Macrophages escape Klotho gene silencing in the mdx mouse model of Duchenne muscular dystrophy and promote muscle growth and increase satellite cell numbers through a Klotho-mediated pathway

Abstract

Duchenne muscular dystrophy (DMD) is a muscle wasting disease in which inflammation influences the severity of pathology. We found that the onset of muscle inflammation in the mdx mouse model of DMD coincides with large increases in expression of pro-inflammatory cytokines [tumor necrosis factor-α (TNFα); interferon gamma (IFNγ)] and dramatic reductions of the pro-myogenic protein Klotho in muscle cells and large increases of Klotho in pro-regenerative, CD206+ macrophages. Furthermore, TNFα and IFNγ treatments reduced Klotho in muscle cells and increased Klotho in macrophages. Because CD206+/Klotho+ macrophages were concentrated at sites of muscle regeneration, we tested whether macrophage-derived Klotho promotes myogenesis. Klotho transgenic macrophages had a pro-proliferative influence on muscle cells that was ablated by neutralizing antibodies to Klotho and conditioned media from Klotho mutant macrophages did not increase muscle cell proliferation in vitro. In addition, transplantation of bone marrow cells from Klotho transgenic mice into mdx recipients increased numbers of myogenic cells and increased the size of muscle fibers. Klotho also acted directly on macrophages, stimulating their secretion of TNFα. Because TNFα is a muscle mitogen, we tested whether the pro-proliferative effects of Klotho on muscle cells were mediated by TNFα and found that increased proliferation caused by Klotho was reduced by anti-TNFα. Collectively, these data show that pro-inflammatory cytokines contribute to silencing of Klotho in dystrophic muscle, but increase Klotho expression by macrophages. Our findings also show that macrophage-derived Klotho can promote muscle regeneration by expanding populations of muscle stem cells and increasing muscle fiber growth in dystrophic muscle.

Figure 1.

Proinflammatory cytokines reduce Klotho expression in muscle in vivo. (A) QPCR analysis of Klotho expression in quadriceps muscles over the course of mdx pathology. Values normalized to 1-month-old wild-type mice. N = 5 per data set. * indicates significant difference from 1-month-old wild-type muscles at P < 0.05. # indicates significant difference from age-matched, wild-type muscles at P < 0.05. n = 5 per data set. (B) Western blot of Klotho in mdx muscle homogenates collected over the course of pathology. Relative Klotho levels are shown for full-length, transmembrane (120 kDa) and cytosolic/secreted (60 kDa) Klotho. “Loading” shows membrane used for western blotting that was stained with Ponceau Red to show relative loading of each lane. (C) QPCR analysis of TNFα expression in quadriceps muscles over the course of mdx pathology. Values normalized to 1-month-old wild-type mice. N = 5 per data set. * indicates significant difference from 1-month-old wild-type muscles at P < 0.05. # indicates significant difference from age matched, wild-type muscles at P < 0.05. (D) QPCR analysis of IFNγ expression in quadriceps muscles over the course of mdx pathology. Values normalized to 1-month-old wild-type mice. N = 5 per data set. * indicates significant difference from 1-month-old wild-type muscles at P < 0.05. # indicates significant difference from age matched, wild-type muscles at P < 0.05. (E) QPCR analysis of Klotho expression in quadriceps muscle of 5-month-old wild-type or TNFα–null mice. Values normalized to wild-type control mice. N = 5 per data set. * indicates significant difference from age-matched, wild-type muscles at P < 0.05. (F) QPCR analysis of Klotho expression in myotubes stimulated with TNFα and IFNγ in vitro. Values normalized to untreated control myotubes. * indicates significant difference from untreated control myotubes at P < 0.05. (G) Western blot of myotube extracts for Klotho. “Load” shows membrane used for western blotting that was stained with Ponceau Red to show relative loading of each lane. “Klotho” shows 60 kDa cytosolic/secreted Klotho in western blot of membrane. (H) Western blot of myotube samples treated with proinflammatory cytokines. Myotubes were treated with IFNγ or TNFα or IFNγ and TNFα together or with vehicle-only controls (Cont.). The image labeled “Loading” shows membrane used for western blotting that was stained with Ponceau Red, to show relative loading of each lane. The anti-Klotho reactive band labeled “60 kDa” shows cytosolic/secreted Klotho in western blot of the membrane.

