Structural requirements for basolateral sorting of the human transferrin receptor in the biosynthetic and endocytic pathways of Madin-Darby canine kidney cells

The polarized sorting of membrane proteins expressed in cultured hippocampal neurons using viral vectors

One model of neuronal polarity (Dotti and Simons, 1990) proposes that neurons and polarized epithelia use similar mechanisms to sort membrane proteins. To explore this hypothesis, we used viral vectors to express proteins in cultured neurons and assessed their distribution using quantitative immunofluorescence microscopy. Basolateral epithelial proteins were polarized to dendrites; more significantly, mutations of sequences required for their basolateral targeting in epithelia also disrupted dendritic targeting. Unexpectedly, apical proteins were not polarized to axons but were expressed at roughly equal amounts in dendrites and axons. These data provide strong evidence that targeting of basolateral and dendritic proteins depends on common mechanisms. In contrast, the sorting of proteins to the axon requires signals that are not present in apical proteins.

A novel method of cell-specific mRNA transfection

In this study, we developed a cell-specific mRNA transfection system using streptavidin-protein A (ST-PA) fusion protein and monoclonal antibodies (mAbs). We previously reported that ST-PA fusion protein and mAb complexes can transfer certain biotinylated proteins into specific cell types. At this time, we combined an in vitro transcribed biotinylated and self-replicating Sindbis virus genomic RNA with ST-PA fusion protein and mAbs. In the presence of cationic liposomes, to prevent RNA degradation, this complex is able to transfect a reporter gene to specific cancer cells in a mAb does-dependent manner. Even in the absence of cationic liposomes, biotinylated mRNA, ST-PA fusion, and mAb complexes can transfer some types of cancer cell suspension cultures. This cell-specific transfection system is a novel method of introducing various mRNAs into cells that results in high levels of transient protein expression.

Carbohydrate-mediated Golgi to cell surface transport and apical targeting of membrane proteins

Polarized expression of most epithelial plasma membrane proteins is achieved by selective transport from the Golgi apparatus or from endosomes to a specific cell surface domain. In Madin-Darby canine kidney (MDCK) cells, basolateral sorting generally depends on distinct cytoplasmic targeting determinants. Inactivation of these signals often resulted in apical expression, suggesting that apical transport of transmembrane proteins occurs either by default or is mediated by widely distributed characteristics of membrane glycoproteins. We tested the hypothesis of N-linked carbohydrates acting as apical targeting signals using three different membrane proteins. The first two are normally not glycosylated and the third one is a glycoprotein. In all three cases, N-linked carbohydrates were clearly able to mediate apical targeting and transport. Cell surface transport of proteins containing cytoplasmic basolateral targeting determinants was not significantly affected by N-linked sugars. In the absence of glycosylation and a basolateral sorting signal, the reporter proteins accumulated in the Golgi complex of MDCK as well as CHO cells, indicating that efficient transport from the Golgi apparatus to the cell surface is signal-mediated in polarized and non-polarized cells.

Mechanisms of neuronal polarity

The mechanisms that permit neurons to establish axons and dendrites involve an interplay between a cell's genetic program and signals in its environment. Recent experiments have identified some of the important extracellular molecules that regulate dendritic development and have furthered our understanding of the endogenous cell biological mechanisms that underlie protein sorting. Some of the signaling pathways that allow extracellular cues to regulate neuronal morphogenesis are also being elucidated.