Dwight A. Towler

Conditional inactivation of FGF receptor 2 reveals an essential role for FGF signaling in the regulation of osteoblast function and bone growth

Human craniosynostosis syndromes, resulting from activating or neomorphic mutations in fibroblast growth factor receptor 2 (FGFR2), underscore an essential role for FGFR2 signaling in skeletal development. Embryos harboring homozygous null mutations in FGFR2 die prior to skeletogenesis. To address the role of FGFR2 in normal bone development, a conditional gene deletion approach was adopted. Homologous introduction of cre recombinase into the Dermo1 (Twist2) gene locus resulted in robust expression of CRE in mesenchymal condensations giving rise to both osteoblast and chondrocyte lineages. Inactivation of a floxed Fgfr2 allele with Dermo1-cre resulted in mice with skeletal dwarfism and decreased bone density. Although differentiation of the osteoblast lineage was not disturbed, the proliferation of osteoprogenitors and the anabolic function of mature osteoblasts were severely affected.

Calcific Aortic Valve Disease: Not Simply a Degenerative Process A Review and Agenda for Research From the National Heart and Lung and Blood Institute Aortic Stenosis Working Group Executive Summary: Calcific Aortic Valve Disease - 2011 Update

Calcific aortic valve disease (CAVD) encompasses the range of disease from initial alterations in the cell biology of the leaflets to end-stage calcification resulting in left ventricular outflow obstruction. The first detectable macroscopic changes in the leaflets, seen as calcification, or focal leaflet thickening with normal valve function, is termed aortic valve sclerosis, but it is likely that the initiating events in the disease process occur much earlier. Disease progression is characterized by a process of thickening of the valve leaflets and the formation of calcium nodules – often including the formation of actual bone – and new blood vessels, which are concentrated near the aortic surface. End stage disease, e.g. calcific aortic stenosis, is characterized pathologically by large nodular calcific masses within the aortic cusps that protrude through the outflow surfaces into the sinuses of Valsalva, interfering with opening of the cusps. For decades, this disease was thought to be a passive process in which the valve degenerates with age in association with calcium accumulation. Moreover, although calcific aortic valve disease is more common with age, it is not an inevitable consequence of aging. Instead, CAVD appears to be an actively regulated disease process that cannot be characterized exclusively as “senile” or “degenerative.” The NHLBI convened a group of scientists from different fields of study, including cardiac imaging, molecular biology, cardiovascular pathology, epidemiology, cell biology, endocrinology, bioengineering, and clinical outcomes, to review the scientific studies from the past decade in the field of CAVD. The purpose was to develop a consensus statement on the current state of translational research related to CAVD. Herein, we summarize recent scientific studies and define future directions for research to diagnose, treat and potentially prevent this complex disease process.

Msx2 promotes cardiovascular calcification by activating paracrine Wnt signals

In diabetic LDLR-/- mice, an ectopic BMP2-Msx2 gene regulatory program is upregulated in association with vascular calcification. We verified the procalcific actions of aortic Msx2 expression in vivo. CMV-Msx2 transgenic (CMV-Msx2Tg(+)) mice expressed 3-fold higher levels of aortic Msx2 than nontransgenic littermates. On high-fat diets, CMV-Msx2Tg(+) mice exhibited marked cardiovascular calcification involving aortic and coronary tunica media. This corresponded to regions of Msx2 immunoreactivity in adjacent adventitial myofibroblasts, suggesting a potential paracrine osteogenic signal. To better understand Msx2-regulated calcification, we studied actions in 10T1/2 cells. We found that conditioned media from Msx2-transduced 10T1/2 cells (Msx2-CM) is both pro-osteogenic and adipostatic; these features are characteristic of Wnt signaling. Msx2-CM stimulated Wnt-dependent TCF/LEF transcription, and Msx2-transduced cells exhibited increased nuclear beta-catenin localization with concomitant alkaline phosphatase induction. Msx2 upregulated Wnt3a and Wnt7a but downregulated expression of the canonical inhibitor Dkk1. Dkk1 treatment reversed osteogenic and adipostatic actions of Msx2. Teriparatide, a PTH1R agonist that inhibits murine vascular calcification, suppressed vascular BMP2-Msx2-Wnt signaling. Analyses of CMV-Msx2Tg(+) mice confirmed that Msx2 suppresses aortic Dkk1 and upregulates vascular Wnts; moreover, TOPGAL(+) (Wnt reporter); CMV-Msx2Tg(+) mice exhibited augmented aortic LacZ expression. Thus, Msx2-expressing cells elaborated an osteogenic milieu that promotes vascular calcification in part via paracrine Wnt signals.

