Spontaneous regeneration of the seriously injured sympathetic pathway projecting to the prostate over a long period in the dog

Nerve-tumor crosstalk in tumor microenvironment: From tumor initiation and progression to clinical implications

The autonomic nerve system (ANS) innervates organs and tissues throughout the body and maintains functional balance among various systems. Further investigations have shown that excessive activation of ANS not only causes disruption of homeostasis, but also may promote tumor formation. In addition, the dynamic interaction between nerve and tumor cells in the tumor microenvironment also regulate tumor progression. On the one hand, nerves are passively invaded by tumor cells, that is, perineural invasion (PNI). On the other hand, compared with normal tissues, tumor tissues are subject to more abundant innervation, and nerves can influence tumor progression through regulating tumor proliferation, metastasis and drug resistance. A large number of studies have shown that nerve-tumor crosstalk, including PNI and innervation, is closely related to the prognosis of patients, and contributes to the formation of cancer pain, which significantly deteriorates the quality of life for patients. These findings suggest that nerve-tumor crosstalk represents a potential target for anti-tumor therapies and the management of cancer pain in the future. In this review, we systematically describe the mechanism by which nerve-tumor crosstalk regulates tumorigenesis and progression.

Targeting tumor innervation: premises, promises, and challenges

A high intratumoral nerve density is correlated with poor survival, high metastasis, and high recurrence across multiple solid tumor types. Recent research has revealed that cancer cells release diverse neurotrophic factors and exosomes to promote tumor innervation, in addition, infiltrating nerves can also mediate multiple tumor biological processes via exosomes and neurotransmitters. In this review, through seminal studies establishing tumor innervation, we discuss the communication between peripheral nerves and tumor cells in the tumor microenvironment (TME), and revealed the nerve-tumor regulation mechanisms on oncogenic process, angiogenesis, lymphangiogenesis, and immunity. Finally, we discussed the promising directions of ‘old drugs newly used’ to target TME communication and clarified a new line to prevent tumor malignant capacity.

The Adrenergic Nerve Network in Cancer

The central and autonomic nervous systems interact and converge to build up an adrenergic nerve network capable of promoting cancer. While a local adrenergic sympathetic innervation in peripheral solid tumors influences cancer and stromal cell behavior, the brain can participate to the development of cancer through an intermixed dysregulation of the sympathoadrenal system, adrenergic neurons, and the hypothalamo-pituitary-adrenal axis. A deeper understanding of the adrenergic nerve circuitry within the brain and tumors and its interactions with the microenvironment should enable elucidation of original mechanisms of cancer and novel therapeutic strategies.

Progenitors from the central nervous system drive neurogenesis in cancer

Autonomic nerve fibres in the tumour microenvironment regulate cancer initiation and dissemination, but how nerves emerge in tumours is currently unknown. Here we show that neural progenitors from the central nervous system that express doublecortin (DCX⁺) infiltrate prostate tumours and metastases, in which they initiate neurogenesis. In mouse models of prostate cancer, oscillations of DCX⁺ neural progenitors in the subventricular zone-a neurogenic area of the central nervous system-are associated with disruption of the blood-brain barrier, and with the egress of DCX⁺ cells into the circulation. These cells then infiltrate and reside in the tumour, and can generate new adrenergic neurons. Selective genetic depletion of DCX⁺ cells inhibits the early phases of tumour development in our mouse models of prostate cancer, whereas transplantation of DCX⁺ neural progenitors promotes tumour growth and metastasis. In humans, the density of DCX⁺ neural progenitors is strongly associated with the aggressiveness and recurrence of prostate adenocarcinoma. These results reveal a unique crosstalk between the central nervous system and prostate tumours, and indicate neural targets for the treatment of cancer.

Nerve regeneration: might this be the only solution for functional problems of the urinary tract?

PURPOSE OF REVIEW

To assess the potential role of nerve regeneration in restoring urinary tract function, the rapidly developing and exciting area of central and peripheral nerve repair and regeneration is reviewed, with particular reference to papers in which animal models of nerve damage resulting in urogenital dysfunction have been used. The difficulties and potential of these techniques for therapeutic application to human subjects with functional problems of the urinary tract are discussed.

RECENT FINDINGS

Methods for encouraging regeneration of cut axons and directed growth in the inhibitory environment of the central nervous system are being extensively explored. The recent discovery of the potential of olfactory ensheathing cells has proved a significant advance. Olfactory ensheathing cells are a type of glial cell which can be harvested from the olfactory mucosa. Transplantation of these cells, in conjunction with a biodegradable synthetic nerve guide or conduit, has been shown to restore urinary tract function after spinal cord injury. Artificial, biodegradable conduits have also restored bladder and spermatic duct function after sympathetic nerve damage. Other adjuvants facilitating the process of axonal recovery include the use of neurotrophins to accelerate and guide the formation of new nerve-fibre growth.

SUMMARY

These revolutionary technologies may, in the future, provide a means of treating urinary tract dysfunction with some types of aetiology, including acute spinal cord injury, and injury to nerves following pelvic surgery. It is, however, less likely that these treatments will be used successfully in the near future in patients in which the neural damage is long term, or associated with death of post-ganglionic neurons.