Preparation and maintenance of single-cell micro-island cultures of basal forebrain neurons

Flexibility of in vitro cortical circuits influences resilience from microtrauma

Background

Small clusters comprising hundreds to thousands of neurons are an important level of brain architecture that correlates single neuronal properties to fulfill brain function, but the specific mechanisms through which this scaling occurs are not well understood. In this study, we developed an in vitro experimental platform of small neuronal circuits (islands) to probe the importance of structural properties for their development, physiology, and response to microtrauma.

Methods

Primary cortical neurons were plated on a substrate patterned to promote attachment in clusters of hundreds of cells (islands), transduced with GCaMP6f, allowed to mature until 10-13 days in vitro (DIV), and monitored with Ca²⁺ as a non-invasive proxy for electrical activity. We adjusted two structural factors-island size and cellular density-to evaluate their role in guiding spontaneous activity and network formation in neuronal islands.

Results

We found cellular density, but not island size, regulates of circuit activity and network function in this system. Low cellular density islands can achieve many states of activity, while high cellular density biases islands towards a limited regime characterized by low rates of activity and high synchronization, a property we summarized as "flexibility." The injury severity required for an island to lose activity in 50% of its population was significantly higher in low-density, high flexibility islands.

Conclusion

Together, these studies demonstrate flexible living cortical circuits are more resilient to microtrauma, providing the first evidence that initial circuit state may be a key factor to consider when evaluating the consequences of trauma to the cortex.

Autaptic Cultures: Methods and Applications

Neurons typically form daisy chains of synaptic connections with other neurons, but they can also form synapses with themselves. Although such self-synapses, or autapses, are comparatively rare in vivo, they are surprisingly common in dissociated neuronal cultures. At first glance, autapses in culture seem like a mere curiosity. However, by providing a simple model system in which a single recording electrode gives simultaneous access to the pre- and postsynaptic compartments, autaptic cultures have proven to be invaluable in facilitating important and elegant experiments in the area of synaptic neuroscience. Here, I provide detailed protocols for preparing and recording from autaptic cultures (also called micro-island or microdot cultures). Variations on the basic procedure are presented, as well as practical tips for optimizing the outcomes. I also illustrate the utility of autaptic cultures by reviewing the types of experiments that have used them over the past three decades. These examples serve to highlight the power and elegance of this simple model system, and will hopefully inspire new experiments for the interrogation of synaptic function.

Leucine‐rich repeat kinase 2 phosphorylation on synapsin I regulates glutamate release at pre‐synaptic sites

Leucine-rich repeat kinase 2 (LRRK2) is a large multidomain scaffolding protein with kinase and GTPase activities involved in synaptic vesicle (SV) dynamics. While its role in Parkinson's disease has been largely investigated, little is known about LRRK2 physiological role and until now few proteins have been described as substrates. We have previously demonstrated that LRRK2 through its WD40 domain interacts with synapsin I, an important SV-associated phosphoprotein involved in neuronal development and in the regulation of neurotransmitter release. To test whether synapsin I is substrate for LRRK2 and characterize the properties of its phosphorylation, we used in vitro kinase and binding assays as well as cellular model and site-direct mutagenesis. Using synaptosomes in superfusion, patch-clamp recordings in autaptic WT and synapsin I KO cortical neurons and SypHy assay on primary cortical culture from wild-type and BAC human LRRK2 G2019S mice we characterized the role of LRRK2 kinase activity on glutamate release and SV trafficking. Here we reported that synapsin I is phosphorylated by LRRK2 and demonstrated that the interaction between LRRK2 WD40 domain and synapsin I is crucial for this phosphorylation. Moreover, we showed that LRRK2 phosphorylation of synapsin I at threonine 337 and 339 significantly reduces synapsin I-SV/actin interactions. Using complementary experimental approaches, we demonstrated that LRRK2 controls glutamate release and SV dynamics in a kinase activity and synapsin I-dependent manner. Our findings show that synapsin I is a LRRK2 substrate and describe a novel mechanisms of regulation of glutamate release by LRRK2 kinase activity.

Ephrin-B3 controls excitatory synapse density through cell-cell competition for EphBs

Cortical networks are characterized by sparse connectivity, with synapses found at only a subset of axo-dendritic contacts. Yet within these networks, neurons can exhibit high connection probabilities, suggesting that cell-intrinsic factors, not proximity, determine connectivity. Here, we identify ephrin-B3 (eB3) as a factor that determines synapse density by mediating a cell-cell competition that requires ephrin-B-EphB signaling. In a microisland culture system designed to isolate cell-cell competition, we find that eB3 determines winning and losing neurons in a contest for synapses. In a Mosaic Analysis with Double Markers (MADM) genetic mouse model system in vivo the relative levels of eB3 control spine density in layer 5 and 6 neurons. MADM cortical neurons in vitro reveal that eB3 controls synapse density independently of action potential-driven activity. Our findings illustrate a new class of competitive mechanism mediated by trans-synaptic organizing proteins which control the number of synapses neurons receive relative to neighboring neurons.

