In the pituitary of mammals, the GnRH receptor (GnRHR) plays a primary role in the control of reproductive function. It is further expressed in the hippocampus, where its function, however, is not well defined. By quantitative RT-PCR analyses, we demonstrate herein that the onset of GnRHR gene (Gnrhr) expression in the rat hippocampus was unexpectedly delayed as compared to the pituitary and only occurred after birth. Using a previously described transgenic mouse model bearing the human placental alkaline phosphatase reporter gene under the control of the rat Gnrhr promoter, we established a positive correlation between the temporal pattern of Gnrhr mRNA levels and promoter activity in the hippocampal formation. The gradual appearance of human placental alkaline phosphatase transgene expression occurred simultaneously in the hippocampus and interconnected structures such as the lateral septum and the amygdala, coinciding with the establishment of hippocampo-septal projections. Analysis of transcription factors together with transient transfection assays in hippocampal neurons indicated that the combinatorial code governing the hippocampus-specific expression of the Gnrhr is distinct from the pituitary, likely involving transactivating factors such as NUR77, cyclic AMP response element binding protein, and Finkel-Biskis-Jinkins murine osteosarcoma virus oncogene homolog. A silencing transcription factor acting via the -3255/-1135 promoter region of the Gnrhr may be responsible for the transcriptional repression observed around birth. Finally, GnRH directly stimulated via activation of its receptor the expression of several marker genes of neuronal plasticity such as Egr1, synaptophysin, and spinophilin in hippocampal primary cultures, suggesting a role for GnRHR in neuronal plasticity. Further characterization of these mechanisms may help unravel important functions of GnRH/GnRHR signaling in the brain.
The effects of Bmp-4 on interdigital cell death were investigated in the mouse. Affi-Gel beads, loaded with recombinant Bmp-4 protein, were transplanted into the interdigital tissues of day 12.5 hindlimb, ex utero. It was established that Bmp-4 could induce precocious interdigital cell death. Using in situ hybridization, the expression patterns of bmp-4 and alk-6 receptor were established. Both genes were found coexpressed in the interdigital region of 12.5- and 13. 5-day hindlimbs. This suggests that Bmp-4 may act in an autocrine fashion. We have also studied the effects of Bmp-4 on 12.5-day interdigital tissue cultures. In all specimens examined, the interdigital tissues produced cartilage instead of participating in cell death. The addition of exogenous Bmp-4 to the interdigital cultures did not induce apoptosis but instead enhanced chondrogenesis. The discrepancy between the effects of Bmp-4 in vitro and ex utero was attributed to the presence of digits. When a flanking digit was left attached to the interdigital tissues, in vitro, Bmp-4 promoted apoptosis instead of chondrogenesis. In sum, the results suggest that Bmp-4 is a multifunctional protein and its effect on the interdigital tissues is dependent on the modulating influence of the digits.
Much of what we currently know about digit morphogenesis during limb development is deduced from embryonic studies in the chick. In this study, we used ex utero surgical procedures to study digit morphogenesis during mouse embryogenesis. Our studies reveal some similarities; however, we have found considerable differences in how the chick and the mouse autopods respond to experimentation. First, we are not able to induce ectopic digit formation from interdigital cells as a result of wounding or TGFbeta-1 application in the mouse, in contrast to what is observed in the chick. Second, FGF4, which inhibits the formation of ectopic digits in the chick, induces a digit bifurcation response in the mouse. We demonstrate with cell marking studies that this bifurcation response results from a reorganization of the prechondrogenic tip of the digit rudiment. The FGF4 effect on digit morphogenesis correlates with changes in the expression of a number of genes, including Msx1, Igf2, and the posterior members of the HoxD cluster. In addition, the bifurcation response is digit-specific, being restricted to digit IV. We propose that FGF4 is an endogenous signal essential for skeletal branching morphogenesis in the mouse. This work stresses the existence of major differences between the chick and the mouse in how digit morphogenesis is regulated and is thus consistent with the view that vertebrate digit evolution is a relatively recent event. Finally, we discuss the relationship between the digit IV bifurcation restriction and the placement of the metapterygial axis in the evolution of the tetrapod limb.
In the pituitary of mammals, the GnRH receptor (GnRHR) plays crucial roles in the neuroendocrine control of reproductive function. This receptor is specifically expressed by the gonadotrope cells scattered among the five other endocrine cell types constituting the anterior pituitary; it is also expressed in other organs, such as the gonads and brain where its function is not well defined. To gain insight into GnRHR function, distribution, and regulation, several transgenic approaches have been developed using a range of reporter genes under the control of the mouse, rat, or ovine GnRHR gene (Gnrhr) promoters. Comprehensive reviews of the literature, together with recent results obtained in our laboratory, illustrate how these transgenic models highlight the endocrine as well as the neural facet of GnRHR function. In this review, the endocrine aspect will be discussed with regard to the pituitary and gonad function, whereas the neural aspect will be discussed with regard to hippocampal formation and the oculomotor pathway, the latter constituting an unpreviously described site of Gnrhr promoter activity. These approaches should help elucidate the properties of the mammalian GnRH system.
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