Molecular/genetic manipulation of extrinsic axon guidance factors for CNS repair and regeneration

Abstract

During development, guidance molecules play a key role in the formation of complex circuits required for neural functions. With the cessation of development, this exuberant growth process slows and stabilizes, and inhibitory molecules expressed by glia prevent initial attempts for axonal regeneration. In this review, we discuss the expression patterns and relative contribution of several guidance molecules on the regenerative process. Injury to the immature CNS or species capable of regenerating exhibit a complete or partial recapitulation of their developmental guidance patterns, whereas similar injuries to adult mammals results in altered expression that acts to further hinder regeneration. Manipulations of guidance molecules after injury have been used to control detrimental effects of axon sprouting and target regenerating axons within the spinal cord.

Introduction

The function of the nervous system is based on complex networks formed by neurons during development, and accordingly any deficit or alternation in neural circuitry caused by injury leads to impaired function. During normal development, this highly precise network is established by temporal and spatial regulation of guidance molecules that direct the growth of axons along specific pathways to reach the target (Osterfield et al., 2003, Dickson, 2002). This process depends on the ability of growth cones (the leading edge of the axon) to sample and respond to multiple guidance cues in the environment (Song and Poo, 2001). In contrast, after injury to the adult mammalian central nervous system (CNS), the ability of axons to regenerate and reestablish specific projections is very limited. Axons fail to regenerate for multiple reasons, including the expression of myelin inhibitory proteins, reduced intrinsic growth potential of axons, upregulation of inhibitory extracellular matrix at the injury site and alterations in developmentally important guidance molecules (Silver and Miller, 2004). During the past 15 years, major advances in neutralizing glial inhibitory molecules and increasing the intrinsic growth ability of axons has lead to regeneration past the lesion site. Still, little is known about the accuracy in reconstruction of damaged circuits with functional indices being the final arbiter for circuit repair. Likewise, functional recovery is most likely due to the formation of novel circuits from sprouting and regenerating axons that undergo regional as well as supraspinal plasticity and rarely reconstruction of the original circuit (Maier and Schwab, 2006).

During development, axon pathfinding and target recognition are guided by both positive (permissive and attractive) and negative (inhibitory and repulsive) cues, which operate at short- or long-range (Tessier-Lavigne and Goodman, 1996). This gives rise to four different guidance mechanisms: contact attraction, chemoattraction, contact repulsion and chemorepulsion. Specific guidance decisions reflect the combination of all four mechanisms. The growth cone integrates signals of multiple cues presented simultaneously in the environment and reacts to the relative balance of the attractive and repulsive forces (Tessier-Lavigne and Goodman, 1996). The expression of these guidance molecules is under tight genetic control, orchestrating complex and precise patterning of connections in the CNS. The precision by which the guidance program is maintained within the normal adult CNS or recapitulated after injury has only recently begun to be studied. Many guidance factors and their receptors persist into adulthood; however, their distribution, particularly after injury, show incongruities with developmental expression patterns (Koeberle and Bahr, 2004). These variations in patterning could compromise accurate reconstruction of circuits. In addition, the environment in the adult CNS after injury is much more hostile to regenerating axons than during development. Axons navigate through a terrain that is replete with debris, reactive immune cells and cytokines and contains high levels of inhibitory molecules, reactive astrocytes, extracellular matrix (ECM) and reactive oxygen species—all of which could alter growth cone responsiveness to endogenous cues (Silver and Miller, 2004). Although many factors including neurotrophins, morphogens and extracellular matrix molecules are involved in axon growth and guidance, we will focus on the best-characterized of these ligand-receptor pairs that belong to the families of ephrins, netrins, semaphorins and slits.