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Hoechst 33342 HSC Staining and Stem Cell Purification Protocol
(see Goodell, M., et al. (1996) J Exp Med 183, 1797-806) The Hoechst purification was established for murine hematopoietic stem cells (HSC) onnormal C57Bl/6 bone marrow (NBM). We suggest that initial experiments beperformed using this marrow exactly as we describe in order to establish the procedurein your laboratory and to definitively identify the side population (SP) on the flowcytometer.
Hoechst Staining of C57Bl/6 bone marrow
Note that the ability to discriminate Hoechst SP cells is based on the differential efflux of
Hoechst 33342 by a multi-drug-like transporter. This is an active biological process.
Therefore, optimal resolution of the profile is obtained with great attention to the
staining conditions. The Hoechst concentration, staining time, and staining
temperature are all CRITICAL. Likewise, when the staining process is over, the cells
should be maintained at 4oC in order to prohibit further dye efflux. If you adhere
rigorously to the protocol below, you should easily find SP cells.
1) Ensure that a water bath is at precisely 37o C (check this with a thermometer!). Pre- warm DMEM+ (see below) while preparing the bone marrow.
2) Using mice 5-8 weeks of age, prepare bone marrow from femurs and tibias and 3) Count the nucleated cells accurately. We find an average of 5 x 107 nucleated cells per C57Bl/6 mouse. This number varies from strain to strain.
4) Spin bone marrow down. Resuspend at 106 cells per ml in pre-warmed DMEM+.
5) Add Hoechst to a final concentration of 5 µg/ml (a 200x dilution of the stock).
6) Mix the cells well, and place in the 37oC water bath for 90 minutes EXACTLY. Make sure the staining tubes are well submerged in the bath water to ensure that thetemperature of the cells is maintained at 37oC. Tubes should be mixed several timesduring the incubation.
We find staining large amounts of bone marrow most convenient in Corning 250 mlpolypropylene centrifuge tubes. Because of the sensitivity of the staining totemperature, DO NOT use a water bath which is constantly fluctuating intemperature due to heavy use. Water baths next to your tissue culture hoods are constantly dropping temperature as your friends put 500 ml bottles of ice coldmedium in, or worse, 500 ml of frozen serum.
7) After 90 minutes, spin the cells down in the COLD and re-suspended in COLD
8) At this point samples may be run directly on the FACS or further stained with antibodies*. All further manipulations MUST be performed at 4oC to prohibit
leakage of the Hoechst dye from the cells. Magnetic enrichments may also be
employed at this stage if the entire procedure is carried out at 4oC. Alternatively,
perform magnetic enrichments prior to Hoechst staining.
9) At the end of the staining, resuspend bone marrow cells in cold HBSS+ containing 2 µg/ml propidium iodide (PI) for dead cell discrimination. This is not required to
see the SP cells, but will help. Hoechst is somewhat toxic to the bone marrow, and
the PI will allow you exclude the dead cells from the profile (see Figure 1).
*Antibody Staining of Hoechst-stained cells
In order to confirm to your satisfaction that you have the correct population, you maywant to co-stain the Hoechst-stained bone marrow with antibodies. We find the mouseSP population to be very homogeneous with respect to cell surface markers. Werecommend staining with two antibodies, one which positively stains most SP cells(Sca-1 or c-kit) and one which does not stain SP cells but stains a large fraction of thebone marrow (for example Gr-1, or B220). These antibodies are available fromPharmingen. We suggest Sca-1-FITC and Gr-1-PE. Figure 2 shows typical staining ofwhole marrow and SP cells with these markers.
Hoechst 33342
We obtain from Sigma (called Bis-Benzimide) as a powder and resuspend at 1 mg/ml in
water, filter sterilize, and freeze in small aliquots. Since Hoecsht is not costly, there is
no reason to reuse old or frozen dye.
HBSS+
Hanks Balanced Salt Solution (from Gibco) with 2% Fetal Calf Serum and 10 mM
HEPES buffer (Gibco).
DMEM+
DMEM (Gibco) with 2% Fetal Calf Serum and 10mM HEPES buffer (Gibco).
Propidium Iodide
We obtain from Sigma. Our frozen stock is at 10 mg/ml in water. Our working stock
(covered with aluminum foil and kept in the fridge) is at 200 micrograms/ml in PBS.
Final concentration of PI in your sample should be 2 micrograms/ml.
Other Species
The optimal Hoechst-staining protocols are similar for multiple species. We found 90 minutes to be optimal for mouse SP cells, whereas 120 minutes is optimal for
human, rhesus, and swine cells. Follow the protocol exactly as described above, but
stain the bone marrow for 120 minutes.
Storage of Cells
If bone marrow preparation and sorting cannot be performed the same day, we
recommend that the BM be kept in the refrigerator overnight prior to Hoechst staining.
In our experience, cell viability is best when unficolled bone marrow is kept at 4oC.
[Note that mouse marrow does not need to be ficolled] In the morning, ficolled or un-
ficolled bone marrow may be warmed to 37oC, resuspended at 106 cells/ml, and stained
with Hoechst as described above. We do not recommend plating the bone marrow on
tissue culture plastic and leaving in the incubator overnight.
Flow Cytometry
Set UpWe have now used this set-up on multiple cytometers from both BD (Facstar-plus andVantage) and Cytomation (MoFlow). You need an ultraviolet laser to excite theHoechst dye and propidium iodide. A second laser can be used to excite additionalfluorochromes (eg. FITC and phycoerythrin with a 488 laser).
