A knockout mouse model gives us clues regarding the important normal functions of both full-length and secreted fragments of the -amyloid precursor protein in the central nervous system.In 1906 Alois Alzheimer examined the brain of a profoundly demented patient and observed the presence of intraneuronal fibrils (neurofibrillary tangles) and 'miliary foci of a peculiar substance in the cerebral cortex' (senile plaque). 1 Diagnosis of the disease, later named after Alzheimer, still rests on the identification of tangles and plaques in postmortem brain tissue. Although Alzheimer's disease (AD) was initially thought to be a rare disorder, it has become a serious public health issue. To understand the pathology of and to direct potential therapies for AD requires characterizing the major proteins associated with AD brain lesions. To this end, we are studying -amyloid precursor protein (PP), the precursor to the -amyloid (A) that is the major protein constituent of AD senile plaques.Because PP is abundantly expressed in brain cells and because potential therapeutic strategies are aimed at altering PP levels it is important to determine the physiological role of PP in brain. Although many putative roles have been attributed to PP (recently reviewed in Reference 2), 2 it is crucial to determine if diminishing PP affects normal neuronal physiology. A mouse model that is deficient in PP has been developed and exhibits phenotypic abnormalities including: reactive gliosis, a reduced forelimb grip strength, and decreased locomotion. 3 While the reasons underlying the phenotypic changes in the PP-deficient mice have not been thoroughly explored, the model does argue that PP is important for normal nervous system function. We began our investigations to determine the biology of neuronal PP and have recently shown that PP contributes to neuronal viability and neuron differentiation in vitro. 4 By comparing the survival of cultured hippocampal neurons from wild-type (WT) and PP-deficient knockout (KO) mice, we determined that WT neurons survive better than do KO neurons in all culture conditions tested.Correspondence: RG Perez, PhD, Allegheny University of the Health Sciences, Center for Neuroscience Research, 10th Floor, South Tower, 320 E North Avenue, Pittsburgh, PA 15212, USA Because WT neurons express full-length PP while KO neurons do not, and because the presence of different factors in the media did not rescue KO neurons to the same level as WT controls, it appears that neuron survival is enhanced by the expression of cell-associated full-length PP.The advantage of using this unique model system is that it allows for selectively teasing apart the relative contributions of cell-associated PP, on neurons, from the effects of soluble forms of PP. To test for differences in process outgrowth in response to astrocytederived soluble factors we co-cultured WT and KO neurons with astrocytes obtained from WT or KO mice. WT (but not KO) astrocytes release soluble PP fragments 5 including the long N-terminal fr...