In this study, we have presented evidence for an essential role of Lrp1b
during embryonic development. From two different Lrp1b
null alleles, no viable offspring or embryos were obtained. Although blastocyst outgrowths appeared normal, we were unable to identify viable homozygous Lrp1b
mutant embryos at or beyond E8.5, suggesting that loss of Lrp1b
causes early embryonic lethality and underscoring the importance of this gene for embryonic development. We have previously reported that mice carrying a truncated form of Lrp1b
exclusively expressing a secreted ECD, are born at normal Mendelian ratios and are phenotypically essentially normal. In this earlier study, we had used insertion of a ‘neomycin’ cassette to replace the transmembrane domain at exon 88 of Lrp1b
, resulting in the predicted truncation of the receptor and the secretion of a fully folded and functionally apparently intact ECD 
. Under physiological conditions, LRP1b
is anchored through its transmembrane domain in the cell membrane where it can undergo regulated intramembrane proteolysis (RIP) 
. The ECD of LRP1b
is cleaved by several metalloproteinases, including ADAM17 and other members of the ADAM family, in the initial step of receptor processing and leads to shedding into the extracellular space where its function has not yet been determined 
. Subsequently, γ-secretase activity mediates the release the intracellular domain from the membrane. LRP1 and other members of the LDL receptor gene family are known to bind a wide variety of ligands, including growth factors, membrane receptors, the amyloid precursor protein, bacterial toxins, and other proteins 
. Given their structural similarities, LRP1b
is also likely to bind a comparable spectrum of ligands. In fact, the amyloid precursor protein 
, Pseudomonas exotoxin A 
and some other ligands have been reported to also bind to the ECD of LRP1b. Our gene targeting study to disrupt Lrp1b
by duplicating internal exons of the gene suggests that the ECD can function independently from the membrane anchored receptor to regulate critical developmental processes required for embryonic viability. The shedding of the ECD into the extracellular space might therefore serve as a soluble ligand scavenger. This event presumably preserves a critical signaling threshold at an early stage of embryonic development.
For other members of the LDL receptor gene family, it has been demonstrated that the cleavage of the extracellular domain can occur in the native receptor 
. Interestingly, we have found a comparable rescue of a severe perinatally lethal phenotype by a truncated form of Lrp4
, where only the ECD remains expressed 
. Here, we confirmed Lrp4 ECD expression in this mutant mouse strain and present in vitro
evidence that Lrp4 undergoes regulated intramembraneous processing (RIP) by cleavage and shedding of the ECD by metalloproteases and ICD release after γ-secretase cleavage. Both steps have important physiological functions in other LDL gene family members including signal modulation and transcriptional inhibition.
Furthermore, our in vitro results suggest that Lrp4 ECD can negatively modulate Wnt signaling. Whether this happens through cooperation with inhibitory ligands or scavenging of activating ligands extracellularly remains to be determined. It also remains presently unclear whether shedding occurs in vivo and on which physiological processes this may impact. However, anchorage-independent modulation of extracellular conditions seems to play a crucial role in preserving a threshold for proper cellular signal input. No specific signaling mechanisms, which are modulated by Lrp1b are currently known. This hypothesis thus requires further confirmation once such pathways have been identified.
Deletion of Lrp4
causes perinatal death due to an inability to form neuromuscular junctions and subsequent respiratory failure 
. This phenotype is mitigated in the truncated Lrp4
receptor expressing only the ECD, allowing the animal to breathe and move, despite general muscular weakness and hypotrophy. Another prominent phenotype, involving abnormal distal limb development, appears to be identical in the null and hypomorph 
There are several reports of LRP1b
being deleted or epigenetically silenced in a variety of human tumors 
. The exact mechanistic role of LRP1b
in tumor suppression and development has remained elusive. The previously reported functional insights into tumor suppression at the molecular level overlap with its close relative LRP1
. They include the regulation of uPA, uPAR and PDGF receptor tyrosine kinase 
. However, the lack of mutations in LRP1 
indicate important functions that have diverged from those of LRP1b
. These differences could be attributed to the distinct selective pressure on the LRP1b
gene in the process of tumor development. It is thus possible that the same unknown mechanisms that are regulated by the Lrp1b ECD are involved in tumorigenesis as well as development.
While the release of the intracellular domain and its effect on inflammatory signaling and proliferation has been described for both LRP1b 
and LRP1 
, no such independent function has been described for the isolated ECDs of either receptor. Our data, obtained from two distinct mouse models suggest that the ECD of Lrp1b
can function to some extent to maintain signaling homeostasis even in the absence of membrane integration. In analogy to LRP1, this might occur through binding of soluble ligands in the extracellular space 
In summary, we have reported an essential role for Lrp1b in embryonic development and propose a novel role for Lrp1b and Lrp4 as signal modulators through ligand scavenging (). Further elucidation of the molecular functions of the LRP1b and LRP4 ECDs has the potential to provide novel and functionally significant insights into the role of LRP1b in embryogenesis and cancer.
Summary of known mutations and their respective phenotypes.