VegT activation of Sox17 at the midblastula transition alters the response to nodal signals in the vegetal endoderm domain.

In Xenopus, the prospective endoderm and mesoderm are localized to discrete, adjacent domains at the beginning of gastrulation, and this is made evident by the expression of Sox17 in vegetal blastomeres and Brachyury (Xbra) in marginal blastomeres. Here, we examine the regulation of Sox17alpha expression and the role of Sox17alpha in establishing the vegetal endodermal gene expression domain. Injection of specific inhibitors of VegT or Nodal resulted in a loss of Sox17alpha expression in the gastrula. However, the onset of Sox17alpha expression at the midblastula transition was dependent on VegT, but not on Nodal function, indicating that Sox17alpha expression is initiated by VegT and then maintained by Nodal signals. Consistent with these results, VegT, but not Xenopus Nodal-related-1 (Xnr1), can activate Sox17alpha expression at the midblastula stage in animal explants. In addition, VegT activates Sox17alpha in the presence of cycloheximide or a Nodal antagonist, suggesting that Sox17alpha is an immediate-early target of VegT in vegetal blastomeres. Given that Nodal signals are necessary and sufficient for both mesodermal and endodermal gene expression, we propose that VegT activation of Sox17alpha at the midblastula transition prevents mesodermal gene expression in response to Nodal signals, thus establishing the vegetal endodermal gene expression domain. Supporting this idea, Sox17alpha misexpression in the marginal zone inhibits the expression of multiple mesodermal genes. Furthermore, in animal explants, Sox17alpha prevents the induction of Xbra and MyoD, but not Sox17beta or Mixer, in response to Xnr1. Therefore, VegT activation of Sox17alpha plays an important role in establishing a region of endoderm-specific gene expression in vegetal blastomeres.

Vegetal localization of maternal mRNAs is disrupted by VegT depletion.

VegT is an essential maternal regulator of germ layer specification in Xenopus. The localization of VegT mRNA to the vegetal cortex of the oocyte during oogenesis ensures its inheritance by vegetal and not animal cells, and directs the differentiation of vegetal cells into endoderm. Similarly localized mRNAs, Vg1 and Bicaudal-C, are also inherited by vegetal cells, while germ plasm-associated mRNAs, such as Xcat2, become incorporated into vegetally derived primordial germ cells. Although mRNA localization is clearly important for tissue specification, the mechanism of mRNA anchoring to the oocyte vegetal cortex is not understood. Here, we examine the role of VegT in cortical localization. We report that depletion of VegT mRNA caused the release of Vg1 mRNA from the vegetal cortex and a reduction of Vg1 protein, without affecting the total amount of Vg1 transcript. Furthermore, we found that Bicaudal-C and Wnt11 mRNAs were also dispersed, but not degraded, by VegT depletion, while the localization of Xcat2 and Xotx1 mRNAs was unaffected. This effect was specific to the loss of VegT mRNA and not VegT protein, since a morpholino oligo against VegT, that blocked translation without degrading mRNA, did not disperse the vegetally localized mRNAs. Therefore, a subset of localized mRNAs is dependent on VegT mRNA for anchoring to the vegetal cortex, indicating a novel function for maternal VegT mRNA.

Mesendoderm induction and reversal of left-right pattern by mouse Gdf1, a Vg1-related gene.

TGFbeta signals play important roles in establishing the body axes and germ layers in the vertebrate embryo. Vg1 is a TGFbeta-related gene that, due to its maternal expression and vegetal localization in Xenopus, has received close examination as a potential regulator of development in Xenopus, zebrafish, and chick. However, a mammalian Vg1 ortholog has not been identified. To isolate mammalian Vg1 we screened a mouse expression library with a Vg1-specific monoclonal antibody and identified a single cross-reactive clone encoding mouse Gdf1. Gdf1 is expressed uniformly throughout the embryonic region at 5.5-6.5 days postcoitum and later in the node, midbrain, spinal cord, paraxial mesoderm, lateral plate mesoderm, and limb bud. When expressed in Xenopus embryos, native GDF1 is not processed, similar to Vg1. In contrast, a chimeric protein containing the prodomain of Xenopus BMP2 fused to the GDF1 mature domain is efficiently processed and signals via Smad2 to induce mesendoderm and axial duplication. Finally, right-sided expression of chimeric GDF1, but not native GDF1, reverses laterality and results in right-sided Xnr1 expression and reversal of intestinal and heart looping. Therefore, GDF1 can regulate left-right patterning, consistent with the Gdf1 loss-of-function analysis in the mouse (C. T. Rankin, T. Bunton, A. M. Lawler, and S. J. Lee, 2000, Nature Genet. 24, 262-265) and a proposed role for Vg1 in Xenopus. Our results establish that Gdf1 is posttranslationally regulated, that mature GDF1 activates a Smad2-dependent signaling pathway, and that mature GDF1 is sufficient to reverse the left-right axis. Moreover, these findings demonstrate that GDF1 and Vg1 are equivalent in biochemical and functional assays, suggesting that Gdf1 provides a Vg1-like function in the mammalian embryo.

The rehabilitative management of acute severe combined demyelination: a case report.

Demyelination involving both the peripheral and central nervous system, seen with Acute Severe Combined Demyelination, results in a combination of the deficits from Acute Inflammatory Demyelinating Polyneuropathy and Acute Disseminated Encephalomyelitis. The unique medical, physical, psychosocial and functional challenges this disorder presents to the rehabilitation team have not been described. A-70-year old individual with a 3-week course of ascending paralysis, dysesthesias, urinary retention, insomnia and cognitive deficits is presented. An individualized program of rehabilitation entailing principles of spinal cord injury, brain injury and geriatric rehabilitation was implemented. Significant improvements in upper extremity weakness, pain, bladder control, sleep disturbance and cognition were achieved, however, lower extremity weakness and truncal control deficits remained after a 10-week rehabilitation program. This case report describes the rehabilitative management of an adult with Acute Severe Combined Demyelination, highlighting the unique aspects of intervention.

A review of prescriptive authority for nurse practitioners.

Nurse practitioners in many states continue to struggle to gain prescriptive authority. Independent prescriptive authority holds in a small number of states. In states that grant prescriptive privileges, nurse practitioners battle strict regulations and mandatory supervision by a physician. The article reviews prescriptive authority for practitioners, interpretation of legislation, barriers to prescribing practice, and current trends.

Scanning electron microscopic study of microaneurysms in the diabetic retina.

We describe the clinical and pathologic appearance of the retinal microcirculation in a patient with a 22-year history of diabetes mellitus. A vascular cast of one eye was prepared and studied with scanning electron microscopy. The three-dimensional views obtained give insight into the pathogenesis of microaneurysms.