Journal of Personalized Medicine Review CD24 for Cardiovascular Researchers: A Key Molecule in Cardiac Immunology, Marker of Stem Cells and Target for Drug Development Eyal Sagiv * and Michael A. Portman Division of Pediatric Cardiology, Seattle Children’s Hospital and the University of Washington School of Medicine, Seattle, WA 98105, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-206-987-6916; Fax: +1-206-987-3839 Abstract: The study of the membrane protein, CD24, and its emerging role in major disease processes, has made a huge leap forward in the past two decades. It appears to have various key roles in oncogenesis, tumor progression and metastasis, stem cell maintenance and immune modulation. First described in the 1980s as the homologous human protein to the mouse HSA (Heat Stable Anti- gen), it was reported as a surface marker in developing hematopoietic cell lines. The later discovery of its overexpression in a large number of human neoplasms, lead cancer researchers to discover its various active roles in critical checkpoints during cancer development and progression. Targeting CD24 in directed drug development showed promising results in cancer treatment. More recently, the chimeric CD24-Fc protein has shown exciting results in clinical trials as a specific modulator of auto-inflammatory syndromes. This report is aimed to summarize the relevant literature on CD24 and tie it together with recent advancements in cardiovascular research. We hypothesize that CD24 is a promising focus of research in the understanding of cardiovascular disease processes and the Citation: Sagiv, E.; Portman, M.A. development of novel biological therapies. CD24 for Cardiovascular Researchers: A Key Molecule in Cardiac Immunology, Keywords: CD24; dilated cardiomyopathy; vasculitis; immune modulator; multisystem inflamma- Marker of Stem Cells and Target for tory syndrome in children (MIS-C) Drug Development. J. Pers. Med. 2021, 11, 260. https://doi.org/ 10.3390/jpm11040260 1. Background Academic Editors: Nadir Arber and CD24 is a membrane protein that is composed of a short peptide backbone with Shiran Shapira no transmembrane domain and is anchored to the membrane by glycosyl-phosphatidyl- inositol (GPI) [1]. It is heavily glycosylated in a pattern that can change between different Received: 15 February 2021 cells, perhaps accounting for its various interactions and roles. The human CD24 gene Accepted: 29 March 2021 Published: 1 April 2021 is located on chromosome 6q21. The initial cloning and mapping of the CD24 gene was challenging due to the presence of homologous areas in the human genome, especially two Publisher’s Note: MDPI stays neutral highly observed sequences in chromosome location 15q21 and Yq11, most likely non-coding with regard to jurisdictional claims in pseudo-genes [2]. The conservation of gene copies in human genome, highly homologous published maps and institutional affil- to its sequences in other species, is suggestive of a functional role with evolutionary 0 iations. significance. The human gene encodes a long mRNA molecule with a relatively long 5 UTR (untranslated region) and a reading frame of 243 base pairs. The 81 amino acids peptide undergoes significant modifications in the endoplasmic reticulum, including cleavage of membrane- directing region and GPI anchor enhancing peptide tail. The mature peptide is a sequence of 31 amino acids of a molecular weight that ranges from 30 to 75 kDa Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. depending on the attached glycans. It is directed through the GPI anchor to be expressed in This article is an open access article lipid-enriched areas that harbor other mucins and signal receptors, known as ‘lipid rafts’ [3]. distributed under the terms and The human CD24 protein was found on the outer cell membrane of multiple cell conditions of the Creative Commons lines [4]. It was initially reported to be expressed in the healthy human in B cell populations Attribution (CC BY) license (https:// during specific phases of maturation and absent on mature plasma cells. It was then shown creativecommons.org/licenses/by/ to be expressed on other immune cells including T cells and dendritic cells, during regula- 4.0/). tory phases of innate immune response [5]. The commercial availability of an anti-CD24 J. Pers. Med. 2021, 11, 260. https://doi.org/10.3390/jpm11040260 https://www.mdpi.com/journal/jpm J. Pers. Med. 2021, 11, 260 2 of 8 monoclonal antibody for tissue staining and the development of microarray expression assays lead to an influx of research work finding CD24 overexpression in various human malignancies [6]. This includes a subset of hematologic malignancies–non-Hodgkin B-cell lymphomas, acute lymphoblastic leukemia (ALL) and