Cholinergic innervation of cerebral cortex in organotypic slice cultures: sustained basal forebrain and transient striatal cholinergic projections

A Bridge Between in vitro and in vivo Studies in Neuroscience: Organotypic Brain Slice Cultures

In vitro and in vivo models are efficiently used systems in neuroscience research to study the brain in normal or pathological conditions. There are many advantages to these systems, yet they also have significant limitations. In vitro cell cultures offer the opportunity to investigate the cell basics or primary response of a cell population against any treatment. However, these models do not always predict in vivo behavior. In vivo animal studies constitute the most realistic platform for research and therapeutic approaches, yet they are laborious, open to secondary complications and painful or stressful for the animals from an ethical point of view. Organotypic brain slice cultures provide an in vivo-like environment since they maintain three-dimensional cytoarchitecture of the brain thus enable to study many cell types in one system and allow precise control of the microenvironment. In this review, we will focus on the history and key features of organotypic brain slice cultures as well as its preparation.

Regioselective biolistic targeting in organotypic brain slices using a modified gene gun

Transfection of DNA has been invaluable for biological sciences and with recent advances to organotypic brain slice preparations, the effect of various heterologous genes could thus be investigated easily while maintaining many aspects of in vivo biology. There has been increasing interest to transfect terminally differentiated neurons for which conventional transfection methods have been fraught with difficulties such as low yields and significant losses in viability. Biolistic transfection can circumvent many of these difficulties yet only recently has this technique been modified so that it is amenable for use in mammalian tissues.

New modifications to the accelerator chamber have enhanced the gene gun's firing accuracy and increased its depths of penetration while also allowing the use of lower gas pressure (50 psi) without loss of transfection efficiency as well as permitting a focused regioselective spread of the particles to within 3 mm. In addition, this technique is straight forward and faster to perform than tedious microinjections. Both transient and stable expression are possible with nanoparticle bombardment where episomal expression can be detected within 24 hr and the cell survival was shown to be better than, or at least equal to, conventional methods. This technique has however one crucial advantage: it permits the transfection to be localized within a single restrained radius thus enabling the user to anatomically isolate the heterologous gene's effects. Here we present an in-depth protocol to prepare viable adult organotypic slices and submit them to regioselective transfection using an improved gene gun.

Optimized heterologous transfection of viable adult organotypic brain slices using an enhanced gene gun

Background

Organotypic brain slices (OTBS) are an excellent experimental compromise between the facility of working with cell cultures and the biological relevance of using animal models where anatomical, morphological, and cellular function of specific brain regions can be maintained. The biological characteristics of OTBS can subsequently be examined under well-defined conditions. They do, however, have a number of limitations; most brain slices are derived from neonatal animals, as it is difficult to properly prepare and maintain adult OTBS. There are ample problems with tissue integrity as OTBS are delicate and frequently become damaged during the preparative stages. Notwithstanding these obstacles, the introduced exogenous proteins into both neuronal cells, and cells imbedded within tissues, have been consistently difficult to achieve.

Results

Following the ex vivo extraction of adult mouse brains, mounted inside a medium-agarose matrix, we have exploited a precise slicing procedure using a custom built vibroslicer. To transfect these slices we used an improved biolistic transfection method using a custom made low-pressure barrel and novel DNA-coated nanoparticles (40 nm), which are drastically smaller than traditional microparticles. These nanoparticles also minimize tissue damage as seen by a significant reduction in lactate dehydrogenase activity as well as propidium iodide (PI) and dUTP labelling compared to larger traditional gold particles used on these OTBS. Furthermore, following EYFP exogene delivery by gene gun, the 40 nm treated OTBS displayed a significantly larger number of viable NeuN and EYFP positive cells. These OTBS expressed the exogenous proteins for many weeks.

Conclusions

Our described methodology of producing OTBS, which results in better reproducibility with less tissue damage, permits the exploitation of mature fully formed adult brains for advanced neurobiological studies. The novel 40 nm particles are ideal for the viable biolistic transfection of OTBS by reducing tissue stress while maintaining long term exogene expression.

