Mapping Trends and Identifying Hotspots in Research on Mesenchymal Stem Cells in Cardiovascular Disease

Background: Mesenchymal stem cells (MSCs) have important research value and broad application prospects in the cardiovascular disease. This study in order to provide information on the latest progress, evolutionary path, frontier research hotspots and future research developmental trends in this eld. Methods: A knowledge map was generated by CiteSpace and VOSviewer analysis software based on data obtained from the literature on MSCs in the cardiovascular eld. Results: The USA and China ranked at the top in terms of the percentage of articles, accounting for 34.306% and 28.550%, respectively. The institution with the highest number of research publications in this eld was the University of Miami, followed by the Chinese Academy of Medical Sciences and Harvard University. The research institution with the highest ACI value was Harvard University, followed by the Mayo Clinic and the University of Cincinnati. The top three subjects in terms of the number of published articles were cell biology, cardiovascular system cardiology and research experimental medicine. The journal with the most publications in this eld was Circulation Research, followed by Scientic Reports and Biomaterials. The direction of research on MSCs in the cardiovascular system was divided into four parts: (1) tissue engineering and gene and material research, (2) cell transplantation and signal transduction pathway research, (3) assessment of the ecacy of the use of stem cells from different sources in the treatment of acute myocardial infarction, and (4) myocardial hypertrophy, heart failure and myocardial infarction regeneration and repair research. Tissue research is the hotspot and frontier in this eld. Conclusion: MSC research has presented a gradual upward trend in the cardiovascular eld. Multidisciplinary intersection is a characteristic of this eld. Engineering and materials disciplines are particularly valued and have received attention from researchers. The progress in multidisciplinary research will provide motivation and technical support for the development of this eld. engineering and gene and materials research; cell transplantation and signal transduction pathway research; assessment of the ecacy of stem cells from different sources in the treatment of acute myocardial infarction, myocardial hypertrophy and heart failure; research of myocardial infarction and regenerative repair. the long-term fate mesenchymal


Introduction
Mesenchymal stem cells (MSCs) are widely perceived as a class of adult pluripotent stem cells with multiple differentiation potentials, which derived from mesoderm and neuroectoderm and do not express hematopoietic-related markers [1]. The important biological characteristics of MSCs include its low level of expression of human leukocyte antigen class I molecules and CD40, CD40 ligand, CD80 or CD86, which is required to induce effector T cells [2]. MSCs have low immunogenicity and immunoregulatory effects, which can affect every cell of the immune system through cell-cell interactions and paracrine effects [3]. Based on these special biological characteristics, MSCs have important research value and broad application prospects.
Recent studies have shown that the use of MSCs has made great progress in cardiovascular basic and clinical research. MSCs induce the differentiation of cardiomyocytes and vascular endothelial cells. Bonnet found that BMSCs induced in vitro can express platelet-derived growth factor receptor (PDGFR), smooth muscle myosin heavy chain 11 (SMMHC11) and myoglobulin light chain 2 (MLC2), which is similar to that observed in aortic smooth muscle cells, and BMSCs have similar electrophysiological activity and contraction ability [4]. MSCs inhibit myocardial brosis; for example, MSC exosomes reduce brosis of the heart by inhibiting the proliferation of broblasts, promoting the synthesis of metalloproteinases, and stimulating angiogenesis in the infarct area [5]. MSCs also promote angiogenesis. Rahbarghazi transplanted MSCs into infarcted myocardium in rabbits and found that the surrounding area of infarcted myocardium mainly differentiated into cardiomyocytes, endothelial cells and smooth muscle cells, and the microvessel density signi cantly increased [6]. MSCs can effectively treat myocardial infarction, dilated cardiomyopathy, heart failure and other conditions. For example, Lee et al proved that intravenous injection of bone marrow MSCs was safe, mild, effective, and had a long-lasting effect [7]. Chin et al con rmed that autologous bone marrow-derived MSCs were safe and effective in treating DCM [8]. Bartunek et al found that MSC therapy did not produce myocardial toxicity, which signi cantly increased the left ventricular ejection fraction, reduced the end-systolic volume, and increased the 6-minute walking distance of HF patients [9].
Bibliometrics uses the literature system and bibliometric characteristics as the research object and conducts quantitative and qualitative analysis of the literature [10]. It allows the quantitative measurement of the pro le distribution as well as the relationships and clustering of studies [11]. In addition to describing and predicting the development of a particular research area, this type of analysis can also compare the contributions of different countries, institutions, journals, and scholars [12]. This type of analysis technology is playing an increasingly important role in developing guidelines and evaluating research trends [13]. Many scholars have used this method of literature analysis in various elds of medicine, such as spinal surgery research [14], health information research [15], biological signaling molecule research [16], neurogenetics research [17] and endocrine disease research [18].
This research is based on data regarding the literature on MSCs in the cardiovascular eld which uses CiteSpace and VOSviewer to form a corresponding knowledge map and recognize the knowledge base. The study provides information on the latest progress, evolutionary path, frontier research hotspots and future research developmental trends in this eld.

