Neuronal human BACE1 knockin induces systemic diabetes in mice

Diabetic complications can lead to cognitive dysfunction and are acknowledged risk factors of Alzheimer’s disease, but little is known about the reverse scenario. Here, we demonstrate that the major amyloidogenic enzyme, BACE1, is sufficient to increase the risk of diabetes development when expressed in neurons only. Severe impairment in systemic glucose homeostasis and insulin sensitivity was evident in PLB4 mice from the age of 4 months onwards and progressively deteriorated with age. We previously reported that PLB4 mice develop mild cognitive deficits between 4 and 6 months. Our current findings therefore indicate that neuronal BACE1 induces global metabolic dysregulation along with brain inflammation and amyloidosis-related cognitive decline. Overall, the diabetic profile of PLB4 mice agrees with the improved glucose clearance and insulin sensitivity in mice lacking murine Bace1 globally [17, 30], but pinpoints neuronal BACE1 as the major driver of this alteration.

Although both Bace1 knockout and our hBACE1 knockin mice were leaner compared with WT controls, there were differential effects on adiposity and leptin signalling. Deletion of Bace1 decreased adiposity and improved sensitivity to leptin, while neuronal hBACE1 expression promoted adipogenesis and induced hyperleptinaemia. Hence, it seems plausible that manipulation of (neuronal) BACE1 affects body weight via changing metabolic efficiency, adipose composition and signalling. Importantly, leptin production and signalling are regulated by neuronal PTP1B [24]. We propose that the elevated central PTP1B expression in PLB4 forebrains may explain the hyperleptinaemic profile and compensatory BAT hyperactivity in these mice, which confirms that leptin has opposing effects on WAT and BAT [31].

Further, while proinsulin synthesis appeared unchanged in PLB4 mice compared with controls, heightened levels of amylin indicate an attempt to downregulate hyperglycaemia in the insulin-deficient state, and suggests that pancreatic function was at least partially preserved in 8-month-old PLB4 mice. Glucose intolerance and the fatty liver phenotype in PLB4 mice were further associated with drastically elevated levels of serum GIP. Although GIP exhibits insulinotropic properties under physiological conditions, its incretin effects are thought to be blunted in the diabetic state, and elevated levels may promote fatty acid accumulation and induction of proinflammatory cytokines [32].

The fatty liver phenotype of PLB4 mice corresponded with high plasma triacylglycerol and elevated levels of phospholipids such as phosphatidylcholine and lysophosphatidylcholine, which are typically observed in type 2 diabetic patients [26]. Altered plasma lipid composition was also recently proposed to predict early memory impairments, thus potentially offering a novel approach to identify Alzheimer’s disease [27, 33, 34].

Although lipid accumulation was also affected in neuronal tissue from symptomatic PLB4 mice, there was a poor species match for plasma, suggesting that (1) plasma markers may not be indicative of changes in brain lipid composition and (2) that the increase in neuronal lipids probably originated from the CNS. PLB4 mice had pronounced elevation of several classes of phospholipids such as phosphatidylethanolamine, lysophosphatidylethanolamine and phosphatidylserine, which are major components of neuronal membrane bilayers. Such alterations are reported in human Alzheimer’s disease brains [35] and are proposed to affect mitochondrial function, signal transduction and receptor activation, hence interfering with neurotransmission and neuronal integrity [36, 37].

Importantly, brain ceramide levels were substantially increased in PLB4 mice compared with controls. This sphingomyelin precursor is of particular interest because it increases naturally with age [38] and at an early stage of human Alzheimer’s disease [39]. Furthermore, ceramides were previously shown to regulate BACE1 protein expression and promote APP β-site cleavage [40]. Although the mechanisms through which BACE1 may upregulate ceramide accumulation are largely unknown, initial evidence suggests that Aβ peptides may activate sphingomyelinase [41]. It therefore seems plausible that the introduction of BACE1 promotes ceramide biogenesis, and vice versa. In support of this, a recent study demonstrated that intracerebroventricular ceramide infusions induce lipotoxicity and hypothalamic ER stress associated with increased eIF2α and PERK phosphorylation, sympathetic inhibition, reduced weight gain and altered energy balance in rats [42].

Cerebral hypometabolism revealed via ¹⁸FDG-PET imaging is common in early dementia patients as well as diabetic patients with or without mild cognitive impairment [43], and may ultimately contribute to cognitive pathology. We found that reduced glucose utilisation in PLB4 mice was associated with poor neuronal insulin sensitivity. Altered IR and PTP1B expression occurred in PLB4 mice at an advanced stage of systemic insulin resistance, hepatic dysfunction and Aβ-associated cognitive impairment [13]. Similar elevations in neuronal PTP1B were observed in response to HFD feeding in other Alzheimer’s disease models [6], while deletion of neuronal PTP1B improved IR signalling and protected against HFD-induced obesity and insulin resistance [24].

Increased S6K phosphorylation in PLB4 vs WT brains and elevated expression of its substrate, rpS6, suggest an increased demand for protein and lipid synthesis. Additionally, the ribosomal element is regulated by eIF2, offering an alternative route for its modification [44]. Elevated rpS6 phosphorylation along with increased brain RBP4 levels were previously found in Alzheimer’s disease mice on a HFD [6]. Neuronal pathways that mediate RBP4’s action and toxicity are yet to be investigated, but have been linked to proinflammatory cytokines in macrophages and to activation of JNK, a major ER stress kinase [29].

Recent studies revealed reduced hypothalamic volume and accelerated atrophy of orexin neurons in early Alzheimer’s disease [45, 46]. In contrast to diet-induced obese and diabetic models, PLB4 mice displayed increased Pomc and Mc4r mRNA levels, suggesting a hypothalamic shift toward appetite suppression and increased energy expenditure. The increase in hypothalamic Mc4r and Pomc transcription in PLB4 mice further contrasts with the recently demonstrated Aβ-oligomer-induced elevation in Npy mRNA (but not in Pomc mRNA), which was not associated with changes in circulating leptin levels [47]. Here, elevated melanocortin transcription in the hypothalamus of PLB4 mice may be a downstream effect of persistently increased circulating levels of leptin and systemic hyperglycaemia.

An advanced state of ER stress was confirmed in the hypothalamus of PLB4 mice, resembling that induced by HFD feeding [48]. Pharmacologically induced hypothalamic ER stress resulted in systemic diabetes in mice [49]; this was also recently illustrated for Aβ oligomer infusions [47]. The pathology of PLB4 mice therefore agrees with a scenario of BACE1-driven elevations of the ER stress marker CHOP and the protein translation regulator eIF2α, which suggest induction of an integrated stress response. Mechanistically, neuronal expression of human BACE1 may promote hypothalamic ER stress via ceramide lipotoxicity, activation of eIF2α and an integrated stress response [42], in addition to driving Aβ production [13].

In conclusion, we demonstrate that neuronal expression of human BACE1 causes systemic diabetic complications. We propose that increased levels of central BACE1 promotes metabolic disturbance via inducing hypothalamic impairment, ER stress, and Aβ and lipid accumulation, leading to neuronal damage, insulin resistance, hepatic deficits and global glucose dyshomeostasis. The comorbid phenotype of the PLB4 mouse provides insight into the complex mechanistic interactions between diabetes and Alzheimer’s disease. As an extension to the hypothesis that diabetic complications promote the onset and progression of Alzheimer’s disease, we suggest that the reverse scenario may also apply.