Selective permeability of different connexin channels to the second messenger inositol 1,4,5-trisphosphate

Abstract

Intercellular propagation of signals through connexin32-containing gap junctions is of major importance in physiological processes like nerve activity-dependent glucose mobilization in liver parenchymal cells and enzyme secretion from pancreatic acinar cells. In these cells, as in other organs, more than one type of connexin is expressed. We hypothesized that different permeabilities towards second messenger molecules could be one of the reasons for connexin diversity. In order to investigate this, we analyzed transmission of inositol 1,4,5-trisphosphate-mediated calcium waves in FURA-2-loaded monolayers of human HeLa cells expressing murine connexin26, -32 or -43. Gap junction-mediated cell coupling in different connexin-transfected HeLa cells was standardized by measuring the spreading of microinjected Mn(2+) that led to local quenching of FURA-2 fluorescence. Microinjection of inositol 1,4,5-trisphosphate into confluently growing HeLa connexin32 transfectants induced propagation of a Ca(2+) wave from the injected cell to neighboring cells that was at least three- to fourfold more efficient than in HeLa Cx26 cells and about 2.5-fold more efficient than in HeLa Cx43 transfectants. Our results support the notion that diffusion of inositol 1,4,5-trisphosphate through connexin32-containing gap junctions is essential for the optimal physiological response, for example by recruiting liver parenchymal cells that contain subthreshold levels of this short lived second messenger.

Role of network connectivity in intercellular calcium signaling

“…Examples of real cellular configurations in vitro are shown in Fig. 1 and 2 in (27) and Fig. 9 and 11 in (8).…”

“…It also appears that physiological concentrations of Ca 2+ can reversibly close gap junctions but this response takes about 30 s (24). Thus, because Ca 2+ waves propagate faster than the proposed closure times (25), the two possible intercellular messengers (Ca 2+ and IP 3 ) ( 26), (27) can diffuse to adjacent cells to propagate the wave before Ca 2+ closes the gap junctions. Therefore, it would seem that a more accurate description of the gap junction permeability requires taking into account its dependence on the cytosolic calcium concentration, rather than treating it as a constant as in the earlier models (15), (17), (19), (20).…”

“…However, in reality, the structure of even flat tissues is significantly more complicated. Even small areas of tissue can exhibit different topological types of cell connections between each other (8), (27). In addition, in the case of local stimulation in which only one cell (not the entire tissue) is stimulated, it is important to know which cell is stimulated (for the same structure of cell connections).…”

Function and Plasticity of Electrical Synapses in the Mammalian Brain: Role of Non-Junctional Mechanisms

“…Gap junction’s channels are permeable to ions as well as small molecules, thus supporting not only the electrical communication between cells but also biochemical signaling through second messengers, such as Ca 2+ , cAMP, and inositol 1,4,5- trisphosphate [ 18 , 19 , 20 , 21 , 22 , 23 , 24 ]. As other membrane channels, they present voltage gating processes, as their conductance is sensitive to the voltage difference between cells (transjunctional voltage, V J ), and to a lesser extent, to the membrane potential ( V m ) [ 5 , 25 ].…”

The Amino Terminal Domain and Modulation of Connexin36 Gap Junction Channels by Intracellular Magnesium Ions

“…Gap junction (GJ) channels, formed of connexin (Cx) protein subunits, serve as intercellular communication pathways that enable the efficient propagation of electrical signals and the flux of small ions, amino acids, metabolites and peptides between cells ( Niessen et al, 2000 ; Neijssen et al, 2005 ; Bedner et al, 2006 ). There are 20 Cxs expressed in rodents and 21 in humans and the Cx36 isoform is the main Cx expressed in neurons thereby constituting the bulk of the electrical synapses in the adult brain, as well as in retinal circuits ( Connors and Long, 2004 ).…”