Pulmonary arterial hypertension (PAH) is a deadly disease associated with systemic sclerosis (SSc-PAH) and is dependent on several vascular cell types. However, key systems of molecular cross-talk remain enigmatic. Previously, we defined a key regulatory axis between the transcriptional coactivators YAP/TAZ with the enzyme glutaminase, establishing a new paradigm of how glutamine metabolism is related to vascular stiffness in PAH. Yet, crucial questions remain. What are the triggers that activate YAP/TAZ to initiate PAH and do they originate from separate cell types? Downstream of those triggers, can vascular glutamine metabolism serve as a diagnostic indicator of SSc-PAH? Our unpublished data demonstrate that platelets from SSc patients show dysregulated mitochondrial energetics which correlate with the severity of vascular manifestations in these patients and cause increased glutaminolysis in pulmonary adventitial fibroblasts. Hypothesis: Altered platelet energetics signal to PA fibroblasts, resulting in specific alterations of glutamine metabolism to control collagen deposition, vascular stiffness, and SSc-PAH. In this translational proposal, we will define the presence of dysregulated platelet energetics and fibroblast glutamine metabolism in SSc-PAH. We will also determine if fibroblast glutamine uptake can be targeted for the development of more effective diagnostics. Aim 1) Determine whether dysregulated platelet energetics is associated with the severity of vascular symptoms in SSc and with lung vascular fibroblast glutaminolysis in SSc-PAH. We postulate that dysfunctional platelet energetics can serve as a biomarker of progressive vascular damage in SSc and is associated with increased glutaminolysis in pulmonary adventitial fibroblasts in SSc-PAH. To investigate, platelets will be isolated from SSc-PAH, SSc, and control patients and energetic profiles will be analyzed and correlated with measures of clinical vascular measures. In parallel, single cell RNA sequencing of human SSc- PAH vs. control explant lungs will determine if platelet energetic transcriptional profiles correlate with vascular fibroblast glutaminolytic profiles. Aim 2) Utilize 18F-fluoroglutamine PET imaging to measure glutamine uptake in SSc-PAH vs. controls. We found increased glutamine uptake into pulmonary vessels and right ventricle in rodent PAH, as quantified by PET imaging of a 18F-FGln tracer and by spectral (MIMS) imaging of 15N-glutamine. In a first-in-human study, we will investigate 18F-FGln PET/CT in SSc-PAH patients vs. SSc alone and non-diseased controls. This aim will define the relevance of glutamine metabolism in human SSc-PAH and the potential of 18F-FGln to serve as a novel diagnostic tracer for SSc-PAH. Significance: Our multi-disciplinary team is uniquely positioned to define the clinical relevance of a platelet-to-fibroblast metabolism pathway critical for inducing vascular stiffening and PAH. We will leverage those findings to embark on a first-...