Nitric Oxide Synthase Inhibitors into the Clinic at Last

The 1998 Nobel Prize in Medicine and Physiology for the discovery of nitric oxide, a nitrogen containing reactive oxygen species (also termed reactive nitrogen or reactive nitrogen/oxygen species) stirred great hopes. Clinical applications, however, have so far pertained exclusively to the downstream signaling of cGMP enhancing drugs such as phosphodiesterase inhibitors and soluble guanylate cyclase stimulators. All clinical attempts, so far, to inhibit NOS have failed even though preclinical models were strikingly positive and clinical biomarkers correlated perfectly. This rather casts doubt on our current way of target identification in drug discovery in general and our way of patient stratification based on correlating but not causal biomarkers or symptoms. The opposite, NO donors, nitrite and enhancing NO synthesis by eNOS/NOS3 recoupling in situations of NO deficiency, are rapidly declining in clinical relevance or hold promise but need yet to enter formal therapeutic guidelines, respectively. Nevertheless, NOS inhibition in situations of NO overproduction often jointly with enhanced superoxide (or hydrogen peroxide production) still holds promise, but most likely only in acute conditions such as neurotrauma (Stover et al., J Neurotrauma 31(19):1599-1606, 2014) and stroke (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016; Casas et al., Proc Natl Acad Sci U S A 116(14):7129-7136, 2019). Conversely, in chronic conditions, long-term inhibition of NOS might be too risky because of off-target effects on eNOS/NOS3 in particular for patients with cardiovascular risks or metabolic and renal diseases. Nitric oxide synthases (NOS) and their role in health (green) and disease (red). Only neuronal/type 1 NOS (NOS1) has a high degree of clinical validation and is in late stage development for traumatic brain injury, followed by a phase II safety/efficacy trial in ischemic stroke. The pathophysiology of NOS1 (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016) is likely to be related to parallel superoxide or hydrogen peroxide formation (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016; Casas et al., Proc Natl Acad Sci U S A 114(46):12315-12320, 2017; Casas et al., Proc Natl Acad Sci U S A 116(14):7129-7136, 2019) leading to peroxynitrite and protein nitration, etc. Endothelial/type 3 NOS (NOS3) is considered protective only and its inhibition should be avoided. The preclinical evidence for a role of high-output inducible/type 2 NOS (NOS2) isoform in sepsis, asthma, rheumatic arthritis, etc. was high, but all clinical development trials in these indications were neutral despite target engagement being validated. This casts doubt on the role of NOS2 in humans in health and disease (hence the neutral, black coloring).

Immunolocalisation of nitric oxide synthase isoforms in the ductuli efferentes and epididym

The present research used immunohistochemistry to analyse the detection and localisation of nitric oxide synthase (NOS) isoforms in the ductuli efferentes and epididymis of prepubertal and adult alpaca. In the ductuli efferentes and epididymis of prepubertal and adult animals, nNOS and eNOS were similarly expressed in epithelial lining cells, conversely differences were observed in the immunopresence of iNOS. Our data provide evidence that NOS isoforms may have roles in reproductive functions and in the developmental processes of the excurrent duct system in the alpaca.

Region-specific localization of NOS isoforms and NADPH-diaphorase activity in the intratesticular and excurrent duct systems of adult domestic cats (Felis catus)

Nitric oxide (NO) is produced by nitric oxide synthases (NOSs) and plays an important role in all levels of reproduction from the brain to the reproductive organs. Recently, it has been discovered that all germ cells and Leydig cells in the cat testis exhibit stage-dependent nuclear and cytoplasmic endothelial (eNOS) and inducible (iNOS)-NOS immunoreactivity and cytoplasmic nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) reactivity. As a continuation of this finding, in this study, cellular localization of NADPH-d and immunolocalization and expression of all three NOS isoforms were investigated in the intratesticular (tubuli recti and rete testis), and excurrent ducts (efferent ductules, epididymal duct and vas deferens) of adult cats using histochemistry, immunohistochemistry and western blotting. NADPH-d activity was found in the midpiece of the spermatozoa tail and epithelial cells of all of ducts, except for nonciliated cells of the efferent ductules. Even though the immunoblotting results revealed similar levels of nNOS, eNOS and iNOS in the caput, corpus and cauda segments of epididymis and the vas deferens, immunostainings showed cell-specific localization in the efferent ductules and region- and cell-specific localization in the epididymal duct. All of three NOS isoforms were immunolocalized to the nuclear membrane and cytoplasm of the epithelial cells in all ducts, but were found in the tail and the cytoplasmic droplets of spermatozoa. These data suggest that NO/NOS activity might be of importance not only for the functions of the intratesticular and excurrent ducts but also for sperm maturation.