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Abstract:

ADAM10 is a member of the “A Disintegrin And Metalloprotease “ family that are type I transmembrane proteins with metalloprotease, integrin-binding, intracellular signaling and cell adhesion activities. ADAMs play an important role in the development of the nervous system, where they regulate cell proliferation, migration and differentiation, as well as axonal growth and myelination.ADAM10 is expressed in oligodendrocytes and a subset of developing neurons and brain nuclei and in differentiating gray matter. Adam10 knockout (KO) mice die at E 9.5 due to major developmental defects in development of somites, vasculogenesis and brain, which suggests that ADAM10 is crucial for early embryogenesis and neurodevelopment. In contrast to other metalloproteases, ADAM10 is particularly important for cleavage-dependent activation. It is thought to be responsible for the shedding of several cell-surface proteins in the brain, such as Ephrins, L1 adhesion molecule and N-cadherin.ADAM10 also seems to control the proteolytic processing of the Notch receptor, which mediates lateral inhibition during neurogenesis. Signaling of the Notch receptor is dependent on three types of proteolytic events. Both ADAM10 and ADAM17 have been implicated in the second cleavage of the Notch receptor. However in vivo only Adam10 KO mice display Notch-1 loss-of-function phenotypes, whereas Adam17 KO do not. From medical point of view the most important putative substrate of ADAM10 is the Amyloid precursor protein (APP). It is widely believed that the Abeta peptide, one of the major components of the amyloid plaques, triggers a cascade causative for the neurodegeneration in Alzheimer’s disease (AD). This peptide is generated from the larger APP by two sequential cleavages, beta-secretase and gamma-secretase. However in the non-amyloidogenic pathway, APP is cleaved within the Abeta domain by alpha-secretase. Processing of APP by alpha-secretase(s) is postulated to be protective in the context of AD because the enzyme(s) cleave within the Abeta sequence, thereby preventing the production of Abeta. Secondly increasing alpha-secretase activity would be neuroprotective via the increased release of neuroprotective soluble APPalpha ectodomains.However the nature of the metalloprotease responsible for the physiologic ectodomain processing of APP is not completely clear. Overexpression studies and in vitro cleavage assays indicate that ADAM9, ADAM10 and ADAM17 are able to act as alpha-secretases. However, Adam9- deficient mice do not show an impaired APP shedding in hippocampal neurons. ADAM17 is also unlikely to act as an essential alpha-secretase since it is predominantly expressed in endothelial cells and astrocytes in the human CNS, while ADAM10 is highly expressed in neurons in the mouse CNS. Consistent with these findings, overexpression of ADAM10 in an Alzheimer’s mouse model resulted in a decrease in beta-amyloid deposition and cognitive deficits, but sincetransgenic overexpression of proteins creates a highly unphysiological situation, formal proof that ADAM10 is the crucial neuronal alpha-secretase needs an additional loss of function approach.To investigate the function of ADAM10 in brain development and in APP and Notch signaling, we generated Adam10 conditional knockout (cKO) mice using a nestin Cre promotor, limiting ADAM10 inactivation to neural progenitor cells (NPC) and NPC-derived neurons and glial cells. The cKO mice die perinatally with a disrupted neocortex and a severely reduced ganglionic eminence, due to precocious neuronal differentiation resulting in an early depletion of progenitor cells. Premature neuronal differentiation is associated with aberrant neuronal migration and a disorganized laminar architecture in the neocortex. Neurospheres derived from Adam10 cKO mice also have a disrupted sphere organisation and segregated more neurons at the expense of astrocytes.We found that notch-1 processing was affected leading to down regulation of several notch regulated genes in Adam10 cKO brains, in accordance with the central role of ADAM10 in this signaling pathway and explaining the neurogenic phenotype. Finally, we found that alpha-secretase-mediated processing of APP was largely reduced in these neurons, demonstrating that ADAM10 represents the most important APP alpha-secretase in the brain. Our study reveals that ADAM10 plays a central role in the developing brain by controlling mainly notch-dependent pathways but likely also by reducing surface shedding of other neuronal membrane proteins including APP.