A Fluorescent Intravital Imaging Approach to Study Load-Induced Calcium Signaling Dynamics in Mouse Osteocytes.

Bone tissue is exquisitely sensitive to differences in mechanical load magnitude. Osteocytes, dendritic cells that form a syncytium throughout the bone, are responsible for the mechanosensory function of bone tissue. Studies employing histology, mathematical modeling, cell culture, and ex vivo bone organ cultures have greatly advanced the understanding of osteocyte mechanobiology. However, the fundamental question of how osteocytes respond to and encode mechanical information at the molecular level in vivo is not well understood. Intracellular calcium concentration fluctuations in osteocytes offer a useful target for learning more about acute bone mechanotransduction mechanisms. Here, we report a method for studying osteocyte mechanobiology in vivo, combining a mouse strain with a fluorescently genetically encoded calcium indicator expressed in osteocytes with an in vivo loading and imaging system to directly detect osteocyte calcium levels during loading. This is achieved with a three-point bending device that can deliver well-defined mechanical loads to the third metatarsal of living mice while simultaneously monitoring fluorescently indicated calcium responses of osteocytes using two-photon microscopy. This technique allows for direct in vivo observation of osteocyte calcium signaling events in response to whole bone loading and is useful in the endeavor to reveal mechanisms in osteocyte mechanobiology.

Alterations to the gut microbiome impair bone tissue strength in aged mice.

Whole bone strength and resistance to fracture are determined by a combination of bone quantity and bone quality - key factors in determining risk for osteoporosis and age-related fractures. Recent preclinical studies have shown that alterations to the gut microbiome can influence bone quantity as well as bone tissue quality. Prior work on the gut microbiome and bone has been limited to young animals, and it is unknown if the gut microbiome can alter bone tissue strength in aged animals. Here we ask if alterations to the constituents of the gut microbiome influence bone strength in older mice (12-24 months of age). Male C57BL/6J mice raised on a standard chow diet until 12 months of age were assigned to one of three diets: high glycemic, low glycemic, or low glycemic diet containing antibiotics (ampicillin and neomycin) to modify the constituents of the gut microbiome. The group fed the low glycemic diet containing antibiotics showed reductions in whole bone strength that could not be explained by geometry, indicating reduced bone tissue strength (p < 0.007). The high glycemic diet group had larger bone cross-sectional area and moment of inertia and a corresponding greater bone strength as compared to the low glycemic groups, however tissue strength did not noticeably differ from that of the low glycemic group. These findings demonstrate that modifying the gut microbiome in aged mice can alter bone tissue quality.

Components of the Gut Microbiome That Influence Bone Tissue-Level Strength.

Modifications to the constituents of the gut microbiome influence bone density and tissue-level strength, but the specific microbial components that influence tissue-level strength in bone are not known. Here, we selectively modify constituents of the gut microbiota using narrow-spectrum antibiotics to identify components of the microbiome associated with changes in bone mechanical and material properties. Male C57BL/6J mice (4 weeks) were divided into seven groups (n = 7-10/group) and had taxa within the gut microbiome removed through dosing with: (i) ampicillin; (ii) neomycin; (iii) vancomycin; (iv) metronidazole; (v) a cocktail of all four antibiotics together (with zero-calorie sweetener to ensure intake); (vi) zero-calorie sweetener only; or (vii) no additive (untreated) for 12 weeks. Individual antibiotics remove only some taxa from the gut, while the cocktail of all four removes almost all microbes. After accounting for differences in geometry, whole bone strength was reduced in animals with gut microbiome modified by neomycin (-28%, p = 0.002) and was increased in the group in which the gut microbiome was altered by sweetener alone (+39%, p < 0.001). Analysis of the fecal microbiota detected seven lower-ranked taxa differentially abundant in animals with impaired tissue-level strength and 14 differentially abundant taxa associated with increased tissue-level strength. Histological and serum markers of bone turnover and trabecular bone volume per tissue volume (BV/TV) did not differ among groups. These findings demonstrate that modifications to the taxonomic components of the gut microbiome have the potential to decrease or increase tissue-level strength of bone independent of bone quantity and without noticeable changes in bone turnover. © 2021 American Society for Bone and Mineral Research (ASBMR).

The Gut Microbiome and Bone Strength.

PURPOSE OF REVIEW: Osteoporosis is commonly diagnosed through the clinical assessment of bone quantity using bone mineral density; however, the primary clinical concern is bone fragility. Bone fragility is determined by both bone quantity and bone quality. Over the past decade, the gut microbiome has emerged as a factor that can regulate diseases throughout the body. This review discusses how microbial organisms and their genetic products that inhabit the gastrointestinal tract influence bone quantity, bone quality, and bone strength. RECENT FINDINGS: Recent studies have shown that the gut microbiome regulates bone loss during estrogen depletion and glucocorticoid treatment. A series of studies has also shown that the gut microbiome influences whole bone strength by modifying bone tissue quality. The possible links between the gut microbiome and bone tissue quality are discussed focusing on the effects of microbiome-derived vitamin K. We provide a brief introduction to the gut microbiome and how modifications to the gut microbiome may lead to changes in bone. The gut microbiome is a promising target for new therapeutic approaches that address bone quality in ways not possible with current interventions.

A gastric resident drug delivery system for prolonged gram-level dosing of tuberculosis treatment.

Multigram drug depot systems for extended drug release could transform our capacity to effectively treat patients across a myriad of diseases. For example, tuberculosis (TB) requires multimonth courses of daily multigram doses for treatment. To address the challenge of prolonged dosing for regimens requiring multigram drug dosing, we developed a gastric resident system delivered through the nasogastric route that was capable of safely encapsulating and releasing grams of antibiotics over a period of weeks. Initial preclinical safety and drug release were demonstrated in a swine model with a panel of TB antibiotics. We anticipate multiple applications in the field of infectious diseases, as well as for other indications where multigram depots could impart meaningful benefits to patients, helping maximize adherence to their medication.