Volume 53 (6): 745-751, 2005 Copyright ©The Histochemical Society, Inc. Pep-1 as a Novel Probe for the In Situ Detection of Hyaluronan
Department of Dermatology, University of Texas Southwestern Medical Center, Dallas, Texas Correspondence to: Dr. Mark E. Mummert, Department of Dermatology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-9069. E-mail: mark.mummert{at}utsouthwestern.edu
Hyaluronan (HA) is expressed by most tissues, including skin. Localization of HA in the skin is assessed by histology with HA-binding protein (HABP) serving as the probe. Reports have suggested that HA expression in skin is altered in a number of diseases. However, interlaboratory variations in HABP staining profiles, even in normal skin, suggest a need to standardize methods and/or identify new probes. We report the staining patterns of a HA-binding peptide (termed "Pep-1") in human and mouse skin. After acetone fixation, Pep-1 stained HA in the intercellular spaces of the epidermis, whereas staining in the dermis was weak and diffuse in both human and mouse skin. HABP staining of the epidermis and dermis were comparable in human skin but failed to stain the vital epidermis of mouse skin. In human skin, Pep-1 stained the basal, spinous, and granular layers, whereas HABP failed to stain the basal layer. Precipitation of HA in situ resulted in dermal staining but weak staining in the epidermis for HABP and Pep-1. Our results may suggest that Pep-1 is sensitive to HA conformation. Furthermore, Pep-1 may represent a new probe to study HA expression in the skin. (J Histochem Cytochem 53:745751, 2005)
Key Words: hyaluronan hyaluronan-binding protein Pep-1 extracellular matrix
HYALURONAN (HA) is a large glycosaminoglycan composed of N-acetylglucosamine and glucuronic acid subunits. HA has been detected in nearly every tissue with its highest concentration in the skin (Fraser et al. 1997
Cutaneous HA metabolism is normally a tightly regulated process. However, abnormally high concentrations of HA in the skin have been reported for a number of diseases including psoriasis (Wells et al. 1991
Analyses of the concentration and polymer size of HA are typically determined by biochemical methods such as chromatography (Okamoto et al. 1999
Recently, we have developed a HA-binding peptide (termed Pep-1) using the phage display technique (Mummert et al. 2000
Skin Samples Human skin samples were from discarded foreskins following circumcisions. Mouse ear skin was from C57BL/6 mice maintained in our breeding colonies. Animal experiments were approved by the Institutional Review Board at University of Texas Southwestern Medical Center.
Peptides and HABP
Sample Preparation and Staining Procedure
Glycosaminoglycan Inhibition Assay
Other Fixation Techniques Permanent sections were prepared by embedding acetone-fixed ear skin in paraffin. Sections were cut to 8-µm thickness, deparaffinized, blocked with glycine and BSA as above, and stained with Pep-1 or the scrambled peptide after pretreatment with Streptomyces hyaluronidase or mock treated with buffer alone. Skin sections were stained with Pep-1 and the scrambled peptide control as above.
Quantitative Analysis
Staining Profiles of HABP and Pep-1 in Human Foreskin Significant variations in HABP staining profiles of human skin have been reported. Moreover, there are discrepancies between biochemical findings (i.e., high concentrations of dermal HA) (Sakai et al. 2000 We found that HABP stained the dermis of human foreskin uniformly and with high intensity (Figure 1). In contrast, HABP predominantly stained HA in the spinous and granular layers of the epidermis, whereas staining in the basal layer was relatively weak or absent. No staining was detected in the stratum corneum. Quantitative analyses showed insignificant differences (p>0.05) between the dermal and epidermal (spinous and granular layers) integrated fluorescence intensities (Figure 2). Pretreatment of cryostat sections with Streptomyces hyaluronidase, an enzyme that specifically degrades HA, significantly (p<0.01) reduced HABP staining in both the epidermis and dermis, showing HA specificity (Figure 1 and Figure 2). Unlike HABP, Pep-1 showed diffuse and relatively weak staining of HA in the dermal compartment (Figure 1). With the exception of the stratum corneum, Pep-1 stained all of the epidermal layers, although the intensity of the staining appeared to be in the order basal layer > spinous layer > granular layer. However, integrated fluorescence intensities were not significantly different among the various epidermal layers as assessed by ANOVA (p>0.05) (data not shown). On the other hand, comparisons of integrated fluorescence intensities between the epidermis and dermis showed that Pep-1 stained the epidermis significantly better than the dermis (Figure 2; p<0.01). We next evaluated the specificity of Pep-1 staining by pretreatment of slides with Streptomyces hyaluronidase. Qualitative (Figure 1) and quantitative (Figure 2) analyses showed that digestion of HA from skin sections nearly completely abrogated Pep-1 staining, indicating HA specificity (Figure 1 and Figure 2).
Staining Profiles of HABP and Pep-1 in Mouse Skin Mice remain important models for many human diseases including selected dermatologic disorders (i.e., atopic dermatitis) (Yagi et al. 2002 HABP stained the dermis of mouse ear skin uniformly and with high intensity (Figure 3 and Figure 4). On the other hand, HABP stained the epidermis mainly in the stratum corneum and basal lamina with relatively weak or no binding in the vital epidermis. Pretreatment of slides with Streptomyces hyaluronidase significantly reduced the intensity of staining in the dermis, whereas the stratum corneum and basal lamina was partially resistant to enzymatic digestion (Figure 3 and Figure 4). Finally, preincubation of HABP with HA polymers (1.5 mg/ml) significantly reduced HA staining, further showing the specificity of HABP for HA (data not shown).
