Pathophysiology of Diabetic Erectile Dysfunction: Potential Contribution of Vasa Nervorum and Advanced Glycation Endproducts

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REVIEW Pathophysiology of diabetic erectile dysfunction: potential contribution of vasa nervorum and advanced glycation endproducts

S Cellek1, NE Cameron2, MA Cotter2 and A Muneer3

Erectile dysfunction (ED) due to diabetes mellitus remains difficult to treat medically despite advances in pharmacotherapeutic approaches in the field. This unmet need has resulted in a recent re-focus on the pathophysiology, in order to understand the cellular and molecular mechanisms leading to ED in diabetes. Diabetes-induced ED is often resistant to PDE5 inhibitor treatment, thus there is a need to discover targets that may lead to novel approaches for a successful treatment. The aim of this brief review is to update the reader in some of the latest development on that front, with a particular focus on the role of impaired neuronal blood flow and the formation of advanced glycation endproducts.

International Journal of Impotence Research (2013) 25, 1–6; doi:10.1038/ijir.2012.30; published online 23 August 2012 Keywords: advanced glycation endproducts; diabetes mellitus; erectile dysfunction; hypoxia; microvessel; nitric oxide; vasa nervorum

INTRODUCTION disease and is likely to precede a coronary event by 5 years.8,9 Penile erection is the result of relaxation of smooth muscle in the Combined with the prevalence of obesity and metabolic syndrome cavernous body and associated blood vessels.1 Smooth muscle in these men, it is apparent that disease progression is likely to relaxation is mediated primarily by nitric oxide (NO), which is a reduce the efficacy of conventional pharmacotherapeutic options gaseous and labile mediator, yet one of the most potent for the treatment of ED in diabetic men. This has meant that endogenous smooth muscle relaxants. NO is synthesised by diabetic men form a large group of patients undergoing end-stage neuronal NO synthase (nNOS) in the autonomic postganglionic treatment in the form of penile prosthesis surgery. parasympathetic nerves (nitrergic nerves)2,3 and by endothelial Our knowledge regarding the pathophysiology of ED in nitric oxide synthase in the endothelium lining the blood vessels diabetes has gradually increased since the 1990s. As NO is the and cavernosal sinusoids. Nerve impulses in response to sexual main erectile mediator, the field has focussed on how diabetes stimulus are carried from the spinal cord to the hypogastric plexus alters the nitrergic and endothelial NO function. The aim of this where the cell bodies of the nitrergic nerves are located. Once paper is to update the reader with some of the latest development activated, the nitrergic neurones within the hypogastric plexus on that front, with a particular focus on the role of impaired transmit action potential through their axons to the penile neuronal blood flow, and the formation of advanced glycation end vasculature. These nitrergic axons then release high quantities of products. NO on to the nearby smooth muscle cells. NO diffuses rapidly into the smooth muscle cells, causing relaxation by increasing the intracellular concentrations of cGMP. The relaxation of the A NEED FOR TEMPORAL ANALYSIS cavernosal and arterial smooth muscle results in an increase in Thetoneofpenilevascularandcavernosal smooth muscle is blood flow into the penis. In turn, this causes shear stress on the regulated by vasodilators such as NO and vasoactive intestinal pep- endothelial lining, which promotes phosphorylation and tide, and by vasoconstrictors such as noradrenaline and endothelins. prolonged activation of endothelial nitric oxide synthase leading An imbalance in favour of the vasoconstrictors has been demon- to long-lasting release of NO from the endothelium to maintain the strated in both type 1 and 2 diabetes.9,10 Such an imbalance seems smooth muscle relaxation.4 As the intracavernosal pressure reaches to be caused by several factors, all playing equal roles, such as the level of the systemic arterial blood pressure, the subtunical decreased NO production from either nitrergic nerves or endothe- venules are compressed, which results in a rigid erection. lium,10 low responsiveness of the smooth muscle to vasodilators,1 Diabetes mellitus is one of the predominant risk factors of upregulation of the receptors for vasoconstrictor mediators such as erectile dysfunction (ED) and also one of the most difficult to endothelin11 and increased vasoconstrictor production and/or treat.5,6 Approximately 50% of diabetic men will suffer from ED release and hyper-responsiveness to vasoconstrictors.12 within 10 years of the diagnosis.7 ED presenting in these men is It is important to study the progression of ED in a temporal widely regarded as a manifestation of more systemic vascular fashion in order to gain a better understanding of the

