Synapse-to-neuron ratio is inversely related to neuronal density in mature neuronal cultures

Ontogeny of biochemical, morphological and functional parameters of synaptogenesis in primary cultures of rat hippocampal and cortical neurons

Background

Synaptogenesis is a critical neurodevelopmental process whereby pre- and postsynaptic neurons form apposed sites of contact specialized for chemical neurotransmission. Many neurodevelopmental disorders are thought to reflect altered patterns of synaptic connectivity, including imbalances between excitatory and inhibitory synapses. Developing rapid throughput approaches for assessing synaptogenesis will facilitate toxicologic and drug screening studies of neurodevelopmental disorders. The current study describes the use of high-content imaging to quantify the ontogeny of excitatory and inhibitory synapses using in vitro models of neurodevelopment. These data are compared to biochemical and functional measures of synaptogenesis.

Results

The ontogenetic patterns of synapse formation were compared between primary rodent hippocampal and cortical neurons over 28 days in vitro (DIV). As determined by ELISA, the increase in synaptophysin expression levels as cultures matured was similar between hippocampal and cortical cultures. High-content imaging of immunoreactivity of excitatory and inhibitory synaptic biomarkers demonstrated an overall greater number of synapses in hippocampal relative to cortical neurons with marked differences in the pattern of inhibitory synapse development between these two neuronal cell types. Functional assays revealed that both the mean firing rates and mean bursting rates were significantly increased in cortical cultures relative to hippocampal cultures. This difference may reflect decreased inhibitory synaptic tone in cortical versus hippocampal cultures.

Conclusions

These data demonstrate differences and similarities in the ontogeny of synaptogenesis between hippocampal and cortical neurons, depending on the biological level examined. Assessment of synaptophysin protein levels by ELISA showed a general increase in synapse formation in both cell types with increasing time in culture, while high-content imaging was able to delineate cell type-dependent differences in formation of excitatory versus inhibitory synapses. The functional significance of differences in the balance of excitatory to inhibitory synapses was confirmed by the assessment of network activity using microelectrode arrays. These results suggest that high-content imaging and microelectrode arrays provide complementary approaches for quantitative assessment of synaptogenesis, which should provide a robust readout of toxicologic and pharmacologic effects on this critical neurodevelopmental event.

Network dynamics of 3D engineered neuronal cultures: a new experimental model for in-vitro electrophysiology

Despite the extensive use of in-vitro models for neuroscientific investigations and notwithstanding the growing field of network electrophysiology, all studies on cultured cells devoted to elucidate neurophysiological mechanisms and computational properties, are based on 2D neuronal networks. These networks are usually grown onto specific rigid substrates (also with embedded electrodes) and lack of most of the constituents of the in-vivo like environment: cell morphology, cell-to-cell interaction and neuritic outgrowth in all directions. Cells in a brain region develop in a 3D space and interact with a complex multi-cellular environment and extracellular matrix. Under this perspective, 3D networks coupled to micro-transducer arrays, represent a new and powerful in-vitro model capable of better emulating in-vivo physiology. In this work, we present a new experimental paradigm constituted by 3D hippocampal networks coupled to Micro-Electrode-Arrays (MEAs) and we show how the features of the recorded network dynamics differ from the corresponding 2D network model. Further development of the proposed 3D in-vitro model by adding embedded functionalized scaffolds might open new prospects for manipulating, stimulating and recording the neuronal activity to elucidate neurophysiological mechanisms and to design bio-hybrid microsystems.

