In this study, we have shown that Lrp4 functions as a critical regulator of kidney development in both mouse and human. In mice, complete absence of functional Lrp4 leads to uni- or bilateral kidney agenesis caused by a delay in the formation of the ureteric bud. In other mouse models, e.g. the limb deformity (ld
) mutation or Danforth's short tail
) mice 
, delayed invasion of the ureteric bud into the receptive mesenchyme results in mesenchymal apoptosis and kidney agenesis 
. The fact that normal kidneys do develop in a subset of Lrp4 null embryos suggests that the signaling capacity of the bud and the receptivity of the mesenchyme is unaffected by loss of this gene. However, the range of phenotypes observed in humans, from complete agenesis to hypoplasia, along with the expression of Lrp4 mRNA in multiple cell types of the kidney throughout the embryonic period suggest this molecule may have additional roles in kidney development, or that other factors exist, which can modify the phenotype.
The precise mechanism for Lrp4 action during kidney development is still unclear. During kidney development, tissue-tissue interactions between the metanephric mesenchyme and the UB are critical and rely on the integration and regulation of several signaling pathways. Wnt signaling is crucial for UB branching and has been shown to be regulated by Lrp4 in other systems 
. Intriguingly, a mouse model with UB-specific overexpression of activated β-catenin presents with a very similar phenotype to the Lrp4 mutant (). However, analysis of the Wnt pathway activity has failed to reveal significant changes in Lrp4 mutants, possibly due to high baseline activity in wildtype animals.
An alternative yet equally plausible scenario is that Lrp4 is involved in the modulation of Bmp signaling. We have found that, like other members of the LDL receptor gene family, Lrp4 is capable of modulating TGF-β related signaling 
. In this study, we have confirmed novel binding partners for Lrp4 including the Bmp regulating ligand Gremlin1 (). As Gremlin1 knockout mice display a phenotype of bilateral kidney agenesis (reportedly due to ectopic Bmp4 activity) 
, an attractive model is that Lrp4 cooperates with Gremlin to inhibit Bmp4 activity. However, similar to the case with β-catenin signaling, we were unable to detect significant changes in the expression of the Bmp targets, pSmad1, 4 and 8. It is therefore possible that Lrp4 acts through an unrelated pathway or perhaps through only partial modulation and integration of both Bmp and Wnt signaling.
Normal kidney formation occurs in a hypomorphic Lrp4 mutant, where only a secreted extracellular domain is expressed, adding additional insight into the mechanism of Lrp4 during ureteric budding 
. These findings suggest that whatever factor Lrp4 is normally interacting with in the kidney, it is occurring extracellularly and most likely does not require endocytosis of the receptor. Possible mechanisms include quenching of Wnt and BMP modulators, such as Gremlin1 () by the secreted extracellular domain.
In summary, we have identified Lrp4 as a critical factor for UB outgrowth and kidney formation in the mouse. We have also shown that mutations in Lrp4 lead to the same or very similar developmental malformations as seen in human LRP4 deficient patients with Cenani-Lenz syndrome, further underscoring the importance of Lrp4 for human genetics and medicine.