Micropatterned substrates coated with neuronal adhesion molecules for high-content study of synapse formation

Studying the roles of different proteins and the mechanisms involved in synaptogenesis is hindered by the complexity and heterogeneity of synapse types, and by the spatial and temporal unpredictability of spontaneous synapse formation. Here we demonstrate a robust and high-content method to induce selectively presynaptic or postsynaptic structures at controlled locations. Neurons are cultured on micropatterned substrates comprising arrays of micron-scale dots coated with various synaptogenic adhesion molecules. When plated on neurexin-1β-coated micropatterns, neurons expressing neuroligin-1 exhibit specific dendritic organization and selective recruitment of the postsynaptic scaffolding molecule PSD-95. Furthermore, functional AMPA receptors are trapped at neurexin-1β dots, as revealed by live-imaging experiments. In contrast, neurons plated on SynCAM1-coated substrates exhibit strongly patterned axons and selectively assemble functional presynapses. N-cadherin coating, however, is not able to elicit synapses, indicating the specificity of our system. This method opens the way to both fundamental and therapeutic studies of various synaptic systems.

A Bcl-xL-Drp1 complex regulates synaptic vesicle membrane dynamics during endocytosis

Following exocytosis, the rate of recovery of neurotransmitter release is determined by vesicle retrieval from the plasma membrane and by recruitment of vesicles from reserve pools within the synapse, the latter of which is dependent on mitochondrial ATP. The Bcl-2 family protein Bcl-x_L, in addition to its role in cell death, regulates neurotransmitter release and recovery in part by increasing ATP availability from mitochondria. We now find, however, that, Bcl-x_L directly regulates endocytotic vesicle retrieval in hippocampal neurons through protein/protein interaction with components of the clathrin complex. Our evidence suggests that, during synaptic stimulation, Bcl-x_L translocates to clathrin-coated pits in a calmodulin-dependent manner and forms a complex of proteins with the GTPase Drp1, Mff and clathrin. Depletion of Drp1 produces misformed endocytotic vesicles. Mutagenesis studies suggest that formation of the Bcl-x_L-Drp1 complex is necessary for the enhanced rate of vesicle endocytosis produced by Bcl-x_L, thus providing a mechanism for presynaptic plasticity.

Single neuron capture and axonal development in three-dimensional microscale hydrogels

Autapse is an unusual type of synapse generated by a neuron on itself. The ability to monitor axonal growth of single neurons and autapse formation in three-dimensions (3D) may provide fundamental information relating to many cellular processes, such as axonal development, synaptic plasticity and neural signal transmission. However, monitoring such growth is technically challenging due to the requirement for precise capture and long-term analysis of single neurons in 3D. Herein, we present a simple two-step photolithography method to efficiently capture single cells in microscale gelatin methacrylate hydrogel rings. We applied this method to capture and culture single neurons. The results demonstrated that neural axons grew and consequently formed axonal circles, indicating that our method could be an enabling tool to analyze axonal development and autapse formation. This method holds great potential for impact in multiple areas, such as neuroscience, cancer biology, and stem cell biology.

A high-throughput method for generating uniform microislands for autaptic neuronal cultures

Generating microislands of culture substrate on coverslips by spray application of poly-d lysine is a commonly used method for culturing isolated neurons that form self (autaptic) synapses. This preparation has multiple advantages for studying synaptic transmission in isolation; however, generating microislands by spraying produces islands of non-uniform size and thus cultures vary widely in the number of islands containing single neurons. To address these problems, we developed a high-throughput method for reliably generating uniformly shaped microislands of culture substrate. Stamp molds formed of poly(dimethylsiloxane) (PDMS) were fabricated with arrays of circles and used to generate stamps made of 9.2% agarose. The agarose stamps were capable of loading sufficient poly D-lysine and collagen dissolved in acetic acid to rapidly generate coverslips containing at least 64 microislands per coverslip. When hippocampal neurons were cultured on these coverslips, there were significantly more single-neuron islands per coverslip. We noted that single neurons tended to form one of three distinct neurite-arbor morphologies, which varied with island size and the location of the cell body on the island. To our surprise, the number of synapses per autaptic neuron did not correlate with arbor shape or island size, suggesting that other factors regulate the number of synapses formed by isolated neurons. The stamping method we report can be used to increase the number of single-neuron islands per culture and aid in the rapid visualization of microislands.

Inhibited cell spreading on polystyrene nanopillars fabricated by nanoimprinting and in situ elongation

Polymer nanopillars (40-80 nm in diameter and 100 nm in pitch) were fabricated at high density over large areas directly on bulk tissue culture polystyrene plates using nanoimprint lithography. Nanoporous Si molds for imprinting were generated by transfer from an anodic alumina membrane. Ultrahigh aspect ratio polymer nanopillars were formed in a novel procedure using controlled elongation of the imprinted pillars during mold release. The resulting nanopillar arrays show significant changes in surface wettability upon brief O(2) plasma treatment. Human dermal fibroblasts were cultured on the nanopillar surfaces in order to study cell-substrate interaction at the nanoscale. The nanopillar topography shows strong effects on the cell morphology, with pillars of widely varying aspect ratios and surface energies resisting cell spreading. This effect on cell behavior can be rationalized in terms of the cells' requirement to form micron-scale focal adhesions. The study indicates that at the nanoscale, physical factors can supersede the effects of chemical factors on the cell-substratum interaction.