INTRODUCTION
Mesangiogenic Progenitor cells (MPCs)
    
    Mesangiogenic Progenitor cells (MPCs) has been firstly described in 2008 in human bone marrow mononuclear cell (hBM-MNC) cultures intended to isolate mesenchymal stromal cells (MSCs) in animal-free conditions \cite{Petrini_2009}. In this work, we substituted fetal calf serum (FBS), usually applied for the isolation/expansion of MSCs, with human autologous serum (hAS). The serum supplement switching did not affect hBM-MSC colture, however in comparison to standard FBS-supplemented cultures the application of hAS lead to the co-isolation of rounded, highly rifrangent and firmly attached cells , with a frequency that varied from 0.10 to 0.01% of total plated cells. After enzymatic digestion these cells remain attached and could be re-cultured in hAS for several weeks without signs of proliferation. Conversely, if re-cultured in FBS or pooled human cord blood sera, these rounded cells generated a new confluence of MSCs highly similar to those obtained from the primary culture in terms of proliferative, clonogenic and differentiative potential. As these "fried egg"-shaped cells also showed angiogenic and partially cardiomyogenic potential, were firstly named Mesodermal Progenitor Cells (MPCs). Later, we also developed a selective culture method able to isolate MPCs with high grade of purity and pushing the yield to around 1% of total plated cells, applying hydrophobic non gas-treated plastics as culture surface \cite{Trombi_2009}.  The establishment of an optimal culture condition for isolation of MPCs from hBM allowed a more extensive characterization of these interesting cells. In 2010, a specific MPC phenotype was defined to distinguishing them, by flow cytometry from standard MSCs resulting SSClowSSEA-4negCD105brightMSCA-1+CD90bright, while MPCs were SSChighSSEA-4+CD105dimMSCA-1negCD90neg. Interestingly, pluripotency-associated gene, as OCT-4 and NANOG, expression were detected in MPCs only as well as consistent expression of nestin \cite{Pacini_2010}.  Further investigations on integrins profile later revealed the expression of specific  α and  β chains on MPCs. In particular these cells showed podosome-like structures sustained by integrins αL (CD11a), αM (CD11b), αX (CD11c) and β2 (CD18) that were absent on MSCs, extending their phenotype characterization and functionally sustaining an increased adhesion onto activated and non activated endothelium \cite{Pacini2013}.
    More rigorous studies and MPC differentiation capability demonstrated that their mesengenic commitment take place trough a SSA-4+CD105intCD90bright early intermediate precursor,  named early MSCs, activating a specific Wnt5/Calmodulin signaling pathway \cite{Fazzi2011}. After culturing MPCs for a week in mesengenic culture conditions, usually identified as "passage one MSCs" (P1-MSCs) cultures, cells can be detached and further expanded (P2-MSCs), definitively acquiring a classic or late MSCs phenotype and proliferation/differentiation features. Interestingly, P2-MSCs culture were independent to the activation of the Wnt5/Calmodulin signaling pathway and were not affected by Calmidazolium Chloride (CLMDZ), a potent and specific Calmodulin inhibitor, able to completely suppress the differentiation of primary MPC cultures into P1-MSCs, instead. Moreover, CLMDZ did not affect the endothelial differentiation of MPCs resulting highly specific inhibitor of the MPC differentiation into early MSCs only.
    Minimal criteria to characterize MPC cultures has been definitively identified in 2016 \cite{Montali2016}. Alongside the above-mentioned lack of MSC markers  as CD73 also and specific integrin profile, phenotype should include  CD31 and surprisingly CD45, from weak to brightly expressed on MPC. A percentage of CD73negCD90negCD45+CD31+of 95% has been indicated as the lower purity limit to consider the primary culture as a MPC culture. However, more stringent functional characterization is represented by assaying the differentiation potential of these culture. Together with the demonstration that MPC are able to differentiate into P2-MSC, able then to form calcium deposit in osteogenic medium and intracellular lipid-droplets in adipogenic medium, MPCs should be able of angiogenic sprouting from 3D-spheroids cultured in VEGF-rich medium. At this time, as the cardiomyogenic differentiation could not be definitively demonstrated, MPCs were then re-named as Mesangiogenic Progenitor Cells, maintaining the same acronym. More recently, the MPC angiogenic potential and endothelial differentiation capability was clearly reported also in vivo, applying MPC constructs on chicken chorioallantoic membrane \cite{Montali2017}. In this work, we also showed that MPCs are able to uptake acetylated low density lipoproteins (Ac-LDL) and trans-endothelial migration but not directly form capillary-like tubes (CLTs) in gels composed of of extracellular matrix proteins. These CLTs were obtainable only if MPCs were firstly let sprout from 3D-spheroids, under VEGF stimulus in a 2-steps differentiation protocol similarly to the P1 and P2 steps of mesengenic differentiation. Interestingly, we also demonstrated that the mesengenic and angiogenic potentials of MPCs are mutually exclusive as P2-MSC completely lost any angiogenic potential. figura delle schema di differenziazione ampliato. AGGIUNGI risultati bortezomib dal lavoro MM