Here we present a protocol that is based on spinal cord slices cultured on multi-electrode arrays to study functional regeneration of propriospinal connections in vitro.
Adult higher vertebrates have a limited potential to recover from spinal cord injury. Recently, evidence emerged that propriospinal connections are a promising target for intervention to improve functional regeneration. So far, no in vitro model exists that grants the possibility to examine functional recovery of propriospinal fibers. Therefore, a representative model that is based on two organotypic spinal cord sections of embryonic rat, cultured next to each other on multi-electrode arrays (MEAs) was developed. These slices grow and, within a few days in vitro, fuse along the sides facing each other. The design of the used MEAs permits the performance of lesions with a scalpel blade through this fusion site without inflicting damage on the MEAs. The slices show spontaneous activity, usually organized in network activity bursts, and spatial and temporal activity parameters such as the location of burst origins, speed and direction of their propagation and latencies between bursts can be characterized. Using these features, it is also possible to assess functional connection of the slices by calculating the amount of synchronized bursts between the two sides. Furthermore, the slices can be morphologically analyzed by performing immunohistochemical stainings after the recordings.
Several advantages of the used techniques are combined in this model: the slices largely preserve the original tissue architecture with intact local synaptic circuitry, the tissue is easily and repeatedly accessible and neuronal activity can be detected simultaneously and non-invasively in a large number of spots at high temporal resolution. These features allow the investigation of functional regeneration of intraspinal connections in isolation in vitro in a sophisticated and efficient way.
Organotypic slice cultures of the central nervous system (CNS) have been used extensively to study various physiological and pathological phenomena reaching from neuronal development to neurodegeneration. Compared to dissociated cell cultures, organotypic slices offer the advantage of more complexity with an intact cytoarchitecture and local synaptic circuitry. At the same time, the tissue can be analyzed in an isolated way without the need to implicate the intricacy of the whole in vivo context. Moreover, easy and repeated access to the cells of choice is warranted, the extracellular environment can be precisely controlled and usually, in vitro models are less costly than their in vivo counterparts. In recent years, various studies presented striking similarities between the developmental profiles of long term cultures versus the corresponding living animals 1,2. It has been shown that neuronal circuits of various CNS regions express spontaneous network activity during development and that organotypic slices partially maintain this phenomenon 3–7. Isolated spinal cord preparations have been particularly used to investigate rhythm generation 6 and the formation of neuronal circuits 1.
A way to record the spontaneous activity of organotypic slices is to culture them on top of multi-electrode arrays (MEAs). These devices contain multiple electrodes that can monitor the extracellular potentials generated by action potentials from numerous cells simultaneously (multi-unit recording). The high temporal resolution of the MEA recordings can be used to reconstruct the precise activity dynamics within a given neuronal circuit. Additionally, in contrast to patch-clamping the technique is non-invasive, which permits long-term measurements and results in the fact that neurons are not affected in their behavior.
For the purpose of this study, the combination of organotypic spinal cord co-cultures and multisite recordings was used to investigate functional regeneration within the spinal cord. Since adult higher vertebrates have limited potential to recover from damage of the spinal cord, different strategies have been studied to promote regeneration after spinal cord injury. So far, the corticospinal tract has usually been the model system of choice for such investigations. These experiments are typically time-consuming, costly and require large animal cohorts due to high variability within single groups. Moreover, evidence suggests that propriospinal connections (neurons that are entirely confined within the spinal cord) can play a pivotal role in the recovery process following spinal cord lesions 8. These fibers are difficult to study in vivo without the interference of ascending and descending fiber tracts. Bonnici and Kapfhammer 9 used longitudinal spinal cord slice cultures of postnatal mice as an alternative approach. After performing mechanical lesions they analyzed semi-quantitatively the morphological regeneration of axons between spinal cord segments. They observed less but not none axons crossing the lesion site in cultures cut at a mature age. In contrast, cultures that were lesioned at a younger stage displayed a high amount of axonal regeneration 5 – 7 days after the damage. However, proof for functional connections was not presented.
