Species-specific blood-brain barrier permeability in amphibians.

Background: The blood-brain barrier (BBB) is a semipermeable interface that prevents the non-selective transport into the central nervous system. It controls the delivery of macromolecules fueling the brain metabolism and the immunological surveillance. The BBB permeability is locally regulated depending on the physiological requirements, maintaining the tissue homeostasis and influencing pathological conditions. Given its relevance in vertebrate CNS, it is surprising that little is known about the BBB in Amphibians, some of which are capable of adult CNS regeneration.

Results: The BBB size threshold of the anuran Xenopus laevis (African clawed toad), as well as two urodele species, Ambystoma mexicanum (axolotl) and Pleurodeles waltl (Iberian ribbed newt), was evaluated under physiological conditions through the use of synthetic tracers. We detected important differences between the analyzed species. Xenopus exhibited a BBB with characteristics more similar to those observed in mammals, whereas the BBB of axolotl was found to be permeable to the 1 kDa tracer. The permeability of the 1 kDa tracer measured in Pleurodeles showed values in between axolotl and Xenopus vesseks. We confirmed that these differences are species-specific and not related to metamorphosis. In line with these results, the tight junction protein Claudin-5 was absent in axolotl, intermediate in Pleurodeles and showed full-coverage in Xenopus vessels. Interestingly, electron microscopy analysis and the retention pattern of the larger tracers (3 and 70 kDa) demonstrated that axolotl endothelial cells exhibit higher rates of macropinocytosis, a non-regulated type of transcellular transport.

Conclusions: Our study demonstrated that, under physiological conditions, the blood-brain barrier exhibited species-specific variations, including permeability threshold, blood vessel coverage, and macropinocytosis rate. Future studies are needed to test whether the higher permeability observed in salamanders could have metabolic and immunological consequences contributing to their remarkable regenerative capacity.