Integrative erectile biology. The effects of age and disease on gap junctions and ion channels and their potential value to the treatment of erectile dysfunction

Gap Junctions and Ageing

Gap junctions, comprising connexin proteins, create conduits directly coupling the cytoplasms of adjacent cells. Expressed in essentially all tissues, dynamic gap junction structures enable the exchange of small molecules including ions and second messengers, and are central to maintenance of homeostasis and synchronized excitability. With such diverse and critical roles throughout the body, it is unsurprising that alterations to gap junction and/or connexin expression and function underlie a broad array of age-related pathologies. From neurological dysfunction to cardiac arrhythmia and bone loss, it is hard to identify a human disease state that does not involve reduced, or in some cases inappropriate, intercellular communication to affect organ function. With a complex life cycle encompassing several key regulatory steps, pathological gap junction remodeling during ageing can arise from alterations in gene expression, translation, intracellular trafficking, and posttranslational modification of connexins. Connexin proteins are now known to "moonlight" and perform a variety of non-junctional functions in the cell, independent of gap junctions. Furthermore, connexin "hemichannels" on the cell surface can communicate with the extracellular space without ever coupling to an adjacent cell to form a gap junction channel. This chapter will focus primarily on gap junctions in ageing, but such non-junctional connexin functions will be referred to where appropriate and the full spectrum of connexin biology should be noted as potentially causative/contributing to some findings in connexin knockout animals, for example.

PDGF-mediated PI3K/AKT/β-catenin signaling regulates gap junctions in corpus cavernosum smooth muscle cells

Erectile dysfunction (ED) is the most common sexual disorder that men report to healthcare providers. Gap junctions (GJs) are thought to be responsible for synchronous shrinkage of corpus cavernosum smooth muscle cells (CCSMCs), and play thus an important role in the maintenance of an erection. Hypoxia has been suggested as a pathological mechanism underlying ED. Here we demonstrate that hypoxia increased the expression of platelet-derived growth factor (PDGF) and the main GJ component connexin (Cx)43 in CCSMCs. Inhibiting PDGF receptor (PDGFR) activity decreased Cx43 expression. Treatment with different concentrations of PDGF increased the levels of phosphorylated protein kinase B (AKT), β-catenin, and Cx43, whereas inhibition of PDGFR or activation of phosphatidylinositol 3 kinase (PI3K)/AKT signaling altered β-catenin and Cx43 expression. Meanwhile, silencing β-catenin resulted in the downregulation of Cx43. These results demonstrate that PDGF secretion by CCSMCs and vascular endothelial cells is enhanced under hypoxic conditions, leading to increased Cx43 expression through PI3K/AKT/β-catenin signaling and ultimately affecting GJ function in ED. Thus, targeting this pathway is a potential therapeutic strategy for the treatment of ED.

Animal models of erectile dysfunction

Erectile dysfunction (ED) is a prevalent male sexual dysfunction with profound adverse effects on the physical and the psychosocial health of men and, subsequently, on their partners. The expanded use of various types of rodent models has produced some advances in the study of ED, and neurophysiological studies using various animal models have provided important insights into human sexual dysfunction. At present, animal models play a key role in exploring and screening novel drugs designed to treat ED.

Inhibition of histone deacetylase (HDAC) by 4-phenylbutyrate results in increased junctional conductance between rat corpora smooth muscle cells

4-phenylbutyrate (4-PB) has been shown to increase the protein content in a number of cells types. One such protein is Connexin43 (Cx43). We show here that 4-phenylbutyrate exposure results in significantly elevated cell to cell coupling, as determined by dual whole cell patch clamp. Incubation with 5 mM 4PB for 24 h or more nearly doubles junctional conductance. Interestingly, mRNA levels for Cx43 declined with exposure to 4-PB while western blot analysis revealed not significant change in protein levels. These data are most consistent with stabilization of the existing Cx43 pool or alterations in the number of functional channels within an existing pool of active and silent channels. These data represent a baseline for testing the efficacy of increased connexin mediated coupling in a variety of multicellular functions including erectile function.

