Here, we present a protocol for simple isolation of specific groups of live neuronal cells expressing green fluorescent protein from transgenic Caenorhabditis elegans lines. This method enables a variety of ex vivo studies focused on specific neurons and has the capacity to isolate cells for further short-term culturing.
During the aging process, many cells accumulate high levels of damage leading to cellular dysfunction, which underlies many geriatric and pathological conditions. Post-mitotic neurons represent a major cell type affected by aging. Although multiple mammalian models of neuronal aging exist, they are challenging and expensive to establish. The roundworm Caenorhabditis elegans is a powerful model to study neuronal aging, as these animals have short lifespan, an available robust genetic toolbox, and well-cataloged nervous system. The method presented herein allows for seamless isolation of specific cells based on the expression of a transgenic green fluorescent protein (GFP). Transgenic animal lines expressing GFP under distinct, cell type-specific promoters are digested to remove the outer cuticle and gently mechanically disrupted to produce slurry containing various cell types. The cells of interest are then separated from non-target cells through fluorescence-activated cell sorting, or by anti-GFP-coupled magnetic beads. The isolated cells can then be cultured for a limited time or immediately used for cell-specific ex vivo analyses such as transcriptional analysis by real time quantitative PCR. Thus, this protocol allows for rapid and robust analysis of cell-specific responses within different neuronal populations in C. elegans.
Over the past several decades, the metazoan model organism Caenorhabditis elegans has been a tremendous asset in the study of neurons, neuronal circuitry and its role in physiological and behavioral responses, and aging-associated neurodegenerative diseases. A unique feature of C. elegans is that the animals are transparent, allowing for the lineage of all adult somatic cells to be mapped1. C. elegans also harbors manageable quantity of neurons, leading to the morphology and connectivity of the nervous system being well understood2. The invariant cell lineage, short lifespan, and abundance of high-throughput genetic tools available for C. elegans make it an ideal model organism for the study of aging within different neuronal populations.
Aging of neurons and neurodegenerative disorders are complex, and each disorder has unique pathological signatures associated with it. However, for many of these disorders, such as Parkinson’s and Alzheimer’s disease, a common feature is the progressive load of misfolded proteins3,4,5. In both of these diseases, proteins misfold and aggregate within the cell to cause toxicity, ultimately leading to cell death4,5. For proper aging of neurons, the homeostasis of mitochondria, more specifically protein homeostasis, is important, as perturbations and dyshomeostasis can lead to neurodegeneration6,7,8. Cells are equipped with various mechanisms to maintain protein homeostasis, one being the unfolded protein response (UPR)―a classical pathway where signals from endoplasmic reticulum (ER) activate intracellular signal cascades, leading to a transcriptional response9. Akin to this UPRER, metazoans display a similar response to loss of mitochondrial protein homeostasis, the so-called mitochondrial UPR (UPRmt)7,8. C. elegans appears to be the most basal organism to have a confirmed and well-defined UPRmt pathway7.
The function/dysfunction of specific neuronal populations within a larger network can be difficult to assess due to their intrinsic connectivity and complexity3. However, there is often a need to study distinct neuronal populations due to cell type-specific diseases with complex pathologies, such as those associated with impaired cellular protein homeostasis. In C. elegans, specific neuronal populations can be genetically manipulated allowing for facile observation in vivo. The nervous system of C. elegans primarily consists of neurons, with a small percentage of glial cells. In adult hermaphrodite worms, there are approximately 302 neurons, subdivided into over 100 different classes2,10. Neurons such as cholinergic neurons predominate in neuromuscular junctions, while dopaminergic neurons are primarily involved in sensation10,11. As both motor activity and sensory abilities decline with age, it is important to detail mechanistic insights about defects in these individual neurons11,12. As such, there is a need for a simple and robust method to isolate intact cells of interest for subsequent ex vivo studies.
Here, we describe an optimized effective method for the rapid isolation of specific neuronal cells expressing green fluorescent protein (GFP) from C. elegans. This isolation method can be performed on larval, juvenile or adult worms. Isolation of cells from larval worms was previously published by Zhang et al.13 and will not be discussed here. An important note here is that all worms need to be at the same life stage to prevent over-digestion of the animal or contamination from animals in different life stages. Through both enzymatic and mechanical disruption of the nematode cuticle, an exoskeleton high in collagen and other structural proteins, a wide variety of cells can be isolated13,14. Cells can then be isolated via flow cytometry or antibody labeled magnetic beads. Typically, RNA is isolated via a phenol and guanidine isothiocyanate method to ensure the enrichment of desired cell population15. With much care these isolated cells can be maintained in a culture flask or multi-well dish. This method represents a unique and powerful tool in the study of specific neurons and has the capacity to isolate live and functional cells for further culturing.