Diet-induced diabetes activates an osteogenic gene regulatory program in the aortas of low density lipoprotein receptor-deficient mice

Vascular calcification is common in people with diabetes and its presence predicts premature mortality. To clarify the underlying mechanisms, we used low density lipoprotein receptor-deficient (LDLR −/−) mice to study vascular calcification in the ascending aorta. LDLR −/− mice on a chow diet did not develop obesity, diabetes, atheroma, or vascular calcification. In contrast, LDLR −/− mice on high fat diets containing cholesterol developed obesity, severe hyperlipidemia, hyperinsulinemic diabetes, and aortic atheroma. A high fat diet without cholesterol also induced obesity and diabetes, but caused only moderate hyperlipidemia and did not result in significant aortic atheroma formation. Regardless of cholesterol content, high fat diets induced mineralization of the proximal aorta (assessed by von Kossa staining) and promoted aortic expression ofMsx2 and Msx1, genes encoding homeodomain transcription factors that regulate mineralization and osseous differentiation programs in the developing skull. Osteopontin(Opn), an osteoblast matrix protein gene also expressed by activated macrophages, was up-regulated in the aorta by these high fat diets. In situ hybridization showed that peri-aortic adventitial cells in high fat-fed mice expressMsx2. Opn was also detected in this adventitial cell population, but in addition was expressed by aortic vascular smooth muscle cells and macrophages of the intimal atheroma. High fat diets associated with hyperinsulinemic diabetes activate an aortic osteoblast transcriptional regulatory program that is independent of intimal atheroma formation. The spatial pattern ofMsx2 and Opn gene expression strongly suggests that vascular calcification, thought to be limited to the media, is an active process that can originate from an osteoprogenitor cell population in the adventitia.

Aortic Msx2-Wnt Calcification Cascade Is Regulated by TNF-α–Dependent Signals in Diabetic Ldlr−/− Mice

Objective—Aortic calcification is prevalent in type II diabetes (T2DM), enhancing morbidity and tracking metabolic syndrome parameters. Ldlr−/− mice fed high-fat “Westernized” diets (HFD) accumulate aortic calcium primarily in the tunica media, mediated via osteogenic morphogens and transcriptional programs that induce aortic alkaline phosphatase (ALP). Because elevated TNF-&agr; is characteristic of obesity with T2DM, we examined contributions of this inflammatory cytokine. Methods and Results—HFD promoted obesity, hyperglycemia, and hyperlipidemia, and upregulated serum TNF-&agr; in Ldlr−/− mice. Serum haptoglobin (inflammatory marker) was increased along with aortic expression of BMP2, Msx2, Wnt3a, and Wnt7a. Dosing with the TNF-&agr; neutralizing antibody infliximab did not reduce obesity, hypercholesterolemia, or hyperglycemia; however, haptoglobin, aortic BMP2, Msx2, Wnt3a, and Wnt7a and aortic calcium accumulation were downregulated by infliximab. Mice with vascular TNF-&agr; augmented by a transgene (SM22-TNF&agr;Tg) driven from the SM22 promoter upregulated aortic Msx2, Wnt3a, and Wnt7a. Furthermore, SM22-TNF&agr;Tg;TOPGAL mice exhibited greater aortic &bgr;-galactosidase reporter staining versus TOPGAL sibs, indicating enhanced mural Wnt signaling. In aortic myofibroblast cultures, TNF-&agr; upregulated Msx2, Wnt3a, Wnt7a, and ALP. ALP induction was inhibited by Dkk1, an antagonist of paracrine Wnt actions. Conclusions—TNF-&agr; promote aortic Msx2-Wnt programs that contribute to aortic calcium accumulation in T2DM.

