Alteration of behavioural phenotype in mice by targeted disruption of the progranulin gene

Abstract

Sexual differentiation of the brain in rodents is achieved by estrogens, which are converted from androgens in the brain, during the perinatal period. We have identified the progranulin (PGRN) gene as one of the sex steroid-inducible genes that may be involved in masculinization of the rat brain. In the present study, we generated a line of mice with targeted disruption of the PGRN gene, and investigated male sexual behaviour, aggression and anxiety. PGRN-deficient mice exhibited a decrease in ejaculation incidence, while the latency and frequency of both mount and intromission were unchanged. For the aggressive behaviour test, the resident–intruder paradigm was used, and PGRN-deficient mice exhibited enhanced aggressiveness. In wild-type mice, males exhibited lower levels of anxiety than females by the open field test, while male PGRN-deficient mice exhibited an elevated level of anxiety and sex difference in anxiety was not observed. In addition, mRNA expression of the serotonergic receptor 5-HT1A, which could be related to the inhibition of aggression and anxiety, was significantly reduced in the hippocampus of PGRN-deficient mice after aggressive encounters. On the other hand, deficiency of the PGRN gene did not affect serum testosterone concentrations. These results suggest that PGRN gene plays a role in establishing sexual dimorphic behaviours at least partially by modulating the brain serotonergic system.

Introduction

It is well established that the mammalian brain possesses sexually dimorphic structure, which is reflected by functional sexual differences in the control of specific types of behaviour and endocrine patterns [1]. In rodent species, such as rats and mice, sexual dimorphism of the brain is established during the perinatal period, which is known as the critical period for brain sexual differentiation. During this period, androgens secreted from the testis masculinize the brain in males, while the female brain is not exposed to androgens and thereby develops into the “default” female type. In the brain, androgens are converted to estrogens by aromatase and are then involved in the masculinization of the brain through the facilitation of transcription of various factors [32].

In an attempt to identify genes that are involved in sexual differentiation of the brain, we have isolated the progranulin (PGRN) gene as an androgen-inducible gene in neonatal rat hypothalamus [45]. The expression of PGRN mRNA in the hypothalamus of males is maintained at high levels throughout the critical period, yet abruptly declines in females after birth. As estrogens can induce PGRN gene expression in the hypothalamus [48], androgens probably induce PGRN gene expression after being converted to estrogens. We further observed that infusion of an antisense oligodeoxynucleotide for the PGRN gene into the neonatal male rat brain significantly suppressed male sexual behaviours after maturation [46], suggesting that the PGRN gene is indeed involved in the process of masculinization of the rat brain. Recently, we have also found that PGRN is involved in estrogen-induced neurogenesis in the adult rat hippocampus [11].

In the mouse, the PGRN gene spans approximately 6.3 kbp and contains 13 exons [3]. PGRN mRNA has been demonstrated in various tissues and organs including the reproductive organs, gastrointestinal tract, endocrinal organs and neural tissues [9], [17]. Peptides of approximately 6 kDa called granulins [6], [8] also known as epithelins [42] are derived from PGRN. Both PGRN and granulins have growth-modulating effects on many cell types in culture [7], [42]. For example, PGRN mediates the mitogenic effect of estrogens on MCF-7 cells, a human breast cancer cell line [30], which supports the above-mentioned notion that transcription of the PGRN gene is induced by estrogens. In addition, it has been recently reported that mutations in PGRN cause tau-negative, ubiquitin-positive frontotemporal dementia (FTD) in humans [4], [16], suggesting that PGRN is involved in neuronal survival.

To elucidate the physiological roles of PGRN/granulins in vivo, especially their roles in the brain, we generated mice lacking the PGRN gene in the present study. Here, we describe their phenotypes regarding sexually dimorphic behaviours, such as sexual behaviour, aggression and anxiety.