Regional alterations in purkinje cell density in patients with autism

Neuropathological studies, using a variety of techniques, have reported a decrease in Purkinje cell (PC) density in the cerebellum in autism. We have used a systematic sampling technique that significantly reduces experimenter bias and variance to estimate PC densities in the postmortem brains of eight clinically well-documented individuals with autism, and eight age- and gender-matched controls. Four cerebellar regions were analyzed: a sensorimotor area comprised of hemispheric lobules IV-VI, crus I & II of the posterior lobe, and lobule X of the flocculonodular lobe. Overall PC density was thus estimated using data from all three cerebellar lobes and was found to be lower in the cases with autism as compared to controls, an effect that was most prominent in crus I and II (p<0.05). Lobule X demonstrated a trend towards lower PC density in only the males with autism (p = 0.05). Brain weight, a correlate of tissue volume, was found to significantly contribute to the lower lobule X PC density observed in males with autism, but not to the finding of lower PC density in crus I & II. Therefore, lower crus I & II PC density in autism is more likely due to a lower number of PCs. The PC density in lobule X was found to correlate with the ADI-R measure of the patient's use of social eye contact (R² = -0.75, p = 0.012). These findings support the hypothesis that abnormal PC density may contribute to selected clinical features of the autism phenotype.

Differential labeling of cell-surface and internalized proteins after antibody feeding of live cultured neurons

In order to demonstrate the cell-surface localization of a putative transmembrane receptor in cultured neurons, we labeled the protein on the surface of live neurons with a specific primary antibody raised against an extracellular portion of the protein. Given that receptors are trafficked to and from the surface, if cells are permeabilized after fixation then both cell-surface and internal protein will be detected by the same labeled secondary antibody. Here, we adapted a method used to study protein trafficking ("antibody feeding") to differentially label protein that had been internalized by endocytosis during the antibody incubation step and protein that either remained on the cell surface or was trafficked to the surface during this period. The ability to distinguish these two pools of protein was made possible through the incorporation of an overnight blocking step with highly-concentrated unlabeled secondary antibody after an initial incubation of unpermeabilized neurons with a fluorescently-labeled secondary antibody. After the blocking step, permeabilization of the neurons allowed detection of the internalized pool with a fluorescent secondary antibody labeled with a different fluorophore. Using this technique we were able to obtain important information about the subcellular location of this putative receptor, revealing that it was, indeed, trafficked to the cell-surface in neurons. This technique is broadly applicable to a range of cell types and cell-surface proteins, providing a suitable antibody to an extracellular epitope is available.

The influence of neuronal density and maturation on network activity of hippocampal cell cultures: a methodological study

It is known that cell density influences the maturation process of in vitro neuronal networks. Neuronal cultures plated with different cell densities differ in number of synapses per neuron and thus in single neuron synaptic transmission, which results in a density-dependent neuronal network activity. Although many authors provided detailed information about the effects of cell density on neuronal culture activity, a dedicated report of density and age influence on neuronal hippocampal culture activity has not yet been reported. Therefore, this work aims at providing reference data to researchers that set up an experimental study on hippocampal neuronal cultures, helping in planning and decoding the experiments. In this work, we analysed the effects of both neuronal density and culture age on functional attributes of maturing hippocampal cultures. We characterized the electrophysiological activity of neuronal cultures seeded at three different cell densities, recording their spontaneous electrical activity over maturation by means of MicroElectrode Arrays (MEAs). We had gather data from 86 independent hippocampal cultures to achieve solid statistic results, considering the high culture-to-culture variability. Network activity was evaluated in terms of simple spiking, burst and network burst features. We observed that electrical descriptors were characterized by a functional peak during maturation, followed by a stable phase (for sparse and medium density cultures) or by a decrease phase (for high dense neuronal cultures). Moreover, 900 cells/mm² cultures showed characteristics suitable for long lasting experiments (e.g. chronic effect of drug treatments) while 1800 cells/mm² cultures should be preferred for experiments that require intense electrical activity (e.g. to evaluate the effect of inhibitory molecules). Finally, cell cultures at 3600 cells/mm² are more appropriate for experiments in which time saving is relevant (e.g. drug screenings). These results are intended to be a reference for the planning of in vitro neurophysiological and neuropharmacological experiments with MEAs.

