Here we present a simple and efficient method to isolate live meiotic and post-meiotic germ cells from adult mouse testes. Using a low-cytotoxicity, violet-excited DNA binding dye and fluorescence-activated cell sorting, one can isolate highly enriched spermatogenic cell populations for many downstream applications.
Isolation of meiotic spermatocytes is essential to investigate molecular mechanisms underlying meiosis and spermatogenesis. Although there are established cell isolation protocols using Hoechst 33342 staining in combination with fluorescence-activated cell sorting, it requires cell sorters equipped with an ultraviolet laser. Here we describe a cell isolation protocol using the DyeCycle Violet (DCV) stain, a low cytotoxicity DNA binding dye structurally similar to Hoechst 33342. DCV can be excited by both ultraviolet and violet lasers, which improves the flexibility of equipment choice, including a cell sorter not equipped with an ultraviolet laser. Using this protocol, one can isolate three live-cell subpopulations in meiotic prophase I, including leptotene/zygotene, pachytene, and diplotene spermatocytes, as well as post-meiotic round spermatids. We also describe a protocol to prepare single-cell suspension from mouse testes. Overall, the procedure requires a short time to complete (4-5 hours depending on the number of needed cells), which facilitates many downstream applications.
Spermatogenesis is a complex process wherein a small population of spermatogonial stem cells sustain continuous production of a large number of sperm throughout adult life1,2. During spermatogenesis, dynamic chromatin remodeling takes place when spermatogenic cells undergo meiosis to produce haploid spermatids3,4,5. Isolation of meiotic spermatocytes is essential for molecular investigation, and several different approaches to isolate meiotic spermatocytes have been established, including sedimentation-based separation6,7 and fluorescence-activated cell sorting (FACS)8,9,10,11,12,13,14,15,16,17. However, these methods have technical limitations. While sedimentation-based separation yields a large number of cells5,6,7, it is labor intensive. The established FACS-based method uses Hoechst 33342 (Ho342) to separate meiotic spermatocytes based on DNA content and light scattering properties and requires FACS cell sorters equipped with an ultraviolet (UV) laser8,9,10,11. Alternative FACS-based methods require transgenic mouse lines that express florescent proteins, synchronization of spermatogenesis12, or cell fixation and antibody labeling that is not compatible with isolation of live cells13. While there is another alternative method using a cell-permeable DNA binding dye, DyeCycle Green stain14,15,16,17, this method is recommended for the isolation of spermatogenic cells from juvenile testis. Therefore, there is a critical need to develop a simple and robust isolation method for live meiotic spermatocytes that can be applied to any mouse strain of any age and that can be performed using any FACS cell sorter.
Here we describe such a long-sought cell isolation protocol using the DyeCycle Violet (DCV) stain. DCV is a low cytotoxicity, cell-permeable DNA binding dye structurally similar to Ho342 but with an excitation spectrum shifted toward the violet range18. In addition, DCV has a broader emission spectrum compared to DCG. Thus, it can be excited by both UV and violet lasers, which improves the flexibility of equipment, allowing the use of an FACS cell sorter not equipped with a UV laser. The DCV protocol presented here uses two-dimensional separation with DCV blue and DCV red, mimicking the advantage of the Ho342 protocol. With this advantage, our DCV protocol allows us to isolate highly enriched germ cells from the adult testis. We provide a detailed gating protocol to isolate live spermatogenic cells from adult mouse testes of one mouse (from two testes). We also describe an efficient and quick protocol to prepare single-cell suspension from mouse testes that can be used for this cell isolation. The procedure requires a short time to complete (preparation of single cell suspension – 1 hour, dye staining – 30 min, and cell sorting – 2-3 hours: total – 4-5 hours depending on the number of needed cells; Figure 1). Following cell isolation, a wide range of downstream applications including RNA-seq, ATAC-seq, ChIP-seq, and cell culture can be completed.
This protocol follows the guidelines of the Institutional Animal Care and Use Committee (protocol no. IACUC2018-0040) at Cincinnati Children’s Hospital Medical Center.
