Wnt5a regulates hematopoietic stem cell proliferation and repopulation through the Ryk receptor

Wnt5a signaling increases IL-12 secretion by human dendritic cells and enhances IFN-γ production by CD4+ T cells

Wnt5a is a secreted pleiotropic glycoprotein produced in an inflammatory state by a wide spectrum of ubiquitous cell populations. Recently, we demonstrated that Wnt5a skews the differentiation of human monocyte derived dendritic cells (moDCs) to a tolerogenic functional state. In this study we focus our interest on the role of this Wnt ligand after DC differentiation, during their maturation and function. We show that the expression of Wnt receptors is tightly regulated during the life cycle of DCs suggesting a differential responsiveness to Wnt signaling conditioned by their differentiation stage and the maturational stimuli. Furthermore, we confirm that Wnt5a is the main non-canonical Wnt protein expressed by DCs and its production increases upon specific stimuli. Exogenous Wnt5a improved the endocytic capacity of immature DCs but it is not a stimulatory signal on its own, slightly affecting the maturation and function of DCs. However, knocking down Wnt5a gene expression in maturing DCs demonstrates that DC-derived Wnt5a is necessary for normal IL-12 secretion and plays a positive role during the development of Th1 responses. Wnt5a acts both in autocrine and paracrine ways. Thus, human naive CD4(+) T cells express Wnt receptors and, the addition of Wnt5a during CD3/CD28 stimulation enhances IL-2 and IFN-γ production. Taken together these results suggest a time-dependent role for Wnt5a during inflammatory responses conditioned by the differentiation stage of cellular targets.

The Wnt receptor Ryk reduces neuronal and cell survival capacity by repressing FOXO activity during the early phases of mutant huntingtin pathogenicity

The Wnt receptor Ryk is an evolutionary-conserved protein important during neuronal differentiation through several mechanisms, including γ-secretase cleavage and nuclear translocation of its intracellular domain (Ryk-ICD). Although the Wnt pathway may be neuroprotective, the role of Ryk in neurodegenerative disease remains unknown. We found that Ryk is up-regulated in neurons expressing mutant huntingtin (HTT) in several models of Huntington's disease (HD). Further investigation in Caenorhabditis elegans and mouse striatal cell models of HD provided a model in which the early-stage increase of Ryk promotes neuronal dysfunction by repressing the neuroprotective activity of the longevity-promoting factor FOXO through a noncanonical mechanism that implicates the Ryk-ICD fragment and its binding to the FOXO co-factor β-catenin. The Ryk-ICD fragment suppressed neuroprotection by lin-18/Ryk loss-of-function in expanded-polyQ nematodes, repressed FOXO transcriptional activity, and abolished β-catenin protection of mutant htt striatal cells against cell death vulnerability. Additionally, Ryk-ICD was increased in the nucleus of mutant htt cells, and reducing γ-secretase PS1 levels compensated for the cytotoxicity of full-length Ryk in these cells. These findings reveal that the Ryk-ICD pathway may impair FOXO protective activity in mutant polyglutamine neurons, suggesting that neurons are unable to efficiently maintain function and resist disease from the earliest phases of the pathogenic process in HD.

Regulation of hematopoiesis by activators and inhibitors of Wnt signaling from the niche

Hematopoietic stem cells (HSCs) are a rare population of somatic stem cells that have the ability to regenerate the entire mature blood system in a hierarchical way for the duration of an adult life. Adult HSCs reside in the bone marrow niche. Different niche cell types and molecules regulate the balance of HSC dormancy and activation as well as HSC behavior in both normal and malignant hematopoiesis. Here, we describe the interplay of HSCs and their niche, in particular the involvement of the Wnt signaling pathway. Although the prevailing notion has been that malignant transformation of HSCs is the main cause of leukemia, evidence is mounting that disruption of niche regulation by transformed hematopoietic cells, which may overexpress Wnt signaling or intrinsic stromal defects in gene expression, is at least a collaborative factor in leukemogenesis. Thus, insights into the normal and altered functions of niche components will help to obtain a better understanding of normal and malignant hematopoiesis and how environmental factors affect these processes.