MicroRNAs (miRNAs) are a group of small noncoding RNAs capable of regulating specific gene expression. Let-7 miRNA was first discovered in Caenorhabditis elegans and it is highly conserved in human tissues. The human let- 7 family of miRNA contains 12 members of miRNA. Today, these members have become the most studied miRNAs and they have attracted attention of researchers in various fields, such as development, stem cell biology, aging, and metabolism. Furthermore, there is a large body of evidence linking the loss of let-7 expression and the development of poorly differentiated, aggressive cancers. In addition to the canonical biogenesis pathway, let-7 has been found to be regulated by protein factors, such as RNA binding proteins previously identified as regulators of protein-coding mRNAs. Moreover, the direct interaction between miRNAs has recently been identified as a novel pathway to control let-7 expression. In this review, we discuss the multifaceted roles of let-7 and provide an overview of its regulation at multiple levels.

Osteoporosis is one of the most common bone pathologies. A number of novel molecules have been reported to increase bone formation including cysteine-rich protein 61 (CYR61), a ligand of integrin receptor, but mechanisms remain unclear. It is known that bone morphogenetic proteins (BMPs), especially BMP-2, are crucial regulators of osteogenesis. However, the interaction between CYR61 and BMP-2 is unclear. We found that CYR61 significantly increases proliferation and osteoblastic differentiation in MC3T3-E1 osteoblasts and primary cultured osteoblasts. CYR61 enhances mRNA and protein expression of BMP-2 in a time-and dose-dependent manner. Moreover, CYR61-mediated proliferation and osteoblastic differentiation are significantly decreased by knockdown of BMP-2 expression or inhibition of BMP-2 activity. In this study we found integrin ␣ v ␤ 3 is critical for CYR61-mediated BMP-2 expression and osteoblastic differentiation. We also found that integrin-linked kinase, which is downstream of the ␣ v ␤ 3 receptor, is involved in CYR61-induced BMP-2 expression and subsequent osteoblastic differentiation through an ERK-dependent pathway. Taken together, our results show that CYR61 up-regulates BMP-2 mRNA and protein expression, resulting in enhanced cell proliferation and osteoblastic differentiation through activation of the ␣ v ␤ 3 integrin/integrin-linked kinase/ ERK signaling pathway.Bone is a mineralized tissue that underlies multiple mechanical and metabolic functions of the skeleton (1). Bone functions include maintaining blood calcium levels, providing mechanical support to soft tissues and serving as levers for muscle action, supporting hematopoiesis, and housing the brain and spinal cord (2). Formation and maintenance of bone tissue are regulated in a sophisticated fashion by boneforming osteoblasts and bone-resorbing osteoclasts (3). Development and differentiation of these two cell types are under tight regulation by a number of endogenous substances such as hormones, growth factors, and cytokines (4). These factors are individually secreted through endocrine, paracrine/autocrine, and neurocrine systems, with subsequent interaction essential to the delicate balance between bone-forming and -resorbing cells in the marrow microenvironment. An imbalance between the two cell types leads to pathogenesis of certain bone diseases including osteopetrosis and osteoporosis (5, 6).Osteoporosis is the most common human metabolic bone disorder characterized by progressive and age-dependent bone loss and increasing bone fracture risk. It is an important public health issue in postmenopausal women; if untreated, more than half of white women will experience fractures during their lifetime. Between 30 and 50% of women and 15-30% of men will suffer a fracture related to osteoporosis in their lifetime (7). Fractures increase morbidity and mortality and impose a financial burden on the community (8). A most compelling therapeutic need for osteoporosis at the present time is a drug that will substantially increase bone formation...

Oxygen is essentially required by most eukaryotic organisms as a scavenger to remove harmful electron and hydrogen ions or as a critical substrate to ensure the proper execution of enzymatic reactions. All nucleated cells can sense oxygen concentration and respond to reduced oxygen availability (hypoxia). When oxygen delivery is disrupted or reduced, the organisms will develop numerous adaptive mechanisms to facilitate cells survived in the hypoxic condition. Normally, such hypoxic response will cease when oxygen level is restored. However, the situation becomes complicated if hypoxic stress persists (chronic hypoxia) or cyclic normoxia-hypoxia phenomenon occurs (intermittent hypoxia). A series of chain reaction-like gene expression cascade, termed hypoxia-mediated gene regulatory network, will be initiated under such prolonged or intermittent hypoxic conditions and subsequently leads to alteration of cellular function and/or behaviors. As a result, irreversible processes occur that may cause physiological disorder or even pathological consequences. A growing body of evidence implicates that hypoxia plays critical roles in the pathogenesis of major causes of mortality including cancer, myocardial ischemia, metabolic diseases, and chronic heart and kidney diseases, and in reproductive diseases such as preeclampsia and endometriosis. This review article will summarize current understandings regarding the molecular mechanism of hypoxia in these common and important diseases.

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Mitochondrial dynamics regulate the quality and morphology of mitochondria. Calcium (Ca2+) plays an important role in regulating mitochondrial function. Here, we investigated the effects of optogenetically engineered Ca2+ signaling on mitochondrial dynamics. More specifically, customized illumination conditions could trigger unique Ca2+ oscillation waves to trigger specific signaling pathways. In this study, we found that modulating Ca2+ oscillations by increasing the light frequency, intensity, and exposure time could drive mitochondria toward the fission state, mitochondrial dysfunction, autophagy, and cell death. Moreover, illumination triggered phosphorylation at the Ser616 residue, but not the Ser637 residue of the mitochondrial fission protein, dynamin-related protein 1 (DRP1), via the activation of Ca2+-dependent kinases, CaMKII, ERK, and CDK1. However, optogenetically engineered Ca2+ signaling did not activate calcineurin phosphatase to dephosphorylate DRP1 at Ser637. In addition, light illumination had no effect on the expression levels of the mitochondrial fusion proteins, mitofusin (MFN)-1 and MFN2.Taken together, this study provides an effective and innovative approach to altering Ca2+ signaling for controlling mitochondrial fission with a more precise resolution than pharmacological approaches in the temporal dimension.