Targeting sensory neurons to restore bone marrow regenerative potential in diabetes

Diabetes accounts for ~6% of general mortality, with 50% of diabetes-associated deaths being caused by cardiovascular and neuropathic complications. Moreover, diabetic patients recover much worst from a heart attacks or stroke compared to non-diabetic patients. It is now clear that a global deficit in reparative mechanisms plays a key role in such a poor outcome. Hence, interest is growing on new therapies that improve the spontaneous ability to regenerate tissues damaged by aging and disease. One important source of regenerative cells in the adulthood is the marrow contained in bones. These cells are regularly released into the circulation to replace other cells that have completed their lifecycle. This process is normally best appreciated for cells with high turnover, like red and white cells of the blood. However, also cells with low turnover, like vascular cells and cardiac cells, need to be replaced with new cells. The replacement of cardiovascular cells becomes an urgent need when an acute injury intervenes. The mechanisms regulating the activation and release of new cells from the marrow after an injury are poorly understood, but it is well acknowledged that they are defective in patients with diabetes. Since diabetes alters nerve function and perception of pain, we propose that this dysfunction can compromise the ability of bone marrow to sense the stress and respond adequately. We have collected preliminary data in support of this theory: 1. the bone marrow is highly innervated with neuronal fibres, 2. the nerve fibres release signals that are sensed by regenerative cells of the marrow via specific receptors expressed on their surface, 3. variations in the intensity of these signals push cells outside the marrow in the circulation, 4. likewise, after a heart attack, cells responsive to neuronal signalling are released to the circulation, 5. transplanting those specific cells directly into damaged tissues accelerate recovery, 6. those regenerative cells are depleted and insensitive to stress signals in diabetes. Now, we propose to extend our investigation through three pieces of work. First, we will characterize the anatomical distribution of different neuronal fibres in the marrow and of cells responsive to neuronal signals in models of diabetes, second we will investigate the defective transmission of the signalling from nerves to cells of the marrow, and third we will compare the effect of classical drugs that reduce glucose levels in blood with a specific therapy targeting the neuronal signalling. We will eventually investigate if the two approaches are complementary. If our hypothesis is proved true, there is a real possibility that new effective regenerative treatments become available to attenuate the damaging action of diabetes.

In diabetic patients, circulating progenitor cells (PC) are reduced and accumulate epigenetic changes that compromise their function. Our overarching hypothesis is that diabetes reduces the regenerative potential by affecting PC in the bone marrow (BM) niche. We showed that microangiopathy affects the BM of diabetic animals, creating a harsh microenvironment incompatible with PC homeostasis. We now propose that another common complication, e.g. sensory neuropathy, impinges on BM homeostasis and compromises the egression of regenerative PC upon ischaemia. Although it is known that diabetic patients suffer silent ischemia, due to altered heart innervation, the possibility that sensory neuropathy extends to BM was not investigated. Pilot data indicate that sensory fibres are present in BM together with PC that express neuropeptide receptors. Exposure to neuropeptides enhances PC motility and allows isolation of PC endowed of potent proangiogenic activity. Furthermore, this sub-fraction is enriched within the cell population mobilized after myocardial infarction in non-diabetic patients. Neuronal fibres are reduced in diabetic BM, this defect being associated with blunted response of marrow PC to neuropeptides and decreased nerve growth factor (NGF), which is essential for sensory nerve function/regeneration. This project aims to: 1.Characterize structural alterations underpinning the impaired neuronal control of PC mobilization, 2.Unravel the molecular mechanisms of this deficit and 3.Restore neurotrophic signalling by targeted delivery of NGF to neuronal cells thereby rescuing BM capacity to respond to noxious stimuli. We will also investigate if NGF adds to the protection afforded with metabolic control. Therefore, this proposal has the twofold ambition to clarify key mechanisms of diabetic complications and translate this knowledge into new therapies for devastating complications insufficiently controlled by current treatments.

The proposed study will identify new cellular and molecular mechanisms behind diabetes-associated complications. Furthermore, it will introduce novel treatments able to rescue the neuronal control of regenerative progenitor cell release, which is remarkably depressed in diabetic patients.
There are 2.8 million people diagnosed with diabetes in the UK and an estimated 850,000 people who have the condition without being aware of it. Fifty percent of deaths associated to diabetes are due to cardiovascular or neuropathic complications. There is an association between reduced numbers and dysfunction of circulating progenitor cells and severity of cardiovascular and neuropathic complications, which led to the proposal of progenitor cells being a potential biomarker of clinical outcome. The other way round, common diabetic complications, like microangiopathy and sensory neuropathy, could directly affect bone marrow homeostasis and dampen the capacity to respond to stress. Hence, this study will provide important insights into the global impact of sensory neuropathy on the maintenance of homeostasis, including regular hematopoiesis under normal conditions and activation of regenerative response under stress. Although the diabetic bone marrow might be able to afford physiologic functions, the induction of stress or tissue injury could reveal substantial defects in the activation and liberation processes due to dampened neuronal modulation. Addressing this question is timely and clinically relevant not only to ischaemic complications, but also to a large spectrum of human diseases, including hematopoietic disorders, chronic inflammation and aging-related diseases.
Treatment of diabetes and its complications is well-standardized; however recent trials show that common anti-diabetic agents and tight metabolic control may increase cardiovascular mortality. Moreover, a large proportion of diabetic patients manifest complications despite of adequate metabolic control. Therefore, novel treatments are eagerly awaited to complement the action of glucose-lowering drugs. Experimental studies showed that NGF is an ideal candidate for treatment of sensory neuropathy, but a recent clinical trial using systemic recombinant NGF has suspended the final decision, due the limited bioavailability following intermittent administration. Targeted delivery of human NGF gene by new viral vectors increases significantly bioavailability thereby enhancing the therapeutic impact. It should be pointed out that meaningful translation of preclinical results is often precluded by inadequate study design. In this respect, our study will consider the additional benefit of the new proposed approach over conventional treatments, an aspect that being neglected in preclinical studies leads to over-estimation of therapeutic advantage. Addressing the neurotrophic potential of NGF in regenerative medicine will further strengthen the excellence of our team in the field
The study provides an excellent opportunity for academic career development of involved researcher, Dr. Amadesi. Furthermore, liaison with Prof. Baker (Glasgow University) and Prof. Rankin (Imperial College, London) will implement our strategy to create permanent collaborative ties between the University of Bristol and other prestigious UK University.