Effect of icarisid II on diabetic rats with erectile dysfunction and its potential mechanism via assessment of AGEs, autophagy, mTOR and the NO-cGMP pathway

Erectile dysfunction (ED) is a major complication of diabetes mellitus. Icariin has been shown to enhance erectile function through its bioactive form, icarisid II. This study investigates the effects of icarisid II on diabetic rats with ED and its potential mechanism via the assessment of advanced glycosylation end products (AGEs), autophagy, mTOR and the NO-cGMP pathway. Icarisid II was extracted from icariin by an enzymatic method. In the control and diabetic ED groups, rats were administered normal saline; in the icarisid II group, rats were administered icarisid II intragastrically. Erectile function was evaluated by measuring intracavernosal pressure/mean arterial pressure (ICP/MAP). AGE concentrations, nitric oxide synthase (NOS) activity and cGMP concentration were assessed by enzyme immunoassay. Cell proliferation was analysed using methyl thiazolyl tetrazolium assay and flow cytometry. Autophagosomes were observed by transmission electron microscopy, monodansylcadaverine staining and GFP-LC3 localisation. The expression of NOS isoforms and key proteins in autophagy were examined by western blot. Our results have shown that Icarisid II increased ICP/MAP values, the smooth muscle cell (SMC) growth curve, S phase and SMC/collagen fibril (SMC/CF) proportions and decreased Beclin 1 (P<0.05). Icarisid II significantly increased the proliferative index and p-p70S6K(Thr389) levels and decreased the numbers of autophagosomes and the levels of LC3-II (P<0.01). Icarisid II decreased AGE concentrations and increased cGMP concentration, NOS activity (P<0.05) and cNOS levels (P<0.01) in the diabetic ED group. Therefore, Icarisid II constitutes a promising compound for diabetic ED and might be involved in the upregulation of SMC proliferation and the NO-cGMP pathway and the downregulation of AGEs, autophagy and the mTOR pathway.

Emerging gene and stem cell therapies for the treatment of erectile dysfunction

Erectile dysfunction is a prevalent condition that leads to significant morbidity and distress, not just for affected men but also for their partners. Very few currently available treatments ameliorate the underlying causes of the disorder and 'cure' the disease state. Much recent effort has been focused on the development of gene and cell-based approaches to rectify the molecular and tissue defects responsible for ED. Gene therapy has been investigated in animal models as a means to restore normal function to the penis; at this time, however, only one human trial has been published in the peer-reviewed literature. Recent gene therapy studies have focused on the modulation of enzymes associated with the NOS/cGMP pathway, and supplementation of trophic factors, peptides and potassium channels. Stem cell therapy has been a topic of interest in more recent years but there are currently very few published reports in animal models and none in human men. Although stem cell therapy offers the potential for restoration of functional tissues, legitimate concerns remain regarding the long-term fate of stem cells. The long-term safety of both gene and stem cell therapy must be thoroughly investigated before large-scale human studies can be considered.

New Insights into Hyperglycemia-induced Molecular Changes in Microvascular Cells

Hyperglycemia is the most prevalent characteristic of diabetes and plays a central role in mediating adverse effects on vascular cells during the progression of diabetic vascular complications. In diabetic microangiopathy, hyperglycemia induces biochemical and molecular changes in microvascular cells that ultimately progress to retinal, renal, and neural complications and extends to other complications, including advanced periodontal disease. In this review, we describe changes involving basement membrane thickening, tissue remodeling, gap junctions, inflammation, cytokines, and transcription factors, and their effects on the pathogenesis of diabetic microvascular complications. The majority of the changes described relate to retinal microangiopathy, since ultrastructural, structural, and biochemical alterations have been well-characterized in this tissue.

Age-related morphological changes in smooth muscle and collagen content in human corpus cavernosum

INTRODUCTION

Aging process has been related to erectile dysfunction (ED) possibly due to morphological changes in corpus cavernosum among many other causes.

AIM

To evaluate smooth muscle and collagen content in human corpus cavernosum and to correlate it to age.

METHODS

Cadaveric human cavernosal tissue was collected during the period of 1 year. Morphological analysis of a whole corpus cavernosum was performed in tissue sections stained with Masson's trichromic method to differentiate smooth muscle (red) from collagen (blue) content.

MAIN OUTCOME MEASURES

Analysis was performed with specialized micrographs image analysis software. Pearson's correlation test was used to establish correlation between corpus cavernosum morphology (smooth muscle and collagen content) and age.

RESULTS

A total sample of 89 tissues from different male cadavers were analyzed. The average age of the sample was 49.2 ± 19.1 years, with a range between 14 and 90 years. There was a statistically significant inverse correlation between age and the percentage of smooth muscle content (P = 0.012), direct correlation between age and percentage of collagen content (P = 0.019), and inverse correlation between age and the ratio of smooth muscle : collagen content (P = 0.007).

CONCLUSIONS

Age-related morphological changes in terms of smooth muscle and collagen content are observed in human corpus cavernosum as a possible contributing factor to the development of ED.

