Specification of Neuronal Polarity Regulated by Local Translation of CRMP2 and Tau via the mTOR-p70S6K Pathway

Mammalian target of rapamycin (mTOR) is an important regulator of neuronal development and functions. Although it was reported recently that mTOR signaling is critical for neuronal polarity, the underlying mechanism remains unclear. Here, we describe the molecular pathway of mTOR-dependent axon specification, in which the collapsing response mediator protein 2 (CRMP2) and Tau are major downstream targets. The activity of mTOR effector 70-kDa ribosomal protein S6 kinase (p70S6K) specifically increases in the axon during neuronal polarity formation. The mTOR inhibitor rapamycin suppresses the translation of some neuronal polarity proteins, including CRMP2 and Tau, thereby inhibiting axon formation. In contrast, constitutively active p70S6K up-regulates the translation of these molecules, thus inducing multiple axons. Exogenous CRMP2 and Tau facilitate axon formation, even in the presence of rapamycin. In the 5′-untranslated region of Tau and CRMP2 mRNAs, we identified a 5′-terminal oligopyrimidine tract, which mediates mTOR-governed protein synthesis. The 5′-terminal oligopyrimidine tract sequences of CRMP2 and Tau mRNAs strongly contribute to the up-regulation of their translation in the axon in response to the axonal activation of the mTOR-p70S6K pathway. Taken together, we conclude that the local translation of CRMP2 and Tau, regulated by mTOR-p70S6K, is critical for the specification of neuronal polarity.To function, developing neurons must establish axonal-dendritic polarity. The polarization processes have been well studied in cultured hippocampal neurons (1, 2). They first form lamellipodia and small protrusion veils shortly after plating (stage 1) and then extend several immature neurites within 12–24 h (stage 2). One of the neurites rapidly elongates and becomes the axon, whereas the others become dendrites (stage 3). During these processes, a number of molecules coordinately control the axon specification (3). These molecules spatially regulate cytoskeletal networks of actin filaments and microtubules for specification and maintenance of a single axon.Mammalian target of rapamycin (mTOR)² is a serine/threonine protein kinase and mainly controls protein synthesis via phosphorylation of its downstream targets, such as eukaryotic translation initiation factor 4E (eIF-4E)-binding protein 1 (4E-BP1) and 70-kDa ribosomal S6 protein kinase (p70S6K) (4). In the nervous system, mTOR plays important roles in neuronal survival, dendritic arbor formation, synaptic plasticity, learning, and memory (5). Notably, in the regulation of synaptic plasticity, a number of mRNAs are locally translated via activation of the mTOR pathway in response to extracellular signals (5–7). Recently, two research groups reported that the mTOR pathway is also critical for the formation of neuronal polarity (8, 9). They proposed that the mTOR pathway controls the synthesis of polarity proteins, such as synapses of the amphid-detective (SAD) kinases (8) and Rap1 (9), and therefore regulates the neuronal polarity. However, the underlying molecular mechanism remains to be fully understood. Here, we demonstrate that p70S6K is necessary and sufficient for the mTOR-controlled axon formation as a downstream effector of mTOR. The mTOR-p70S6K pathway regulates translation of several neuronal polarity genes, including SAD kinases and Rap1. In particular, the expressions of collapsing response mediator protein 2 (CRMP2) and microtubule-associated protein Tau are highly dependent on mTOR-governed protein synthesis. We further identified a 5′-terminal oligopyrimidine (5′-TOP) tract, which mediates mTOR-governed protein synthesis (10), in the 5′-UTRs of CRMP2 and Tau mRNAs. In the axon of developing neurons, the translation of CRMP2 and Tau mRNAs is spatially controlled by the mTOR-p70S6K pathway via the 5′-TOP sequences, resulting in accumulation of CRMP2 and Tau proteins in the axon. Thus, our study substantially improves understanding of molecular mechanisms underlying the mTOR-dependent development of neuronal polarity.