Isolation of Proteoglycans from Tendon

Kathryn G. Vogel and Julie A. Peters

1. Introduction

Proteoglycans make up less than 1% of the dry weight of a dense connective tissue such as tendon (1). Most of this proteoglycan is a small molecule called decorin. Because decorin has only one glycosaminoglycan chain, it cannot be separated from other proteins by the CsCl density-gradient centrifugation method that was originally used to purify aggrecan from cartilage.

The basic approach described in this chapter is to extract proteoglycans from the tissue with 4 M guanidine, a solution that will denature collagen and disrupt most kinds of noncovalent molecular interactions. The cross-linked collagen of adult tendon remains insoluble during this extraction, allowing the proteoglycans and other soluble proteins to be separated from the bulk of the tissue by centrifugation. Proteoglycans are then separated from other extracted proteins by ion-exchange chromatography, taking advantage of the anionic nature of the glycosaminoglycan chain.

This method has been used to quantitate and isolate proteoglycans from the tensile (proximal) and compressed (distal) regions of bovine deep flexor tendon. These mechanically distinct regions of flexor tendon are characterized by differences in proteoglycan amount and type (2). The method is equally applicable to isolation of proteoglycans from human tendon or from other dense connective tissues (3). Once isolated, the large proteoglycans can be separated from smaller ones by sieve chroma-tography. These isolated proteoglycans and their unique core proteins and glycosami-noglycan chains are of sufficient purity to then be examined by specific analytical techniques or used in functional assays.

2. Materials

2.1. Quantitation of Glycosaminoglycans

1. Single-edge razor blades.

2. Heated water bath.

From: Methods in Molecular Biology, Vol. 171: Proteoglycan Protocols Edited by: R. V. Iozzo © Humana Press Inc., Totowa, NJ

3. Microcentrifuge.

4. Lyophilizer.

5. Papain digestion solution: 0.05 M sodium acetate, 5 mML-cysteine HCl, 10 mM EDTA, pH 5.5. Add 50 ^g/mL papain just before starting digestion.

6. DEAE cellulose (Whatman, DE52).

7. Disposable columns (Bio-Rad Poly Prep, 0.8 x 4 cm).

8. Column elution solutions: 0.02 M HCl, 1 M HCl.

9. Uronic acid detection reagent (orcinol reagent): 2 g of orcinol (5-methylresorcinol; 3,5-dihydroxytoluene monohydrate) in 50 mL of 1.5% FeCl3, 850 mL of conc. HCl, and 150 mL of water (4). Orcinol is sensitive to light and is irritating to eyes, the respiratory system, and skin. Dissolve orcinol in FeCl3 solution, then add HCl and water. Use caution and work in a fume hood when adding water to concentrated HCl.

10. Uronic acid standards: stock solution of 100 ^g/mL of glucuronolactone diluted to standards of 5 ^g/mL, 10 ^g/mL, 20 ^g/mL, and 30 ^g/mL.

11. Pyrex test tubes.

12. Heat block.

13. Spectrophotometer.

2.2. Isolation of Proteoglycans

1. Single-edge razor blades.

2. Tissue-grinding device (we have used a food mill made by Arthur Thomas, Incorporated, Philadelphia, PA).

3. Lyophilizer.

4. 16-mL centrifuge tubes with sealing screw cap (Nalge Nunc).

5. 4 Mguanidine buffer: 4 Mguanidine, 0.05 Msodium acetate, pH 6.0. Add protease inhibitors just before starting extraction: 5 mM benzamindine, 5 mM phenylmethylsulfonyl fluoride, and 1 mM N-ethylmaleimide.

6. Nutator (Clay-Adams) or other rocking device.

7. Centrifuge (Beckman J2, JA-17 rotor).

8. 7 M urea buffer: 7 M urea, 0.05 M sodium acetate, pH 6.0.

9. 7 M urea buffer plus protease inhibitors: shortly before use add 5 mM benzamindine, 5 mM phenylmethylsulfonyl fluoride, 1 mM N-ethylmaleimide.

