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© 2001 The Journal of Bone and Joint Surgery, Inc.
Brain and Bone: Central Regulation of Bone MassA New Paradigm in Skeletal Biology
Investigation performed at the Department of Trauma and Reconstructive
Surgery, Hamburg University School of Medicine, Hamburg, Germany
Bone-remodeling is the cellular process used by vertebrates to maintain a nearly constant bone mass between the end of puberty and the time of cessation of gonadal function. Body weight, fertility, and bone formation are regulated, at least in part, by the same hormone, leptin, which exerts its control through hypothalamic pathways. Bone-remodeling disorders such as osteoporosis are, in part, hypothalamic diseases, and modulation of central signaling pathways can be used to overcome the skeletal consequences of gonadal failure and to potentially restore bone mass. Bone mass is of critical importance for skeletal integrity and skeletal function. A sufficient bone stock is required for locomotion, for protection of inner organs, as a reservoir of vital ions, and as the scaffold for skeletal repair and osteosynthesis. Bone-remodeling is the physiological process used by vertebrates to maintain a constant bone mass between the end of puberty and the time of cessation of gonadal function. In addition to the well-characterized and critical local regulation of bone-remodeling, a central control of bone formation has been shown in recent genetic studies1. This central regulation involves leptin, an adipocyte-secreted hormone that controls body weight, reproduction, and bone-remodeling following binding to its receptor located on hypothalamic nuclei. This novel physiological concept may shed light on the etiology of osteoporosis and help to identify new therapeutic strategies for the treatment of that disease and its associated clinical problems, such as delayed fracture-healing. Our understanding of the biology of the skeleton, like that of virtually every other subject in biology, has been transformed by recent advances in human, mouse, and chick genetics. These advances, together with findings by embryologists studying chickens, have radically enhanced our comprehension of the developmental biology of the vertebrate skeleton2-4. In contrast, we have added very little to our understanding of skeletal physiology. Some of the many largely unanswered questions about skeletal physiology include: • Why and how do we stop growing? • Why and how are bones and teeth the only organs to mineralize under physiological conditions? • Why is osteoporosis mainly a disease affecting women? • How is bone mass maintained at a nearly constant level between the end of puberty and the arrest of gonadal function? This review will deal with the final question. The two clinical observations that are the basis for this review are that cessation of gonadal function favors the development of osteoporosis while obesity protects against it. Our working hypothesis has been that these two observations suggest that body mass, bone-remodeling, and reproduction are somehow controlled by the same endocrine mechanisms. Since body weight and reproduction are known to be controlled centrally, it is reasonable to hypothesize that bone-remodeling may be controlled centrally as well. In addition to the large body of evidence indicating the existence of a local regulation of bone-remodeling (which will not be discussed here), genetic evidence demonstrates the existence of a central regulation of bone-remodeling; this evidence is consistent with the concept that the two clinical observations mentioned above are mechanistically linked1-5. This central regulation is not accessory, as its modulation can overcome the deleterious effect of cessation of gonadal function on bone-remodeling and prevent osteoporotic bone loss.
Bone-remodeling is the physiological process by which constant bone mass is maintained in vertebrates between the end of puberty and the time of cessation of gonadal function6. This is an unusual process as bone is the only organ that contains a cell type, the osteoclast, whose only function is to remove or resorb the tissue that supports it7-9. Indeed, every day and simultaneously at multiple locations, bone is resorbed by osteoclasts and then replaced with new bone laid down by the osteoblasts. The fact that this process occurs simultaneously in multiple locations in the skeleton, together with the well-documented role of cells of the osteoblast lineage that direct osteoclast differentiation, has been viewed as proof that bone-remodeling is primarily an autocrine/paracrine process10-12. Extensive experimental evidence has demonstrated that this regulation does exist12,13, and this review will not address it in detail. For instance, many cytokines present in the extracellular matrix or synthesized by bone cells have been shown to be involved in bone-remodeling. This research has culminated recently in the discovery of osteoprotegerin (OPG), which is an inhibitor of osteoclast differentiation14,15, and the discovery of the receptor activator of NF-kB ligand (RANKL), which is an osteoclast differentiation factor16. Genetic evidence has demonstrated that RANKL and its soluble receptor OPG play a critical role during osteoclast differentiation and can act in a paracrine way16-19. Yet it is also well known that hormones such as sex steroid and parathyroid hormones, among others, can affect bone resorption20,21, and OPG as well as RANKL can also act in a systemic manner14,16. These latter observations suggest that there is also an endocrine regulation of bone-remodeling. These two types of regulation—namely, systemic and local—are not antagonistic but should be viewed as synergistic. Moreover, clinical evidence, reproducible in mutant mouse strains, suggests that there is a systemic regulation of bone formation. In osteopetrotic patients, the osteoclasts are either absent or nonfunctional, yet bone formation is not halted. The same is true in animal models of osteopetrosis. Mice lacking the src gene, which is essential for osteoclast function, and thus showing no osteoclastic activity still have ongoing bone formation and eventually severe osteopetrosis develops (Fig. 1)22-24. The simplest explanation for the maintenance of bone formation activity in the absence of bone resorption is that there is a systemic regulation.
