Disentangling the Effect of Brain Insulin Resistance on Brain Health

The BIR-BrainHealth project investigates the role of brain insulin resistance (BIR) as a key mechanism linking diabetes to cognitive decline and dementia risk. By combining cognitive testing with advanced neuroimaging (MRI and PET), metabolic profiling, and multi-omics analyses, the study aims to uncover how BIR affects brain physiology, energy metabolism, and neurovascular function. The present trial registration specifically covers the MRI components of the project and does not include PET imaging.

Diabetes significantly increases the risk of mild cognitive impairment and dementia. The exact link between diabetes and cognitive decline is yet to be fully understood. Early cognitive decline in diabetes suggests a unique pathogenetic trajectory. One hypothesized mechanism involves impaired insulin signaling/transport in the brain, referred to as brain insulin resistance (BIR). Evidence indicates that in some individuals with diabetes, neuronal insulin signaling is abnormal, resulting in dysregulated cerebral metabolism and function. Another possible pathway is cerebrovascular pathology, which may compromise cerebral perfusion and energy metabolism. The present project will investigate the relationship between BIR and cerebrovascular dysfunction, and their impact on cognition and brain function.

We will set up a multicenter study in Denmark and Germany. Initially the study will include 50 participants with type 1 diabetes, 50 participants with type 2 diabetes, and 50 control participants without diabetes. All participants will undergo a comprehensive clinical evaluation to characterize diabetes duration, severity, and systemic comorbidities. The cognitive performance of the participants will be assessed using a neuropsychological test battery of following standardized tests:

• Rey Auditory Verbal Learning Test (RAVLT)
• Trail Making Test (TMT) part A and B
• Symbol Digit Modalities Test (SDMT)
• RBANS Digit Span forward (Version A)
• Wechsler Adult Intelligence Scale III Letter-Number Sequencing test (WAIS-LNS)
• Verbal Fluency test (phonetic and semantic)
• Grooved Pegboard
• Rapid Visual Processing (RVP) test from the Cambridge Neuropsychological Test Automated Battery (CANTAB) using A' (RVP-A) and mean latency for correct responses
• The Montreal Cognitive Assessment (MoCA)

Tests will be grouped by cognitive domains, and each domain score is calculated as the mean of the individual component z-scores. A cognitive composite (global score) will be derived from the mean of the domains: learning and memory, executive function and working memory, processing speed, and attention.

Magnetic resonance imaging (MRI) will be performed to assess BIR and structural brain changes, including atrophy. All MRI-scans will be performed on a research-optimized 3 Tesla MRI scanner. BIR will be evaluated by measuring changes in cerebral perfusion from administration of intranasal insulin delivered via nasal spray.

Blood, urine and stool will be collected for biomarker development and characterization of patients with BIR. A multi-omics approach will be applied to identify reliable biomarkers of BIR, incorporating lipidomics, metabolomics, proteomics, and transcriptomics. Lipidomic analysis will quantify approximately 650 lipid species, including phospholipids, lysophos-pholipids, ceramides, sphingolipids, diacylglycerols, and triacylglycerols, using liquid chromatography-mass spectrometry. Metabolomic profiling will measure approximately 300 metabolites, including citric acid cycle intermediates, short-chain fatty acids, ketones, and amino acids, using gas chromatography-mass spectrometry. Proteomic analysis will be conducted using a validated, clinically applicable mass spectrometry method, and transcriptomic profiling will be performed using RNA-based methods. The participants will be genotyped using the Illumina Infinium Global Screening Array (GSA v2.4), with exclusion of rare variants and clinically relevant mutations listed by the American College of Medical Genetics, in accordance with the guidelines of the National Science Ethics Committee on genomic research.

A subset of participants will undergo additional advanced MRI-scanning for assessments of cerebro-vascular function. Brain perfusion, blood-brain barrier integrity, and capillary perfusion distribution will be measured using dynamic contrast-enhanced MRI. Cerebrovascular reactivity will be measured by perfusion-weighted MRI during inhalation of air enriched with 5% CO2, a potent cerebral vasodilator. Cerebral blood flow responses to neuronal activation induced by visual stimulation will be assessed using perfusion-weighted MRI. Lastly, in a subset of participants the brain glucose sensing will be assessed using functional MRI (fMRI). fMRI will be measured prior to, during and following oral ingestion of water containing 75 grams of glucose. The glucose drink is given through a straw while the participants are lying in the scanner and will be consumed over a period of 5 minutes. Blood samples will be collected throughout the scan for measurements of blood glucose.