Targeting proteoglycan receptor PTP[sigma] restores sensory function after spinal cord dorsal root injury

Abstract

Following dorsal root crush injury, the lesioned axons may fail to regenerate across the dorsal root entry zone (DREZ), which contains a high amount of extracellular matrix secreted by activated astrocytes. As an inhibitory transitional zone between dorsal root and spinal cord, the DREZ resembles a glial scar barrier for axonal regeneration and remyelination after injury, causing deficits of sensorimotor functions. Chondroitin sulfate proteoglycan (CSPG) as a vital extracellular matrix (ECM) is highly expressed within glial scar. High-affinity binding of the GAG chains of CSPG to neuronal receptor Protein tyrosine phosphatase sigma (PTPσ) can block axonal regrowth. ISP (Intracellular Sigma Peptide) is a mimetic of the PTPσ wedge region that is able to bind the PTPσ wedge domain as an antagonist of CSPG-derived inhibitory signal. Previous studies showed that extracellular-regulated-kinase (Erk) (with its downstream signal cAMP-response-element-binding protein (CREB)) and RhoA (with its downstream signal Collapsin response mediator protein 2 (CRMP2)) are major signaling molecules downstream of CSPG and PTPσ and associated closely with axonal regeneration and myelination. Therefore, this study investigated the effect of systemic applications of ISP as treatment of C5-T1 dorsal root crush model. The objectives of this study are that systemic application of ISP could enhance sensory functional recovery after dorsal roots crush injury through promoting axonal regeneration and remyelination via regulating Erks/CREB and RhoA/CRMP2 signaling pathways. It is hypothesized that systemic application of ISP promotes sensory functions restoration in vivo, and ISP application enhances axonal regeneration and myelination both in vitro and in vivo through modulating Erks/CREB and RhoA/CRMP2 signaling pathways. This study showed that ISP was able to restore the injured animals’ sensory functions without any thermal and mechanical hyperalgesia through regulating the Erk/CREB and RhoA/CRMP2 signaling pathways. On morphological analyses, treatment with ISP could facilitate various sensory axons subtypes including CGRP+ and IB4+ fibers to grow across the inhibitory DREZ and project into their destined laminae within spinal cord dorsal horn without any over-expression in the contralateral dorsal horn. In vitro dorsal root ganglion (DRG) culture experiments showed that the Erk/CREB and RhoA/CRMP2 pathways were involved in the process of ISP-induced DRG neurites regrowth in the presence of CSPG. Furthermore, ISP promoted axonal myelination in vivo which was confirmed on in vitro oligodendrocytes/Schwann cells-DRG cocultured assays. Erk/CREB and RhoA/CRMP2 signaling pathways were shown to be regulated by ISP to participate in ISP enhanced axonal myelination. In conclusion, the present study showed that ISP could increase sensory functional recovery through promoting axonal regeneration and remyelination after dorsal roots crush injury. This study provided new evidence to support future clinical testing of ISP as a novel therapeutic method in nerve injury.