Mechanisms of TSC-mediated Control of Synapse Assembly and Axon Guidance

Abstract

Tuberous sclerosis complex is a dominant genetic disorder produced by mutations in either of two tumor suppressor genes, TSC1 and TSC2; it is characterized by hamartomatous tumors, and is associated with severe neurological and behavioral disturbances. Mutations in TSC1 or TSC2 deregulate a conserved growth control pathway that includes Ras homolog enriched in brain (Rheb) and Target of Rapamycin (TOR). To understand the function of this pathway in neural development, we have examined the contributions of multiple components of this pathway in both neuromuscular junction assembly and photoreceptor axon guidance in Drosophila. Expression of Rheb in the motoneuron, but not the muscle of the larval neuromuscular junction produced synaptic overgrowth and enhanced synaptic function, while reductions in Rheb function compromised synapse development. Synapse growth produced by Rheb is insensitive to rapamycin, an inhibitor of Tor complex 1, and requires wishful thinking, a bone morphogenetic protein receptor critical for functional synapse expansion. In the visual system, loss of Tsc1 in the developing retina disrupted axon guidance independently of cellular growth. Inhibiting Tor complex 1 with rapamycin or eliminating the Tor complex 1 effector, S6 kinase (S6k), did not rescue axon guidance abnormalities of Tsc1 mosaics, while reductions in Tor function suppressed those phenotypes. These findings show that Tsc-mediated control of axon guidance and synapse assembly occurs via growth-independent signaling mechanisms, and suggest that Tor complex 2, a regulator of actin organization, is critical in these aspects of neuronal development.

Rheb in neuronal degeneration, regeneration, and connectivity

“…S6K regulated overall synapse growth (synapse bouton size, active zone number, and density) but not synaptic enlargement measured by the number of synaptic boutons per muscle area (Cheng et al, 2011a). However, Rheb overexpression in the motor neuron of the Drosophila larval stage or in NMJ did induce a doubling of the synapse size in terms of number of synaptic boutons per muscle area as well as an increase in synaptic function (Knox et al, 2007). Interestingly, rapamycin treatment reduced overall growth but did not suppress the Rheb-induced synaptic enlargement.…”

“…Drosophila neuromuscular junction (NMJ) was used to determine the role of TSC complex-Rheb-mTORC1 signaling on synapse assembly and function (Knox et al, 2007;Dimitroff et al, 2012;Natarajan et al, 2013). S6K regulated overall synapse growth (synapse bouton size, active zone number, and density) but not synaptic enlargement measured by the number of synaptic boutons per muscle area (Cheng et al, 2011a).…”

“…Interestingly, rapamycin treatment reduced overall growth but did not suppress the Rheb-induced synaptic enlargement. This Rheb-induced synaptic enlargement was shown to depend upon bone morphogenetic protein (BMP) signaling mediated by wishful thinking, a BMP type II receptor (Knox et al, 2007). BMP pathways are important for normal NMJ growth in Drosophila.…”

“…BMP pathways are important for normal NMJ growth in Drosophila. Animals bearing mutations in the wishful thinking gene show a very small NMJ with dramatically compromised synaptic function (Knox et al, 2007). The synaptic enlargement by over-expression of Rheb at the NMJ results in altering the balance between the number of synaptic boutons and the size of the underlying muscle which disrupts the normal growth and development (Dimitroff et al, 2012).…”

“…Axon targeting has been studied extensively using the Drosophila visual system (Tayler and Garrity, 2003;Knox et al, 2007;Dimitroff et al, 2012). A study on the role of Rheb in the mushroom bodies of Drosophila shows that over-expression of Rheb in the mushroom bodies results in enlarged axonal lobes (Brown et al, 2012).…”