NRN1 and Alzheimer disease: Other synaptic signalling molecules positively associated with cognitive fitness in AD include neuritin-1 (NRN1)11–13,16 and brain-derived neurotrophic factor (BDNF)17,18, whereas proteins associated with cognitive frailty include gut-derived cholecystokinin (CCK) and the chromogranins A/B (CHGA/B)11,19.