The role of raptor during brain development and in adult forebrain neurons

mTOR is a serine/threonine protein kinase that appears in two functionally distinct multi-protein complexes. mTOR together with the scaffold protein raptor forms mTOR complex1 (mTORC1) that is inhibited in its function by rapamycin. mTORC1 controls a wide range of cellular processes, including protein synthesis, ribosome biogenesis, cell growth, gene transcription, autophagy and metabolism. In the developing CNS, mTORC1 was shown to be involved in axonal outgrowth and navigation, dendritic arborization as well as spine and filopodia formation. In the adult brain, the dysfunction of mTORC1 signaling has been linked to several neurodegenerative and mental disorders. In addition, mTORC1 has also been suggested to be involved in synaptic plasticity and the process of learning and memory. To study the role of mTORC1 during development as well as in the adult stage of the brain, we deleted the mTORC1-defining component raptor by crossing floxed rptor mice with mice expressing the Cre-recombinase under the control of the Nestin promoter (RAbKO mice) or under the control of the α-CamKII promoter (RcKO mice), respectively.
Analysis of the RAbKO mice revealed that several aspects of brain development are critically controlled by mTORC1. RAbKO mice showed a pronounced microcephaly which is evenly expressed in all brain structures and gets apparent at E17.5. The observed change in brain size is likely due to reduced cell size and cell number. The latter is potentially the result of two mechanisms that are altered in RAbKO mice. First, raptor-depletion results in an increased incidence of apoptosis at late embryonic stages. Secondly, RAbKO mice show reduced proliferation at E17.5 and prolonged cell cycle length earlier during development. Furthermore, RAbKO mice show deficits in glial differentiation, probably mediated by altered STAT3 signaling that is observed in those mice. Moreover, ablation of mTORC1 activity during brain development affects cortical and hippocampal layering.
Due to the immediate postnatal lethality of RAbKO mice, a second knockout-mouse strain based on α-CamKII-Cre mediated recombination was generated to further analyze the postnatal role of mTORC1 in the brain (RcKO mice). The data that were obtained from this mouse model indicate that mTORC1 is involved in cell size maintenance in adult neurons but does not affect apoptosis in the adult brain. Further, RcKO mice display a distinct deficit in learning and memory in the Morris water maze. Under long inter-training-interval conditions where consolidation takes place within a time window that relies on de novo protein synthesis, the learning and memory performance of RcKO mice is reduced. This deficit can be overcome by an intensive training phase with short inter-training-intervals. Additionally, RcKO mice do exhibit impairments in fear extinction learning whereas the learning as well as context- and cue-memory are not affected in the fear conditioning paradigm in RcKO mice compared to control. These behavioral phenotypes are resembled by impaired E-LTP and L-LTP maintenance in RcKO mice. In addition to these plasticity-related aspects, also basal synaptic function of CA1 hippocampal neurons is altered in RcKO mice.
In summary, the data obtained from the two mouse models provide evidence that mTORC1 signaling controls several aspects of brain development and adult brain function. While during embryogenesis mainly cellular processes such as proliferation, differentiation and growth are affected by raptor deficiency, in the adult brain – besides the cell size control – primarily synaptic and plasticity-related functions depend on mTORC1 signaling.