Characterisation of developmental cues in early postnatal AgRP neurons

Abstract

Body weight regulation and the control of food intake are topics that continue to receive much attention as countries around the world grapple with alarming rates of obesity and the associated poorer health outcomes. Central to this has been two antagonistic populations of neurons, the agouti-related protein (AgRP)-expressing neurons and the proopiomelanocortin (POMC)-expressing neurons, which originate from the hypothalamic arcuate nucleus (ARC). Over the past two decades, there has been increasing interest in how these neurons develop, and how developmental insults may impact on their formation and later life function. As a component of this, it has been recognised that the adipocyte hormone leptin plays a critical role in modulating the development of the projections of AgRP and POMC neurons to other brain regions associated with body weight regulation. In rodent models, much of this circuitry elaboration occurs during the first few postnatal weeks.

The research presented in this thesis followed two lines of enquiry, with the aim of better understanding the normal postnatal development of AgRP neurons in a mouse model. Initially, I focused on the role of leptin in these neurons’ development, by using qPCR to investigate whether several putative transcriptional targets of leptin signalling, Ephb3, Fez1, Ngfr, Plxnb1, and Unc119, were indeed leptin regulated in postnatal day 8 ARC. However, no evidence was found to support this hypothesis. In the second half of this thesis, the novel in situ hybridisation technique, RNAscope, was used to survey the hypothalamic expression of three ligand-receptor pairs, Ntn1 and Dcc, Wnt5a and Fzd5, and Slti1 and Robo1, that I reasoned may be involved in AgRP neuron elaboration. All three pairs were found to be expressed in the early postnatal hypothalamus, and various patterns of expression were described in the ARC and paraventricular hypothalamus on postnatal days 2, 8, 12 and 16.

As the incidence of obesity in the developed world reaches epidemic proportions, understanding the normal development of the neural circuitry responsible for regulating food intake and maintaining energy balance is of increasing public health importance.