PROJECT SUMMARY/ABSTRACT We propose to define the molecular mechanisms of two underappreciated dementia syndromes known as Familial Danish Dementia and Familial British Dementia. Both syndromes have highly similar pathologies to Alzheimer’s Disease (AD), with onset of dementia as early as 40 years of age. While a majority of AD is age- related, sporadic, and can be caused by a myriad of events or dysfunctions, the genetic cause of the Familial dementia syndromes is identified as mutations in the gene that encodes for an integral membrane protein, BRI-2. Normally, maturation of BRI-2 involves proteolytic processing that generates a 23-residue peptide. However, the BRI2-derived peptides generated from the mutant BRI-2 proteins are each extended by 11 residues. The extensions differ between Danish and British dementia because the causative mutations differ. Notably, the earliest manifestation of Danish dementia syndrome is the appearance of cataracts at ages as early as 20 years, while no ocular symptoms are reported for British dementia patients. The Danish dementia peptide has been shown to bind to the major protein chaperone responsible for maintaining lens transparency, -crystallin (HSPB4/HSPB5) and to interfere with its ability to function as a chaperone against model client proteins, while the British dementia peptide was a much less effective inhibitor of lens chaperone function. Both Danish and British patients suffer from early-onset dementia, suggesting a shared ability to accelerate aggregation processes that lead to amyloid plaques and neurofibrillary tangles. This dichotomy provides a powerful strategy for defining the key features responsible for 1) protein aggregation that leads to development of cataracts in Danish patients and 2) accelerated aggregation and plaque formation in both Danish and British patients. We propose to employ protein reagents and experimental approaches developed in the ongoing parent grant to investigate how familial dementia peptides interfere with 1) HSPB4/HSPB5 chaperone function towards a lens-specific protein client (D-crystallin) and 2) HSPB1/HSPB5 chaperone function towards the amyloidogenic protein, Tau. Our approaches include: aggregation assays using bona fide protein clients under physiologically relevant conditions, quantitative determination of intermolecular interactions, mapping of interactions at the residue-level by NMR and UV-crosslinking/mass spectrometry, among others. The chaperones under investigation are constitutively expressed in lens, neurons, and brain and likely serve to stave off harmful protein aggregation for most of an individual’s lifetime. Understanding the molecular mechanism of these familial dementias and how chaperone function is disrupted provides a novel path to understand a broad array of AD/ADRDs and potentially to shore up intrinsic systems such as the small heat shock protein chaperones to delay and inhibit protein aggregation and amyloid formation throughout an in...