1Department of Translational Medicine, Cranfield Health, Cranfield University, Bedfordshire, UK; 2School of Medical Sciences, University of Aberdeen, Aberdeen, UK and 3Institute of Urology, University College London, London, UK. Correspondence: Dr S Cellek, Department of Translational Medicine, Cranfield Health, Cranfield University, Bedfordshire, MK43 0AL, UK. E-mail: s.cellek@cranfield.ac.uk Received 8 May 2012; revised 2 July 2012; accepted 20 July 2012; published online 23 August 2012 Vasa nervorum and AGEs in diabetic ED S Cellek et al 2 pathophysiology of the condition. Although such longitudinal large ones such as the sciatic nerve to small ones such as the studies investigating the mechanisms at different time points cavernous nerve, have their own vasa nervorum. Earlier studies of the disease are few,13 they provide invaluable information into have shown that vasa nervorum of the peripheral nerve trunks can our understanding of diabetic ED. Similar longitudinal studies be divided into three groups: epineurial, perineurial and from other fields within or outside the diabetes field14 also have endoneurial.35 Epineurial and perineurial blood vessels form a been extremely useful in broadening our comprehension of complex network known as the epineurial plexus.35 The epineurial pathophysiological events. plexus has prominent arteriovenous shunts, supplies the endoneurial vascular compartment and is innervated by autonomic nerves such as sympathetic and peptidergic nerves.36 THE STORY BEGINS WITH HYPERGLYCAEMIA However, such detailed information is not available for the vasa At the early stages of diabetes, before the patient has symptoms of nervorum of the autonomic ganglia, although ganglia have a any complications such as ED and may not even be aware of the much greater metabolic demand than peripheral nerve trunk.35 underlying diagnosis and high blood glucose levels, there are two Whether the vasa nervorum of autonomic ganglia can be divided very important factors that trigger the cascade of events: high into sub-compartments like in the peripheral nerve trunk is not blood glucose and low insulin (low insulin concentration in Type 1 known. Moreover, details on the innervation of ganglion vasa diabetes and low insulin action due to receptor resistance in nervorum are sparse. We have recently shown that the rat major Type 2). The effect of high glucose on endothelial NO-dependent pelvic ganglia are surrounded by numerous small diameter blood vasodilation has been debated: using isolated human blood vessels, vessels (20-120 mm), which are innervated by noradrenergic and high glucose concentrations have been shown to have varying nitrergic nerve fibres.37 A drop in blood flow by as much as 50% is effects on endothelial NO-dependent relaxations depending on the observed in the autonomic ganglia as early as 1 week after type of the vascular bed.15,16 In humans, early reports have shown diabetes induction,32,33 although such an early decrease in blood that elevation of blood glucose concentrations does not affect the flow is not associated with ED initially. Thus drawing parallels with forearm blood flow.16,17 Later, it has been found that the reason for vasa nervorum in nerve trunks, it is likely that such a reduction in glucose-induced vasodilatation could be because of increased perfusion would cause ganglionic neurons to be exposed to a insulin levels as a direct response to glucose; insulin is known hypoxic environment. to elicit vasodilation directly.17 When such studies were repeated using agents like octreotide to inhibit insulin secretion, hence removing the influence of insulin on vasodilation, high glucose CONSEQUENCES OF HYPOXIA IN AUTONOMIC GANGLIA AND concentrations caused an acute and transient reduction in AXONS endothelium-dependent vasodilation.18–22 This has been attribu- Hypoxia can trigger several neuronal events both in the cell body ted to increased vasoconstrictor mediators22 and decreased and axon of the neuron: disturbances in membrane conductivity, endothelial NO production23 as a direct response to hyper- impaired nerve conduction and action potential generation, and glycaemia. Therefore we can deduce that acute and direct