Skeletal Muscle Expression of the Adhesion-GPCR CD97: CD97 Deletion Induces an Abnormal Structure of the Sarcoplasmatic Reticulum but Does Not Impair Skeletal Muscle Function

Abstract

CD97 is a widely expressed adhesion class G-protein-coupled receptor (aGPCR). Here, we investigated the presence of CD97 in normal and malignant human skeletal muscle as well as the ultrastructural and functional consequences of CD97 deficiency in mice. In normal human skeletal muscle, CD97 was expressed at the peripheral sarcolemma of all myofibers, as revealed by immunostaining of tissue sections and surface labeling of single myocytes using flow cytometry. In muscle cross-sections, an intracellular polygonal, honeycomb-like CD97-staining pattern, typical for molecules located in the T-tubule or sarcoplasmatic reticulum (SR), was additionally found. CD97 co-localized with SR Ca2+-ATPase (SERCA), a constituent of the longitudinal SR, but not with the receptors for dihydropyridine (DHPR) or ryanodine (RYR), located in the T-tubule and terminal SR, respectively. Intracellular expression of CD97 was higher in slow-twitch compared to most fast-twitch myofibers. In rhabdomyosarcomas, CD97 was strongly upregulated and in part more N-glycosylated compared to normal skeletal muscle. All tumors were strongly CD97-positive, independent of the underlying histological subtype, suggesting high sensitivity of CD97 for this tumor. Ultrastructural analysis of murine skeletal myofibers confirmed the location of CD97 in the SR. CD97 knock-out mice had a dilated SR, resulting in a partial increase in triad diameter yet not affecting the T-tubule, sarcomeric, and mitochondrial structure. Despite these obvious ultrastructural changes, intracellular Ca2+ release from single myofibers, force generation and fatigability of isolated soleus muscles, and wheel-running capacity of mice were not affected by the lack of CD97. We conclude that CD97 is located in the SR and at the peripheral sarcolemma of human and murine skeletal muscle, where its absence affects the structure of the SR without impairing skeletal muscle function.

International Union of Basic and Clinical Pharmacology. XCIV. Adhesion G Protein–Coupled Receptors

“…Recently, ADGRE5 (CD97) was detected not only in the sarcolemma as other Adhesion GPCRs but also in the sarcoplasmatic reticulum of myocytes. Adgre5 (Cd97)-knockout mice showed a dilated sarcoplasmatic reticulum; yet, despite this severe ultrastructural alteration, the mice had no overt skeletal muscle phenotype (Zyryanova et al, 2014), which is similar to as Adgrb1 (Bai1)-and Adgrg1 (Gpr56)-deficient mice (Hochreiter-Hufford et al, 2013;Wu et al, 2013) and may indicate compensatory mechanisms.…”

“…Human ADGREs (EGF-TM7 receptors) are useful markers for granulocytes, with ADGRE1 (EMR1) found on eosinophils and ADGRE3 (EMR3) on mature polymorphogenic granulocytes Matmati et al, 2007;Legrand et al, 2014). In contrast, ADGRE5 (CD97) is not restricted to myeloid cells but also found on lymphoid, epithelial, muscle, and other cell types (Eichler et al, 1994;Jaspars et al, 2001;Aust et al, 2006;Veninga et al, 2008;Zyryanova et al, 2014).…”

“…ADGRE5 (CD97) was the first Adhesion GPCR identified to be involved in cancer (Aust et al, 1997). Widely distributed in normal cells, ADGRE5 (CD97) is regularly induced, upregulated, or post-translationally modified in corresponding malignancies (Aust et al, 1997Steinert et al, 2002;Wobus et al, 2004;Ward et al, 2012;Wu et al, 2012;Mirkowska et al, 2013;Safaee et al, 2013;Zyryanova et al, 2014). In thyroid cancer, ADGRE5 (CD97) expression levels correlate with dedifferentiation (Aust et al, 1997;Ward et al, 2012), and in colorectal cancer, this Adhesion GPCR is overexpressed at the invasion front Galle et al, 2006).…”

The Adhesion GPCR CD97/ADGRE5 inhibits apoptosis

“…The frequent induction, upregulation, and/or biochemical modification of CD97 in tumor cells, compared to the corresponding normal cells (Aust et al, 1997(Aust et al, , 2002(Aust et al, , 2006Steinert et al, 2002;Wobus et al, 2004;Coustan-Smith et al, 2011;Bonardi et al, 2013;Mirkowska et al, 2013;Ward et al, 2013;Zyryanova et al, 2014), suggests that this Adhesion GPCR is involved in basic cellular and molecular processes during tumorigenesis. Here, we demonstrated that CD97 overexpression inhibits intrinsic apoptosis, mediated by serum starvation or staurosporine, as well as extrinsic apoptosis, induced by TNF/CHX treatment.…”

“…In the corresponding malignant tumors, CD97 is found to be induced or upregulated (Aust et al, 1997(Aust et al, , 2002(Aust et al, , 2006Steinert et al, 2002;Wobus et al, 2004;Ward et al, 2013). In leio-and rhabdomyosarcoma, CD97 is in part N-glycosylated, whereas normal muscle cells express the "naked" CD97 protein core (Aust et al, 2006;Zyryanova et al, 2014), indicating tumor-specific post-translational modification of the receptor. Expression levels of CD97 correlate with dedifferentiation in thyroid carcinoma (Aust et al, 1997;Ward et al, 2013).…”