Bone is a highly dynamic tissue. Every year approximately 10% of an individual’s skeleton is resorbed and new bone is formed, which means that every 10 years your bones are made of entirely new material. We call this process “bone remodeling”. As we age, the balance between bone resorption and bone formation changes, leading to relatively more bone being removed and less bone being formed – this is called osteopenia, which refers to low bone mass, and is a normal consequence of aging. When the balance tilts excessively toward the loss of bone, we refer to it as osteoporosis.
|Shuler F. ORTHOPEDICS. 2012|
Osteoporosis is a disease characterized by bone fragility and an increased incidence of broken bones, which results when an individual’s bones become thinner and more brittle. Osteoporosis has long been thought to mainly affect elderly women; however, with the increasing use of prescription medicines such as glucocorticoids, and the large number of people leading unhealthy lifestyles, the incidence of osteoporosis is predicted to significantly increase in the future. In 2002, approximately 43 million people had either osteoporosis or osteopenia, and in 2020 this number is predicted to grow to nearly 61 million people.
Alterations in bone remodeling – the coupled action of bone resorption and bone formation – that lead to osteoporosis are a result of changes in the activities of the cells that carry out these processes. Bones are made up of three main types of cells: osteoblasts, osteoclasts, and osteocytes. Osteoblasts are responsible for forming new bone, while osteoclasts eat away (resorb) the old or damaged bone. Osteocytes are osteoblasts that become entombed within the newly formed bone matrix, and they are the most abundant cell type accounting for nearly 90% of the cells. Osteocytes are the main regulators of the osteoblasts and osteoclasts. Osteocytes are among the main producers of the cytokine receptor activator of nuclear factor kappa-B ligand (RANKL) and the decoy cytokine receptor osteoprotegerin (OPG). The ratio of RANKL:OPG controls osteoclast formation because OPG is able to bind to RANKL and prevent its binding to the RANK receptor. On the osteoclast precursor surface, the cytokine RANKL binds to the RANK receptor and activates osteoclast differentiation and function of the osteoclasts, leading to increased bone resorption. The osteocytes embedded in the bone connect to each other through outgrowths called cannaliculi, creating networks within the bone that allow for the bone to sense mechanical stimuli and transmit signals between the cells.
Connexin (Cx) 43, a key protein involved in the formation of gap junctions, which are intercellular channels between the cells that allow for cell-to-cell communication. As an individual ages, the levels of Cx43 decrease and the number of dead osteocytes increases. Animal models with an osteocyte-specific deletion of Cx43 display increased osteocyte cell death, empty lacunae (the spaces in the bone cortex normally occupied by living osteocytes), and an increased number of osteoclasts along the bone surface. Experiments studying MLO-Y4 osteocytic cells lacking Cx43 also found an increase in cell death. Transfection of the Cx43 back into these osteocytic cells was sufficient to prevent this increase in cell death observed in this cell line. Osteocytes lacking Cx43 undergo a specific form of programmed cell death called apoptosis. The process of apoptosis is initiated through the action of multiple caspase proteins, including caspase-3. This increase in osteocyte apoptosis leads to the release of specific molecules and signals, which are involved in communicating with the osteoblasts and osteoclasts.
To study the effects that osteocyte apoptosis has on osteoclast recruitment, we collected the conditioning media (the media containing growth factors that is added to cells) from Cx43-silenced and control MLO-Y4 cells that were either untreated or treated with DEVD, a caspase-3 inhibitor. This conditioning media was then used to treat non-adherent bone marrow cells that were treated with m-CSF (macrophage colony stimulating factor) and RANKL to induce osteoclast differentiation. This study found that blocking osteocyte apoptosis reduced the levels of soluble RANKL and prevented the increase in osteoclast recruitment and activity associated with osteocyte cell death.
The overall findings from this study suggest that Cx43 is required to maintain osteocyte viability and show that the increased osteoclast activity observed in Cx43 silenced osteocytes is a result of the increased osteocyte apoptosis. These findings provide a potential way in which osteocyte apoptosis could be targeted to prevent bone fragility in individuals with low bone mass.
Currently, the majority of osteoporosis drugs on the market work to maintain bone mass through inhibiting the activity of the bone resorbing osteoclasts. While these drugs are effective at preventing further bone loss, they do not reverse the bone loss that has already occurred before treatment has begun. This is because bone formation and resorption are coupled in bone remodeling*, so inhibition of resorption also decreases the amount of formation. The findings from this study provide evidence that specifically targeting osteocytes could allow for a therapeutic method to prevent bone loss and maintain bone mass through mechanisms that do not involve completely inhibiting the activity of osteoclasts.
* The uncoupled action of bone formation and bone resorption is referred to as “bone modeling”, which is one of the ways that bones can change their shape as we grow during childhood and adolescence.
Contributed by: Hannah Davis