Mice expressing a dominant-negative Ret mutation phenocopy human Hirschsprung disease and delineate a direct role of Ret in spermatogenesis

Abstract

spermatogenesis has been suggested following the analysis of mice that overexpress GDNF in the testes (Gianino et al., 2003;Heuckeroth et al., 1999;Meng et al., 2000;Rossi et al., 1999). Unfortunately, the perinatal lethality of Ret-null animals has made it impossible to study the postnatal roles of Ret, and to develop animal models that mimic Ret-related human diseases. This has hindered the molecular understanding of these diseases, and the subsequent development and testing of novel treatment strategies.Ret signaling is mediated by the binding of the GFLs [GDNF, neurturin (NRTN), artemin (ARTN) and persephin (PSPN)] with their cognate glycophosphatidylinositol (GPI)-anchored GFRα1-4 co-receptor, and their subsequent interaction with the Ret extracellular domain (Baloh et al., 2000). These interactions result in functional GFL-GFRα-Ret complexes and the autophosphorylation of key Ret tyrosine residues that harbor consensus sequences for adaptor proteins. Recruitment of these adaptor proteins to phosphorylated Ret tyrosine residues activates multiple signaling pathways, including the mitogen-activated protein kinase (MAPK) and the phosphoinositide 3-kinase (PI3K)-AKT pathways that regulate cell survival, proliferation, migration and axonalThe Ret receptor tyrosine kinase mediates physiological signals of glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and is essential for postnatal survival in mice. It is implicated in a number of human diseases and developmental abnormalities. Here, we describe our analyses of mice expressing a Ret mutant (RetDN) with diminished kinase activity that inhibits wildtype Ret activity, including its activation of AKT. All Ret DN/+ mice died by 1 month of age and had distal intestinal aganglionosis reminiscent of Hirschsprung disease (HSCR) in humans. The Ret DN/+ proximal small intestine also had severe hypoganglionosis and reduction in nerve fiber density, suggesting a potential mechanism for the continued gastric dysmotility in postsurgical HSCR patients. Unlike Ret-null mice, which have abnormalities in the parasympathetic and sympathetic nervous systems, the Ret DN/+ mice only had defects in the parasympathetic nervous system. A small proportion of Ret DN/+ mice had renal agenesis, and the remainder had hypoplastic kidneys and developed tubulocystic abnormalities postnatally. Postnatal analyses of the testes revealed a decreased number of germ cells, degenerating seminiferous tubules, maturation arrest and apoptosis, indicating a crucial role for Ret in early spermatogenesis.

Maintenance of spermatogenesis requires TAF4b, a gonad-specific subunit of TFIID

“…Transgenic mice expressing a dominant-negative c-Ret mutation have normal testes on the day of birth. However, by post-natal day 10, mutant c-Ret mice have fewer germ cells, a phenotype similar to that observed in Taf4b-null males (Jain et al 2004). The observed deficiency of c-Ret in Taf4b-null males coupled with the evidence that GDNF signaling is important to germ cell stem cell function directly implicates a role for TAF4b in these germ cell stem cells.…”

Male and female differential reproductive rate could explain parental transmission asymmetry of mutation origin in Hirschsprung disease

“…20 The question of a sperm-specific fitness explaining the parental transmission asymmetry, that is, a selective advantage for spermatozoids not carrying a RET CDS mutation, could also be raised, as the RET gene is known to play a crucial role in early spermatogenesis. 21 However, a selective advantage, 22 and not disadvantage, has been described for spermatogonia carrying a RET mutation.…”

The many faces of RET dysfunction in kidney

“…or through other genes encompasses majority if not all of CAKUT phenotypes. Parts of this figure are adapted from Jain et al 40,42 with permission.…”

“…Scale bar = 5 mm in top row, and 600 μm in bottom row. The figure is adapted from Jain et al 40 with permission.…”

“…However, additional studies during mesonephric and early metanephric kidney development will likely provide insights into the developmental basis for these results including defining whether UB sprouts from the right position, and problems associated with mesonephros degeneration, insertion of WD into cloaca and and activate PI3K/MAPK pathway). 40 This mouse presents with varying degrees of abnormal kidneys including bilateral hypoplasia, unilateral hypoplasia and unilateral agenesis (Fig. 3).…”