Since the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) initiated a global pandemic resulting in an estimated 775 million infections with over 7 million deaths, it has become evident that COVID-19 is not solely a pulmonary disease. Emerging evidence has shown that, in a subset of patients, certain symptoms − including chest pain, stroke, anosmia, dysgeusia, diarrhea and abdominal pain – all indicate a role of vascular, neurological and gastrointestinal (GI) pathology in the disease process. Many of these disease processes persist long after the acute disease has been resolved, resulting in ‘long COVID’ or post-acute sequelae of COVID-19 (PASC). The molecular mechanisms underlying the acute and systemic conditions associated with COVID-19 remain incompletely defined. Appropriate animal models provide a method of understanding underlying disease mechanisms at the system level through the study of disease progression, tissue pathology, immune system response to the pathogen and behavioral responses. However, very few studies have addressed PASC and whether existing models hold promise for studying this challenging problem. Here, we review the current literature on cardiovascular, neurological and GI pathobiology caused by COVID-19 in patients, along with established animal models of the acute disease manifestations and their prospects for use in PASC studies. Our aim is to provide guidance for the selection of appropriate models in order to recapitulate certain aspects of the disease to enhance the translatability of mechanistic studies.

Background: Peripheral artery disease is associated with significant morbidity and mortality. Mechanical revascularization strategies are a mainstay of treatment but are often limited by the anatomic complexity of atherosclerotic lesions. Therapeutic angiogenesis has fallen short of being impactful due to fundamental gaps in our understanding of postdevelopmental angiogenesis. Methods: Using a preclinical model of peripheral artery disease involving acute vascular injury by femoral artery ligation along with cellular and molecular studies of VEGF-A expression, we sought to further understand the early role of macrophages in inflammatory angiogenesis and arteriogenesis. Results: Macrophage depletion studies revealed that the optimal levels of tissue VEGF-A expression, endothelial cell recruitment, and blood flow recovery were dependent on early macrophage recruitment. Proangiogenic VEGF-A expression was highest in macrophages polarized towards an inflammatory phenotype. Myeloid VEGF-A-deletion, while having no impact on the potent inflammatory cytokine, IL-1β, led to reductions in ischemic tissue VEGF-A, endothelial cell recruitment, and blood flow recovery due to impaired angiogenesis and arteriogenesis. Transplant of inflammatory polarized macrophages rescued the myeloid VEGF-A-deletion phenotype, leading to full blood flow recovery. Conclusions: Macrophages are a necessary and sufficient source of tissue VEGF-A during inflammatory-driven angiogenesis and arteriogenesis in response to vascular injury. Although further study is needed, cell-based therapeutic angiogenesis strategies involving the polarization of macrophages toward an inflammatory state, in order to produce high levels of proangiogenic VEGF-A, may be quite effective for improving revascularization in the context of PAD.

Aging and age-related diseases like peripheral artery disease (PAD) is associated with impairedinflammatory angiogenesis responses to injury. Critical ischemic limb and wound injury animalmodels, have shown impact of inflammatory monocytes/macrophages as a major source ofproangiogenic mediators leading to new arterial growth and healing. In hind limb ischemia mousemodel of PAD, we have recently demonstrated the loss of macrophage IL-1β expression leads todecreased VEGF-A (total) and VEGF-A165a expression (proangiogenic isoform), while VEGF-A165b (inhibitory isoform) was increased, under the regulation of transcription factors SignalTransducer and Activator of Transcription 3 (STAT3) and Nuclear Factor Kappa B (NF-κB).Previous studies on aged mice have shown impaired of blood flow recovery and reduced VEGF-A expression. Moreover, recombinant VEGF-A administration did not fully overcome the footperfusion and prevent limb salvage. The mechanisms of reduced VEGF-A expression in aged animals remains incomplete. Our data from advanced aged (104-week-old) mice demonstratereduced blood flow recovery after excision of the femoral artery and reduced VEGF-A and VEGF-A165a in ischemic muscle tissue compared to young (12-week-old) while IL-1β and VEGF-A165bwere increased. Bone marrow-derived-macrophages (BMDMs) from 104-week-old alsodemonstrate decreased VEGF-A and VEGF-A165a expression. IL-1β andVEGF-A165b were foundincreased. Lastly, polarized BMDMs from 12-week-old demonstrate increased VEGFR2expression in inflammatory “M1” macrophages while VEGFR1 was increased in alternativelyactivated “M2" macrophages. Interestingly, 104-week-old demonstrate significantly increasedVEGFR1 expression while VEGFR2 was reduced compared to 12-week-old. We hypothesize thatthe regulation of IL-1β-VEGF-A axis and the type of VEGFR1/R2 involved in the signalingcomplex determine the fate of angiogenesis responses to injury in the context of advanced age.Our goals will be to define mechanisms whereby aging is associated with uncoupling of IL-1β andVEGF-A expression then, how VEGF-A isoforms and VEGFR1/2 signaling complex axis regulateinflammatory angiogenesis.