Hypercontractility and impaired sildenafil relaxations in the BKCa channel deletion model of erectile dysfunction

Erectile dysfunction (ED) can be elicited by a variety of pathogenic factors, particularly impaired formation of and responsiveness to nitric oxide (NO) and the downstream effectors soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase I (PKGI). One important target of PKGI in smooth muscle is the large-conductance, Ca2+ -activated potassium (BKCa) channel. In our previous report (42), we demonstrated that deletion of the BKCa channel in mice induced force oscillations and led to reduced nerve-evoked relaxations and ED. In the current study, we used this ED model to explore the role of the BKCa channel in the NO/sGC/PKGI pathway. Electrical field stimulation (EFS)-induced contractions of corpus cavernosum smooth muscle strips were significantly enhanced in the absence of BKCa channel function. In strips precontracted with phenylephrine, EFS-induced relaxations were converted to contractions by inhibition of sGC, and this was further enhanced by loss of BK channel function. Sildenafil-induced relaxations were decreased to a similar extent by inhibition of sGC or BKCa channels. At concentrations >1 microM, sildenafil caused relaxations independent of inhibition of sGC or BKCa channels. Sildenafil did not affect the enhanced force oscillations that were induced by the loss of BKCa channel function. Yet, these oscillations could be completely eliminated by blocking L-type voltage-dependent Ca2+ channels (VDCCs). These results suggest that therapeutically relevant concentrations of sildenafil act through cGMP and BKCa channels, and loss of BKCa channel function leads to hypercontractility, which depends on VDCCs and cannot be modified by the cGMP pathway.

Gene therapy for male erectile dysfunction

The potential clinical advantages of a gene transfer therapy-based approach to treatment of genitourinary smooth muscle-based disorders are several: potential single therapy for restoration of normal bladder or erectile function; eliminating the need for daily medication; use in combination with other therapies to reduce dose requirements and side effects; and development of mechanism-based, patient-specific treatment approaches. With the safe administration of hMax-K to men with erectile dysfunction in the first human phase 1 trial and the initiation of the phase 1 trial of hMaxi-K for patients who have detrusor overactivity, we have entered an exciting new era in the development of safe enduring therapies for genitourinary disorders.

hMaxi-K Gene Transfer in Males with Erectile Dysfunction: Results of the First Human Trial

Eleven patients with moderate to severe erectile dysfunction (ED) were given a single-dose corpus cavernosum injection of hMaxi-K, a "naked" DNA plasmid carrying the human cDNA encoding hSlo (for human slow-poke), the gene for the alpha, or pore-forming, subunit of the human smooth muscle Maxi-K channel. Three patients each were given 500, 1000, and 5000 microg, and two patients were given 7500 microg, of hMaxi-K and monitored for 24 weeks. The primary objectives of this phase I study were safety and tolerability of escalating hMaxi-K doses as assessed by clinical evaluations and laboratory tests. Secondary efficacy objectives were measured primarily by use of the International Index of Erectile Function (IIEF) scale. Patient responses were validated by partner responses. There were no serious adverse events and no dose-related adverse events attributed to gene transfer for any patient at any dose or study visit. No clinically significant changes from baseline were seen in physical evaluations (general and genitourinary), hematology, chemistry, and hormone analyses, or in cardiac events evaluated by repeated electrocardiograms. Importantly, no plasmid was detected in the semen of patients at any time after the injections. Patients given the two highest doses of hMaxi-K had apparent sustained improvements in erectile function (EF) as indicated by improved IIEF-EF domain scores over the length of the study. One patient given 5000 microg and one given 7500 microg reported EF category improvements that were highly clinically significant and were also maintained through the 24 weeks of study. Efficacy conclusions cannot be drawn from results of a phase I trial with no control group. However, the promising primary safety outcomes of the study and preliminary indications of effectiveness provide evidence that hMaxi-K gene transfer is a viable approach to the treatment of ED and that further studies investigating the efficacy of hMaxi-K in patients with ED should be performed.

Diabetes-induced changes in the alternative splicing of the slo gene in corporal tissue

OBJECTIVES

Erectile dysfunction is a common diabetic complication. Preclinical studies have documented that the Slo gene (encoding the BK or Maxi-K channel alpha-subunit) plays a critical role in erectile function. Therefore, we determined whether diabetes induces changes in the splicing of the Slo gene relevant to erectile function.

METHODS

Reverse transcriptase-polymerase chain reaction was used to compare Slo splice variant expression in corporal tissue excised from control and streptozotocin (STZ)-induced diabetic Fischer F-344 rats. Splice variants were sequenced, characterized by patch clamping, and fused to green fluorescent protein to determine cellular localization. The impact of altered Slo expression on erectile function was further evaluated in vivo.

RESULTS

A novel Slo splice variant (SVcyt, with a cytoplasmic location) was predominantly expressed in corporal tissue from control rats. STZ-diabetes caused upregulation of a channel-forming transcript SV0. Preliminary results suggest that SV0 was also more prevalent in the corporal tissue of human diabetic compared with nondiabetic patients. The change in isoform expression in STZ-treated rats was partially reversed by insulin treatment. Intracorporal injection of a plasmid expressing the SV0 transcript, but not SVcyt, restored erectile function in STZ-diabetic rats.