10. Dialysis tubing (MWCO: 12,000-14,000).

11. DEAE cellulose (Whatman, DE52).

12. Disposable columns, Bio-Rad Poly-Prep, 0.8 x 4 cm.

13. Column elution solutions: 7 Murea buffer + 0.1 MNaCl; 7 Murea buffer + 0.2 MNaCl; 7 M urea buffer + 0.8 M NaCl.

2.3. SDS Polyacrylamide Gel Electrophoresis of Proteoglycans

1. 2-mL screw-capped centrifuge tubes.

3. Microcentrifuge.

4. Enzyme digestion buffer: 0.1 M Tris base, brought to pH 8.0 with acetic acid.

5. Chondroitinase ABC (Seikagaku, 5 U/vial) resuspended in 0.5 mL of 0.1 M Tris-HCl, pH 8.0 mL (10 U/mL).

6. Gel sample buffer: for 100-mL, combine 50 mL stacking gel buffer, 5 g sodium dodecyl sulfate, 20 g glycerol, 4 mg bromophenol blue, and water to volume.

7. Electrophoresis apparatus and power source.

Table 1

Water Content of Adult Flexor Tendons

Table 1

Water Content of Adult Flexor Tendons

Tissue

1

2

3

Tens.

65.6

61.5

62.7

65.2

62.1

63.4

X =

63.7

+ 1.7 %

66.6

62.1

63.7

Comp.

70.2

70.6

73.1

70.8

67.3

71.1

X =

= 71.0

+ 1.7 %

70.9

70.9

73.7

Samples of fresh tendon from animals between 1 and 2 years of age were cut into small pieces with a razor blade, weighed, lyophilized, and weighed again to determine the percent of tendon wet weight that was water. Triplicate samples of about 130 mg each were assessed. Tens., tissue from the tensile/ proximal region of tendon; Comp., tissue from the compressed/distal region of tendon. Samples are from the front (1) and rear (2) leg of one animal and the rear (3) leg of another. Note that the water content is lower in the tensile tendon.

8. Polyacrylamide: 30% acrylamide, 0.8% bis-acrylamide.

9. Separating gel buffer: 0.75 M Tris, 4 mM EDTA, pH 8.8.

10. Stacking gel buffer: 0.25 M Tris, 4 mM EDTA, pH 6.8.

11. 0.5% ammonium persulfate.

12. TEMED.

13. Electrode buffer stock: 0.1 M Tris, 0.768 M glycine, 8 mM EDTA, pH 8.7. After adjusting pH (if necessary), add 0.4% SDS. The upper buffer is diluted 1/4; the lower buffer is diluted 1/8.

14. Gel fixative: 50% methanol, 7% acetic acid, 43% dH2O.

15. Coomassie blue staining solution: 0.1% Coomassie Brilliant Blue R-250, 40% methanol, 10% acetic acid, 50% dH2O.

16. Gel destain: 25% methanol, 7% acetic acid, 68% dH2O.

17. Alcian blue staining solution: 0.5% Alcian blue (8GX, Sigma Chemical Co), 7% acetic acid, 93% dH2O.

18. Alcian blue destain solution: 7% acetic acid, 93% dH2O.

3. Methods

3.1. Quantitation of Glycosaminoglycans

1. Dissect tendon tissue free of muscle and fat, cut into chunks about 3 mm on a side, weigh, lyophilize, weigh again (see Note 1 and bovine tendon water content, Table 1).

2. Add papain digestion solution to tissue at 50 mg dry tissue/1 mL enzyme in 2-mL screw cap tubes. Incubate at 65°C until tissue pieces have disappeared (6-18 h, see Note 2).

3. Centrifuge and remove the supernatant. Dilute supernatant with equal volume of dH2O to assure low salt concentration.

4. Pour columns containing 0.3 mL DE-52 in water (see Note 3).

5. Apply 1 mL of diluted sample to DEAE cellulose in column; collect the flow-through (see Note 4).

6. Rinse column with 20 mL of 0.02 M HCl to remove loosely adherent molecules. Elute the glycosaminoglycans into a clean tube with 2 mL of 1 M HCl.