Data collection
SCI-E and SSCI of the core database of the document information index database Web of Science were selected as the target databases for source document retrieval. The search formula was set to TS = ("cardiovascular" OR "heart" OR "circulation") AND TS = (mesenchymal stem cells), and the dates of the search were from 2010-01-01 to 2020-03-31, which resulted in a total of 3455 records. There were 8 types of documents among the 3455 records. As shown in Table 1, there were 2380 articles, which accounted for 72.187% of the total number of records, making articles the most common type of literature. Reviews ranked second, as there were 755 reviews, accounting for 22.900% of the total. The other 8 document types were meeting abstracts (98), editorial materials (55), book chapters (35), proceedings papers (32), early access (20), letters (5), corrections (3) and news items (1). VOSviewer and CiteSpace were used to analyze the 3384 exported articles. VOSviewer constructs a map based on the cooccurrence matrix. The construction of the map is a three-step process. In the rst step, the similarity matrix is calculated based on the cooccurrence matrix. In the second step, the VOS mapping technique is applied to the similarity matrix to construct a map. Finally, in the third step, the map is translated, rotated and re ected [19]. The term cooccurrence graph in VOSviewer only includes terms that appear in the title and are abstracted at least 50 times under the binary count [20]. The purpose of the algorithm is to ensure that terms that occur more frequently have larger bubble images and that terms with high similarity are close to each other [21]. CiteSpace is a web-based Java application that focuses on detecting and analyzing the evolution of research frontiers and the relationship between research frontiers and their knowledge base.
CiteSpace also examines the internal connections between different research frontiers [22]. It is used to capture keywords associated with strong citation bursts, which can be used as predictors of research frontiers.

Country/Region distribution
As shown in Table 2 As shown in Figure 2, Countries with close cooperation can be mainly divided into two types. The USA and China showed the greatest cooperation with South Korea, Japan, Canada and Australia. The Germany and Italy worked more closely with England, Netherlands, France, Switzerland and Spain.

Distribution of authors and research institutions
As shown in   As shown in Table 4  As shown in Figure 4, the University of Miami cooperated closely with Harvard University, Pittsburgh University and Zhejiang University. Sun Yat-sen University cooperated closely with Fudan University.

