Impaired memory and olfactory performance in NaS_i-1 sulphate transporter deficient mice

Abstract

In the present study, NaS_i-1 sulphate transporter knock-out (Nas1−/−) mice, an animal model of hyposulphataemia, were examined for spatial memory and learning in a Morris water maze, and for olfactory function in a cookie test. The Nas1−/− mice displayed significantly (P < 0.05) increased latencies to find an escape platform in the reversal learning trials at 2 days but not 1 day after the last acquisition trial in a Morris water maze test, suggesting that Nas1−/− mice may have proactive memory interference. While the wild-type (Nas1+/+) mice showed a significant (P < 0.02) decrease in time to locate a hidden food reward over four trials after overnight fasting, Nas1−/− mice did not change their performance, resulting in significantly (P < 0.05) higher latencies when compared to their Nas1+/+ littermates. There were no significant differences between Nas1−/− and Nas1+/+ mice in the cookie test after moderate food deprivation. In addition, both Nas1−/− and Nas1+/+ mice displayed similar escape latencies in the acquisition phase of the Morris water maze test, suggesting that learning, motivation, vision and motor skills required for the task may not be affected in Nas1−/− mice. This is the first study to demonstrate an impairment in memory and olfactory performance in the hyposulphataemic Nas1−/− mouse.

Introduction

Inorganic sulphate (SO₄²⁻) is involved in many metabolic and cellular processes and is essential for numerous physiological functions [23]. Sulphate conjugation is an important step in the biotransformation of xenobiotics [10] and in the metabolism of certain neurotransmitters, including serotonin, dopamine and norepinephrine (NE) (reviewed in [43]). In addition, sulphonation of structural components, such as glycosaminoglycans and cerebroside sulphate, is essential for the maintenance of normal structure and function of tissues [28]. Disturbances in sulphate metabolism have been associated with human syndromes and diseases including metachromatic leukodystrophy, Hunter's syndrome, Morquio's syndrome, Maroteaux-Lamy syndrome, Sanfilippo's syndrome and multiple-sulphohydrolase deficiency (reviewed in [38]). Interestingly, behavioural problems are a common feature in these disorders [2], and animal models of Metachromatic leukodystrophy, Hunter's and Sanfilippo's syndromes, exhibit abnormalities in learning, memory and neuromotor function [9], [20], [44]. Recently, Han and co-workers reported a decreased sulphatide content in brain tissue and cerebroside fluid derived from Alzheimer's patients [11], [12]. The sulphatide deficiency in these patients was detected at the earliest stage of the disease and was proposed to occur prior to the appearance of clinical symptoms [11].

Despite the importance of sulphate in the body, plasma SO₄²⁻ levels are rarely measured clinically and little is known about the consequences of disturbed sulphataemia. To date, our knowledge is limited to the findings of markedly reduced plasma sulphate levels in Alzheimer's disease (AD), Parkinson's disease (PD), Motor Neurone disease (MND) and autistic disorder patients [14], [42]. However, the role of hyposulphataemia in the neurological dysfunctions in AD, PD, MND and autistic individuals has not yet been studied. We isolated the human sodium sulphate cotransporter, NaS_i-1, which is primarily expressed in the kidney [19] where it is proposed to maintain blood SO₄²⁻ levels [23]. We have also cloned the mouse NaS_i-1 gene, Nas1 [3], and recently generated a Nas1 knock-out (Nas1−/−) mouse that lacks a functional NaS_i-1 protein [6]. The Nas1−/− mice exhibit increased urinary SO₄²⁻ excretion and hyposulphataemia, highlighting the essential role of NaS_i-1 in maintaining blood sulphate levels.

We have recently shown that Nas1−/− mice have behavioural abnormalities that were suggestive of a decrease in object-induced anxiety [7], highlighting the consequences of inactivating NaS_i-1 on behaviour. In the present study, we compared the spatial memory and learning, and olfactory performance of Nas1−/− and Nas1+/+ mice in a Morris water maze [26] and a cookie test [47], respectively. Three pieces of information prompted us to study the spatial memory and learning, and olfactory performance of Nas1−/− mice: (1) abnormal memory and learning in animal models of disturbed sulphate metabolism (e.g., Hunter's and Sanfilippo's syndrome and metachromatic leukodystrophy) [9], [20], [44]; (2) the correlation of reduced serum sulphate levels in Nas1−/− mice [6] and neuropsychiatric disorders, including Alzheimer's, Parkinson's, Motor Neurone disease and autism [14], [42], which have memory [4], [25], [30] and olfactory [13], [31], [32], [46] abnormalities as a clinical feature; and (3) the presence of high olfactory sulphotransferase activity in mice [24], [39]. The present study is the first to identify changes in spatial learning and memory, and olfactory function in the hyposulphataemic mice lacking a functional NaS_i-1 protein.