Cytokine control of neuronal phenotype

Diffusible proteins regulate neural development at a variety of stages. Using a novel neuronal culture assay, I have identified several cytokines that regulate the expression of neurotransmitters and neuropeptides in sympathetic neurons. These cytokines fall into two families. The first group is termed the neuropoietic cytokines, while including CDF/LIF, CNTF, OSM and GPA, induces expression of the same set of neuropeptide mRNAs in cultured sympathetic neurons. These four factors not only exhibit similar biological activities; they also share a predicted secondary structure and bind to a signal-transducing receptor subunit in common with IL-6 and IL-11. The latter two cytokines display a weaker activity in this assay. In addition, I find that several members of the TGF-β superfamily, activin A, BMP-2, and BMP-6, have a selective overlap with the neuropoietic family in the spectrum of neuropeptides that these cytokines induce in sympathetic neurons. Different patterns of neuropeptides induced by the TGF-β family members, however, demonstrate that the activities of these cytokines are distinct from those of the neuropoietic family. Another 30 cytokines are without detectable effect in this neuronal assay.

Activin A induces a set of neurotransmitters and neuropeptides that is somewhat similar to the phenotype of sympathetic neurons innervating sweat glands in rat footpads. In situ hybridization and RNase protection were carried out to test whether activins were involved in the phenotypic transition when sympathetic neurons contact sweat glands. I find that activin mRNA is present in both cholinergic and noradrenergic targets. Moreover, homogenates of footpads do not contain activin-like activity in the neuronal assay in vitro. Taken together, these data do not support activins as the best candidates for the sweat gland factor.

Several novel factors that regulate neuropeptide expression exist in heart cell conditioned medium. I attempted to purify these factors in collaboration with Dr. Jane Talvenheimo. Our results suggest that these factors are sensitive to the storage conditions used. Several modifications of purification strategy are discussed.

Function of microRNA-146a and NF-κB in physiologic and pathologic hematopoiesis

During inflammation and infection, hematopoietic stem and progenitor cells (HSPCs) are stimulated to proliferate and differentiate into mature immune cells, especially of the myeloid lineage. MicroRNA-146a (miR-146a) is a critical negative regulator of inflammation. Deletion of the gene encoding miR-146a—expressed in all blood cell types—produces effects that appear as dysregulated inflammatory hematopoiesis, leading to a decline in the number and quality of hematopoietic stem cells (HSCs), excessive myeloproliferation, and, ultimately, to exhaustion of the HSCs and hematopoietic neoplasms. Six-week-old deleted mice are normal, with no effect on cell numbers, but by 4 months bone marrow hypercellularity can be seen, and by 8 months marrow exhaustion is becoming evident. The ability of HSCs to replenish the entire hematopoietic repertoire in a myelo-ablated mouse also declines precipitously as miR-146a-deficient mice age. In the absence of miR-146a, LPS-mediated serial inflammatory stimulation accelerates the effects of aging. This chronic inflammatory stress on HSCs in deleted mice involves a molecular axis consisting of upregulation of the signaling protein TRAF6 leading to excessive activity of the transcription factor NF-κB and overproduction of the cytokine IL-6. At the cellular level, transplant studies show that the defects are attributable to both an intrinsic problem in the miR-146a-deficient HSCs and extrinsic effects of miR-146a-deficient lymphocytes and non-hematopoietic cells. This study has identified a microRNA, miR-146a, to be a critical regulator of HSC homeostasis during chronic inflammatory challenge in mice and has provided a molecular connection between chronic inflammation and the development of bone marrow failure and myeloproliferative neoplasms. This may have implications for human hematopoietic malignancies, such as myelodysplastic syndrome, which frequently displays downregulated miR-146a expression.