Activin type IIA decoy receptor and intermittent parathyroid hormone in combination overturns the bone loss in disuse-osteopenic mice

Abstract

Damage of the lower motor neuron cell bodies or their axons results in reduced or abolished voluntary movement accompanied by a substantial loss of bone and muscle mass. Intermittent parathyroid hormone 1–34 (PTH) (teriparatide) is one of the most potent bone-anabolic treatment regimens. ActRIIA-mFc is an activin type IIA decoy receptor that increases bone mass mediated by inhibition of the activin receptor signaling pathway. We investigated whether PTH or ActRIIA-mFc alone or in combination could prevent loss of bone and muscle mass induced by injecting botulinum toxin A (BTX) into the right hind limb in mice. Seventy-two 16-week-old female C57BL/6 mice were allocated to the following groups: Baseline, Control, BTX, BTX + ActRIIA-mFc (10 mg/kg), BTX + PTH (100 μg/kg), and BTX + ActRIIA-mFc + PTH. The mice were sacrificed after three weeks of disuse and treatment. In contrast to monotherapy with PTH, ActRIIA-mFc alone or in combination with PTH was able partly or completely to prevent disuse-induced loss of whole femoral bone mass, trabecular thickness, and bone strength. Moreover, an additive effect of ActRIIA-mFc and PTH on areal bone mineral density and trabecular bone volume was found. In summary, ActRIIA-mFc and PTH in combination were more effective in preventing disuse-induced bone loss and deterioration of trabecular micro-architecture than either treatment alone.

Introduction

Damage of the lower motor neuron cell bodies or their axons results in reduced or abolished voluntary movement, which is accompanied by substantial loss of muscle and bone mass. The most common cause of lower motor neuron damage in adults is trauma or amputation of peripheral nerves [[1], [2], [3], [4]]. Both in vivo animal and human studies, where disuse is caused by either peripheral nerve damage or spinal cord injury results in a substantial and significant loss of muscle and bone mass that quickly materialize after the injury [1,2,5,6]. In rodents, femoral bone mineral density (BMD) is reduced by up to 15% after only 4 weeks of disuse [5,7,8]. Similarly, dramatic reductions in BMD are seen in humans after spinal cord injury, where tibial BMD is reduced by up to 8% after only six weeks [9,10].

In order to counteract the rapid muscle and bone loss after lower motor neuron damage, potent muscle and bone-anabolic treatments are needed. Human parathyroid hormone (PTH) is a polypeptide secreted from the principal cells in the parathyroid glands that participate in the regulation of the calcium and phosphate content in the blood [11,12]. Intermittent administration of PTH (1–34) (teriparatide) is a potent bone-anabolic treatment regimen acting through the PTH 1 receptor. The intracellular signaling pathway for PTH in osteoblasts has previously been described in detail by Datta et al. and others [[13], [14], [15], [16], [17], [18], [19], [20], [21]]. A simplified intracellular signaling pathway of intermittent PTH is shown in Fig. 1.

A growing body of studies in rodents with lower motor neuron axon damage or spinal cord injury has revealed that systemic treatment with intermittent PTH (1–34) increases BMD, trabecular thickness, and bone strength, but not muscle mass or muscle strength [6,7,[22], [23], [24]]. Human studies with PTH have been performed predominantly in post-menopausal women, whereas only lately studies of patients with spinal cord injuries have emerged [[25], [26], [27], [28]].

Recently, it has been unraveled that several members of the transforming growth factor beta (TGF-β) superfamily plays an important role in regulating both muscle and bone mass. The mechanism revolves around the TGF-β superfamily members activin A and myostatin (GDF8). Activin A is proposed to be a negative regulator of bone mass, while both myostatin and activin A are believed to be negative regulators of skeletal muscle mass [[29], [30], [31]]. Both ligands are inherently connected by their common mechanism of pathway initiation through the activin receptor complex (Fig. 1). As a consequence, inhibitors of the activin receptor signaling pathway (IASPs) have emerged as new and promising experimental therapies for treatment of muscle and bone loss [[32], [33], [34]].

As disuse not only leads to a substantial and rapid osteopenia, but also to a pronounced sarcopenia, treatments regimens targeting both is warranted. We have previously shown that the combination of growth hormone (GH) and intermittent PTH (1–34) can prevent BTX-induced induced osteopenia and attenuate the concomitant sarcopenia [6]. However, due to GHs widespread anabolic effects on multiple tissue in the body, caution should be exercised before employing GH-treatment in a clinical setting. IASPs, which presumably do not have the same side effects as GH, are obvious candidates for replacing GH in a combination therapy. Moreover, intermittent PTH (1–34) and IASPs acts through different pathways indicating the possibility for an additive osteoanabolic effect (Fig. 1). Furthermore, combining the IASP Activin II A decoy receptor with intermittent PTH (1–34) may allow use of a lower dose of PTH without compromising the bone gain and obliterating the risk of adverse effects related to PTH like hypercalcemia. Therefore, the present study aimed to investigate the ability of the activin decoy receptor ActRIIA-mFc and PTH (1–34) to prevent disuse-induced bone and muscle loss, alone or in combination.