1. Equipment and reagent setup for the preparation of testicular cell suspension
2. Animal dissection and preparation of testicular cell suspension
3. Cell staining
4. Flow cytometry and experimental gates
5. Sort male germ cell subpopulations
6. Purity analysis of sorted cells
A representative result of this sorting protocol is shown in Figure 3. The total sorting time of two testes (one mouse) is usually around 3 hours, which is dependent on the concentration of cell suspension and the sorting speed. After sorting, the purity of spermatocytes is confirmed by immunostaining of SYCP3 and γH2AX (Figure 3A). The representative purity of sorted L/Z, P, D spermatocyte fractions are around 80.4%, 90.6%, and 87.6%, respectively (Figure 3C). We have determined the substages based on the criteria we published previously19. Briefly, in the leptotene and zygotene stage, synapsis between homologous chromosomes is incomplete, which is indicated by thin threads of SYCP3 staining. Broad γH2AX domains are observed throughout the nuclear chromatin due to programmed DNA double-strand breaks. In the pachytene stage, homologous chromosomes have completely synapsed, and γH2AX specifically accumulates on the sex chromosomes. In the diplotene stage, homologous chromosomes progressively undergo desynapsis. The purity of RS is confirmed by nucleus staining with DCV (Figure 3B). RS can be precisely judged with DNA staining: a unique DAPI-intense chromocenter surrounded by euchromatin; or combine with specific markers, such as Sp56 that is expressed within the developing acrosomal granule of spermatid and histone variant H1T that is highly expressed in nucleus after the mid-pachytene stage (Figure 3B).
RS purity is around 90.1% after sorting (Figure 3C). The sample size of purity analysis is over 1,000 cells for each experiment; the purity of L/P and D populations is averaged from 6 independent experiments; the purity of P and RS populations is averaged from 3 independent experiments. The viability of these isolated cells is usually over 95% (Figure S1). The total yield of each fraction from a single adult mouse is estimated and listed in Fig 3C, which provides sufficient cells for various downstream analyses. Recently, we have used this protocol to isolate wild-type pachytene spermatocytes for ChIP-seq analysis20,21.
Reagent | Ingredient | Stock concentration | Volume |
(HBSS base) | |||
Dissociation Buffer | DMEM | – | 2 ml |
(DMEM base) | FBS | – | 40 μl |
Hyaluronidase | 100 mg/ml | 30 μl | |
DNase I | 10 mg/ml | 50 μl | |
Collagenase Type I | 100 mg/ml | 40 μl | |
Recombinant Collagenase | 14000 unit/ml | 100 μl | |
FACS buffer | PBS | – | 980 ml |
(PBS base) | FBS | – | 20 ml |
Table 1: Reagent Recipe. The dissociation buffer must be prepared right before use. Prewarm DMEM before starting dissection. The enzyme stocks can be prepared any time before the experiment and stored at -20 °C. FACS buffer needs to be vacuum-filtered and stored at 4 °C; prewarm to room temperature before use.
Figure 1: Workflow of murine spermatogenic cells isolation on DCV-based sorting. This image illustrates the general procedure, from tissue dissociation to FACS sorting, to harvesting of isolated spermatogenic cells within one day. Please click here to view a larger version of this figure.
Figure 2: FACS analysis of adult murine testicular cells based on DCV fluorescence and light scattering. (A) Acquired unstained cells in the first decade of a DCV-blue histogram plot (left side of the red bar). (B)(C) Debris and non-single cell excluded by light scattering. (D) Unstained cell and side population exclusion based on DCV fluorescence. (E) DNA content determination based on DCV-blue fluorescence. Left peak (green) and right peak (pink) correspond to 1C and 4C populations. (F) Gating on 1C and 4C testicular populations based on DCV-blue/DCV-red fluorescence. (G) Precise gating on 4C testicular populations. (H) Back-gating of Gate 4C from the DCV plot on an FSC/BSC plot. Based on regions of minimal overlap on the FSC/BSC plot, the L/Z, P, and D gates are created to enrich their respective spermatocyte populations. (I) Color dot plot showing the L/Z, P, and D populations are in continuous order within Gate 4C. (J) Back-gating of Gate 1C from the DCV plot on an FSC/BSC plot. RS gate was created to enrich round spermatid population with uniform size, resulting in greater purity of populations during sorting. Please click here to view a larger version of this figure.