Table 2

Uronic Acid Content of Adult Flexor Tendons (^g uronic acid/mg dry weight)

Table 2

Uronic Acid Content of Adult Flexor Tendons (^g uronic acid/mg dry weight)

Tissue

1

2

3

Tens.

3.1

2.6

2.1

2.8

2.2

2.3

X =

2.6 + 0.4

3.3

2.2

2.4

Comp.

7.1

7.1

6.0

5.8

6.8

6.3

X =

6.4 + 0.8

5.2

7.9

5.7

Samples of dry tendon were digested with papain, passed over a small column of DEAE cellulose, and the isolated glycosaminoglycans assessed for uronic acid content. Note that tissue from the compressed region of tendon contains a higher amount of uronic acid.

7. Determine the amount of GAG uronic acid spectrophotometrically using the orcinol reagent and uronic acid standards (4). Combine 0.5 mL of glycosaminoglycan sample or standard with 1.5 mL of orcinol reagent in Pyrex test tube, place in heat block at 100°C for 20 min, cool to room temperature, read OD at 670 nm within 30 min, and calculate micrograms of uronic acid per milligram of tissue dry weight (see Notes 5-7, and tendon uronic acid content, Table 2)

3.2. Isolation of Proteoglycans

1. Chop the tissue into chunks ~3 mm on a side and freeze immediately with liquid nitrogen (see Note 8).

2. Powder the frozen chunks of tissue (see Note 9).

3. Extract tissue in 4 M guanidine buffer + protease inhibitors for 24 h at 4°C with rocking. A good ratio of tissue wet weight to extraction fluid is about 1/20 (see Note 10). The efficiency of proteoglycan extraction from powdered tendon was higher than extraction from tissue chunks (see Tables 3 and 4).

4. Centrifuge in Beckman JA-17 fixed-angle rotor at 3440g (5000 rpm) for 10 min; carefully remove the supernatant.

5. Repeat the extraction with fresh buffer, centrifuge, and combine supernatants (see Note 11).

6. Dialyze the extract into 7 M urea buffer + 0.1 M NaCl (see Notes 12 and 13).

7. Apply 0.5 mL of dialyzed extract to a 1-mL column of DEAE cellulose equilibrated in 7 M urea buffer (see Note 14).

8. Rinse the column with 8 mL of 7 M urea buffer + 0.1 M NaCl followed by 3 mL of 7 M urea buffer + 0.2 M NaCl.

9. Elute proteoglycans with 1.5 mL of 7 M urea buffer + 0.8 M NaCl (see Note 15).

3.3. SDS Polyacrylamide Gel Electrophoresis of Proteoglycans

1. Precipitate the proteoglycans in ethanol to prepare samples for gel electrophoresis. For example, mix 50 ^L of sample from the ion-exchange column with 400 ^L of ethanol in a small centrifuge tube. Let stand at -20°C for several hours or in -70°C freezer for at least 1h.

Table 3

Extraction of Proteoglycan from Adult Flexor Tendons

_Tensile___Compressed_

Chunks Powder Chunks Powder

7 M Urea Total

4 M Guanidine Total

41 35 76

195 110 305

176a

63 28 91

285 131 416

748 239 987

41 172

624 122 746

1335 112 1447

aDue to tissue swelling, 15 mL of solution was added.

Chunks or powdered tendon was extracted with 7 M urea, 7 M urea + 0.1 M NaCl, or 4 M guanidine. In each case the extraction started with 200 mg dry weight of tissue and 8 mL of extraction solution. After 24 h the extracts were centrifuged, the supernatants removed, and an additional 8 mL of extraction solution added to the pellet for another 24 h. The amount of uronic acid in the supernatant of each extraction is shown, in micrograms. 1, first extraction; 2, second extraction of same tissue.

2. Spin samples in the microcentrifuge in cold for 10 min at 10,000 rpm, remove supernatant by suction, rinse pellet and tube with 1 mL cold ethanol, let stand at -20°C for at least 1h, and spin again. Dry pellet with brief lyophilization (see Note 16).