Pep-1 showed a staining pattern similar to human skin, with diffuse and relatively weak staining in the dermis and high-intensity staining in the epidermis (Figure 3 and Figure 4). Evaluation of the epidermal and dermal integrated fluorescence intensities showed that the Pep-1 staining was significantly weaker in the dermis compared with the epidermis (Figure 4; p<0.01). Again, the scrambled peptide control failed to bind cryostat sections, showing the requirement of the Pep-1 sequence for molecular interaction with HA. Finally, Streptomyces hyaluronidase treatment significantly reduced Pep-1 intensities showing HA specificity. We should note that Pep-1 staining patterns were not significantly altered when hyaluronidase digestions were performed in the presence of protease inhibitors (Ripellino et al. 1985
Effect of Pep-1 Concentration on HA Staining Patterns
Specificity of Pep-1 for HA To assess the specificity of Pep-1 for HA, we conducted inhibition assays using HA polymers and oligomers, chondroitin sulfate A, and dermatan sulfate. As shown in Figure 6, HA oligomers and polymers nearly completely abrogated binding of Pep-1 to cryostat sections. In contrast, two related glycosaminoglycans (chondroitin sulfate A and dermatan sulfate) failed to significantly inhibit Pep-1 binding to skin when used at the same inhibitor concentration. These results suggest that Pep-1 binds specifically to HA among the potentially available glycosaminoglycans.
Impact of Fixation on HA Staining Patterns Because the method of fixation has been shown to alter HABP binding patterns in skin, we assessed the impact of paraformaldehyde/CPC fixation on Pep-1 binding to mouse ear skin. Paraformaldehyde/CPC fixation markedly altered the binding pattern of Pep-1. In contrast to acetone-fixed specimens, Pep-1 stained with high intensity throughout the dermis but showed weak to no binding in the epidermis. On the other hand, the scrambled peptide control showed little or no staining throughout the skin (Figure 7A). Interestingly, pretreatment of skin sections with Streptomyces hyaluronidase showed only partial abrogation of Pep-1 binding. Similarly, HABP binding to paraformaldehyde/CPC-fixed sections was only partially sensitive to pretreatment with Streptomyces hyaluronidase (Figure 7A). The failure of hyaluronidase pretreatment to completely remove Pep-1 and HABP staining may reflect the inability to completely remove CPC-precipitated HA from tissue sections (Lin et al. 1997
Because paraffin sections retain details of tissue morphology better than frozen sections, we also ascertained the capacity of Pep-1 to stain deparaffinized skin. As shown in Figure 7B Pep-1, but not the scrambled peptide control, bound throughout the epidermis but diffusely in the dermis. However, the intensity of Pep-1 staining was less than for cryostat sections. Again, Pep-1 binding was reduced after pretreatment with Streptomyces hyaluronidase showing HA specificity.
Localization of HA in skin and other tissues is conventionally determined using HABP as the standard probe. A number of studies using HABP have been reported for normal skin as well as skin from patients with local or systemic diseases. As discussed in the introduction, significant variations in HABP staining patterns even in normal skin have been reported. In an effort to develop alternative HA probes for histology, we have compared the staining profiles of HABP and Pep-1 in human and mouse skin. Perhaps the most striking finding was that HABP uniformly stained the dermis with high intensity, whereas Pep-1 showed diffuse dermal staining with relatively weak intensity after acetone fixation. In contrast, fixation of tissues with paraformaldehyde/CPC resulted in intense dermal staining for both HABP and Pep-1. How can we explain the discordant staining profiles observed in these studies? One possible reason for differences in dermal staining between HABP and Pep-1 after acetone fixation is that Pep-1 may have a markedly slower diffusion rate in the dermal compartment compared with HABP. However, given the relatively small size of Pep-1 compared with HABP, one would expect Pep-1 to more easily diffuse into the tissue for staining, not vice versa.
Recent biophysical studies have shown that HA adopts distinct conformations based on polymer length (Scott and Heatley 2002
Can differences in HA conformations explain why Pep-1 stains the dermis after paraformaldehyde/CPC fixation? Recently, Cowman et al. (2005) In terms of utility, Pep-1 may allow expression of epidermal HA to be studied in a number of pathologic states in fine detail after acetone fixation. Pep-1 may be of special value for studying epidermal HA expression in experimental conditions in mice, where the thin epidermis can be especially difficult to distinguish from the underlying dermis. Lastly, comparisons of Pep-1 and HABP staining profiles in conjunction with biophysical and biochemical analyses may shed new light on the relationship between HA structure and function in situ. Based on our experimental observations, we suggest that Pep-1 may represent a unique reagent to probe HA conformation.
Finally, we should emphasize that the synthetic nature of Pep-1 may represent an advantage over HABP, which is derived from a complex biological material (i.e., cartilage) (Tengblad 1979
This work was supported by National Institutes of Health Grant R01 AR-48840. We would like to thank Patricia Adcock for her secretarial assistance and Dr. Akira Takashima for his thoughtful comments and suggestions.
Received for publication July 30, 2004; accepted January 20, 2005
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