Adaptation of Sensory Neurons to Hyalectin and Decorin Proteoglycans

Abstract

Proteoglycans are abundantly expressed in the pathways of developing and regenerating neurons, yet the responses of neurons to specific proteoglycans are not well characterized. We have shown previously that one chondroitin sulfate proteoglycan (CSPG), aggrecan, is potently inhibitory to sensory axon extension in short-term assays and that over time, embryonic neurons adapt to aggrecan-mediated inhibition through the transcriptional upregulation of integrin expression (Condic et al., 1999). Here, we have compared the response of embryonic sensory neurons to structurally distinct CSPGs that belong to either the hyalectin (or lectican) family of large, aggregating proteoglycans or the decorin (or small leucine-rich proteoglycan) family of smaller proteoglycans. Both of these structurally diverse proteoglycan families are expressed in developing embryos and inhibit outgrowth of embryonic sensory neurons in short-term cultures. These results document a previously uncharacterized inhibitory function for the decorin-family proteoglycan biglycan. Interestingly, embryonic neurons adapt to these diverse proteoglycans over time. Adaptation is associated with upregulation of select integrin ␣ subunits in a proteoglycan-specific manner. Overexpression of specific integrin ␣ subunits improves neuronal regeneration on some but not all decorin-family CSPGs, suggesting that neurons adapt to inhibition mediated by closely related proteoglycans using distinct mechanisms. Our findings indicate that CSPGs with diverse core proteins and distinct numbers of chondroitin sulfate substitution sites mediate a similar response in sensory neurons, suggesting that increased integrin expression may be an effective means of promoting axonal regeneration in the presence of diverse inhibitory proteoglycan species in vivo.

Isolation and Culture of Dissociated Sensory Neurons From Chick Embryos

“…These cultures can be used for specifically analyzing growth cones in addition to analyzing the entire neuron. Previously, this procedure has been used to evaluate growth cone velocity and behaviors, such as growth cone collapse 8,9,11,13,16 . Lower plating densities on these coverslips results in dead neurons that are not adhered to the substrata.…”

“…For example, intracellular levels of cAMP and surface levels of integrins are significantly different in neurons plated on these two substrata 7,8 . Additional studies have shown that other molecules, including fibronectin and chondroitin sulfate proteoglycans, impact the expression of cell surface molecules and neuronal motility [7][8][9][10][11] . In addition, soluble molecules such as neurotrophins and neurotropins also impact cell membrane composition and neuronal motility [12][13][14][15][16] and can be easily and accurately manipulated in a cell culture model.…”

“…This procedure has been used to make significant breakthroughs in neurobiology, including axon outgrowth 5,7,8,10,11,[16][17][18][19][20][21] and was modified from a procedure designed to isolate ganglion cells 22 . There are several advantages to this approach.…”

Extrinsic cellular and molecular mediators of peripheral axonal regeneration

“…Laminin, the major component of the extracellular matrix (ECM) in the PNS, exhibits clear stimulatory effects on axonal growth and is able to counteract inhibitory myelin influences (Lemons et al 2005). Numerous studies have demonstrated that laminin not only improves neurite growth but also mediates Schwann cell migration, proliferation, or (re)myelination Condic 2008: Tucker andMearow 2008;Chen et al 2007;Previtali et al 2001).…”

Integrins and the extracellular matrix: Key mediators of development and regeneration of the sensory nervous system

“…Injured axons are exposed to a number of inhibitory molecules expressed at the DREZ and in the glial scar that develops following CNS injury (Schwab, 1996;Schwab, 2010). CSPGs are ECM proteins expressed in the developing and adult nervous system, which inhibit neurite outgrowth in vitro (Snow and Letourneau, 1992;Snow et al, 1996;Condic et al, 1999;Snow et al, 2001;Lemons et al, 2005). Upregulation of CSPGs after central injury is correlated with the failure of axonal regeneration in the adult CNS (Hoke and Silver, 1996).…”