Unfocused laser illumination kills dye-targeted mouse neurons by selective photothermolysis

Development of fetal hippocampal grafts in intact and lesioned hippocampus

Functional recovery observed in Parkinson's disease patients following grafting of fetal substantia nigra has encouraged the development of similar grafting therapy for other neurological disorders. Fetal hippocampal grafting paradigms are of considerable significance because of their potential to treat neurological disorders affecting primarily hippocampus, including temporal lobe epilepsy, cerebral ischemia, stroke, and head injury. Since many recent studies of hippocampal transplants were carried out with an aim of laying the foundation for future clinical applications, an overview of the development of fetal hippocampal transplants, and their capability for inducing functional recovery under different host conditions is timely. In this review, we will summarize recent developments in hippocampal transplants, especially the anatomical and/or functional integration of grafts within the host brain under specific host conditions, including a comparison of intact hippocampus with various types of hippocampal lesions or injury. Improvements in grafting techniques, methods for analysis of graft integration and graft function will be summarized, in addition to critical factors which enhance the survival and integration of grafted cells and alternative sources of donor cells currently being tested or considered for hippocampal transplantation. Viewed collectively, hippocampal grafting studies show that fetal hippocampal tissue/cells survive grafting, establish both afferent and efferent connections with the host brain, and are also capable of ameliorating certain learning and memory deficits in some models. However, the efficacy of intracerebral fetal hippocampal grafts varies considerably in different animal models, depending on several factors: the mode of donor tissue preparation, the method of grafting, the state of host hippocampus at the time of grafting, and the placement of grafts within the hippocampus. Functional improvement in many models appeared to be caused partially by re-establishment of damaged circuitry and partially by a trophic action of grafts. However, exact mechanisms of graft-mediated behavioral recovery remain to be clarified due to the lack of correlative analysis in the same animal between the degree of graft integration and behavioral recovery. Issues of mechanisms of action, degree of restoration of host circuitry and amelioration of host pathological conditions will need to be sorted out clearly prior to clinical use of fetal hippocampal transplants for susceptible neurological conditions.

Selective laser-activated lesioning of prelabeled fetal hippocampal grafts by intracellular photolytic chromophore

Selective removal of grafted tissue is critical to assess the functional role of that tissue in the host, yet is technically difficult for well-dispersed neural grafts. We labeled fetal hippocampal cells with both a nuclear marker (5'-bromodeoxyuridine) and a cytoplasmic marker (latex microspheres) before grafting into normal adult hippocampus. A nontoxic chromophore, chlorin e6, was conjugated on to the surface of latex microspheres of the treatment group. Chlorin e6 produces cytotoxic singlet oxygen only during photoactivation. Grafted animals received transcranial exposure to various intensities of near-infrared laser light. Following laser exposure, grafts were observed in all groups except in transplants prelabeled with chlorin e6 latex microspheres. An optimal laser exposure of 50-100 J/cm2 (4-8 min) was found to selectively remove only the chlorin e6-containing grafted cells. With increasing doses of laser illumination, non-specific lesions of the host tissue surrounding the graft were induced. Quantitative graft analysis, in the absence of laser exposure, indicated that the survival of grafted cells was similar between control transplants labeled with latex microspheres alone and grafts labeled with latex microspheres plus chlorin e6. This is further evidence that chlorin e6 by itself was not toxic without laser exposure. The results clearly demonstrate that singlet oxygen-induced cell photolysis can result in selective, non-invasive removal of dispersed grafted cells located in adult hippocampus. This technique may facilitate defining specific mechanisms of action of grafted cells which mediate functional recovery in different host conditions.

Fluorescent labeling of dissociated fetal cells for tissue culture

The ability to pre-label cells used in transplantation experiments would have the potential benefits of identification of cell type and associated processes and the analysis of graft migration in the host. We have used an in vitro tissue culture system as a model to test several fluorescent dyes for this application. Fetal rat hippocampal tissue (E17-E18) was dissociated and incubated in the presence of carboxyfluorescein ester (CFSE), rhodamine-B dextran amine (RBD), DiI, or rhodamine-labeled latex microspheres. Cells were cultured in defined medium for up to 1 month. Cells labeled with CFSE were initially bright but faded over several days. RBD labeled the soma of cells, but fluorescence intensity was lost over a period of a few weeks. Cells labeled with DiI possessed brilliant staining of neuronal processes for weeks. Latex microspheres brightly labeled the soma but not the processes of neurons; fluorescent debris and sterility were problems with this label. We conclude that CFSE and DiI have significant potential usefulness in vitro as markers of cell viability and process formation with mammalian fetal CNS cells, whereas RBD is much less permanent. Latex microspheres may be suitable for pre-labeling of cells for transplantation if purification and sterility can be enhanced over present preparations.

