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Damage Removal by Alternate Proteasome in ESCs

“Removal of damaged proteins during ES cell fate specification requires the proteasome activator PA28”

Embryonic stem cells (ESCs) have been shown to contain relatively high levels of ‘damaged’ proteins such as carbonylated proteins (Hernebring et al 2006), which are eliminated upon differentiation and are also observed to accumulate in aging cells, (Hernebring et al 2006). Additionally, immunoproteasome subunit expression, required for the production of peptide antigens for display by antigen-presenting cells (Strehl et al) has been noted to be significantly altered during ESC differentiation (Atkinson and Collin et al), suggesting that the specific proteasomal subunits may be dynamically regulated during ESC self-renewal and differentiation. Now, in Scientific Reports, researchers from the laboratory of  Thomas Nyström at the Department of Chemistry and Molecular Biology, University of Gothenburg, Sweden have shown that damaged protein removal during ESC differentiation is associated with the induction of the proteasome activator PA28, normally associated with the immunoproteasome (Hernebring et al).

Dilution of protein damage by increased growth rate was firstly ruled out, as the growth rate of ESCs decreases during differentiation, while a reduction of damaged proteins due to a reduced rate of damage formation was also ruled out. Enhancement of the rate of damage removal could also reduce protein damage levels; and indeed, inhibition of proteasome function (using the proteasome-specific inhibitor epoxomicin) led to an increase in damage levels, suggesting that proteasome activity during early ESC differentiation is required to keep protein damage low. Upon assessment of proteasome subunit expression it was discovered that, while the levels of subunits of the 20S core proteasome, which mediates proteolysis, (β5 and a mixture of α-subunits) and 19S regulator (Rpn7), which mediated protein entry, were similar in undifferentiated and differentiated cells. However levels of the immunoproteasome associated PA28 subunits PA28α and β and the 20Si immunoproteasome subunit β5i became markedly elevated upon differentiation. Additional experiments demonstrated that the PA28-20S complex was hardly detectable in undifferentiated cells, but its incidence increased dramatically upon differentiation.   PA28 was also found to physically interact with the 20S core and the activity of the PA28-20S complex was induced 5-fold upon early differentiation, rising beyond that of the 26s proteasome. Knockdown of PA28α mRNA in differentiating cells abolished protein damage removal, while treatment in undifferentiated cells did not affect damage levels. Specificity of this effect to the loss of activity of PA28 rather than 20Si or 26S (20s and 19s regulator) was shown through the lack of alteration of β5i levels and the lack of an increase in poly-ubiquitinated proteins, as would be expected for a reduction in 19S/26S proteasome, in response to a decrease in PA28a.

The immunoproteasome normally functions in antigenic processing (Strehl et al) and so what specific role does it play in ESC pluripotency and differentiation? This study suggests that increased proteolytic activity in cells expressing PA28 is the main mode by which this proteasome activator aids the removal of protein damage during ESC differentiation. However, other modes of action have also been attributed to the immunoproteasome, such as protection against oxidative stress and the clearance of protein aggregates (Hussong et al, Li et al, Pickering et al and Seifert et al). Together, this suggests that the proteasome functions to provide an appropriate cellular environment to allow for exit from self-renewal and cellular differentiation, and connected to this suggestion, the authors end with an interesting proposition; can induction of the PA28 activator counteract the accumulation of damaged proteins in aging cells/tissues and can this counteract age-related disease?

 

References

  • Atkinson, SP. And Collin, J. et al. A putative role for the immunoproteasome in the maintenance of pluripotency in human embryonic stem cells. Stem Cells 30, 1373-84 (2012)
  • Hernebring, M. et al. Elimination of damaged proteins during differentiation of embryonic stem cells. Proc Natl Acad Sci U S A 103, 7700–7705 (2006)
  • Strehl, B. et al. Interferon-c, the functional plasticity of the ubiquitin–proteasome system, and MHC class I antigen processing. Immunological Reviews 207, 19–30 (2005)
  • Li, J. et al. Enhancement of proteasome function by PA28a overexpression protects against oxidative stress. FASEB Journal 25, 883–893 (2011)
  • Hussong, S. A. et al. Immunoproteasome deficiency alters retinal proteasome’s response to stress. Journal of Neurochemistry 113, 1481–1490 (2010)
  • Pickering, A. M. et al. The immunoproteasome, the 20S proteasome and the PA28ab proteasome regulator are oxidative-stress-adaptive proteolytic complexes. Biochemical Journal 432, 585–594 (2010).
  • Seifert, U. et al. Immunoproteasomes Preserve Protein Homeostasis upon Interferon-Induced Oxidative Stress. Cell 142, 613–624 (2010)

Study originally appeared in Scientific Reports.

Stem Cell Correspondent Stuart P. Atkinson reports on those studies appearing in current journals that are destined to make an impact on stem cell research and clinical studies.