Therapeutic potential of FGF19 in combatting osteosarcopenia: effects on muscle strength and bone health in aged male mice

Abstract

Osteosarcopenia, characterized by the coexistence of osteopenia/osteoporosis and sarcopenia, represents a significant health concern in geriatrics, with an increased risk of falls and fractures. The enterokine fibroblast growth factor 19 (FGF19) was recently shown to prevent muscle weakness in preclinical models. This study investigated the therapeutic potential of FGF19 in mitigating bone and muscle deterioration in aged male mice. Twenty-one-month-old C57BL/6 male mice received daily injections of human recombinant FGF19 (0.1 mg/kg) for 21 days. Histological and functional analyses revealed a shift toward larger muscle fibers in FGF19-treated mice as well as an increased muscle strength, without affecting muscle mass. In parallel, X-ray microtomography showed that FGF19 had no overt negative impact on bone, with a range of modest, site-specific, and opposing effects. In the distal femur metaphysis FGF19, it reduced cortical thickness, but significantly increased bone cross-sectional area, with an overall increased polar moment of inertia, a geometrical parameter linked to favorable mechanical properties. It also elevated cortical bone porosity in the same region. There were no significant effects on trabecular bone or cortical bone parameters in the proximal femur side at the lesser trochanter level nor at the femoral midshaft or in the tibia. In the L2 vertebra, cortical porosity decreased. Histomorphometry of trabecular bone and analysis of transcriptional output of selected genes in femurs revealed only minor changes in bone cellular activities and gene expression after three weeks of treatment. In conclusion, FGF19 treatment increased muscle strength in aged male mice, without negatively impacting aging bone.

Keywords: FGF19; aging; bone microarchitecture; muscle strength; osteosarcopenia.

Graphical Abstract

Figure 1

FGF19 treatment enhances muscle strength and liver weight. (A) Panel of figures showing, from left to right, the initial body weight of the mice (in g), the body weight after 21 days of treatment with FGF19 or vehicle solution (in g), and the food intake measured throughout the treatment period (in g/mouse/d). (B) Tissue weights collected at sacrifice, with liver weight shown on the left (in g) and combined gastrocnemius and tibialis anterior (TA) muscle weight on the right (in g). Inset shows tibialis and gastrocnemius weights, separately. (C) Length of the femurs and tibias collected at sacrifice (in mm). (D) Parameters representing muscle strength during the grip strength test, with normalized muscle strength (relative to body weight, in g/g) on the left, and maximum strength on the right (in g). Data are presented as mean ± SEM. All data were analyzed using Mann–Whitney U tests. ^* indicates p < .05, and ^*** indicates p < .001. Mice treated with the vehicle solution are represented in black, while mice treated with FGF19 (0.1 mg/kg) are represented in gray.

Figure 2

FGF19 treatment shifts muscle fiber distribution toward larger fibers. Panel of figures representing muscle fiber analysis in the soleus (A) and tibialis anterior (TA) (B) muscles of mice. From left to right: distribution frequency of muscle fibers by cross-sectional area (in μm²), mean fiber area (in μm²), percentage of fibers with an area smaller than 1100 μm², and percentage of fibers with an area greater than 1100 μm². This value corresponds to the peak of the unimodal distribution for control groups for both muscles, shown in the histograms. Therefore, it represents the most common fiber size under basal conditions in aged mice. Data are presented as mean ± SEM. All data were analyzed using Mann–Whitney U tests. ^* indicates p < .05. Mice treated with the vehicle solution are represented in black, while mice treated with FGF19 (0.1 mg/kg) are represented in gray.

Figure 3

FGF19 treatment affects bone microarchitectural parameters at the distal metaphysis of the femur. Panel of figures representing bone microarchitecture analysis of the distal femur at the level of the metaphysis: trabecular bone (A) and cortical bone (B). Parameters that describe trabecular bone are not affected by FGF19 treatment. Changes in total volume and marrow area suggest alterations in bone size at this site, accompanied by changes in cortical porosity and thickness. Abbreviations: BV/TV, bone volume/total (bone volume fraction); Tb.Th, trabecular thickness; Tb.N, trabecular number; Tb.Sp, trabecular separation; TV, total volume of trabecular bone; Ct.Ar, cortical area; Ma.Ar, marrow area; Ct.Po, cortical porosity; Ct.Th, cortical thickness; TMD, tissue mineral density. Data are presented as mean ± SEM. All data were analyzed using Mann–Whitney U tests. ^* indicates p < .05; ^** indicates p < .01. Mice treated with the vehicle solution are represented in black, while mice treated with FGF19 (0.1 mg/kg) are represented in gray.

Figure 4

Enlarged distal femur in FGF19-treated mice. (A) Heat map showing bone total cross-sectional area inside the periosteal surface distal to the growth plate in vehicle injected vs FGF19 injected aged mice. Bone cross-sectional area is represented from larger cross-sectional area (light) to narrower cross-section (dark) in μm². FGF19 injected aged mice have larger bone cross-sectional area for up to roughly 200 slices proximal to the growth plate, when compared to vehicle injected aged mice as evident from the heat map. This analysis was done using 2D slice by slice analysis of distal metaphysis below the growth plate, with each slice taken at 10.5 μm interval. Distal to the top. (B) Additional bone microarchitectural parameters at the distal metaphysis of femur: Tt.Ar (total cross-sectional area inside the periosteal surface); pMOI (polar moment of inertia); total volume (defined by the outer periostal surface). In (B) data are presented as mean ± SEM. All data were analyzed using Mann–Whitney U tests. ^** indicates p < .01; ^*** indicates p < .001. Mice treated with the vehicle solution are represented in black, while mice treated with FGF19 (0.1 mg/kg) are represented in gray.

Figure 5

No effect of FGF19 treatment on bone microarchitectural cortical parameters at the midshaft and at the proximal trochanter in the femur. Panel of figures representing bone microarchitecture analysis of cortical bone at the femur diaphysis (midshaft, A) and at a proximal site at the level of the trochanter (B). No significant difference could be recorded, indicating no effect on overall bone size, nor on cortical parameters at these two sites: Tt.Ar (total cross-sectional area inside the periosteal surface); Ct.Ar (cortical area); Ma.Ar (marrow area); Ct.Po (cortical porosity); Ct.Th (cortical thickness); TMD (tissue mineral density). Data are presented as mean ± SEM. All data were analyzed using Mann–Whitney U tests. Mice treated with the vehicle solution are represented in black, while mice treated with FGF19 (0.1 mg/kg) are represented in gray.

Figure 6

Bone histomorphometry on frontal sections of femurs shows no difference in bone forming and resorbing activities after three weeks of FGF19 treatment. (A) Goldner staining (left); quantification of osteoblast surface (Ob.S) over bone surface (BS) (right). (B) TRAP-staining (left and center); quantification of osteoclast surface (Oc.S) over BS (right). (A and B) The image at the center is a higher magnification (taken with 40X vs 10X objective) of the one on the left, corresponding to the area defined by the colored rectangle on the left. Scalebars correspond to 250 μm. Data in graphs are presented as mean ± SEM. All data were analyzed using Mann–Whitney U tests. Mice treated with the vehicle solution are represented in black, while mice treated with FGF19 (0.1 mg/kg) are represented in gray.