Insights into foraminiferal influences on microfabrics of microbialites at Highborne Cay, Bahamas.

Microbialites, which are organosedimentary structures formed by microbial communities through binding and trapping and/or in situ precipitation, have a wide array of distinctive morphologies and long geologic record. The origin of morphological variability is hotly debated; elucidating the cause or causes of microfabric differences could provide insights into ecosystem functioning and biogeochemistry during much of Earth's history. Although rare today, morphologically distinct, co-occurring extant microbialites provide the opportunity to examine and compare microbial communities that may be responsible for establishing and modifying microbialite microfabrics. Highborne Cay, Bahamas, has extant laminated (i.e., stromatolites) and clotted (i.e., thrombolites) marine microbialites in close proximity, allowing focused questions about how community composition relates to physical attributes. Considerable knowledge exists about prokaryotic composition of microbialite mats (i.e., stromatolitic and thrombolitic mats), but little is known about their eukaryotic communities, especially regarding heterotrophic taxa. Thus, the heterotrophic eukaryotic communities of Highborne stromatolites and thrombolites were studied. Here, we show that diverse foraminiferal communities inhabit microbialite mat surfaces and subsurfaces; thecate foraminifera are relatively abundant in all microbialite types, especially thrombolitic mats; foraminifera stabilize grains in mats; and thecate reticulopod activities can impact stromatolitic mat lamination. Accordingly, and in light of foraminiferal impacts on modern microbialites, our results indicate that the microbialite fossil record may reflect the impact of the radiation of these protists.

Rac signaling in osteoblastic cells is required for normal bone development but is dispensable for hematopoietic development.

Hematopoietic stem cells (HSCs) interact with osteoblastic, stromal, and vascular components of the BM hematopoietic microenvironment (HM) that are required for the maintenance of long-term self-renewal in vivo. Osteoblasts have been reported to be a critical cell type making up the HSC niche in vivo. Rac1 GTPase has been implicated in adhesion, spreading, and differentiation of osteoblast cell lines and is critical for HSC engraftment and retention. Recent data suggest a differential role of GTPases in endosteal/osteoblastic versus perivascular niche function. However, whether Rac signaling pathways are also necessary in the cell-extrinsic control of HSC function within the HM has not been examined. In the present study, genetic and inducible models of Rac deletion were used to demonstrate that Rac depletion causes impaired proliferation and induction of apoptosis in the OP9 cell line and in primary BM stromal cells. Deletion of Rac proteins caused reduced trabecular and cortical long bone growth in vivo. Surprisingly, HSC function and maintenance of hematopoiesis in vivo was preserved despite these substantial cell-extrinsic changes. These data have implications for therapeutic strategies to target Rac signaling in HSC mobilization and in the treatment of leukemia and provide clarification to our evolving concepts of HSC-HM interactions.

Lifelong Excess in GH Elicits Sexually Dimorphic Effects on Skeletal Morphology and Bone Mechanical Properties.

Excess in growth hormone (GH) levels, seen in patients with acromegaly, is associated with increases in fractures. This happens despite wider bones and independent of bone mineral density. We used the bovine GH (bGH) transgenic mice, which show constitutive excess in GH and insulin-like growth factor 1 (IGF-1) in serum and tissues, to study how lifelong increases in GH and IGF-1 affect skeletal integrity. Additionally, we crossed the acid labile subunit (ALS) null (ALSKO) to the bGH mice to reduce serum IGF-1 levels. Our findings indicate sexually dimorphic effects of GH on cortical and trabecular bone. Male bGH mice showed enlarged cortical diameters, but with marrow cavity expansion and thin cortices as well as increased vascular porosity that were associated with reductions in diaphyseal strength and stiffness. In contrast, female bGH mice presented with significantly smaller-diameter diaphysis, with greater cortical bone thickness and with a slightly reduced tissue elastic modulus (by microindentation), ultimately resulting in overall stronger, stiffer bones. We found increases in C-terminal telopeptide of type 1 collagen and procollagen type 1 N propeptide in serum, independent of circulating IGF-1 levels, indicating increased bone remodeling with excess GH. Sexual dimorphism in response to excess GH was also observed in the trabecular bone compartment, particularly at the femur distal metaphysis. Female bGH mice preserved their trabecular architecture during aging, whereas trabecular bone volume in male bGH mice significantly reduced and was associated with thinning of the trabeculae. We conclude that pathological excess in GH results in sexually dimorphic changes in bone architecture and gains in bone mass that affect whole-bone mechanical properties, as well as sex-specific differences in bone material properties. © 2022 American Society for Bone and Mineral Research (ASBMR).

Induction of somatopause in adult mice compromises bone morphology and exacerbates bone loss during aging.

Somatopause refers to the gradual declines in growth hormone (GH) and insulin-like growth factor-1 throughout aging. To define how induced somatopause affects skeletal integrity, we used an inducible GH receptor knockout (iGHRKO) mouse model. Somatopause, induced globally at 6 months of age, resulted in significantly more slender bones in both male and female iGHRKO mice. In males, induced somatopause was associated with progressive expansion of the marrow cavity leading to significant thinning of the cortices, which compromised bone strength. We report progressive declines in osteocyte lacunar number, and increases in lacunar volume, in iGHRKO males, and reductions in lacunar number accompanied by ~20% loss of overall canalicular connectivity in iGHRKO females by 30 months of age. Induced somatopause did not affect mineral/matrix ratio assessed by Raman microspectroscopy. We found significant increases in bone marrow adiposity and high levels of sclerostin, a negative regulator of bone formation in iGHRKO mice. Surprisingly, however, despite compromised bone morphology, osteocyte senescence was reduced in the iGHRKO mice. In this study, we avoided the confounded effects of constitutive deficiency in the GH/IGF-1 axis on the skeleton during growth, and specifically dissected its effects on the aging skeleton. We show here, for the first time, that induced somatopause compromises bone morphology and the bone marrow environment.

