Extracellular matrix molecules, including laminin, affect the development of enteric neurons and accumulate in the aganglionic colon of 1s/1s mice. Quantitative Northern analysis revealed that mRNAs encoding the β1 and γ1 subunits of laminin and collagens α1(IV) and α2(IV) are increased in the colons of 1s/1s mice. Transcripts of laminin α1 were evaluated quantitatively with reverse transcription and the competitive polymerase chain reaction (RT-cPCR). The abundance of laminin α1 transcripts was developmentally regulated, but greater in the 1s/1s than the wild-type colon at each age examined. In situ hybridization revealed that transcripts in the colon encoding laminin α1 and β1 and collagen α2(IV) were initially expressed in the endoderm, but by E15, expression shifted to cells of the colonic mesenchyme (1s/1s > wild type) where crest-derived cells migrate. The expression of laminin α1 was examined in the totally aganglionic intestine of E15 and newborn c-ret -/- mice, to determine whether an increase occurs when neurogenesis fails independently of the 1s/1s defect. RT-cPCR revealed no difference from control in mRNA encoding laminin α1 in the c-ret -/- colon in either E15 or newborn animals. The accumulation of immunohistochemically demonstrable laminin that is prominent in the newborn 1s/1s colon could not be detected in that of c-ret -/- animals. These observations suggest that transcripts encoding laminin-1 and collagen (IV) are increased in the colon and surrounding pelvic mesenchyme of 1s/1s mice because of an intrinsic lesion, rather than a secondary consequence of aganglionosis. The data are compatible with the hypothesis that the increased expression of laminin-1 contributes to the failure of crest-derived cells to complete their colonization of the 1s/1s colon.
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The authors thank Drs. M. Kurkinen and Y. Yamada for generously supplying cDNA probes encoding collagens α1(IV) and α2(IV) and the α1, β1, and g1 subunits of laminin-1, respectively. Antibodies to collagen (IV) were generously supplied by Dr. Hynda Kleinman. We also thank Ms. Edith Abreu, Ms. Wanda Setlik, and Mr. Lei Chen for excellent technical assistance. This work was supported by NIH Grants HD 21032, NS 15547, HD 28184, and CA 23767.