Antipsychotics Increase Akt signaling to mTORC1-Dependent Protein Synthesis

Haloperidol, a first-generation antipsychotic, has a known rapid signaling effect that increases phosphorylation of protein kinase B (or Akt). Haloperidol and other antipsychotics in its class are dopamine receptor type 2 (D₂R) antagonists. Previous attempts to unravel the mechanisms of D₂R antagonists have looked at the way in which Akt phosphorylates and inhibits glycogen synthase kinase 3 beta because it is part of a signaling pathway that has been implicated in other behavioral disorders.¹

In the case of disorders such as schizophrenia, haloperidol provides patient benefit; however, lithium—which specifically targets the glycogen synthase kinase 3 beta signaling pathway—does not. This is indicative that there may be a different mechanism involved in the efficacy of haloperidol, which is commonly used to treat disorders such as schizophrenia and treatment-resistant depression. A current investigation by Bowling et al. (2014) is the first to describe the mechanism of action of haloperidol as an antipsychotic and to identify morphological changes and proteins that are synthesized as a result of haloperidol use.²

The study initially used cultures of striatal neurons due to their known D₂R abundance and, therefore, their predicted interaction with haloperidol. Using a 20 nM concentration of haloperidol, the researchers were able to significantly increase phosphorylation of Akt in a subset (40%) of the striatal neurons, those being the neurons that tested positive for D₂R. Conversely, pre-treating the striatal neurons with either an Akt inhibitor or rapamycin (an mTORC1 inhibitor), they decreased phosphorylation. Further to this, they verified that amisulpride, a second-generation antipsychotic, also activated the Akt-mTORC1, as identified using a Western blot with antibodies for phosphorylated S6 ribosomal protein (a marker for activity within the mTORC1 pathway).

Using surface sensing of translation, or SUNsET—a nonradioactive, fluorescence-activated cell sorting-based assay—with a puromycin tag, Bowling et al. identified that haloperidol was able to induce the synthesis of new proteins. Specifically, the team identified 269 newly synthesized proteins in cells that increased in response to a 5-hour exposure to haloperidol and 139 proteins that decreased in abundance. At 48 hours they identified 3,209 proteins. These results were congruous with mRNA studies done by Thoreen et al. on mTORC1-mediated mRNA translation.³

Analysis of the peptides extracted from the haloperidol-treated neurons using a nano liquid chromatograph coupled to an LTQ Orbitrap mass spectrometer (Thermo Scientific), along with an EASY-nLC 1000 liquid chromatograph coupled to a Q Exactive hybrid quadrupole-Orbitrap mass spectrometer (Thermo Scientific), revealed an increase in the amounts of S6 and eEF2 (a protein synthesis factor) with haloperidol. Analysis of the mass spectrometry data indicted that 17 of the 44 proteins that increased as a result of haloperidol were associated with the cytoskeleton and 10 were associated with mTORC1-dependent translation. The increase in cytoskeletal proteins was consistent with an observed increase in neuronal branching. The converse attenuation of the action of haloperidol by expression of dominant-negative 4E-BPAA or by treatment of the cultures with S6K1-targeted shRNA was consistent with the morphological changes affected by mTORC1 signaling.

Bowling et al. are the first to demonstrate that the action of haloperidol is not via the assumed glycogen synthase kinase 3 beta pathway, but rather by activation of the Akt-mTORC1 pathway; in so doing, it induces synthesis of cytoskeletal proteins that correlate with increased morphological complexity within neurons of the brain. Additionally, the ability of haloperidol to improve patient symptoms rapidly may result from the increase in protein abundance associated with translation being persistent, following an initial dose. This persistence may then be priming the neurons to enable rapid protein synthesis and subsequent change in the proteomic profile in response to successive doses.

 

References

1. Jope, R.S., and Roh, M.S. (2006) “Glycogen synthase kinase-3 (GSK3) in psychiatric diseases and therapeutic interventions,” Current Drug Targets, 7(11) (pp. 1421–34).

2. Bowling, H., et al. (2014) “Antipsychotics Activate mTORC1-Dependant Translation to Enhance Neuronal Morphological Complexity,” Science Signaling, 7(308), doi: 10.1126/scisignal.2004331.

3. Thoreen, C.C., et al. (2012) “A unifying model for mTORC1-mediated regulation of mRNA translation,” Nature, 485(7396) (pp. 109–113).

Post Author: Miriam Pollak. Miriam specialised in neuroscience as an undergraduate but traded in lab work for a post graduate degree in science communication.

She has since had a career that has spanned science communication, science education and communications management.

However, Miriam has found her bliss balancing her love of writing and disseminating medical research with managing a multimillion dollar research budget for a childhood cancer charity in Australia.