Mechanism of Awareness of Hypoglycemia: Perception of Neurogenic (Predominantly Cholinergic) Rather Than Neuroglycopenic Symptoms

We sought 1) to determine which symptoms of hypoglycemia are reproducible, 2) to pharmacologically distinguish neurogenic (autonomic) from neuroglycopenic symptoms, and 3) to test the hypothesis that awareness of hypoglycemia is the result of perception of neurogenic rather than neuroglycopenic symptoms. Awareness of hypoglycemia and 19 symptoms were quantitated in 10 normal, young adults, each studied on four occasions in random sequence, during 1) clamped euglycemia (∼ 5 mM), 2) clamped hypoglycemia (∼ 2.5 mM), 3) clamped hypoglycemia with combined α- and β-adrenergic blockade (phentolamine and propranolol), and 4) clamped hypoglycemia with pan-autonomic blockade (phentolamine, propranolol and atropine). Significant (ANOVA, P < 0.001) treatment effects on the awareness of hypoglycemia (“blood sugar low”) were noted. No change occurred in the score for this during euglycemia, but the mean ± SE increase was 2.1 ± 0.4 during hypoglycemia. This increase was not reduced significantly by adrenergic blockade (1.6 ± 0.5), but was reduced significantly and substantially (∼ 70%) by pan-autonomic blockade (0.6 ± 0.3). Significant neurogenic symptoms included shaky/tremulous (P < 0.001), heart pounding (P < 0.001), and nervous/anxious (P = 0.002), all adrenergic; and sweaty (P < 0.001), hungry (P < 0.001), and tingling (P = 0.009), all cholinergic. Significant neuroglycopenic symptoms, those produced by hypoglycemia but not reduced by pan-autonomic blockade, included warm (P < 0.001), weak (P = 0.011), difficulty thinking/confused (P = 0.004), and tired/drowsy (P = 0.003). We conclude that muscarinic cholinergic mechanisms mediate an important and previously uncharacterized component of the neurogenic symptoms of hypoglycemia and awareness of hypoglycemia. Awareness of hypoglycemia is largely, perhaps exclusively, the result of perception of neurogenic rather than neuroglycopenic symptoms.

Teriparatide (human parathyroid hormone (1-34)) inhibits osteogenic vascular calcification in diabetic low density lipoprotein receptor-deficient mice.

Cardiovascular calcification is a common consequence of diabetes. High fat diets induce diabetes and arterial calcification in male low density lipoprotein receptor (LDLR) –/– mice; calcification occurs via Msx2 signaling that promotes the osteogenic differentiation of arterial myofibroblasts. We studied regulation of arterial osteogenesis by human parathyroid hormone (PTH) (1–34) (also called teriparatide) in LDLR –/– mice fed diabetogenic diets for 4 weeks. LDLR –/– mice were treated with vehicle or 0.4 mg/kg of PTH(1–34) subcutaneously five times/week. Gene expression was determined from single aortas and hind limb RNA by fluorescence reverse transcription-PCR. Valve calcification was determined by histological staining of cardiac sections using image analysis to quantify valve leaflet mineralization. PTH(1–34) increased bone mineral content (by dual energy x-ray absorptiometry) in LDLR –/– mice, with induction of osseous osteopontin (OPN) expression and serum OPN levels (>150 nm); PTH(1–34) did not significantly change serum glucose, lipids, body weight, or fat mass. PTH(1–34) suppressed aortic OPN and Msx2 expression >50% and decreased cardiac valve calcification 80% (8.3 ± 1.5% versus 1.4 ± 0.5%; p < 0.001). Of the known circulating regulators of vascular calcification (OPN, osteoprotegerin, and leptin), PTH(1–34) regulated only serum OPN. We therefore studied actions of PTH(1–34) and OPN in vitro on cells induced to mineralize with Msx2. OPN (5–50 nm) reversed Msx2-induced mineralization. PTH(1–34) inhibited mineralization by 40% and down-regulated Msx2 in aortic myofibroblasts. PTH(1–34) inhibits vascular calcification and aortic osteogenic differentiation via direct actions and potentially via circulating OPN. PTH(1–34) exerts beneficial actions at early stages of macrovascular disease responses to diabetes and dyslipidemia.

