrupted BBB. No HRP reaction was seen at days 7 and 10 in groups receiving fetal forebrain tissue whether solid or cell suspension. Solid grafts of SCG consistently demonstrated HRP leakage from the first through the tenth day. However, cell suspension of SCG established a BBB by 7 days. These results suggest that within the solid grafts of CNS and PNS tissue, the permeability of the vessels is dictated by the transplanted tissue itself. When cell suspensions of the same tissue are introduced, host CNS tissue dominates as the local environment resulting in non-leaky vasculature within the graft.

Vascularization and microvascular permeability in solid versus cell-suspension embryonic neural grafts

Vascularization and microvascular permeability were assessed in a comparative study of solid (organized) and cell-suspension (dissociated) fetal nigral grafts implanted in the dopamine-deprived striatum of adult rats. Both graft types were analyzed by chromogen detection of intravenously injected horseradish peroxidase (HRP), which outlined vessel walls, and, in cases in which the blood-brain barrier was compromised, permeated the graft and host parenchyma. Survival of graft-derived dopaminergic cells was assessed using tyrosine hydroxylase (TH) immunocytochemistry. Glial reactivity to cell-suspension grafts was similarly assessed with an antibody directed against glial fibrillary acidic protein. Morphometry revealed significantly higher microvessel density in the cell-suspension grafts (p < 0.001), which effectively equaled that found in the contralateral striatum despite rather prominent surrounding glial reactivity. Capillaries in the cell-suspension grafts were not permeable to blood-borne HRP at postimplantation study times of 7, 14, and 30 days whereas, in the solid grafts, permeability in some cases could be detected for up to 30 days. Large numbers of cells immunoreactive for TH were seen in cell-suspension grafts; in contrast, few if any were found in the majority of solid transplants. The multiple-fragment solid graft implant model used clinically compares poorly with the cell-suspension model because it lacks consistency in early revascularization and shows a greater (albeit temporary) tendency for blood-brain barrier dysfunction. Delayed and inadequate vascularization of the solid graft is likely to account for graft failure more often than in the cell-suspension graft. Similarly, a certain critical number of specific grafted cells are required to achieve sufficient expression to bring about a favorable response in the disabled host, and this expression appears to be achieved less consistently with the solid implant technique.

Allografts of CNS tissue possess a blood-brain barrier: III. Neuropathological, methodological, and immunological considerations

Development of a blood-brain barrier (BBB) within mammalian CNS grafts, placed either intracerebrally or peripherally, has been controversial. Published data from this laboratory have emphasized the presence or the absence of a BBB within solid mammalian tissue or cell suspension grafts is determined intrinsically by the graft and not by the surrounding host parenchyma (e.g., brain, kidney, testis, etc.). Nevertheless, correctly interpreting whether or not a BBB exists within brain grafts is manifested by methodologies employed to answer the question and by ensuing neuropathological and immunological consequences of intracerebral grafting. The present study addresses these issues and suggests misinterpretation for the absence of a BBB in brain grafts can be attributed to: (1) rupture of interendothelial tight junctional complexes in vessels of CNS grafts fixed by perfusion of the host; (2) damage to host vessels and BBB during the intracerebral grafting procedure; (3) graft placement in proximity to inherently permeable vessels (e.g., CNS sites lying outside the BBB) supplying the subarachnoid space/pial surface and circumventricular organs such as the median eminence, area postrema, and choroid plexus; and (4) graft rejection associated with antigen presenting cells and the host immune response. The latter is prevalent in xenogeneic grafts and exists in allogeneic grafts with donor-host mismatch in the major and/or minor histocompatibility complex. CNS grafts between non-immunosuppressed outbred donor and host rats of the same strain (e.g., Sprague Dawley or Wistar rats) can be rejected by the host; these grafts exhibit populations of immunohistochemically identifiable major histocompatibility complex class I+ and class EE+ cells (microglia, macrophages, etc.) and CD4+ T-helper and CD8+ T-cytotoxic lymphocytes. PC12 cell suspension grafts placed within the CNS of non-immunosuppressed Sprague Dawley rats are rejected similarly. Donor cells from solid CNS grafts placed intracerebrally and stained immunohistochemically for donor major histocompatibility complex (MHC) class I expression are identified within the host spleen and lymph nodes; these donor MHC expressing cells may initiate the host immune response subsequent to the cells entering the general circulation through host cerebral vessels damaged during graft placement. Rapid healing of damaged cerebral vessels is stimulated with exogenously applied basic fibroblast growth factor, which may prove helpful in reducing the potential entry of donor cells to the host circulation. These results have implication clinically for the intracerebral grafting of human fetal CNS cell suspensions.

