# Direct and Indirect Downstream Pathways That Regulate Repulsive Guidance Effects of FGF3 on Developing Thalamocortical Axons

## TL;DR

This study explores how FGF3 repels developing thalamocortical axons through direct and indirect signaling pathways, revealing key molecular mechanisms.

## Contribution

The paper identifies the PC-PLC and PI3K pathways as mediators of FGF3's direct and indirect repulsive effects on axons.

## Key findings

- The PC-PLC pathway is essential for FGF3's direct chemorepulsion of thalamocortical axons.
- FGF3 indirectly induces repulsion by increasing Slit1 expression in the diencephalon.
- The PI3K pathway mediates the indirect repulsive effect of FGF3 through FGFR1.

## Abstract

The thalamus is an important sensory relay station. It integrates all somatic sensory pathways (excluding olfaction) and transmits information through thalamic relay neurons before projecting to the cerebral cortex via thalamocortical axons (TCAs). Emerging evidence has shown that FGF3, a member of the morphogen family, is an axon guidance molecule that repels TCAs away from the hypothalamus and into the internal capsule so that they subsequently reach different regions of the cortex. However, current studies on FGF-mediated axon guidance predominantly focus on phenomenological observations, with limited exploration of the underlying molecular mechanisms. To address this gap, we investigated both direct and indirect downstream signaling pathways mediating FGF3-dependent chemorepulsion of TCAs at later developmental stages. Firstly, we used pharmacological inhibitors to identify the signaling cascade(s) responsible for FGF3-triggered direct chemorepulsion of TCAs, in vitro and in vivo. Our results demonstrate that the PC-PLC pathway is required for FGF3 to directly stimulate the asymmetrical repellent growth of developing TCAs. Then, we found the FGF3-mediated repulsion can be indirectly induced by Slit1 because the addition of FGF3 in the culture media induced an increase in Slit1 expression in the diencephalon. Furthermore, by using downstream inhibitors, we found that the indirect repulsive effect of FGF3 is mediated through the PI3K downstream pathway of FGFR1.