The Hoechst dye is excited with the UV laser at 350 nm and its fluorescence ismeasured with a 450/20 BP filter (Hoechst Blue) and a 675 EFLP optical filter (HoechstRed) (Omega Optical, Brattleboro VT). A 610 DMSP is used to separate the emissionwavelengths. Propidium iodide (PI) fluorescence is also measured through the 675EFLP (having been excited at 350 nm). Note that PI is much BRIGHTER than theHoechst red signal. Hoechst blue is the standard analysis wavelength for Hoechst33342 DNA content analysis. We have tried other filter sets/combinations. Whileothers work sufficiently, we have found these to give the best results.
Running
Hoechst-stained cells are placed on the flow cytometer and preferably kept cold by the
use of a chilling apparatus. It is not necessary to establish live gates on forward vs. side
scatter parameters. First, the Hoechst BLUE vs. RED profile is displayed, with BLUE
(450 BP filter) on the vertical axis and RED (675 LP) on the horizontal axis. With the
detectors in LINEAR mode, the voltages are adjusted so that the red blood cells are seen
in the lower left corner and the dead cells line up on a vertical line to the far right (very
bright for the PI wavelength, thus dead cells: SEE FIGURE 1). The bulk of the rest of
the cells can be centered. It should be possible to identify a major G0-G1 population
with S-G2M cells going off to the upper right corner.
Once you can see a profile similar to that shown in Figure 1, draw a live gate to exclude
the red and dead cells. Then, collect a large file within this window. In order to
identify the SP region definitively, 50,000-100,000 events MUST be collected within
this live gate. The SP region should appear as shown in the figure. The prevalence is
LOW: it is around 0.05% of whole bone marrow in the mouse. In human samples, the
prevalence is lower (0.03% of ficolled marrow).
ConfirmationIn order to confirm that you have identified the right cells, you can 1) block thepopulation with verapamil, or 2) co-stain with antibodies. Verapamil is used at 50µM(buy it from Sigma, and make a 100x stock in 95% ethanol), and is included during theentire Hoechst staining procedure. To confirm the mouse SP population, goodantibodies to co-stain with are Sca-1 and Gr-1 or another lineage antigen. Figure 2shows whole C57Bl/6 bone marrow and SP cells stained with Gr-1 and Sca-1.
For human SP cells, you can also block with verapamil. For antibody staining, you can use CD34 and some other marker. The most consistent feature of human SPcells is their lack of CD34 expression. They express low but variable level of CD38.
Other tips for optimal resolution of the multiple Hoechst populations
Since analysis of the Hoechst dye is performed in linear mode, we have found that good C.V.s are critical. We perform alignments in linear mode with particles
which have a very tight distribution (e.g. DNA Check beads from Coulter).
Furthermore, we have used the UV laser in the "first" position for optimal C.V.s (this
has the added benefit of allowing thresholding on DNA (Hoechst blue) and thus red
blood cells are irrelevant). However, this is not necessary.
In keeping with having good C.V.s, the sample differential pressure must be as low as possible. Preferably, the maximum sample differential pressure is calibratedwith your alignment particle. In other words if your C.V. for your alignment particle is3% with a low differential pressure then determine the maximum differential pressurethat will still give you good % C.V. s and do not ever exceed that pressure.
Finally, a relatively high power on the UV laser gives the best CVs. We find 50- 100 mW to give the best Hoechst signal. Less power will suffice, but the populationsmay not be as clearly resolved.
Other comments about Hoechst Fluorescence
Many people have asked us why we even see Hoechst fluorescence in the far red (>675nm). This is indeed surprising. None of the great flow cytometry textbooks documentHoechst fluorescence out this far. We do NOT think that this represents a separateemission peak for Hoechst fluorescence, but rather the fact that Hoechst stains cellsVERY brightly, and we still manage to detect significant signal this far because theoverall quantity of signal is so great. However, although you can easily detect a signal, it is not very bright in the red wavelengths, relative to the blue. The voltage on our redPMT is usually cranked fairly high.
Note that the red signal is NOT propidium iodide. PI positive cells are even brighterthan these Hoechst-red cells and can be seen lining up at the far right of the profile (seeFigure 1). Of course, if you also have a 488 laser running, you will also see these PIpositive cells in the PE and PI channels until you gate them out on the basis of theHoechst profile.
Given that we DO see red Hoechst fluorescence, what is going on? And why are somany populations resolved? Hoechst is doing several things at once: 1) It IS a DNA binding dye, and can be used for DNA cell cycle analysis. Some of the cells that reach into the upper right of your plot are in S-G2M. And if you had ahomogeneous population of cells, you should get a simple cell cycle profile if youlook at Hoechst fluorescence at only one wavelength (usually the blue).
2) Hoechst is pumped out by hematopoietic stem cells. That is why we see the LOW Hoechst fluorescence in the SP population 3) Hoechst also has some property that we don’t understand, that we think of as a chromatin effect. If you do a literature search, you can find out more about howHoechst binds AT base pairs and think about how the binding and emission spectramight be affected by chromatin conformation The best paper we have found that explores this dual wavelength phenomenon in anydetail is: Watson, J. V.; Nakeff, A.; Chambers, S. H.; Smith, P. J. (1985) Flow cytometric
fluorescence emission spectrum analysis of Hoechst-33342-stained DNA in chicken
thymocytes. Cytometry 6 310-5.
GOOD LUCK!
MOUSE BONE MARROW
NO GATE UV 405/30
100,000 events collectedin the live gate shown: allows the SP to be better defined and cleans up the FSC/SSC view Mouse Bone Marrow (C57Bl/6)
Whole BM NO STAIN (Gated from Fig 1)
Whole BM: Sca-PE+ Gr1-FITC (Gated from Fig 1))
Gr1-FITC
Sca-1-PE
Gr1-FITC17
SCA-1-PE <PE>

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