Introduction

Skeletal integrity is maintained by the process of bone remodeling, i.e., the balanced removal of old bone matrix by bone-resorbing osteoclasts and the deposition of new bone tissue by bone-forming osteoblasts [1]. Since the early work of Julius Wolff in 1892 [2], it has been well established that the skeleton represents a dynamic tissue undergoing adaptive changes in response to loading to better withstand these loads (“Wolff’s law”). There are several clinical examples of the anabolic response of bone to loading, one of them being the observation that the bone mass is markedly higher in the dominant arm than in the nondominant arm in tennis players [3]. In recent years, a number of essential molecular load-sensation pathways to build the optimal amount of bone required for adequate stability have been uncovered. Osteocytes, which represent terminally differentiated osteoblasts embedded into the bone matrix, have gained research attention due to their ability to sense loading and translate mechanical signals into biochemical signals to orchestrate bone remodeling [4]. The discovery of osteocyte-secreted proteins such as sclerostin has already led to new osteoporosis treatments [5, 6], and more recently, other molecular osteocytic and nonosteocytic proteins and nanostructures involved in the process of mechanosensation, such as the calcium channel Piezo1, were revealed [80], thereby confirming clinical observations. Disuse in rats attenuated the bone anabolic response to teriparatide treatment [81]. In another experimental study in dogs, the cortical bone loss observed in disuse osteoporosis could only partially be counteracted by bisphosphonates, which indicates that the strong osteoclastogenic stimulus of disuse may not fully be antagonized by antiresorptive agents [82]. Together, these results suggest that mechanical loading is of critical importance in optimizing the treatment effects of bone-specific agents. Of note, an emerging model to study bone and its adaption to physical activity is the zebrafish [83]. Modification of the physical exercise intensity in a swim tunnel experiment led to rapid bone formation and increased bone volume and mineralization [84].

There have been a number of studies in rodents focusing on mechanical loading, bone remodeling and especially osteocyte mechanobiology. In mice, spaceflight led to both trabecular and cortical deterioration along with increased numbers of cortical empty osteocyte lacunae [85]. Early studies found evidence of osteocytic changes following immobilization, including increased osteocyte apoptosis [86]. More recently, it was demonstrated that osteocyte apoptosis following weightlessness (i.e., unloading by tail suspension) in mice triggers osteocytic RANKL production along with activation of both cortical and trabecular bone resorption [87]. While osteocyte apoptosis preceded the recruitment of osteoclastic cells and bone loss [88], another study by the same group found that inhibition of osteocyte apoptosis did not prevent unloading-induced bone loss although osteocytic RANKL production was sufficiently inhibited [89]. Tatsumi et al. developed a transgenic mouse model with the ablation of osteocytes through the injection of diphtheria toxin [90]. Importantly, these mice did not develop unloading-induced bone loss, underlining the osteocyte’s function in sensing mechanical loading and in the development of disuse osteoporosis. It is interesting to note that other models, including Botox-induced immobilization, were shown to lead to significant changes in bone mechanical and structural properties without affecting cortical osteocyte lacunar properties in rats [91]. A reason for these diverging findings, and especially the lack of osteocytic changes in the latter work, may be due to the almost nonexistent intracortical remodeling in the rats studied as well as the fact that osteocyte apoptosis was not studied [91]. Overall, it is likely that certain factors influence how osteocytes respond to unloading, for example, cortical vs. trabecular bone compartments, local loading history, and remodeling rates.

In the search for molecular mechanisms involved in the pathogenesis of disuse osteoporosis, several other important signaling pathways and proteins involved in the process of mechanically driven bone remodeling have been discovered (Fig. 4). Indeed, matrix-embedded osteocytes represent the skeletal cell type that most likely plays a major role during mechanotransduction, i.e., the process of converting external mechanical forces into biochemical responses. This is achieved by sensing local mechanical signals and responding to these signals both directly and indirectly [92]. Notably, both mechanically induced bone formation and disuse-induced bone loss and skeletal fragility may be mediated by osteocytes. Osteocytes are embedded in a lacuno-canalicular system (LCS), which becomes evident in acid-etched bone with subsequent scanning electron microscopy. Inside the LCS reside the osteocytes and the dendritic processes that originate from the osteocyte cell body. The osteocyte-secreted protein sclerostin was originally discovered through the identification of loss-of-function mutations in the SOST gene in individuals with the genetic high-bone-mass disorder sclerosteosis (van Buchem disease) [93]. Further research has demonstrated that increased sclerostin expression is primarily caused by mechanical unloading in mice, which leads to low bone formation [94]. While in vitro studies have revealed that disuse promotes osteocyte apoptosis and mechanical stimulation by fluid shear stress promotes osteocyte survival [95], it has further been demonstrated that increased sclerostin expression is specifically caused by unloading in osteocytic cells [96]. Translating these findings into therapeutic concepts, it is most likely that the recently approved sclerostin antibody romosozumab [6, 97] is especially effective in disuse osteoporosis, but the efficacy of romosozumab in association with conditions of disuse osteoporosis has to be investigated in future clinical studies.