Therefore, the development of a representative in vitro model of propriospinal fibers that allows the investigation of functional regeneration can be a valuable tool to extend our knowledge about regenerative processes in the spinal cord.
Animal care was in accordance with guidelines approved by Swiss local authorities (Amt für Landwirtschaft und Natur des Kantons Bern, Veterinärdienst, Sekretariat Tierversuche, approval Nrs. 52/11 and 35/14). These guidelines are in agreement with the European Community Directive 2010/63/EU.
Note: Work in a laminar flow hood with sterile instruments and solutions for all steps 1 – 5 including sub-steps.
1. Preparation of MEAs
Note: MEAs are composed of a glass substrate, micro-fabricated metal electrodes and a SU-8 polymer insulation layer (also see Tscherter et al. 3). For the purpose of this study, commercially available MEAs were ordered with a customized electrode array layout (Figure 1 A&B). The 68 electrodes are arranged in a rectangular grid that is split into two zones by a 300 µm wide groove free of electrodes, electrical leads and insulation. Each electrode is 40 x 40 µm in size and they are spaced apart by 200 µm from center to center. Four large ground electrodes are positioned around the recording site. They differ from other commercially available standard MEAs in their size (21 mm x 21 mm) and they have no fixed recording chamber.
2. Ingredients Required for the Preparation and Growth of Organotypic Cultures
3. Spinal Cord Tissue Dissection
Note: Procedure yields, depending on the number of embryos, spinal cord slices for about 25 – 35 co-cultures and is prepared inside a laminar flow hood under sterile conditions.
4. Mounting Spinal Cord Tissue Slices on MEAs
5. Mechanical Lesions
6. Electrophysiological Recordings of Spontaneous Activity
7. Data Analysis
Note: For the detection of the extracellularly recorded action potentials use for each electrode a detector based on standard deviations and a subsequent discriminator. This procedure is described in detail in Tscherter et al. 3.
8. Fixation of the Cultures and Immunohistochemistry
To investigate the potential for functional recovery of the co-cultures derived from the spinal cord of E14 rat embryos, lesions were performed in a time window of 8 to 28 DIV. 2 to 3 weeks later, the spontaneous neuronal activity was recorded with the MEAs (Figure 1 C&D). The extracellularly recorded action potentials were identified offline for each individual electrode (Figure 1E). From these data raster plots were generated (Figure 1F), followed by network activity plots to visualize the total activity separately per side (Figure 1G). In each slice the spontaneous activity is usually organized in bursts. If the slices are functionally connected, these bursts can propagate from one slice to the other. Therefore, the amount of synchronized bursts between the two slices was calculated to quantitatively measure functional regeneration. For a detailed description of how the transformation into the different plots is performed and how the formula for calculating the percentage of synchronized activity was derived, please see Heidemann et al. 10.
In all experiments, the activity was first recorded under standard conditions. In a second step, synaptic inhibition was blocked by adding strychnine (1 µM) and gabazine (10 µM) to the extracellular bath solution. This disinhibition leads usually to a more regular activity pattern with prolonged bursts and burst intervals, which facilitates the definition of bursts and thus the determination of burst synchronization between the slices.
Cultures lesioned at a young age (7 – 9 DIV) displayed a high amount of synchronized activity 2-3 weeks after lesion whereas cultures lesioned at later stages (>19 DIV) showed a distinct reduction in the ability to regenerate (Figure 2 A – F). These findings indicate that the regeneration ability of functional connections in spinal cord slice cultures decreases from a high level, representing the embryonic state in vivo, to a low level, representing the further developed state in vivo, within three weeks in culture.
