Loss of mechanosensitivity in a mouse model for gerodermia osteodysplastica due to an altered lacuno-canalicular osteocyte network

 

Introduction:

With increasing age bone tissue loses its capacity to respond to mechanical loading. But whether this is also true for premature aging disorders is unknown.

Methods:

We investigated the effect of a two week in vivo tibia loading protocol in the GorabPrx1 mouse model for the progeroid disorder gerodermia osteodysplastica. The osteocyte lacuno-canalicular network was investigated by rhodamine staining and confocal microscopy.

Results:

Compared to control animals in GorabPrx1 mutants only half the force (-5.4 N) was needed to reach 1200 micro strain, illustrating the osteoporotic phenotype. After loading control mice showed a robust 3-fold increase of the mineral apposition rate, which resulted in a greater cortical area and in a doubling of the trabecular bone volume fraction (7% vs. 14%). Unexpectedly, this anabolic effect was completely abolished in the GorabPrx1 mutants. Instead, 4D microCT reconstruction revealed an elevated and undirected bone turnover at basal level, which did not change after mechanical loading. In search for an explanation for this loss of mechanoresposiveness we focused our attention to the osteocytes, which were almost doubled in number in the GorabPrx1 mutants (1133 1/mm2 vs. 2130 1/mm2). However, mutant osteocytes displayed an abnormal morphology. A quantification of the lacuno-canalicular network revealed a reduction of the number of canaliculi per lacuna (80 vs. 40) resulting in a lower canalicular density and connectivity. After knock-down of Gorab in MLO-Y4 osteocyte-like cells we did not observe any alteration of calcium influx after mechanical stimulation.

Discussion:

Therefore, we conclude that loss of Gorab does not impair osteocyte function per se, but that the altered lacuno-canalicular network is not capable of proper strain amplification. These morphological changes are probably secondary to the impaired in glycosylation of ECM proteins observed in GorabPrx1 bone tissue, which also impact on collagen fibril formation.