Volume 52 (7): 967-977, 2004 Copyright ©The Histochemical Society, Inc. Corticotropin-releasing Factor 2 Receptor Localization in Skeletal Muscle
Research Division, Procter & Gamble Pharmaceuticals, Mason, Ohio (SS,JSL,RTH,RJI), and Department of Pediatrics and Medicine, McMaster University, Hamilton, Ontario, Canada (MT) Correspondence to: Dr. Robert J. Isfort, Research Division, Procter & Gamble Pharmaceuticals, Health Care Research Center, 8700 MasonMontgomery Road, Mason, OH 45040-9317. E-mail: isfort.rj{at}pg.com
Our objective in this study was to localize the corticotropin-releasing factor 2 receptor (CRF2R) in rodent and human skeletal muscle. We found CRF2R protein to be abundant in neural tissues in skeletal muscle, including large nerve fibers and bundles, neural tissue associated with mechanoreceptors, muscle spindles, and the Golgi tendon organ. CRF2R protein was also abundant in blood vessels in skeletal muscle. CRF2R protein was also observed, although with less abundance, in the endo/perimysial regions in skeletal muscle. The localization of the CRF2R to blood vessels is consistent with the CRF2R-mediated vascular phenomena observed previously, but the observation of CRF2R in neural tissue in skeletal muscle is a novel finding with an unknown function. (J Histochem Cytochem 52:967977, 2004)
Key Words: corticotropin-releasing factor receptor skeletal muscle neural tissues
TWO CORTICOTROPIN-RELEASING FACTOR RECEPTORS have been identified, the corticotropin-releasing factor 1 receptor (CRF1R) and the corticotropin-releasing factor 2 receptor (CRF2R), both of which belong to G protein coupled, seven transmembrane receptor family (Dautzenberg and Hauger 2002
Recently we have discovered that activation of the CRF2R modulates skeletal muscle mass under physiological and pathological conditions (Hinkle et al. 2003
Six-week-old male and female SpragueDawley rats (Charles River; Raleigh, NC) and C57Bl6 mice (Charles River) were housed singly and acclimatized to the conditions of the facility for 1 week before use. Mice and rats had access to lab chow and water ad libitum. Animals were subjected to standard conditions of humidity, temperature, and a 12-hr light cycle. All studies described in this report were conducted in compliance with the US Animal Welfare Act and rules and regulations of the State of Ohio Departments of Health, and were approved by the local IUCAC. Rat and mouse lower leg muscles (tibialis anterior and medial gastrocnemius muscles) were obtained by removing the indicated muscles and immediately placing the muscles in optimal cutting temperature (OCT) compound (Sakura Finetek; Torrance, CA). Tissues in OCT compound were frozen in liquid nitrogen-cooled isopentane and held at 70C. Human vastus lateralis muscle biopsies from healthy young adults, frozen in OCT, were obtained from Dr. Mark Tarnopolsky (McMaster University; Hamilton, Ontario, Canada). Human muscle biopsies were obtained under the approval from the McMaster University Ethics Committee. Two anti-CRF2R antibodies were obtained from Santa Cruz Biotechnology (Santa Cruz, CA) and were used throughout this study. The CRFR (C-20, sc-1757) antibody reacts with both CRFR1 and CRFR2, while the CRFR2 (N-20, sc-1826) antibody is specific for CRFR2. Anti-CRFR1 and -CRFR2 (RDI-CRFR1CabG, RDI-RTCRFR2NabG) antibodies (Research Diagnostics; Flanders, NJ) were used to validate results obtained with the Santa Cruz products. The specificity of each CRFR antibody was established by running preadsorption controls. Each antibody was preincubated for 4 hr with a 5000-fold molar excess of its corresponding peptide (sc-1757P for C20 and sc-1826P for N20) or an irrelevant peptide. The antigenantibody mixture was centrifuged at 100,000 x g for 30 min with the supernatant used in place of primary antibody in the normal immunocytochemistry protocol. Preadsorbed samples were processed in parallel with normal CRFR antiserum to optimize comparative staining. Immunocytochemical analysis was performed on 7-µm frozen sections by indirect labeling, as follows. Cryostat sections were attached, the slides were warmed to room temperature, sections were encircled with a PAP pen, and were hydrated in PBS. Sections were permeabilized in PBS/0.1% Triton X-100 for 30 min, aspirated, and covered with blocking buffer of 10% normal donkey serum (NDS) in BSA buffer [2% bovine serum albumin (BSA)/0.1 M phosphate buffer, pH 7.4] for 1 hr at room temperature (RT). Primary antibodies were diluted in BSA buffer, and sections were incubated directly, after aspiration of blocking solution, for 1 hr at RT. Slides were washed three times for 10 min in PBS, then incubated with fluorescein-conjugated donkey secondary antibodies (Jackson Immunoresearch Labs; West Grove, PA) at 5 µg/ml in BSA buffer with 0.5 µM diamidinophenylindole dihydrochloride (DAPI; Molecular Probes, Eugene, OR) for 1 hr at RT. Sections were washed twice for 10 min in PBS with a final wash in 0.1 M Tris buffer, pH 8.5. Sections were coverslipped under PPD glycerol (10% 1 M Tris, 90% glycerol, pH 8.5, containing 1 mg/ml p-phenylenediamine), sealed with nailpolish, and stored at 20C. Tissue sections were examined with a Nikon Microphot FXA (Melville, NY). Images were captured with a Spot II digital camera (Diagnostic Instruments; Sterling Heights, MI) and processed using Metamorph 4.6 imaging software (Universal Imaging; Downingtown, PA).