3. Solubilize pellet in 25 ^L of gel sample buffer and heat to 100°C for 5 min in a heat block. If the samples are to be reduced, add dithiothreitol or ß-mercaptoethanol to the gel sample buffer before heating.

4. The core protein of proteoglycans having chondroitin sulfate or dermatan sulfate glycosaminoglycan chains can be seen after removal of the glycosaminoglycans by digestion with chondroitinase ABC (see Note 17). Resuspend the dry proteoglycan pellet in 25 ^L of digestion buffer, add 1 ^L (0.01 unit) of enzyme, and incubate at 37°C for 1 h. Add 25 ^L of 2X gel sample buffer and heat to 100°C for 5 min in the heat block.

5. Pour 4-16% gradient gel, 1.5 mm thick (5, see Notes 18 and 19). For one gel, make solutions containing 4% and 16% acrylamide. For 4%: 6.75 mL separating buffer, 1.8 mL acrylamide + bis, 4.14 mL water, 135 ^L 10% SDS, 13.5 ^L TEMED, and 675 ^L 0.5% ammonium persulfate. For 16%: 3.38 ^L 2X separating buffer, 7.2 mL acrylamide + bis, 2.12 mL water, 135 ^L 10% SDS, 13.5 ^L TEMED, and 675 ^L 0.5% ammonium persulfate. The 16% solution is stirred while pumping it between glass plates, as it is diluted by the 4% solution.

6. Pour 4% stacking gel using a 15-lane comb.

14

Vogel and Peters

Table 4

Efficiency of Proteoglycan Extraction3

Tensile

Compressed

7 M urea

16%

12%

7 M urea + 0.1 M NaCl

65%

51%

4 M Guanidine

108%

100%

a Uronic acid in the papain digest of each tissue was set to equal 100% tens. = 467 |g, comp. = 1454 |g.

a Uronic acid in the papain digest of each tissue was set to equal 100% tens. = 467 |g, comp. = 1454 |g.

The amount of uronic acid solublized by two sequential 24-h extractions in each extraction solution (Table 3) is compared to the amount of uronic acid measured after papain digestion of tissue from the same animal (sample 2, Table 2). Note that adding 0.1 M NaCl to 7 M urea greatly increased the amount of proteoglycan that was solublized from the tissue. Virtually all proteoglycan was solublized by 4 M guanidine.

7. Load 10- 50 |L of sample into each lane, as appropriate. Put molecular weight standards in one lane. If samples were digested with chondroitinase ABC, it is useful to prepare one sample containing only 1 |L of enzyme and digestion buffer.

8. Run electrophoresis at 8 mA for approximately 18 h (see Note 20). Cooling to 16°C may help uniformity but is not necessary. If desired, carry out Western blot procedures immediately after electrophoresis.

9. Put the gel into gel fixative solution for 1 h.

10. Cover the gel with Coomassie blue staining solution for 1h with gentle rocking. Pour off the stain. Cover the gel with several changes of destain solution until the gel background is clear.

11. To stain for proteoglycans, cover the gel in Alcian blue staining solution for 1 h. Pour off the stain and destain with 7% acetic acid until gel background is clear (see Fig. 1 and Notes 21 and 22).

4. Notes

1. Many plastic tubes will lose weight during lyophilization. It is best to remove dry tissue from the tube to obtain dry weight.

2. Tissue digestion can be encouraged by shaking the tube and by adding additional papain. The digest may appear cloudy.

3. Precycle DE52 through washes in 0.5 NHCl, dH2O, 0.5 NNaOH, and then rinse in dH2O until filtrate is near pH 7. Used resin can be recovered by the same treatment and used again.

4. The DE52 resin is kind; the columns will not run dry even when no fluid remains above the resin. Columns can be placed in a tube to collect eluent and simply moved to the next tube for the next elution fluid.

5. Place a glass marble on each tube while in the heat block to diminish evaporation of acid. Cool tubes quickly and uniformly by transferring the tubes to a rack and placing the rack in a pan of water.

