We have recently published that a NGF receptor, p75, is a potential therapeutic target for the spinal cord injured: motor function after spinal cor injury (SCI) improved after oral delivery of LM11A-31, a small molecule that was designed to target p75. Effective in vivo targeting of p75 by LM11A-31 was confirmed by observation of a block in binding of a pathological proNGF to p75, suggesting that the drug can be used in other systems where p75 exerts similar pathological effects upon interaction with proNGF or other proneurotrophins. We have published that ProNGF levels increase rapidly after injury to the CNS, being released into the cerebrospinal fluid. Unlike mature NGF that binds both TrkA and p75, proNGF binds p75 selectively, thereby activating p75's downstream apoptotic cascade. NGF has been implicated in bladder dysfunction after SCI as well as in overactive bladder and interstitial cystitis/painful bladder syndromes. Under these conditions and after SCI, a significan increase in NGF levels is observed in the urine, suggesting that NGF can be a biomarker for general bladder dysfunction and may be targeted to improve bladder function. Despite the long-standing interest, however, studies that aimed at neutralizing NGF action has led to mixed results. We believe a more detailed understanding of the roles that NGF and its receptors play in bladder biology will be necessary before we focus on NGF and related neurotrophins as therapeutic target for alleviating bladder dysfunction. We have found that proNGF and proBDNF are released into the urine shortly after SCI, while mature NGF is released days later. These proneurotrophins appear to induce apoptosis of umbrella cells that are responsible for establishing the permeability barrier, as LM11A-31 administration led to retention of umbrella cells after SCI. We thus hypothesize that p75 that is expressed in umbrella cells play a role in apoptosis of umbrella cells after SCI. In addition to umbrella cells, we found that p75 is also expressed in proliferating progenitors of the urothelium during the period of urothelial hyperplasia after SCI. LM11A-31 administration attenuated this hyperplasic response, suggesting that proneurotrophins and p75 play an additional role in regulating SCI-induced hyperplasia. We thus hypothesize that p75 plays dual roles in the regeneration of the urothelium after SCI, (1) by inducing apoptosis of the umbrella cells shortly after SCI, and (2) by regulating proliferation of the progenitor cells, thereby replenishing the lost umbrella cells. Under this hypothesis, we propose to determine whether p75 induces death of umbrella cells in the urothelium after SCI (Aim 1), which will be addressed by conditional p75 mice, in which p75 is selectively deleted in umbrella cells; to determine the role of p75 in proliferation of the urothelium after SCI (Aim 2), which will be addressed by analyzing mice after SCI in which p75 is deleted in urothelial progenitors; and to determine whether LM11A-31 improves bladder function after SCI (Aim 3), which will be addressed using novel optical mapping approaches, single-unit afferent nerve recording, permeability measurements and cystometrograms (CMG) with simultaneous urethral sphincter electromyograms (EMG). We believe the outcome of this study will change our current understanding on neurotrophin action in the bladder and energize the existing effort to target neurotrophin in improving bladder function in various urological conditions.
Loss of bladder control has been known to be mediated in part by neuroplasticity induced by NGF. As NGF is implicated in the pathophysiology of urological conditions, there has been a continuous effort to target NGF as a therapy. Neutralizing NGF action has led to mixed results, however, in part due to insufficient understanding of the role that NGF and its receptors, TrkA and p75, play in bladder biology. We found that p75 expression is regulated in the bladder by spinal cord injury in a dynamic fashion, and propose to investigate its role in the bladder using mouse genetics. Our study will be complemented with a small molecule inhibitor that targets p75 selectively, LM11A-31, which we have recently characterized to be non-toxic, non-pain inducing but effective in promoting motor coordination after SCI in mice. We believe the outcome of this study will change our current understanding on neurotrophin action in the bladder and energize the existing effort to target neurotrophins in improving bladder function in various urological conditions.
| Ryu, Jae Cheon; Tooke, Katharine; Malley, Susan E et al. (2018) Role of proNGF/p75 signaling in bladder dysfunction after spinal cord injury. J Clin Invest 128:1772-1786 |
