The Sp1 family of transcription factors are often involved in the regulated expression of TATA-less genes, frequently enhancing gene transcription. In this paper, we demonstrate that an Sp1-binding element inhibits the expression of the megakaryocyte-specific ␣ IIb gene in all cell lines tested and that this inhibition is actively overcome only in megakaryocyte-like cell lines. We had noted previously in primary megakaryocytes that a 50-base pair (bp) deletion from ؊150 to ؊101 bp in the rat ␣ IIb promoter region resulted in increased expression. We now show that deletion of this region markedly increased expression in both megakaryocytic and nonmegakaryocytic cell lines, eliminating the tissue specificity of the ␣ IIb promoter. Electrophoretic mobility shift assays (EMSA) defined a single complex, which bound to a ؊145 to ؊125 bp subregion. Point mutations within this region, localized the critical point of binding around bases ؊136/؊135, and expression studies showed that introduction of the ؊136/؊135 mutation into the rat ␣ IIb promoter had a comparable result to that seen with the 50-bp deletion. EMSA studies with the homologous human ␣ IIb promoter region gave an identical migrating band. Southwestern blots of HeLa nuclear proteins with both the rat ؊145 to ؊125 DNA and its human homologue bound to a single ϳ110-kDa protein, the known molecular weight of Sp1. Confirmation that this region of the ␣ IIb gene promoter bound Sp1 was accomplished using EMSA studies with an Sp1 consensus probe, antiSp1 and -Sp3 antibodies, and recombinant Sp1 protein.Further support for the role of Sp1 in the silencing of the ␣ IIb promoter was obtained using a Gal4 binding site substitution for the silencer region of ␣ IIb and co-expression of near full-length Sp1/Gal4 fusion protein expression vectors. Ectopic reinsertion of the ؊150 to ؊101 bp region, back into the ؊150 to ؊101 bp deleted promoter, enhanced rather than decreased expression, suggesting that Sp1's inhibitory role at ؊136/؊135 depends on its local interactions. In summary, we believe that we have identified a cross-species, non-consensus Sp1-binding site that binds Sp1 and that acts as a silencer of ␣ IIb expression in many cell lines. A model is presented as to how this Sp1-binding silencer element contributes to the megakaryocyte-specific expression of ␣ IIb gene.Platelets have a central role in thrombus formation. These anuclear cytoplasmic fragments are derived from bone marrow megakaryocytes and are highly differentiated (1). One of the specialized features found on the platelet membrane is the ␣ IIb /␤ 3 (also known as glycoprotein IIb/IIIa or CD41b) integrin receptor (2). This receptor is densely packed on the platelet surface. Following platelet activation, this receptor binds fibrinogen and plays an important role in platelet aggregation (2). Normally, ␣ IIb /␤ 3 is only found on developing megakaryocytes and circulating platelets. This is due to the tissue-specific nature of the ␣ IIb subunit. While ␤ 3 is expressed in several different cell types (3), ...

Development of the vertebrate inner ear is characterized by a series of genetically programmed events involving induction of surface ectoderm, preliminary morphogenesis, specification and commitment of sensory, nonsensory and neuronal cells, as well as outgrowth and restructuring of the otocyst to form a complex labyrinth. Hmx2, a member of the Hmx homeobox gene family, is coexpressed with Hmx3 in the dorsolateral otic epithelium. Targeted disruption of Hmx2 in mice demonstrates the temporal and spatial involvement of Hmx2 in the embryonic transition of the dorsal portion (pars superior) of the otocyst to a fully developed vestibular system. In Hmx2 null embryos, a perturbation in cell fate determination in the lateral aspect of the otic epithelium results in reduced cell proliferation in epithelial cells, which includes the vestibular sensory patches and semicircular duct fusion plates, as well as in the adjacent mesenchyme. Consequently, enlargement and morphogenesis of the pars superior of the otocyst to form a complex labyrinth of cavities and ducts is blocked, as indicated by the lack of any distinguishable semicircular ducts, persistence of the primordial vestibular diverticula, significant loss in the three cristae and the macula utriculus, and a fused utriculosaccular chamber. The developmental regulators Bmp4, Dlx5 and Pax2 all play a critical role in inner ear ontogeny, and the expression of each of these genes is affected in the Hmx2 null otocyst suggesting a complex regulatory role for Hmx2 in this genetic cascade. Both Hmx2 and Hmx3 transcripts are coexpressed in the developing central nervous system including the neural tube and hypothalamus. A lack of defects in the CNS, coupled with the fact that not all of the Hmx2-positive regions in developing inner ear are impaired in the Hmx2 null mice, suggest that Hmx2 and Hmx3 have both unique and overlapping functions during embryogenesis.