Nodal/SMAD2 and Bone Morphogenic Protein 4 RNA Expression in Microgravity. Implications for Spacial Orientation Signaling in Hematopoesis and the Implications for the Hematosuppressive Effects of Space Flight

Introduction: Nodal proteins are members of the TGF alpha family which direct right left orientation in the developing embryo. Nodal/SMAD2 signaling controls BMP4 expression and with canonical Wnt signaling, regulates the posteriorization of the primitive streak, development of the mesoderm, development of early hematopoetic stem cells, and the property of self renewal in adult hematopoiesis. This study investigated the extent that microgravity alters expression of these spatially orienting proteins of BMP4, Nodal, Wnt, and SMAD2 thus influencing the anemia, thrombocytopenia, and immune alteration associated with space flight.

Method: The Gene Expression Omnibus Database (GEO) of the National Institutes of Health was accessed to define RNA expression of Nodal, SMAD2, Wnt and BMP4 in hematopoetic cells in microgravity. Initial queries for "Low Gravity" studies resulted in 565 investigations within the GEO Omnibus. Of the 565 studies, the GSE136939, GSE101102, and the GSE101309 datasets were explored which assessed the lymphoid and macrophage cell lines TK6, U937, and Jurkat. RNA expression with normal and microgravity environments were compared. In the U937 cells hypergravity environments were also studied. Nodal, Lefty, Wnt, BMP4, as well as the NODAL signaling cascade proteins of SMAD 1/5, SMAD2, SMAD3, and the inhibitory SMAD 7. Affymetrix Expression Arrays were utilized for the TK^ and Jurkat cells assays. The U937 cell lines were assayed with NimbleGen Human Gene Assay Arrays.

Results: Significant decreases in BMP4 expression were seen in microgravity environments (P value, <0.005) where SMAD2 expression was significantly elevated (P value <0.05) in microgravity. Conversely, hypergravity effects on U937 cells resulted in a reverse expression pattern. (BMP4 expression increased while SMAD2 decreased (P value <0.01) ). Comparisons of 14, 12, and 4 expression assays were made in U937, Jurkat, and TK6 cells respectively. LEFTY, NODAL, Wnt, SMA1/5/3/7 expression assays all showed changes in expression, but the small sample numbers resulted in poor statistical power and insignificant P values to detect differences in the RNA expression assessed. Comparisons of the gravitational pressures and time spent in these environments for the cellular cultures resulted in insignificant statistical differences.

Conclusions: Microgravity induces decreases in BMP4 RNA expression and increases in SMAD2 RNA expression in TK6, U937, and Jurkat cell as defined by the GSE101309, GSE101102 and GSE136939 datasets. BMP4 is required for stem cell posteriorization, HSC number, and adult hematopoetic self renewal. This diminishment in BMP4 expression in microgravity may result in diminished HSC number and self renewal capacity. This qualitative change in gene expression in microgravity and may be a significant component of the anemia, thrombocytopenia, and immune alterations of space flight. Moreover such changes in RNA expression in microgravity implicates the importance of hematopoetic spacial orientation signals in normal hematopoiesis. Therapeutic targeting of the BMP/SMAD2 pathway in refractory multilineage hematopoetic diseases may prove to be fruitful. Statistical interpretation of the data is hindered by the small number of samples, the cell type, hematopoetic maturation stage of the samples, and microenvironmental differences in the studies utilized in these datasets. If this result is confirmed in future and larger studies, it would have significant biologic and therapeutic implications for long term space flight and the developing concept of spacial hematopoiesis.