Astrocyte-endothelial interaction: physiology and pathology

The blood-brain barrier of higher vertebrates is formed by the layer of endothelial cells lining the brain microvessels. The close anatomical association between endothelial cells and perivascular astrocytic end feet suggests cooperation between these cell types in forming and maintaining the blood-brain barrier. This review considers evidence from grafting experiments, developmental studies and culture models of the brain endothelium, concerning the inductive influences acting on the endothelium, and from endothelial cells acting on perivascular astrocytes. Examples from pathology and neurotoxicology which may involve breakdown of induction are also considered.

Allografts of CNS tissue possess a blood-brain barrier. II. Angiogenesis in solid tissue and cell suspension grafts

Angiogenesis and patency of blood vessels were analyzed qualitatively in solid CNS and peripheral tissue syngeneic, allogeneic, and xenogeneic grafts and in individual cell suspension grafts of astrocytes, fibroblasts, PC12, and three additional tumor cell lines placed intracerebrally in adult host mice. Postgrafting survival times were 1 day through 4 weeks. The patency of graft vessels was determined in sections from immersion-fixed tissues incubated to reveal the endogenous peroxidase activity of host red cells trapped within the lumen of blood vessels. Additionally, horseradish peroxidase (HRP) was administered intravenously to live hosts; HRP labels host brain and graft vessels on the luminal surface and reveals the presence or absence of a blood-brain barrier (BBB) within the grafts. The origins of blood vessels supplying solid tissue xenografts were identified immunohistochemically with primary antibodies against host (athymic AKR mice) and donor (fetal Lewis rats) major histocompatibility complex (MHC) class I. Blood vessels supplying solid CNS grafts at 1-7 days post-transplantation were identified ultrastructurally and possessed interendothelial tight junctional complexes; however, they were not perfused with either host blood or blood-borne HRP prior to 8 days. Graft vessels at 10 days were outlined consistently by peroxidase-positive red cells in immersion-fixed material and labeled with blood-borne HRP. These vessels provided a BBB to the circulating HRP and exhibited interendothelial tight junctions. Evidence of angiogenesis within solid anterior pituitary grafts and the variety of cell suspension grafts was obtained prior to 3 days post-transplantation in immersion-fixed preparations; the vessels, with the notable exception of those supplying astrocyte cell suspensions, failed to present a BBB to blood-borne peroxidase. Endothelia in the solid pituitary allografts and the PC12 cell grafts were highly fenestrated and exhibited open interendothelial junctions; those in the tumor and fibroblast cell grafts, for the most part, appeared nonfenestrated, and many possessed open interendothelial junctional complexes. Immunostaining for host and donor MHC class I revealed that donor blood vessels predominate over host vessels in CNS xenografts and supply pituitary xenografts exclusively; in both preparations, donor vessels were not identified within the host CNS. Because cell suspension grafts were derived from endothelia-free preparations grown in culture, blood vessels supplying these grafts were necessarily of host CNS origin and manifested a morphological transformation from a BBB to a non-BBB endothelium. The data suggest that angiogenesis in solid CNS grafts placed into the adult host CNS, compared to similarly placed solid peripheral tissue/cell suspension grafts, is not rapid.(ABSTRACT TRUNCATED AT 400 WORDS)