high-risk acute myeloid leukemia (AML)–but more abundant in carcinoma tumors, including breast, ovarian, colon, hepato- cellular, prostate, pancreatic, non-small cell lung, bladder and prostate cancers and others. It has no physiologic expression in the healthy adjacent tissues. In the heart, Chen et al. also reported overexpression of CD24 in cardiac adenocarcinoma tissue, but not in the adjacent mucosa [7]. CD24 was found to be expressed at early stages of the malignant transformation, like intestinal polyps [8], and to confer poorer prognosis [9–11]. In vitro and animal models show the significance of CD24 expression in tumor initiation and cell proliferation, drug- resistance, adhesion to vascular endothelium and metastases forma- tion. More recently, CD24 was shown to have an intriguing role in tumor evasion from phagocytosis, hypothesized to act as a ‘don’t eat me signal’ [12]. The functional role in signal pathways and cell communication of a small peptide with no transmembrane domain remains challenging to decipher. Several protein ligands and binding molecules were identified over the years. CD24 can interact with important adhesion molecules, including P-selectin, E-selectin, and L1CAM (CD171) and activate integrins, thus perhaps participating in leukocyte extravasation in inflammation and cancer metastasis. It was more recently shown to bind the immune cell receptors Siglec-5 and Siglec-10 that have an immune- inhibitory role and possibly anti- phagocytic [6]. Intracel- lular pathways were shown to be activated in response to changes in CD24 expression, directly or indirectly, including CXCR-4, Src or PI3K/Akt kinase proto-oncogenes. Di- rected biological therapies that block CD24 are thus studied as promising novel cancer treatment modalities, using monoclonal antibodies, chimeric molecules or siRNA [13–16]. We would suggest that exploring the role of CD24 in cardiovascular disease processes may lead to targeted drug development in our research field as well. 2. CD24 as a Marker for Progenitor Cells and Its Implications for Post-Ischemic Myocardial Regeneration Studied most extensively in the field of cancer research, CD24 is suggested to be a marker for circulating cancer stem cells (CSCs), a small subset of cancer cells that retains differentiation potency and low proliferative profile. CRCs are defined as responsible for tumor initiation, metastases and resistance to most cancer therapies that target cell proliferation. Studying CRCs in breast carcinoma, Al-Hajj et al. suggested that CD44+CD24– identified cells with preserved potency and tumor initiating ability while CD24 expression marks epithelial- to- mesenchymal transition (EMT) [17]. Following their lead, other studies in different cancer types found CD24 expression to be a marker for the CSCs themselves, overexpressed on cells that maintain tumor initiating abilities [18–21]. These findings are of a particular interest to cardiovascular research. The embryonic cellular pathways associated with CSCs are similar to the fingerprint of mesenchymal transition that characterizes cardiac stem cells. CD24+ CSCs were shown to have active Notch pathway signaling and high expression of SOX-2, OCT-4 and NANOG [6,22], accepted descriptors of successful in vitro derivation of cardiac mesenchymal stem cells (MSCs). CD24 overexpression was shown to trigger Notch-1 signaling and induce EMT [23]. Human cardiomyocytes respond to injury by hypertrophy and proliferation of local supportive tissues that lead to scarring rather than regeneration of contractile cells that lack a sufficient ability to proliferate [24]. Pilot studies suggest that direct myocardial injection of MSCs from autologous de-differentiated source may assist in post-ischemic regenera- tion [25]. In children too, using bone-marrow derived MSCs administered intra-coronary to infants with hypoplastic left heart syndrome (HLHS) is trialed with the goal to support recovery and preserve of systolic function in the single right ventricle [26]. Improved and reliable methodologies to identify circulating MSCs or produce them in the lab are studied and new surface markers are in need. In describing a new protocol for deriving MSCs from human embryonic stem cells (hESCs), Lian et al. [27] used gene expression and cell sorting J. Pers. Med. 2021, 11, 260 3 of 8 analysis to prove that CD24 is a reliable marker for pluripotent hESCs. They showed that CD24 expression is eliminated in successful MSC differentiation while continues to be positive on other cell lines that are inaccurately derived from hESCs, and thus it is a valu- able selection marker. Supporting this conclusion, perhaps, Da Sacco et al. [28] identified CD24+ cells in human amniotic fluid to be metanephric progenitor mesenchymal