White arrow in (A) shows that fibrillin-2 is also concentrated around cells

White arrow in (A) shows that fibrillin-2 is also concentrated around cells. but only sparse/filamentous fibres of fibrillin-1 and LTBP-2. Dual immunostaining revealed that in this region, fibrillin-2 was highly colocalised with elastin. The LTBP-2 network co-localised well with that of fibrillin-1 in all regions and indeed is reported to bind strongly to fibrillin-1. However, interestingly LTBP-2 but not fibrillin-1 or fibrillin-2 was removed by hyaluronidase but not collagenase pre-digestion. Our results suggest that fibrillin-2 and LTBP-2 could play an important role in disc function. Keywords:Fibrillin-2, LTBP-2, fibrillin-1, elastic fibres, disc == Introduction == The intervertebral disc plays an important biomechanical role, providing the spine with flexibility and carrying load. Morphologically, the disc is composed of a central region, the nucleus pulposus (NP), surrounded laterally by the annulus fibrosus (AF) and longitudinally by the cartilaginous endplates, which lie between the disc and the vertebral bodies (White & Panjabi, 1978). The biomechanical behaviour of the disc ultimately depends on the composition and organisation of its macromolecular constituents. The major constituents are fibrillar collagens and the large aggregating proteoglycan aggrecan; their role in the biomechanical responses of disc has long been of interest (Eyre et al. 2002;Roughley et al. 2002;Urban & Roberts, 2003). Minor constituents also play important roles in maintaining disc integrity and function; of these, the distribution and organisation of the network of elastic fibres has recently attracted attention (Yu et al. 2002,2005;Smith & Fazzalari, 2006;Hayes et al. 2011). Elastic fibres in general consist of a central core of elastin surrounded by microfibrils, of which fibrillins are the main constituent (Greenlee et al. 1966;Sakai et al. 1986;Montes, 1996). Three types exist in human tissues (fibrillin 13) with fibrillin-1 most widely distributed and extensively investigated (Ramirez PS-1145 et al. 2004;Hubmacher et al. 2006;Ramirez & Sakai, 2010). In the disc, the organisation of the elastic network, shown by immunostaining elastin and fibrillin-1, varies with region; in the annulus fibrosus, elastic fibres are aligned with collagen fibres within the lamellae and encircle collagen bundles PS-1145 (Yu et al. 2002), whereas in the NP, fibrillin-1 is concentrated around the cells with elastin fibres, seen mainly in the interterritorial matrix (Yu et al.2007). There is no information on the distribution of fibrillin-2 in the post-natal disc. Indeed, fibrillin-2 and fibrillin-3 are thought to be mainly involved in the early stage of microfibril formation during embryonic development, fibrillin-1 being the main component of microfibrils in a mature tissue PS-1145 (Zhang et al. 1994,1995;Corson et al. 2004). However, fibrillin-2 has recently PS-1145 been reported to be present in post-natal tissues PR22 (Cain et al. 2006;Charbonneau et al. 2010). Microfibrils are also associated with many other matrix proteins, including latent transforming growth factor binding proteins (LTBPs). There are four forms of LTBPs, LTBP14 (Annes et al. 2003;Hyytiainen et al. 2004), all of which, apart from LTBP-2, can bind covalently to transforming growth factor (TGF)- (Rifkin, 2005). LTBP-2 is reported to co-localise with fibrillin-1 in human embryonic lung fibroblasts and aorta (Vehvilainen et al. 2009) and in human foetal disc (Hayes et al. 2011). Its distribution in adult disc is unknown. The distribution of microfibrils is of interest as their role is not only biomechanical but biological in that they regulate the signaling of growth factors, particularly TGF- (Ramirez et al. 2004). The main aim of the current study is to investigate the distribution of fibrillin-2 and LTBP-2 in adult disc tissue, with the ultimate aim to understand their roles in disc function. == Materials and Methods == == Specimen preparation == Fresh bovine tails were obtained from three adult steers (1824 months) from a local abattoir within 3 h of slaughter. Different regions of the disc were dissected out and immediately snap-frozen and stored at 80 C until used. For the central region, the PS-1145 nucleus pulposus (NP), frozen sections were cut transversely at a thickness of 20 m. The annulus fibrosus (AF) is formed from concentric collagen lamellae encircling the NP. The lamellae are formed from bundles of collagen fibres extending obliquely from one vertebral body to the next with their angle to the axis of the spine alternating between adjacent lamellae. To reflect the organisation of this structure adequately, frozen AF tissue sections at 20 m thickness were cut obliquely at about 45 to the axis of the disc using a cryostat microtome; each section thus showed cross-sectional and in-plane views of adjacent lamellae.Figure 1illustrates specimen preparation. The sections were mounted on microscope slides (VWR International Ltd, UK) and stored at 20 C for further use. == Fig. 1. == Schematic showing orientation of specimens from the different disc regions. NP, nucleus pulposus; AF, annulus fibrosus; IAF, inner.