Elucidating a role for Mn-dependent IGF/insulin biology and autophagy in Huntington's disease pathology

Vanderbilt University, Nashville TN

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Abstract

ABSTRACT: Huntington?s disease (HD) is a devastating neurodegenerative disease caused by a CAG expansion in the Huntingtin (HTT) gene giving rise to the mutant protein (mHTT) and is characterized by cognitive, psychiatric, and motor symptoms. Mn is an essential metal for all biological systems by acting as a cofactor for several biologically indispensable enzymes and known to activate several other key cell signaling pathways. Recently, a striatal-specific deficit in Mn uptake has been observed several HD models and cell lines. Furthermore, this deficit has been found to underlie several enzymatic and cell signaling defects observed in HD including a Mn-dependent defect in ATM/p53 signaling. Interestingly, other critical cell signaling pathways which are defective in HD (AKT and mTOR) are also highly responsive to cellular Mn levels. Additionally, treatment with IGF-1, an upstream activator of AKT, has been shown to be highly neuroprotective in HD cell and mouse models. The proposed mechanism of these neuroprotective effects are increased mitochondrial function and autophagy via upregulation of AKT and downstream pathways. Intriguingly, prior studies have shown that Mn can act as an insulin-mimetic, activating several of the same metabolic kinases and pathways that IGF/Insulin activate. My preliminary data show that HD models exhibit decreased Mn-induced AKT/mTOR signaling and reduced Mn-induced autophagy. Additionally, our data provide evidence that Mn-induced AKT/mTOR signaling is IGF1-receptor-dependent. Indeed, the kinase domain of IGF and insulin receptors have been shown in vitro to use Mn as a co-factor (with some differences in the enzymatic properties versus magnesium); and these receptors signal to AKT/mTOR. We hypothesize that reduced Mn bioavailability in HD models deprives cell signaling pathways dependent or responsive to Mn as a cofactor. Specifically, we propose: 1) Mn acts on the IGF receptor to induce AKT/mTOR signaling; 2) this is interaction is diminished in HD models due to reduced bioavailable Mn; 3) downstream processes of AKT/mTOR such as autophagy will also be affected in a Mn- dependent manner; 4) both wildtype (WT) and mHTT modify Mn homeostasis which then contribute to AKT/mTOR and autophagy defects via interactions with the IGF/insulin signaling. We propose a highly mechanistic set of experiments aimed at dissecting the intricate connections between mHTT, IGF/insulin biology, and manganese. In Specific Aim 1, we will utilize neuronal, immortalized cell lines, patient-derived lymphoblasts, and neuroprogenitors derived from patient-derived iPSC lines to determine the role of Mn on IGF/insulin signaling by analysis of receptor expression, phosphorylation, translocation, downstream activity, and ligand concentration after treatment with Mn and/or IGF. We will also determine if this mechanism is perturbed in HD giving rise to reduced AKT/mTOR signaling. In Specific Aim 2, we will explore the basis by which Mn influences autophagy, a process regulated by IGF/insulin, and whether this is also perturbed in HD. For these experiments, we will use a wide-array of methodology to assess autophagy outcome measures.