Diabetes mellitus (DM) related microangiopathy that attacks the human kidney is the leading cause of chronic kidney disease (CKD) worldwide and will result in diabetic kidney disease (DKD) (1) and even end-stage kidney disease (ESKD) requiring renal dialysis or transplantation, thereby causing severe social and family burden (2). It is reported that approximately 30–40% of DM patients will eventually progress to DKD during their disease course (3). With the increasing incidence of DM worldwide, DKD-affected patients are also elevated (4). In China, significantly increased prevalence and incidence of DKD are observed, and it is estimated that the number of DM related DKD is about 24.3 million (5). Given the large number of patients, precise screening of high-risk patients with DM will be critical. Moreover, although proteinuria could be used effectively for monitoring DKD development and progression, its relatively late time window might hinder effective therapeutic intervention. In a hospital laboratory setting, blood-based testing, which requires minimal invasiveness, could be a better candidate for biomarker selection. In our previous studies, we obtained extracellular vesicles (EVs) from the plasma of DM and DKD patients and identified that EV-derived mRNA (AEBP1) (6) and miRNAs (miRNA-615-3p and miRNA-3147) (7) could differentiate DKD from DM and were involved in disease progression. However, EV isolation requires a commercial kit and a high-speed centrifugation device; it is necessary to discover novel biomarkers inexpensively. Genes (including mRNAs and miRNAs) and protein expression of blood-based biomarkers have been reported for DKD differential diagnosis; however, most of the current studies use the blood-derived nucleated cells, such as lymphocytes, monocytes, and neutrophils, for expression level evaluation. Less attention has been paid to the platelets, another part of blood composition, and a kind of anucleate cells. Compared with the time-consuming Ficoll reagent-based density gradient centrifugation and the expensive magnetic beads required for the isolation of blood-derived nucleated cells, platelet isolation is simple, high-yielding, and inexpensive, requiring only an inexpensive low-speed centrifugation device. Moreover, platelets have been implicated in several human diseases, including cancer (8), cardiovascular disease (9), and diabetes (10). Recently, increased evidence has shown that platelets play a role in the onset and progression of DKD (11). Multiple platelet-derived proteins are elevated in DKD and promote inflammation and fibrosis (12–15). Then the problem is to determine which genes or proteins to examine to obtain the diagnostic or high-risk differentiation information for DKD. With the development of high-throughput sequencing and protein mass technology, information on platelet-related genes and proteins can be easily obtained from multiple databases. Xie et al. (8) established a database called Platelet Expression Atlas (PEA) and described all the available platelet data from patients with different kinds of diseases, among which DM is included. In current clinical practice, renal biopsy, the gold standard for DKD diagnosis, is an invasive method for tissue. Although blood is a mixture composed of nucleated (white blood cells) and anucleate cells (red blood cells and platelets), platelet isolation can be achieved directly by simple centrifugation. Therefore, it is obvious that a platelet sample could be easily obtained (9). Moreover, previous studies (10, 11) indicate that platelets contributed to the development and progression of DM and renal disease. Building on our prior experience with miRNAs, we proposed evaluating the role of platelet-derived miRNAs in the diagnosis and treatment of DKD.

