"Molecular chaperones are known as protein folding factors, but then in the last couple of years we identified more and more co-chaperones that can actually link chaperones to the degradation machinery," Höhfeld explained. Protein degradation is really the main focus of his research, trying to understand this balance between protein folding and degradation and how it's regulated.
Höhfeld was drawn to molecular chaperones in graduate school 20 years ago. Ten years ago Höhfeld and colleagues found that HSPBP1 was involved in regulating the degradation of the Cystic Fibrosis Transmembrane Regulator (CFTR), that when mutated can lead to cystic fibrosis.2 Those studies were done in HeLa cells, and the next step for Höhfeld was to see the physiological role of HSPBP1 in an animal model. "Initially we came from a basic science background so we wanted to know the real function of these proteins at the organismal level," he said. The surprise was that mouse HSPBP1 was critical not in lung function but spermatogenesis.
"For this study we looked at the physiological role of the co-chaperone [HSPBP1] by generating a knockout mouse," Höhfeld said. They noticed the male mice lacking HSPBP1 were sterile. "You make knockout animals and it's always very exciting when you look for phenotypes. Suddenly we ended up in the testis, I never would have expected that... It turns out that a key role of this protein is in fulfilled in testis and it's involved in spermatogenesis," Höhfeld said.
"Molecular chaperones themselves use HSPBP1 to become stabilized in the testis," Höhfeld said. He explained that after the chaperones are made HSPBP1 is able to regulate their abundance. HSPBP1 does this by inhibiting the addition of ubiquitin, a cellular flag signaling the protein is garbage, to the chaperone proteins. So with HSPBP1 around, chaperones can live longer in the cell. This is important in the testis where spermatogenesis depends on the expression of chaperones.
Could mutations in HSPBP1 lead to infertility in men? Höhfeld explained that in mice "there is another protein that has very similar biochemical functions, and the other protein is not present in the testis, so therefore we have a phenotype in the testis. But in the brain the other component comes in and takes over the function of HSPBP1. In humans we don't know yet if both proteins are present in the testis."
As intriguing as he finds the questions, Höhfeld regrets this line of investigation has reached the end of its road for his lab. "We will stop here now because it took us very long to get the mice characterized," he said. But Höhfeld hopes that others, particularly cancer researchers, will pick up where he's leaving off, as tumor cells and spermatocytes have similarities worth investigating. "Both cell types, spermatocytes and tumor cells, apparently depend on high level chaperone expression that exerts antiapoptotic and survival function," Höhfeld said.
1Rogon, C., Ulbricht, A., Hesse, M., Alberti, S., Vijayaraj, P., Best, D., Adams, I., Magin, T., Fleischmann, B., & Hohfeld, J. (2014). HSP70-binding protein HSPBP1 regulates chaperone expression at a posttranslational level and is essential for spermatogenesis Molecular Biology of the Cell, 25 (15), 2260-2271 DOI: 10.1091/mbc.E14-02-0742
2Alberti S, Böhse K, Arndt V, Schmitz A, Höhfeld J. (2004). The Cochaperone HspBP1 Inhibits the CHIP Ubiquitin Ligase and Stimulates the Maturation of the Cystic Fibrosis Transmembrane Conductance Regulator Molecular Biology of the Cell, 15 (9), 4003-4010 DOI: 10.1091/mbc.E04-04-0293