A method for the three-dimensional reconstruction of Neurobiotin™-filled neurons and the location of their synaptic inputs

Here, we describe a robust method for mapping the number and type of neuro-chemically distinct synaptic inputs that a single reconstructed neuron receives. We have used individual hypoglossal motor neurons filled with Neurobiotin by semi-loose seal electroporation in thick brainstem slices. These filled motor neurons were then processed for excitatory and inhibitory synaptic inputs, using immunohistochemical-labeling procedures. For excitatory synapses, we used anti-VGLUT2 to locate glutamatergic pre-synaptic terminals and anti-PSD-95 to locate post-synaptic specializations on and within the surface of these filled motor neurons. For inhibitory synapses, we used anti-VGAT to locate GABAergic pre-synaptic terminals and anti-GABA-A receptor subunit α1 to locate the post-synaptic domain. The Neurobiotin-filled and immuno-labeled motor neuron was then processed for optical sectioning using confocal microscopy. The morphology of the motor neuron including its dendritic tree and the distribution of excitatory and inhibitory synapses were then determined by three-dimensional reconstruction using IMARIS software (Bitplane). Using surface rendering, fluorescence thresholding, and masking of unwanted immuno-labeling, tools found in IMARIS, we were able to obtain an accurate 3D structure of an individual neuron including the number and location of its glutamatergic and GABAergic synaptic inputs. The power of this method allows for a rapid morphological confirmation of the post-synaptic responses recorded by patch-clamp prior to Neurobiotin filling. Finally, we show that this method can be adapted to super-resolution microscopy techniques, which will enhance its applicability to the study of neural circuits at the level of synapses.

Neurient: an algorithm for automatic tracing of confluent neuronal images to determine alignment

A goal of neural tissue engineering is the development and evaluation of materials that guide neuronal growth and alignment. However, the methods available to quantitatively evaluate the response of neurons to guidance materials are limited and/or expensive, and may require manual tracing to be performed by the researcher. We have developed an open source, automated Matlab-based algorithm, building on previously published methods, to trace and quantify alignment of fluorescent images of neurons in culture. The algorithm is divided into three phases, including computation of a lookup table which contains directional information for each image, location of a set of seed points which may lie along neurite centerlines, and tracing neurites starting with each seed point and indexing into the lookup table. This method was used to obtain quantitative alignment data for complex images of densely cultured neurons. Complete automation of tracing allows for unsupervised processing of large numbers of images. Following image processing with our algorithm, available metrics to quantify neurite alignment include angular histograms, percent of neurite segments in a given direction, and mean neurite angle. The alignment information obtained from traced images can be used to compare the response of neurons to a range of conditions. This tracing algorithm is freely available to the scientific community under the name Neurient, and its implementation in Matlab allows a wide range of researchers to use a standardized, open source method to quantitatively evaluate the alignment of dense neuronal cultures.

Calcium regulation of spontaneous and asynchronous neurotransmitter release

The molecular machinery underlying action potential-evoked, synchronous neurotransmitter release, has been intensely studied. It was presumed that two other forms of exocytosis, delayed (asynchronous) and spontaneous transmission, were mediated by the same voltage-activated Ca(2+) channels (VACCs), intracellular Ca(2+) sensors and vesicle pools. However, a recent explosion in the study of spontaneous and asynchronous release has shown these presumptions to be incorrect. Furthermore, the finding that different forms of synaptic transmission may mediate distinct physiological functions emphasizes the importance of identifying the mechanisms by which Ca(2+) regulates spontaneous and asynchronous release. In this article, we will briefly summarize new and published data on the role of Ca(2+) in regulating spontaneous and asynchronous release at a number of different synapses. We will discuss how an increase of extracellular [Ca(2+)] increases spontaneous and asynchronous release, show that VACCs are involved at only some synapses, and identify regulatory roles for other ion channels and G protein-coupled receptors. In particular, we will focus on two novel pathways that play important roles in the regulation of non-synchronous release at two exemplary synapses: one modulated by the Ca(2+)-sensing receptor and the other by transient receptor potential cation channel sub-family V member 1.

