Anabolic Response to Skeletal Loading in Mice with Targeted Disruption of

Pleiotrophin Gene

C. Kesavan and S. Mohan

JLP VA Medical Center, Loma Linda Univ

Loma Linda, CA, 92357

ASBMR 2008

Acknowledgements

• US Army Research

• Veterans administration

The authors have no conflict of interest

Load

Load

Good Responder

Poor Responder

Skeletal Response to Loading Differs in Different Individuals

Variation in skeletal response to loading is genetically determined

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6 7 8 9Load (N)

% In

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vB

MD

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Four point bending device

C57BL/6J

C3H/HeJ

Four point bending IncreasesBMD in C57BL but not in C3H mice

P<0.001

Kesavan et al., 2005

P<0.01

ML increases pleiotrophin (PTN) expression-Whole

genome microarray analysis

PTN signals through β-catenin and PI3K

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800,000

1,200,000

Unloaded Loaded

PT

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p<0.0001

Xing et al., 2005

PTN/HB-GAM/OSF

SyndecanRprpζ

β-catenin PI3K

Proliferation/survival/Migration

Time course changes in PTN expression in the loaded bones of C57BL Mice

Ap<0.05 vs. 2- and 4- days of loading (One-way ANOVA)

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0 2 4 6 8 10 12 14

p<0.05

p<0.001

p<0.0001

Days (Duration)

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C57BL C3H

Greater PTN expression in response to ML in good responder C57BL versus poor

responder C3H mice

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vs.

un

load

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P<0.0001

P<0.05

P<0.05

Mean ± SD; N = 6

Hypothesis

PTN plays a role in mediating anabolic effects of mechanical

loading on bone formation

9N load produces equivalent amount of strain in the tibia of PTN KO and

littermate control mice

• CSA (mm2) 4.69 ± 0.21 4.56 ± 0.24

• BMD (mg/cm3) 838 ± 40 881 ± 51

• Strain (µe) 6351 ± 414 6310 ± 726

at 9N load

Control PTN KO

Mean ± SD, N = 7

Skeletal changes in response to loading in PTN KO and control mice

*p<0.05 vs. unloaded bones, N = 7

Pe

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BMC PC BMD

Control KO* *

* ** *

Micro-CT analysis of skeletal response to loading

Control

PTN KO

Unloaded Loaded

Mean ± SD, N = 7

BV/TV (%) 3.88 ± 2.6Tb.N (1/mm) 1.56 ± 0.76Tb.Th (mm) 0.037 ± 0.0.01Tb.S (mm) 0.76 ± 0.28

BV/TV (%) 2.81 ± 1.5Tb.N (1/mm) 1.61 ± 0.36Tb.Th (mm) 0.036 ± 0.0.01Tb.S (mm) 0.57 ± 0.21

Greater midkine expression in response to ML in PTN KO and Control mice

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Control PTN KO

Fo

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ncr

ease

vs.

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P<0.05

P<0.05

P<0.001

N=6

Summary

• PTN expression is increased in response to ML but decreased during osteoblast differentiation

• Lack of PTN does not influence peak bone mass

• PTN disruption in mice although caused a small decrease in the amount of new bone formed in response to ML, this difference was not statistically significant

• Disruption of PTN gene in mice caused significant increase in midkine, member of PTN family that shares similar functional properties

Conclusions

1) Disruption of PTN alone is not sufficient to impair skeletal anabolic response to external loading in mice

2) Disruption of both PTN and midkine may be necessary to fully evaluate the role of heparin binding growth associated molecules in mediating anabolic effects of mechanical loading in bone.

Thank you

Skeletal Un loadingSkeletal Loading