Differentiation of osteogenic lineage cells is highly organized biological process which underlies in skeletal elements developing, in embryo, bone tissue remodeling and healing in adult. Many signaling networks and transcription factors as well as co-factors gathered MCSs differentiation into mature osteocyte [26]. Numerous research also pointed out that mircoRNAs has crucial function in osteogenesis and bone tissue remodeling targeting the key signaling pathways and transcriptional factors (Tabl. 1). However, different paper show that different miR or set of miRs are driving the hMSCs osteogenic differentiation. On one hand, a different biological model has been used to explain this variety of results, on the other – nature of genes expression regulation by miRs. Indeed, bioinformatic analysis revealed that a single miRNA can regulate up to several hundred target genes and vice versa, the same 3′-UTR of a gene can be targeted by many different miRs [30, 31]. In our opinion, it is depending on miRs or its targets tissue specific expression, hence on cells type.
Table 1
MicroRNAs regulates osteogenic differentiation via transcription factors and signaling networks.
miRNAs
|
Targets
|
Expression level
|
Reference
|
Pro-osteogenic differentiation miRNAs
|
miR-27a
|
PPARγ, GREM1
|
Up
|
Rev [38]
|
miR-21
|
P-Akt, HIF-1α
|
Up
|
Rev [38]
|
miR-217
|
DKK1
|
Up
|
Rev [38]
|
miR-26a
|
Runx2, OC, GSK3β
|
Up
|
Rev [38]
|
miR-148a
|
IGF1
|
Down
|
Rev [38]
|
miR-200b
|
Cx43, VEGF-A
|
Down
|
Rev [38]
|
miR-335-5p
|
DKK1
|
Up
|
Rev [38]
|
miR-92a
|
Smad6 3′-UTR
|
Up
|
Rev [38]
|
miR-9
|
Runx2, ERK
|
Up
|
Rev [38]
|
miR-199b-5p
|
GSK3β
|
Up
|
Rev [38]
|
miR-26a/b
miR-29b
|
CDK6, HDAC4
|
Up
|
[39]
|
miR-1249-5p
|
PDX1/PI3K/Akt
|
Up
|
[40]
|
Anti-osteogenic differentiation miRNAs
|
miR-124
|
Sp7
|
Up
|
Rev [38]
|
miR-23a
|
CXCL12, LRP5, Runx2 3′UTR
|
Up
|
Rev [38]
|
miR-23a-5p
|
MAPK13
|
Up
|
Rev [38]
|
miR-23b
|
Runx2 3′UTR
|
Up
|
Rev [38]
|
miR-214
|
BMP2 3′UTR, FGF, p-JNK, p-p38
|
Up
|
Rev [38]
|
miR-214-5p
|
COL4A1
|
Up
|
Rev [38]
|
miR-144-3p
|
Smad4
|
Up
|
Rev [38]
|
miR-383
|
Satb2
|
Up
|
Rev [38]
|
miR-96
|
Osterix
|
Up
|
Rev [38]
|
miR-183-5p
|
Hmox1
|
Up
|
Rev [38]
|
miR-217
|
Runx 2 3′UTR
|
Up
|
Rev [38]
|
miR-133a-3p
|
MEG3
|
Up
|
Rev [38]
|
miR-10
|
Runx 2 3′UTR
|
Up
|
Rev [38]
|
miR-503-5p
|
Runx2, ALP
|
Up
|
Rev [38]
|
miR-205
|
SATB2, Runx2
|
Up
|
Rev [38]
|
miR-141, miR-200a
|
SVCT2 3′UTR
|
Up
|
Rev [38]
|
miR-204
|
Runx2 3′UTR
|
Up
|
Rev [38]
|
miR-138
|
ALP, RUNX2
|
Up
|
Rev [38]
|
miR-338-3p
|
Runx2, Fgfr2
|
Up
|
Rev [38]
|
miR-31
|
Satb2
|
Up
|
Rev [38]
|
miR-138
|
FAK, ERK1/2, RUNX2
|
Up
|
Rev [38]
|
miR-27a
|
Sp7
|
Up
|
Rev [38]
|
miR-381
|
WNT5A, FZD3
|
Up
|
[22]
|
miR-889
|
WNT7A
|
Up
|
[23]
|
miR204
|
BMP2
|
Up
|
[41]
|
Here we provide meta-analysis of experimental data and identify the line of miRs which are significantly involved in hMCSs osteogenic differentiation. The results of our analysis demonstrate complex involvement of different miRs in osteogenic hMSCs differentiation and show that each step of osteoblasts differentiation is orchestrated by a few miRs via similar molecular mechanisms (Fig. 5). Vice versa, the same miRs regulate different signaling pathways, regulating osteogenesis.