Anatomical and physiological localization of prelabeled grafts in rat hippocampus

Dissociated rat fetal hippocampal cells were grafted into normal adult rats. The fetal cells were incubated with one of a number of fluorescent compounds at the time of the dissociation to facilitate identification of the individual grafted cells. The fluorescent labels which were analyzed for this purpose included rhodamine latex microspheres, Cascade blue latex beads, rhodamine-dextran-amine, DiI, and carboxyfluorescein ester. The labeled cells were stereotaxically placed as a suspension into normal rat host hippocampi. The rats were sacrificed 2 to 6 weeks after the grafting for in vitro physiological recordings, and the prelabeled grafts were located using fluorescence optics. During intracellular recordings neurons within the prelabeled grafts were injected with Lucifer yellow to visualize the morphology and integration of neuronal processes within the host. Following the recordings the host slices with the grafts were fixed in 4% paraformaldehyde for anatomical analysis. The ability to prelabel cellular grafts allows subsequent anatomical and physiological analysis of the integration of grafted neurons at the resolution of a single neuron. Such an analysis will improve our understanding of the survival, differentiation, migration, and integration of the grafted neurons and their potential to replace lost function in the lesioned hippocampus.

Targeted neuronal lesion induced by photosensitizing dyes

Free radical-induced phototoxicity mediated by laser irradiation was investigated in the rabbit facial nerve. Azure-C, mesoporphyrin, or the dye conjugated to the protein carrier horseradish peroxidase were injected into the levator alae nasi muscle. Two to 7 days after uptake and laser exposure, nerve sections showed varying degrees of cellular modifications including: severe membrane degradation and associated lipid peroxide granules, distended mitochondria, and mitochondrial loss. Immunoblots of homogenates from treated nerves revealed specific changes in neurofilament and myelin basic protein. The site specific damage produced in vivo by photosensitizing dye resembles abnormalities in aging neurons and in Batten's disease, both hypothesized to be cases of free radical-peroxidation reactions. These reactions differ from those found in transection and crush lesions.

Progressive incorporation of propidium iodide in cultured mouse neurons correlates with declining electrophysiological status: a fluorescence scale of membrane integrity

We describe a visual assay of neuronal electrophysiologic status for use with cultured neurons, based on the exclusion of propidium iodide (PI) by intact cellular membranes. We use this fluorescent dye, which binds to nucleic acids, at concentrations suitable for long-term exposure to neurons without toxicity. We correlate the progressive loss of resting membrane potential and the progressive inability to generate stimulated action potentials by cultured mouse dorsal root ganglion neurons with increasing incorporation of PI. The scoring system used to gauge incorporation of PI is rapid and highly reproducible using a standard fluorescence microscope. Applications exist for studies of neuronal toxicity, survival, and electrophysiology in vitro.

Non-invasive laser microsurgery selectively damages populations of labeled mouse neurons: dependence on incident laser dose and absorption

Selective photothermolysis (SP) is a novel technique by which brief, unfocused laser pulses are selectively absorbed by, and cause selective thermal damage to, endogenously pigmented structures. This report describes the use of an exogenous non-fluorescent chromophore (Procion blue) to effect cellular damage by SP. Cultured dorsal root ganglia neurons were selectively labeled with Procion blue and subjected to varying doses of laser illumination. Progressive cellular damage was assessed by leakage of propidium iodide through damaged membranes. The neurons targeted via an exogenous chromophore sustained damage which was proportionate to laser dose and chromophore concentration. The results of these experiments demonstrated that the rate and extent of neuronal damage can be controlled by adjusting either the incident dose of laser energy or the amount of target chromophore within cells. Selective photothermolysis will provide an experimental tool for neurobiologists in particular and will find general use within the biomedical field.