Accumulation of the amyloid ss-protein (Ass) in brain, either as parenchymal plaques or cerebrovascular deposits, is a key pathological feature of patients with Alzheimer's disease (AD) and several related disorders. The Ass peptides are derived from the amyloid ss-protein precursor (AssPP) by sequential proteolytic cleavages by ss- and -secretase enzymes. The factors that either promote or impede Ass assembly into fibrillar structures that deposit in brain remain largely undefined. Recent in vitro work from our laboratory has shown that myelin basic protein (MBP), a prominent component of myelin in brain, is a potent inhibitor of Ass fibrillar assembly and can protect cultured primary neurons from the toxic effects of Ass. Although the spatial deposition of Ass in brain is consistent with this finding (i.e. brain white matter rich in MBP is largely devoid of fibrillar Ass deposits) and there is a relationship between decreased MBP levels and increased Ass levels it remains unknown if MBP does indeed influence Ass assembly and accumulation in vivo. Several well-characterized human AssPP transgenic mouse models have been generated that develop AD-like fibrillar amyloid deposits. To study the consequences of the absence of MBP on fibrillar amyloid assembly and deposition in these established AssPP transgenic models one could breed them onto an MBP gene knockout background. Such a model, known as the shiverer mouse, exists but comes with the significant shortcomings in that they do not form myelin and die within several months after birth. Unfortunately, human AssPP transgenic mouse models require aging well beyond several months to develop significant pathologic amyloid formation. Instead of knocking out expression of the entire MBP protein a more sophisticated approach that we plan to employ will be to mutate a highly specific domain on the MBP protein to disable a specific function. To this end, in the R21 Phase of this application we propose to generate a novel knock in mouse model where we will introduce alanine mutations into a specific KRG motif in the endogenous mouse Golli-MBP gene. Our recent studies have identified this specific KRG motif as an essential element for binding to Ass peptides and inhibiting their fibrillar assembly. The resulting new knock in model will produce MBP that lacks the ability to bind Ass peptides and inhibit their assembly. These novel MBP- KRG/AAA knock in mice will be generated and initially characterized for viability, growth, behavior, and myelination. After successful completion of the R21 Phase of this application we plan to proceed to the R33 Phase where we propose to cross the MBP-KRG/AAA knock in mice with two different human AssPP transgenic mouse models that develop fibrillar amyloid deposition. The crossed mouse lines will be aged and quantitatively evaluated for the acumulation, asembly, and deposition of Ass peptides and the resulting downstream pathological and behavioral consequences. Completion of these studies will provide new insight into potential physiological mechanisms that govern pathogenic amyloid assembly and may lead to new avenues for intervention into this pathologic process.