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Osteoblast recruitment and bone formation enhanced by cell matrix-associated heparin-binding growth-associated molecule (HB-GAM).

Imai S, Kaksonen M, Raulo E, Kinnunen T, Fages C, Meng X, Lakso M, Rauvala H - J. Cell Biol. (1998)

Bottom Line: We show here that heparin-binding growth-associated molecule (HB-GAM), an extracellular matrix-associated protein that enhances migratory responses in neurons, is prominently expressed in the cell matrices that act as target substrates for bone formation.The HB-GAM transgenic mice develop a phenotype characterized by an increased bone thickness.HB-GAM may thus play an important role in bone formation, probably by mediating recruitment and attachment of osteoblasts/osteoblast precursors to the appropriate substrates for deposition of new bone.

View Article: PubMed Central - PubMed

Affiliation: Department of Anatomy, Shiga University of Medical Science, Shiga-ken, 520-2192, Japan. simai@belle.shiga-med.ac.jp

ABSTRACT
Bone has an enormous capacity for growth, regeneration, and remodeling. This capacity is largely due to induction of osteoblasts that are recruited to the site of bone formation. The recruitment of osteoblasts has not been fully elucidated, though the immediate environment of the cells is likely to play a role via cell- matrix interactions. We show here that heparin-binding growth-associated molecule (HB-GAM), an extracellular matrix-associated protein that enhances migratory responses in neurons, is prominently expressed in the cell matrices that act as target substrates for bone formation. Intriguingly, N-syndecan, which acts as a receptor for HB-GAM, is expressed by osteoblasts/osteoblast precursors, whose ultrastructural phenotypes suggest active cell motility. The hypothesis that HB-GAM/N-syndecan interaction mediates osteoblast recruitment, as inferred from developmental studies, was tested using osteoblast-type cells that express N-syndecan abundantly. These cells migrate rapidly to HB-GAM in a haptotactic transfilter assay and in a migration assay where HB-GAM patterns were created on culture wells. The mechanism of migration is similar to that previously described for the HB-GAM-induced migratory response of neurons. Our hypothesis that HB-GAM/N-syndecan interaction participates in regulation of osteoblast recruitment was tested using two different in vivo models: an adjuvant-induced arthritic model and a transgenic model. In the adjuvant-induced injury model, the expression of HB-GAM and of N-syndecan is strongly upregulated in the periosteum accompanying the regenerative response of bone. In the transgenic model, the HB-GAM expression is maintained in mesenchymal tissues with the highest expression in the periosteum. The HB-GAM transgenic mice develop a phenotype characterized by an increased bone thickness. HB-GAM may thus play an important role in bone formation, probably by mediating recruitment and attachment of osteoblasts/osteoblast precursors to the appropriate substrates for deposition of new bone.

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(A) Bone phenotype of the HB-GAM  transgenic mice. Femora from 1-yr-old transgenic (a and c) and nontransgenic (b and d) littermates. (a) HB-GAM is abundantly expressed  by osteocytes of the transgene-positive femur,  and it is widely distributed along the bone surface (arrows). (b) HB-GAM expression in the  transgene-negative femur is confined to the epiphyseal growth plate (arrowheads), which persists throughout life in rodents. Note that the  transgene-positive mouse (a) has developed a  much wider growth plate (arrowheads). (c)  HB-GAM is expressed in the transgene-positive  femur by the osteocytes of cortical bone (arrowheads) and localized to the bone surface (arrows)  in a similar manner to that found during regeneration (see Fig. 4 h). (d) HB-GAM is not expressed either by the osteocytes or on the bone  surface (arrow) in the transgene-negative femur.  The cortical bone of the transgene-positive femur (cb′ in c) is much thicker than that of the  transgene-negative femur (cb in d). (B) Histomorphometric analysis of femora of transgene-negative (Non tg) and transgene-positive mice  (Tg). The parameters followed the recommended nomenclature (Parfitt et al., 1987). (a)  Cortical bone volume (BT/TV,%): expressed as  the percentage of the cortical tissue area divided  by the cross-sectional area. (b) Cancellous bone  volume (BT/TV,%): expressed as the percentage  of metaphyseal area occupied by cancellous bone  excluding the cortices. (c) Bone marrow space  (mm2): area of metaphyseal marrow cavity excluding cancellous bone. Bars represent means ±  SEM. Statistical differences between the groups  were assessed by Student's t test (*P < 0.05,  **P < 0.01, n = 6 and 8 for nontransgenic and  transgenic mice, respectively).
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Figure 8: (A) Bone phenotype of the HB-GAM transgenic mice. Femora from 1-yr-old transgenic (a and c) and nontransgenic (b and d) littermates. (a) HB-GAM is abundantly expressed by osteocytes of the transgene-positive femur, and it is widely distributed along the bone surface (arrows). (b) HB-GAM expression in the transgene-negative femur is confined to the epiphyseal growth plate (arrowheads), which persists throughout life in rodents. Note that the transgene-positive mouse (a) has developed a much wider growth plate (arrowheads). (c) HB-GAM is expressed in the transgene-positive femur by the osteocytes of cortical bone (arrowheads) and localized to the bone surface (arrows) in a similar manner to that found during regeneration (see Fig. 4 h). (d) HB-GAM is not expressed either by the osteocytes or on the bone surface (arrow) in the transgene-negative femur. The cortical bone of the transgene-positive femur (cb′ in c) is much thicker than that of the transgene-negative femur (cb in d). (B) Histomorphometric analysis of femora of transgene-negative (Non tg) and transgene-positive mice (Tg). The parameters followed the recommended nomenclature (Parfitt et al., 1987). (a) Cortical bone volume (BT/TV,%): expressed as the percentage of the cortical tissue area divided by the cross-sectional area. (b) Cancellous bone volume (BT/TV,%): expressed as the percentage of metaphyseal area occupied by cancellous bone excluding the cortices. (c) Bone marrow space (mm2): area of metaphyseal marrow cavity excluding cancellous bone. Bars represent means ± SEM. Statistical differences between the groups were assessed by Student's t test (*P < 0.05, **P < 0.01, n = 6 and 8 for nontransgenic and transgenic mice, respectively).

