The TSC1 Tumor Suppressor Hamartin Interacts with Neurofilament-L and Possibly Functions as a Novel Integrator of the Neuronal Cytoskeleton

Abstract

Tuberous sclerosis complex, an autosomal dominant disease caused by mutations in either TSC1 or TSC2, is characterized by the development of hamartomas in a variety of organs. The proteins encoded by TSC1 and TSC2, hamartin and tuberin, respectively, associate with each other forming a tight complex. Here we show that hamartin binds the neurofilament light chain and it is possible to recover the hamartin-tuberin complex over the neurofilament light chain rod domain spanning amino acids 93-156 by affinity precipitation. Homologous rod domains in other intermediate filaments such as neurofilament medium chain, ␣-internexin, vimentin, and desmin are not able to bind hamartin. In cultured cortical neurons, hamartin and tuberin co-localize with neurofilament light chain preferentially in the proximal to central growth cone region. Interestingly, in the distal part of the growth cone hamartin overlaps with the ezrin-radixin-moesin family of actin binding proteins, and we have validated the interaction of hamartin with moesin. These results demonstrate that hamartin may anchor neuronal intermediate filaments to the actin cytoskeleton, which may be critical for some of the CNS functions of the hamartin-tuberin complex, and abolishing this through mutations in TSC1 or TSC2 may lead to certain neurological manifestations associated with the disease.

Hamartin and tuberin: Working together for tumour suppression

“…21 As NF-L is capable of retaining tuberin, presumably through hamartin, it is probable that NF-L is 1 of the components of the TSC1/TSC2 complex, consistent with the observation of Nellist et al 3 that the molecular weight of TSC1/TSC2 complex is much higher than expected 330 kDa. Association of hamartin with the ERM proteins and NF-L may argue for integration of the actin skeleton and neurofilament network.…”

“…As other intermediate filaments, such as desmin, a-internexin and vimentin, do not show association to hamartin, it is possible that such an association with intermediate filaments is restricted to neurons. As Haddad et al 21 suggests, the binding of NF-L and ERM proteins by hamartin may play an important role in neuronal migration, which could partly explain abnormalities of this process that are often postulated to be the cause of characteristic brain lesions in tuberous sclerosis, sometimes defined as ''neuronal migration disorder''.…”

Rheb in neuronal degeneration, regeneration, and connectivity

“…It has long been known that PI3K-Akt signaling plays a role in neuronal polarization in vitro (Arimura and Kaibuchi, 2007). Previous investigations show that TSC complexRheb-mTORC1 components are localized along growth cones of axons of cortical neurons in culture (Haddad et al, 2002;Choi et al, 2008). The presence of TSC complex and Rheb-mTORC1 pathway components as well as the activation of local translation of proteins that are required for axonal growth, together support the axon outgrowth in response to extracellular signals like Ephrins (Nie et al, 2010;Gracias et al, 2014).…”

“…TSC complex may regulate soma and spine size via a mTOR dependent rapamycinsensitive pathway and spine length via a mTOR independent rapamycin-insensitive pathway (Tavazoie et al, 2005). TSC complex regulates the cytoskeleton via ERM (ezrin, radixin, and moesin) proteins, neurofilament light chain and activation of Rho family of proteins (Haddad et al, 2002). Whether this regulation also involves mTORC2 and its regulation of cytoskeleton (Jacinto et al, 2004) still needs to be investigated.…”

“…Whether this regulation also involves mTORC2 and its regulation of cytoskeleton (Jacinto et al, 2004) still needs to be investigated. TSC complex also regulate cofilin and induces alteration in the spine length (Haddad et al, 2002;Luo 2002;Tavazoie et al, 2005).…”