CONCLUSIONS

Alternative splicing of the Slo transcript may represent an important compensatory mechanism to increase the ease with which relaxation of corporal tissue may be triggered as a result of a diabetes-related decline in erectile capacity.

Overexpression of arginase in the aged mouse penis impairs erectile function and decreases eNOS activity: influence of in vivo gene therapy of anti-arginase

Since both increased nitric oxide (NO) synthase (NOS) abundance and diminished NO signaling have been reported in the aging penis, the role of NO in the adaptations of aging remains controversial. Here we tested the hypothesis that arginase, an enzyme that competes with NOS for the substrate l-arginine, contributes to erectile dysfunction with advanced age in the B6/129 mouse strain. Arginase protein abundance, mRNA expression, and enzyme activity were elevated in aged compared with young penile endothelial cells. In addition, endothelial NOS (NOS3) protein abundance was greater in aged versus young penile endothelial cells, whereas NOS activity and cGMP levels were reduced. Calcium-dependent l-arginine-to-l-citrulline conversion and cGMP formation increased significantly in aged mouse penes in the presence of the arginase inhibitor 2(S)-amino-6-boronohexanoic acid (ABH). However, there was no effect on l-arginine-to-l-citrulline conversion or cGMP accumulation in the endothelium from young mouse penes. To assess the functional role of arginase in the inhibition of NOS pathway responsiveness in the penis, we evaluated the effects of ABH and an adeno-associated virus encoding an antisense sequence to arginase I (AAVanti-arginase) on erectile function in vivo. ABH and AAVanti-arginase enhanced endothelium-dependent erectile responses in the aged mice without altering endothelium-independent responses. Paralleling our in vitro observations, ABH or AAVanti-arginase did not affect vascular responses in the young mice. Inhibition of the arginase pathway improves endothelial function in the aging penile circulation, suggesting that the arginase pathway may be exploited to improve erectile dysfunction associated with aging.

Effects of streptozotocin-induced diabetes on bladder and erectile (dys)function in the same rat in vivo

OBJECTIVE

To establish the methods, feasibility and utility of evaluating the impact of diabetes on bladder and erectile function in the same rat, as more than half of diabetic patients have bladder dysfunction, and half of diabetic men have erectile dysfunction, but the severity of coincident disease has not been rigorously assessed.

MATERIALS AND METHODS

In all, 16 F-344 rats had diabetes induced by streptozotocin (STZ), and were divided into insulin-treated (five) and untreated (11), and compared with age-matched controls (10), all assessed in parallel. All STZ rats were diabetic for 8-11 weeks. Cystometric studies were conducted on all rats, with cavernosometric studies conducted on a subset of rats.

RESULTS

There were insulin-reversible increases in the following cystometric variables; bladder weight, bladder capacity, micturition volume, residual volume, micturition pressure and spontaneous activity (P < 0.05, in all, one-way analysis of variance, anova). Cavernosometry showed a diabetes-related, insulin-reversible decline in the cavernosal nerve-stimulated intracavernosal pressure (ICP) response at all levels of current stimulation (P < 0.05, in all one-way anova). Plotting erectile capacity (i.e. ICP) against bladder capacity showed no correlation between the extent of the decline in erectile capacity and the magnitude of the increase in bladder capacity.

CONCLUSIONS

These studies extend previous work to indicate that the extent of diabetes-related bladder and erectile dysfunction can vary in the same rat. As such, these findings highlight the importance of evaluating the impact of diabetes on multiple organ systems in the lower urinary tract. Future studies using this model system should lead to a better understanding of the initiation, development, progression and coincidence of these common diabetic complications.

Connexin43: emerging role in erectile function

Connexins, that have their main function as part of gap junction channels, are proteins expressed in a large number of tissues such as endocrine, nervous, vascular, and muscular tissues. Gap junctions are implicated in tissue homeostasis and control of cell proliferation and differentiation. Interestingly, mutations of connexin genes have been reported in several human diseases (peripheral neuropathies, cardiovascular and dermatological diseases, hereditary cataract, and deafness) and altered expression of connexins have been associated with tumoral progression. Today, several lines of study argue for a critical role of gap junctions in corporal smooth muscle relaxation and erectile response. The present review highlights the emerging role of connexin43, one of these membranous proteins, in the physiology and physiopathology of human erectile function and its possible medical application.

