TRANSPORT PHYSIOLOGY Altered expression of tight junction proteins in mammary epithelium after discontinued suckling in mice Alexander G. Markov & Natalia M. Kruglova & Yulia A. Fomina & Michael Fromm & Salah Amasheh Received: 13 June 2011 /Revised: 9 September 2011 /Accepted: 19 September 2011 # Springer-Verlag 2011 Abstract Milk production is modulated by the paracellular barrier function of tight junction (TJ) proteins located in the mammary epithelium. The aim of our study was the molecular analysis of TJs in native lactating murine mammary gland epithelium as this process may strongly challenge epithelial barrier properties and regulation. Mammary gland tissue specimens from lactating control mice and animals after a 20-h interruption of suckling were prepared; histological analyses were performed by light and electron microscopy; and expression of TJ proteins was detected by PCR, Western blotting, immunofluorescent staining, and confocal laser scanning microscopy. Discontinuation of suckling resulted in a substantial accumulation of milk in mammary glands, an increase of alveolar size, and a flattening of epithelial cells without effects on inflammatory indicators. In control tissues, PCR and Western blots showed signals for occludin, and claudin-1, -2, -3, -4, -5, -7, -8, -15, and -16. After a 20-h accumulation of milk, expression of two sealing TJ proteins, claudin-1 and -3, was markedly increased, whereas two TJ proteins involved in cation transport, claudin-2 and -16, were reduced. Real-time PCR validated increased transcripts of claudin-1 and claudin-3. During extension of mammary glands in the process of lactation, claudin-1 and -3 are markedly induced and claudin-2 and -16 are decreased. Volume and composition of milk might be strongly dependent on this counter-regulation of sealing claudins with permeability-mediating claudins, indicating a physiological process of a tightening of TJs against a back- leak of solutes and ions from the alveolar lumen. Keywords Mammary gland . Lactation . Claudins . Occluding Abbreviations TJ Tight junction Introduction Mammary gland alveolar epithelium consists of cuboidal cells which are responsible for milk synthesis and secretion. Prerequisite for vectorial transport through epithelia is a paracellular barrier function which is provided by the tight junction (TJ) [6, 41]. TJs have been identified in the apicolateral region of alveolar cuboidal cells and are involved in the selectivity and regulation of the passage of solutes through the paracellular pathway, maintaining secretion formation in alveoli [32]. TJs are organized in strands, and within these strands, several tetraspan TJ proteins have been reported to contribute to barrier function, namely occludin [16], tricellulin [22, 26], marvel D3 [38], and the family of claudins [14]. Among these TJ proteins, the claudin family has been demonstrated to primarily determine barrier properties but, in contrast, also selective paracellular permeability to ions, larger solutes, and water in a wide A. G. Markov : N. M. Kruglova : Y. A. Fomina Biological and Soil Faculty, St. Petersburg University, Universitetskaya nab. 7/9, 199034 St. Petersburg, Russia M. Fromm : S. Amasheh (*) Institute of Clinical Physiology, Charité Campus Benjamin Franklin, 12200 Berlin, Germany e-mail: salah.amasheh@charite.de Pflugers Arch - Eur J Physiol DOI 10.1007/s00424-011-1034-2