Nurr1 is not essential for the development of prepulse inhibition deficits induced by prenatal immune activation

Abstract

Inflammation-induced disruption of fetal neurodevelopmental processes has been linked to the precipitation of long-lasting behavioral abnormalities and associated neuropathology. Recent longitudinal investigations in prenatal immune activation models have revealed developmental correspondences between the ontogeny of specific dopaminergic neuropathology and the postnatal onset of distinct forms of dopamine-dependent functional abnormalities implicated in schizophrenia. Two examples of such developmental correspondences are increased expression of the orphan nuclear receptor Nurr1 (NR4A2) in ventral midbrain areas and disruption of prepulse inhibition of the acoustic startle reflex, with both the neuroanatomical and behavioral effects emerging only in adult but not pre-pubertal subjects exposed to prenatal maternal inflammation. In the present study, we tested the hypothesis that Nurr1 may be a critical molecular mediator of prepulse inhibition deficits induced by prenatal immune activation. To this end, we compared the effects of prenatal immune challenge on adult PPI in wild-type (wt) mice and mice with a heterozygous constitutive deletion of Nurr1 (Nurr1+/−) using a well established mouse model of maternal immune activation by exposure to the viral mimetic poly(I:C) (=polyriboinosinic–polyribocytidilic acid). We found that prenatal poly(I:C) treatment on gestation day 9 was similarly effective in disrupting prepulse inhibition in adult wt and Nurr1+/− mice. Prenatal poly(I:C) treatment also generally increased midbrain Nurr1-positive cells and counteracted the genetically driven Nurr1 deficit in the substantia nigra. Our data thus suggest that at least under the present experimental conditions, Nurr1 is not essential for the development of prepulse inhibition deficits induced by prenatal immune activation.

Introduction

Prenatal maternal exposure to infection is a noticeable environmental risk factor of schizophrenia and related psychotic disorders (Brown and Derkits, 2010). The precise neuroimmunological and neurodevelopmental mechanisms involved in this association remain largely elusive. However, one prevalent hypothesis suggests that infection/inflammation-induced disruption of fetal neurodevelopmental processes may predispose the organisms to long-lasting changes in subsequent brain and behavioral development, thereby increasing the risk of psychotic disturbances in later life (Gilmore and Jarskog, 1997, Meyer et al., 2009). This hypothesis has been substantiated by numerous investigations in experimental rodent models demonstrating the emergence of altered fetal brain development (Meyer et al., 2008a; Vuillermot et al., 2010) and multiple long-term brain and behavioral abnormalities relevant to schizophrenia following prenatal exposure to infection and/or immune activation (reviewed in Meyer and Feldon, 2010). Recent molecular studies further underscore the essential role of prenatal cytokine-associated inflammatory processes in mediating the effects of maternal infection on the offspring: Blocking the actions of pro-inflammatory cytokines such as IL-1β or IL-6 in the pregnant maternal host by genetic or pharmacological interventions, or genetically enforced over-expression of the anti-inflammatory cytokine IL-10, prevents the long-term brain and behavioral consequences of prenatal viral-like immune activation in mice (Girard et al., 2010, Meyer et al., 2008b; Smith et al., 2007).

We have recently carried out a longitudinal study in which the neuropathological effects of prenatal viral-like-infection in mice were followed up from late fetal development to adult stages of life (Vuillermot et al., 2010). This approach has revealed remarkable developmental correspondences between the ontogeny of specific dopaminergic neuropathology and the postnatal onset of distinct forms of dopamine-dependent functional abnormalities implicated in schizophrenia. Two prominent examples of such developmental correspondences are increased expression of the transcription factor Nurr1 (NR4A2) in ventral midbrain areas and disruption of prepulse inhibition (PPI) of the acoustic startle reflex, with both the neuroanatomical and behavioral effects emerging only in adult but not pre-pubertal subjects exposed to prenatal maternal inflammation (Vuillermot et al., 2010). Impaired sensorimotor gating in the form of attenuated PPI is a robust pathological finding in schizophrenia (Braff et al., 2001), and has also been reported in other neuropsychiatric disorders such as obsessive–compulsive disorder, bipolar disorder, Huntington’s disease, and autism (Perry et al., 2001, Perry et al., 2007, Swerdlow et al., 1993, Swerdlow et al., 1995).

Nurr1 is a member of the orphan steroid hormone receptor family with pivotal functions in both the final differentiation and survival of ventral mesencephalic dopaminergic precursor neurons (Zetterström et al., 1996, Saucedo-Cardenas et al., 1998, Kadkhodaei et al., 2009). It is expressed in at least 95% of all mesencephalic dopamine neurons from early fetal development onwards and together with other seminal transcription factors such as Pitx3 and Lmx1b, it exerts a number of functions in post-mitotic and mature mesencephalic dopamine neurons, including regulation of tyrosine hydroxylase (TH), dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) expression (Saucedo-Cardenas et al., 1998; Smidt and Burbach, 2007; Kadkhodaei et al., 2009).

Taken together, it is intriguing to speculate that the post-pubertal increase in Nurr1 expression may be a critical molecular mechanism required for the delayed onset of prenatal inflammation-induced PPI deficits. This possibility seems reasonable in view of the fact that prenatal inflammation-induced PPI deficits in mice are dopamine-dependent (Vuillermot et al., 2010) and that Nurr1 is highly essential for the development and post-mitotic functions of dopaminergic neurons. Here, we directly tested the hypothesis that Nurr1 may be a critical molecular mediator of the prenatal inflammation-induced PPI deficits by comparing the effects of prenatal inflammation on adult PPI and midbrain Nurr1 expression in wild-type (wt) mice and mice with a heterozygous constitutive deletion of Nurr1 (Nurr1+/−). According to our previous research (e.g., Meyer et al., 2005, Meyer et al., 2008a, Meyer et al., 2008b; Vuillermot et al., 2010), we used a model of prenatal immune challenge that is based on maternal administration of the viral mimic poly(I:C) (=polyriboinosinic–polyribocytidilic acid), a synthetic analog of double-stranded RNA that mimics a cytokine-associated viral-like acute phase response (Kimura et al., 1994, Traynor et al., 2004). This model system is known to robustly induce PPI deficits in adult rodents (Dickerson et al., 2010, Meyer et al., 2005, Meyer et al., 2008b; Shi et al., 2003, Smith et al., 2007, Vuillermot et al., 2010, Wolff and Bilkey, 2008) and has further been shown to increase midbrain Nurr1 expression in adult mice (Vuillermot et al., 2010).