Figure 2.

Macrophages in dystrophic muscle in vivo and in vitro express Klotho. (A) A cross-section of 4-week-old mdx muscle labeled with antibodies to Klotho shows mononucleated cells in an inflammatory lesion express KL (red) although no Klotho is detectible in neighboring, healthy muscle fibers. (B) A cross-section of 4-week-old mdx muscle labeled with antibodies to Klotho (red) and CD206 (green). No Klotho is detectible in muscle fibers. Most labeled cells in the inflammatory lesion (between brackets) appear yellow or orange, indicating that they express both Klotho and CD206. CD206+ macrophages (green) that are within the muscle in the perimysium (between arrows), but not within the inflammatory lesion do not express Klotho. Nuclei appear blue (DAPI). Bars = 40 µm. (C) Flow cytometry of BMDMs labeled with anti-F4/80 after growth in M-CSF supplemented media show that over 93% of the cells differentiated into mature monocytes/macrophages. (D) Western blot of homogenates of BMDMs with anti-Klotho or with anti- Klotho that had been preabsorbed with full length Klotho shows that the 60 kDa protein in macrophages is Klotho. (E) Western blot of homogenates of BMDMs that were untreated, or treated with Klotho siRNA or scrambled RNA controls show that the 60 kDa protein in macrophage extracts is Klotho. (F–H) BMDMs treated with proinflammatory cytokines TNFα and IFNγ are activated to express elevated levels of TNFα (F), iNOS (G) and a strong trend for increased expression of IFNγ (H). * indicates significant difference from unstimulated control. (I,J) BMDMs treated with anti-inflammatory cytokines IL10 and IL4 are activated to express elevated levels of CD206 (I) and CD163 (J). * indicates significant difference from unstimulated control. (K) Stimulation of BMDMs with either proinflammatory (Th1; TNFα and IFNγ) or anti-inflammatory (Th2; IL10 and IL4) cytokines increases Klotho expression. * indicates significant difference from unstimulated control at P < 0.05.

Figure 3.

Klotho increases the expression of pro-inflammatory and anti-inflammatory cytokines. (A–D) Conditioned media from BMDMs stimulated in vitro with Klotho were assayed by ELISA for levels of TNFα (A), IFNγ (B), IL10 (C) and IL4 (D). * indicates significant difference from cytokine concentration in control cultures at P < 0.05. (E–G) Expression levels of TNFα (E), IFNγ (F) and IL10 (G) mRNA in 6-month-old quadriceps muscles from wild-type, mdx and KL+/mdx mice. N = 8 per control data set. N = 6 per Klotho treated data set. * indicates significant difference from expression in wild-type control muscles at P < 0.05. # indicates significant difference from expression in mdx muscles at P < 0.05.

Figure 4.

Klotho and CD206 expressing macrophages are co-regulated in vivo. (A) QPCR assay of CD206 expression levels in 6-month-old quadriceps muscle from mdx and KL+/mdx mice. N = 5 per data set. * indicates significant difference from expression in control muscles at P < 0.05. (B) Numbers of CD68+ macrophages and CD206+ macrophages in quadriceps muscles from 6-month-old mdx and KL+/mdx mice. N = 5 per data set. * indicates significant difference from expression in mdx muscles at P < 0.05. (C) Numbers of CD206+ macrophages in quadriceps muscles from 6-month-old mdx and IL10 null mutant mdx mice. N = 5 per data set. * indicates significant difference from expression in mdx muscles at P < 0.05. (D) QPCR assay of Klotho expression levels in 6-month-old quadriceps muscle from mdx and IL10 null mutant mdx mice. N = 5 per data set. * indicates significant difference from expression in mdx muscles at P < 0.05.

Figure 5.