Fig. 4
figure 4

Schematic model of the molecular pathways involved in skeletal unloading. Mechanosensation in osteocytes is mediated by the lacuno-canalicular system (LCS) and via ion channels of the Piezo family (primarily Piezo1), among others. Inactivation of Piezo1 leads to decreased Wnt1 expression. Furthermore, sclerostin secretion by osteocytes is increased in response to unloading, which inhibits Wnt/β-catenin and results in suppressed osteoblast activity. Unloading also leads to increased osteocyte apoptosis (partly via Wnt/β-catenin signaling) and increased RANKL expression, promoting increased bone resorption

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The mechanosensitive function of the skeleton was recently evidenced by the identification of mechanically activated ion channels of the Piezo family, specifically Piezo1 in osteoblasts and osteocytes [7, 8]. Interestingly, the deletion of Piezo1 reduced the expression of Wnt1 [7], which is a key regulator of bone formation [98]. By a systematic analysis of mice with inactivated Piezo proteins in different skeletal cell populations, our group confirmed that Piezo1 acts as a mechanosensor in osteocytes and demonstrated that it represents an essential osteogenic differentiation factor during endochondral bone formation [9]. The latter finding supports the assumption that various cell types are involved in mechanotransduction, which is highly relevant not only for bone remodeling but also for developmental and regenerative processes [99]. Together, these collective data suggest that Piezo1 plays a crucial role in mechanosensation and that Piezo channels could be a novel therapeutic target for osteoanabolic treatment in mechanical unloading-induced bone loss.

Another molecular example of musculoskeletal mechanotransduction, more specifically the close interaction between muscle and bone during this process, is the identification of the myokine irisin, a protein that is derived from muscle in response to exercise. Irisin injection increased cortical bone mass in mice, partly through suppression of sclerostin expression [100]. In this way, irisin prevented not only muscular atrophy but also the development of disuse osteoporosis accompanied by decreased osteocyte apoptosis [101]. In human subjects, circulating irisin levels were associated with the risk of osteoporotic fracture [102]. Beyond this associative evidence, there is currently no strong clinical evidence demonstrating that muscle or bone modulate each other’s tissue response to load changes.

Finally, the molecular mechanosensors in osteocytes are the subject of ongoing research, as recently discussed in a detailed review article [92]. Briefly summarized (and as partly outlined above), osteocytic mechanosensors include the osteocyte cytoskeleton and dendritic processes within the lacuno-canalicular system (LCS), as well as ion channels (e.g., Piezo1). However, the extracellular matrix (ECM), primary cilia, integrin-based focal adhesions and connexin-based intercellular junctions have also been identified to contribute to mechanotransduction in osteocytes. During skeletal development and mechanical stimulation, there is interactive communication between osteocytes and the ECM, which is mainly composed of proteoglycans, glycoproteins, and hyaluronic acid. One example highlighting the role of the ECM during the osteocyte mechanotransduction process is perlecan (HSPG2), a proteoglycan that is localized in the pericellular space of osteocytes within the LCS [103]. Importantly, perlecan-deficient mice showed dysfunctional bone formation in response to loading [104]. Primary cilia are solitary, rigid structures that span from the cell body into the extracellular space [105]. Interestingly, primary cilia translate extracellular mechanical stimuli into cellular responses via an ion-channel-independent mechanism [106]. In addition to these important structures responsible for mechanotransduction in osteocytes, integrin-based focal adhesions play an important role in this complex process. Integrins are transmembrane receptors with an extracellular and a cytoplasmic domain composed of an α and β subunit. Both in vitro and in vivo studies have revealed the involvement of the different subunits during mechanotransduction [107]. Finally, cellular communication via hemichannels and gap junctions, which are composed of the protein connexin, has been found to be essential during mechanotransduction. While multiple types of connexins are expressed in bone cells, Cx43 is the major candidate for mechanotransduction in osteocytes. The cell-specific deletion of Cx43 in osteoblasts and osteocytes led to an attenuated response to both loading and unloading [108, 109].

Concluding Remarks

Although a number of fundamental questions concerning the clinical, molecular and therapeutic aspects of disuse osteoporosis remain unanswered, the combined effort of both clinical and basic research has revealed the microstructural and mechanistic basis of the profound negative effects of disuse on skeletal integrity. Clinical studies with real or modeled unloading conditions, including spinal cord injury, limb suspension, bed rest and microgravity (spaceflight), have demonstrated that early bone loss is characterized by a relative increase in bone resorption compared to decreased bone formation. As a consequence, combined cortical and trabecular microstructural deterioration was noted in most studies, with subtle disease-, sex-, site- and compartment-specific differences that can be accurately imaged using high-resolution CT-based techniques such as HR-pQCT. In a broader sense, disuse osteoporosis also includes clinical conditions of bone loss in response to local skeletal unloading, such as after surgery or around implants. Furthermore, full and timely recovery from bone loss may not be achieved by loading, but bone loss may be counteracted by high-frequency physical exercise programs and bone-specific agents such as bisphosphonates/denosumab and teriparatide. The use of animal models has allowed the cellular and molecular mechanisms of disuse-induced bone loss to be elucidated. Matrix-embedded osteocytes have been identified as the primary mechanoresponsive cell type translating mechanical into biochemical signals to orchestrate mechanically induced bone formation and remodeling, which is in fact a regulatory process with high complexity also involving neighboring bone cells. Osteocyte-specific proteins such as sclerostin and Piezo1 are involved in this process of mechanotransduction, and new therapeutic options have already been introduced; however, their efficacy needs to be clarified in further experiments and clinical trials acknowledging the mechanical environment.