effect decreased axonal transport. It is therefore not surprising that of hyperglycaemia could be a transient reduction in blood flow in a nitrergic function in the isolated human corpus cavernosum healthy individual. In diabetic patients or individuals with impaired deteriorates in hypoxic conditions.38 However it should be noted glucose tolerance, the recovery from reduced vasodilation has been that the corpus cavernosum does not contain the neuronal cell shown to be delayed.20,24 bodies of the nitrergic nerves. To date, the effect of hypoxia on Although high glucose concentrations impair nitrergic nitrergic neurons obtained either from human hypogastric plexus function acutely and reversibly in vitro,24,25 ED occurs much later or rat major pelvic ganglia has not been studied. In the absence of in diabetic animal models.5,13,26–28 These observations have led the direct evidence, inferences can be made from studies in other researchers to conclude that endothelial dysfunction precedes parts of the peripheral and in the central nervous systems. Acute nitrergic dysfunction. There seems to be a lag period between hypoxia or ischaemia alters the expression of endothelial nitric endothelial and ED; the latter coinciding with nitrergic dysfunction. oxide synthase and nNOS in the blood vessels and central In perspective, it takes a minimum of 4 weeks for a rat to develop neurons, respectively.39–45 In the nodose ganglion, which primarily erectile and nitrergic dysfunction,13,27,28 whereas endothelial holds sensory neurons, acute hypoxia increases nNOS protein dysfunction can potentially develop in days as a direct expression 2-4 folds.45 Upregulation of nNOS in the central consequence of hyperglycaemia, as mentioned above. So one neurons is thought to be a compensatory mechanism to replenish question arising is: ‘What pathophysiological role could endothelial the blood flow by vasodilation and neovascularization. Whether a dysfunction play in the early stages of the development of ED?’. similar phenomenon occurs in the autonomic ganglia remains to One potential answer comes from another area of the diabetes be investigated. field, the study of blood flow to peripheral nerves. Within days of Although acute hypoxia may increase nNOS expression transi- induction of experimental diabetes, blood flow to major nerve ently, nitrergic function deteriorates as the hypoxia is prolonged. In trunks such as sciatic nerve has dropped.13,27,29–31 Recently such a chronic hypoxia models where animals were exposed to hypoxia dramatic decrease in blood flow has also been shown in the for extended periods, a significant decrease in nNOS expression superior cervical32 and in the major pelvic ganglia of diabetic rats.33 and NO production has been shown for nerves innervating the The latter is equivalent to hypogastric plexus in the human34 and cerebral arteries46 and the central neurons.47 Interestingly chronic contains the cell bodies of the neurons that innervate the bladder outlet obstruction decreases nNOS expression in the urogenital organs, including penis. Thus it is plausible that an intramural neurons of the bladder, which has been attributed to the early decrease in blood flow to the ganglia in response to the chronic hypoxia caused by the urethral ligation.48,49 Although diabetic state, including elevated glucose, may be the first step sympathetic neurons have received greater attention than nitrergic triggering a series of events. neurons within the ganglia, existing studies suggest that chronic hypoxia causes a nitrergic–sympathetic imbalance in favour of sympathetic system, which may contribute to conditions associated THE ROLE OF THE VASA NERVORUM with chronic hypoxia such as hypertension and chronic respiratory The ganglia where the cell bodies of the autonomic nerves are failure.49 located are surrounded by small-diameter blood vessels (arterioles Acute and chronic hypoxia are associated with reduced nerve and venules) known as vasa nervorum, which supply the blood conduction velocity and neuronal action potential generation.50 necessary for the function of the neurons. All nerve trunks, from In the central neurons, the reduction of nerve cell excitability

