Impairment of osteoblast differentiation due to proliferation-independent telomere dysfunction in mouse models of accelerated aging

Bone cell senescence: mechanisms and perspectives

Age-related bone loss is in large part the consequence of senescence mechanisms that impact bone cell number and function. In recent years, progress has been made in the understanding of the molecular mechanisms underlying bone cell senescence that contributes to the alteration of skeletal integrity during aging. These mechanisms can be classified as intrinsic senescence processes, alterations in endogenous anabolic factors, and changes in local support. Intrinsic senescence mechanisms cause cellular dysfunctions that are not tissue specific and include telomere shortening, accumulation of oxidative damage, impaired DNA repair, and altered epigenetic mechanisms regulating gene transcription. Aging mechanisms that are more relevant to the bone microenvironment include alterations in the expression and signaling of local growth factors and altered intercellular communications. This review provides an integrated overview of the current concepts and interacting mechanisms underlying bone cell senescence during aging and how they could be targeted to reduce the negative impact of senescence in the aging skeleton.

Mouse models of telomere dysfunction phenocopy skeletal changes found in human age-related osteoporosis

A major medical challenge in the elderly is osteoporosis and the high risk of fracture. Telomere dysfunction is a cause of cellular senescence and telomere shortening, which occurs with age in cells from most human tissues, including bone. Telomere defects contribute to the pathogenesis of two progeroid disorders characterized by premature osteoporosis, Werner syndrome and dyskeratosis congenital. It is hypothesized that telomere shortening contributes to bone aging. We evaluated the skeletal phenotypes of mice with disrupted telomere maintenance mechanisms as models for human bone aging, including mutants in Werner helicase (Wrn^−/−), telomerase (Terc^−/−) and Wrn^−/−Terc^−/− double mutants. Compared with young wild-type (WT) mice, micro-computerized tomography analysis revealed that young Terc^−/− and Wrn^−/−Terc^−/− mice have decreased trabecular bone volume, trabecular number and trabecular thickness, as well as increased trabecular spacing. In cortical bone, young Terc^−/− and Wrn^−/−Terc^−/− mice have increased cortical thinning, and increased porosity relative to age-matched WT mice. These trabecular and cortical changes were accelerated with age in Terc^−/− and Wrn^−/−Terc^−/− mice compared with older WT mice. Histological quantification of osteoblasts in aged mice showed a similar number of osteoblasts in all genotypes; however, significant decreases in osteoid, mineralization surface, mineral apposition rate and bone formation rate in older Terc^−/− and Wrn^−/−Terc^−/− bone suggest that osteoblast dysfunction is a prominent feature of precocious aging in these mice. Except in the Wrn^−/− single mutant, osteoclast number did not increase in any genotype. Significant alterations in mechanical parameters (structure model index, degree of anistrophy and moment of inertia) of the Terc^−/− and Wrn^−/−Terc^−/− femurs compared with WT mice were also observed. Young Wrn^−/−Terc^−/− mice had a statistically significant increase in bone-marrow fat content compared with young WT mice, which remained elevated in aged double mutants. Taken together, our results suggest that Terc^−/− and Wrn^−/−Terc^−/− mutants recapitulate the human bone aging phenotype and are useful models for studying age-related osteoporosis.

5-azacytidine improves the osteogenic differentiation potential of aged human adipose-derived mesenchymal stem cells by DNA demethylation

The therapeutic value of adipose-derived mesenchymal stem cells (Ad-MSCs) for bone regeneration is critically discussed. A possible reason for reduced osteogenic potential may be an age-related deterioration of the Ad-MSCs. In long term in vitro culture, epigenomic changes in DNA methylation are known to cause gene silencing, affecting stem cell growth as well as the differentiation potential. In this study, we observed an age-related decline in proliferation of primary human Ad-MSCs. Decreased Nanog, Oct4 and Lin28A and increased Sox2 gene-expression was accompanied by an impaired osteogenic differentiation potential of Ad-MSCs isolated from old donors (>60 a) as compared to Ad-MSCs isolated from younger donors (<45 a). 5-hydroxymethylcytosine (5 hmC) and 5-methylcytonsine (5 mC) distribution as well as TET gene expression were evaluated to assess the evidence of active DNA demethylation. We observed a decrease of 5 hmC in Ad-MSCs from older donors. Incubation of these cells with 5-Azacytidine induced proliferation and improved the osteogenic differentiation potential in these cells. The increase in AP activity and matrix mineralization was associated with an increased presence of 5 hmC as well as with an increased TET2 and TET3 gene expression. Our data show, for the first time, a decrease of DNA hydroxymethylation in Ad-MSCs which correlates with donor-age and that treatment with 5-Azacytidine provides an approach which could be used to rejuvenate Ad-MSCs from aged donors.