1. Preparation and collection of aged worms for cell isolation
NOTE: Explained below is the isolation of cholinergic neurons from the transgenic unc-17::GFP strain (OH13083) obtained from the Caenorhabditis Genetics Center (CGC) strain repository at the University of Minnesota. It is imperative to maintain sterile conditions to prevent contamination from fungi or bacteria.
2. Isolation of GFP-positive cells via flow cytometry or anti-GFP magnetic beads
NOTE: There are two methods that can be deployed to isolate specific cell types. The first method is to use fluorescence-activated cell sorting (FACS), while the second method uses antibody-conjugated magnetic beads to pool cells expressing distinct plasma membrane-localized proteins. The latter method requires the localization of GFP to the plasma membrane, thus being available for interaction with the antibody-coupled magnetic bead.
3. Fluorescent imaging of isolated GFP-positive cells
4. Extraction of RNA from isolated cells
NOTE: Extraction of RNA should be performed immediately after cell isolation to ensure high quality of material. Isolated RNA can be used to produce cDNA and subsequent measurement of transcript levels by quantitative PCR (qPCR). All tubes, tips, and reagents should be RNase-free, and workspace should be ethanol-washed prior to RNA extraction.
The protocol described here allows for specific isolation of unc-17::GFP-positive cholinergic neurons from the roundworm C. elegans for subsequent ex vivo studies such as cell type-specific gene expression profiling and eventual short-term culturing for patch-clamp electrophysiology measurements.
Figure 1 shows unc-17::GFP-positive cholinergic neurons in their normal setting. The unc-17 gene is expressed throughout the C. elegans body and codes for a vesicular acetylcholine transmembrane transporter17. The expression of this gene can be observed in head, midbody, and tail neurons and neuron projections.
Figure 2A shows a pictorial overview of the neuron isolation procedure outlined in the methods section. Step one is to synchronize and culture the worms. Additionally, Figure 2B shows representative fluorescence micrographs of unc-17::GFP-positive neurons after isolation as well as respective negative control from unmodified N2 wild type animals. Isolated cells can stay alive up to 3 days, when supplemented with Leibovitz's L-15 medium and 10% FBS.
Figure 3 shows representative data of qPCR from GFP expressing cholinergic neurons as well as GFP-expressing muscle cells. After successful sorting of cells by either method, RNA was isolated via a phenol and guanidine isothiocyanate method after which cDNA was produced using a synthesis kit. Expression of cholinergic neuron-specific genes, tph-1 and snb-1, a tryptophan hydroxylase and synaptic vesicle transporter, respectively, is elevated in unc-17::GFP isolated cells but is not present in myo-3::GFP isolated cells. The inverse is true with the expression of the muscle-specific myosin heavy chain transcript of myo-2. Expression of the latter gene is limited to isolated muscle cells but not the isolated cholinergic cells.
Figure 1: Reconstructed image of cholinergic neurons in one-day-old wild type transgenic animals expressing unc-17::GFP reporter. The image was assembled from four separate confocal fluorescence images of the same worm, covering the length of the entire animal. The scale bar of each individual image is 50 μm. Please click here to view a larger version of this figure.
Figure 2: Workflow of cell isolation and representative GFP-expressing cells isolated from unc-17::GFP strain. (A) Schematic workflow depiction for the isolation of specific cell populations enriched in unc-17::GFP-expressing cholinergic neurons from respective C. elegans transgenic strains. (B) Representative fluorescence images showing the presence of GFP signal in the isolated unc-17::GFP-positive cells. Note that the image shows a single focal plane. Scale bar = 10 μm. Figure 2B is reproduced with permission from the Journal of Cell Science and has been published by Germany et al.12. Please click here to view a larger version of this figure.
Figure 3: Representative qPCR analysis of unc-17-neuron specific markers (tph-1, unc-47 and snb-1) in whole-animal versus unc-17::GFP reporter cell isolates confirming specific enrichment of cholinergic neurons of interest. Additional negative controls used here were myo-3::GFP reporter-expressing muscle cells, which had been isolated following the same protocol. Data show mean values ± SD (n = 3 biological replicates). *p < 0.05; **p < 0.01; ***p < 0.001 by paired t test. This figure has been modified from Germany et al.12. Please click here to view a larger version of this figure.
The roundworm C. elegans is a well-established and powerful model to study neuronal health and disease2. With ample genetic tools to manipulate these animals and manageable quantity of precisely mapped various neuron types, a great deal of data can be collected with a relatively small quantity of material. Here, we outline an optimized method to isolate distinct neurons from whole animals. By disrupting the worm’s outer cuticle proteins, a slurry of various cell types can be isolated, including neurons and muscular cells5. These isolated cells can be used in a multitude of unique experiments. The ability to use either cell sorting through flow cytometry or antibody-labeled magnetic beads allows for an adaptive technique dependent of the parameters of specific experiment. Extracting RNA from a specific C. elegans cell type also allows for a more thorough examination and a mechanistic understanding of various cellular processes.
