Ontogeny of steroid receptor coactivators in the hippocampus and their role in regulating postnatal HPA axis function

Abstract

The function and regulation of the hypothalamic–pituitary–adrenal (HPA) axis during ontogeny differs markedly from the situation in adult animals. Postnatally mice undergo a so-called stress hypo-responsive period, which is characterized by a relative inability of mild stressors to induce a marked corticosterone response. Steroid receptor coactivators (SRCs) have been shown to influence the function of the HPA axis in adult animals by interacting with steroid receptors as the mineralocorticoid and the glucocorticoid receptor. Here we test the hypothesis that expression changes of the three identified SRC genes (SRC1, SRC2 and SRC3) correlate with differences in HPA axis activity during postnatal development. First, we mapped the ontogeny of the three SRCs during postnatal development in the hippocampus. We found a time- and region-specific regulation of gene expression, which was specific for each SRC. However, there was no relation between the age-dependent stress system activity and the expression levels of the SRCs. Further, we studied the acute regulation of the three SRCs following maternal deprivation in 9-day-old wild-type or CRH receptor type 1 (CRHr1) knockout mice. Under these conditions, no differential expression of any of the tested SRCs could be detected. Thus, while it seems likely that their varying abundance throughout postnatal life affects steroid receptor function in the different hippocampal subregions, acute changes of HPA axis activity or reactivity are not mediated by hippocampal changes in expression of this coactivator family.

Introduction

The function of the developing HPA axis in the neonatal mouse is characterized by specific features, which are different from the adult situation (Levine, 2002). Peripheral stress hormones as ACTH and corticosterone only respond to a subset of stressors (e.g., cold, ether), while stressors relayed via the limbic system do not elicit a marked hormonal response (Baram et al., 1997, Walker et al., 1991). This (at least partial) hypo-responsiveness of the HPA axis, termed stress hypo-responsive period (SHRP), is strongly dependent on maternal signals (Cirulli et al., 1992, Schmidt et al., 2002, van Oers et al., 1998) and glucocorticoid feedback (Sapolsky and Meaney, 1986, Schmidt et al., 2005, Walker et al., 1990).

It could be hypothesized that the feedback sensitivity or regulation during the postnatal period in the mouse may be influenced by the presence or absence of co-regulatory proteins, which affect glucocorticoid receptor (GR) or mineralocorticoid receptor (MR) signaling and thereby may contribute to the specific function of the HPA axis in the neonate. A prominent class of these proteins are the steroid receptor coactivators (SRC). In vitro, SRCs interact with a number of nuclear receptors (among which MR and GR) and modulate transcription via modification of chromatin structure and by recruitment of more general coactivators, such as CBP/p300. In the mouse three different SRC genes have been identified (SRC-1, SRC-2 and SRC-3) (Rosenfeld and Glass, 2001).

The SRCs are good candidates for modulation of glucocorticoid sensitivity in the brain: they are expressed in a cell-specific manner in different brain nuclei (Camacho-Arroyo et al., 2005), have been shown to modulate the transcriptional effects of MR and GR in a gene and receptor-specific manner (Meijer et al., 2005), and may be subject to regulation by external stimuli like stress (Bousios et al., 2001, Kurihara et al., 2002). Sex steroid signaling in the hypothalamus has been shown to depend on SRC function in several species (Apostolakis et al., 2002, Auger et al., 2000, Charlier and Balthazart, 2005, Molenda-Figueira et al., 2006). Recently, Winnay et al. (2006) showed an altered function of the hypothalamic–pituitary–adrenal axis in SRC-1-deficient mice, including resistance of the POMC gene to suppression by dexamethasone, demonstrating the importance of this steroid coactivator in vivo.

There are not many studies on the expression and function of SRCs during development. Xu and colleagues report that deletion of the SRC-3 gene results in postnatal growth retardation and delayed puberty, suggesting a functional role of the SRC-3 during development (Xu et al., 2000). With regard to the developing brain, the pattern of expression of SRC-1 has been shown to be rather dynamic in a brain area specific way in several species (Mitev et al., 2003, Setiawan et al., 2004). SRC-1 knockout mice, in which there is an apparent partial compensation by SRC-2, there is a mild disturbance in cerebellar development, leading to small motor impairments in later life (Nishihara et al., 2003). However, nothing is known of SRCs in relation to glucocorticoid feedback regulation during the SHRP.

To test the hypothesis of an involvement of SRCs in HPA axis regulation during ontogeny, we first mapped the ontogeny of SRC 1, SRC-2 and SRC 3 in the mouse hippocampus throughout postnatal development. We focused our attention to the hippocampal region because it shows the most abundant expression of all three SRCs in adult animals, is significantly involved in negative feedback regulation of the HPA axis and can be a target for long-term programming of steroid signaling by early life events. In a different experiment, we tested the hypothesis that the SRCs are regulated in response to stimuli that profoundly affect the function of the HPA axis and glucocorticoid signaling in the neonate. To this aim we studied the regulation of these three steroid receptor coactivators following maternal deprivation in 9-day-old wild-type or CRH receptor type 1 (CRHr1) knockout mice (Schmidt et al., 2004, Schmidt et al., 2003b), as both maternal deprivation and the lack of CRHr1 greatly alter glucocorticoid signaling in the neonatal brain.