Long-term functional engraftment of mesenchymal progenitor cells in a mouse model of accelerated aging

Age-related osteoporosis is characterized by a decrease in bone-forming capacity mediated by defects in the number and function of osteoblasts. An important cellular mechanism that may in part explain osteoblast dysfunction that occurs with aging is senescence of mesenchymal progenitor cells (MPCs). In the telomere-based Wrn(-/-) Terc(-/-) model of accelerated aging, the osteoporotic phenotype of these mice is also associated with a major decline in MPC differentiation into osteoblasts. To investigate the role of MPC aging as a cell-autonomous mechanism in senile bone loss, transplantation of young wild-type whole bone marrow into Wrn(-/-) Terc(-/-) mutants was performed and the ability of engrafted cells to differentiate into cells of the osteoblast lineage was assessed. We found that whole bone marrow transplantation in Wrn(-/-) Terc(-/-) mice resulted in functional engraftment of MPCs up to 42 weeks, which was accompanied by a survival advantage as well as delays in microarchitectural features of skeletal aging.

Sclerostin is expressed in osteoclasts from aged mice and reduces osteoclast-mediated stimulation of mineralization

Osteoclast-mediated bone resorption precedes osteoblast-mediated bone formation through early adulthood, but formation fails to keep pace with resorption during aging. We previously identified several factors produced by osteoclasts that promote bone formation. In this study, we determined if osteoclast-produced factors contribute to the impaired bone formation with aging. We previously found that mice between the ages of 18 and 22 months develop age-related bone loss. Bone marrow-derived pre-osteoclasts were isolated from 6-week, 12-month, and 18- to 24-month-old mice and differentiated into osteoclasts in vitro. Conditioned media were collected and compared for osteoblast mineralization support. Conditioned medium from osteoclasts from all ages was able to support mineralization of bone marrow stromal cells. Concentrating the conditioned medium from 6-week-old and 12-month-old mouse marrow cells-derived osteoclasts enhanced mineralization support whereas concentrated conditioned medium from 18- to 24-month-old mouse marrow-derived osteoclasts repressed mineralization compared to base medium. This observation suggests that an inhibitor of mineralization was secreted by aged murine osteoclasts. Gene and protein analysis revealed that the Wnt antagonist sclerostin was significantly elevated in the conditioned media from 24-month-old mouse cells compared to 6-week-old mouse cells. Antibodies directed to sclerostin neutralized the influences of the aged mouse cell concentrated conditioned media on mineralization. Sclerostin is primarily produced by osteocytes in young animals. This study demonstrates that osteoclasts from aged mice also produce sclerostin in quantities that may contribute to the age-related impairment in bone formation.

R-Spondin 1 promotes vibration-induced bone formation in mouse models of osteoporosis

Bone tissue adapts to its functional environment by optimizing its morphology for mechanical demand. Among the mechanosensitive cells that recognize and respond to forces in the skeleton are osteocytes, osteoblasts, and mesenchymal progenitor cells (MPCs). Therefore, the ability to use mechanical signals to improve bone health through exercise and devices that deliver mechanical signals is an attractive approach to age-related bone loss; however, the extracellular or circulating mediators of such signals are largely unknown. Using SDS-PAGE separation of proteins secreted by MPCs in response to low-magnitude mechanical signals and in-gel trypsin digestion followed by HPLC and mass spectroscopy, we identified secreted proteins up-regulated by vibratory stimulation. We exploited a cell senescence-associated secretory phenotype screen and reasoned that a subset of vibration-induced proteins with diminished secretion by senescent MPCs will have the capacity to promote bone formation in vivo. We identified one such vibration-induced bone-enhancing (vibe) gene as R-spondin 1, a Wnt pathway modulator, and demonstrated that it has the capacity to promote bone formation in three mouse models of age-related bone loss. By virtue of their secretory status, some vibe proteins may be candidates for pre-clinical development as anabolic agents for the treatment of osteoporosis.

