Tuberous sclerosis complex (TSC) is an autosomal dominant disease characterized by the presence of benign tumors called hamartomas, which can affect virtually every organ system of the body including the brain (where hamartomas are known as cortical tubers)1
. Most TSC patients also develop epilepsy, and 25–50% are diagnosed with autism spectrum disorders. Although it has been proposed that the cortical tubers cause seizures and cognitive deficits, increasing evidence suggests a poor correlation between cortical tubers and the incidence of epilepsy or autism in TSC patients2
. Furthermore, animal models of TSC have increased susceptibility to seizures in the absence of cortical tubers, supporting the notion that tubers are not responsible for epilepsy. Hence, other mechanisms – such as miswiring of neuronal connections – may contribute to the pathogenesis of epilepsy, autism and intellectual disabilities in TSC patients.
TSC is caused by mutations in either of two genes: TSC1
, whose protein products form a complex that plays a major role in the phosphatidylinositol 3-kinase (PI3K)-Akt-mTOR pathway. Binding of a growth factor such as insulin to its cell surface receptor leads to activation of PI3K, which in turn activates the Akt kinase. Phosphorylation of TSC2 by Akt releases the inhibitory effect of the TSC1/TSC2 complex on the Ras family GTPase, Rheb3, 4
. Rheb and its downstream effector mTOR are master regulators of cell growth. When Rheb is activated, the protein synthesis machinery is turned on, most likely via mTOR, and cell growth programs are initiated. Therefore, the TSC1/TSC2 complex keeps cell size in check by inhibiting mTOR-mediated mRNA translation. Cells with insufficient TSC1 or TSC2 function grow beyond their normal size and form hamartomas, but the pathophysiology of the neurological symptoms in TSC patients remains poorly understood.
The establishment of neural circuits in vivo
requires a precise interplay between extending axons and guidance cues in their environment. One of the best-characterized axon pathways in the central nervous system is the projection of retinal ganglion cells (RGCs) from the eye to their targets in the brain. Many proteins – such as neurotrophins, semaphorins, slits and ephrins – regulate retinal axon pathfinding and topographic mapping in target regions such as the dorsal lateral geniculate nucleus (dLGN)5
. Interactions between EphA receptors and ephrin-A ligands expressed in gradients in retinal neurons and across the dLGN play prominent roles in initial topographic map formation in the dLGN6
. Spontaneous retinal activity then contributes to map refinement during postnatal stages7–9
Binding of ephrin ligands triggers Eph receptor clustering, autophosphorylation and downstream signaling cascades that cause cytoskeletal rearrangements and changes in cell adhesion10
. Through these mechanisms, Eph receptors control axon turning, retraction and branching. Local regulation of protein synthesis and degradation in the axon also contributes to the rapid changes in growth cone dynamics that occur during axonal navigation11–15
. Both repulsive and attractive cues can alter local protein translation in an mTOR-dependent manner, suggesting that guidance cues might affect axon growth and navigation at least in part by modulating mTOR activity14, 16
We have identified a new role for Tsc1/Tsc2 in axon guidance by using mouse models of TSC. We found that components of the Tsc-mTOR pathway are highly expressed in developing RGC axons and that Tsc2+/− mice, which have elevated mTOR activity in RGCs, develop aberrant retinogeniculate projections. Consistent with this phenotype, in vitro Tsc2+/− RGCs are less sensitive to ephrin-A repulsive effects. Furthermore, EphA receptor signaling inhibits the mTOR pathway and reduces local protein synthesis in neurons. Our findings reveal a new mode of regulation of the Tsc-mTOR pathway by cell surface receptor tyrosine kinases through the ERK1/2 kinases and shed light on the mechanism by which EphA receptors control mTOR activity and growth cone dynamics.