With KEGG pathway and GO analysis we found that let-7a, mir-17, mir-21 and mir-101 in hMSC are likely to be involved in transforming growth factor β (TGF-β) signaling where mir-17 is predicted to inhibit the TGF-β cytoplasmic mediator activity. At the same time, mir-29b, let-7a and mir-101 were predicted to regulate the platelet-derived growth factor (PDGF). Aadditionally, mir-101 was predicted to target the fibroblasts growth factor (FGF). Mir-29b was predicted to activate the vascular endothelial growth factor (VEGF) and maintains the cellular response to TGF-β and FGF. Together up-regulated PDGF, TGF-β, FGF and VEGF create appropriate microenvironment for hMSCs to enter in osteogenic lineage differentiation, namely promote collagen synthesis and fibril organization. GO analysis additionally shows important regulatory role of let-7a and mir-29b in collagen synthesis and fibril organization. Having summarized our data, we suppose that balance between high level of let-7a, mir-21, mir-29b, mir-101 and low level of mir-17 is necessary for microenvironment formation and hMSCs turn on into osteogenic lineage (Fig. 5).
Widely known TGF-β/BMP activates SMAD-dependent and non-SMAD-dependent signaling which results in main osteogenic transcriptional factor Runx2 expression [12]. TGF-β–SMAD signaling raises MSCs proliferation, chemotaxis, and early differentiation of osteoprogenitors, where BMP–SMAD signaling regulate almost every stage of osteoblast differentiation and maturation [12]. During osteoprogenitors development BMP–SMAD also activates the Runx2 and promotes hMSCs differentiation. We suggest that let-7a is mostly involves in TGF-β modulation, where mir-26a leads to all steps of osteoblast development and maturation via BMP–SMAD targeting. Additionaly, TGF-β/BMP–SMAD dependent pathway is supposed to be regulated by lowering in mir-17 which targets SMAD7, SMAD6 and SMAD1. But, surprisingly, overexpressed miRs mir-21 and mir-101 also targets SMAD proteins (SMAD7 and SMAD6) inhibiting SMAD dependent transcriptions.
However, non-SMAD-dependent pathway (JNK, ERK, p38 MAPK, Pi3K-Akt signaling) also promote osteogenesis and main transcriptional factors (Runx2, Osx, Dlx5 and other) expression [12-15, 26]. Mentioned signaling can activate the Runx2 expression in hMSCs. Moreover, all these signaling together with mentioned above BMP pathway regulates the Osterix expression Runx2 independently. With DAVID Bioinformatics Resources we found that non-SMAD-dependent pathway activity is orchestrated by interplaying between different up- and downregulated miRs. As we mentioned above let-7a targets TGF-β–SMAD signaling and is crucial for entering of hMSCs into osteogenesis, however this microRNA regulates the TGF-β–non-SMAD-dependent signaling including MAPK, PI3K-Akt, cGMP-PKG, Jak-STAT, Hippo and AMPK positively affected their activity. Our data showed that let-7a is versatile regulator of osteogenesis and high level of its expression maintain osteoprogenitors differentiation as well as osteoblast maturation and metabolism (via AMPK pathway).
Our data show that not only let-7a, but MAPK, ErbB/MAPK and Pi3K-Akt signaling are predictably regulated by mir-29b, mir-101, mir-143 and mir-218 that are also expressed at higher level during osteogenic differentiation. Additionaly MAPK and Pi3K-Akt signaling supposed to be regulated by let-7a and mir-26a via cGMP-PKG signaling pathway and mir-21, mir-101, mir-29b and mir-218 through Ras signaling pathway. However, GO analysis revealed that mir-21 can affect negatively the ERK1/1 and Ras signaling pathway via inhibiting of MAPK activity. As shown, Ras/MAPK signaling branch activity is associated with osteogenesis inhibiting [32], so mir-21 may inactivate Ras/MAPK signaling promoting in such way osteogenic differentiation. We found that mir-101 and mir-218 also are possible inactivators of MAPK-cascade and Jak-STAT signaling. Thus, our research indicates that high content of let-7a, mir-21, mir-26a, mir-29b, mir-143 can be specifically involved in non-SMAD dependent signaling and promote osteogenic TF expression (Fig. 5). On the other side non-SMAD dependent signaling modulated by miRs shows significantly lower expression in osteogenesis. Namely, mir-17, mir-222, mir-138 and mir-31 are supposed to be involved in non-SMAD-dependent pathway (i.e., MAPK, Pi3K-Akt, and canonical Wnt signaling) regulation. However, GO analysis indicates that mir-17 positively regulates Wnt and p38-MAPK signaling cascades where inhibits TGF-β signaling. Hence, we suppose that mir-17 level diminishing is necessary for TGF-β/non-SMAD dependent signaling activity. Our data show that mir-31 also positively regulates MAPK-cascade, however low level of its content points that this microRNA does not require for MAPK-dependent expression of osteogenic TF.
