An "empty bone marrow with depletion in the HSPC number" is one of the characteristics of patients with AA. The current understanding of the disease etiopathology includes cytotoxic T-cell activation and abnormal bone marrow microenvironment4–6,11,12,33. BM-MSC are the key cells of the BM microenvironment that supports HSPC functions by direct cell contact and paracrine release of growth factors and EVs5,6,13–16. We have previously demonstrated that BM-MSC EVs from AA patients hamper the proliferative, colony-forming capacity and enhance apoptosis of HSPC25. However, the exact mechanism by which AA BM-MSC EVs exert their functional effect on HSPC is unclear. Filling this gap, the present study revealed that AA BM-MSC-derived EVs have altered expression of miRNAs that could directly impact genes/pathways involved in HSPC cell proliferation, colony-formation capacity, apoptosis, etc., thus highlighting that the EVs of BM-MSC of AA patients mediate impaired HSPC functions via the transfer of encapsulated miRNAs.
BM-MSC plays a vital role in maintaining hematopoietic homeostasis via the release of EVs, which are critical mediators of cellular communication13,15,16. However, studies investigating the role of BM-MSC EVs in AA are scarce. It has been shown that BM-MSC EVs from AA patients induced apoptosis and reduced the cellular proliferation and colony forming ability of HSPC25. Another study has shown that EVs from healthy BM-MSC potentially ameliorate bone marrow failure symptoms in the murine AA model36. These findings imply that BM-MSC EVs significantly regulate HSPC functions under normal or pathological hematopoiesis conditions. Mechanistically, EVs are shown to impart their functional effect via the transfer of miRNA cargoes to HSPC, thereby modulating the target genes and pathways in HSPC regulating normal or malignant hematopoiesis21,22,24,34–36. These miRNAs are small non-coding RNAs that play a crucial role in shaping the hematopoietic landscape during developmental and adult hematopoiesis37,38. Thus, our hypothesis was that miRNAs carried by the AA BM-MSC EVs have a role in inhibiting the hematopoietic functions of HSPC in the BM niche. An altered expression of miRNAs in T-cell39 and plasma-derived exosomes29 has been reported in AA. However, studies on miRNAs in AA BM-MSC EVs is scarce. Using the NGS based miRNA profiling, the present study demonstrates that EVs from AA BM-MSC have a distinct miRNA profile compared to their normal counterparts. A total of 57 miRNAs, including precursor and mature miRNAs, were significantly deregulated in AA BM-MSC EVs. These results align with the previous studies, which have reported differential expression of miRNAs in the BM-MSC derived EVs from hematopoietic disorders such as AML22, MDS23,36, and multiple myeloma24. Additionally, the EV miRNA expression profile of our study was in alignment with the BM-MSC exosomal miRNA expression profile as reported by Ferguson et al., 201839. However, it was completely different from the miRNA expression profile of plasma-derived exosomes as reported by Giudice et al.,201829. Thus, highlighting that there could be a disparity in miRNA packaging in the EVs from different sources, signifying the presence/existence of a selective mechanisms for intercellular communication by EVs from different sources.
Since miRNAs from BM-MSC derived EVs impart their regulatory role by altering the genes and associated pathways at the post-transcriptional level, as described in hematological malignancies24,35,36. We tried to elucidate the functional role of these AA BM-MSC EVs encapsulated DE miRNAs by pathway enrichment analysis. To date two studies have been conducted to study the differential gene expression in HSPC in AA patients using a single cell genomics approach40,41. These studies demonstrated that HSPC from AA patients have upregulated genes involved in cell death, cytokine signalling, and immune responses and downregulated genes involved in cell cycle and cell differentiation. Further, the HSPC of AA patients had several changes linked to DNA damage and repair41. However, whether EV miRNAs impart these functional defects in HSPC is elusive in AA. Our GSEA analysis of significantly altered miRNAs showed enrichment of apoptosis and DNA damage response pathways. Additionally, pathways analysis showed enrichment of several pathways such as apoptosis, MAPK, PI3K/Akt, mTOR, chemokine receptor signalling, etc., which are crucial for maintaining hematopoietic homeostasis42. Our findings here provide with an insight that miRNAs enriched in AA BM-MSC EVs could potentially dampen HSPC functions in the microenvironment through impediment of these pathways, which is in alignment with our previous finding where we showed that AA BM-MSC EVs promoted apoptosis in healthy HSPC25. Concomitantly, our in-silico analysis of miRNA-mRNA interaction revealed that these EV miRNAs targeted 235 HSPC genes and their pathways analysis showed significant enrichment of pathways involved in cell survival and cell death. Altogether, these findings highlight the involvement of EV miRNAs of AA BM-MSC in the regulation of HSPC through cell survival and apoptotic mechanisms. However, the findings need to be established in different model systems, such as in-vivo and 3D scaffold, to study the in-depth interaction of these cells in the hematopoietic niche43,44.
To further comprehend the complex regulation and functional importance of HSPC genes in AA patients and role of BM-MSC EVs miRNA in their regulation, we performed hub gene analysis to identify the top 10 hub HSPC genes and their associated pathways. The functional analysis of Hub genes showed significant enrichment of pathways linked with cell cycle, survival, and cell death pathways. We made a Venn diagram to identify the pathways that were consistently enriched in dysregulated EV miRNAs, target HSPC and, hub genes. We found eight intersecting pathways: chemokine, MAPK, PI3K-Akt, Ras, mTOR, GABAergic synapse, cholinergic synapses and apoptotic signalling pathways targeted Supplementary Fig.S5 and Table S4. The role of the hub HSPC genes and their intervening pathways has been previously reported to be involved in regulating HSPC via different mechanisms as summarized in Fig. 7 and Supplementary Table S5 however, the role of these genes and associated miRNAs needs experimental validation in AA.
Overall, in continuation of our previous study25, we now explicitly demonstrate that EVs from AA BM-MSC have differential regulation of miRNAs. We further demonstrate that these altered miRNAs can potentially target HSPC genes of AA patients and attenuate normal HSPC functions such as their proliferation, differentiation, and apoptosis by targeting several pathways. However, these miRNA-mRNA regulatory mechanisms need to be established in in-vitro and in-vivo settings to understand their role in AA pathobiology. Also, more studies are warranted on studying the functionality of BM-MSC derived EV-miRNA in a larger patient cohort and in the development of newer therapeutic strategies such as administration of engineered BM-MSC EVs in AA patients for the management of the disease. Nonetheless, our results also paved the way for exploring new signalling pathways such as GABAergic synapses, cholinergic synapses, and mTOR, which can be targeted for rescuing HSPC function in AA patients. Together, these findings provide valuable insight into the fact that the BM-MSC-derived EVs encapsulated miRNAs and their gene regulatory pathways are potentially involved in suppressing HSPC functions in AA. Thus, in the next step it would be important to investigate the pathways targeted by the differentially expressed miRNAs in the BM-MSC EVs of AA patients, which may be potential alternative therapeutic targets in AA.