Co-localization analyses were performed with the following landmark antibodies and filter sets appropriate for fluorescein and Cy-3:
CRF2R localization in rat, mouse and human skeletal muscle was performed using two antibodies, one that was CRFR-specific with equivalent affinity for both CRF1R and CRF2R (C20) and a second that was specific for CRF2R only. Staining was considered positive if both antibodies stained the same structures. Both of these antibodies were crossreactive with mouse, rat, and human CRF2R, allowing cross-species comparison. In addition, we attempted to localize the CRF2R to specific substructures in skeletal muscle using antibodies specific for proteins found in neurons (neurofilament and PGP9.5) (Krikler et al. 1992 -smooth muscle actin) (Nora et al. 1998 -bungarotoxin (Molecular Probes) to co-localize CRF2R staining to the neuromuscular junction. With these tools, we observed CRF2R protein staining in large nerve bundles and intrafusal fibers of muscle, where it co-localized with neuronal markers PGP9.5 and neurofilament protein staining (Figures 1, 2, and 3)
. At neuromuscular junctions, CRF2R staining co-localized with bungarotoxin (Figure 4)
. Rhodamine-conjugated -bungarotoxin was used to delineate the postsynaptic membrane of the neuromuscular junction, where it binds tightly to the acetylcholine receptors (Blumer et al. 2001
CRF2R protein staining was also observed at the myotendinous junction, where it is co-expressed with nestin and tubulin (Figure 5) . CRF2R protein staining was enriched on blood vessels; CRF2R staining co-localized with smooth muscle actin staining (Figure 6) . Finally, CRF2R staining was found in the endo- and perimysium surrounding rat, mouse, and human muscle fibers (Figure 7) . These connective tissue regions contain small arterioles, capillaries, venules, fibroblasts, neural processes, and satellite cells. CRF2R staining in all the human skeletal muscle samples was similar (five males and five females). Because of sampling limitations for human skeletal muscle, we were unable to evaluate CRF2R staining at myotendinous junction, neuronal, and vascular regions (samples were needle biopsies taken from the belly of the vastus lateralis muscle). Finally, C20 and N20 antibody specificity was evaluated by blocking antibody binding with peptide immunogen compared with staining in which the antibody was blocked with an irrelevant peptide. The blocking data were used in conjunction with our control data to establish staining and antibody specificity (Figure 8) .
In this study we showed that the CRF2R in skeletal muscle localized to neural structures, blood vessels, myotendinous junctions, and endomysial/perimysial spaces. Interestingly, expression of the CRF2R in skeletal muscle myocytes was not observed. This raises the possibility that the effects of CRF2R activation on skeletal muscle mass may be an indirect effect, potentially secondary to activation of the CRF2R on either nerves, blood vessels, myotendinous junction, or endo/perimysium. Current understanding of CRF2R effects on skeletal muscle does not provide conclusive evidence about which of these sites of CRF2R expression is critical for the skeletal muscle mass effects of CRF2R activation. However, it can be inferred from the data presented here that the effects of CRF2R activation on skeletal muscle mass are an indirect myocyte effect. One possibility that is consistent with known CRFR biology in other tissues is that activation of the CRF2R results in the release of trophic factors from nerve endings, factors that directly increase skeletal muscle mass. In particular, it is known that members of the CRH family of ligands function as neurohumoral factors (Garzon et al. 1985
Do CRF2Rs present on blood vessels or in the endo/perimysium function to modulate skeletal muscle mass? CRF2Rs present on blood vessels are known to have an important role in the control of blood pressure and vasodilatation (Coste et al. 2000 In conclusion, CRF2R protein has been localized to neural structures, myotendinous junction, blood vessels, and endo/perimysial space in skeletal muscle. CRF2R expression was not observed on myocytes, even though treatment of skeletal muscle in vivo resulted in increased myocyte mass. The mechanism underlying the effects of CRF2R activation on regulating skeletal muscle mass is presently unknown.
1 Present address: Eyetech Pharmaceuticals Inc., Eyetech Research Center, Woburn, MA. Received for publication February 10, 2004; accepted February 24, 2004
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