Insulin receptor deficits in schizophrenia and in cellular and animal models of insulin receptor dysfunction

Abstract

Schizophrenia is associated with abnormalities in glucose metabolism that may lead to insulin resistance and a 3 fold higher incidence of type II diabetes mellitus. The goal of the present studies was to assess the role of insulin-dependent Akt signaling in schizophrenia and in animal and cellular models of insulin resistance. Our studies revealed a functional decrease in insulin receptor (IR)-mediated signal transduction in the dorsolateral prefrontal cortex (BA46) of medicated schizophrenics relative to control patients using post-mortem brain material. We found ∼ 50% decreases in the content and autophosphorylation levels of IRβ and ∼ 76–78% decreases in Akt content and activity (pSer⁴⁷³-Akt). The inhibition of IRβ signaling was accompanied by an elevated content of glycogen synthase kinase (GSK)-3α and GSK-3β without significant changes in phospho-Ser^21/9 GSK-3α/β levels. A cellular model of insulin resistance was induced by IRβ knockdown (siRNA). As in schizophrenia, the IRβ knockdown cells demonstrated a reduction in the Akt content and activity. Total GSK-3α/β content remained unaltered, but phospho-Ser^21/9 GSK-3α/β levels were reduced indicating a net increase in the overall enzyme activity similar to that in schizophrenia. Insulin resistance phenotype was induced in mice by treatment with antipsychotic drug, clozapine. Behavioral testing showed decreases in startle response magnitude in animals treated with clozapine for 68 days. The treatment resulted in a functional inhibition of IRβ but the Akt activation status remained unaltered. Changes in GSK-3α/β were consistent with a net decrease in the enzyme activity, as opposed to that in schizophrenia. The results suggest that alterations in insulin-dependent Akt signaling in schizophrenia are similar to those observed in our cellular but not animal models of insulin resistance. In animal model, clozapine ameliorates IRβ deficits at the GSK-3α/β level, which may justify its role in treatment of schizophrenia. Our studies suggest that aberrant IR function may be important in the pathophysiology of schizophrenia.

Introduction

Schizophrenia is a disorder characterized by a 20% higher mortality rate than the general population. Contributing factors may be a number of medical conditions, including an increased risk for Type II diabetes mellitus (Dynes, 1969, Felker et al., 1996, Mukherjee et al., 1996). Schizophrenia has been associated with impaired glucose metabolism such as glucose intolerance and insulin resistance. For example, a family history of type II diabetes has been found in 18–19% of schizophrenic patients (Mukherjee et al., 1989) as compared to 1.2–6.3% in the population at large (Adams and Marano, 1995). Review of current literature also underlines prevalence of diabetes and impaired glucose tolerance in patients with schizophrenia (Bushe and Holt, 2004).

Many atypical antipsychotic drugs, including clozapine and olanzapine, interfere with glucose metabolism (Dwyer et al., 2001, Dwyer et al., 2003, Lindenmayer et al., 2003). They reduce glucose transport in blood cells and neuron-like PC12 cells (Dwyer et al., 1999). They may also induce diabetes (Henderson et al., 2005a). Clozapine- and olanzapine-treated subjects display a significant insulin resistance and impairment of glucose effectiveness as compared to risperidone-treated patients (Henderson et al., 2005b). Conventional drugs are less likely to cause metabolic syndrome and weight changes in schizophrenia or psychiatric patients (Mackin et al., 2005). It has been speculated that the glycemic state of schizophrenic patients contributes to their psychotic symptoms or modulates the incidence of drug side effects. A study using a large national PORT database found that prevalence of diabetes in schizophrenia exceeds that in the general population well before the widespread use of the new (atypical) antipsychotic drugs (Dixon et al., 2000). More recent studies confirm this observation and demonstrate impaired fasting glucose tolerance and higher insulin resistance in drug-naïve patients with schizophrenia (Ryan et al., 2003). These findings suggest that impaired glucose metabolism is associated with schizophrenia rather than only a side-effect of drug treatment.

Stimulation of insulin receptors triggers phosphorylation of tyrosine receptor kinase and activation of a downstream signal transduction pathway coupled to phosphatidylinositol 3-kinase (PI3K) and protein kinase B/Akt (Akt) (Fig. 1). Akt is a multifunctional kinase regulating anti-apoptotic activities, cellular growth and glucose metabolism (Chang et al., 2003, Coffer et al., 1998, Franke et al., 1997, Hemmings, 1997). Akt regulates glucose metabolism by phosphorylation and downregulation of GSK-3α/β, which stimulates glycogen and protein synthesis. Moreover, it couples glucose metabolism to oxidative phosphorylation via mitochondria-bound hexokinase (Gottlob et al., 2001). It also phosphorylates tau protein (Ksiezak-Reding et al., 2003), which may play a role in microtubule-based axonal transport (Mandelkow et al., 2003).

GSK-3β is a multifunctional Ser/Thr kinase and a key regulator of several signaling pathways involved in cellular growth and development, including insulin- and Wnt-dependent signaling (Dajani et al., 2003, Doble and Woodgett, 2003, Jope and Johnson, 2004). GSK-3β is constitutively active in resting cells and becomes inhibited in response to external signals. Insulin inhibits GSK-3 via Akt-dependent phosphorylation at Ser²¹ (GSK-3α) and Ser⁹ (GSK-3β). GSK-3 is also phosphorylated by unidentified kinase(s) or autophosphorylation at Tyr²⁷⁹ (GSK-3α) and Tyr²¹⁶ (GSK-3β). The role of Tyr phosphorylation implicated in GSK-3 activation is not totally clear since the mutation of Tyr²¹⁶ to phenylalanine does not impair GSK-3β activity (Itoh et al., 1995).

Studies linking insulin receptor function with Akt signaling in schizophrenia have been limited. Recently, convergent evidence has demonstrated a decrease in the total Akt1 protein in brain of schizophrenic patients and identified Akt1 as a potential susceptibility gene (Emamian et al., 2004). In the present studies, we examined the relationship between the insulin receptor and Akt signaling in postmortem brain tissue from schizophrenic patients, in a cellular model of insulin receptor knockdown and in mice with aberrant function of brain insulin receptors. Our results revealed a significant downregulation of insulin receptors and inhibition of Akt activity suggesting drastically suppressed insulin-dependent Akt signaling in schizophrenia.