Restoring bladder function by promoting neuronal regeneration
Brian Anthony Rooney
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Following dorsal root injury, dorsal root ganglion neurons can regenerate their severed axons but are unable to cross the dorsal root entry zone (DREZ) due to the presence of an inhibitory environment produced in part by chondroitin sulfate proteoglycans (CSPGs) manufactured by reactive astrocytes and oligodendrocytes. We examined the influence of the GDNF family neurotrophin artemin on the neuroarchitecture and extracellular matrix composition of the dorsal spinal cord following a bilateral dorsal root crush (DRC) at L6 and S1. Our functional assessment involved recording from the bladder and external urethral sphincter (EUS). Systemic artemin treatment resulted in significant increases in the laminar fine fiber population and reinnervation of the sacral parasympathetic nucleus. In a tracer study, where AAV-GFP was injected into the dorsal root ganglia of crushed roots, artemin treatment resulted in an the reappearance of GFP-filled afferents in the medial dorsal horn and of GFP-filled rostrocaudally oriented afferent cross-sections in the dorsal column, which we interpret as regeneration. To find out if sensorimotor functional improvements resulted from reafferentation of the dorsal spinal cord, we carried out urodynamic analysis of the bladder and EUS. Artemin treatment resulted in significant reductions in dysfunctional EUS electromyographic (EMG) activity, and normalization of bladder threshold volumes and residual volumes. To examine how artemin facilitates regeneration of primary afferents across the DREZ, we studied post-injury expression of CSPGs in the DREZ following artemin administration. Artemin had no effect on the post-injury increase in the CSPGs neurocan or NG2 in the DREZ. However, there was a delay in the post injury increase in phosphacan in the DREZ in the artemin treatment group. There was a rightward shift in the expression profile of phosphacan in the DREZ as a result of artemin treatment. This data accompanied by recent work reporting the influence of artemin on neuronal RhoA activity suggests that artemin encourages regeneration across the DREZ by ramping up the intrinsic regenerative capacity of injured afferents to overcome the inhibitory barrier rather than breaking the barrier down.