Schizophrenia

Schizophrenia is another disorder in which STEP61 levels are elevated and is a major focus of the current work in the laboratory. The project began under the leadership of Dr. Nikisha Carty (currently at Evotec Pharmaceuticals). Dr. Carty found that STEP61 levels were elevated in dorsal lateral prefrontal cortex and cingulate from two different SZ cohorts and in a phencyclidine model of SZ (Carty et al., 2012). The increase in STEP61 expression was correlated with a decrease in the Tyr phosphorylation of several substrates (e.g., ERK1/2 and GluN2B) and a loss of GluN1/GluN2B complexes from synaptoneurosome membranes. The findings also showed that three different classes of neuroleptics used in the treatment of SZ resulted in the phosphorylation and inactivation of STEP, as well as an increase trafficking of NMDAR complexes back to synaptic membranes.

These findings were significant for two reasons. They suggest that high levels of STEP activity are responsible for a decrease of glutamate signaling due to internalization of glutamate receptors, consistent with the glutamate hypothesis of SZ (Balu and Coyle, 2015). The second important finding was that current drugs used for the treatment of SZ work, at least in part, through the inactivation of STEP and trafficking of glutamate receptors back to synaptic membranes (reviewed in Goebel-Goody et al., 2012).

The project is now led by Jian Xu, PhD and our collaborators at Mt. Sinai (led by Kristen Brennand, PhD). We are testing the hypothesis that lowering STEP activity by using either genetic techniques or pharmacologic inhibition of STEP with TC-2153 will reverse the biochemical and cognitive deficits present in Nrg1+/- mice. In addition, preliminary data generated by Dr. Brennand has now shown that STEP61 expression is elevated in excitatory forebrain neurons derived from human induced pluripotent stem cell from SZ patients compared to unaffected controls. (http://icahn.mssm.edu/research/labs/brennand-laboratory)

Additional related projects include:

• What is the mechanism for the increase in STEP protein levels in schizophrenia (microRNAs, disruption of the normal ubiquitination and degradation of STEP)?
• What are the effects of other neuromodulators (e.g., serotonin, acetylcholine, dopamine, nicotine) on STEP expression levels?

Figure 2: STEP KO mice are less sensitive to PCP-induced behavioral and cognitive deficits. (a) WT (n=10) and STEP KO (n=8) were acutely injected with PCP (2.5-20 mg/kg, i.p.) and analyzed for locomotor activity in an open field task. A repeated-measures ANOVA indicated a main effect of dose (P < 0.001) and a significant dose by genotype interaction (P < 0.02). Tukey’s post hoc indicated that WT was significantly different from STEP KO at the PCP doses indicated (*P < 0.05). (b) Exploration time of the novel and familiar objects during the 24-h delay object recognition retention trial (n = 12 for all groups). Both saline-treated WT and STEP KO mice spent significantly more time with the novel object than the chance value of 15 sec (one-sample t-test; *P < 0.05). WT mice treated subchronically with PCP (5 mg/kg, i.p. twice daily for 7 consecutive days) did not spend more time than chance with the novel object. In contrast, STEP KOs spent significantly more time with the novel object (one-sample t-test; *P < 0.05).