Organotypic vibrosections: novel whole sagittal brain cultures

In vitro cell culture models are of enormous importance in neuroscience research and organotypic brain slices are found to be a potent model very close to the in vivo situation. Brain slices can be cultured as single slices or as co-slices. However, there is need to culture whole brain sections, containing the complex functional architecture. The aim of the present study was to develop and characterize whole brain sagittal slice cultures (200μm organotypic vibrosections) from postnatal day 8 rats. We show that sagittal vibrosections can be cultured for several weeks and they maintain survival of cholinergic and dopaminergic neurons, as well as a strong capillary network. Partly long-distance cortico-striatal and cortico-hippocampal nerve fibers were found using Mini-Ruby neurotracing. Dopaminergic nerve fibers extended from the mesencephalon, but in the striato-nigral tract and in the striatum only strong dense varicosities were found. The model also allows to study pathological triggers, such as e.g. hydrogen peroxide markedly increased propidiumiodide-positive nuclei in the hippocampus. In conclusion, our novel model provides an easy potent whole sagittal brain culture system that allows to study cholinergic and dopaminergic neurons together but also in close interaction with all other cells of the brain and with capillaries. It will be a great challenge in future to use this model to re-construct whole pathways. This vibrosection model may partly represent a close adult in vivo situation, which allows to study neurodegeneration and neuroprotection of cholinergic and dopaminergic neurons, which plays an important role in Alzheimer's and Parkinson's disease, respectively.

Brain slices as models for neurodegenerative disease and screening platforms to identify novel therapeutics

Recent improvements in brain slice technology have made this biological preparation increasingly useful for examining pathophysiology of brain diseases in a tissue context. Brain slices maintain many aspects of in vivo biology, including functional local synaptic circuitry with preserved brain architecture, while allowing good experimental access and precise control of the extracellular environment, making them ideal platforms for dissection of molecular pathways underlying neuronal dysfunction. Importantly, these ex vivo systems permit direct treatment with pharmacological agents modulating these responses and thus provide surrogate therapeutic screening systems without recourse to whole animal studies. Virus or particle mediated transgenic expression can also be accomplished relatively easily to study the function of novel genes in a normal or injured brain tissue context.In this review we will discuss acute brain injury models in organotypic hippocampal and co-culture systems and the effects of pharmacological modulation on neurodegeneration. The review will also cover the evidence of developmental plasticity in these ex vivo models, demonstrating emergence of injury-stimulated neuronal progenitor cells, and neurite sprouting and axonal regeneration following pathway lesioning. Neuro-and axo-genesis are emerging as significant factors contributing to brain repair following many acute and chronic neurodegenerative disorders. Therefore brain slice models may provide a critical contextual experimental system to explore regenerative mechanisms in vitro.

Effects of ciliary neurotrophic factor and leukemia inhibiting factor on oxytocin and vasopressin magnocellular neuron survival in rat and mouse hypothalamic organotypic cultures

Organotypic cultures of mouse and rat magnocellular neurons (MCNs) in the hypothalamo-neurohypophysial system (HNS) have served as important experimental models for the molecular and physiological study of this neuronal phenotype. However, it has been difficult to maintain significant numbers of the MCNs, particularly vasopressin MCNs, in these cultures for long periods. In this paper, we describe the use of the neurotrophic factors, leukemia inhibiting factor (LIF) and ciliary neurotrophic factor (CNTF) to rescue rat vasopressin (Avp)- and oxytocin (Oxt)-MCNs from axotomy-induced, programmed cell death in vitro. Quantitative data are presented for the efficacy of the LIF family of neurotrophic factors on the survival of MCNs in three nuclei, the paraventricular (PVN), supraoptic (SON), and accessory (ACC) nuclei in the mouse and rat hypothalamus.

A role for neurotrophin-3 in targeting developing cholinergic axon projections to cerebral cortex

This study examined the relationship between expression of neurotrophin-3 (NT-3) and the ingrowth of cholinergic axonal projections in cerebral cortex. Patterns of expression of NT-3 (defined by beta-galactosidase reporter expression in heterozygous offspring of transgenic NT-3(lacZneo/+) mice) revealed that limbic cortical regions (including frontal, cingulate, and insular cortex, as well as the dentate gyrus) express NT-3 and that these cortical regions receive early and relatively dense cholinergic axons (stained for acetylcholinesterase, AChE). Using the dentate gyrus as a model system, studies revealed that expression of the NT-3 reporter parallels, and precedes by approximately 2 days, the ingrowth of AChE positive cholinergic axons. Studies of forebrain organotypic slice cultures demonstrate that basal forebrain-derived cholinergic axons extend into cortical regions in a pattern that mimics the pattern of expression of the NT-3 reporter. Similarly, chimeric co-cultures, combining wild type septum with a slice of hippocampus from heterozygous NT-3(lacZneo/+) mice, demonstrate that cholinergic axons grow into regions of the dentate gyrus that express the NT-3 reporter. Hemisphere slice cultures made from NT-3 knockout mice reveal cholinergic axonal growth into cortex, but these axons do not form the regional pattern characteristic of slice cultures made from wild type or heterozygous NT-3(lacZneo/+) mice. Further, chimeric co-cultures made using slices of wild type septum combined with slices of hippocampus from NT-3 knockout mice demonstrate robust cholinergic axonal growth into the hippocampus, but the cholinergic axons do not form the characteristic preterminal pattern associated with the dentate gyrus. Slice cultures from limbic cortical tissue from the NT-3 null mice do not display exaggerated levels of cell death. In aggregate, these data support the hypothesis that expression of NT-3 by cortical neurons serves to attract basal forebrain cholinergic projections to their target cells in cerebral cortex.

