The role of megalin (LRP-2/Gp330) during development.

Megalin (LRP-2/GP330), a member of the LDL receptor family, is an endocytic receptor expressed mainly in polarised epithelial cells. Identified as the pathogenic autoantigen of Heymann nephritis in rats, its functions have been studied in greatest detail in adult mammalian kidney, but there is increasing recognition of its involvement in embryonic development. The megalin homologue LRP-1 is essential for growth and development in Caenorhabditis elegans and megalin plays a role in CNS development in zebrafish. There is now also evidence for a homologue in Drosophila. However, most research concerns mammalian embryogenesis; it is widely accepted to be important during forebrain development and the developing renal proximal tubule. Megalin is also expressed in lung, eye, intestine, uterus, oviduct, and male reproductive tract. It is found in yolk sacs and the outer cells of pre-implantation mouse embryos, where interactions with cubilin result in nutrient endocytosis, and it may be important during implantation. Models for megalin interaction(s) with Sonic Hedgehog (Shh) have been proposed. The importance of Shh signalling during embryogenesis is well established; how and when megalin interacts with Shh is becoming a pertinent question in developmental biology.

FITC-induced murine pulmonary inflammation: CC10 up-regulation and concurrent Shh expression.

We describe an immunohistochemical study of the acute and chronic effects of fluorescein isothiocyanate (FITC) on Sonic hedgehog (Shh) expression and Clara cell secretory protein (CC10) up-regulation in murine lung. FITC was dissolved in PBS and instilled non-surgically into adult mouse lungs via the trachea. During the acute phase (120h) of the FITC response, CC10 staining within Clara cells increased markedly but the protein did not leak into the tissue spaces or the airways, and no fibrosis was apparent. An immune response was evident, characterised by infiltrating T and B lymphocytes. There was no concomitant expression of Shh. During the chronic phase (6 months post-instillation), significant tissue degeneration was observed in the airways. There was moderate to severe fibrosis in the lung fields that stained positively for FITC and significant inflammatory cell infiltrate was observed. Shh was expressed, and CC10 showed multiple sites of diffuse staining consistent with release from Clara cells into alveolar air spaces. PBS controls showed no fibrosis after 6 months, but there was positive Shh staining below the airway epithelia and minimal extracellular CC10 staining. The results may throw some light on the role of CC10 in pulmonary inflammation. The relationship of Shh expression and CC10 leakage to lung damage and repair is discussed.

Signalling by glial cell line-derived neurotrophic factor (GDNF) requires heparan sulphate glycosaminoglycan.

Glial cell line-derived neurotrophic factor, GDNF, is vital to the development and maintenance of neural tissues; it promotes survival of sympathetic, parasympathetic and spinal motor neurons during development, protects midbrain dopaminergic neurons from apoptosis well enough to be a promising treatment for Parkinson's disease, and controls renal and testicular development. Understanding how GDNF interacts with its target cells is therefore a priority in several fields. Here we show that GDNF requires glycosaminoglycans as well as the already-known components of its receptor complex, c-Ret and GFRalpha-1. Without glycosaminoglcyans, specifically heparan sulphate, c-Ret phosphorylation fails and GDNF cannot induce axonogenesis in neurons, in PC-12 cells, or scatter of epithelial cells. Furthermore, exogenous heparan sulphate inhibits rather than assists GDNF signalling. The involvement of heparan sulphates in GDNF signalling raises the possibility that modulation of heparan expression may modulate signalling by GDNF in vivo.

Genes and proteins in renal development.

Metanephric kidney development begins with the formation of the metanephrogenic mesenchyme; this event depends on the prior action in the intermediate mesoderm of transcription factors such as Lim-1, Pax-2, Eya-1, and Foxc-1. Once it has formed, the mesenchyme secretes GDNF; this induces the nearby wolffian duct to produce a ureteric bud which invades the metanephrogenic mesenchyme and begins to arborize. Ureteric bud development and branching depends on the transcription factor Emx-2, the GDNF-cRet and probably the HGF/cMet, signalling systems, and the intracellular regulatory molecules formin IV and timeless. Proteins of the BMP family modulate ureteric bud branching and keep bud development in step with that of other tissue types. Proteins and glycosaminoglycans of the matrix, and their receptors, and also required. The metanephrogenic mesenchyme has a default fate of apoptosis and is dissuaded from suicide by factors secreted from the bud such as TGF-alpha, TIMP-2, EGF, and FGF-2. Other factors such as LIF and TGFbeta2 cooperate with these to induce clumps of mesenchyme cells to differentiate into nephrons, while BMP-7 appears to lead them instead to form stroma. As nephrons form, they express critical transcription factors such as WT-1, Pax-2, and Hoxa11 and d11, condense, and secrete Wnt-4. Wnt-4 acts in an autocrine loop to stimulate its own synthesis and is required for cells to differentiate into epithelia; its action is antagonized by sFRP-1, secreted by stroma, but this antagonism is itself inhibited by sFRP-2 made by the developing nephron. This system probably acts both to limit the spread of Wnt-4's influence and also to commit responding cells to their epithelial fate. As nephrons mature, regions of them differentiate to perform specific physiological functions, a process that requires the proteins WT-1, Lmx-1b, Notch-2, Jagged-1, and Hnf-1.