Osteocytes detect the need for mechanical adaptation and microdamage repair and signal to osteoblasts or osteoclasts, leading to bone gain or loss. Although long-lived, osteocytes undergo apoptosis, and their viability contributes to bone strength by mechanisms dependent and independent of changes in bone mineral density. During the preceding funding period, it was found that, similar to hormonal and pharmacological stimuli, mechanical forces regulate osteocyte lifespan. Thus, mechanical stimulation in vitro prevents osteocyte apoptosis via an integrin/Src/ERK pathway that requires the integrity of caveolae and a ligand-independent function of the estrogen receptor (ER). Conversely, reduced mechanical forces in vivo increase osteocyte apoptosis, and this event temporally precedes, and is spatially associated with, osteoclast-mediated resorption and the subsequent loss of mineral and strength. It was also found that during aging bone strength decreases before a reduction in bone mineral density, and osteocyte apoptosis increases. Based on this knowledge, it is proposed that mechanical stimuli trigger ERK-dependent signals that sustain osteocyte survival via a signalsome assembled in caveolae and composed of integrins and signaling molecules, including the ERs. Conversely, diminished mechanical forces?from reduced physical activity with age?eliminate the signals that maintain osteocyte viability, thereby leading to apoptosis. Dying osteocytes in turn recruit osteoclasts to the vicinity. Both mechanisms, disruption of the integrity of the osteocyte network and increased osteoclastic bone resorption, contribute to the age-related decline in bone strength and mass. To validate these hypotheses, in Aim 1 will be defined in vitro the role of the ER and caveolin-1 in ERK-mediated anti-apoptosis induced by mechanical stimulation;and whether other survival signaling pathways act in concert to control osteocyte death.
In Aim 2, it will be determined in vivo whether mechanically induced survival signaling is disrupted by unloading and during aging, and established whether unloading- or aging-induced osteoclast-mediated resorption and loss of bone and strength are ameliorated by inhibiting osteocyte apoptosis by 1) constitutively activating the ERK pathway in OG2- MEK-SP mice, 2) overexpressing the anti-apoptotic protein Bcl-2 in DMP1-Bcl2 mice, and 3) treating with a unique bisphosphonate that inhibits osteocyte apoptosis without affecting osteoclasts.
In Aim 3, it will be established whether induction of osteocyte apoptosis is sufficient to trigger osteoclast recruitment and whether the kinetics of the osteoclastogenic response to osteocyte apoptosis are altered with aging. These studies will advance our understanding of the mechanistic basis for the profound role of mechanical forces, or lack of thereof, in skeletal health and disease, and will establish the contribution of osteocyte apoptosis to the loss of bone strength that ensues with aging. We expect that this work will provide new avenues for the treatment of bone fragility in conditions of reduced physical activity, such as in the elderly or during the temporary immobilization of bed rest, space flight, and motor paralysis.
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