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Trisomy 21 affects virtually every organ system and results in the complex clinical presentation of Down syndrome (DS). Patterns of differences are now being recognized as patients’ age and these patterns bring about new opportunities for disease prevention and treatment. Low bone mineral density (BMD) has been reported in many studies of males and females with DS yet the specific effects of trisomy 21 on the skeleton remain poorly defined. Therefore we determined the bone phenotype and measured bone turnover markers in the murine DS model Ts65Dn. Male Ts65Dn DS mice are infertile and display a profound low bone mass phenotype that deteriorates with age. The low bone mass was correlated with significantly decreased osteoblast and osteoclast development, decreased bone biochemical markers, a diminished bone formation rate and reduced mechanical strength. The low bone mass observed in 3 month old Ts65Dn mice was significantly increased after 4 weeks of intermittent PTH treatment. These studies provide novel insight into the cause of the profound bone fragility in DS and identify PTH as a potential anabolic agent in the adult low bone mass DS population.

Microtubule organization and lysosomal secretion are both critical for the activation and function of osteoclasts, highly specialized polykaryons that are responsible for bone resorption and skeletal homeostasis. Here, we have identified a novel interaction between microtubule regulator LIS1 and Plekhm1, a lysosome-associated protein implicated in osteoclast secretion. Decreasing LIS1 expression by shRNA dramatically attenuated osteoclast formation and function, as shown by a decreased number of mature osteoclasts differentiated from bone marrow macrophages, diminished resorption pits formation, and reduced level of CTx-I, a bone resorption marker. The ablated osteoclast formation in LIS1-depleted macrophages was associated with a significant decrease in macrophage proliferation, osteoclast survival and differentiation, which were caused by reduced activation of ERK and AKT by M-CSF, prolonged RANKL-induced JNK activation and declined expression of NFAT-c1, a master transcription factor of osteoclast differentiation. Consistent with its critical role in microtubule organization and dynein function in other cell types, we found that LIS1 binds to and colocalizes with dynein in osteoclasts. Loss of LIS1 led to disorganized microtubules and aberrant dynein function. More importantly, the depletion of LIS1 in osteoclasts inhibited the secretion of Cathepsin K, a crucial lysosomal hydrolase for bone degradation, and reduced the motility of osteoclast precursors. These results indicate that LIS1 is a previously unrecognized regulator of osteoclast formation, microtubule organization, and lysosomal secretion by virtue of its ability to modulate dynein function and Plekhm1.