Experimental diabetes alters connexin43 derived gap junction permeability in short-term cultures of rat corporeal vascular smooth muscle cells

PURPOSE

Intercellular communication through gap junctions was assessed in 8 to 10-week STZ diabetic rats to evaluate diabetes related effects on gap junctional conductance and permeability in short-term cultures of corporeal myocytes.

MATERIALS AND METHODS

Rats were made diabetic by a single intraperitoneal injection of STZ. Eight to 10 weeks later erectile function was evaluated in vivo and corporeal tissue was harvested to isolate corporeal myocytes. Dual whole cell patch clamp studies of intercellular communication through connexin43 (Cx43) derived gap junction channels were done in short-term, ie passages 0 to 2, cultured corporeal myocytes excised from STZ diabetic rats with documented erectile impairment as well as in myocytes from age matched control rats.

RESULTS

No differences in macroscopic junctional conductance, single channel conductance or open probability were detected between myocytes from age matched control and STZ diabetic rats, confirming the lack of diabetes related alterations in Cx43 gating or conductance. However, fluorescence dye transfer experiments revealed a marked 3-fold increase in Cx43 mediated junctional permeability in the absence of any detectable change in Cx43 protein expression.

CONCLUSIONS

These data suggest that an alteration in the selectivity filter of Cx43 in diabetic animals affects the permeability of specifically sized and charged solutes. To our knowledge these studies provide the first evidence of a diabetes related increase in intercellular permselectivity in corporeal myocytes and, thus, they may have important implications for diabetes related erectile dysfunction.

The first human trial for gene transfer therapy for the treatment of erectile dysfunction: preliminary results

OBJECTIVE

To test the safety of a single intracavernous injection of a plasmid vector (hMaxi-K) that expresses the hSlo gene, that encodes the alpha-subunit of the Maxi-K channel, for the treatment of erectile dysfunction (ED).

METHODS

Six men, thus far have fulfilled the entry criteria of the protocol and had gene transfer with hMaxi-K. Three received a dose of 500 microg and three received a dose 1000 microg of the gene product, injected intracavernously as naked DNA. Dosing at 5000 microg and higher is planned.

RESULTS

The primary end point of the phase I trial is safety. No drug-related adverse events or significant laboratory changes have occurred after the gene transfer. Moreover, there is no evidence of the gene in semen at the one copy per mug total DNA in any of the participants.

CONCLUSION

Preliminary results indicate that, in a single dose escalation study, ion channel gene transfer with hMaxi-K can be administered safely to men with ED without adverse events.

Gene transfer for the therapy of erectile dysfunction: progress in the 21st century

Gene transfer represents the next potential era of advancement in medicine for the prevention of the effects of aging or for treatment of genetic or acquired disease. For gene transfer to be a practical successor to today's oral and minimally invasive therapies, the product must have a high safety profile and a long duration of effectiveness to correct the need for on-demand administration. Several types of vectors have been used in preclinicals studies, but because of widely publicized adverse events, progress using viral vectors in humans has been limited. There is a current phase I human trial using naked DNA as the vector with the maxi-K gene to modify cellular contractility. Preliminary results in the safety trial thus far have shown no treatment-related adverse events, no transfer to the semen, and the possibility of efficacy in one participant.

Impact of aging on penile hemodynamics in men responding normally to prostaglandin injection: A power Doppler study

AIM

Hemodynamic changes with aging in the penile arterial circulation, including the helicine arteries, were evaluated with power Doppler imaging in erectile dysfunction patients with a normal response to prostaglandin injection.

METHODS

In 36 patients with erectile dysfunction and no definite vascular risk factors, 72 corpora were examined using power Doppler imaging. Patients were classified by age: younger than 40 years, from 40 to 49, or 50 and older. Hemodynamic variables in the cavernous arteries, helicine arteries and dorsal arteries were measured after intracavernous injection of prostaglandin E1 (20 microg).

RESULTS

All cavernous arteries had a maximum peak systolic velocity of greater than 35 cm/s and a minimum end-diastolic velocity of less than 0 cm/s. Mean peak systolic velocity in the cavernous artery differed between groups (P = 0.016), especially between the younger than 40 age group and the 40-49 age group. Peak systolic velocity correlated negatively with age (P = 0.0048). In the helicine arteries and dorsal arteries, peak systolic velocity did not differ between groups and showed no correlation with age. End-diastolic velocity, resistance index and acceleration time did not differ between groups, or correlate with age for any artery.

CONCLUSIONS

Hemodynamic change with aging occurs predominantly in the cavernous arteries, where peak systolic velocity decreases. The arterial circulation beyond the cavernous arteries, including the helicine arteries, as well as veno-occlusive mechanisms, may have an important role in maintaining erectile function in aging.

