Urea, the toxic end-product of protein catabolism, is elevated in end-stage renal disease (ESRD), although it is unclear whether or how it contributes to disease. Urea can promote the carbamylation of proteins on multiple lysine side chains, including human albumin, which has a predominant carbamylation site on lysine 549. The proportion of serum albumin carbamylated on Lys-549 (%C-Alb) correlated with time-averaged blood urea concentrations and was twice as high in ESRD patients than in non-uremic subjects (0.90% vs. 0.42%, P<0.0001). Baseline %C-Alb was higher in ESRD subjects who died within 1-year than in those who survived longer than 1 year (1.01% vs. 0.77%, P<0.001) and was associated with an increased risk of death within 1 year (HR of 3.76, 95% CI: 2.20–6.43, P<0.0001). These findings were validated in an independent cohort of diabetic ESRD subjects (HR 3.73, 95% CI: 2.00–6.96, P<0.001). Decreased concentrations of serum amino acids correlated with higher %C-Alb in ESRD patients, and mice with diet-induced amino acid deficiencies exhibited greater susceptibility to albumin carbamylation than did chow-fed mice. In vitro studies showed that amino acids such as cysteine, histidine, arginine, lysine, as well as other nucleophiles such as taurine, inhibited cyanate-induced C-Alb formation at physiologic pH and temperature. Together, these results suggest that chronically elevated urea promotes carbamylation of proteins in ESRD, and that serum amino acid concentrations may modulate this protein modification. In summary, we have identified serum %C-Alb as a risk factor for mortality in patients with ESRD and propose that this risk factor may be modifiable with supplemental amino acid therapy.

Lithium is the first-line therapy for bipolar disorder. However, its therapeutic target remains controversial. Candidates include inositol monophosphatases, glycogen synthase kinase-3 (GSK-3), and a β-arrestin-2/AKT/ protein phosphatase 2A (β-arrestin-2/AKT/PP2A) complex that is known to be required for lithium-sensitive behaviors. Defining the direct target(s) is critical for the development of new therapies and for elucidating the molecular pathogenesis of this major psychiatric disorder. Here, we show what we believe to be a new link between GSK-3 and the β-arrestin-2 complex in mice and propose an integrated mechanism that accounts for the effects of lithium on multiple behaviors. GSK-3β (Gsk3b) overexpression reversed behavioral defects observed in lithium-treated mice and similar behaviors observed in Gsk3b +/-mice. Furthermore, immunoprecipitation of striatial tissue from WT mice revealed that lithium disrupted the β-arrestin-2/Akt/PP2A complex by directly inhibiting GSK-3. GSK-3 inhibitors or loss of one copy of the Gsk3b gene reduced β-arrestin-2/Akt/PP2A complex formation in mice, while overexpression of Gsk3b restored complex formation in lithium-treated mice. Thus, GSK-3 regulates the stability of the β-arrestin-2/Akt/PP2A complex, and lithium disrupts the complex through direct inhibition of GSK-3. We believe these findings reveal a new role for GSK-3 within the β-arrestin complex and demonstrate that GSK-3 is a critical target of lithium in mammalian behaviors.