Functional Properties of Extracellular Matrix

胶原蛋白网络在决定软骨和其他细胞外基质的功能特性方面起着至关重要的作用。胶原蛋白网络对被困在其中的渗透活性蛋白多糖施加收缩压力，就像气球的橡胶膜对其中包含的气体施加静水压力的方式大致相同。然而，到目前为止，还不可能独立于细胞外基质中的其他成分来测量胶原网络的重要物理性质。最近，我们设计了一种新的方法来确定胶原网络结构本身的性质，如其体积弹性系数。这种新的方法需要(A)将软骨组织基质建模为由两个不同相组成的复合材料：胶原网和困在其中的蛋白多糖(PG)溶液，(B)施加各种已知水平的平衡渗透应力，以及(C)使用物理化学原理和独立的实验来独立地确定PG和胶原蛋白相的有用的“压力-体积”关系。在初步研究中，我们使用这种方法测定了天然和经胰酶处理的正常人软骨标本以及骨关节炎(OA)关节软骨标本中胶原网络和PG相的压力-体积曲线。在正常和胰酶处理的标本中，胶原网络硬度没有变化，而在骨性关节炎标本中，胶原网络硬度降低。我们的发现强调了胶原网络在限制正常软骨水化和确保基质中高PG浓度方面的作用，这两者都是软骨有效负荷所必需的，但在骨关节炎中消失了。这些数据还表明，胶原网络僵硬的丧失，而不是PGs的丢失或修饰，可能是导致随后观察到的OA软骨解体的早期事件。最近，金伯利·波特启动了显微磁共振成像(MRI)研究，旨在通过尝试将中空纤维生物反应器中生长的软骨组织的化学成分与各种可测量的MRI参数相关联，来非侵入性地估计我们的软骨肿胀数学模型的参数。Ferenc Horkay现在正在开发一种仪器，它将使我们能够研究极薄的软骨切片的膨胀特性，使我们能够从关节表面获得其功能特性随深度的剖面。

The collagen network plays a critical role in determining functional properties of cartilage and other extracellular matrices. The collagen network exerts a retractive stress on the osmotically active proteoglycans trapped within it in much the same way a balloon's rubber membrane exerts a hydrostatic pressure of the gas contained within it. Until now, however, it has not been possible to measure important physical properties of the collagen network independently of other constituents within the extracellular matrix. Recently, we devised a new methodology to determine structural properties of the collagen network per se, such as its bulk modulus. This new approach entails (a) modeling the cartilage tissue matrix as a composite material consisting of two distinct phases: a collagen network and a proteoglycan (PG) solution trapped within it, (b) applying various known levels of equilibrium osmotic stress, and (c) using physical-chemical principles and independent experiments to determine useful "pressure-volume" relations for both the PG and collagen phases independently. In pilot studies, we used this approach to determine pressure-volume curves for the collagen network and the PG phases in native and in trypsin treated normal human cartilage specimen, as well as in cartilage specimen from osteoarthritic (OA) joints. In both normal and trypsin-treated specimen, collagen network stiffness appeared unchanged,whereas in the OA specimen, collagen network stiffness decreased. Our findings highlight the role of the collagen network in limiting normal cartilage hydration, and in ensuring a high PG concentration in the matrix, both of which are essential for effective load bearing in cartilage, but are lost in OA. These data also suggest that the loss of collagen network stiffness, and not the loss or modification of PGs may be the incipient event leading to the subsequent disintegraton of cartilage observed in OA. Recently, Kimberlee Potter has initiated microscopic Magnetic Resonance Imaging (MRI) studies designed to estimate parameters of our mathematical model of cartilage swelling noninvasively by attempting to relate the chemical composition of cartilage tissue grown in a hollow-fiber bioreactor to various measurable MRI parameters. Ferenc Horkay is now developing an instrument that will enable us to study swelling properties of extremely thin cartilage sections, permitting us to obtain a profile its functional properties with depth from a joint's articular surface.