KEY MESSAGE

Mesenchymal stem cells respond to low magnitude mechanical signals (vibration). R-Spondin 1 is upregulated by mechanical signals and secreted. R-Spondin 1 promotes bone formation in three mouse models of osteoporosis.

The role of the JAK-STAT pathway and related signal cascades in telomerase activation during the development of hematologic malignancies

Telomerase, comprising a reverse transcriptase protein (TERT) and an RNA template, plays a critical role during senescence and carcinogenesis; however, the mechanisms by which telomerase is regulated remain to be elucidated. Several signaling pathways are involved in the activation of TERT at multistep levels. The JAK-STAT pathway is indispensable for mediating signals through growth factor and cytokine receptors during the development of hematopoietic cells, and its activity is frequently upregulated in hematological malignancies. Here, we review the role of the JAK-STAT pathway and related signaling cascades in the regulation of telomerase in hematological malignancies.

Functional impairment of bone formation in the pathogenesis of osteoporosis: the bone marrow regenerative competence

The skeleton is a high-renewal organ that undergoes ongoing cycles of remodeling. The regenerative bone formation arm ultimately declines in the aging, postmenopausal skeleton, but current therapies do not adequately address this deficit. Bone marrow is the primary source of the skeletal anabolic response and the mesenchymal stem cells (MSCs), which give rise to bone matrix-producing osteoblasts. The identity of these stem cells is emerging, but it now appears that the term 'MSC' has often been misapplied to the bone marrow stromal cell (BMSC), a progeny of the MSC. Nevertheless, the changes in BMSC phenotype associated with age and estrogen depletion likely contribute to the attenuated regenerative competence of the marrow and may reflect alterations in MSC phenotype. Here we summarize current concepts in bone marrow MSC identity, and within this context, review recent observations on changes in bone marrow population dynamics associated with aging and menopause.

The changing balance between osteoblastogenesis and adipogenesis in aging and its impact on hematopoiesis

Osteoblasts (OBs) and adipocytes (APs) share a common mesenchymal ancestor. It is now clear that mesenchymal stem cell (MSC) maturation along the OB lineage comes at the expense of adipogenesis and vice versa. During aging, this balance increasingly favors the formation of APs. Hematopoiesis also slowly declines during the aging process. The role of OB lineage cells in hematopoiesis has been studied, but less is known about how APs regulate hematopoiesis. A few studies have demonstrated a negative relationship between APs and hematopoiesis; however, there is also evidence that brown adipose tissue (BAT) may promote hematopoiesis. This review will examine the current knowledge of how adipogenesis and osteogenesis change with aging and the implications of this changing environment on hematopoeisis.

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

The organ of Corti in the mammalian inner ear is comprised of mechanosensory hair cells (HCs) and nonsensory supporting cells (SCs), both of which are believed to be terminally post-mitotic beyond late embryonic ages. Consequently, regeneration of HCs and SCs does not occur naturally in the adult mammalian cochlea, though recent evidence suggests that these cells may not be completely or irreversibly quiescent at earlier postnatal ages. Furthermore, regenerative processes can be induced by genetic and pharmacological manipulations, but, more and more reports suggest that regenerative potential declines as the organ of Corti continues to age. In numerous mammalian systems, such effects of aging on regenerative potential are well established. However, in the cochlea, the problem of regeneration has not been traditionally viewed as one of aging. This is an important consideration as current models are unable to elicit widespread regeneration or full recovery of function at adult ages yet regenerative therapies will need to be developed specifically for adult populations. Still, the advent of gene targeting and other genetic manipulations has established mice as critically important models for the study of cochlear development and HC regeneration and suggests that auditory HC regeneration in adult mammals may indeed be possible. Thus, this review will focus on the pursuit of regeneration in the postnatal and adult mouse cochlea and highlight processes that occur during postnatal development, maturation, and aging that could contribute to an age-related decline in regenerative potential. Second, we will draw upon the wealth of knowledge pertaining to age related senescence in tissues outside of the ear to synthesize new insights and potentially guide future research aimed at promoting HC regeneration in the adult cochlea.