Basal forebrain cholinergic cell attachment and neurite outgrowth on organotypic slice cultures of hippocampal formation

Distributions of somata and neurites of cholinergic neurons were studied after seeding dissociated cells onto organotypic slice cultures. Slice cultures were made from hippocampal formation and adjacent cortical regions from rats or mice. Dissociated cell suspensions of basal forebrain tissue from rat or mouse fetuses were seeded onto the slice cultures. Combined cultures were maintained for 1-21 days in vitro. Cultures processed for acetylcholinesterase (AChE) histochemistry demonstrated non-random patterns of cholinergic cells and their neurites. Labeled cells appeared most frequently in the molecular layer of the dentate gyrus, and in the deeper layers of cortical regions adjacent to the hippocampus. Neurites extending from these labeled cells appeared to target the dentate molecular layer and the cortical subplate layer. By 4 days in vitro, AChE-positive basal forebrain cells display several short and thick neurites that appear to be dendrites, and one long process that appears to be an axon. By 5 days in vitro, dendrites are well developed; by 7 days the presumed axon has extended widely over the cortical target zone. These neurites are maintained through 3 weeks in culture. Distributions of cells varied with the age of the slice. AChE-labeled cells were not seen overlying hippocampal tissue when dissociated cells were seeded on slice cultures made from day 0 rats, but a few labeled cells were seen when seeded on slices from day 2 rats. Clear non-random patterns of labeled cells and neurite outgrowth were seen on slice cultures from day 5 or older pups. The non-random distribution seen with AChE-positive neurons was not seen using other techniques that labeled all cells (non-selective fluorescent labels) or all neurons; these techniques resulted in labeled cells scattered apparently homogenously across the slice culture.These studies demonstrate a non-random pattern of attachment or differentiation of basal forebrain cholinergic neurons when these cells are seeded onto cultured cortical slices; this pattern mimics the normal patterns of basal forebrain cholinergic projections to these cortical regions. These data suggest that the factors that normally guide basal forebrain-derived cholinergic axons to their target cells in vivo are present and detectable in this model system.

Electrophysiological recordings of patterned rat brain stem slice neurons

Dissociated neuronal cultures on substrates patterned with extracellular matrix (ECM) proteins have yielded much information in the past. However, although the culture of brain slices has many advantages over dissociated neuronal cultures, its feasibility on patterned substrates has not been demonstrated to date. In the present study, neuronal outgrowth from brain stem slices onto homogeneous control substrates, and onto laminin structures of grid- and line-shape was achieved. Cultures were evaluated by means of phase contrast microscopy, antibody staining, and patch-clamp measurements. Only patterns with line sizes of more than 4 microm yielded satisfactory neuronal outgrowth. The size of the nodes in the pattern influenced the nodal compliance of the spreading cells and the amount of unstructured overgrowth. Best grid patterns were 4 microm lines and 10 microm nodes, best line patterns were 4 microm lines and 20 microm nodes. On patterned substrates, average sodium and potassium currents were reduced by approximately 50% compared to controls, whereas area-normalized ion-currents were in the same order of magnitude. This indicates that as a consequence of the pattern-enforced geometrical confinement, neurons tend to have a smaller surface. In addition, neurons on patterned substrates were rapidly covered with glial overgrowth. This was shown by antibody staining.

Local proliferation of microglia cells in response to neocortical injury in vitro

Studies examined whether increased numbers of microglia following neural damage result from induced mitotic activity of resident microglia in situ. Organotypic slice cultures of neocortex were maintained for 1 week prior to placement of lesions. Increased numbers of OX-6 or tomato lectin labeled microglial cells were detected 1-8 days following lesions. Administration of BrdU to the cultures demonstrated lectin and BrdU double labeled microglial cells, conclusively demonstrating that a portion of the microglial cells were generated by local mitotic activity in situ.