International Journal of Impotence Research (2013), 1 – 6 & 2013 Macmillan Publishers Limited Vasa nervorum and AGEs in diabetic ED S Cellek et al 3 reversible phase irreversible phase

DIABETES Axonal hyperglycemia transport dyslipidemia nNOS defect accumulation in cell body Irreversible Vasoconstriction Hypoxia Nitrergic erectile in vasa nervorum in ganglia Nerve degeneration nNOS dysfunction conduction depletion at defect nerve terminals Endothelial NO dysfunction in production the penis at nerve Point of terminals no return

Endothelial NO production in penis

ED symptoms

AGEs

TIME

Figure 1. The schematic representation of temporal effect of microvascular deficit and advanced glycation endproducts (AGEs) on erectile function with particular emphasis on nitrergic nerves. in hypoxia is primarily because of increased K þ -conductance. normoglycaemia with insulin.56,57 A somewhat similar Thus, the nerve cells are hyper-polarised and the input resistance phenomenon can be observed in patients who were newly reduced, causing higher threshold and reduction of synaptic diagnosed with Type 1 or 2 diabetes.58–61 Autonomic nerves potentials.51,52 In the peripheral nerves, hypoxia elicited follow a similar pattern: nitrergic dysfunction, nNOS depletion in reductions in both sensory and motor conduction velocities the axons and accumulation in the ganglia can be reversed using similar to those observed in diabetic animals.53 insulin within the first 10 weeks of diabetes in rats, whereas there Another consequence of hypoxia on the central and peripheral is no loss of neurons.13,14 We have previously coined the term nerves is a reduction in axonal (axoplasmic) transport. The ‘reversible phase’ followed by ‘a point of NO return’ (Figure 1).26 neurons transport the proteins, lipids, vesicles and organelles to Thus, it is important to diagnose and start treatment within the distal parts of their axons via axonal transport. Considering that an first few years of human diabetes. The two largest trials in the axon can be as long as 1-2 m (for example, in the sciatic nerve diabetes field: The United Kingdom Prospective Diabetes Study trunk) compared with its neuronal body (B10 mm), an efficient (UKPDS62) and the Diabetes Control and Complications Trial two-way transport system akin to a very busy highway is required (DCCT63) have answered the question of whether blood glucose to meet the metabolic and functional needs of a neuron. control is beneficial for people with diabetes with respect to the Disruption of axonal transport can result in acute accumulation development of complications. It definitely is. Although improved of proteins in the cell body, mitochondrial dysfunction in the distal glycaemic control reduces the incidence of diabetic compli- parts of the nerve and loss of neuronal function which can cations,62,63 physiological levels of control are difficult to achieve. eventually lead to axonal and/or neuronal degeneration. Diabetes In addition, as diabetes duration increases, complications develop is known to alter axonal transport.54 Rats exposed to hypoxia for and confounders such as chronic hypertension and dyslipidaemia up to 24 h have increased nNOS protein in the peripheral contribute to the complications, thus lessening the beneficial ganglia.39 Interestingly, patients with chronic mountain sickness impact of tight glycaemic control.64 who suffer from ‘‘burning feet and hands’’ sensation at high altitudes (low oxygen) exhibited mild sensory neuropathy which was attributed to the disruption of axonal transport.55 We have HOW DO THE REVERSIBLE CHANGES BECOME IRREVERSIBLE? previously shown that nNOS protein is depleted in the axons and Data suggest that this could be because of several factors: accumulates in the autonomic ganglia of diabetic rats.13,14 Primarily, the ongoing impact of hypoxia and nerve dysfunction; Overall it can be hypothesised that, hypoxia secondary to secondarily, owing to the accumulation of advanced glycation diabetes-induced dysfunction of vasa nervorum supplying the endproducts (AGEs). AGEs are adducts formed on proteins and nerve trunks and ganglia alters neuronal electrophysiology and lipids following non-enzymatic glycation and oxidisation after axonal transport (Figure 1). Such effects can happen acutely and exposure to aldose sugars and their metabolites. Similar adducts are often reversible as noted in both animal and human studies may be formed by lipid-oxidation products. AGEs are formed mentioned above. For example, patients who return to lower naturally in human body, and they are also ingested in food as a altitude after a trip in the mountains report improvement in their result of cooking processes. They accumulate with age on long- ‘burning feet and hands’ symptoms within weeks.55 lived proteins such as collagen. Diabetes is considered an accelerated form of aging65 because of this elevated AGE accumulation.66 In addition, it is becoming increasingly apparent CONSEQUENCES OF NERVE DYSFUNCTION AND AXONAL that relatively rapid intracellular AGE formation, primarily as a TRANSPORT DEFECT result of increased methylglyoxal levels resulting from triose Reduced conduction velocity develops in motor and sensory metabolism, has marked effects in the pathophysiology of diabetic nerves within 2–3 weeks of diabetes induction in rats.29 During complications, including endothelial dysfunction.67–71 this phase, there are no obvious structural alterations to the AGEs are implicated in the pathophysiology of both micro- and nerves and the deficit is largely reversible on the establishment of macrovascular complications.72 They can induce oxidative stress in