Role of adenosine triphosphate-dependent potassium channels in canine penile erection

OBJECTIVES

To define the physiologic role and hemodynamic features of nitric oxide (NO) and the adenosine triphosphate (ATP)-dependent K(+) (K(ATP)) channel in canine penile erection.

METHODS

Mongrel dogs were anesthetized, and penile erection was induced by electrical stimulation of the pelvic nerve. Changes in the intracavernous pressure (ICP) were measured with a transducer.

RESULTS

The basal ICP was 12.8 +/- 5.0 mm Hg. Pelvic nerve stimulation (5 to 20 V, 5 to 15 Hz, for 1-minute intervals) significantly increased the ICP to 86.2 +/- 11.4 mm Hg (n = 5, P <0.05). Treatment with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (10 mg/kg intravenously) abolished this increase (15.4 +/- 5.0 mm Hg, n = 5). Intracavernous injection of the K(ATP) channel opener cromakalim (3 and 10 microg) increased the ICP (103 +/- 14.4 mm Hg and 106 +/- 12.1 mm Hg, respectively; n = 4). This response was abolished by the prior intracavernous injection of the selective K(ATP) channel-specific blocker glibenclamide (10 mg). Glibenclamide did not affect the increase in ICP induced by electric stimulation of the pelvic nerve (88 +/- 24.2 mm Hg).

CONCLUSIONS

Our results indicate that relaxation of canine cavernous smooth muscle and penile tumescence are mediated by NO. The failure of glibenclamide to affect the increase in ICP induced by pelvic nerve stimulation suggests that ATP-dependent K(+) channels probably do not play a physiologic role in canine penile erection.

PROCEEDINGS OF THE BALTIMORE SMOOTH MUSCLE MEETING: IDENTIFYING RESEARCH FRONTIERS AND PRIORITIES FOR THE LOWER URINARY TRACT

PURPOSE

The myocyte is a major parenchymal cell of the lower urinary tract (LUT) in men and women. Significant phenotypic diversity ensures that myocytes subserve their important role in the physiologically distinct tissues and organs of the LUT, including the ureters, bladder, urethra, prostate, penis, vagina and myometrium. Coordinated contraction and relaxation of myocytes is required for normal organ function, while alterations in myocyte structure/function are implicated in the etiology of various LUT diseases/disorders. LUT diseases/disorders will continue to increase in an ever aging American population. The purpose of the Baltimore Smooth Muscle Meeting was to begin to identify some research frontiers and priorities.

MATERIALS AND METHODS

A 1-day conference of some of the leading world experts in smooth muscle research was held at American Urological Association headquarters. These experts gave presentations in their areas of expertise and extensively discussed their work. This report details those interactions.

RESULTS

There is astonishing diversity in the contribution of the myocyte to LUT physiology and dysfunction. Novel tools, technologies and ideas have produced increased understanding and identified new frontiers.

CONCLUSIONS

An improved understanding of urogenital myocyte physiology, function and dysfunction is required better to elucidate disease mechanisms and develop novel therapeutics. The First Annual Baltimore Smooth Muscle Meeting provided the first step in this direction. More coordinated LUT myocyte funding initiatives, the further development of research resources, tools and technologies, and exploration of the urogenital system as a model system for studying systems biology and integrative physiology are among the highest research priorities.

Intracorporal injection of hSlo cDNA restores erectile capacity in STZ-diabetic F-344 rats in vivo

The ability of gene transfer with the pore-forming subunit of the human maxi-K channel ( hSlo) to ameliorate the decline in erectile capacity commensurate with 12–24 wk of streptozotocin (STZ)-diabetes was examined in 181 Fischer-344 rats. A 2-mo period of STZ-diabetes was induced before gene transfer, and erectile capacity was evaluated by measuring the intracavernous pressure response (ICP) to cavernous nerve (CN) stimulation (ranging from 0.5 to 10 mA). In the first series of experiments, ANOVA revealed increased CN-stimulated ICP responses at 1 and 2 mo postinjection of 100 μg pcDNA- hSlo compared with control values. A second series of experiments further examined the dose dependence and duration of gene transfer. The ICP response to submaximal (0.5 mA) and maximal (10 mA) nerve stimulation was evaluated 3 or 4 mo postinjection of a single dose of pcDNA- hSlo ranging from 10 to 1,000 μg. ANOVA again revealed that hSlo overexpression was associated with increased CN-stimulated ICP responses compared with responses in corresponding control animals. Histological studies revealed no immune response to the presence of hSlo. PCR analysis documented that expression of both plasmid and transcript were largely confined to the corporal tissue. In the third series of pharmacological experiments, hSlo gene transfer in vivo was associated with iberiotoxin-sensitive relaxation responses to sodium nitroprusside in corporal tissue strips in vitro. The latter data indicate that gene transfer produces functional maxi-K channels that participate in the modulation of corporal smooth muscle cell tone. Taken together, these observations suggest a fundamental diabetes-related change in corporal myocyte maxi-K channel regulation, expression, or function that may be corrected by expression of recombinant hSlo.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Causes of erectile dysfunction

