Neuronal cilia and appetite regulation in Alms1 mutant mice

Abstract

The foz/foz mouse is a murine model of Alstr{u00F6}m syndrome, a monogenetic disorder characterised in humans by childhood obesity, hearing loss, blindness, hyperinsulinaemia, early-onset type 2 diabetes and liver disease. In 2006, research from the host laboratory reported that foz/foz mice inherit a spontaneous mutation (foz) , an 11 base pair deletion in exon 8 of the Alms1 gene, and develop a similar phenotype to patients suffering from Alstr{u00F6}m syndrome. Thus, foz/foz mice are obese and exhibit high circulating insulin and leptin levels as well as fatty liver disease and metabolic syndrome. The purpose of the studies presented in this thesis was to further characterise the pathogenesis of obesity in foz/foz mice, by studying the role of Alms1 and hypothalamic appetite-regulating neuropeptide expression during the development of obesity.

ALMS1 has been shown to localise at the base of primary cilia in what is termed the basal body or centrosome. Primary cilia are ubiquitously expressed hair-like organelles. Therefore, the first approach was to study primary cilia in the hypothalamus as well as hypothalamic Alms1 gene expression and Alms1 localisation in foz/foz and wildtype (WT) mice from birth until the obese phenotype is evident. At birth, foz/foz mice showed similar number of ciliated hypothalamic neurons to WT mice. However, from weaning and onwards the number of cilia was significantly decreased in foz/foz mice. In addition, while cilia were present in primary neuronal cultures from foz/foz and WT mice, Alms1 was only detected in WT neurons, appearing as two perinuclear dots at the base of cilia.

After weaning, serum leptin levels become greatly elevated in foz/foz compared to WT mice. Leptin decreases appetite by acting in the hypothalamus and inducing or inhibiting the release of appetite-regulating neuropeptides. A detailed study of key hypothalamic neuropeptides demonstrated no differences in their gene and/or protein expression or localisation between foz/foz and WT mice. This failure of elevated leptin levels to decrease appetite and body weight is defined as leptin resistance. Further studies were therefore performed on hypothalamic leptin receptor (Ob-R) expression and signalling pathways to characterise leptin resistance in foz/foz mice. These results demonstrated induction of two proteins, SOCS3 and PTP1B, which negatively regulate Ob-R signalling and have been implicated in leptin resistance.

Taken together, the data presented in this thesis strongly support the proposal that foz/foz mice develop leptin resistance, which correlates molecularly with over-expression of at least two negative feedback regulators of Ob-R. In addition, the post-natal reduction in ciliated hypothalamic neurons in foz/foz mice, in combination with the lack of Alms1 detection are consistent with the proposal that primary cilia stability/maintenance could be impaired as a consequence of the Alms1 mutation. In conclusion, foz/foz mice provide new opportunities for studying the role of Alms1 and neuronal cilia in appetite regulation, particularly with respect to the onset of leptin resistance. A better understanding of Alms1, cilial stability and behavioural responses that underlie obesity could provide clues to novel therapeutic approaches to combat more common forms of obesity. Show more