There are critical steps discussed here which must be followed closely: 1) aging and harvesting well-fed worms from 25 µM FuDR plates; 2) time in SDS-DTT incubation or treatment with protease mixture; 3) selection of GFP-positive cells. First, during aging of the worms, it is imperative to be diligent to ensure that any eggs laid do not hatch, as this will cause age heterogeneity in the collected worms, causing significant variability in data. The wash steps when harvesting worms for isolation are also necessary to ensure that any dead or immature animals are not collected. Second, excessive incubation of worms in the SDS-DTT buffer can cause undesired breakdown of important cellular components and toxicity to the worms, preventing isolation of target cell types. Maintaining cell viability is essential for successful RNA extraction from or further short-term culturing of isolated cells. Because the majority of neuronal cells are delicate, overly harsh treatment during either the SDS-DTT buffer or the protease mixture treatment can result in massive damage to these cells. If a stress response pathway is the target of the experiment, conditions of incubations may be slightly altered as the highly reductive power of DTT may cause enhanced expression of certain stress response genes. In this case, it is important to successfully wash cell pellets with L-15-supplemented medium to halt the reaction and quickly re-supply the cells with nutrients. Finally, carefully choosing which selection step is most appropriate to the cell type of interest will allow for maximum recovery of these cells. Likewise, considering cellular localization of cell type-specific markers may help researchers choose the most optimal cell enrichment approach. Using flow cytometry will yield a more homogenous and viable cell culture, whereas using anti-GFP magnetic beads is less time consuming and labor intensive.
It is important to note that the method described here does have some limitations. First, robust expression of GFP in desired cell types should be verified to ensure suitable isolation by FACS or antibody-coupled magnetic beads. Second, even gentle cuticle disruption can lead to a poor yield or even loss of certain body wall cells, or other low-abundance cell populations such as dat-1 dopaminergic neurons. When choosing cell types, the more abundant cell type can usually provide a more successful yield. Nonetheless, this method can be used for the isolation of less abundant cell type populations as well.
In summary, the procedure discussed in this article presents an effective method of extraction and isolation of individual cell types from the roundworm C. elegans. The method is robust enough to allow for the extraction of RNA, and subsequent gene profiling, and sensitive enough to allow for cultivation of the isolated cells. Although the exact methodology utilized to isolate specific groups of cells may vary between different cell types, the workflow described in this report is generally effective and can be applied to virtually all kinds of worm cells.
The authors have nothing to disclose.
We thank Dr. Jennifer Fox and the members of the Khalimonchuk laboratory for insightful comments. We acknowledge the support from the National Institutes of Health (R01 GM108975 to O.K. and T32 GM107001-01A1 to E.M.G.).
6-well plate | Fisher Scientific | 12-556-004 | |
Agar, Molecular Biology Grade | VWR | A0930 | |
CaCl2 | Sigma | C5670 | |
Chloroform | Sigma | 496189 | |
Contess Automated Cell Counter | Invitrogen | Z359629 | |
DTT | USBiological | D8070 | |
Ethanol | Decon Labs | 2701 | |
FBS | Omega Scientific | FB-02 | |
Fluorodeoxyuridine | Sigma | F0503 | |
HEPES | Sigma | H3375 | |
Isopropanol | VWR | BDH1133 | |
KCl | Amresco | O395 | |
KH2PO4 | USBiological | P5110 | |
Kimwipes | Kimberly-Clark Professionals | 7552 | |
Leibovitz's L-15 Medium | Gibco | 21083027 | |
MgCl2 | Sigma | M8266 | |
MgSO4 | USBiological | M2090 | |
Na2HPO4·7H2O | USBiological | S5199 | |
NaCl | VWR | X190 | |
NaOCl (Bleach) | Clorox | ||
NaOH | Amresco | O583 | |
Penicillin-Streptomicen | Fisher Scientific | 15140122 | |
Peptone Y | USBiological | P3306 | |
Pronase E | Sigma | 7433 | protease mixture from Streptomyces griseus |
SDS | Amresco | O227 | |
SMT1-FLQC fluorescence stereomicroscope | Tritech Research | ||
Sucrose | USBiological | S8010 | |
SuperScript IV One-Step synthesis kit | ThermoFisher | 12594025 | |
TRIzol | Invitrogen | 15596026 | phenol and guanidine isothiocyanate solution |
Trypan Blue Stain | Invitrogen | T10Z82 | |
α-GFP magnetic beads | MBL | D153-11 |