Disease relevance of Mad

• The transactivating activity resides in the conserved carboxy-terminal domain of Smad1 and is disrupted by a nonsense mutation that corresponds to null mutations found in Mad and in the related gene DPC4, a candidate tumour-suppressor gene in human pancreatic cancer [1].

High impact information on Mad

• Dpp signals via the SMAD proteins Mad and Medea [2].
• Dad shares weak homology with Drosophila Mad (Mothers against dpp), a protein required for transduction of Dpp signals [3].
• Expression of Dad together with either Mad or the activated receptor rescues phenotypic defects induced by each protein alone [3].
• Drosophila Mad binds to DNA and directly mediates activation of vestigial by Decapentaplegic [4].
• The subsequent refinement of this pattern to the dorsal-most cells, however, correlates with high levels of p-Mad that accumulate in the same region during late blastoderm [5].

Biological context of Mad

• When the inhibitory activity of Sog is removed, Mad phosphorylation is expanded [6].
• However, Mad is essential in the germline for oogenesis whereas Medea is dispensable [7].
• This recognition of overlapping binding sites provides a potential explanation for why the G/C-rich Mad binding site consensus differs the Smad3/Smad4 binding site consensus [8].
• Based on these results, we propose that Mad is a highly conserved and essential element of the DPP signal transduction pathway [9].
• The zinc-finger transcription factor Schnurri (Shn) has been implicated as a co-factor for Mad, based on its DNA-binding ability and evidence of signaling dependent interactions between the two proteins [10].

Anatomical context of Mad

• Assay of Mad's role in the DPP-dependent events of embryonic midgut development demonstrates that Mad is required for any response of the visceral mesoderm or endoderm to DPP signals from the visceral mesoderm [9].
• Furthermore, Mad phosphorylation occurs in regions facing the presynaptic active zones of neurotransmitter release within the postsynaptic subsynaptic reticulum (SSR) [11].
• Signals of Dpp are transmitted from the cell membrane to the nucleus by Medea and Mad, both belonging to the Smad protein family [12].
• Furthermore, the spider Dpp signal appeared to induce graded levels of the phosphorylated Mothers against dpp (Mad) protein in the nuclei of germ disc epithelial cells [13].
• In Mad mutant embryos or in embryos expressing dominant negative constructs of the two type I Dpp receptors in the trachea the number of cells expressing fusion cell-specific marker genes is reduced and fusion of the dorsal branches is defective [14].

Associations of Mad with chemical compounds

• Binding was also affected by alanine substitutions in Mad and Med at a subset of basic residues within and flanking helix 2, indicating a contribution to binding of the GRCGNC and GTCT sites [15].
• Recent studies revealed that Mothers against dpp (Mad) in Drosophila and its homologs play important roles in the intracellular signal transduction of the serine/threonine kinase receptors [16].

Physical interactions of Mad

• Finally, we show that Med and Mad directly bind to the bam silencer in vitro [17].
• In this study we show that Shn acts as a DNA-binding Mad cofactor in the nuclear response to Dpp [18].

Regulatory relationships of Mad

• We show that a weak, homozygous-viable sog mutant is enhanced to lethality by reduction in the activities of the Smad family members Mad or Medea, and that the lethality is caused by defects in the molecular specification and subsequent cellular differentiation of the dorsal-most cell type, the amnioserosa [19].
• Experiments utilizing Mad transgenes regulated by tissue-specific promoters show that MAD is required specifically in cells responding to DPP [9].
• Brinker also competes with activated Mad in vivo, blocking the stimulation of the Ubx enhancer in response to simultaneous Dpp signalling [20].

Other interactions of Mad

• Overexpression of eIF4A decreases Dpp signalling and causes loss of Mad and phospho-Mad [21].
• Furthermore, Brk was able to compete with Mad for occupancy of this binding site [8].
• Tkv induces homo-oligomerization of Mad, and Dad inhibits this step [22].
• Our data are consistent with a model in which Shn acts as a cofactor for Mad [23].
• From this stage onward, activation by Scw is no longer required, and Dpp suffices to induce high levels of pMad [6].

Analytical, diagnostic and therapeutic context of Mad

• Xenopus cDNAs homologous to the Drosophila Mad gene and C. elegans CEM genes have been cloned and functionally analyzed by microinjection into frog embryos [24].

References