In mammals, cells contain a group of proteins known as “specificity factors” that acting during gene expression. Specificity proteins, or Sps, bind to DNA at specific sequences and activate gene expression at those genes that possess the binding target for the Sps. Sps control the expression of a many genes, including house keeping, tissue-specific, development-specific and cell-cycle-regulated genes. There are nine different Sps that have been discovered in mammals, and they are numbered Sp1 to Sp9. For a good article on Sps, see Suske G, Bruford E, Philipsen S (2005) Mammalian SP/KLF transcription factors: bring in the family. Genomics 85: 551–556.
Several of the Sps have been shown to play rather central roles during development. For example, mouse embryos that lack Sp1, die very early (around embryonic day 10.5). Mice that do not have functional Sp3, develop until the end of pregnancy but die immediately after birth due to respiratory failure. Such embryos also show impaired skeletal bone ossification, tooth development, heart development, and abnormal organization of the placenta. Newborn Sp4-deficient mice either die within the first month of life or are severely growth-retarded, and males do not breed and females show delayed in sexual maturation. Sp4 is also required for the development of the conduction system in the heart.
Of the Sps, Sp2 has been very poorly characterized. For this reason, Troy Ghashghaei at North Carolina State University has investigated the role of Sp2 in neural stem cells.
In collaboration with Jon Horowitz, a colleague at the Center for Comparative Medicine and Translational Research, they made mouse neural stem cells that lacked functional Sp2. The neural stem cells without Sp2 were able to divide, but the progeny were unable to differentiate into neurons. Instead the neural stem cells simply divided over and over without ever forming neurons.
This result was unexpected, since Horowitz had shown in an earlier publication that overproducing Sp2 did something similar in skin stem cells. Instead of dividing and forming new skin cells, skin stem cells that expressed excessive amounts of Sp2 continued to divide and without forming new skin cells. Instead they formed tumors (Kim TH, et al., Cancer Res. 2010 Nov 1;70(21):8507-16. doi: 10.1158/0008-5472.CAN-10-1213).
In Horowitz’s view: “We believe that Sp2 must play a fundamental role in the lives of normal stem cells. Trouble ensues when the mechanisms that regulate its activity are overwhelmed due to its excess abundance.”
However, this recent work shows that in a different system, the absence of Sp2 has much the same effect – prevent of stem cells from producing progeny that differentiates into mature cell types and continued, uncontrolled proliferation.
Of this Ghashghaei said: “It’s interesting that both an overabundance of this protein and a total lack of it result in similar disruptions in how stem cells divide. So while this work confirms that Sp2 is absolutely necessary for stem cell function, a lot of questions still remain about what exactly it is regulating, and whether it is present in all stem cells or just a few. We also need to find out if Sp2 deletion or overabundance can produce brain tumors in our mice as in the skin.”
Ghashghaei continued: “Lastly, we are very interested in understanding how Sp2 regulates a very important decision a stem cells has to make: whether to produce more of itself or to produce offspring that can become neurons or skin cells. We hope to address these questions in our future research, because these cellular mechanisms have implications for cancer research, neurodevelopmental diseases and regenerative medicine.”
See Liang H, Xiao G, Yin H, Hippenmeyer S, Horowitz JM, Ghashghaei HT. Neural development is dependent on the function of specificity protein 2 in cell cycle progression. Development. 2013 Feb;140(3):552-61. doi: 10.1242/dev.085621.