What kinds of connections are actually involved in burst synchronization between the two slices? Besides propriospinal connections, the fibers could also arise from motoneurons or dorsal root ganglion neurons. It has been shown previously that dorsal root ganglion neurons are not relevant for the functional connectivity between the spinal cord slices 10. However, the involvement of motoneurons remained a possibility. Since motoneurons are known to form cholinergic connections to spinal cord neurons through nicotinic receptors 13, the activity of spinal cord co-cultures was recorded at 8 DIV, a time when the cultures show highly synchronized activity 10, and the nicotinic antagonist Mecamylamine (MEC, 100 µM) was added to the extracellular bath solution. A participation of motoneurons in the connection between the two slices would result in a decreased percentage of synchronized activity. However, this was not the case (MEC: 91.0 ± 4.5%, n = 7; control: 93.6 ± 4.6%, n = 7, p >0.05, Wilcoxon matched pairs test). These results suggest that cholinergic synapses do not contribute to the functional connection between slices. However, since motoneurons have also been shown to release glutamate at synapses within the CNS 11, 15, these experiments do not entirely exclude their involvement.
To identify motoneurons immunohistochemical stainings against the non-phosphorylated neurofilament H with SMI-32 were performed. They revealed several labelled large cell bodies typical for motoneurons, distributed over the slices (Figure 3A). The SMI-32 antibody has been reported to label the cell bodies of motoneurons 12 and this finding holds also true for the used cultures 13. Also, stainings of neuronal cell bodies in general, e.g., against b-III-tubulin or NeuN as well as glial cell bodies, e.g., astrocytes with GFAP antibody are in line with other reports and match the morphological descriptions of these cell types (Figure 3 B – D). Nevertheless, the labelling of axons with the SMI-32 antibody is concerning (Figure 3E). Against expectations, a huge network of fibers appears when using this antibody and it looks similar to the SMI-31 staining that labels phosphorylated neurofilament H (Figure 3F). One interpretation of this result could be that the developmental regulation of neurofilament phosphorylation/dephosphorylation is not as tight in the used cultures compared to the in vivo situation. A mixed occurrence of phosphorylated and non-phosphorylated neurofilaments in the same axon could explain this finding. Another possibility is that the SMI-32 labelled axons arise from projection neurons. Tsang and colleagues 16 have shown that e.g., Clark’s column and intermediolateral cell column neurons are stained by SMI-32 besides motoneurons. Pronounced neurite proliferation of such neurons could also explain the abundance of SMI-32 positive fibers.
Figure 1. Display and Analysis of Spontaneous Activity. (A) Diagram of a MEA. The platinum covered electrodes are depicted in black, the transparent wires in red and the groove in the middle of the MEA in yellow. (B) Close-up of the electrode array located in the center of the MEA. The diagrams are kindly provided by Dr. M. Heuschkel. (C) Bright-field image of an 8 DIV old culture. The slices have grown and fused along the sides facing each other. The yellow bar represents the electrode- and insulation-free groove of the MEA. Scale bar = 400 µm (D) Timeline of experiments. Two spinal cord slices of E14 rat embryos are placed next to each other on MEAs. Within a few days, the slices grow and fuse along the sides facing each other. In a time frame of 8 – 28 DIV, complete lesions are performed through the fusion site. Two to three weeks later the spontaneous activity is recorded and the cultures are fixed for immunohistochemical stainings. (E) Spontaneous activity traces of each individual electrode of a 23 DIV old culture. For clearer visualization, only every second trace is illustrated. Orange traces depict activity that has been recorded from the slice on the right side, blue traces from the left side. Most of the bursts are synchronized between the two. The arrow points to a burst occurring in the left slice that only partially propagated to the right slice. The activity in the right slice however did not reach the chosen threshold of at least 25% of the averaged maximal peak activity of the according side and therefore, is not detected as a burst. Magnifications on the right depict the last synchronized burst pair. (F) Raster plot of the activity shown in (E). (G) Network activity plot with defined bursts (bars below baseline) of the activity shown in (E). Adapted from Heidemann et al. 10. Please click here to view a larger version of this figure.