Distribution of disciplines in the literature
As shown in Table 5, the top three disciplines in terms of the number of published articles were cell biology (30.664%), cardiovascular system cardiology (20.534%) and research experimental medicine (20.140%).
Additional disciplines represented in the literature were engineering (9.827%), materials science (9.160%), biochemistry and molecular biology (8.280%), biotechnology and applied microbiology (7.158%), pharmacology and pharmacy (6.946%), transplantation (4.974%), chemistry (3.063%) and other disciplines, indicating that the research performed in this eld was broad and that the research methods were diverse.   The POSEIDON Randomized Trial". In this article, Hare et al con rmed that intracardiac injection of allogeneic and autologous MSCs could treat ischemic cardiomyopathy effectively and relatively safely [24].
The third most cited article was "Whole-Organ Tissue Engineering: Decellularization and Recellularization of Three-Dimensional Matrix". In this article, Badylak explained that the combination of three-dimensional bioscaffold materials and cell transplantation was a promising tissue engineering strategy and a method for the regeneration of functional organs for medical replacement [25]. The above articles could be regarded as constituting an important theoretical basis and providing clinical evidence for research in this eld.
According to the types of the articles, 8 of the most highly cited articles were reviews, and 7 were monographs.
Based on the publication dates of the most highly cited articles, the most highly cited articles were published from 2011 to 2013, followed by 2015 to 2016. These periods can be regarded as representing the two stages of the development of this eld. Based on the numbers of cooperating institutions and countries, there were 10 articles involving more than three institutions and 6 articles involving groups in at least two countries.

Research hotspot analysis
Keywords re ect the core content of the article and can be used to identify the evolving research frontiers related to the knowledge eld [26]. As shown in Table 8, in addition to mesenchymal stem cells and heart, the keywords with a high frequency of occurrence were heart transplantation (582), differentiation (535), myocardial infarction (482), in vitro (473), therapy (472) and progenitor cells (458). As shown in Figure 5, in the keyword cooccurrence network map, the thicker the connection between the nodes is, the more frequently the two keywords appear together. The keywords formed 4 clusters, which represented the four major research directions in the eld. Stromal cells, tissue, tissue engineering, regenerative medicine, gene expression, scaffolds and extracellular matrix were the key words that had a high correlation with MSCs, as shown in the blue cluster. This indicates that this clustering was predominated by tissue engineering, genes and material research. The yellow cluster was associated with transplantation, expression, therapy and differentiation in the main body and mainly explored the pathways of cell transplantation and signal transduction. The green cluster was mainly associated with progenitor cells, endothelial progenitor cells and acute myocardial infarction and mostly explored the e cacy of stem cells from different sources in the treatment of acute myocardial infarction, cardiac hypertrophy and heart failure. The red cluster was associated with myocardial infarction, repair and regeneration, which indicated that myocardial infarction, regeneration and repair were the main research topics.

Combined evolutionary path
In Figure 6, the year corresponding to each of the keywords is the rst year it appeared in the analyzed data

Research frontier identi cation
In Table 9, the timeline is depicted as a blue line, while burst detection is shown as a red segment on the blue timeline that indicates the start year, end year, and duration of the burst. In particular, we are interested in the key words with research signi cance, which re ect the evolutionary trend of this eld. Endothelial progenitor cells showed the strongest burst strength, followed by ischemic cardiomyopathy, cardiovascular disease and endothelial cells. The terms progenitor cells, marrow stromal cells, and heart function appeared for the rst time recently but persisted for a shorter period of time. The burst times of the terms delivery, ischemic heart and ischemic cardiomyopathy were consistent. Tissue is the current research frontier in this eld and is currently within the burst period.

