AUTHOR=Khrystoforova Iryna , Shochat-Carvalho Chen , Harari Ram , Henke Katrin , Woronowicz Katherine , Harris Matthew P. , Karasik David 

TITLE=Zebrafish mutants reveal unexpected role of Lrp5 in osteoclast regulation

JOURNAL=Frontiers in Endocrinology

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2022.985304

DOI=10.3389/fendo.2022.985304

ISSN=1664-2392

ABSTRACT=<p>Low-density Lipoprotein Receptor-related Protein 5 (<italic>LRP5</italic>) functions as a co-receptor for Wnt ligands, controlling expression of genes involved in osteogenesis. In humans, loss-of-function mutations in <italic>LRP5</italic> cause Osteoporosis-Pseudoglioma syndrome, a low bone mass disorder, while gain-of-function missense mutations have been observed in individuals with high bone mass. Zebrafish (<italic>Danio rerio</italic>) is a popular model for human disease research, as genetic determinants that control bone formation are generally conserved between zebrafish and mammals. We generated <italic>lrp5-</italic> knock-out zebrafish to study its role in skeletogenesis and homeostasis. Loss of <italic>lrp5</italic> in zebrafish leads to craniofacial deformities and low bone mineral density (total body and head) at adult ages. To understand the mechanism and consequences of the observed phenotypes, we performed transcriptome analysis of the cranium of adult <italic>lrp5</italic> mutants and siblings. Enrichment analysis revealed upregulation of <italic>genes</italic> significantly associated with hydrolase activity: <italic>mmp9, mmp13a, acp5a</italic>. <italic>acp5a</italic> encodes Tartrate-resistant acid phosphatase (TRAP) which is commonly used as an osteoclast marker, while Matrix metalloprotease 9, Mmp9, is known to be secreted by osteoclasts and stimulate bone resorption. These genes point to changes in osteoclast differentiation regulated by <italic>lrp5</italic>. To analyze these changes functionally, we assessed osteoclast dynamics in mutants and observed increased TRAP staining, significantly larger resorption areas, and developmental skeletal dysmorphologies in the mutant, suggesting higher resorptive activity in the absence of Lrp5 signaling. Our findings support a conserved role of Lrp5 in maintaining bone mineral density and revealed unexpected insights into the function of Lrp5 in bone homeostasis through moderation of osteoclast function.</p>