T2DM progresses from compensated insulin resistance to beta cell failure resulting in uncompensated hyperglycemia, a process replicated in the Zucker diabetic fatty (ZDF) rat. Our earlier findings (Nature Medicine 19:1132, 2013) implicate pro-inflammatory macrophages infiltrating pancreatic islets in a paracrine mechanism of beta cell loss, and identify macrophage-expressed CB1 receptors as a therapeutic target in T2DM. In a follow up study, we have addressed the question whether CB1 receptors have an obligatory role in the development of T2DM, by generating CB1 receptor knockout rats on a ZDF genetic background, using the zinc finger technology. ZDF-Cnr1 rats retain the inactivating mutation of the leptin receptor present in the parent ZDF strain, and also lack CB1 receptors globally. Unlike their ZDF littermates that develop T2DM, ZDF-Cnr1 rats remain normoglycemic due to preservation of pancreatic beta cells and beta-cell function. They are also protected from the nephropathy and dyslipidemia affecting their ZDF littermates, which were reported earlier (PNAS 111:E5420-28, 2014). Adoptive transfer of ZDF-Cnr1 bone marrow to ZDF rats also protects them from the development of hyperglycemia and T2DM, but not from the nephropathy or dyslipidemia. These findings confirm that obligatory role of CB1 receptors on bone marrow-derived macrophages in the beta-cell loss of diabetic ZDF rats, but indicates that role of CB1 receptors in other cell types in the associated nephropathy and dyslipidemia. This work has been published in Diabetes during the current reporting period. We have developed novel, dual-target compounds for the treatment of various pathologies associated with fibrosis. The hybrid CB1R/iNOS inhibitor MRI-1867, which we had earlier tested and found effective in the treatment of experimental models of liver fibrosis (JCI INsight, 2016), has also been tested in different forms of pulmonary fibrosis, in collaboration with Drs. William Gahl and Bernadette Gochuico from NHGRI. Pulmonary fibrosis (PF) is a life-threatening disease with poor prognosis and in need of novel treatment strategies, as currently available single target therapies have yielded modest success. For complex and multi-factorial diseases such as PF, targeting multiple pathways may improve therapeutic efficacy. We discovered that the endocannabinoid/CB1R system is overactive in the lung of patients with idiopathic PF and mice with bleomycin-induced PF where it contributes to disease progression, marking CB1R as a novel therapeutic target. Moreover, we demonstrate that engaging the secondary target iNOS by a peripherally restricted, orally bioavailable hybrid CB1R/iNOS inhibitor improves anti-fibrotic efficacy over inhibition of CB1R alone, and minimizes CNS side effects caused by brain-penetrant CB1R antagonists. This work has been published in JCI Insight during the current reporting period. The hybrid CB1R/iNOS inhibitor MRI-1867 has also been tested in the pale ear mouse, an experimental model of type 1 Hermansky-Pudlak syndrome (HPS), a rare genetic disease associated with pulmonary fibrosis, and showed anti-fibrotic efficacy. The NIH (NCATS) TRENDS program (Therapeutics for Rare and Neglected Diseases has partnered with our groups to conduct IND-enabling toxicology and ADME work required for the clinical testing of MRI-1867 in HPS. Previously, we have documented the therapeutic efficacy of the peripherally restricted CB1R antagonist/inverse agonist, JD-5037, in a mouse model of high-fat diet-induced obesity (DIO)/metabolic syndrome (Cell Metabolism 2012). The compound was as effective as its brain-penetrant counterpart not only in improving peripheral carbohydrate and lipid metabolism of DIO mice, but also in reducing their food intake. This latter effect could be partially attributed to reversing the leptin resistance of the obese mice by reducing leptin production by adipose tissue and leptin clearance via the kidney, so that the hypophagic effect of endogenous leptin could manifest again. In a recently published study we have explored the central signaling pathway(s) through which leptin acted, once its efficacy was restored following peripheral CB1R blockade by JD-5037. We found that leptin treatment or an increase in endogenous leptin following fasting/refeeding induced STAT3 phosphorylation in neurons in the arcuate nucleus (ARC) in lean and JD5037-treated DIO mice, but not in vehicle-treated DIO animals. Co-localization of pSTAT3 in leptin-treated mice was significantly less common with NPY+ than with POMC+ ARC neurons. The hypophagic effect of JD5037 was absent in melanocortin-4 receptor (MC4R) deficient obese mice or DIO mice treated with a MC4R antagonist, but was maintained in NPY-/- mice kept on a high-fat diet. We concluded that peripheral CB1R blockade in DIO restores sensitivity to endogenous leptin, which elicits hypophagia via the re-activation of melanocortin signaling in the ARC. This work has been published in Molecular Metabolism (2017)