Cell-matrix interactions in neuronal regeneration : focus on integrins and laminins

Abstract

The central and peripheral nervous system (CNS and PNS, respectively) differ in their ability to regenerate after injury. This difference has been attributed both to intrinsic differences in the properties of the neurons and to the extracellular environment. Several studies have provided evidence that PNS regeneration is dependent upon the interaction between members of the extracellular protein family known as laminins and their cellular receptors called integrins. Neurons projecting to the PNS such as motoneurons upregulate their expression of integrins after axonal injury. This has been interpreted as a sign of a switch from a transmission mode to a regenerative mode where focus is to regrow the injured axon and reestablish the lost connection with its target.

The dorsal root ganglion (DRG) neurons are unique in the sense that they have two branches with different regenerative ability. The peripheral branch conveys information from sensory organs and the central branch projects from the DRG into the spinal cord. The peripheral branch can regenerate after injury, whereas the central branch cannot. We determined that integrin mRNA is upregulated after peripheral, but not central branch injury, indicating that this could partly explain the poor outcome after central branch injury. We also examined the regulation of neuronal integrin expression after a pure CNS injury: axotomy of neurons located in the red nucelus. In doing so, we could not detect similar upregulations as described for sensory or motoneurons. Interestingly, we could se an increase in integrin beta1 after axotomy if the neurotrophic factor NT-3 was administered. The combined results of these experiments give further support to the notion that integrin upregulation is an important feature of neuronal regeneration. Also, the results indicate that integrin production can be manipulated by trophic factors and could therefore constitute a possible way to increase the regenerative ability of CNS neurons.

The most abundant laminins in the peripheral nerve are laminin-2 (alpha2beta1gamma1) and laminin-8 (alpha4beta1gamma1). Loss of laminin-2 leads to a hereditary muscular dystrophy, affecting both muscles and peripheral nerves. We examined, for the first time, the phenotype of peripheral nerves of mice deficient for laminin-8. We found that these mice have a pronounced dysmyelinating phenotype with thinner sheaths and bundles of unsorted and unmyelinated axons. Using cell cultures, we found that Schwann cells adhere to laminin-8 via integrin alpha6beta1 and that they grow longer processes on laminin-8 compared to laminin-2 which might explain the inability of Schwann cells to properly sort and ensheath axons in laminin-8 mutant mice.

Next, we examined the growth of adult DRG neurons on four laminin isoforms. Growth was extensive on laminin-1 and -10, but very poor on laminin-2 and -8 where addition of nerve growth factor was necessary to achieve substantial growth. By use of function blocking antibodies, we saw that DRG neurons interact with laminin-1 and -2 via integrins alpha7beta and alpha3beta1 and with laminin-8 and -10 via alpha6beta1. Finally, in order for integrins to be functional in neuronal outgrowth, they need to be transported to the tip of the growing neurite. A motorprotein called myosin-X was recently shown to be essential for growth of cellular processes by binding and transporting integrins. We therefore examined the regulation of myosin-X in neurons and found that the mRNA was upregulated in both sensory and motoneurons after axotomy. Reducing protein levels in a motoneuron cell line by use of RNA interference decreased neurite growth on laminin. These results indicate that myosin-X could be important for successful axonal regeneration.

In summary, the results presented here show that integrins are upregulated after injury by neurons with a regenerative capability. Also, the interaction between laminins and integrins are of fundamental importance for neurite growth from peripheral neurons and for myelination of the peripheral nerve. Finally, the motor protein myosin-X is upregulated after axonal injury and is essential for integrin-mediated neurite growth.