Mentions: In the transgene-positive mice, HB-GAM was abundantly expressed by the osteocytes dispersed in the cortical bone (Fig. 8 A, c). HB-GAM was localized along the surface of the cortical bone (Fig. 8 A, a) in a similar manner to that found during the adult periosteal ossification (see Fig. 5 h). It thus appears that HB-GAM is localized because of binding sites on the bone surface, which may stabilize the secreted protein and present it in a surface-bound form to the motile osteoblasts/osteoblast precursors. In contrast, the transgene-negative mice were devoid of HB-GAM expression (Fig. 8 A, d) with the exception of the epiphyseal growth plate (Fig. 8 A, b).


Osteoblast recruitment and bone formation enhanced by cell matrix-associated heparin-binding growth-associated molecule (HB-GAM).

Imai S, Kaksonen M, Raulo E, Kinnunen T, Fages C, Meng X, Lakso M, Rauvala H - J. Cell Biol. (1998)

(A) Bone phenotype of the HB-GAM  transgenic mice. Femora from 1-yr-old transgenic (a and c) and nontransgenic (b and d) littermates. (a) HB-GAM is abundantly expressed  by osteocytes of the transgene-positive femur,  and it is widely distributed along the bone surface (arrows). (b) HB-GAM expression in the  transgene-negative femur is confined to the epiphyseal growth plate (arrowheads), which persists throughout life in rodents. Note that the  transgene-positive mouse (a) has developed a  much wider growth plate (arrowheads). (c)  HB-GAM is expressed in the transgene-positive  femur by the osteocytes of cortical bone (arrowheads) and localized to the bone surface (arrows)  in a similar manner to that found during regeneration (see Fig. 4 h). (d) HB-GAM is not expressed either by the osteocytes or on the bone  surface (arrow) in the transgene-negative femur.  The cortical bone of the transgene-positive femur (cb′ in c) is much thicker than that of the  transgene-negative femur (cb in d). (B) Histomorphometric analysis of femora of transgene-negative (Non tg) and transgene-positive mice  (Tg). The parameters followed the recommended nomenclature (Parfitt et al., 1987). (a)  Cortical bone volume (BT/TV,%): expressed as  the percentage of the cortical tissue area divided  by the cross-sectional area. (b) Cancellous bone  volume (BT/TV,%): expressed as the percentage  of metaphyseal area occupied by cancellous bone  excluding the cortices. (c) Bone marrow space  (mm2): area of metaphyseal marrow cavity excluding cancellous bone. Bars represent means ±  SEM. Statistical differences between the groups  were assessed by Student's t test (*P < 0.05,  **P < 0.01, n = 6 and 8 for nontransgenic and  transgenic mice, respectively).
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Figure 8: (A) Bone phenotype of the HB-GAM transgenic mice. Femora from 1-yr-old transgenic (a and c) and nontransgenic (b and d) littermates. (a) HB-GAM is abundantly expressed by osteocytes of the transgene-positive femur, and it is widely distributed along the bone surface (arrows). (b) HB-GAM expression in the transgene-negative femur is confined to the epiphyseal growth plate (arrowheads), which persists throughout life in rodents. Note that the transgene-positive mouse (a) has developed a much wider growth plate (arrowheads). (c) HB-GAM is expressed in the transgene-positive femur by the osteocytes of cortical bone (arrowheads) and localized to the bone surface (arrows) in a similar manner to that found during regeneration (see Fig. 4 h). (d) HB-GAM is not expressed either by the osteocytes or on the bone surface (arrow) in the transgene-negative femur. The cortical bone of the transgene-positive femur (cb′ in c) is much thicker than that of the transgene-negative femur (cb in d). (B) Histomorphometric analysis of femora of transgene-negative (Non tg) and transgene-positive mice (Tg). The parameters followed the recommended nomenclature (Parfitt et al., 1987). (a) Cortical bone volume (BT/TV,%): expressed as the percentage of the cortical tissue area divided by the cross-sectional area. (b) Cancellous bone volume (BT/TV,%): expressed as the percentage of metaphyseal area occupied by cancellous bone excluding the cortices. (c) Bone marrow space (mm2): area of metaphyseal marrow cavity excluding cancellous bone. Bars represent means ± SEM. Statistical differences between the groups were assessed by Student's t test (*P < 0.05, **P < 0.01, n = 6 and 8 for nontransgenic and transgenic mice, respectively).
