Trafficking pathways of Cx49-GFP in living mammalian cells

Tyrosine-dependent basolateral targeting of human connexin43-eYFP in Madin-Darby canine kidney cells can be disrupted by the oculodentodigital dysplasia mutation L90V

Polarized membrane sorting of connexin 43 (Cx43) has not been well-characterized. Based on the presence of a putative sorting signal, YKLV(286-289), within its C-terminal cytoplasmic domain, we hypothesized that Cx43 is selectively expressed on the basolateral surface of Madin-Darby canine kidney (MDCK) cells in a tyrosine-dependent manner. We generated stable MDCK cell lines expressing human wild-type and mutant Cx43-eYFP, and analyzed the membrane localization of Cx43-eYFP within polarized monolayers using confocal microscopy and selective surface biotinylation. We found that wild-type Cx43-eYFP was selectively targeted to the basolateral membrane domain of MDCK cells. Substitution of alanine for Y286 disrupted basolateral targeting of Cx43-eYFP. Additionally, substitution of a sequence containing the transferrin receptor internalization signal, LSYTRF, for PGYKLV(284-289) also disrupted basolateral targeting. Taken together, these results indicate that Y286 in its native amino acid sequence is necessary for targeting Cx43-eYFP to the basolateral membrane domain of MDCK cells. To determine whether the F52dup or L90V oculodentodigital dysplasia-associated mutations could affect polarized sorting of Cx43-eYFP, we analyzed the expression of these Cx43-eYFP mutant constructs and found that the L90V mutation disrupted basolateral expression. These findings raise the possibility that some oculodentodigitial dysplasia-associated mutations contribute to disease by altering polarized targeting of Cx43.

Expression of both N- and C-terminal GFP tagged huCD36 and their discrepancy in OxLDL and pRBC binding on CHO cells

CD36, an 88 kDa membrane glycoprotein, is found in several cell types and it has been characterized to have two hydrophobic domains at their N- and C-termini which are essential for protein folding and targeting. In this study, we first tagged the green fluorescent protein (GFP) to both the N- and C-termini of huCD36 and investigated their cellular expression and influences on lipoprotein and plasmodium falciparium parasitized erythrocytes (pRBC) binding. Our work revealed that huCD36 proteins are expressed normally irrespective of the GFP tag presence at either the N- or C-termini. However, the two recombinant proteins showed discrepancy in uptake and surface-binding of OxLDL but they did not affect pRBC binding. These results suggested that the interaction between oxLDL and CD36 could be blocked using recombinant proteins and this may be useful in potential control of the trafficking of modified lipoproteins into monocytes leading to atherogenesis.

The connexin turnover, an important modulating factor of the level of cell-to-cell junctional communication: comparison with other integral membrane proteins

The constituent proteins of gap junctions, called "connexins" (Cxs) in chordates, are generally renewed several times a day, in approximately the same rate range as many other integral plasma membrane proteins and the proteins of other channels, other intercellular junctions or different membrane receptors. This permanent renewal turns on a fine-tuned balance among various processes, such as gene transcription, mRNA stability and processing, protein synthesis and oligomerization, posttranslational modifications, transport to the plasma membrane, anchoring to the cytoskeleton, connexon aggregation and docking, regulation of endocytosis and controlled degradations of the proteins. Subtle changes at one or some of these steps would represent an exquisite level of regulation that extends beyond the rapid channel opening and closure events associated with channel gating; membrane channels and receptors are constantly able to answer to physiological requirements to either up- or downregulate their activity. The Cx turnover rate thereby appears to be a key component in the regulation of any protein, particularly of gap junctional proteins. However, the physiological stimuli that control the assembly of Cxs into gap junctions and their degradation remain poorly understood.

The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.

Gap Junction Coupling and Apoptosis in GFSHR-17 Granulosa Cells

Recently, we found that intracellular washout of cGMP induces gap junction uncoupling and proposed a link between gap junction uncoupling and stimulation of apoptotic reactions in GFSHR-17 granulosa cells. In the present report we show that an inhibitor of guanylyl cyclase, ODQ, reduces gap junction coupling and promotes apoptotic reactions such as chromatin condensation and DNA strand breaks. To analyze whether gap junction uncoupling and induction of apoptotic reactions are related, the cells were treated with heptanol and 18 beta-GA, two known gap junction uncouplers. Gap junction coupling of GFSHR-17 cells could be restored if the incubation time with the gap junction uncouplers was less than 10 min. A prolonged incubation time irreversibly suppressed gap junction coupling and caused chromatin condensation as well as DNA degradation. The promotion of apoptotic reactions by heptanol or 18 beta-GA was not observed in cells with low gap junction coupling like HeLa cells, indicating that the observed genotoxic reactions are not caused by unspecific effects of gap junction uncouplers. Additionally, it was observed that heptanol or 18 beta-GA did not induce a sustained rise of [Ca(2+)](i). The effects of gap junction uncouplers could not be suppressed by the presence of 8-Br-cGMP. It is discussed that irreversible gap junction uncoupling can be mediated by cGMP-dependent as well as cGMP-independent pathways and in turn could lead to stimulation of apoptotic reactions in granulosa cells.