Loss of chaperone-mediated autophagy does not alter age-related bone loss in male mice.

Chaperone-mediated autophagy (CMA) is a lysosome-dependent degradation pathway that eliminates proteins that are damaged, partially unfolded, or targeted for selective proteome remodeling. CMA contributes to several cellular processes, including stress response and proteostasis. Age-associated increase in cellular stressors and decrease in CMA contribute to pathologies associated with aging in various tissues. CMA contributes to bone homeostasis in young mice. An age-associated reduction in CMA was reported in osteoblast lineage cells; however, whether declining CMA contributes to skeletal aging is unknown. Herein we show that cellular stressors stimulate CMA in UAMS-32 osteoblastic cells. Moreover, the knockdown of an essential component of the CMA pathway, LAMP2A, sensitizes osteoblasts to cell death caused by DNA damage, ER stress, and oxidative stress. As elevations in these stressors are thought to contribute to age-related bone loss, we hypothesized that declining CMA contributes to the age-associated decline in bone formation by sensitizing osteoblast lineage cells to elevated stressors. To test this, we aged male CMA-deficient mice and controls up to 24 months of age and examined age-associated changes in bone mass and architecture. We showed that lack of CMA did not alter age-associated decline in bone mineral density as measured by dual x-ray absorptiometry (DXA). Moreover, microCT analysis performed at 24 months of age showed that vertebral cancellous bone volume, cortical thickness, and porosity of CMA-deficient and control mice were similar. Taken together, these results suggest that reduction of CMA does not contribute to age-related bone loss.

CRISPR interference provides increased cell type-specificity compared to the Cre-loxP system.

Cre-mediated recombination is frequently used for cell type-specific loss of function (LOF) studies. A major limitation of this system is recombination in unwanted cell types. CRISPR interference (CRISPRi) has been used effectively for global LOF in mice. However, cell type-specific CRISPRi, independent of recombination-based systems, has not been reported. To test the feasibility of cell type-specific CRISPRi, we produced two novel knock-in mouse models that achieve gene suppression when used together: one expressing dCas9::KRAB under the control of a cell type-specific promoter and the other expressing a single guide RNA from a safe harbor locus. We then compared the phenotypes of mice in which the same gene was targeted by either CRISPRi or the Cre-loxP system, with cell specificity conferred by Dmp1 regulatory elements in both cases. We demonstrate that CRISPRi is effective for cell type-specific LOF and that it provides improved cell type-specificity compared to the Cre-loxP system.