Automated quantification of synapses by fluorescence microscopy

The quantification of synapses in neuronal cultures is essential in studies of the molecular mechanisms underlying synaptogenesis and synaptic plasticity. Conventional counting of synapses based on morphological or immunocytochemical criteria is extremely work-intensive. We developed a fully automated method which quantifies synaptic elements and complete synapses based on immunocytochemistry. Pre- and postsynaptic elements are detected by their corresponding fluorescence signals and their proximity to dendrites. Synapses are defined as the combination of a pre- and postsynaptic element within a given distance. The analysis is performed in three dimensions and all parameters required for quantification can be easily adjusted by a graphical user interface. The integrated batch processing enables the analysis of large datasets without any further user interaction and is therefore efficient and timesaving. The potential of this method was demonstrated by an extensive quantification of synapses in neuronal cultures from DIV 7 to DIV 21. The method can be applied to all datasets containing a pre- and postsynaptic labeling plus a dendritic or cell surface marker.

Ultrastructural characterization of rat neurons in primary culture

Few studies have addressed the ultrastructure and morphology of neurons in primary pure culture. We therefore use immunohistochemistry and electron microscopy to investigate the ultrastructure of cultured neurons during extended incubation in vitro. Rat cerebral cortex neurons were cultured in Neurobasal™ medium. Adherent cells developed as networks of single neurons or clusters depending on the plating density. Almost all surviving cells were neurons as demonstrated by neurofilament immunolabeling. The number of cultured neurons increased substantially to 14-21 days in vitro (DIV) and then plateaued and subsequently declined. From DIV 1-10 neurons extended large neurites, followed by the development of fine and dense neurites, and neurones survived until DIV 30-50. Notably, numerous mitochondria were observed along fibrous elements within neurites, suggestive of active intracellular trafficking. Electron microscopy also revealed that multiple types of synapses were formed between neurons. These ultrastructural results confirm previous reports of electrophysiological activity in cultured neurons. However many neurons contained distorted mitochondria and abnormal organelles including multilamellar vesicles and multivesicular myeloid bodies. The proportion of neurons containing abnormal organelles increased significantly in culture medium supplemented with antibiotics. On long-term culture neuronal death and apoptotic nuclei were observed. Despite the presence of abnormal organelles, the ultrastructure of cultured neurons was very similar to that of in vivo neurons; in vitro culture therefore provides a useful tool for studies on neuronal development, aging, and neurotransmission.

Synergistic NGF/B27 gradients position synapses heterogeneously in 3D micropatterned neural cultures

Native functional brain circuits show different numbers of synapses (synaptic densities) in the cerebral cortex. Until now, different synaptic densities could not be studied in vitro using current cell culture methods for primary neurons. Herein, we present a novel microfluidic based cell culture method that combines 3D micropatterning of hydrogel layers with linear chemical gradient formation. Micropatterned hydrogels were used to encapsulate dissociated cortical neurons in laminar cell layers and neurotrophic factors NGF and B27 were added to influence the formation of synapses. Neurotrophic gradients allowed for the positioning of distinguishable synaptic densities throughout a 3D micropatterned neural culture. NGF and B27 gradients were maintained in the microfluidic device for over two weeks without perfusion pumps by utilizing a refilling procedure. Spatial distribution of synapses was examined with a pre-synaptic marker to determine synaptic densities. From our experiments, we observed that (1) cortical neurons responded only to synergistic NGF/B27 gradients, (2) synaptic density increased proportionally to synergistic NGF/B27 gradients; (3) homogeneous distribution of B27 disturbed cortical neurons in sensing NGF gradients and (4) the cell layer position significantly impacted spatial distribution of synapses.