Quasi-steady state aerodynamics of the cheetah tail.

During high-speed pursuit of prey, the cheetah (Acinonyx jubatus) has been observed to swing its tail while manoeuvring (e.g. turning or braking) but the effect of these complex motions is not well understood. This study demonstrates the potential of the cheetah's long, furry tail to impart torques and forces on the body as a result of aerodynamic effects, in addition to the well-known inertial effects. The first-order aerodynamic forces on the tail are quantified through wind tunnel testing and it is observed that the fur nearly doubles the effective frontal area of the tail without much mass penalty. Simple dynamic models provide insight into manoeuvrability via simulation of pitch, roll and yaw tail motion primitives. The inertial and quasi-steady state aerodynamic effects of tail actuation are quantified and compared by calculating the angular impulse imparted onto the cheetah's body and its shown aerodynamic effects contribute to the tail's angular impulse, especially at the highest forward velocities.

Effect of bisphosphonates on the rapidly growing male murine skeleton.

Bisphosphonates are effective for preventing and treating skeletal disorders associated with hyperresorption. Their safety and efficacy has been studied in adults where the growth plate is fused and there is no longitudinal bone growth and little appositional growth. Although bisphosphonate use in the pediatric population was pioneered for compassionate use in the treatment of osteogenesis imperfecta, they are being increasingly used for the treatment and prevention of bone loss in children at risk of hyperresorptive bone loss. However, the effect of these agents on the growing skeleton in disorders other than osteogenesis imperfecta has not been systematically compared. Studies were, therefore, undertaken to examine the consequences of bisphosphonate administration on the growth plate and skeletal microarchitecture during a period of rapid growth. C57Bl6/J male mice were treated from 18 to 38 days of age with vehicle, alendronate, pamidronate, zoledronate, or clodronate at doses selected to replicate those used in humans. Treatment with alendronate, pamidronate, and zoledronate, but not clodronate, led to a decrease in the number of chondrocytes per column in the hypertrophic chondrocyte layer. This was not associated with altered hypertrophic chondrocyte apoptosis or vascular invasion at the growth plate. The effects of pamidronate on trabecular microarchitecture were less beneficial than those of alendronate and zoledronate. Pamidronate did not increase cortical thickness or cortical area/total area relative to control mice. These studies suggest that bisphosphonate administration does not adversely affect skeletal growth. Long-term investigations are required to determine whether the differences observed among the agents examined impact biomechanical integrity of the growing skeleton.

Sclerostin antibody inhibits skeletal deterioration due to reduced mechanical loading.

Sclerostin, a product of the SOST gene produced mainly by osteocytes, is a potent negative regulator of bone formation that appears to be responsive to mechanical loading, with SOST expression increasing following mechanical unloading. We tested the ability of a murine sclerostin antibody (SclAbII) to prevent bone loss in adult mice subjected to hindlimb unloading (HLU) via tail suspension for 21 days. Mice (n = 11-17/group) were assigned to control (CON, normal weight bearing) or HLU and injected with either SclAbII (subcutaneously, 25 mg/kg) or vehicle (VEH) twice weekly. SclAbII completely inhibited the bone deterioration due to disuse, and induced bone formation such that bone properties in HLU-SclAbII were at or above values of CON-VEH mice. For example, hindlimb bone mineral density (BMD) decreased -9.2% ± 1.0% in HLU-VEH, whereas it increased 4.2% ± 0.7%, 13.1% ± 1.0%, and 30.6% ± 3.0% in CON-VEH, HLU-SclAbII, and CON-SclAbII, respectively (p < 0.0001). Trabecular bone volume, assessed by micro-computed tomography (µCT) imaging of the distal femur, was lower in HLU-VEH versus CON-VEH (p < 0.05), and was 2- to 3-fold higher in SclAbII groups versus VEH (p < 0.001). Midshaft femoral strength, assessed by three-point bending, and distal femoral strength, assessed by micro-finite element analysis (µFEA), were significantly higher in SclAbII versus VEH-groups in both loading conditions. Serum sclerostin was higher in HLU-VEH (134 ± 5 pg/mL) compared to CON-VEH (116 ± 6 pg/mL, p < 0.05). Serum osteocalcin was decreased by hindlimb suspension and increased by SclAbII treatment. Interestingly, the anabolic effects of sclerostin inhibition on some bone outcomes appeared to be enhanced by normal mechanical loading. Altogether, these results confirm the ability of SclAbII to abrogate disuse-induced bone loss and demonstrate that sclerostin antibody treatment increases bone mass by increasing bone formation in both normally loaded and underloaded environments.

Biomechanical consequences of rapid evolution in the polar bear lineage.

The polar bear is the only living ursid with a fully carnivorous diet. Despite a number of well-documented craniodental adaptations for a diet of seal flesh and blubber, molecular and paleontological data indicate that this morphologically distinct species evolved less than a million years ago from the omnivorous brown bear. To better understand the evolution of this dietary specialization, we used phylogenetic tests to estimate the rate of morphological specialization in polar bears. We then used finite element analysis (FEA) to compare the limits of feeding performance in the polar bear skull to that of the phylogenetically and geographically close brown bear. Results indicate that extremely rapid evolution of semi-aquatic adaptations and dietary specialization in the polar bear lineage produced a cranial morphology that is weaker than that of brown bears and less suited to processing tough omnivorous or herbivorous diets. Our results suggest that continuation of current climate trends could affect polar bears by not only eliminating their primary food source, but also through competition with northward advancing, generalized brown populations for resources that they are ill-equipped to utilize.