Development of therapeutics for the treatment of diabetic brain complications

Abstract

Type-2 diabetes (T2D) is characterized by hyperglycemia and hyperlipidemia, resulting in impaired insulin production and insulin resistance in peripheral tissues. Several studies have demonstrated an association between diabetes and central nervous system complications such as stroke and Alzheimer’s disease. Due to the fact that T2D is one of the fastest growing chronic illnesses, there is an urgent need to improve our knowledge on the pathogenic mechanisms to why diabetes leads to brain complications as well as to identify novel drugable targets for therapeutic use.

Project 1: studies I-II Pre-clinical studies have shown that adult neurogenesis is impaired in diabetic animal models. We hypothesized that diabetes leading to neurogenesis impairment plays a role in the development of neurological complications. If so, normalizing neurogenesis in diabetes/obesity could be therapeutically useful in counteracting neurological dysfunction. The aim of studies I-II was to establish an in vitro system where to study the effect of a diabetic milieu on adult neurogenesis. Furthermore, we determined the potential role of pituitary adenylate cyclase-activating polypeptide (PACAP) and galanin to protect adult neural stem cells (NSCs) from these diabetic-like conditions. Moreover, we determined whether apoptosis and the unfolded protein response (UPR) were induced by diabetic-like conditions and whether their regulation was involved in the PACAP/galanin-mediated protective effect. Finally, we studied the potential regulation of PACAP and galanin receptors in NSCs in response to diabetic-like conditions in vitro and ex vivo.

The viability of NSCs isolated from the mouse brain subventricular zone (SVZ) was assessed in presence of a diabetic milieu, as mimicked by high palmitate and glucose, which characterize diabetic glucolipotoxicity. The results show that high palmitate and glucose impair NSC viability in correlation to increased apoptosis (Bcl-2, cleaved caspase-3) and UPR signaling (CHOP, BIP, XBP1, JNK phosphorylation). We also show that PACAP and galanin counteract glucolipotoxicity via PAC1 receptor and GalR3 activation, respectively. Furthermore, we also report that PACAP and galanin receptors are regulated by diabetes in NSCs in vitro and in the SVZ ex vivo.

Project 2: study III T2D is a strong risk factor for stroke and no therapy based on neuroprotection is currently available. Exendin-4 (Ex-4) is a glucagon-like peptide-1 receptor (GLP-1R) agonist in clinical use for the treatment of T2D, which has also been shown to mediate neuroprotection against stroke pre-clinically. However, the applicability of a therapy based on Ex-4 has not been investigated in a pre-clinical setting with clinical relevance.

The aim of this study was to determine the potential efficacy of Ex-4 against stroke in T2D rats by using a drug administration paradigm and a dose that mimics a diabetic patient on Ex-4 therapy. Moreover, we investigated inflammation and neurogenesis as potential cellular mechanisms at the basis of Ex-4 efficacy. T2D Goto-Kakizaki (GK) rats were treated peripherally for 4 weeks with daily clinical doses of Ex-4 (0.1, 1, 5 !g/kg body weight) before inducing stroke by transient middle cerebral artery occlusion. The Ex-4 treatment was continued for 2-4 weeks thereafter. The severity of ischemic damage was measured by evaluation of stroke volume and by stereological counting of neurons in the striatum and cortex. Evaluation of stroke-induced inflammation, stem cell proliferation and neurogenesis was also quantitatively assessed by immunohistochemistry. We show that peripheral administration of Ex-4 counteracts ischemic brain damage in T2D GK rats. The results also show that Ex-4 decreased microglia infiltration and increased stroke-induced neural stem cell proliferation and neuroblast formation, while stroke-induced neurogenesis was not affected by Ex-4 treatment.

Together, our data in project 1 show that we have established an in vitro assay where to study the molecular mechanism on how diabetes impact adult neurogenesis. Furthermore, our results show that this assay has the potential to be developed into a screening platform for the identification of molecules that can regulate adult neurogenesis under diabetes. In project 2, we show neuroprotective efficacy against stroke by Ex-4 in a T2D rat model, by using a pre-clinical setting with clinical relevance. Ex-4 is an anti-diabetic drug in clinical use that has been reported to show limited side effects. Thus, at least in theory stroke patients should be able to easily receive this treatment, probably with minimal risks.