👤 Vladimir Sytnyk

Convergence of amyloid precursor protein (APP) and β-site APP cleaving enzyme 1 (BACE1) in endosomes initiates the production of amyloid-β (Aβ) peptides that accumulate in brains of Alzheimer's disease patients. APP and BACE1 are segregated in neurons, and mechanisms triggering their convergence have remained poorly understood, limiting therapeutic attempts to reduce Aβ production. Neural cell adhesion molecule 2 (NCAM2) is a cell surface localized protein, which increases Aβ levels via mechanisms that are not known. We show that APP binds to the extracellular domain of NCAM2. The intracellular domain of NCAM2 binds to the Rab11 adaptor protein Rab11-FIP5. The NCAM2/APP complex is endocytosed from the cell surface and targeted to BACE1-containing Rab11-positive recycling endosomes where it is processed. Convergence of APP with BACE1 is increased in transfected CHO cells and neurons expressing NCAM2. Consequently, the levels of amyloidogenic APP cleavage products are increased in cells expressing NCAM2. In NCAM2-deficient neurons, APP accumulates at the cell surface and in early endosomes, and APP levels in recycling endosomes are reduced. Aβ production is increased by Aβ oligomers and neuronal activity, and we show that the binding of NCAM2 to APP is increased in neurons treated with Aβ oligomers or after activation of synaptic NMDA receptors. Together, our data indicate that NCAM2 binds to APP and promotes APP targeting from the neuronal cell surface to recycling endosomes where APP is cleaved by BACE1. This novel mechanism regulating the convergence of APP and BACE1 in neurons can contribute to Aβ accumulation in Alzheimer's disease. Show less

The neural cell adhesion molecule 2 (NCAM2) regulates axonal organization in the central nervous system via mechanisms that have remained poorly understood. We now show that NCAM2 increases axonal levels of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), a protease that regulates axonal guidance. In brains of NCAM2-deficient mice, BACE1 levels are reduced in hippocampal mossy fiber projections, and the infrapyramidal bundle of these projections is shortened. This abnormal axonal organization correlates with impaired short-term spatial memory and cognitive flexibility in NCAM2-deficient male and female mice. Self-grooming, rearing, digging and olfactory acuity are increased in NCAM2-deficient male mice, when compared with littermate wild-type mice of the same sex. NCAM2-deficient female mice also show increased self-grooming, but are reduced in rearing, and do not differ from female wild-type mice in olfactory acuity and digging behavior. Our results indicate that errors in axonal guidance and organization caused by impaired BACE1 function can underlie the manifestation of neurodevelopmental disorders, including autism as found in humans with deletions of the NCAM2 gene. Show less

Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), also known as β-secretase, is an aspartic protease. The sorting of this enzyme into Rab11-positive recycling endosomes regulates the BACE1-mediated cleavage of its substrates, however, the mechanisms underlying this targeting remain poorly understood. The neural cell adhesion molecule 2 (NCAM2) is a substrate of BACE1. We show that BACE1 cleaves NCAM2 in cultured hippocampal neurons and NCAM2-transfected CHO cells. The C-terminal fragment of NCAM2 that comprises the intracellular domain and a small portion of NCAM2's extracellular domain, associates with BACE1. This association is not affected in cells with inhibited endocytosis, indicating that the interaction of NCAM2 and BACE1 precedes the targeting of BACE1 from the cell surface to endosomes. In neurons and CHO cells, this fragment and BACE1 co-localize in Rab11-positive endosomes. Overexpression of full-length NCAM2 or a recombinant NCAM2 fragment containing the transmembrane and intracellular domains but lacking the extracellular domain leads to an increase in BACE1 levels in these organelles. In NCAM2-deficient neurons, the levels of BACE1 are increased at the cell surface and reduced in intracellular organelles. These effects are correlated with increased levels of the soluble extracellular domain of BACE1 in the brains of NCAM2-deficient mice, suggesting increased shedding of BACE1 from the cell surface. Of note, shedding of the extracellular domain of Sez6, a protein cleaved exclusively by BACE1, is reduced in NCAM2-deficient animals. These results indicate that the BACE1-generated fragment of NCAM2 regulates BACE1 activity by promoting the targeting of BACE1 to Rab11-positive endosomes. Show less