MicroRNAs (miRNAs) are a class of small non-coding RNAs that regulate gene expression by degrading or inhibiting the translation of their target messenger RNAs (14, 15). Since miRNA is a key regulator of cell homeostasis (16, 17), its dysfunction is an important part of cell and organ damage (18, 19). In DKD, miRNAs from different sources have been widely studied for their roles in the pathogenesis of DKD and serve as potential biomarkers for disease prediction (20). Peng et al. (21) found that urine-free miRNA-29a was significantly increased in DKD patients and was positively correlated with the degree of urinary albumin, which may be an alternative biomarker for DKD. Jia et al. (22) found that urinary exosome-derived miR-192 levels increased in the early DKD microalbuminuria group, suggesting that miR-192 may be a non-invasive tool for detecting early DKD in patients with T2DM. The results of a bioinformatic analysis showed that six miRNAs, including miR-21-5p, miR-29a-3p, etc., are involved in apoptosis, fibrosis, extracellular matrix accumulation, and other pathways associated with DKD pathogenesis and can be used as potential therapeutic targets against the progression of DKD (23). An animal study demonstrated that renal tissue-derived miRNA-337 increased expression in mice with DKD and caused podocyte damage by upregulating IL-6 and IL-18 levels (24). Obvious, recent advances in miRNA research have not only deepened understanding of the pathophysiological complexities of kidney disease, but also laid the foundation for the discovery of new diagnostic biomarkers (25), such as miR-3137, miR-4270 (26), miR-21, and miR-495 (27). However, until now, the miRNAs associated with DKD were basically derived from urinary exosomes, although several platelet-derived miRNAs have been shown to be early biomarkers of type 2 diabetes (T2DM) (28, 29), many platelet-derived miRNAs have been confirmed to be associated with diabetes (30–32), few studies have investigated whether platelet-derived miRNAs can be further used to evaluate the progression of DKD. Nührenberg et al. (33) performed miRNA analyses from the platelets of DM and controls, thereby providing the basis of our study.

miR-145-5p has been found to be associated with human cancers, regulating the proliferation of cancer cells, such as non-small cell lung cancer, colorectal adenocarcinoma, prostate cancer, and glioma (34–37). Urinary exosome-derived miR-145-5p was found to promote podocyte apoptosis by inhibiting Srgap2 and then activating RhoA/ROCK signaling pathway, suggesting that exosome miR-145-5p may play a role in the pathological process of DKD and serve as a non-invasive biomarker of DKD (38). Moreover, another study found that the target genes of miR-145-5p were located in the cytoskeleton and exosomes and were involved in 11 signaling pathways, including the mitogen-activated protein kinase (MAPK) pathway, transforming growth factor-beta (TGF-β) pathway, Forkhead Box O (FOXO) pathway, Ras pathway, and advanced glycation end-products (AGE)–receptor for AGE system (AGE–RAGE) pathway in diabetic complications (39). Among these pathways, TGF-β and MAPK signaling are considered to be the core of renal fibrosis in the late stage of DKD (40), which, in turn, suggests the possible role of miR-145-5p in renal fibrosis. Few studies have reported miR-6805-3p. Wang et al. showed that cancer cell-derived exosome contains miR-6805-3p, which had the ability to increase the expression of MAPK1, thereby promoting cervical cancer angiogenesis and tumor growth (41). A previous study (38) using urinary EVs also found that miR-145-5p was an upregulated pattern in DKD patients, which is consistent with our study. In this study, we found that platelet-derived miR-145-5p and miR-6805-3p were upregulated in DKD patients compared with DM and healthy control subjects, indicating a distinct DKD phenotype among DM patients. Moreover, we found that increased levels of platelet-derived miR-6805-3p were associated with DKD staging differentiation, suggesting its involvement in DM to DKD progression. miR-6805-3p was significantly higher than that in the N and DM groups, which was consistent with previous findings. There were some limitations in this study. First, due to time constraints, the sample size of the present study is relatively small, and all samples were from a single center. Second, the lack of long-term clinical follow-up and dynamic evaluation of miR-145-5p and miR-6805-3p levels fails to analyze the effects of miRNAs on patient prognosis. In addition, the limited availability of experimental tools, such as knock-out mice or kidney tissues, results in a failure to analyze the detailed mechanisms involved by these two miRNAs. In the near future, expanded clinical data with larger sample sizes and more in-depth mechanisms research are needed to verify the conclusions presented here and to explore the molecular mechanisms by which platelet-derived miRNAs affect the pathogenesis of DKD.

Platelet-derived miR-145-5p and miR-6805-3p may be novel non-invasive biomarkers for the diagnosis of DKD. Monitoring of miR-145-5p and miR-6805-3p may contribute to intervention and delay in the progression of DKD, based on their advantages, including simplicity, rapidity, and efficacy. In the near future, multicenter long-term longitudinal follow-up studies are needed to clarify their role in monitoring the condition and prognosis of DKD.