Metabolism and physiology

In my PhD in the Suarez and Fernandez-Hernando's laboratories at Yale University, I have discovered that lanosterol, the first synthesized sterol downstream of squalene, is an important regulator of immune functions in macrophages (Araldi et al, 2017).

Macrophages perform critical functions in both innate immunity and in maintaining cholesterol homeostasis. In my PhD thesis, I report the cross-talk between these two physiological processes, and that lanosterol is an important player in regulating innate immune responses. Activation of toll-like receptor 4 (TLR-4) in mouse macrophages causes lanosterol to accumulate. In turn, genetic or pharmacological lanosterol accumulation in macrophages, suppresses IFNg-mediated STAT-1 activation, interferon stimulated gene expression and cytokine secretion. The effects of lanosterol accumulation on macrophage functions are selective in that lanosterol accumulation increases membrane fluidity, potentiating phagocytosis of bacteria, and in vivo it enhances survival to endotoxemic shock and infection by Listeria monocytogenes.

My data shows that lanosterol, a component of the cholesterol biosynthetic pathway that accumulates in response to LPS, is an endogenous selective regulator of macrophage immunity.

In my future independent research, I would like to study what is the function of other cholesterol biosynthesis intermediates in development and disease.

After graduating from the Scuola Normale Superiore, I studied hypoxia and bone development during an internship at the Harvard Medical School under the supervision of Professor Ernestina Schipani (now at UPenn).

The transcription factors hypoxia-inducible factor-1 (HIF1a) and HIF2 mediate cellular responses to hypoxia. The fetal growth plate is an avascular tissue that needs hypoxia-driven pathways for proper development. During my work in the Schipani laboratory I have contributed to study the role of the hypoxia signaling machinery in limb bud mesenchyme and bone development.

HIF1 is a survival factor for growth plate chondrocytes and it is necessary for timely differentiation of mesenchymal cells into chondrocytes. In the growth plate, ablation of VEGF, an angiogenic factor and HIF target, impairs fetal cartilage differentiation and the phenotype resembles the loss of HIF1a. We aimed at

testing whether the defects in HIF1a knockout growth plates were merely due to loss of VEGF and we could demonstrate that VEGF overexpression is not able to rescue HIF1a loss in fetal cartilage (Maes et al, 2012).

While loss of HIF1 has a devastating effect on chondrocyte differentiation and growh plate survival, HIF2 does not play a major role in limb bud development. Mice with limb bud specific loss of HIF2 are viable and, prenatally, display only a modest and transient growth plate phenotype, which fully resolves postnatally

(Araldi et al, 2011).

During my stay in the Schipani Laboratory, I have carried out the initial characterization of another mouse model lacking the E3 ubiquitine ligase Von Hippel-Lindau (VHL) in limb bud mesenchyme or osteoblasts. VHL degrades the HIFs proteins in an hypoxia-dependent fashion. Continuous activation of hypoxia-driven pathways in mesenchymal progenitors of the limb bud, driven by the loss of VHL, is sufficient to generate ectopic cartilage in the soft tissue surrounding the growth plate (Mangiavini et al, 2014). It also causes aggressive fibrosis of the synovial joints, formation of fibroblastic tumors in proximity of skeletal elements, and dwarfism (Mangiavini et al, 2015). The dwarfism is a consequence of both impaired proliferation and delayed hypertrophy of growth plate chondrocytes. Conditional loss of VHL from osteoblasts, on the other hand, caused HIF2 gain-of-function-driven polycythemia by stimulating erythropoietin (EPO) production and secretion by cells of the osteoblast lineage (Rankin et al, 2012).

Overall my studies in the Schipani’s laboratory contributed to elucidating the role of HIF2 and the other hypoxia mediators in bone development.