Discussion
Extrapulmonary manifestations of SARS-CoV-2 infection encompass ocular presentations, with documented instances of retinal involvement. Retinal disorders associated with COVID-19 have been extensively reported[7, 23]. These ocular manifestations often include vaso-occlusive events characterized by flame-shaped retinal hemorrhages, cotton wool spots, and sectoral pallor. Although AMN is inherently rare, an elevated prevalence of AMN has been observed among individuals afflicted with SARS-CoV-2, and the underlying mechanisms driving this association remain to be elucidated[24, 25]. To our knowledge, this is the first study to offer a comprehensive overview of the metabolic profile of serum in AMN-SARS-CoV-2, thereby unveiling intricate associations between these metabolites and alterations in coagulation markers. Our findings significantly enhance the current understanding of the relationship between SARS-CoV-2 infection and AMN, providing valuable insights to guide future research and clinical applications.
COVID-19 patients frequently exhibit diverse pathological mechanisms associated with microvascular dysfunction and thrombogenesis, encompassing microvascular blood clots and thromboembolism across the pulmonary and other organ systems. Accordingly, changes in coagulation parameters are often observed, including elevated levels of circulating fibrin degradation products and von Willebrand factor, APTT, and thrombocytopenia in hospitalized and critically ill COVID-19 patients, which may hold prognostic relevance[26]. Most importantly, extensive microthrombi can be disseminated throughout organs such as the heart, kidneys, and liver in COVID-19 patients, indicative of multi-organ thrombotic microangiopathy[2, 27]. This prothrombotic state appears to contribute to the vascular complications observed in COVID-19 patients. The underlying mechanisms encompass direct viral-mediated injury to endothelial cells, excessive activation of inflammatory responses, and perturbed activation of the coagulation cascade. In parallel, AMN represents a microvascular disorder typified by microvascular impairment and inflammatory responses, culminating in intravascular clot formation, leading to microvascular occlusion and tissue ischemia[8]. This study detected significant changes in peripheral blood levels of procalcitonin, TAT complex, APTT, and FDP in AMN-SARS-CoV-2 patients compared to those without AMN-SARS-CoV-2. These changes underscore the involvement of coagulation markers in AMN pathogenesis within the context of SARS-CoV-2 infection and indicate the intricate interplay of coagulation processes in the emergence and progression of AMN amidst SARS-CoV-2 infection.
Subsequent application of the LC-MS method uncovered distinct alterations in serum metabolites of AMN-SARS-CoV-2 patients. Interestingly, dysregulations in amino acid metabolism have been documented in COVID-19 patients[15]. The current investigation consistently identified amino acid metabolism perturbations in AMN-SARS-CoV-2. Specifically, 18 amino acid-related metabolites were upregulated, while 6 were downregulated.
Arginine, a semi-essential amino acid, is a critical regulator of immune and vascular cell functions. The dysregulated metabolism of arginine in the context of COVID-19 fosters dysfunctions in immune and endothelial cells, coupled with proliferation and migration of vascular smooth muscle cells, inflammation, vasoconstriction, thrombus formation, arterial thickening, fibrosis, and heightened stiffness[28]. This multifaceted cascade can culminate in dire outcomes, including vascular occlusion, multi-organ failure, and mortality. Within this study, noteworthy modifications in the arginine metabolism pathway were observed in AMN-SARS-CoV-2 cases in comparison to those without AMN-SARS-CoV-2. This metabolic shift involved several differential metabolites, such as ornithine, citrulline, L-proline, and ADAM. Notably, ornithine and citrulline occupy crucial roles as intermediate metabolites within the urea cycle, constituting an integral facet of arginine metabolism. The conversion of arginine can lead to either citrulline and nitric oxide (NO) via NOS or ornithine and urea through the action of the enzyme ARG1[29]. Moreover, prior research has revealed the close connection between dysregulated ornithine cycling, inflammation, and coagulation, which potentially unveils a mechanism contributing to COVID-19 pathogenesis[30]. Correspondingly, the study identified a notable elevation in ornithine levels in AMN-SARS-CoV-2 patients, significantly correlating with coagulation markers such as PCT, FDP, and Fbg. Thus, ornithine potentially assumes a pivotal role within the AMN-SARS-CoV-2 pathogenic framework, especially in relation to coagulation mechanisms. This emphasizes exploring strategies to modulate ornithine metabolism, which could conceivably ameliorate patient conditions and mitigate complications.