Inflammation and the Osteogenic Regulation of Vascular Calcification: A Review and Perspective

Arterial biomineralization processes have been afflicting humans for ≥5 millennia, as realized in 2003 via the computed tomographic imaging of Otzi, the intriguing “ice mummy” discovered in the Tyrolean Alps.1 Patchy abdominal atherosclerotic calcification was readily detected in the postmortem of this ≈40-year-old hunter of the early Copper Age, by 2000 years a predecessor of King Tutankhamen.1 Today, an epidemic of vascular calcification is emerging within our aging and dysmetabolic populace.2,3 Although vascular calcification was once considered only a passive process of dead and dying cells, work from laboratories worldwide has now highlighted that arterial biomineralization is an actively regulated form of calcified tissue metabolism.4,5 Moreover, as in skeletal development – where unique biology controls matrix mineralization in membranous bone, endochondral bone, dentin, and enamel,6,7 mechanistic diversity exists in the pathobiology of vascular calcium deposition.2,4,5,8 Five common forms of vascular calcification, each possessing unique histoanatomic characteristics and clinical settings with overlapping yet distinct molecular mechanisms, have been described to date4,5,9 (Table 1⇓⇓). Although we touch on the subject, the reader is referred to other contemporary reviews for in-depth consideration of pathogenic differences.2,4,5 View this table: Table 1. Common Histoanatomic Forms of Vascular Calcification and Clinical Settings 2,4,5,11,99 View this table: Table 1. Continued View this table: Table 1. Continued In this brief review and perspective, we recount recent data that emphasize inflammation and oxidative stress signaling as key contributors to the pathogenesis of vascular mineral deposition.10 Furthermore, we highlight differences between the low-density lipoprotein receptor (LDLR)-deficient and apolipoprotein E (apoE)-deficient murine models (Table 2) that help articulate the multifaceted contributions of dyslipidemia, diabetes mellitus, and uremia to arterial calcium deposition.2,4,11 We end by summarizing the importance of considering these disease stage- and context-specific contributions arterial mineralization when crafting therapeutic strategies to address the disease burden of vascular …

Calcific Aortic Valve Disease: Not Simply a Degenerative Process A Review and Agenda for Research from the National Heart and Lung and Blood Institute Aortic Stenosis Working Group

Calcific aortic valve disease (CAVD) encompasses the range of disease from initial alterations in the cell biology of the leaflets to end-stage calcification resulting in left ventricular outflow obstruction. The first detectable macroscopic changes in the leaflets, seen as calcification, or focal leaflet thickening with normal valve function, is termed aortic valve sclerosis, but it is likely that the initiating events in the disease process occur much earlier. Disease progression is characterized by a process of thickening of the valve leaflets and the formation of calcium nodules – often including the formation of actual bone – and new blood vessels, which are concentrated near the aortic surface. End stage disease, e.g. calcific aortic stenosis, is characterized pathologically by large nodular calcific masses within the aortic cusps that protrude through the outflow surfaces into the sinuses of Valsalva, interfering with opening of the cusps. For decades, this disease was thought to be a passive process in which the valve degenerates with age in association with calcium accumulation. Moreover, although calcific aortic valve disease is more common with age, it is not an inevitable consequence of aging. Instead, CAVD appears to be an actively regulated disease process that cannot be characterized exclusively as “senile” or “degenerative.” The NHLBI convened a group of scientists from different fields of study, including cardiac imaging, molecular biology, cardiovascular pathology, epidemiology, cell biology, endocrinology, bioengineering, and clinical outcomes, to review the scientific studies from the past decade in the field of CAVD. The purpose was to develop a consensus statement on the current state of translational research related to CAVD. Herein, we summarize recent scientific studies and define future directions for research to diagnose, treat and potentially prevent this complex disease process.

Arterial calcification and bone physiology: role of the bone–vascular axis

Bone never forms without vascular interactions. This simple statement of fact does not adequately reflect the physiological and pharmacological implications of the relationship. The vasculature is the conduit for nutrient exchange between bone and the rest of the body. The vasculature provides the sustentacular niche for development of osteoblast progenitors and is the conduit for egress of bone marrow cell products arising, in turn, from the osteoblast-dependent haematopoietic niche. Importantly, the second most calcified structure in humans after the skeleton is the vasculature. Once considered a passive process of dead and dying cells, vascular calcification has emerged as an actively regulated form of tissue biomineralization. Skeletal morphogens and osteochondrogenic transcription factors are expressed by cells within the vessel wall, which regulates the deposition of vascular calcium. Osteotropic hormones, including parathyroid hormone, regulate both vascular and skeletal mineralization. Cellular, endocrine and metabolic signals that flow bidirectionally between the vasculature and bone are necessary for both bone health and vascular health. Dysmetabolic states including diabetes mellitus, uraemia and hyperlipidaemia perturb the bone–vascular axis, giving rise to devastating vascular and skeletal disease. A detailed understanding of bone–vascular interactions is necessary to address the unmet clinical needs of an increasingly aged and dysmetabolic population.