Temporal relationships of latency and magnitudes of Limulus ventral photoreceptor potentials elicited by two light pulses.

As the dark interval between two stimulating light pulses increases, the latent period of the ensuing receptor potential increases to an asymptote reached about midway during the latency and is, thereafter, constant. The latent period is thus divisible into two segments, the first of which is light sensitive and the second not. The possible significance of these segments has been explored [12]. During the time the latent period is increasing toward its limiting value, the magnitudes of both components of the receptor potential are constant and maximal. The magnitude of the first component of the receptor potential begins to decline at a pulse interval of 15 msec, signify that the contribution of photons incident on the photoreceptor from the second pulse is diminishing. The magnitude of the second component of the receptor potential begins to decline at a pulse interval of 20 msec, suggesting that the contribution of photons delivered by the first pulse is diminishing. These results demonstrate that the two pulse stimulating paradigm reveals two segments of the receptor potential latent period and the ability of the photoreceptor to integrate radiant energy incident during the latent period, as measured by receptor potential magnitude.

Two latent period processes in Limulus ventral photoreceptors.

The latent period of Limulus ventral photoreceptor potentials consists of two contiguous time segments, S1 and S2, which exhibit the following properties: (1) the duration of S1 is inversely dependent on the incident energy provided by the first stimulating light flash; (2) the duration of S1 is prolonged in the presence of GTP-gamma-S [guanosine-5'-0- (3-thiotriphosphate)] or papaverine; (3) the duration of S1 is abbreviated in the presence of sodium vanadate or chlorobutanol; (4) the duration of S2 is abbreviated by light adaptation; and (5) the duration of S2 is prolonged more than S1 in a low temperature environment. The existence of these two components of the Limulus ventral photoreceptor potential latent period is demonstrable both in dark adapted photoreceptors, using single constant intensity light pulses of varying duration, or in partially light adapted photoreceptors, using a stimulating conditioning pulse-test pulse sequence. On the basis of these results it is tentatively concluded that the two segments of the latent period of Limulus ventral photoreceptor potentials are occupied by different processes. The first process, occupying the S1 segment, generates a critical concentration of a photoproduct which, directly or indirectly, eventually alters the conductance of the active photoreceptor membrane. The second process, occupying the S2 segment, is concerned with the initiation of the SPFs which sum to produce the receptor potential. It is possible that the hydrolysis of GTP actively participates in the first process.

The reactive tumor microenvironment: MUC1 signaling directly reprograms transcription of CTGF.

The MUC1 cytoplasmic tail (MUC1.CT) conducts signals from spatial and extracellular cues (growth factor and cytokine stimulation) to evoke a reprogramming of the cellular transcriptional profile. Specific phosphorylated forms of the MUC1.CT achieve this function by differentially associating with transcription factors and redirecting their transcriptional regulatory capabilities at specific gene regulatory elements. The specificity of interaction between MUC1.CT and several transcription factors is dictated by the phosphorylation pattern of the 18 potential phosphorylation motifs within the MUC1.CT. To better appreciate the scope of differential gene expression triggered by MUC1.CT activation, we performed microarray gene expression analysis and chromatin immunoprecipitation (ChIP)-chip promoter analysis and identified the genome-wide transcriptional targets of MUC1.CT signaling in pancreatic cancer. On a global scale, MUC1.CT preferentially targets genes related to invasion, angiogenesis and metastasis, suggesting that MUC1.CT signaling contributes to establishing a reactive tumor microenvironment during tumor progression to metastatic disease. We examined in detail the molecular mechanisms of MUC1.CT signaling that induces the expression of connective tissue growth factor (CTGF/CCN2), a potent mediator of ECM remodeling and angiogenesis. We demonstrate a robust induction of CTGF synthesis and secretion in response to serum factors that is enabled only when MUC1 is highly expressed. We demonstrate the requirement of phosphorylation at distinct tyrosine motifs within the MUC1.CT for MUC1-induced CTGF expression and demonstrate a phosphorylation-specific localization of MUC1.CT to the CTGF promoter. We found that MUC1 reorganizes transcription factor occupancy of genomic regions upstream of the CTGF gene, directing ß-catenin and mutant p53 to CTGF gene regulatory elements to promote CTGF expression and destabilizing the interaction at these regions of the transcriptional repressor, c-Jun. With this example we illustrate the capacity of MUC1.CT to mediate transcription factor activity in a context-dependent manner to achieve wide spread and robust changes in gene expression and facilitate creation of the reactive tumor microenvironment.