Figure 3: Representative result images and statistics of spermatogenic cells obtained from sorting. (A) Immunofluorescence characterization of sorted spermatocytes. Upper panel: DCV staining showing nucleus pattern of the live spermatocytes right after sorting; L/Z (leptotene/zygotene), P (pachytene), and D (diplotene). Lower panel: Confirmation of meiotic substages for each population by immunostaining for SYCP3 (green) and γH2AX (red). (B) Representative DCV image showing nucleus pattern of RS. Scale bars: 50 μm (upper panels), and 10 μm (lower magnified panels). Right Panels: Immunofluorescence confirmation of round spermatids stained with Sp56 and H1T. (C) The purity of L/Z, P, and D were confirmed by immunostaining, sample size was over 1,000 cells for each independent experiment, in total 6 independent experiments. The RS purity was confirmed by nucleus staining with a total of 3 independent experiments. The total cell number of the testicular cell suspension from one 8-week-old WT B6 mouse was around 100 million cells before sorting. Please click here to view a larger version of this figure.
Figure S1: The viability of isolated pachytene spermatocytes. A representative image shows the cell viability of isolated pachytene spermatocytes (Red: PI; Blue: DCV). PI could not be combined with DCV during sorting. However, under microscope, the DCV-red signal was quite low; therefore, PI-positive dead cells were easily distinguished from other live cells. The viability is usually over 95%. Scale bars: 10 μm. Please click here to download this figure.
Figure S2: Incomplete dissociation or debris disturbs gating. The A population (red circle) contains debris and polymer of spermatids (indicated by arrow). The bigger A population will cross with B population (yellow circle) and eventually contaminate the 4C population. Scale bars: 200 μm. Please click here to download this figure.
Here we present a practical and simple protocol to isolate subpopulations of spermatocytes and spermatids from a single adult male mouse. To ensure the reproducibility of this protocol, there are some critical steps that need attention. Before enzyme digestion, wash step aims to remove interstitial cells; after digestion, this step helps to remove spermatozoa and debris. Washing/centrifuging 3 times is important. In our dissociation buffer recipe, the combination of several different enzymes facilitates the dissociation of testes into the single-cell suspension without excessive cell damage. Gently pipetting to avoid causing air bubbles also helps to protect cell integrity. Please check the cell suspension under a microscope after dissociation to make sure the suspension achieves single-cell level. Incomplete dissociation or debris contamination from excessive digestion will affect the purity of sorted cells; as shown in Figure S2, the upright population contains debris and tetramer of spermatids. DCV staining requires incubation in the dark and no washing afterwards.
To troubleshoot potential difficulties on gating and back-gating of spermatocyte subpopulations, as an option for optimization, we recommend using a synchronized wild type mouse to help locate a specific stage of spermatocytes22,23,24. It is also worth noting that some knockout mouse strains with spermatogenesis arrest phenotypes may have uncommon DCV profiles because they are missing some subpopulations. Proper wildtype control is strongly recommended in this case. In addition, this protocol can be potentially applied to adult mice of any age. However, the age of the experimental mouse could be a confounding factor due to the variable proportion of germ cells.
Over the years, several protocols to purify germ cells have been developed. As one of the most popular methods, STA-PUT velocity sedimentation separates germ cells by the BSA gradient and provides a good yield of intact germ cells6,7. However, STA-PUT not only requires special devices that may not be readily available to many laboratories but is also time-consuming and labor-intensive to conduct in a cold room at 4 °C. Unlike STA-PUT, which is suitable for large-scale separation, this FACS-based method could provide high purity and precise fraction for a small-scale experiment. A large-scale sorting using our protocol is possible but will prolong the sorting time significantly and may compromise cell viability. Therefore, STA-PUT is still a practical option when a large number of cells is needed5,25,26.
In comparison with the previous FACS method based on Ho342 dye staining8,9,10,11, our protocol utilizes DCV, which has a broader excitation spectrum and can be applied to most current FACS sorters equipped with a UV or 405 nm violet laser18. Although there is another protocol using DCG14,15,16,17, the difference between our protocol and the DCG protocol is that our DCV protocol uses two-dimensional separation with DCV blue and DCV red, mimicking the advantage of the Ho342 protocol. With this advantage, our DCV protocol allows us to isolate highly enriched germ cells from adult testis. The DCG protocol does not employ two-dimensional separation and is recommended for isolation of germ cells from juvenile mice. The two-dimensional separation can have better resolution to separate the substages of spermatocytes. However, our method is still incapable of isolating leptotene and zygotene spermatocytes separately, as well as “2C” cell types including spermatogonia, preleptotene spermatocytes, and secondary spermatocytes.