6. Dilute the sample with dH20 as needed (1/2 or 1/5) to obtain readings that fall within the range of standards. Authentic chondroitin sulfate was recovered from the columns with efficiency >95%. Multiply by 3.2 to convert amount of uronic acid to chondroitin sulfate.

7. Orcinol reagent is stable for 6 wk when kept in the refrigerator in a dark glass container.

Glass Columns Reagent

Fig. 1. SDS/Polyacrylamide gel electrophoresis of proteoglycans. Proteoglycans were extracted, isolated by ion-exchange chromatography, precipitated with ethanol, incubated without or with added chondroitinase ABC, and loaded onto a 4-16% gradient gel. After electrophoresis the gel was stained with Coomassie blue and Alcian blue. An approximately equal amount of uronic acid (12.5 ^g) from samples extracted with 7 M urea + 0.1 M NaCl and 4 M guanidine was loaded onto each lane; it was not possible to precipitate this much material from the 7 M urea extracts. Tens, from tensile region of tendon; Comp, from compressed region of tendon. Note that a proteoglycan migrating just above 120 kDa (decorin) is the predominant molecule in tensile samples. A proteoglycan that enters the running gel but migrates more slowly (biglycan) is present only in the compressed samples. The core proteins of these small proteoglycans migrate close together at about 45 kDa. Large proteoglycan (aggrecan) is present in the stacking gel of compressed samples but does not form a discrete band. The high-molecular-weight band appearing after chondroitinase ABC digestion of samples from compressed tissue is the aggrecan core protein with some keratan sulfate chains.

Fig. 1. SDS/Polyacrylamide gel electrophoresis of proteoglycans. Proteoglycans were extracted, isolated by ion-exchange chromatography, precipitated with ethanol, incubated without or with added chondroitinase ABC, and loaded onto a 4-16% gradient gel. After electrophoresis the gel was stained with Coomassie blue and Alcian blue. An approximately equal amount of uronic acid (12.5 ^g) from samples extracted with 7 M urea + 0.1 M NaCl and 4 M guanidine was loaded onto each lane; it was not possible to precipitate this much material from the 7 M urea extracts. Tens, from tensile region of tendon; Comp, from compressed region of tendon. Note that a proteoglycan migrating just above 120 kDa (decorin) is the predominant molecule in tensile samples. A proteoglycan that enters the running gel but migrates more slowly (biglycan) is present only in the compressed samples. The core proteins of these small proteoglycans migrate close together at about 45 kDa. Large proteoglycan (aggrecan) is present in the stacking gel of compressed samples but does not form a discrete band. The high-molecular-weight band appearing after chondroitinase ABC digestion of samples from compressed tissue is the aggrecan core protein with some keratan sulfate chains.

8. To freeze tissue chunks for storage, put a few at a time into a half-liter plastic container, add a small amount of liquid nitrogen, cover loosely, and shake the container. This makes it possible subsequently to remove individual chunks of frozen tissue. Be sure to maintain a vent to allow gas to escape during freezing.

Tendon is difficult to powder. To avoid gumming up the mill, it is necessary to keep the grinding surfaces cold by adding liquid nitrogen continually. Some loss of tissue is inevitable. For smaller amounts of tissue one can use a tissue macerator (a shaking steel ball and chamber) cooled with liquid nitrogen.

The ratio of extraction fluid to tissue can be varied, depending on the goal of the extraction. Tensile tendon swells a great deal during extraction, whereas compressed tendon and cartilage do not. Powdered tendon swells more than chunks of tissue. For highest extraction efficiency it is important to have a large supernatant volume and a small pellet after centrifugation. However, a large supernatant volume is not desirable during the subsequent dialysis. With sufficient fluid volume, virtually 100% of the proteoglycan can be removed from powdered tendon with two sequential extractions in 4 M guanidine buffer (see Tables 3 and 4). In contrast, extraction with 7 M urea solublized less that 20% of the proteoglycan. Addition of 0.1 M NaCl to 7 M urea increased extraction efficiency fourfold compared to extraction in 7 M urea alone, but still solublized only 50-65% of the total proteoglycan (see Table 4). Figure 1 indicates that these two extraction solutions solublize the same proteoglycans.

