The AP-1B clathrin adaptor complex plays a key role in the recognition and intracellular transport of many membrane proteins destined for the basolateral surface of epithelial cells. However, little is known about other components that act in conjunction with AP-1B. We found that the Rab8 GTPase is one such component. Expression of a constitutively activated GTP hydrolysis mutant selectively inhibited basolateral (but not apical) transport of newly synthesized membrane proteins. Moreover, the effects were limited to AP-1B–dependent basolateral cargo; basolateral transport of proteins containing dileucine targeting motifs that do not interact with AP-1B were targeted normally despite overexpression of mutant Rab8. Similar results were obtained for a dominant-negative allele of the Rho GTPase Cdc42, previously implicated in basolateral transport but now shown to be selective for the AP-1B pathway. Rab8-GFP was localized to membranes in the TGN-recycling endosome, together with AP-1B complexes and the closely related but ubiquitously expressed AP-1A complex. However, expression of active Rab8 caused a selective dissociation of AP-1B complexes, reflecting the specificity of Rab8 for AP-1B–dependent transport.

Immunocytochemistry, radioimmunoassay, immunoblotting, Northern analysis, and polymerase chain reaction (PCR) technique were applied to investigate the distribution of laminin and its neurite outgrowth domain in brains of neuropathologically verified cases of Alzheimer's disease and Down's syndrome. New antibodies against a neurite outgrowth domain of laminin were characterized and were used in localization of this peptide antigen in the human brain. Laminin was found as large punctate deposits in all plaques in the affected brains. Laminin synthesis was increased as assessed by RNA blotting and immunoblotting, and glial cells were heavily immunoreactive with antibodies for a neurite outgrowth-promoting peptide antigen of the B2 chain of laminin. This peptide antigen not only was produced by glial cells but also was deposited in the brain tissue. As this peptide antigen promotes neurite outgrowth at low concentrations, and is specifically neurotoxic at high concentrations, it may play a synergistic role with other molecules in inducing the sprouting and neurodegeneration occurring in brains of patients with either Alzheimer's disease or Down's syndrome.

Basolateral targeting of membrane proteins in polarized epithelial cells typically requires cytoplasmic domain sorting signals. In the familial hypercholesterolemia (FH)-Turku LDL receptor allele, a mutation of glycine 823 residue affects the signal required for basolateral targeting in MDCK cells. We show that the mutant receptor is mistargeted to the apical surface in both MDCK and hepatic epithelial cells, resulting in reduced endocytosis of LDL from the basolateral/sinusoidal surface. Consequently, virally encoded mutant receptor fails to mediate cholesterol clearance in LDL receptor-deficient mice, suggesting that a defect in polarized LDL receptor expression in hepatocytes underlies the hypercholesterolemia in patients harboring this allele. This evidence directly links the pathogenesis of a human disease to defects in basolateral targeting signals, providing a genetic confirmation of these signals in maintaining epithelial cell polarity.