Expression of the tuberous sclerosis complex gene products, hamartin and tuberin, in central nervous system tissues

Abstract

Tuberous sclerosis complex (TSC) is a common genetic disorder in which affected individuals can develop mental retardation, developmental brain defects, and seizures. Two genetic loci are responsible for TSC: TSC1 on chromosome 9q and TSC2 on chromosome 16p. Here, we report our analysis of TSC1 (hamartin) and TSC2 (tuberin) protein expression in the central nervous system (CNS). Both tuberin and hamartin are expressed in neurons and astrocytes where they physically interact. In the mouse cerebellum in vivo, tuberin predominantly localizes to the perinuclear region of the Purkinje cell, whereas hamartin is distributed along neuronal or astrocytic processes. In contrast, both hamartin and tuberin demonstrate similar neuronal expression patterns in pure neuronal cultures in vitro. Additionally, hamartin is highly expressed in astrocytes in mixed neuron-glia cultures in vitro, suggesting that hamartin may be important for astrocyte growth control. Unlike tuberin, loss of hamartin expression was not observed in sporadic astrocytomas. These results suggest that tuberin and hamartin may differentially contribute to the CNS pathology in TSC.

The tuberous sclerosis-1 (TSC1) gene product hamartin suppresses cell growth and augments the expression of the TSC2 product tuberin by inhibiting its ubiquitination

“…First, hamartin and tuberin display a very similar pattern of expression among di erent tissues (Gutmann et al, 2000;Plank et al, 1999;Wienecke et al, 1997); these patterns were described in one report as`almost identical' (Plank et al, 1999). Such a distribution would be expected if the active forms of these proteins are represented by the pools of each present as a complex, and also if reciprocal stabilization of hamartin and tuberin were important in the physiological regulation of the levels of these proteins in vivo.…”

“…The properties of tuberin and hamartin, as well as their expression and localization in cells and tissues, have been studied through the development and application of speci®c antisera (Gutmann et al, 2000;Nellist et al, 1999;Plank et al, 1998Plank et al, , 1999Wienecke et al, 1996Wienecke et al, , 1997. Tuberin, which contains a Cterminal domain with homology to known GTPase activating proteins (GAPs) for the small GTPase Rap1, has been shown to possess GAP activity for Rap1 and for another small GTPase, Rab5 (Wienecke et al, 1995;Xiao et al, 1997).…”

Heterozygosity for the tuberous sclerosis complex (TSC) gene products results in increased astrocyte numbers and decreased p27-Kip1 expression in TSC2+/− cells

“…Moreover, TSC2 missense mutations have been reported in the GAP domain that would be predicted to abrogate GAP activity (Maheshwar et al, 1997). Consistent with its proposed tumor suppressor function, tuberin expression was reduced in one third of sporadic astrocytomas Gutmann et al, 2000).…”

“…In these experiments, we observed a cooperative astrocyte growth advantage in mice doubly heterozygous for a targeted mutation in both Tsc1 and Tsc2. As tuberin and hamartin have been shown to interact in a variety of cell types (van Slegtenhorst et al, 1998;Plank et al, 1998;Nellist et al, 1999), including astrocytes (Gutmann et al, 2000), it is possible that the combined reduction of tuberin and hamartin signi®cantly reduces the amount of the functional tuberin-hamartin complex required for growth regulation and thus further promotes astrocyte proliferation. In support of this hypothesis, preliminary data suggest that the compound Tsc1;Tsc2 heterozygotes develop tumors at an accelerated pace than either heterozygous animal alone (Onda and Kwiatkowski, manuscript in preparation).…”

“…Second, reduced or absent TSC2 expression is observed in 30% of high grade sporadic astrocytomas Gutmann et al, 1997). Third, Tsc gene expression is modulated by cell contact growth arrest in astrocytes (Gutmann et al, 2000).…”