Mentions: In the transgene-positive mice, HB-GAM was abundantly expressed by the osteocytes dispersed in the cortical bone (Fig. 8 A, c). HB-GAM was localized along the surface of the cortical bone (Fig. 8 A, a) in a similar manner to that found during the adult periosteal ossification (see Fig. 5 h). It thus appears that HB-GAM is localized because of binding sites on the bone surface, which may stabilize the secreted protein and present it in a surface-bound form to the motile osteoblasts/osteoblast precursors. In contrast, the transgene-negative mice were devoid of HB-GAM expression (Fig. 8 A, d) with the exception of the epiphyseal growth plate (Fig. 8 A, b).

Bottom Line: We show here that heparin-binding growth-associated molecule (HB-GAM), an extracellular matrix-associated protein that enhances migratory responses in neurons, is prominently expressed in the cell matrices that act as target substrates for bone formation.The HB-GAM transgenic mice develop a phenotype characterized by an increased bone thickness.HB-GAM may thus play an important role in bone formation, probably by mediating recruitment and attachment of osteoblasts/osteoblast precursors to the appropriate substrates for deposition of new bone.

View Article: PubMed Central - PubMed

Affiliation: Department of Anatomy, Shiga University of Medical Science, Shiga-ken, 520-2192, Japan. simai@belle.shiga-med.ac.jp

ABSTRACT
Bone has an enormous capacity for growth, regeneration, and remodeling. This capacity is largely due to induction of osteoblasts that are recruited to the site of bone formation. The recruitment of osteoblasts has not been fully elucidated, though the immediate environment of the cells is likely to play a role via cell- matrix interactions. We show here that heparin-binding growth-associated molecule (HB-GAM), an extracellular matrix-associated protein that enhances migratory responses in neurons, is prominently expressed in the cell matrices that act as target substrates for bone formation. Intriguingly, N-syndecan, which acts as a receptor for HB-GAM, is expressed by osteoblasts/osteoblast precursors, whose ultrastructural phenotypes suggest active cell motility. The hypothesis that HB-GAM/N-syndecan interaction mediates osteoblast recruitment, as inferred from developmental studies, was tested using osteoblast-type cells that express N-syndecan abundantly. These cells migrate rapidly to HB-GAM in a haptotactic transfilter assay and in a migration assay where HB-GAM patterns were created on culture wells. The mechanism of migration is similar to that previously described for the HB-GAM-induced migratory response of neurons. Our hypothesis that HB-GAM/N-syndecan interaction participates in regulation of osteoblast recruitment was tested using two different in vivo models: an adjuvant-induced arthritic model and a transgenic model. In the adjuvant-induced injury model, the expression of HB-GAM and of N-syndecan is strongly upregulated in the periosteum accompanying the regenerative response of bone. In the transgenic model, the HB-GAM expression is maintained in mesenchymal tissues with the highest expression in the periosteum. The HB-GAM transgenic mice develop a phenotype characterized by an increased bone thickness. HB-GAM may thus play an important role in bone formation, probably by mediating recruitment and attachment of osteoblasts/osteoblast precursors to the appropriate substrates for deposition of new bone.

Show MeSH
Related in: MedlinePlus