A characterization of deficits associated with loss of NCB5OR in the mouse brain

Abstract

The simplest approach to the study of an event is to first consider that of the simplest cause. When investigating the mechanisms governing idiopathic diseases, this generally takes the form of an ab initio genetic approach, often in search of a single genetic culprit. To date, this genetic ‘smoking gun’ has remained elusive for many affected by diabetes mellitus and a number of neurodegenerative diseases. With no single gene, or even subset of genes, found to be causative in all cases, other approaches to studying the etiology and treatment of these diseases seem reasonable. One such approach is considering trends consistently observed in diseases closely correlated with one another. In the cases of diabetes mellitus and neurodegenerative diseases, overlapping themes of mitochondrial influence or dysfunction and iron dyshomeostasis are apparent and relatively consistent. This might suggest that gene networks involved in the maintenance of mitochondrial and iron related pathways are etiologically important. Thus, this dissertation focuses on a reductase, NCB5OR, whose absence has been shown to result in diabetes mellitus, mitochondrial dysfunction, and altered iron metabolism in mice. Specifically, we focus on the effects of NCB5OR deficiency on mouse neural tissue as a means of exploring genes and pathways known to result in these overlapping trends. In order to study the effects of NCB5OR deficiency on neural tissue and pathways we used mice globally deficient for NCB5OR (GKO) and also developed a conditional knockout mouse that inactivates NCB5OR in the cerebellum and midbrain (CKO). Using either of these models, three questions were addressed: What effect does NCB5OR deficiency in the mouse cerebellum and midbrain have on iron homeostasis and locomotor behavior? What effect does loss of NCB5OR in cerebellohypothalamic circuitry have on feeding behavior and metabolism? Does loss of NCB5OR affect major neurotransmitters in the brains of mice globally deficient for NCB5OR? Chapter 1 details background information on the MIND (mitochondria, iron, neurodegeneration, and diabetes) paradigm, which provides the context in which these studies were conducted. Although over 100 neurodegenerative diseases have been classified, the majority of the background presented will use Alzheimer’s disease (AD) as the model complex, idiopathic neurodegenerative disease, providing a focused example of the MIND paradigm framework. Also discussed is the incidence of diabetes accompanied by neuropathy and neurodegeneration along with neurodegenerative disorders prone to development of diabetes. Mouse models containing multiple facets of this overlap are also described alongside current molecular trends attributed to both diseases. A detailed background pertaining to NCB5OR as well as known phenotypes and preliminary observations associated with its absence are presented. Finally, a review of the cerebellum and its contribution to motor and higher order cognitive processes is presented so as to help better understand why the cerebellum was chosen as the primary model system for the study of NCB5OR in neural tissue. Chapters 2-4 address the above three questions with studies aimed at initial characterization of the effects of NCB5OR deficiency. Briefly, results from Chapter 2 demonstrate an altered state of iron homeostasis in the mouse cerebellum devoid of NCB5OR. Additionally, analysis of locomotor behavior revealed altered locomotor activity, proprioception, and sensitivity to harmaline-induced tremor in CKO mice. Chapter 3 explores metabolic and behavioral changes in CKO mice which reveal the complex nature of NCB5OR deficiency on neural pathways that participate in feeding behavior and neural regulation of metabolism. Finally, Chapter 4 presents data that suggest that the absence of NCB5OR does not affect levels of serotonin (5-HT), dopamine (DA), γ-aminobutyric acid (GABA), or glutamate (Glut) in the cerebellum but does increase levels of DA in the frontal cortex of mice globally deficient for NCB5OR. The primary purpose of this work is to contribute to the understanding of the complex nature and etiology of idiopathic neurological disease by providing evidence emphasizing the importance of genes whose function influences iron and metabolic homeostasis. It is hoped that the data presented here helps to shed light on pathways and genetic networks whose composite function could provide insight into the development of complex neurological disease.