Erectile dysfunction (ED) arises as a result of a collision of circumstances among any of a number of factors (e.g., risk factors, causes, probable associations), each with its own primary power to affect the outcome. Furthermore, each of the components has its own timing as part of a complex effort of compensation and adjustment that often obscures the individual details. In the end, ED results from a failure of local tissues or systemic supply and control structures. The power of any individual "cause" to degrade erectile function is an important but as-yet unquantified property. The power of a small abnormality over a long or critical period (e.g., organogenesis), or many small contributions, or multiple risk factors will certainly be greater than the sum of the individual elements. Without a full quantitation of pathways and their potential influence, one can compare the importance of causative factors only in limited ways. Not surprisingly, it is the presence of a multiplicity of unidentified or poorly understood causative factors that accounts in large measure for the current inability to cure and prevent ED. There are two other important properties of a putatively causative factor for ED--reversibility and preventability--and these are strongly influenced by the time of onset and the duration of impact. Thus, a critical understanding that comes from recognizing the importance of the temporal associations of component factors is that the causes of ED in an individual may be guessed at but cannot be fully disclosed by an analysis of a "snapshot" of the disease taken at the time of diagnosis.

The successful long-term treatment of age related erectile dysfunction with hSlo cDNA in rats in vivo

PURPOSE

We have previously reported that 1 intracorporeal injection of 100 microg hSlo/pcDNA reversed the effect of aging on erectile function in a rat model in vivo for at least 2 months. We report our further investigations of the amplitude, duration and physiological relevance of this novel gene transfer approach.

MATERIALS AND METHODS

A total of 191 retired breeder Sprague-Dawley rats were given a single intracavernous injection of phosphate buffered saline, 1,000 microg pcDNA, or 10, 100 or 1,000 microg pcDNA/hSlo. The animals were studied 1 to 6 months after injection. The intracorporeal pressure (ICP) response to cavernous nerve stimulation and immunostaining as well as hematoxylin and eosin staining were done to evaluate effector nerve integrity and tissue histology, respectively.

RESULTS

Gene transfer prevented an age related decrease in resting ICP and a physiologically relevant, significant effect on normalizing erection in vivo, as determined by submaximal (0.5 mA) and maximal (4.0 mA) cavernous nerve stimulation. The effects were observed 1 month after transfection and sustained for 6 months at the 100 and 1,000 microg doses of pcDNA/hSlo (p <0.026).

CONCLUSIONS

The physiological manifestations of gene transfer were detected as an amelioration of the age related decrease in resting ICP, and parallel increase in the magnitude of the cavernous nerve stimulated an ICP response to a level at which visible erections were again observed in this rat model of aging in vivo.

Erectile dysfunction in the elderly: epidemiology, etiology and approaches to treatment

PURPOSE

Erectile dysfunction is experienced at least some of the time by most men who have reached 45 years of age, and it is projected to affect 322 million men worldwide by 2025. The prevalence of erectile dysfunction is high in men of all ages and increases greatly in the elderly.

MATERIALS AND METHODS

This paper reviews the epidemiology of erectile dysfunction with an emphasis on the experience of older men, normal age related changes in the structure and function of the penis that may contribute to increased risk with age, how the accumulation of risk factors with age may contribute to the high prevalence of the disease in older men, and established and emerging therapies. The normal aging process and age related risk factor accumulation contribute to the increased prevalence of erectile dysfunction in the elderly.

RESULTS

Remarkable progress has been made in the treatment of erectile dysfunction. At present inhibition of phosphodiesterase 5 with oral agents such as sildenafil would appear to be the initial treatment of choice. These drugs have been shown to be safe and effective, and sildenafil has demonstrated efficacy in patients with many of the comorbidities observed in older men with erectile dysfunction. New treatments, in particular transfection with genes for key mediators of erectile function that are known to be down-regulated in elderly men, also hold promise.

CONCLUSIONS

Further research into the neural, vascular and molecular mechanisms involved in penile erection will lead to the development of even safer, more effective and more convenient therapies for men with erectile dysfunction.

Gene therapy for erectile dysfunction: where is it going?

PURPOSE OF REVIEW

This review will summarize the most recent preclinical data from the leading US laboratories regarding the application of gene therapy to the treatment of erectile dysfunction. The implications of these findings in the field of gene therapy in general, and more specifically to the treatment of non-life threatening disorders such as erectile dysfunction, will be outlined.

