Overexpression of Smad2 Reveals Its Concerted Action with Smad4 in Regulating TGF- β-Mediated Epidermal Homeostasis

Abstract

Members of the transforming growth factor-beta (TGF-beta) superfamily are critical regulators for epithelial growth and can alter the differentiation of keratinocytes. Transduction of TGF-beta signaling depends on the phosphorylation and activation of Smad proteins by heteromeric complexes of ligand-specific type I and II receptors. To understand the function of TGF-beta and activin-specific Smad, we generated transgenic mice that overexpress Smad2 in epidermis under the control of keratin 14 promoter. Overexpression of Smad2 increases endogenous Smad4 and TGF-beta 1 expression while heterozygous loss of Smad2 reduces their expression levels, suggesting a concerted action of Smad2 and -4 in regulating TGF-beta signaling during skin development. These transgenic mice have delayed hair growth, underdeveloped ears, and shorter tails. In their skin, there is severe thickening of the epidermis with disorganized epidermal architecture, indistinguishable basement membrane, and dermal fibrosis. These abnormal phenotypes are due to increased proliferation of the basal epidermal cells and abnormalities in the program of keratinocyte differentiation. The ectodermally derived enamel structure is also abnormal. Collectively, our study presents the first in vivo evidence that, by providing an auto-feedback in TGF-beta signaling, Smad2 plays a pivotal role in regulating TGF-beta-mediated epidermal homeostasis.

Inhibition of SMAD2 expression prevents murine palatal fusion

“…It is evident that the developmental events are affected by the amount of endogenous SMAD2 (Nomura and Li, 1998;Ito et al, 2001). In Smad2 heterozygous mice with half of the normal amount of Smad2, 20% of embryos and newborn pups showed various abnormalities including craniofacial defects (Nomura and Li, 1998).…”

“…In Smad2 heterozygous mice with half of the normal amount of Smad2, 20% of embryos and newborn pups showed various abnormalities including craniofacial defects (Nomura and Li, 1998). In K14-driven Smad2 transgenic mice, lower levels of Smad2 transgene expression were associated with a less severe phenotype, and a higher level of Smad2 transgene expression correlated with severe changes in skin and ear (Ito et al, 2001). The extent of the improvement of palatal fusion in the TGF-beta3…”

“…2B) caused the failure of palatal fusion in organ culture. It is likely that there is a threshold with regard to the level of SMAD2 necessary to transduce TGF-beta signals in the cell to accomplish different developmental events (Ito et al, 2001). In palatal tissues, the amount of SMAD2 required to complete the disappearance of the MEE from the midline is greater than needed to alter the expression of gene associated with basement membrane degradation such as MMP-13.…”

Characteristic distribution of immunoreaction for estrogen receptor alpha in rat ameloblasts

“…TGF-␤1 controls the proliferation of enamel organ epithelium and regulates ameloblast differentiation during tooth morphogenesis. The signaling responses to TGF-␤ are mediated by their intercellular substrates, the Smad proteins (Ito et al, 2001). In Smad3 knockout mouse, the enamel has been found to be hypomineralized (Yokozeki et al, 2003).…”

Receptor‐regulated and inhibitory Smads are critical in regulating transforming growth factorβ–mediated Meckel's cartilage development

“…In mandibular explants with Smad2 expression level below the threshold of being able to successfully transmit TGF-␤ signaling, the development of Meckel's cartilage is retarded. Haploinsufficiency of Smad2 has been shown to result in craniofacial malformations, affects the inhibitory effect of TGF-␤ on proliferation of enamel organ epithelial cells, and inhibits mandibular molar tooth development (Nomura and Li, 1998;Ito et al, 2001;Ferguson et al, 2001). Taken together, our study provides important evidence that a malformed mandibular primordium-the Meckel's cartilage, as the result of insufficient level of Smad2 expression, may be Fig.…”