Although not a ubiquitous response, the results of this study demonstrate that the brain is capable of releasing large amounts of Hsp72 after prolonged exercise. Although several investigators have demonstrated the induction of Hsp72 synthesis in specific regions of the brain in response to hyperthermia (Walters et al 1998
; Leoni et al 2000
), ischemia (Simon et al 1991
), hypoxia (Murphy et al 1999
), and energy depletion (Imuta et al 1998
), this is the first study, in any species, to demonstrate that the brain is capable of releasing Hsp72 in vivo. Given that the exercise was moderate in nature and that the increased cerebral Hsp72 release occurred in the absence of any changes in cerebral blood flow, it is unlikely that necrosis of specific cells within the brain contributed to the elevated cerebral Hsp72 release. Therefore, it is possible that specific cells within the brain possess an exocytotic pathway for the release of Hsp72 into the extracellular environment, allowing it to perform specific functions.
Although Hsps are quintessentially viewed as intracellular proteins with a vital role in maintaining cellular homeostasis, important extracellular roles for Hsps have been identified. In particular, accumulating evidence supports the hypothesis that Hsp72 released from necrotic cells into the extracellular environment may act as a “danger” signal alerting the immune system to the presence of dying cells (Asea et al 2000
; Vabulas et al 2002
). However, the results presented in this study, a previous study demonstrating that the hepatosplanchnic tissues release Hsp72 during exercise (Febbraio et al 2002a
), and a recent study showing an elevation in the plasma concentration of Hsp72 in rats subjected to tail shock, suggest that Hsp72 release into the systemic circulation can occur independently of cell necrosis. Importantly, it has been demonstrated that cultured rat embryo cells (Hightower and Guidon 1989
) and glial cells (Guzhova et al 2001
) subjected to transient heat shock are able to release Hsp72 independently of cell necrosis in vitro; furthermore, a protein transport mechanism used by the cell to release Hsp72 has been identified recently (Broquet et al 2003
). Taken together, the in vitro and in vivo evidence suggests that in addition to the well-established intracellular “housekeeping” functions of Hsp72 and the immunoregulatory role of Hsp72 released from necrotic cells, cells under stress are able to actively release Hsp72 into the extracellular milieu through a specific protein secretion pathway. Regarding the possible biological role of actively released Hsp72, it has been demonstrated that Hsp72 released from glia are taken up by neuronal cells which may enhance neuronal stress tolerance, and it has been hypothesized that an elevation in the extracellular Hsp72 concentration may facilitate recovery from bacterial inflammation (Campisi et al 2003
). However, the elucidation of the biological role of Hsp72 released from the brain and hepatosplanchnic tissues during exercise awaits further investigation.
It is important to note that in the present study we observed an increase in brain Hsp72 release after exercise in only 3 of 6 subjects; however, the reasons for the subject specificity of this response are unclear. Nonetheless, and importantly, our data clearly demonstrate that the brain is capable of releasing Hsp72 in vivo in response to a stressor. It was not possible in the present study to address the nature of the stimulus by which exercise increases brain Hsp72 release. As discussed above, necrosis of specific cells within the brain is unlikely to have contributed to the elevated cerebral Hsp72 release. The depletion of energy stores, hypoxia, and ischemia have been shown to induce the synthesis of Hsp72 within specific cells of the brain (Simon et al 1991
; Imuta et al 1998
; Murphy et al 1999
). However, we have previously published data from the present study (Nybo et al 2003
) showing that cerebral lactate release and the cerebral respiratory exchange ratio are unchanged during exercise compared with during rest. This would suggest that neither energy depletion nor a relative hypoxia or ischemia within specific cells of the brain are likely to account for the augmented brain Hsp72 release after exercise. However, given that prolonged exercise causes an elevation in brain temperature (Nybo et al 2002
) and that hyperthermia induces the synthesis of Hsp72 within the brain (Walters et al 1998
; Leoni et al 2000
), it is possible that an elevation in brain temperature may contribute to the increased release of Hsp72 from the brain after exercise.
In summary, we have demonstrated that the brain is capable of releasing Hsp72 in response to prolonged exercise; however, this response appears to be subject dependent because we observed an increase in brain Hsp72 release in only 3 of 6 subjects. Importantly, it seems very unlikely that the release of Hsp72 from the brain was due to cell necrosis because cerebral blood flow and oxygen consumption (Walters et al 1998
) were unchanged during exercise compared with values obtained at rest. These data support previous findings (Dekaban 1978
; Imuta et al 1998
; Febbraio et al 2002a
) and suggest that the brain possesses an exocytotic pathway for the release of Hsp72. Future studies will be required to determine the biological significance of this release.