Furthermore, it was observed that ARG1 expression is upregulated in COVID-19 patients depending on disease severity, suggesting the potential involvement of the rewired arginine metabolism catalyzed by ARG1 in unfavorable outcomes for COVID-19 patients[31]. This study unveiled significant upregulation of ARG1 in AMN-SARS-CoV-2 cases, accompanied by a considerable downregulation of NOS and a robust negative correlation between ARG1 and NOS expression levels. NOS and ARG1 play a determining role in the function of vascular cells; NOS protects vessels through the basal release of nitric oxide, which effectively counteracts arterial tone, whereas ARG1’s association lies with endothelial cell (EC) dysfunction and vascular maladies[32, 33]. Indeed, ARG induction has been implicated in endothelial dysfunction across diverse cardiovascular conditions, spanning systemic and pulmonary arterial hypertension, sickle cell disease, diabetes, atherosclerosis, trauma, obesity, aging, myocardial ischemia-reperfusion injury, and hemorrhagic shock. Experimental models have demonstrated that inhibiting or eliminating ARG1 can reinstate endothelial function by enhancing bioavailability[34]. Additionally, researchers have noted that ARG1 stimulates collagen synthesis in smooth muscle cells (SMCs) by channeling arginine metabolism toward proline, stimulating SMC proliferation, migration, and collagen deposition, thereby fostering vascular fibrosis and stiffness that ultimately underlie vascular occlusion[35]. In this context, the study corroborated elevated levels of both ARG1 and L-proline in the serum of AMN-SARS-CoV-2 patients, suggesting the activation of the ARG1-associated arginine metabolism and collagen synthesis pathways within the AMN-SARS-CoV-2 milieu, suggesting ARG1 represents a promising therapeutic target for AMN-SARS-CoV-2. The reduced NOS activity observed could be attributed to ARG1’s upregulation, which competes with NOS for arginine, thus diverting arginine metabolism from NO synthesis. Notably, the study revealed an elevated level of ADMA, an endogenous NOS inhibitor[36], within AMN-SARS-CoV-2 cases, which may further contribute to diminished NOS activity and subsequent reduction in NO production. The cumulative impact of these mechanisms significantly influences vascular health and coagulation processes. Extensive exploration is warranted to unravel the intricate roles and interplay of these mechanisms, which is essential for a comprehensive understanding of the implications of NOS reduction across both physiological and pathological contexts.
To assess the diagnostic relevance of metabolic biomarkers and coagulation parameters within AMN-SARS-CoV-2, a ROC analysis was conducted on pivotal differential metabolites (ornithine, citrulline, L-proline, ADAM) and coagulation abnormality indicators (PCT, Fbg, FDP). The outcomes revealed the individual diagnostic significance of these markers for AMN-SARS-CoV-2, while their combination yielded the most robust diagnostic performance. However, further validation and prospective studies are warranted to establish the efficacy of these biomarkers and their combined utility in diagnosing AMN-SARS-CoV-2.
While this study reveals the potential contributions of metabolism and coagulation markers to the diagnosis and pathogenesis of AMN-SARS-CoV-2, several limitations should be acknowledged. Firstly, the survey excluded individuals not infected by SARS-CoV-2; their inclusion in future investigations could enhance our comprehension of AMN-SARS-CoV-2. Secondly, using serum samples, though fast and convenient, predominantly reflects comprehensive metabolic changes across patients rather than specific metabolic alterations within the ocular domain. Finally, the study’s sample size was limited due to the rarity of AMN as an ocular condition. Although the incidence of AMN increases with SARS-CoV-2 infection, the paucity of AMN-SARS-CoV-2 cases emphasizes the potential benefits of augmenting the sample size through multicenter collaborations.
Taken together, the metabolomic insights delineated in this study offer a glimpse into the serum metabolism of AMN-SARS-CoV-2, pinpointing alterations in urea cycle dynamics and coagulation parameters, along with their interplay, as plausible pathological mechanisms underlying AMN-SARS-CoV-2, enhancing our knowledge of the mechanisms driving this condition. This advances our comprehension of the pathogenesis of AMN-SARS-CoV-2, potentially leading to improved early prognostic predictions and opportunities for therapeutic interventions.