Non-cell-autonomous inhibition of photoreceptor development by Dip3.

We show here that the Drosophila MADF/BESS domain transcription factor Dip3, which is expressed in differentiating photoreceptors, regulates neuronal differentiation in the compound eye. Loss of Dip3 activity in photoreceptors leads to an extra photoreceptor in many ommatidia, while ectopic expression of Dip3 in non-neuronal cells results in photoreceptor loss. These findings are consistent with the idea that Dip3 is required non-cell autonomously to block extra photoreceptor formation. Dip3 may mediate the spatially restricted potentiation of Notch (N) signaling since the Dip3 misexpression phenotype is suppressed by reducing N signaling and misexpression of Dip3 leads to ectopic activity of a N-responsive enhancer. Analysis of mosaic ommatidia suggests that no specific photoreceptor must be mutant to generate the mutant phenotype. Remarkably, however, mosaic pupal ommatidia with three or fewer Dip3(+) photoreceptors always differentiate an extra photoreceptor, while those with four or more Dip3(+) photoreceptors never differentiate an extra photoreceptor. These findings are consistent with the notion that Dip3 in photoreceptors activates a heretofore unsuspected diffusible ligand that may work in conjunction with the N pathway to prevent a subpopulation of undifferentiated cells from choosing a neuronal fate.

Focal adhesion kinase is required, but not sufficient, for the induction of long-term potentiation in dentate gyrus neurons in vivo.

Tyrosine kinase phosphorylation plays an important role in the induction of long-term potentiation (LTP). Focal adhesion kinase (FAK) is a 125 kDa nonreceptor tyrosine kinase that shows decreased phosphorylation in fyn mutant mice, and Fyn plays a critical role in LTP induction. By examining the role of FAK involved in LTP induction in dentate gyrus in vivo with medial perforant path stimulation, we found that both FAK and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) phosphorylation were increased significantly 5 and 10 min after LTP induction, whereas cAMP-responsive element binding protein (CREB) phosphorylation was increased 40 min later. Transfection of the dominant-negative FAK mutant construct HA-FAK(Y397F) impaired LTP, whereas transfection of the constitutively activated form HA-FAK(Delta1-100) reduced the threshold for LTP induction. Transfection of HA-FAK(Delta1-100) by itself did not induce long-lasting potentiation. Further, transfection of the HA-FAK(Y397F) construct decreased FAK, MAPK/ERK, and CREB phosphorylation, and the inhibition of MAPK/ERK decreased CREB phosphorylation. Moreover, blockade of NMDA receptor (NMDAR) did not decrease FAK, MAPK/ERK, and CREB phosphorylation although LTP induction was blunted by NMDAR antagonist. These biochemical changes were not associated with low-frequency stimulation either. Immunoprecipitation results revealed that tyrosine phosphorylation of NR2A and NR2B as well as the association of phosphorylated FAK with NR2A and NR2B was increased with LTP induction. These results together suggest that FAK is required, but not sufficient, for the induction of LTP in a NMDAR-independent manner and that MAPK/ERK and CREB are the downstream events of FAK activation. Further, FAK may interact with NR2A and NR2B to modulate LTP induction.