Transplantation of BMCs from Klotho transgenic mice increases Klotho delivery to mdx muscles and increases satellite cell numbers, muscle fiber size and CD206+ M2 macrophages. (A) QPCR analysis of Klotho expression in tibialis anterior muscles of mdx mice that received BMT from wild-type mice or from Klotho transgenic mice show BMT transplantation from transgenic mice causes a 32-fold greater level of Klotho expression in the muscle. For all panels in Figure 5, white bars are data from mdx mice receiving wild-type BMT. Black bars are data from mdx mice receiving Klotho transgenic BMT. (B,C) BMT of Klotho transgenic BMCs caused significantly higher numbers of satellite cells per muscle fiber (B) and per unit volume of muscle (C), compared with mdx mice receiving wild-type BMT. (D,E) BMT of Klotho transgenic BMCs caused significantly larger muscle fiber cross-sectional areas, compared with mdx mice receiving wild-type BMT. Data are shown as frequency distribution of fiber cross-sectional areas for tibialis anterior muscles from mdx mice receiving wild-type BMT (white bars) or Klotho transgenic BMT (black bars) (D) and shown as mean value of cross-sectional areas of the same groups (E). (F) Transplantation of Klotho transgenic BMCs caused significantly more CD206+ M2 macrophages to occur in mdx muscles at 6 months BMT, compared with mice receiving wild-type BMT. N = 5 per data set. * indicates significant difference from expression in mdx muscles at P < 0.05.

Figure 6.

Klotho increases satellite cell proliferation through a TNFα-mediated mechanism. (A) Myoblast cell numbers per well after growth in culture media only (Control; white bar) or conditioned media from wild-type BMDMs, Klotho transgenic BMDMs (KL Tg) or Klotho hypomorphic mutants (KL mutants). For each sample group grown with conditioned media, 5 wells contained conditioned media only (grey bars), 5 wells contained conditioned media to which isotype control IgG had been added (black bars) and 5 wells contained conditioned media with anti-Klotho (striped bars). * indicates significant difference from control cultures at P < 0.05. # indicates significant difference from isotype control cultures from same conditioned media culture group at P < 0.05. (B) Myoblast cell numbers per well after growth in culture media only or culture media containing recombinant Klotho (KL treated). For each of those treatment groups, 5 wells contained no antibody (white bars), 5 wells contained isotype control antibody (grey bars) and 5 wells contained anti-TNFα (black bars). * indicates significant difference from control cultures at P < 0.05. # indicates significant difference from isotype control cultures at P < 0.05. (C) Protein concentration in isovolumetric homogenates of myotubes grown and stimulated under same conditions as myoblasts in Figure 6C. * indicates significant difference from control cultures at P < 0.05.

Figure 7.

Klotho is expressed by M2 macrophages at sites of muscle regeneration in adult mdx muscles but not senescent mdx muscle. (A) Section of 4-week-old mdx quadriceps muscle labeled with antibodies to CD206 (green) and Klotho (red). Nuclei are labeled with DAPI (blue). Double-positive KL+/CD206+ macrophages are yellow or orange. (B) Section of 3-month-old mdx quadriceps muscle with antibodies to CD206 (green) and Klotho (red). Macrophages in the epimysium at the surface of the muscle (arrows) are KL-/CD206+. Most macrophages in inflammatory site in muscle (center of figure) are KL+/CD206+ (yellow or orange). (C) Section from muscle shown in Figure 6B that was taken adjacent to the section in Figure 6B. Section was stained with antibodies to Klotho (red) or developmental myosin heavy chain (green) to indicate sites of muscle regeneration. Cells in the epimysium at the surface of the muscle (arrows) do not express Klotho. (D) Section of a 3-month-old quadriceps muscle stained with antibodies to Klotho (red) or developmental myosin heavy chain (green) showed the high concentration of M2 macrophages expressing elevated levels of Klotho at sites of muscle regeneration. Central nuclei of large, regenerative muscle fibers are indicated with blue arrows. Macrophages at surface of regenerative fibers express low levels of Klotho (red arrows) (E) Section of 24-month-old mdx quadriceps stained with antibodies to CD206 (green) and Klotho (red) showing that CD206+ macrophages distributed throughout the thickened endomysial tissue (arrows) do not express Klotho. (F) Section of 24-month-old mdx quadriceps stained with antibodies to CD206 (green) and Klotho (red) showing an inflammatory lesion (arrows) occupied by elevated numbers of KL-/CD206+ macrophages. Bars = 50 µm.

Figure 8.

Schematic representation of the regulatory interactions between inflammation and dystrophic muscle cells that are influenced by Klotho. Direct action of pro-inflammatory cytokines on muscle cells reduces Klotho expression (present investigation) which contributes to loss of satellite cells in aging, dystrophic muscle.