The role of cAMP and its downstream targets in neurite growth in the adult nervous system

Abstract

Injured neurons in the adult mammalian central nervous system (CNS) have a very limited capacity for axonal regeneration and neurite outgrowth. This inability to grow new axons or to regrow injured axons is due to the presence of molecules that inhibit axonal growth, and age related changes in the neuron’s innate growth capabilities. Available levels of cAMP are thought to have an important role in linking both of these factors. Elevated levels of cAMP in the developing nervous system are important for the guidance and stability of growth cones. As the nervous system matures, cAMP levels decline and the growth promoting effects of cAMP diminish. It has frequently been demonstrated that increasing neuronal cAMP can enhance neurite growth and regeneration. Some methods used to increase cAMP include administration of cAMP agonists, conditioning lesions, or electrical stimulation. Furthermore, it has been proposed that multiple stages of cAMP induced growth exist, one directly caused by its downstream effector Protein Kinase A (PKA) and one caused by the eventual upregulation of gene transcription.

Although the role cAMP in promoting axon growth is well accepted, the downstream pathways that mediate cAMP-mediated axonal growth are less clear. This is partly because various key studies that explored the link between PKA and axonal outgrowth relied on the PKA inhibitors KT5720 and H89. More recent studies have shown that both of these drugs are less specific than initially thought and can inhibit a number of other signalling molecules including the Exchange Protein Activated by cAMP (EPAC). Consequently, it has recently been shown that a number of intracellular signalling pathways previously attributed to PKA can now be attributed solely to activation of EPAC specific pathways, or the simultaneous co-activation of PKA and EPAC specific pathways. These new studies open the door to new potential treatments for repairing the injured spinal cord.

Introduction

Ever since David and Aguayo [1] first demonstrated that injured central nervous system (CNS) axons can grow into a peripheral nerve graft, a major question has been: why do CNS axons not regenerate within the injured CNS? Three streams of research tackled this question. One stream examined whether the lack of CNS regeneration was due to an inhibitory environment. This research later identified inhibitory factors for neurite outgrowth including myelin associated inhibitors [2], and chondroitin sulfate proteoglycans (CSPGs) in the glial scar and the perineuronal net [3], [4]. The second stream examined whether reduced CNS regeneration was due to a lack of neurotrophic support resulting in the neurons not having the ability to regrow, as proposed by Aguayo himself [1]. The third stream examined the question of whether age related changes might influence the decline in neuronal growth abilities [5], [6]. These streams of research led to three main treatment strategies. Some laboratories attempted to neutralize neurite growth inhibitors (e.g., MAG [7] or Nogo [8], [9];, and CSPGs [10], [11]), while others augmented neurotrophic support by adding growth factors like brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or glial cell-derived neurotrophic factor (GDNF), to the injured spinal cord [12], [13], [14], [15], [16] (Fig. 1). Alternatively, some groups attempted to modulate intracellular signalling pathways with the idea to recreate conditions of the developing CNS in order to allow injured neurons to regrow [5], [6], [12], [17], [18]. As a result of all of this research, it is now well accepted that a combination of all of three factors (growth inhibitors, in sufficient of growth promoting factors and developmental changes) are responsible for the limited neurite outgrowth and repair after a spinal cord injury (SCI) in adult mammals.

With many intrinsic and extrinsic factors converging to prevent regeneration (growth at the tip of lesioned axons) and plasticity (e.g., growth of new collaterals) after SCI, there is a major effort to identify whether there is a common cell signalling pathway responsible for inhibiting neurite growth. This effort has resulted in the knowledge that both the intrinsic and extrinsic factors inhibiting axonal growth after SCI seem to converge on a number of signalling pathways associated with cAMP, calcium/calmodulin, Ras homolog gene family member A (RhoA), and the mechanistic target of rapamycin (mTOR). A number of recent reviews have focused on roles of RhoA [18], [19] and mTOR [6], [20] in axon growth and regeneration; this review will focus on cAMP and its downstream effectors. Specifically, we will discuss the relationship between cAMP and neurite outgrowth in the adult CNS, recently identified cAMP cell signalling pathways linked to axonal growth after SCI, and possible treatment strategies aimed to activate relevant cAMP dependent pathways.