Molecular studies of hypothalamic food intake regulating systems and central myelination in two anorectic mouse models

Abstract

The aim of this thesis is to increase the understanding of the hypothalamic mechanisms that are vital for food intake regulation, in particular as it relates to anorexia, and the molecular mechanisms underlying myelination and nodal/paranodal domain organization. The majority of the thesis concerns the anorectic phenotype of two mouse models for which we have found both great similarities and differences pointing to different mechanisms generating similar symptoms and neuropeptidergic alterations. This thesis starts with immunohistological and electron microscopic investigations of the regulatory effect of the GPI-anchored cell adhesion molecule Contactin-1 in myelination in the central nervous system (CNS) and functional compartmentalization of myelinated nerves (Paper I). With the help of the Cntn1-KO mouse and a Cntn1-KO cross expressing green fluorescent protein in myelin, our analysis revealed several novel functions of Contactin-1 in CNS myelination. These include the demonstration of Contactin-1 expression in oligodendrocytes in vivo and its role in regulating neuron-glia interactions required for myelin membrane extension and myelination. Further, we found that Contactin-1 is essential for the domain organization of myelinated nerves by organizing the attachment of the terminal myelin loops to the axon membrane at the paranode. Contactin-1 is thus a key molecule for forming functional fast propagating high conduction velocity myelinated nerves in the CNS. We continue to explore the roles of Contactin-1 in the CNS by investigating the anorectic and hypothalamic phenotype of the Cntn1-KO in comparison with the anorectic anx/anx mouse (Paper II and IV). In these two studies involving immunohistochemistry and in situ hybridization techniques, we show similarities between the two models in the deviation from the wild type expression levels and location of hypothalamic neuropeptides (NPY, AgRP, α-MSH/POMC and MCH) important for food intake regulation (Paper II). However, further analysis revealed apparent differences with regard to the expression of astroglial and microglial markers in the hypothalamic system, as well as in the hippocampus (Paper IV). A significant upregulation of markers of astroglial and of microglial activation (previously published) was found in the anx/anx hypothalamus, indicating an inflammatory reaction. In contrast, the Cntn1-KO mouse displays no such glial responses in the hypothalamus. We did however detect increased expression of the microglia marker in the hippocampal dentate gyrus of the Cntn1-KO mouse, which we did not see in the anx/anx mouse. Based on previous findings associating the anx/anx mouse with a mitochondrial dysfunction, we explored the possibility of a reduced metabolic rate of hypothalamic neurons (Paper III). Enzymatic assays, ex vivo autoradiography and Western Blot of the anx/anx hypothalamus revealed reduced glucose uptake, reduced cellular metabolic rate both in basal and ischemic conditions and reduced ATP-turnover. The ratio of the metabolic master regulator, AMPK-P/AMPK, was reduced in the anorectic anx/anx hypothalamus. Taken together this is indicative of a hypometabolic state in the hypothalamus of the anx/anx mouse resembling what is seen during hibernation. The two anorectic mouse models have many similarities and many differences making them valuable to further understand the food intake regulating systems. By elucidating molecular pathways the data in this thesis may in the future yield improved understanding of disorders such as Anorexia Nervosa and Multiple Sclerosis.