Since the wide emission spectrum of DCV stain causes leaking to other channels, most of the cell viability dyes like PI and 7AAD cannot be combined due to false positive signals. Other cell viability dyes with emission in far-red or near-infrared channels might be worth trying in the future. But in our experience, sorted cells usually show ≥ 95% viability after 2 hours of FACS sorting (Figure S1), which is sufficient for downstream analysis.
Sorted cells obtained from our procedure can be used for various downstream experiments, including next-generation sequencing analysis (RNA-seq, ATAC-seq, and ChIP-seq). Cells obtained here can also be used for short-term culture27. In conclusion, we provide a simple but efficient protocol including a one-hour single-cell suspension preparation procedure and the detailed gating strategy for FACS based on DCV dye staining, which is suitable for small-scale spermatogenic cell isolation and can be quickly adopted by many investigators, even flow cytometry beginners.
The authors have nothing to disclose.
We thank members of the Namekawa, Yoshida, and Maezawa laboratories for their help; Katie Gerhardt for editing the manuscript; Mary Ann Handel for sharing the H1T antibody, the Cincinnati Children’s Hospital Medical Center (CCHMC) Research Flow Cytometry Core for sharing the FACS equipment supported by NIH S10OD023410; Grant-in-Aid for Scientific Research (KAKENHI; 17K07424) to T.N.; Lalor Foundation Postdoctoral Fellowship to A.S.; AMED-CREST (JP17gm1110005h0001) to S.Y.; the Research Project Grant by the Azabu University Research Services Division, Ministry of Education, Culture, Sports, Science and Technology (MEXT)-Supported Program for the Private University Research Branding Project (2016–2019), Grant-in-Aid for Research Activity Start-up (19K21196), the Takeda Science Foundation (2019), and the Uehara Memorial Foundation Research Incentive Grant (2018) to S.M.; National Institute of Health R01 GM122776 to S.H.N.
1.5 ml tube | Watson | 131-7155C | |
100 mm Petri dish | Corning, Falcon | 351029 | |
15 mL Centrifuge tube | Watson | 1332-015S | |
5 ml polystyrene tube with cell strainer snap cap (35 µm nylon mesh) | Corning, Falcon | 352235 | |
50 mL Centrifuge tube | Watson | 1342-050S | |
60 mm Petri dish | Corning, Falcon | 351007 | |
70 µm nylon mesh | Corning, Falcon | 352350 | |
Cell sorter | Sony | SH800S | |
Centrifuge | |||
Collagenase, recombinant, Animal-derived-free | FUJIFILM Wako Pure Chemical Corporation | 036-23141 | |
Collagenase, Type 1 | Worthington | LS004196 | |
Cover glass | Fisher | 12-544-G | |
Cytospin 3 | Shandon | ||
DAPI (4',6-Diamidino-2-Phenylindole, Dihydrochloride) | Fisher | D1306 | working concentration: 0.1 μg/mL |
Dnase I | Sigma | D5025-150KU | |
Donkey serum | Sigma | S30-M | |
Dulbecco’s phosphate-buffered saline (DPBS) | Gibco | 14190144 | |
Dulbecco's Modified Eagle Medium (DMEM) | Gibco | 11885076 | |
Fetal bovine serum (FBS) | Gibco | 16000044 | |
Hostone H1T antibody | gift from Mary Ann Handel | 1/2000 diluted | |
Hank’s balanced salt solution (HBSS) | Gibco | 14175095 | |
Hyaluronidase from bovine testes | Sigma | H3506-1G | |
Phosphate buffered saline (PBS) | Sigma | P5493-4L | |
Pipettemen | |||
ProLong Gold Antifade Mountant | Fisher | P36930 | |
rH2AX antibody | Millipore | 05-635 | working concentration: 2 μg/mL |
Sperm Fertilization Protein 56 (Sp56) antibody | QED Bioscience | 55101 | working concentration: 0.5 μg/mL |
Sterilized forceps and scissors | |||
Superfrost /Plus Microscope Slides | Fisher | 12-550-15 | |
SYCP3 antibody | Abcam | ab205846 | working concentration: 5 μg/mL |
TWEEN 20 (Polysorbate 20) | Sigma | P9416 | |
Vybrant DyeCycle Violet Stain (DCV) | Invitrogen | V35003 | |
Water bath |