Figure 2. Synchronized versus Not Synchronized Activity. (A, B) Raster plots of cultures lesioned at a young age (at 7 – 9 DIV), showing synchronized activity under standard conditions (A) and under disinhibition (B). (C, D) Raster plots of cultures lesioned at an old age (at >19 DIV), showing not synchronized activity under standard conditions (C) and under disinhibition (D). (E, F) Average percentage of synchronized activity plotted for each individual lesion day recorded under standard conditions (E) and under disinhibition (F). Recordings were taken 2 – 3 weeks after the day of lesion. Adapted from Heidemann et al. 10. Please click here to view a larger version of this figure.
Figure 3. Immunohistochemical Characterization. (A) Staining of motoneuronal cell bodies with SMI-32. (B) β-III tubulin labeling of mature neuronal cell bodies and their processes. (C) Identification of astrocytes with GFAP. (D) Labeling of mature neuronal cell body with NeuN. (E) SMI-32 staining of non-phosphorylated neurofilament H. (F) Phosphorylated neurofilament H identified by SMI-31. Adapted from Heidemann et al. 10. Note that with both, SMI-31 and SMI-32, a large network of fibers is visible in the cultures. Bars A-D = 20 µm, H&F = 100 µm. Please click here to view a larger version of this figure.
Several elements in the presented procedure are fundamental for the accomplishment of accurate and reproducible experiments. All steps during the preparation of the MEAs, solutions and the production of the slice cultures are performed under sterile conditions in a laminar flow hood. Antibiotics are not applied to the nutrient medium because they can affect the spontaneous activity 14. During the MEA coating, the most important part is to make sure that the extracellular matrix gel solution stays cold at all times. Thaw it in the fridge and keep it on ice for the whole procedure to ensure a maximal temperature close to 0 °C. During the preparation of the slices quick isolation and sectioning, as well as paying attention to a low temperature by using ice cold dissection buffers increases the percentage of healthy cultures. Moreover, the plasma/thrombin coagulation is one of the most critical steps. First, make sure that the plasma is properly mixed with the thrombin. Second, pay special attention to removing as much residue as possible to ensure proper attachment of the slices to the MEAs. This step is extremely time-sensitive; if the time frame is too short, too much of the plasma/thrombin mixture is removed. If the time frame is too long, coagulation already happened, increasing the risk of removing the slices along with the residual plasma/thrombin.
If mechanical lesions are planned, it is important to use MEAs with an accurate design. The area where the lesion is intended needs to be free of electrodes, wires and insulation. Otherwise, the electrics of the MEA are going to be damaged and therefore, no recordings can be performed anymore.
Another crucial step concerns the size of the lesion. It has been shown previously that reconnecting two young slices after lesion takes about two days 10. For all experiments, the rule of thumb was used that the space between the slices after lesion should not be bigger than the width of the MEA groove (300 µm). A bigger lesion size might result in a longer time frame for reconnection or, if the lesion is too big, no reconnection at all. To annotate, former experiments revealed that an initial placing of the two slices further away than 1 mm results in a very low connection rate.
Before the start of the recordings, an adjustment time of at least 10 min to RT, instead of 37 °C of the incubator, and to the extracellular solution, instead of the nutrition medium is recommended. Close observation of the activity pattern during these initial minutes ensures that the measured data represent the bursting pattern of the culture appropriately after the recording was started.
For the detection of the activity, data acquisition and further processing, homemade hardware (recording chamber, connections to the amplifiers and amplifiers), programs in LabVIEW, C++ and IgorPro are used. Commercial MEA setups and other software packages are suited as well for these operations. Here, the signals seen by the electrodes are amplified 1001 times, and then digitized at a rate of 6 kHz.
