Ashley C. Kramer

Bone marrow stromal and vascular smooth muscle cells have chemosensory capacity via bitter taste receptor expression.

The ability of cells to detect changes in the microenvironment is important in cell signaling and responsiveness to environmental fluctuations. Our interest is in understanding how human bone marrow stromal-derived cells (MSC) and their relatives, vascular smooth muscle cells (VSMC), interact with their environment through novel receptors. We found, through a proteomics screen, that MSC express the bitter taste receptor, TAS2R46, a protein more typically localized to the taste bud. Expression was also confirmed in VSMCs. A prototypical bitter compound that binds to the bitter taste receptor class, denatonium, increased intracellular calcium release and decreased cAMP levels as well as increased the extracellular release of ATP in human MSC. Denatonium also bound and activated rodent VSMC with a change in morphology upon compound exposure. Finally, rodents given denatonium in vivo had a significant drop in blood pressure indicating a vasodilator response. This is the first description of chemosensory detection by MSC and VSMCs via a taste receptor. These data open a new avenue of research into discovering novel compounds that operate through taste receptors expressed by cells in the marrow and vascular microenvironments.

A model of glucose-6-phosphate dehydrogenase deficiency in the zebrafish

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common genetic defect and enzymopathy worldwide, affecting approximately 400 million people and causing acute hemolysis in persons exposed to prooxidant compounds such as menthol, naphthalene, antimalarial drugs, and fava beans. Mouse models have not been useful because of a lack of significant response to oxidative challenge. We turned to zebrafish (Danio rerio) embryos, which develop ex utero and are transparent, allowing visualization of hemolysis. We designed morpholinos to zebrafish g6pd that were effective in reducing gene expression as shown by Western blot and G6PD enzyme activity, resulting in a brisk hemolysis and pericardial edema secondary to anemia. Titration of the g6pd knockdown allowed us to generate embryos that displayed no overt phenotype until exposed to the prooxidant compounds 1-naphthol, menthol, or primaquine, after which they developed hemolysis and pericardial edema within 48-72 hours. We were also able to show that g6pd morphants displayed significant levels of increased oxidative stress compared with controls. We anticipate that this will be a useful model of G6PD deficiency to study hemolysis as well as oxidative stress that occurs after exposure to prooxidants, similar to what occurs in G6PD-deficient persons.

sdf1 Expression Reveals a Source of Perivascular-Derived Mesenchymal Stem Cells in Zebrafish

There is accumulating evidence that mesenchymal stem cells (MSCs) have their origin as perivascular cells (PVCs) in vivo, but precisely identifying them has been a challenge, as they have no single definitive marker and are rare. We have developed a fluorescent transgenic vertebrate model in which PVC can be visualized in vivo based upon sdf1 expression in the zebrafish. Prospective isolation and culture of sdf1DsRed PVC demonstrated properties consistent with MSC including prototypical cell surface marker expression; mesodermal differentiation into adipogenic, osteogenic, and chondrogenic lineages; and the ability to support hematopoietic cells. Global proteomic studies performed by two‐dimensional liquid chromatography and tandem mass spectrometry revealed a high degree of similarity to human MSC (hMSC) and discovery of novel markers (CD99, CD151, and MYOF) that were previously unknown to be expressed by hMSC. Dynamic in vivo imaging during fin regeneration showed that PVC may arise from undifferentiated mesenchyme providing evidence of a PVC‐MSC relationship. This is the first model, established in zebrafish, in which MSC can be visualized in vivo and will allow us to better understand their function in a native environment. Stem Cells 2014;32:2767–2779

A Functional Bioluminescent Zebrafish Screen for Enhancing Hematopoietic Cell Homing

Summary To discover small molecules that modulate hematopoietic cell homing after adoptive transfer, we created a transgenic zebrafish expressing firefly luciferase downstream of the ubiquitin promoter (ubi:luc) to serve as a hematopoietic donor. Bioluminescence imaging (BLI) was used to detect and follow ubi:luc hematopoietic cells that homed to the marrow as early as 1 day post-transplant. BLI was able to detect the biological effect of prostaglandin E2 on early homing/engraftment of donor hematopoietic cells. This system was utilized in a functional screen of small molecules to enhance homing/engraftment. We discovered a phytosterol, ergosterol, that could increase hematopoietic cell homing in zebrafish and mice. In addition, ergosterol increased CXCR4 expression and promoted expansion of Lin−SCA-1+KIT+ cells in vitro. We have demonstrated the utility of in vivo BLI to non-invasively monitor donor hematopoietic cell activity in adult zebrafish as a functional screen for mediators of cellular homing.

Dermatopontin in Bone Marrow Extracellular Matrix Regulates Adherence but Is Dispensable for Murine Hematopoietic Cell Maintenance

Summary The hematopoietic marrow microenvironment is composed of multiple cell types embedded in an extracellular matrix (ECM). We have explored marrow ECM using mass spectrometry and found dermatopontin (DPT), a small non-collagenous ECM protein, to be present. We found that DPT cooperates with other ECM proteins to promote hematopoietic cell adherence in vitro on plastic as well as OP9 stromal cells. We generated constitutional DPT−/− mice that were viable and had no peripheral lympho-hematopoietic abnormalities. The composition of the marrow of wild-type and DPT−/− mice was equivalent in terms of cellularity, CFU-C, LSK (Lineage−, SCA-1+, KIT+), and LSK-SLAM (LSK, CD48−, CD150+) frequencies. These data suggest that DPT fosters adherence but is not required for steady-state hematopoiesis in vivo. There are likely overlapping cellular adhesion mechanisms that can compensate to maintain the hematopoietic niche in the absence of DPT.

TP53 Modulates Oxidative Stress in Gata1+ Erythroid Cells

Summary Metabolism of oxidative stress is necessary for cellular survival. We have previously utilized the zebrafish as a model of the oxidative stress response. In this study, we found that gata1-expressing erythroid cells contributed to a significant proportion of total-body oxidative stress when animals were exposed to a strong pro-oxidant. RNA-seq of zebrafish under oxidative stress revealed the induction of tp53. Zebrafish carrying tp53 with a mutation in its DNA-binding domain were acutely sensitive to pro-oxidant exposure and displayed significant reactive oxygen species (ROS) and tp53-independent erythroid cell death resulting in an edematous phenotype. We found that a major contributing factor to ROS was increased basal mitochondrial respiratory rate without reserve. These data add to the concept that tp53, while classically a tumor suppressor and cell-cycle regulator, has additional roles in controlling cellular oxidative stress.