AFGF promotes axonal growth in rat spinal cord organotypic slice co-cultures

This study developed a slice culture model system to study axonal regeneration after spinal cord injury. This model was tested in studies of the roles of acidic fibroblast growth factor (aFGF) and peripheral nerve segments in axonal growth between pieces of spinal cord. Transverse sections of P15-P18 Sprague-Dawley rat spinal cord were collected for organotypic slice cultures. Group I consisted of two slices of spinal cord in contact with each other during the culture period. Group II consisted of two slices that were separated by 3 mm and connected by two segments of intercostal nerves. Group III consisted of single slices for studies of neuron survival. Some cultures from each group included aFGF in the culture medium. Bromodeoxyuridine (BrdU) was included in the medium for some cultures. The results showed three principal findings. First, counts of neurofilament-positive cells demonstrated that treatment with aFGF significantly increased the number of surviving neurons in culture. Second, neurofilament immunostaining and DiI tracing demonstrated axons crossing the junction between the two pieces of spinal cord or growing through the intercostal nerve segments, and these axons were seen only in cultures with aFGF treatment. Third, few cells were double stained for neurofilament and BrdU, and these were found only with aFGF treatment. These results demonstrate that (1) organotypic slice cultures present a useful model to study regeneration from spinal cord injury, (2) aFGF rescues neurons and promotes axonal growth in these cultures, and (3) segments of intercostal nerves promote axon growth between slices of spinal cord.

Evidence for target preferences by cholinergic axons originating from different subdivisions of the basal forebrain

Possible target preferences of basal forebrain cholinergic neurons were studied in organotypic slice cultures. Cholinergic neurons in slices of medial septum or substantia innominata send axons into both hippocampus and neocortex when co-cultured together. However, septal cholinergic axons course through adjacent slices of neocortex to reach and branch densely in slices of hippocampus, but septal axons seldom grow beyond adjacent hippocampal tissue to reach neocortex. In contrast, cholinergic axons from substantia innominata commonly grow through hippocampus to reach neocortex, and also grow through neocortex to reach hippocampus, with similar branching densities in each target. The greater density of septal axonal branches in hippocampus than in neocortex suggests a preference of septal axons for the hippocampal target.

Specificity of attachment and neurite outgrowth of dissociated basal forebrain cholinergic neurons seeded on to organotypic slice cultures of forebrain

Development and differentiation of basal forebrain-derived cholinergic neurons were studied using a new technique that combines dissociated cell cultures with organotypic slice cultures. Slices of cerebral cortex or entire forebrain hemispheres were taken from early postnatal rat pups and maintained as organotypic cultures on membranes. Dissociated cell suspensions of basal forebrain tissue, taken from rat or mouse fetuses at gestational day 15-17, were seeded on to the slice cultures. Combined cultures were maintained for two to 14 days in vitro. Cultures processed for acetylcholinesterase histochemical staining demonstrated that stained neurons display regional variation in attachment to the slice, with most attachment occurring on cortex and with no detectable attachment on the caudate-putamen. Regional differences in attachment occur between cortical areas, with medial (cingulate) cortex showing much denser cell attachment than lateral (parietal) cortex, and across cortical layers, with layer I and deep layers showing more attachment than middle cortical layers. Similar patterns were observed on slices from rat brain irrespective of whether rat or mouse dissociated cells were used. Tyrosine hydroxylase-stained dissociated cells from ventral midbrain displayed a different pattern of attachment, with prominent attachment to the caudate putamen and less apparent specificity of regional and cortical laminar attachment. Little evidence of neurite outgrowth occurred during the first two days in vitro, but by four days, acetylcholinesterase-positive basal forebrain cells displayed several short and thick neurites that appeared to be dendrites, and one long process that appeared to be an axon. By seven days in vitro, dendrites are well developed and the presumed axon has extended branches over wide areas of cortex. These studies revealed several different types of cell-tissue interaction. The degree of cell growth and differentiation ranged from robust growth when dissociated cells were seeded on to slice cultures of normal target tissue, to apparently no attachment or growth when cells were seeded on to non-target tissue. This combined technique appears to be a useful method for studies of specificity of cell attachment and patterns of neurite outgrowth.

Cholinergic neurons from different subdivisions of the basal forebrain lack connectional specificity for cerebral cortical target sites in vitro

Basal forebrain cholinergic neurons send their axons to cerebral cortex in a topographically organized projection. Experiments tested the hypothesis that this topographic organization results from target preferences of the cholinergic neurons. Slices containing either medial septum or substantia innominata were grown in co-culture with slices of lateral neocortex and hippocampus. Cholinergic neurons from septum and from substantia innominata projected axons into neocortex and hippocampus, without obvious differences in pattern or density. These data suggest that basal forebrain cholinergic neurons can innervate any portion of the cerebral mantle.