HMGB1 mediates cognitive impairment in sepsis survivors

Abstract

Severe sepsis, a syndrome that complicates infection and injury, affects 750,000 annually in the United States. The acute mortality rate is approximately 30%, but, strikingly, sepsis survivors have a significant disability burden: up to 25% of survivors are cognitively and physically impaired. To investigate the mechanisms underlying persistent cognitive impairment in sepsis survivors, here we developed a murine model of severe sepsis survivors following cecal ligation and puncture (CLP) to study cognitive impairments. We observed that serum levels of high mobility group box 1 (HMGB1), a critical mediator of acute sepsis pathophysiology, are increased in sepsis survivors. Significantly, these levels remain elevated for at least 4 wks after CLP. Sepsis survivors develop significant, persistent impairments in learning and memory, and anatomic changes in the hippocampus associated with a loss of synaptic plasticity. Administration of neutralizing anti-HMGB1 antibody to survivors, beginning 1 wk after onset of peritonitis, significantly improved memory impairments and brain pathology. Administration of recombinant HMGB1 to naïve mice recapitulated the memory impairments. Together, these findings indicate that elevated HMGB1 levels mediate cognitive decline in sepsis survivors, and suggest that it may be possible to prevent or reverse cognitive impairments in sepsis survivors by administration of anti-HMGB1 antibodies.

Figures

Figure 1

Serum HMGB1: Serum samples were harvested from murine sepsis survivors or sham surgery survivors at each indicated time point (n = 4 to 6 mice per time point), and assayed for HMGB1 by Western blot. Densitometry and a standard curve were used for quantitation. Data: mean ± SE. *p < 0.05.

Figure 2

Cognitive and behavioral assessments of sepsis surviving mice. (A) Mice (CLP, n = 20, sham surgery, n = 20) were assessed for their ability to find the exit (indicated by a white box in the top view of the clock maze) in the spatial task. (B) CLP-surviving mice needed significantly more time to exit the maze at 1 month and 4 months following insult (**P < 0.01). (C) Sham surgery and CLP mice behaved equivalently in the primary screen as shown by their scores in muscle and spinal function, spinocerebellar function, sensory function, neuropsychiatric function and autonomic function at 1 month following CLP or sham surgery (see Methods for details). Sham surgery and CLP mice behaved (D) equivalently on the rotarod task, (E) exhibited equivalent locomotion and (F) displayed a similar level of anxiety in the black-white alley.

Figure 3

Preservation of memory function by neutralizing HMGB1 (A) Neutralizing anti-HMGB1 antibody was administered to CLP mice at 1 wk after sepsis. Serum levels of HMGB1 were analyzed at 4 wks after CLP (*P < 0.05). (B) CLP mice were given anti-HMGB1 monoclonal antibody or iso-type control antibody and subjected to the clock maze task. (C) Mice receiving anti-HMGB1 antibody performed significantly better than those receiving control antibody (**P < 0.01).

Figure 4

Memory impairment in mice receiving HMGB1. (A) Mice were given daily injections of HMGB1 or saline and subjected to training on the clock maze task. (B) HMGB1 caused a significant decline in memory function (*P < 0.05).

Figure 5

Neuroanatomical assessments in the hippocampus. (A) Representative cresyl violet-stained sections showed no gross abnormalities in the hippocampus of CLP mice when compared with sham surgery mice. (B) Golgi staining of CA1 neurons in the hippocampus, 4 wks after CLP or sham surgery (bar, 500 μm). (C) Quantification of CA1 apical dendritic spine density (spines per μm, mean ± SD) at 2 wks, 4 wks and 4 months after CLP or sham surgery. The sham group had significantly more spines at the later time points (*P < 0.05, three to four mice in each group; three to six dendritic arbors from each animal). (D) Mice that were treated with anti-HMGB1 antibody (Ab) following CLP exhibited a density of dendritic spines comparable to sham animals, 4 wks after surgery (three animals, six dendritic arbors from each animal). At right, Golgi–stained section of CA1 neuron from a CLP animal that was treated with anti-HMGB1 Ab (bar, 50 μm).