It is necessary to remove 4 M guanidine in order to carry out ion-exchange chromatography. Efficient dialysis is accomplished during three sequential 24-h dialysis steps using 5 volumes of 7 M urea buffer for each step. This will reduce guanidine concentration to less than 0.04 M, a level that does not impede glycosaminoglycan binding to the anion-exchange resin.

Although extraction in 7 M urea + 0.1 M NaCl is less efficient than extraction in 4 M guanidine, it eliminates the need to dialyze samples before ion-exchange chromatography. With a larger extract volume, one can use a larger DEAE cellulose column. The extract can be pumped onto the column and eluted with a continuous gradient of NaCl from 0.1 to 0.8 M. Proteoglycans will elute at about 0.25 M NaCl (6).

The yield of decorin after ion-exchange chromatography and sieve chromatography was about 150 ^g/g wet weight of adult tensile bovine tendon (6).

Precipitate proteoglycans in 8 volumes of ethanol. Precipitation from 7 M urea + NaCl does not present a problem. However, it is sometimes useful to precipitate the 4 M guani-dine extract. In this case it is important to remove all supernatant and rinse the tube carefully. If any guanidine remains in the sample, it will form a nasty precipitate with SDS in the gel sample buffer and make electrophoresis impossible.

The resuspended chondroitinase ABC enzyme from Seikagaku can be kept in the refrigerator for a year.

All electrophoresis buffers should be made with Tris base and brought to proper pH with HCl.

It is not necessary to run gradient gels to see proteoglycan. However, the 4-16% gel is useful for visualizing intact biglycan and decorin and their core proteins on the same gel. Large proteoglycans such as aggrecan will not enter the separating gel. As the gel is running, it is possible to see "crinkly" diffraction lines in the stacking gel of lanes containing the large proteoglycan.

The various concentrations of methanol suggested for destaining solutions are designed to reduce methanol consumption. After destaining in 7% acetic acid, the gel will be somewhat swollen; it will return to size when stored in the solution containing 25% methanol. If the protein bands have faded, just add a few drops of Coomassie blue to this final solution. Gel electrophoresis can be used to visualize intact proteoglycans in a tissue extract, without ion-exchange purification, by staining only with Alcian blue. The gel should be washed with at least three changes of destain solution over a 24-h period to assure complete removal of SDS; then stain the gel with Alcian blue and destain in 7% acetic acid. If SDS remains in the gel, it will bind Alcian blue and make the gel impossible to destain. Do not stain samples of the total extract with Coomassie blue, because this produces a blue smear that will obscure the proteoglycans.

Acknowledgements

Kathryn G. Vogel wishes to thank Dick Heinegard, Lund, Sweden, and Larry Fisher,

Bethesda, Maryland, for teaching her many of the techniques and tips reported here. Supported by the National Institutes of Health, AR36110.

References

1. Vogel, K. G. and Heinegard, D. (1985) Characterization of proteoglycans from adult bovine tendon. J. Biol. Chem. 260, 9298-9306.

2. Vogel, K. G. (1995) Fibrocartilage in tendon: a response to compressive load, in Repetitive Motion Disorders of the Upper Extremity (Gordon, S. L., Blair, S. J., and Fine, L. J., eds.) American Academy Orthopedic Surgery, Rosemont, IL, pp. 205-215.

3. Berenson, M. C., Blevins, F. T., Plaas, A. H. K., and Vogel, K. G. (1996) Proteoglycans of human rotator cuff tendons. J. Orthoped. Res. 14, 518-525.

4. Brown, A. H. (1946) Determination of pentose in the presence of large quantities of glucose. Arch. Biochem. 11, 269-278.

5. Laemmli, U. K. (1970) Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227, 680-685.

6. Vogel, K. G., Sandy, J. D., Pogany, G., and Robbins, J. R. (1994) Aggrecan in bovine tendon. Matrix Biol. 14, 171-179.

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