Novel mechanisms of brain-insulin resistance in patients with major depression and acetylcarnitine deficiency: An in-vivo study with the exosomes nanotechnology

Nasca C, Bigio B, Dobbin J, Watson-Lin K, de Angelis P, Myoraku A, Kautz M, Cochran A, Kocsis J, Mathe A, Lee FS, Murrough F, McEwen BS, Rasgon NL
American College of Neuropsychopharmacology. 2019.


Background: The insulin signaling is critical for neuroplasticity and cerebral metabolism in addition to systemic energy metabolism. Unlike the role of systemic insulin action, the understanding of molecular mechanisms of brain insulin signaling in the neurobiology of major depressive disorder (MDD) has been limited to postmortem brain studies. Exosomes are extracellular nanovesicles, secreted from all cells and carrying biological material important for physiological functions, including neuroplasticity and neurogenesis. A key feature of brain-derived exosomes is that they cross the blood brain barrier and express the membrane surface marker, cell adhesion molecule-1 (LCAM-1), which allows their identification and isolation from the plasma of a subject. This is a very new technology that has opened up the possibility to investigate in-vivo molecular mechanisms otherwise inaccessible in humans, overcoming the limitations of postmortem brain studies. Most insights on the role of brain-derived exosomes come from studies on brain tumor and cognitive disorders, however their role in MDD remains to be fully explored.

Methods: 93 subjects participated in this study (64 subjects suffering from MDD and 29 age- and sex- matched controls) and were recruited at the Department of Psychiatry & Behavioral Sciences at Stanford University and the Mood and Anxiety Disorders Program at the Icahn School of Medicine at Mount Sinai. All patients with MDD were in an acute episode during study participation. At both study sites, the psychiatric examination included the Structured Clinical Interview for DSM-IV (SCID) and the psychiatric scale HDRS-21. Brain-derived exosomes were isolated as described in previous papers. Briefly, total circulating exosomes were isolated from plasma by using a precipitation technology and subsequently enriched for brain origin by using of magnetic beads conjuncted with the L1-CAM protein. Assessment of numbers of both total and brain-derived exosomes was performed by using ExoCet based upon a series of standards calibrated at NanoSight. Protein expression in brain-derived exosomes was measured by ELISA. All groups were evenly divided between the experimental plates to account for any inter-plate variability. LAC levels and systemic insulin resistance as assessed by the Homeostatic Model Assessment of Insulin Resistance (HOMA) were measured as we described in previously papers. Two-tailed t-tests, chi-square, Pearson correlations and multiple regression were used as appropriate to specific analyses. We also developed an algorithm in R to test whether integrated measures of central and systemic insulin signaling predicted depression diagnosis (i.e.: discriminate between subjects with MDD and controls).

Results: By using the novel nanotechnology of exosomes, we show an aberrant secretion (p = 0.036, effect size=0.62) and cargo of brain-derived exosomes (i.e. LCAM+ exosomes) in subjects suffering from MDD as compared to age- and sex-matched controls. Our new data show an increased in-vivo expression of the insulin receptor substrate-1 (IRS-1) in LCAM+ exosomes (p = 0.002) and sex-specific increase in serine phosphorylation of IRS-1 (Men p = 0.3, Women p = 0.02), independently of psychotropic drug treatment. The degree of phosphorylation of IRS1 reflected the severity of depressive symptoms (p = 0.02, r = 0.4). In addition, utilizing machine learning, we show that both central and systemic IR predicted depression diagnosis in near 80% of cases. Furthermore, we replicated in this new study cohort the recent discovery of a deficiency of the glutamatergic modulator of brain plasticity and insulin-sensitizing agent acetyl-L-carnitine (LAC) in subjects suffering from MDD.

Conclusions: We report in-vivo evidence for brain insulin resistance (IR) as a possible biological determinant of MDD with sex-specific relationships with severity of depressive symptoms. Furthermore, integrated measures of both central and systemic IR can serve to predict depression diagnosis with a precision of near 80% as showed by machine learning. The current study also shows the utility of exosomes harvested from the blood of a patient in depressive episode and enriched for brain origin as a tool to study in-vivo molecular mechanisms otherwise inaccessible in the human brain. Together with previous findings of decreased levels of the glutamatergic modulator of brain plasticity and insulin-sensitizing agent acetyl-L-carnitine (LAC) in subjects suffering from MDD (a finding replicated in this study cohort), our translational framework suggest that further mechanistic exploration of the link between LAC deficiency and IR (both central and systemic IR) will aim to develop a novel framework of regulation of brain plasticity and personalized medicine strategies to treat MDD.