Extracellular matrix in spinal cord regeneration: getting beyond attraction and inhibition

INTRODUCTION The expression of matrix molecules in the CNS [1,2] and the role of matrix in development [3–5] and regeneration [6,7] have recently been the subject of excellent reviews. It is evident that matrix proteins, particularly proteoglycans, make major contributions to regenerative failure in the adult CNS. Although a number of growth inhibiting molecules are expressed in CNS myelin and on glia, recent data suggests that these molecules are not sufficient to prevent the regeneration of adult neurons. Adult neurons transplanted into intact [8] or degenerating [9] adult white matter tracts are able to robustly regenerate. Regeneration of adult neurons invariably fails when extending axons encounter the glial scar associated with the injury site, suggesting that molecules expressed in the scar play a critical role in adult regeneration failure. Following injury, a large number of extracellular matrix (ECM) proteins that are known to influence the regeneration of neurons in culture are up-regulated or altered in regions of scarring (Table 1). Many of these same molecules are up-regulated following injury in peripheral nerve, where regeneration occurs [10]. Historically, the thinking in the field of regeneration research has been dominated by the relatively simple view that specific matrix molecules either promote or inhibit the growth of neurons. Therapeutic approaches have been directed towards increasing the expression or the efficacy of the former and minimizing the negative impact of the latter. There are reasons to believe, however, that this view does not fully reflect the complexity of the problem. The relationship between growth cone motility and matrix adhesion is not simply a matter of more being better (see Fig. 1). In addition, the response of neurons to specific molecules no longer appears to be a fixed property of the molecule itself, but rather a complex interaction between the molecule, the precise molecular composition of the environment, the particular type of neuron interacting with that environment and the recent history of the neuron. Here we will briefly discuss the role of cell adhesion in growth-cone migration. We will then consider molecules typically thought to be either ‘growth-promoting’ or ‘growthinhibiting’, and review the evidence that suggests this molecular dualism is an inadequate description of how neurons interact with matrix molecules in development and regeneration. MOTILITYANDMATRIXADHESION Most matrix proteins that are considered to be growthpromoting also promote cell adhesion. Indeed, cell adhesion is thought to be a required first step for migration. The ability of adhesive matrix proteins to both promote and prevent the migration of non-neuronal cells has been well studied (Fig. 1). Theoretical calculations and empirical data indicate that non-neuronal cells will only migrate over a narrow range of matrix concentrations where cells are adhered strongly enough to generate traction, without being so strongly adhered that they are unable to change position [11,12]. Surprisingly, this well-established relationship between motility and adhesion has not been consistently applied to the study of growth-cone migration. In some situations, embryonic neurons appear to be a rare exception to the rule that migration will only occur within a narrow window of matrix concentrations. Growth cones of embryonic neurons efficiently migrate on concentrations of the ECM molecule laminin that vary over several orders of magnitude [13–15]. The ability of neurons to migrate over a wide range of laminin concentrations is due to a very unusual regulation of neuronal receptors for laminin [15]. Laminin receptors are down-regulated in response to high laminin concentrations, thereby reducing neuronal adhesion and allowing an intermediate level of attachment to be maintained over a wide range of absolute ligand concentrations. Laminin-receptor levels in embryonic neurons are modulated by desensitization, i.e., receptor is removed from the surface in response to high ligand levels by endocytosis. Thus, embryonic neurons, like all cells, migrate only at intermediate levels of attachment, but (at least on laminin substrata) embryonic neurons are able to dynamically adjust their levels of attachment by adjusting expression of receptors. While embryonic neurons are able to compensate for a wide range of laminin concentrations, the response of growth cones to other matrix molecules that utilize different receptor systems is unknown. There is evidence that the receptor molecule L1 (also known as Ng-CAM) participates in endocytotic recycling in growth cones [16–18], but whether the absolute levels of L1 compensate for availability of ligand to maintain a constant level of L1-mediated attachment is unknown. Nothing is known about the regulation of other receptors that promote neuronal adhesion to components of scar matrix. Moreover, very little is