Molecular understanding of cytoneme-based Wnt trafficking

Cell-to-cell communication by signaling proteins is essential to orchestrate development and tissue homeostasis in all multicellular organisms. The highly conserved family of Wnt proteins are important guiding cues to control these processes. Fundamental to this complex signaling network are relatively small and defined signaling centers in a given tissue that produce and distribute Wnt proteins. Adjacent, larger groups of cells respond to these spatial and temporal information in a concentration-dependent manner and adjust their transcriptional program. However, a regulated sequence of morphogen activity is required to generate a fine tuned communication network. Therefore, a controlled propagation machinery must ensure accurate signal distribution from the source to the surrounding tissue to initiate the correct developmental path. In this thesis, I consolidated the knowledge of the molecular machinery controlling cytoneme formation in zebrafish development. I expanded this principle to other aspects of Wnt signaling such as cancer growth and tissue homeostasis. Via a screening approach, I identified the receptor tyrosine kinase Ror2 as a promoting factor for cellular protrusions in general and particularly for Wnt8a cytonemes in cultured cells and in vivo. Consistently, I described the novel ligand-receptor pair Wnt8a and Ror2 by measuring the affinity for membrane accumulations and by biophysical imaging applications such as fluorescence correlation spectroscopy. Subsequently, functional interaction and transduction of the Wnt/PCP pathway was demonstrated during zebrafish convergence and extension and during non-canonical reporter activation in Xenopus. Wnt8a and Ror2 are considered to act in mutually repressive pathways, although the autocrine interplay for cytoneme formation to facilitate paracrine Wnt/β-catenin dissemination seems to be conserved. Thus, the model can be applied to other systems: The transcriptional β-catenin level and resulting proliferation of gastric cancer cells can be regulated by Ror2, thereby only disrupting the signal transmitting transport machinery in the source cells. Furthermore, I provided evidence of an ex vivo human stem cell organoid system, where growth and survival require cytoneme-mediated Wnt proteins from isolated myofibroblasts. Remarkably, this setup resembles an innovative approach for stem cell maintenance in the murine intestinal crypt and expands the potential roles of cytonemes in development, tissue homeostasis and diseases

Wnt3 and Wnt3a are required for induction of the mid-diencephalic organizer in the caudal forebrain

Abstract Background A fundamental requirement for development of diverse brain regions is the function of local organizers at morphological boundaries. These organizers are restricted groups of cells that secrete signaling molecules, which in turn regulate the fate of the adjacent neural tissue. The thalamus is located in the caudal diencephalon and is the central relay station between the sense organs and higher brain areas. The mid-diencephalic organizer (MDO) orchestrates the development of the thalamus by releasing secreted signaling molecules such as Shh. Results Here we show that canonical Wnt signaling in the caudal forebrain is required for the formation of the Shh-secreting MD organizer in zebrafish. Wnt signaling induces the MDO in a narrow time window of 4 hours - between 10 and 14 hours post fertilization. Loss of Wnt3 and Wnt3a prevents induction of the MDO, a phenotype also observed upon blockage of canonical Wnt signaling per se. Pharmaceutical activation of the canonical Wnt pathways in Wnt3/Wnt3a compound morphant embryos is able to restore the lack of the MDO. After blockage of Wnt signaling or knock-down of Wnt3/Wnt3a we find an increase of apoptotic cells specifically within the organizer primordium. Consistently, blockage of apoptosis restores the thalamus organizer MDO in Wnt deficient embryos. Conclusion We have identified canonical Wnt signaling as a novel pathway, that is required for proper formation of the MDO and consequently for the development of the major relay station of the brain - the thalamus. We propose that Wnt ligands are necessary to maintain the primordial tissue of the organizer during somitogenesis by suppressing Tp53-mediated apoptosis. </jats:sec

Modeling of Wnt-mediated tissue patterning in vertebrate embryogenesis

During embryogenesis, morphogens form a concentration gradient in responsive tissue, which is then translated into a spatial cellular pattern. The mechanisms by which morphogens spread through a tissue to establish such a morphogenetic field remain elusive. Here, we investigate by mutually complementary simulations and in vivo experiments how Wnt morphogen transport by cytonemes differs from typically assumed diffusion-based transport for patterning of highly dynamic tissue such as the neural plate in zebrafish. Stochasticity strongly influences fate acquisition at the single cell level and results in fluctuating boundaries between pattern regions. Stable patterning can be achieved by sorting through concentration dependent cell migration and apoptosis, independent of the morphogen transport mechanism. We show that Wnt transport by cytonemes achieves distinct Wnt thresholds for the brain primordia earlier compared with diffusion-based transport. We conclude that a cytoneme-mediated morphogen transport together with directed cell sorting is a potentially favored mechanism to establish morphogen gradients in rapidly expanding developmental systems

Cannabigerol is a novel, well-tolerated appetite stimulant in pre-satiated rats

Rationale The appetite-stimulating properties of cannabis are well documented and have been predominantly attributed to the hyperphagic activity of the psychoactive phytocannabinoid, ∆9-tetrahydrocannabinol (∆9-THC). However, we have previously shown that a cannabis extract devoid of ∆9-THC still stimulates appetite, indicating that other phytocannabinoids also elicit hyperphagia. One possible candidate is the non-psychoactive phytocannabinoid cannabigerol (CBG), which has affinity for several molecular targets with known involvement in the regulation of feeding behaviour. Objectives The objective of the study was to assess the effects of CBG on food intake and feeding pattern microstructure. Methods Male Lister hooded rats were administered CBG (30–120 mg/kg, per ora (p.o.)) or placebo and assessed in open field, static beam and grip strength tests to determine a neuromotor tolerability profile for this cannabinoid. Subsequently, CBG (at 30–240 mg/kg, p.o.) or placebo was administered to a further group of pre-satiated rats, and hourly intake and meal pattern data were recorded over 2 h. Results CBG produced no adverse effects on any parameter in the neuromotor tolerability test battery. In the feeding assay, 120–240 mg/kg CBG more than doubled total food intake and increased the number of meals consumed, and at 240 mg/kg reduced latency to feed. However, the sizes or durations of individual meals were not significantly increased. Conclusions Here, we demonstrate for the first time that CBG elicits hyperphagia, by reducing latency to feed and increasing meal frequency, without producing negative neuromotor side effects. Investigation of the therapeutic potential of CBG for conditions such as cachexia and other disorders of eating and body weight regulation is thus warranted