In contrast to the emerging knowledge regarding the existence of a systemic regulation of bone resorption, much less is known about the molecular mechanisms regulating the rate of bone formation by the osteoblasts. The results of one experiment suggest that bone formation is regulated primarily by endocrine signals25. By means of targeted overexpression in osteoblasts of a thymidine kinase gene, Corral et al. were able to induce a nearly complete and reversible osteoblast ablation25. In these transgenic animals, osteoblast ablation led to bone loss, thus illustrating that bone resorption in mice is a function independent of bone formation. However, the most remarkable feature of this model lies elsewhere. Upon osteoblast repopulation, bone mass was restored with an exquisite precision to virtually the same amount as in that in the wild-type age-matched littermates. This extreme precision indicates that somehow the osteoblast has "two speeds" to make bone. It can first quickly deposit a large amount of bone matrix to restore bone mass, and then, once this has been achieved, it can slow down, decreasing the amount of bone matrix deposited, so that bone mass does not increase beyond that in wild-type mice. This "two-speed" model can be viewed as indirect evidence of an endocrine regulation of bone formation, and recognition of this concept has been an incentive to look for circulating molecules regulating the rate of bone formation by the osteoblasts25.
One approach to understanding why bone formation is so tightly regulated is to ask what is deregulated in pathological situations. The most frequent disease affecting bone-remodeling is osteoporosis26. Osteoporosis is characterized by low bone mass with an increased risk of fracture following minimal trauma27. It is the most prevalent disease in developed countries, a fact that emphasizes the importance of understanding, in molecular terms, the regulation of bone homeostasis. Multiple clinical and epidemiological features characterize osteoporosis28,29. Two long-recognized clinical findings—that osteoporosis is often triggered or worsened by the cessation of gonadal function20,30 and that obesity protects individuals from bone loss31-33—suggest a molecular basis for the regulation of bone mass. The latter observation was for a long time poorly understood, as illustrated by the following quote: "Heavier people generally have stronger bones as well as a lower risk of suffering from osteoporotic fractures, and our studies have shown that this is mainly due to the greater proportion of body fat. Increased weight bearing does stimulate further bone growth, and, in women, estrogen is produced by fat cells. However, these facts do not adequately explain the relationship between body weight and bone density, so it is likely that other mechanisms are operative [italics added]."34 Translated into a molecular vocabulary, these two observations may be viewed as suggesting that bone mass, body weight, and gonadal function are regulated by the same secreted molecules.
On the basis of knowledge of the hormonal regulation of body mass, leptin was thought to be the best candidate to fulfill the triple regulatory function mentioned above. Leptin is a polypeptide hormone that is the product of a gene termed ob because of its role in genetically determined obesity. Mice that lack both copies of this gene (ob/ob mice) have been an invaluable tool in obesity research35-37. Obesity is the most visible phenotype of the ob/ob mouse, but it is not the only one. Another prominent phenotype is sterility38,39. Clearly leptin, like most known hormones, has a broad range of action on multiple target organs36,40. Another mutant strain of mouse, the db/db mouse, has a mutation in the gene coding for the leptin receptor (termed db for diabetes) and the same series of phenotypes as the ob/ob mouse41,42. Likewise, fa/fa rats have an inactivating mutation in the gene encoding the leptin receptor (termed fa for fatty), and they are obese and hypogonadic43. Importantly, the obesity and sterility phenotypes of these two mouse and rat mutant strains are recessive. Normally, the influences of the absence of gonadal function and of the obesity of ob/ob and db/db mice on bone integrity should antagonize each other and result in a mild low-bone-mass phenotype. However, both ob/ob and db/db mice have a massive increase in bone mass1 (Fig. 2). The ob/ob and db/db mice are the only known animal models, in any species, in which hypogonadism and high bone mass coexist; thus, in the context of bone physiology, they are an invaluable resource for the study of bone-remodeling and diseases affecting bone-remodeling. This high-bone-mass phenotype is even more surprising because these mice have hypercortisolism, a condition usually leading to a decrease in osteoblast function and to osteoporosis28.
The high-bone-mass phenotype of the ob/ob and db/db mice, which is caused by an increase in bone formation, is not secondary to obesity, as it is observed in young ob/ob mice before they become obese1. More importantly, this phenotype is dominant, being observed in heterozygous mice that harbor one intact (+) and one disrupted gene (ob or db) and thus are termed ob/+ and db/+. The phenotype is specific for the absence of leptin signaling because it is not observed in other mouse models of obesity with intact leptin signaling. The fact that the high-bone-mass phenotype is dominant and the obesity phenotype is recessive demonstrates genetically that the control of bone mass by leptin is not an accidental function of a body-weight-regulating hormone. It indicates rather that the control of bone formation, which is as important as the control of body weight, is a function of leptin. It also demonstrates that the high-bone-mass phenotype is not secondary to any endocrine abnormalities in ob/ob and db/db mice, since ob/+ and db/+ mice do not have such abnormalities.
