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.