& 2013 Macmillan Publishers Limited International Journal of Impotence Research (2013), 1 – 6 Vasa nervorum and AGEs in diabetic ED S Cellek et al 4 the cells either by binding to specific receptors (for example, CONCLUSIONS 73 receptor for AGEs), or as a result of their chemical reactions. It is highly plausible that a microvascular deficit in the vasa AGEs have been shown to reduce endothelium-dependent nervorum of nerve trunks and ganglia is a major trigger for a vasorelaxation by altering the endothelial NO synthase cascade of events that eventually lead to diabetic neuropathy and 74,75 synthesis and have been associated with impaired autonomic neuropathy. ED is a consequence of these events and if endothelium-dependent vasodilation in Type 2 diabetic not treated early may become irreversible. Restoring diminished 76 77,78 patients. AGEs accumulate both in the penis and major blood flow to the vasa nervorum as early as possible before 13,26,79 pelvic ganglia during diabetes. Exposure of human irreversible changes such as fibrosis and neuronal degeneration neuroblastoma cells, which are differentiated into cholinergic- occur should be a key aim of medical management of diabetic nitrergic phenotype to AGEs, induces apoptosis in a NO- neuropathy. Clearly, we still have several challenges to meet: 79 dependent manner. It has also been demonstrated that improving our understanding how the vasa nervorum tone is receptor for AGEs expression is enhanced by ischaemia in the regulated by endothelial and neuronal mediators, and the 80 81 heart and brain. Although the effect of AGEs in combination development of non-invasive blood flow measurement techni- with hypoxia have not been studied specifically in nitrergic ques in human vasa nervorum to further test the validity of this neurons, it is plausible that under hypoxic conditions the hypothesis. detrimental effect of AGEs may be enhanced. Thus, hypoxia, owing to microvascular defects in the ganglia, combined with the cytotoxic effects of accumulating AGEs might be specifically CONFLICT OF INTEREST detrimental to nitrergic neurons. The death of nitrergic neurons would result in irreversible decrease in neuronal numbers in the The authors declare no conflict of interest. ganglia as those neurons are not able to replicate themselves or regenerate. Such a loss of neurons would inevitably push the pathology into the irreversible phase (Figure 1). ACKNOWLEDGEMENTS Studies with inhibitors of AGE formation, such as the reactive The authors wish to thank the European Society for Sexual Medicine (ESSM) for carbonyl scavenger, aminoguanidine, have shown several bene- continuing support. ficial effects in experimental diabetes. 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