Test results of a Stirling engine utilizing heat exchanger modules with an integral heat pipe

The Heat Pipe Stirling Engine (HP-1000), a free-piston Stirling engine incorporating three heat exchanger modules, each having a sodium filled heat pipe, has been tested at the NASA-Lewis Research Center as part of the Civil Space Technology Initiative (CSTI). The heat exchanger modules were designed to reduce the number of potential flow leak paths in the heat exchanger assembly and incorporate a heat pipe as the link between the heat source and the engine. An existing RE-1000 free-piston Stirling engine was modified to operate using the heat exchanger modules. This paper describes heat exchanger module and engine performance during baseline testing. Condenser temperature profiles, brake power, and efficiency are presented and discussed

Bisphosphonates: from bone-targeted therapies to molecular imaging agents of in vivo bone metabolism

Using bisphosphonates as targeting molecules to generate new drug conjugates and bone imaging probes for the assessment of bone metabolisms and the prevention of bone diseas

Acid-Sphingomyelinase Triggered Fluorescently Labeled Sphingomyelin Containing Liposomes in Tumor Diagnosis after Radiation-Induced Stress

In liposomal delivery, a big question is how to release the loaded material into the correct place. Here, we will test the targeting and release abilities of our sphingomyelin-consisting liposome. A change in release parameters can be observed when sphingomyelin-containing liposome is treated with sphingomyelinase enzyme. Sphingomyelinase is known to be endogenously released from the different cells in stress situations. We assume the effective enzyme treatment will weaken the liposome making it also leakier. To test the release abilities of the SM-liposome, we developed several fluorescence-based experiments. In in vitro studies, we used molecular quenching to study the sphingomyelinase enzyme-based release from the liposomes. We could show that the enzyme treatment releases loaded fluorescent markers from sphingomyelin-containing liposomes. Moreover, the release correlated with used enzymatic activities. We studied whether the stress-related enzyme expression is increased if the cells are treated with radiation as a stress inducer. It appeared that the radiation caused increased enzymatic activity. We studied our liposomes' biodistribution in the animal tumor model when the tumor was under radiation stress. Increased targeting of the fluorescent marker loaded to our liposomes could be found on the site of cancer. The liposomal targeting in vivo could be improved by radiation. Based on our studies, we propose sphingomyelin-containing liposomes can be used as a controlled release system sensitive to cell stress

Periarticular Mesenchymal Progenitors Initiate and Contribute to Secondary Ossification Center Formation During Mouse Long Bone Development

Long bone development involves the embryonic formation of a primary ossification center (POC) in the incipient diaphysis followed by postnatal development of a secondary ossification center (SOC) at each epiphysis. Studies have elucidated major basic mechanisms of POC development, but relatively little is known about SOC development. To gain insights into SOC formation, we used Col2-Cre Rosa-tdTomato (Col2/Tomato) reporter mice and found that their periarticular region contained numerous Tomato-positive lineage cells expressing much higher Tomato fluorescence (termed TomatoH) than underlying epiphyseal chondrocytes (termed TomatoL). With time, the TomatoH cells became evident at the SOC invagination site and cartilage canal, increased in number in the expanding SOC, and were present as mesenchymal lineage cells in the subchondral bone. These data were verified in two mouse lineage tracing models, Col2-CreER Rosa-tdTomato and Glil-CreER Rosa-tdTomato. In vitro tests showed that the periarticular TomatoH cells from Col2/Tomato mice contained mesenchymal progenitors with multidifferentiation abilities. During canal initiation, the cells expressed vascular endothelial growth factor (VEGF) and migrated into epiphyseal cartilage ahead of individual or clusters of endothelial cells, suggesting a unique role in promoting vasculogenesis. Later during SOC expansion, chondrocytes in epiphyseal cartilage expressed VEGF, and angiogenic blood vessels preceded TomatoH cells. Gene expression analyses of microdissected samples revealed upregulation of MMPs in periarticular cells at the invagination site and suggested potential roles for novel kinase and growth factor signaling pathways in regulating SOC canal initiation. In summary, our data indicate that the periarticular region surrounding epiphyseal cartilage contains mesenchymal progenitors that initiate SOC development and forms subchondral bone

PDGFRα reporter activity identifies periosteal progenitor cells critical for bone formation and fracture repair

The outer coverings of the skeleton, which is also known as the periosteum, are arranged in concentric layers and act as a reservoir for tissue-specific bone progenitors. The cellular heterogeneity within this tissue depot is being increasingly recognized. Here, inducible PDGFR alpha reporter animals were found to mark a population of cells within the periosteum that act as a stem cell reservoir for periosteal appositional bone formation and fracture repair. During these processes, PDGFR alpha reporter(+) progenitors give rise to Nestin(+) periosteal cells before becoming osteoblasts and osteocytes. The diphtheria toxin-mediated ablation of PDGFR alpha reporter(+) cells led to deficits in cortical bone formation during homeostasis and a diminutive hard callus during fracture repair. After ossicle transplantation, both mouse PDGFR alpha reporter(+) periosteal cells and human Pdgfr alpha(+) periosteal progenitors expand, ossify, and recruit marrow to a greater extent than their counterpart periosteal cells, whereas PDGFR alpha reporter(-) periosteal cells exhibit a predisposition to chondrogenesis in vitro. Total RNA sequencing identified enrichment of the secreted factors Fermt3 and Ptpn6 within PDGFR alpha reporter(+) periosteal cells, which partly underlie the osteoblastogenic features of this cell population

