title: Single molecule force measurements of perlecan/HSPG2: A key component of the osteocyte pericellular matrix.
abstract: Perlecan/HSPG2, a large, monomeric heparan sulfate proteoglycan (HSPG), is a key component of the lacunar canalicular system (LCS) of cortical bone, where it is part of the mechanosensing pericellular matrix (PCM) surrounding the osteocytic processes and serves as a tethering element that connects the osteocyte cell body to the bone matrix. Within the pericellular space surrounding the osteocyte cell body, perlecan can experience physiological fluid flow drag force and in that capacity function as a sensor to relay external stimuli to the osteocyte cell membrane. We previously showed that a reduction in perlecan secretion alters the PCM fiber composition and interferes with bone's response to a mechanical loading in vivo. To test our hypothesis that perlecan core protein can sustain tensile forces without unfolding under physiological loading conditions, atomic force microscopy (AFM) was used to capture images of perlecan monomers at nanoscale resolution and to perform single molecule force measurement (SMFMs). We found that the core protein of purified full-length human perlecan is of suitable size to span the pericellular space of the LCS, with a measured end-to-end length of 170±20 nm and a diameter of 2-4 nm. Force pulling revealed a strong protein core that can withstand over 100 pN of tension well over the drag forces that are estimated to be exerted on the individual osteocyte tethers. Data fitting with an extensible worm-like chain model showed that the perlecan protein core has a mean elastic constant of 890 pN and a corresponding Young's modulus of 71 MPa. We conclude that perlecan has physical properties that would allow it to act as a strong but elastic tether in the LCS.
authors.affiliation: Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA.
authors.forename: Sithara S
authors.initials: SS
authors.lastname: Wijeratne
authors.affiliation: Department of BioSciences, Rice University, Houston, TX 77005, USA.
authors.forename: Jerahme R
authors.initials: JR
authors.lastname: Martinez
authors.affiliation: Department of BioSciences, Rice University, Houston, TX 77005, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Grindel
authors.affiliation: Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA.
authors.forename: Eric W
authors.initials: EW
authors.lastname: Frey
authors.affiliation: Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA.
authors.forename: Jingqiang
authors.initials: J
authors.lastname: Li
authors.affiliation: Department of Mechanical Engineering, University of Delaware, Newark, DE, USA.
authors.forename: Liyun
authors.initials: L
authors.lastname: Wang
authors.affiliation: Department of BioSciences, Rice University, Houston, TX 77005, USA; Department of Bioengineering, Rice University, Houston, TX 77005, USA. Electronic address: farachca@rice.edu.
authors.forename: Mary C
authors.initials: MC
authors.lastname: Farach-Carson
authors.affiliation: Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA; Department of Bioengineering, Rice University, Houston, TX 77005, USA.
authors.forename: Ching-Hwa
authors.initials: CH
authors.lastname: Kiang
mesh_terms: D001857:Bone Matrix / Q000378:metabolism; D017077:Culture Media, Conditioned / Q000737:chemistry; D055119:Elastic Modulus; D019073:HT29 Cells; D019812:Heparan Sulfate Proteoglycans / Q000378:metabolism*; D006801:Humans; D018625:Microscopy, Atomic Force; D010011:Osteocytes / Q000166:cytology / Q000378:metabolism*; D013718:Tensile Strength
chemical_list: D017077:Culture Media, Conditioned; D019812:Heparan Sulfate Proteoglycans; C071927:perlecan
keywords: Atomic force microscopy; Bone; Heparan sulfate proteoglycan; Lacunar canalicular system; Osteocytes; Perlecan