GO analysis reviled that mir-222 and mir-138 can modulate canonical Wnt via noncanonical calcium and Hippo signaling, respectively. Moreover mir-138, based on KEGG pathway analysis, is supposed to negatively regulate canonical Wnt-dependent transcription through Notch signaling pathway. Hence, the lower level of mir-222 and mir-138 in hMSCs maintains the canonical Wnt signaling up-regulation, where the latter one regulates other osteogenic TF expression (Runx2, Osx, Dlx5 and ATF4), strongly stimulates osteoblastogenesis and inhibits adipogenic TF (C/ERB and PPARγ) [10, 13]. Moreover, canonical Wnt involves in osteoblast maturation and metabolism [33]. GO analysis revealed that canonical Wnt activity can be positively regulated by mir-26a, mir-101 and mir-29b which expresses on higher level during osteogenesis, where mir-101 and mir-29b regulate β-catenin binding and β-catenin-TCF complex assembly. Summarizing these data we came to the conclusion that canonical Wnt signaling activity in osteogenesis depends on low content of mir-222, mir-17 and mir-138 and high expression of mir-26a, mir-101 and mir-29b.
Other crucial for osteogenesis signaling is Hippo/Yap driven by a few miRs. Notably, Yap is a co-regulator for main osteogenic TF and act via both SMAD-dependent and non-SMAD-dependent molecular mechanisms, so it is up-regulation is crucial for osteogenic lineage commitment as well as osteoblasts maturation [34,35]. GO and KEGG pathway analysis revealed that Yap dependent transcription can be modulated by up-regulated let-7a, mir-101 and downregulated mir-31, mir-138 and mir-222. Our finding points that mir-138 and mir-222 can regulate Yap/TCF/Lef dependent transcription. Mir-31 and mir-222 are supposed to inhibit Yap/Taz and prevent its transcription activity, where mir-17 modulate TGF-β–SMAD/Yap, BMP–SMAD/Yap and Yap/TCF/Lef dependent signaling. Summarizing we assume that lower level of mir-31, miR-138, mir-222 are important for Yap dependent transcription of main osteogenic TF in hMSC during all steps of osteogenesis (Fig. 5).
Osteoblast differentiation relates to metabolic changes, hence AMPK signaling pathway is one of crucial factors which controls different aspects of metabolism in cells including osteoprogenitors and osteoblast. Moreover, AMPK signaling is required for hMSC to enter the adipogenic and osteogenic lineages through Runx2 and PPARγ and to be able to maintain osteogenesis and osteoblast homeostasis [36]. With bioinformatic analysis we found that AMPK signaling during osteogenesis can be positively modulated by let-7a, mir-29b and mir-218 which are significantly up-regulated (Fig. 5).
Other important signaling involved in osteoblast homeostasis as well as differentiation is Hedgehog signaling pathway, which play dual role in bone homeostasis and required in both bone resorption and formation process. Additionally, Hedgehog signaling induce osteoblast differentiation in BM-MSC through Runx2 and Osx expression [37]. We found that during osteogenesis mir-101 can promote Hedgehog signaling activity.
Osteogenic differentiation, osteoblast maturation and homeostasis are coordinated by several signaling pathways which has many have many crosstalks and common targets genes. In this study, we used bioinformatic approaches to identify specific association between cells signaling and microRNAs up- and downregulated during osteogenic lineage differentiation. We suppose mentioned below miRNA to be specifically involved in hMSC transition into osteogenic differentiation via microenvironment formation (i.e. let-7a, mir-17, mir-21, mir-29b and mir-101) and TGF-β/BMP–SMAD dependent pathway (i.e. let-7a, mir-17, mir-21, mir-26a mir-101) as well as MAPK signaling pathway (i.e. let-7a, mir-21, mir-26a, mir-29b, mir-143 and mir17), while Yap-dependent expression of osteogenic TF is modulated by let-7a, mir-31mir-101, mir-138 and mir-222. We identify mir-17, mir-26a, mir-29b, mir-101, mir-138 and mir-222 as those that can be specifically involved in canonical Wnt signaling dependent osteogenesis as well as in osteoblast maturation and metabolism maintainance together with let-7a, mir-29b and mir-218 which modulate AMPK signaling. Additionally, identified mir-101 is likely involved in osteoblast homeostasis via Hedgehog signaling.
Our data expands knowledge in the field of hMSCs fate and osteogenesis orchestration by miRs, demostrates pro-osteogenic and anti-osteogenic miRs and their potential molecular pathways.