Nitric oxide contributes to rapid sclerostin protein loss following mechanical load.

In response to mechanical loading of bone, osteocytes produce nitric oxide (NO•) and decrease sclerostin protein expression, leading to an increase in bone mass. However, it is unclear whether NO• production and sclerostin protein loss are mechanistically linked, and, if so, the nature of their hierarchical relationship within an established mechano-transduction pathway. Prior work showed that following fluid-shear stress (FSS), osteocytes produce NOX2-derived reactive oxygen species, inducing calcium (Ca) influx.

Age-associated changes in lineage composition of the enteric nervous system regulate gut health and disease.

The enteric nervous system (ENS), a collection of neural cells contained in the wall of the gut, is of fundamental importance to gastrointestinal and systemic health. According to the prevailing paradigm, the ENS arises from progenitor cells migrating from the neural crest and remains largely unchanged thereafter. Here, we show that the lineage composition of maturing ENS changes with time, with a decline in the canonical lineage of neural-crest derived neurons and their replacement by a newly identified lineage of mesoderm-derived neurons.

Osteoblast-lineage calcium/calmodulin-dependent kinase 2 delta and gamma regulates bone mass and quality.

Bone regulates its mass and quality in response to diverse mechanical, hormonal, and local signals. The bone anabolic or catabolic responses to these signals are often received by osteocytes, which then coordinate the activity of osteoblasts and osteoclasts on bone surfaces. We previously established that calcium/calmodulin-dependent kinase 2 (CaMKII) is required for osteocytes to respond to some bone anabolic cues in vitro.

Fluid Shear Stress Induced Sclerostin Degradation and CaMKII Activation in Osteocytes.

Bone is a dynamic tissue that adapts to changes in its mechanical environment. Mechanical stimuli pressurize interstitial fluid in the lacunar-canalicular system within the bone matrix, causing fluid shear stress (FSS) across bone embedded, mechano-sensitive osteocytes. Therefore, modeling this mechanical stimulus is vital for identifying mechano-transduction cascades that contribute to the regulation of mechano-responsive proteins, such as the Wnt/β-catenin antagonist, sclerostin, which is reduced in response to FSS.

The cytoskeleton and connected elements in bone cell mechano-transduction.

Bone is a mechano-responsive tissue that adapts to changes in its mechanical environment. Increases in strain lead to increased bone mass acquisition, whereas decreases in strain lead to a loss of bone mass. Given that mechanical stress is a regulator of bone mass and quality, it is important to understand how bone cells sense and transduce these mechanical cues into biological changes to identify druggable targets that can be exploited to restore bone cell mechano-sensitivity or to mimic mechanical load.

Disparate bone anabolic cues activate bone formation by regulating the rapid lysosomal degradation of sclerostin protein.

The downregulation of sclerostin in osteocytes mediates bone formation in response to mechanical cues and parathyroid hormone (PTH). To date, the regulation of sclerostin has been attributed exclusively to the transcriptional downregulation of the gene hours after stimulation. Using mouse models and rodent cell lines, we describe the rapid, minute-scale post-translational degradation of sclerostin protein by the lysosome following mechanical load and PTH.

Aging, Osteo-Sarcopenia, and Musculoskeletal Mechano-Transduction.

The decline in the mass and function of bone and muscle is an inevitable consequence of healthy aging with early onset and accelerated decline in those with chronic disease. Termed osteo-sarcopenia, this condition predisposes the decreased activity, falls, low-energy fractures, and increased risk of co-morbid disease that leads to musculoskeletal frailty. The biology of osteo-sarcopenia is most understood in the context of systemic neuro-endocrine and immune/inflammatory alterations that drive inflammation, oxidative stress, reduced autophagy, and cellular senescence in the bone and muscle.

TRPV4 calcium influx controls sclerostin protein loss independent of purinergic calcium oscillations.

Skeletal remodeling is driven in part by the osteocyte's ability to respond to its mechanical environment by regulating the abundance of sclerostin, a negative regulator of bone mass. We have recently shown that the osteocyte responds to fluid shear stress via the microtubule network-dependent activation of NADPH oxidase 2 (NOX2)-generated reactive oxygen species and subsequent opening of TRPV4 cation channels, leading to calcium influx, activation of CaMKII, and rapid sclerostin protein downregulation. In addition to the initial calcium influx, purinergic receptor signaling and calcium oscillations occur in response to mechanical load and prior to rapid sclerostin protein loss.

Connexin43 regulates osteoprotegerin expression via ERK1/2 -dependent recruitment of Sp1.

In bone, connexin43 expression in cells of the osteoblast lineage plays an important role in restraining osteoclastogenesis and bone resorption. While there is a consensus around the notion that the anti-osteoclastogenic factor, osteoprotegerin, is a driver of this effect, how connexin43 regulates osteoprotegerin gene expression is unclear. Here, we show that loss of connexin43 decreased osteoprotegerin gene expression and reduced ERK1/2 activation.