RECENT FINDINGS

The preclinical work of several laboratories has clearly documented 'proof-of-concept' for the utility of gene therapy for the treatment of erectile dysfunction. A variety of vectors and several distinct molecular targets have been successfully leveraged. Such observations suggest that numerous potential strategies may exist for gene-based treatments of erectile dysfunction.

SUMMARY

The apparent preclinical success of most, if not all, gene-based strategies for the treatment of erectile dysfunction is consistent with the multifactorial regulatory mechanisms governing the erectile process. The bottleneck in the gene therapy clinical development process therefore apparently will not lie in the ability to identify relevant molecular targets that are amenable to gene therapy for erectile dysfunction, but rather in the safety, specificity and longevity of those targets. That is, the next technical hurdle is to find the strategy(ies) that has the best safety profile, the greatest specificity for altering (increasing) intracavernous pressure 'on demand' and, furthermore, the most appropriate (longest?) half-life. While these criteria may correspond to intuition, finding molecular targets that clear these clinical hurdles may place restrictions on the molecular choices for gene transfer.

The hemodynamics of erection and the pharmacotherapies of erectile dysfunction

The past few years have been witness to a sea of change in understanding the diagnosis and treatment of erectile dysfunction. In the wake of Viagra, effective orally administered therapies are now the expected gold standard. Currently available therapies include both peripherally (at the level of the penis) and centrally (brain, e.g., hypothalamic nuclei) acting compounds. When all modes of pharmacotherapy are considered, ranging from oral, to topical, to intraurethral, to intracavernous injection, it is clear that the vast majority of men with mild to moderate erectile dysfunction can now be effectively treated. However, all forms of pharmacotherapy have the lowest efficacy and greatest side effect profiles (including cardiovascular events) in the patients that need the therapy most, that is, those with relatively severe and longstanding erectile dysfunction. The newer generation of pharmacotherapies, which will likely include gene therapy techniques as well, will therefore have to target this latter group. Improved mechanism-based, perhaps patient-specific therapies are foreseen that will dramatically increase the number of patients seeking treatment, as well as the quality of their lives.

Evaluation and treatment of erectile dysfunction in men with diabetes mellitus

Diabetic men have a more than 3-fold increased prevalence of erectile dysfunction (ED) compared with nondiabetic men. Erectile function is primarily a vascular phenomenon, triggered by neurologic controls and facilitated by appropriate hormonal and psychological components. Recent advances in the understanding of the physiology of penile vasculature and its role in male sexual performance have influenced the clinical approach to ED. The pathophysiological alterations leading to impotence in diabetic men include vasculogenic, neurogenic, and hormonal etiologies. A clinical work-up, including a thorough history and physical examination, is an important aspect of ED management. Biochemical evaluations to rule out secondary causes like hypogonadism and thyroid abnormalities are suggested. Oral medications acting through phosphodiesterase inhibition in penile vasculature have revolutionized treatment of impotence in diabetic men. Because of a high success rate in treating ED of various etiologies, these agents are the treatment of choice for most patients. Safety and efficacy of vacuum-constriction devices, intraurethral suppositories, intracavernosal injections, and other therapies are discussed. A clinical algorithm for the evaluation and management of ED is provided for use in the primary care setting.

Potassium channels and erectile dysfunction

The incidence of erectile dysfunction (ED), defined as the persistent inability to achieve or maintain an erection sufficient for satisfactory sexual performance, increases with age and with risk factors for vascular disease, including smoking, diabetes and hypertension. Penile erection results from an arousal-induced synthesis of nitric oxide (NO) in nonadrenergic-noncholinergic nerves (NANC), endothelial cells and cavernosal smooth muscle cells (SMCs). Vasodilation and relaxation of cavernosal SMCs engorges the corpora cavernosa with blood at arterial pressure. The subcellular mechanism by which tumescence occurs involves NO-induced activation of soluble guanylate cyclase, increased cyclic guanosine monophosphate (cGMP) levels and activation of cGMP-dependent protein kinase (PKG). PKG phosphorylates numerous ion channels and pumps, each promoting a reduction in cytosolic calcium. In particular, PKG activates high-conductance Ca2+(-)sensitive K+ (BKCa) channels, which hyperpolarize the arterial and cavernosal SMC membranes, causing relaxation. This mechanism appears to be compromised with age and with vascular disease, leading to ED. Thus, increasing cavernosal nitric oxide synthase (NOS) expression, cGMP levels and/or BKCa channel expression is an effective therapy for experimental ED. Future therapies may involve augmenting K+ channel expression by gene transfer or increasing channel function through the use of Type 5 phosphodiesterase (Type 5 PDE) inhibitors or phosphatase inhibitors.