Leptin Induces Mitosis and Activates the Canonical Wnt/β-Catenin Signaling Pathway in Neurogenic Regions of Xenopus Tadpole Brain

Abstract

In addition to its well-known role as an adipostat in adult mammals, leptin has diverse physiological and developmental actions in vertebrates. Leptin has been shown to promote development of hypothalamic circuits and to induce mitosis in different brain areas of mammals. We investigated the ontogeny of leptin mRNA, leptin actions on cell proliferation in the brain, and gene expression in the preoptic area/hypothalamus of tadpoles of Xenopus laevis. The level of leptin mRNA was low in premetamorphic tadpoles, but increased strongly at the beginning of metamorphosis and peaked at metamorphic climax. This increase in leptin mRNA at the onset of metamorphosis correlated with increased cell proliferation in the neurogenic zones of tadpole brain. We found that intracerebroventricular (i.c.v.) injection of recombinant Xenopus leptin (rxLeptin) in premetamorphic tadpoles strongly increased cell proliferation in neurogenic zones throughout the tadpole brain. We conducted gene expression profiling of genes induced at 2 h following i.c.v. injection of rxLeptin. This analysis identified 2,322 genes induced and 1,493 genes repressed by rxLeptin. The most enriched Kyoto Encyclopedia of Genes and Genomes term was the canonical Wnt/β-catenin pathway. Using electroporation-mediated gene transfer into tadpole brain of a reporter vector responsive to the canonical Wnt/β-catenin signaling pathway, we found that i.c.v. rxLeptin injection activated Wnt/β-catenin-dependent transcriptional activity. Our findings show that leptin acts on the premetamorphic tadpole brain to induce cell proliferation, possibly acting via the Wnt/β-catenin signaling pathway.

To eat or not to eat: ontogeny of hypothalamic feeding controls and a role for leptin in modulating life-history transition in amphibian tadpoles

“…This, and the lack of adiposity signals due to the absence of fat stores, allows the animal to maximize growth during this critical life-history stage. When lipids accumulate in the tadpole's fat bodies, lep mRNA levels increase, occurring just before the onset of metamorphosis [14]. In mammals, leptin is constitutively secreted by adipocytes in proportion to triglyceride content [15].…”

“…As tadpole metamorphosis progresses, behavioural and neuroendocrine circuits residing in the POA and hypothalamus mature [31]. This development may be influenced by the rising circulating concentration of leptin, because the hormone can induce mitosis in premetamorphic tadpole brain [14]. Leptin is a candidate hormone for signalling to the tadpole's brain information about body condition/energy stores, which is central to the timing of metamorphosis.…”

“…Leptin mRNA was first detectable at NF stage 40, and remained low and constant until just before the onset of metamorphosis at NF stage 50, when it showed a strong increase (sixfold;figure 2a,b); the mRNA for the reference gene rpL8 was stable across development. The increase in lep mRNA during metamorphosis is contributed by the fat bodies[14]. Food restriction of NF stage 50 tadpoles caused a statistically significant decrease in steady-state lep mRNA in fat and liver (figure 2c), showing that the frog lep gene in adipocytes responds similarly to lep of mammals.…”

Leptin and psychiatric illnesses: does leptin play a role in antipsychotic-induced weight gain?

“…Esketamine is a non-competitive antagonist of glutamate receptors N-methyl-D-aspartate (NMDA), and leptin has been found to act on this receptor by directing its synaptic activity [54]. Concerning neurogenesis, previous animal (mice) studies demonstrated that leptin plays a critical role in neuroprotection especially by the regulation of the GSK-3β/β-catenin signaling pathway [55,56].…”

Bulk Electroporation-Mediated Gene Transfer intoXenopusTadpole Brain

“…This technique is also useful for determining the functionality of gene regulatory elements in an in vivo context. For example, plasmids with native or mutant gene regulatory elements driving expression of reporter genes like firefly luciferase or EGFP can be transfected into tadpole brain, followed by biochemical or histochemical analysis of the reporter (Yao et al 2007(Yao et al , 2008Bagamasbad et al 2015;Bender et al 2017). The technique is also used to deliver antisense morpholino oligonucleotides (Bestman and Cline 2014), and may be used with RNAi constructs or CRISPR/Cas9 plasmids to knockdown or knockout genes in targeted cells.…”