How does leptin control bone formation? Does it act through an autocrine, paracrine, or endocrine mechanism? Does it require the presence of fat or does it control bone formation as it controls body weight, by binding to its hypothalamic receptor? Before addressing these crucial questions, one has to summarize the phenotypic features of the ob/ob and db/db mice and the critical implications of this phenotype. These mutant mouse strains do make more bone, but they do it with the same number of osteoblasts as wild-type mice. In other words, this is a functional phenotype, not a phenotype based on cell differentiation. This observation implies that if leptin acts locally it has to do so by means of functional receptors on differentiated primary osteoblasts, not those on osteoblast progenitors. This is an important point as there is clear evidence that the leptin receptor can be observed on immortalized multipotential stromal cell lines in vitro44. A multiplicity of experiments, biochemical, molecular, and genetic, failed to detect any expression of leptin or of a signal transducing receptor in osteoblasts1, thus virtually ruling out an autocrine, paracrine, or endocrine mechanism of regulation, at least in vivo. It was conceivable that, in the absence of leptin, adipocytes release a molecule that favors bone formation. Studies of a transgenic mouse strain deprived of white fat, however, proved the contrary to be true45. These mice, which are called fat-free mice, have a very low level of leptin since they have virtually no adipocytes45. Nevertheless, they have a high-bone-mass phenotype, thus ruling out the possibility that, in the absence of leptin, adipocytes release an activator of bone formation. The remaining possibility to be tested was the most simple and yet the most novel—namely, that leptin controls bone formation following binding to hypothalamic nuclei where the leptin receptor is particularly abundant. Indeed, intracerebroventricular infusion of leptin in ob/ob mice led to a massive and rapid decrease of their bone mass1. Similarly, intracerebroventricular infusion of leptin in wild-type mice led to the development of a severe osteopenic phenotype, demonstrating that bone-remodeling or at least its bone-formation aspect is under the control of the hypothalamus. No leptin could be detected in the serum of these intracerebroventricular infusion-treated animals; this latter control demonstrates unambiguously that leptin can regulate bone formation without direct contact with the osteoblast1. These findings, in line with the mode of regulation of body weight and gonadal function, do not necessarily negate other possible modes of action of leptin yet to be demonstrated in vivo. Rather, they should be viewed as providing investigators with a new conceptual framework with which to better understand bone physiology (Fig. 3).
To date, it is still not known if there is a single linear genetic or biochemical pathway explaining leptin’s role in the control of body weight following binding to its hypothalamic receptor46-49. Likewise, it is not known what gene products convey to the osteoblast the information that leptin delivers to the hypothalamus. Nevertheless, leptin’s actions on body weight and bone mass seem to use different pathways. Indeed, intracerebroventricular infusion of neuropeptide Y (NPY), which is an orexigenic peptide that antagonizes leptin’s action on body weight49, has the same osteopenic effect as leptin itself47,48. This finding suggests that NPY may have different functions in the control of body weight and bone mass. Clearly, one of the challenges ahead will be to identify genes downstream of leptin and other molecules that regulate leptin’s action on bone. Is leptin the only systemic regulator of bone formation? Most likely it is not, as the existence of a negative regulation of bone mass suggests that positive regulators of bone formation may also exist and await identification.
If we examine our initial hypothesis that bone mass, body weight, and reproduction share common regulatory pathways, how do the findings in recent studies relate to the observation that cessation of gonadal function favors osteoporosis and obesity protects against it? It is well known that obese individuals display a state of leptin resistance. The molecular basis of this leptin resistance remains poorly understood, but it results in a partial functional deficiency of leptin, a situation similar to that in the ob/+ and db/+ mice. In vivo analysis of the role of leptin during bone-remodeling has shown that bone-remodeling is as much a centrally controlled process as it is a local one. This central regulation is of paramount importance since its disruption is the only known biological setting in which the deleterious consequences of hypogonadism on bone metabolism are overcome. An implication of this genetic finding is that the most typical and frequent bone-remodeling disease—namely, osteoporosis—is partly a central or hypothalamic disease. As such, the results of the studies noted in this review may be viewed as establishing a novel paradigm in our understanding of bone-remodeling. This does not mean, however, that we now understand everything about bone-remodeling. In particular, these findings cannot explain the low bone mass observed in anorectic patients. Indeed, this shift of concepts raises more questions than it answers. The identification of leptin as a powerful inhibitor of bone formation has potential therapeutic implications. Conceivably, since the high-bone-mass phenotype is dominant and the obesity phenotype is recessive, it should be possible to design drugs acting on this pathway that have a protective effect on skeletal integrity without leading to obesity. Finally, leptin is unlikely to be the only central regulator of bone formation and/or bone mass. Other, yet to be discovered, centrally acting hormones or neurotransmitters may positively or negatively regulate bone formation, bone mass, and possibly other aspects of bone physiology, such as bone resorption or even fracture-healing. Note: The authors are grateful to Dr. G. Karsenty and Dr. P. Ducy. The work on TK-mice and leptin was done in collaboration between our laboratories. The results of this fruitful collaboration led to the discovery of the central aspect of bone-remodeling.
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Introduction
Bone-Remodeling: Different...
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