The presented model can be a helpful tool to examine propriospinal connections because in vivo, they are difficult to study without the interference of ascending and descending fibre tracts. The co-culturing of two spinal cord slices can elegantly circumvent this issue. The cultures provide a microenvironment that can be tightly controlled and easily manipulated. On the other hand, supraspinal and sensory input is of course missing. Additionally, the process of the culture preparation represents a situation of CNS damage. Glial cells like astrocytes and microglia are known to respond to lesions with increased proliferation and therefore, the tissue is to a certain extent reorganized. Relating to the presented model the formation of a glial scar after performing mechanical lesions was not observed. One explanation could be that adaptive mechanisms of astrocytes were already triggered during the preparation process and that lesions of the cultures did not result in a further response. Also, there is no evidence for the involvement of myelin associated inhibitors, e.g., Nogo-A, in the low regeneration potential of cultures lesioned at an old age. However, it was shown that treatment with the phosphodiesterase 4 inhibitor Rolipram, starting directly after performing lesions at 23 DIV increases functional regeneration between the slices 10. These results demonstrate that functional recovery can be increased in old cultures. Noticeably, when counting the number of axons crossing the lesion site, no difference between cultures lesioned at a young age and cultures lesioned at an old age was discovered. These findings suggest that lack of axonal regeneration is not the major reason for the loss of recovery after late lesions in culture and therefore emphasize the need for a model that allows functional analysis of regeneration. Synaptic formation and synaptic plasticity could play a major role in the recovery process 10. These mechanisms gained a lot of attention during recent years and it is nowadays widely accepted that they have a major impact on the outcome after spinal cord injury. Therefore, a model that provides stable conditions to investigate these processes in isolation and in great detail can certainly help to obtain insight about functional regeneration in the spinal cord.
The authors have nothing to disclose.
This study was supported by the Swiss National Foundation (n° 3100AO-120327 and 31003A_140754/1)
Planar multielectrode array | Qwane Biosciences | custom-made according to the design of our lab | |
Nutrient medium | For 100 ml | ||
Dulbeccos modified Eagle's medium | Gibco | 31966-021 | 80 ml |
Horse serum | Gibco | 26050-070 | 10 ml |
distilled, sterile water | 10 ml | ||
Nerve growth factor-7S [5ng/mL] | Sigma-Aldrich | N0513 | 200 µl |
reconstituted in wash solution with 1% BSA | |||
Coating solution | |||
Extracellular matrix gel | BD Biosciences | 356230 | Must stay cold at all times; dilute 1:50 with medium optimized for prenatal and embryonic neurons |
medium optimized for prenatal and embryonic neurons | Gibco | 21103-049 | |
Wash solution | [mg/L] | ||
MgCl2-6H2O | 100 | ||
MgSO4-7H2O | 100 | ||
KCl | 400 | ||
KH2PO4 | 60 | ||
NaCl | 8000 | ||
Na2HPO4 anhydrous | 48 | ||
D-Glucose (Dextrose) | 1000 | ||
According to the protocol on http://www.lifetechnologies.com/ch/en/home/technical-resources/media-formulation.152.html | |||
Extracellular solution (pH 7.4) | [mM] | ||
NaCl | 145 | ||
KCl | 4 | ||
MgCl2 | 1 | ||
CaCl2 | 2 | ||
HEPES | 5 | ||
Na-pyruvate | 2 | ||
Glucose | 5 | ||
Blocking Solution | |||
Detergent: Triton X-100 | 0.3%, harmful if swallowed | ||
Bovine serum albumin | Sigma-Aldrich | A9418 | 1% |
Goat serum | Gibco | 16210-064 | 5% |
Chicken Plasma | Sigma-Aldrich | P2366 | Lyophilized, reconstitute with wash solution |
Gabazine | Sigma-Aldrich | SR-95531 | |
Mecamylamine hydrochloride | Tocris | 2843 | toxic if swallowed |
Micropipette | World Precision Instruments, Inc. | MF28G-5 | |
Paraformaldehyde | harmful, 4% | ||
SMI-31 | Covance | SMI-31P | |
SMI-32 | Covance | SMI-32R-500 | |
Strychnine | Sigma-Aldrich | S0532 | toxic |
Thrombin | Merck Millipore | 1123740001 | irritant, sensitizing, reconstitute with wash solution |
Tissue culture flat tube 10 (= roller tube) | Techno Plastic Products AG | 91243 |