NGF-p75 signaling coordinates skeletal cell migration during bone repair

: Bone regeneration following injury is initiated by inflammatory signals and occurs in association with infiltration by sensory nerve fibers. Together, these events are believed to coordinate angiogenesis and tissue reprogramming, but the mechanism of coupling immune signals to reinnervation and osteogenesis is unknown. Here, we found that nerve growth factor (NGF) is expressed following cranial bone injury and signals via p75 in resident mesenchymal osteogenic precursors to affect their migration into the damaged tissue. Mice lacking Ngf in myeloid cells demonstrated reduced migration of osteogenic precursors to the injury site with consequently delayed bone healing. These features were phenocopied by mice lacking p75 in Pdgfra+ osteoblast precursors. Single-cell transcriptomics identified mesenchymal subpopulations with potential roles in cell migration and immune response, altered in the context of p75 deletion. Together, these results identify the role of p75 signaling pathway in coordinating skeletal cell migration during early bone repair

Spatial transcriptomics reveals a role for sensory nerves in preserving cranial suture patency through modulation of BMP/TGF-β signaling

: The patterning and ossification of the mammalian skeleton requires the coordinated actions of both intrinsic bone morphogens and extrinsic neurovascular signals, which function in a temporal and spatial fashion to control mesenchymal progenitor cell (MPC) fate. Here, we show the genetic inhibition of tropomyosin receptor kinase A (TrkA) sensory nerve innervation of the developing cranium results in premature calvarial suture closure, associated with a decrease in suture MPC proliferation and increased mineralization. In vitro, axons from peripheral afferent neurons derived from dorsal root ganglions (DRGs) of wild-type mice induce MPC proliferation in a spatially restricted manner via a soluble factor when cocultured in microfluidic chambers. Comparative spatial transcriptomic analysis of the cranial sutures in vivo confirmed a positive association between sensory axons and proliferative MPCs. SpatialTime analysis across the developing suture revealed regional-specific alterations in bone morphogenetic protein (BMP) and TGF-β signaling pathway transcripts in response to TrkA inhibition. RNA sequencing of DRG cell bodies, following direct, axonal coculture with MPCs, confirmed the alterations in BMP/TGF-β signaling pathway transcripts. Among these, the BMP inhibitor follistatin-like 1 (FSTL1) replicated key features of the neural-to-bone influence, including mitogenic and anti-osteogenic effects via the inhibition of BMP/TGF-β signaling. Taken together, our results demonstrate that sensory nerve-derived signals, including FSTL1, function to coordinate cranial bone patterning by regulating MPC proliferation and differentiation in the suture mesenchyme

Systemic DKK1 neutralization enhances human adipose-derived stem cell mediated bone repair

: Progenitor cells from adipose tissue are able to induce bone repair; however, inconsistent or unreliable efficacy has been reported across preclinical and clinical studies. Soluble inhibitory factors, such as the secreted Wnt signaling antagonists Dickkopf-1 (DKK1), are expressed to variable degrees in human adipose-derived stem cells (ASCs), and may represent a targetable "molecular brake" on ASC mediated bone repair. Here, anti-DKK1 neutralizing antibodies were observed to increase the osteogenic differentiation of human ASCs in vitro, accompanied by increased canonical Wnt signaling. Human ASCs were next engrafted into a femoral segmental bone defect in NOD-Scid mice, with animals subsequently treated with systemic anti-DKK1 or isotype control during the repair process. Human ASCs alone induced significant but modest bone repair. However, systemic anti-DKK1 induced an increase in human ASC engraftment and survival, an increase in vascular ingrowth, and ultimately improved bone repair outcomes. In summary, anti-DKK1 can be used as a method to augment cell-mediated bone regeneration, and could be particularly valuable in the contexts of impaired bone healing such as osteoporotic bone repair

Lysosomal protein surface expression discriminates fat- from bone-forming human mesenchymal precursor cells

Tissue resident mesenchymal stem/stromal cells (MSCs) occupy perivascular spaces. Profiling human adipose perivascular mesenchyme with antibody arrays identified 16 novel surface antigens, including endolysosomal protein CD107a. Surface CD107a expression segregates MSCs into functionally distinct subsets. In culture, CD107a(low) cells demonstrate high colony formation, osteoprogenitor cell frequency, and osteogenic potential. Conversely, CD107a(high) cells include almost exclusively adipocyte progenitor cells. Accordingly, human CD107a(low) cells drove dramatic bone formation after intramuscular transplantation in mice, and induced spine fusion in rats, whereas CD107a(high) cells did not. CD107a protein trafficking to the cell surface is associated with exocytosis during early adipogenic differentiation. RNA sequencing also suggested that CD107a(low) cells are precursors of CD107a(high) cells. These results document the molecular and functional diversity of perivascular regenerative cells, and show that relocation to cell surface of a lysosomal protein marks the transition from osteo- to adipogenic potential in native human MSCs, a population of substantial therapeutic interest