Discussion
This (1) The research of MSCs in the cardiovascular eld showed a zigzag upward trend.
Stem cell therapy has great potential for use in future regenerative medicine treatment; however, it has some risks and limitations, such as the type of stem cells used, cell proliferation ability, differentiation status, drug delivery route, drug delivery site and the ability for survival of transplanted cells, which will affect the therapeutic effect. The therapeutic characteristics, medical ethics and possible teratogenicity have made the study of MSCs highly controversial [27]. For example, Hegyi proposed that MSCs might form primary cardiac sarcomas and develop into tumors with multiple lineage differentiation [28]. Huang reported that transplantation and differentiation of MSCs led to progressive ventricular dysfunction and other diseases [29].
In view of the risks of research and the limitations of technology, researchers are cautious in carrying out the research work.
(2) The study of cardiovascular MSCs in Europe and America started rst, and Asian countries have paid increasing attention to this area of research.
European and American countries started research in this eld earlier than other countries, and their research is therefore more mature. For example, Harvard University, the Mayo Clinic and University of Cincinnati in the United States have obtained a large number of high-quality research results. Harvard University has mainly studied 3D microcapsules [30] and engineered three-layer scaffolds [31]. The Mayo Clinic has explored the effect of MSCs on left ventricular assist device (LVAD) implantation [32], left ventricular remodeling [33] and heart failure [34] through clinical trials while developing micro ber stents [35] and vascular biomaterials [36].
The paracrine effect of nuclear casein kinase on MSC and MSC-derived extracellular vesicles (EVs) has been studied [37]. The University of Cincinnati explored the cardiac protection mechanism of paracrine MSCs, which involves iPS cells (MiPS) [38], the Wnt11 signaling pathway [39], CXCR4 factor expression [40], the suicide gene [41] and the clusterin Akt/GATA-4 pathway [42].  [43]. The mechanism by which statins and Chinese medicines regulate SDF-1/CXCR4 [44], JAK-STAT [45], RhoA/ROCK [46] and AMPK/eNOS [47] in MSCs has been studied. The Chinese Academy of Medical Sciences also explored the improvement of cardiac function by MSCs regulated by matrix-derived factor 1 (SDF1a) [48] and CXC chemokine receptor 4 (CXCR4) [49]. Sun Yat-sen University researched the effects of exons (Exo) [50], the TGF-β superfamily [51], the long noncoding RNA brave heart (lncRNA-Bvht) [52], apelin [53] and granulocyte colony stimulating factor (G-CSF) [54] on the proliferation, differentiation and vascularization of MSCs. The functions of the transcription factors islet-1 (ISL1) [55] and platelet brin (PRF) [56] in regulating the repair of myocardial infarction by MSCs have been explored. Although basic research on cardiovascular stem cells in China is at the forefront of efforts around the world, progress on clinical studies in China is stagnant, which may be related to the lack of e cient scienti c approval systems and strict regulatory policies.
(3) Multidisciplinary intersection provides power and technical support for the development of this eld.
The literature published in this eld is mainly focused on cell biology, cardiovascular system cardiology and research experimental medicine, as well as engineering, materials science, chemistry, biophysics and other disciplines, which re ects that multidisciplinary intersection is a characteristic of research in this eld.
Engineering and materials disciplines, such as bioengineering [57], tissue engineering [58], genetic engineering [59][60] and biomaterials [61], have received special attention from scholars. The development of these related disciplines will aid in breaking through the limitations of the technical conditions of research in this eld. Heart failure caused by ischemic and nonischemic cardiomyopathy is due to the progressive and complex process of myocardial remodeling. Local compensatory changes at the genetic, molecular, cellular and interstitial levels are accompanied by ventricular dilatation and the impairment of systolic and diastolic function. The consequence is that billions of cardiomyocytes replaced by brous tissue, and the cardiomyocytes and vascular cells are severely injured [62].
Although it has been con rmed that the adult heart contains a small number of active circulating cells, resident stem cells and progenitor cells [63], its inherent ability for self-regeneration is unable to compensate for the loss in cell quality due to heart failure. The need to replace heart cells has aroused great interest in regenerative medicine, especially in the prevention and treatment of heart failure by stem cell transplantation.
(5) Tissue research is a hot spot and frontier area in this eld.
Tissue research refers to tissue engineering and tissue repair. Tissue engineering is an emerging technology that uses cells, engineering methods, materials, and appropriate combinations of biochemical and physicochemical factors to improve or replace biological functions. It aims to reconstruct damaged or diseased organs and tissues in vitro and to transplant tissues in vivo to repair lost or malfunctioning organs or tissues [64]. The ability of the heart to compensate for the loss of functional cardiomyocytes plays a key role in the treatment of heart disease [65]. Therefore, one of the future challenges of cardiovascular therapy is to develop strategies for the regeneration of myocardial contractile function, which can be used for tissue engineering, cell therapy, and reprogramming of scar broblasts [66][67].

Conclusion
MSCs

Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Ethics approval and consent to participate Not applicable Consent for publication Not applicable  Map of keyword clustering