Distribution and synthesis of extracellular matrix proteoglycans, hyaluronan, link proteins and tenascin-R in the rat spinal cord

Chondroitinase ABC treatment opens a window of opportunity for task-specific rehabilitation

Chondroitinase ABC treatment promotes spinal cord plasticity. We investigated whether chondroitinase-induced plasticity combined with physical rehabilitation promotes recovery of manual dexterity in rats with cervical spinal cord injuries. Rats received a C4 dorsal funiculus cut followed by chondroitinase ABC or penicillinase as a control. They were assigned to two alternative rehabilitation procedures, the first reinforcing skilled reaching and the second reinforcing general locomotion. Chondroitinase treatment enhanced sprouting of corticospinal axons independently of the rehabilitation regime. Only the rats receiving the combination of chondroitinase and specific rehabilitation showed improved manual dexterity. Rats that received general locomotor rehabilitation were better at ladder walking, but had worse skilled-reaching abilities than rats that received no treatment. Our results indicate that chondroitinase treatment opens a window during which rehabilitation can promote recovery. However, only the trained skills are improved and other functions may be negatively affected.

Reduced expression of the hyaluronan and proteoglycan link proteins in malignant gliomas

Malignant gliomas have a distinctive ability to infiltrate the brain parenchyma and disrupt the neural extracellular matrix that inhibits motility of axons and normal neural cells. Chondroitin sulfate proteoglycans (CSPGs) are among the major inhibitory components in the neural matrix, but surprisingly, some are up-regulated in gliomas and act as pro-invasive signals. In the normal brain, CSPGs are thought to associate with hyaluronic acid and glycoproteins such as the tenascins and link proteins to form the matrix scaffold. Here, we examined for the first time the expression of link proteins in human brain and malignant gliomas. Our results indicate that HAPLN4 and HAPLN2 are the predominant members of this family in the adult human brain but are strongly reduced in the tumor parenchyma. To test if their absence was related to a pro-invasive gain of function of CSPGs, we expressed HAPLN4 in glioma cells in combination with the CSPG brevican. Surprisingly, HAPLN4 increased glioma cell adhesion and migration and even potentiated the motogenic effect of brevican. Further characterization revealed that HAPLN4 expressed in glioma cells was largely soluble and did not reproduce the strong, hyaluronan-independent association of the native protein to brain subcellular membranes. Taken together, our results suggest that the tumor parenchyma is rich in CSPGs that are not associated to HAPLNs and could instead interact with other extracellular matrix proteins produced by glioma cells. This dissociation may contribute to changes in the matrix scaffold caused by invasive glioma cells.

Differential distribution of aggrecan isoforms in perineuronal nets of the human cerebral cortex

Aggrecan is a component of the CNS extracellular matrix (ECM) and we show here that the three primary alternative spliced transcripts of the aggrecan gene found in cartilage are also present in the adult CNS. Using a unique panel of core protein-directed antibodies against human aggrecan we further show that different aggrecan isoforms are deposited in perineuronal nets (PNNs) and neuropil ECM of Brodmann’s area 6 of the human adult cerebral cortex. According to their distribution pattern, the identified cortical aggrecan isoforms were subdivided into five clusters spanning from cluster 1, comprised isoforms that appeared widespread throughout the cortex, to cluster 5, which was an aggrecan-free subset. Comparison of brain and cartilage tissues showed a different relative abundance of aggrecan isoforms, with cartilage-specific isoforms characterizing cluster 5, and PNN-associated isoforms lacking keratan sulphate chains. In the brain, isoforms of cluster 1 were disclosed in PNNs surrounding small-medium interneurons of layers II–V, small-medium pyramidal neurons of layers III and V and large interneurons of layer VI. Aggrecan PNNs enveloped both neuron bodies and neuronal processes, encompassing pre-terminal nerve fibres, synaptic boutons and terminal processes of glial cells and aggrecan was also observed in continuous ‘coats’ associated with satellite, neuron-associated cells of a putative glial nature. Immunolabelling for calcium-binding proteins and glutamate demonstrated that aggrecan PNNs were linked to defined subsets of cortical interneurons and pyramidal cells. We suggest that in the human cerebral cortex, discrete, layer-specific PNNs are assembled through the participation of selected aggrecan isoforms that characterize defined subsets of cortical neurons.

Molecular control of brain plasticity and repair

Recovery of function after damage to the CNS is limited due to the absence of axon regeneration and relatively low levels of plasticity. Plasticity in the CNS can be reactivated in the adult CNS by treatment with chondroitinase ABC, which removes glycosaminoglycan (GAG) chains from chondroitin sulfate proteoglycans (CSPGs). Plasticity in the adult CNS is restricted by perineuronal nets (PNNs) around many neuronal cell bodies and dendrites, which appear at the closure of critical periods and contain several inhibitory CSPGs. Formation of these structures and the turning off of plasticity is triggered by impulse activity in neurons. Expression of a link protein by neurons is the event that triggers the formation of PNNs. Treatment with chondroitinase removes PNNs and other inhibitory influences in the damaged spinal cord and promotes sprouting of new connections. However, promoting plasticity by itself does not necessarily bring back useful behavior; this only happens when useful connections are stabilized and inappropriate connections removed, driven by behavior. Thus after rodent spinal cord injury, combining a daily rehabilitation treatment for skilled paw function with chondroitinase produces much greater recovery than either treatment alone. The rehabilitation must be specific for the behavior that is to be enhanced because non-specific rehabilitation improves locomotor behavior but not skilled paw function. Plasticity-enhancing treatments may therefore open up a window of opportunity for successful rehabilitation.

Extracellular matrix of the central nervous system: from neglect to challenge

The basic concept, that specialized extracellular matrices rich in hyaluronan, chondroitin sulfate proteoglycans (aggrecan, versican, neurocan, brevican, phosphacan), link proteins and tenascins (Tn-R, Tn-C) can regulate cellular migration and axonal growth and thus, actively participate in the development and maturation of the nervous system, has in recent years gained rapidly expanding experimental support. The swift assembly and remodeling of these matrices have been associated with axonal guidance functions in the periphery and with the structural stabilization of myelinated fiber tracts and synaptic contacts in the maturating central nervous system. Particular interest has been focused on the putative role of chondroitin sulfate proteoglycans in suppressing central nervous system regeneration after lesions. The axon growth inhibitory properties of several of these chondroitin sulfate proteoglycans in vitro, and the partial recovery of structural plasticity in lesioned animals treated with chondroitin sulfate degrading enzymes in vivo have significantly contributed to the increased awareness of this long time neglected structure.