6 October 2006. Repression is in the news this week, with a bevy of papers dealing with new mechanisms for turning off gene expression in neuronal cells, to a variety of ends. A Cell paper from Gabriel Corfas and colleagues at Harvard Medical School in Boston leads the pack with a bonanza of new findings concerning the intersection of presenilins and receptor tyrosine kinase signaling during brain development. From there, a trio of papers deal with aberrant gene expression in Huntington disease, including a second Cell paper, this one from Harvard Medical School researcher Dimitri Krainc and colleagues, showing an intriguing new link among the mutant huntingtin protein, gene regulation and mitochondrial dysfunction early in the disease.
In the first paper, the Corfas group shows that the decision of neural precursor cells to become neurons or astrocytes is regulated by a presenilin-dependent pathway involving cleavage of the ErbB4 receptor tyrosine kinase. When the polypeptide ligand neuregulin binds to ErbB4, they show, the cleavage of the activated receptor by the presenilin-containing γ-secretase liberates the intracellular cytoplasmic domain (E4ICD). This domain translocates to the nucleus and represses the expression of astrocytic genes, thereby steering the cells toward a neuronal fate. Using ErbB4 knockout mice, they found that neuregulin signaling accounts for the classic delay in astrogenesis during development. Finding that presenilins cleave ErbB4 suggests that the pathway could play a role Alzheimer disease pathology, particularly where presenilin mutations are present. Notably, neuregulin and ErbB4 have both been detected in neuritic plaques (Chaudhury et al., 2003).
ErbB4 is a member of the EGF receptor tyrosine kinase family, known for years to signal via a kinase cascade activated by ligand binding. More recently, another potential signaling mechanism was discovered, in which the receptor was proteolytically cleaved by γ-secretase, liberating a cytosolic domain (E4ICD) that underwent nuclear translocation (see ARF related news story and Lee et al., 2002). The processing resembled that of Notch or amyloid precursor protein (APP), right down to the intracellular release of the cytosolic domain and its subsequent association with transcriptional regulatory proteins in the nucleus. Not all Erb4 isoforms undergo such processing, however. It only occurs with ErbB4, and then only with one splice variant that is highly expressed in brain. This pattern suggested a role in the brain for the E4ICD, but just what it did was unclear.
First author S. Pablo Sardi and colleagues have solved that mystery by showing that upon neuregulin binding, the active ErbB4 receptor forms a complex with the signaling adaptor protein TAB2. Upon receptor cleavage, E4ICD-TAB2 complex rapidly moves from the cytosol to the nucleus, where it joins up with the transcriptional co-repressor N-CoR.
The authors found a physiological role for ErbB4 cleavage in neural precursor cells from rats. No matter what they threw at the cells—γ-secretase inhibitors, RNAi to various pathway components, or kinase-dead or non-cleavable variants of ErbB4—the results were consistent: neuregulin binding stimulated presenilin-dependent cleavage of ErB4, and the released E4ICD complexed with TAB2 and N-CoR to shut down astrocytic gene expression and differentiation. In vivo studies with ErbB4 knockout mice backed up this idea, and showed that E4ICD was necessary and sufficient to regulate astrogenesis in the intact developing cortex.
The results show that the γ-secretase activity of presenilins plays an important role in the developing nervous system, where the neuron-to-astrocyte switch is critical for setting up the architecture of the brain. ErbB4 signaling also plays a role in dendrite morphology, neurotransmitter receptor expression, and neuronal survival, suggesting that the pathological alterations in presenilin function associated with familial AD could have an impact via proteins other than APP. Alterations in presenilin activity might even affect adult neurogenesis. ErbB4 could also cross-talk with APP—the APP intracellular domain (AICD) has been shown to affect the activity of TAB2 and N-CoR (Baek et al., 2002).
An accompanying commentary by Joseph Schlessinger, Yale University, New Haven, Connecticut, and Mark Lemmon, University of Pennsylvania, Philadelphia, brings up yet another interesting aspect of the story: the elucidation of a new and potentially more general signaling mechanism for receptor tyrosine kinases. With the demonstration of the dual-protease signaling pathway, they write, “the work sets a standard with which to challenge all other studies of direct nuclear signaling by RTKs.”
A second tale of repression, also appearing in today’s Cell, involves the huntingtin protein, the cause of striatal neuron death in Huntington disease. Both transcriptional repression and altered energy metabolism have been tied to the pathology engendered by polyglutamine-expanded huntingtin. The work, out of Dimitri Krainc’s lab at Harvard Medical School, manages to bring these two aspects together with the demonstration that mutant Htt inhibits the expression of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), a transcriptional coactivator that regulates many genes involved in mitochondrial biogenesis and respiration. Knocking out PGC-1α enhances the toxicity of Htt in mice, and replenishing it by gene transfer blocks neurodegeneration. The identification of PGC-1α as a target of mutant Htt could explain the early defects in energy metabolism seen in the striatal neurons that eventually succumb in Huntington disease.
First author Libin Cui and colleagues first got onto PGC-1α when they found that mice lacking the gene show an HD-like degeneration in the striatum (see ARF related news story). Gene expression analysis showed that metabolic pathways involved in mitochondrial function were among the most significantly decreased in the knockout mice. When they examined the striatum of human HD carriers, or mutant huntingtin knock-in mice, they found lower-than-normal levels of PGC-1α, suggesting an explanation for the metabolic defects observed in Htt cells.
How does huntingtin cause decreased PGC-1α? In co-transfection assays, the polyQ expanded protein decreased PGC-1α promoter activity by disrupting a CREB/ATF4 transcriptional activating complex. Chromatin immunoprecipitation from mouse tissues revealed that mutant Htt, but not wild-type, could be found sitting on the PGC-1α promoter. Interestingly, wild-type huntingtin was found on the promoters of other genes, suggesting that the polyQ expansion resulted in differential targeting of the protein that might explain the alterations in gene expression seen in HD.
The authors conclude, “We hypothesize that in the normal state, PGC-1α regulates metabolic programs and maintains energy homeostasis in the CNS, whereas inhibition of PGC-1α transcription by mutant huntingtin leads to defects in energy metabolism and dysfunction of neurons that are most vulnerable to metabolic stress.” The selectivity for striatal neurons in HD would then reflect their vulnerability to energy depletion.
In addition to CREB/ATF, mutant huntingtin has been shown to repress transcription of Sp1-driven genes by disrupting that factor’s interaction with the general transcriptional activator TAF4. It is also known that fragments of huntingtin accumulate in the nucleus and are responsible for the neurotoxicity of the protein (see ARF related news story). A new paper from the labs of Xiao-Jiang Li and Shi-hua Li at Emory University School of Medicine in Atlanta shows that shorter huntingtin fragments, the ones most likely to be misfolded, are more potent at interfering with Sp1 and inhibiting transcriptional activation. First author Jonathan Cornett and colleagues found that shorter soluble fragments preferentially coprecipitated with Sp1 from cells and brains of HD mice, and were better at suppressing Sp1 activation of a reporter gene. Aggregated fragments did not seem to bind Sp1. The avidity of soluble Htt for Sp1 appeared to result from its misfolded state, since co-transfection of the chaperone HSP40, which helps keep Htt in its native conformation, prevented the inhibition of Sp1. The findings appear in a paper in press in the September 29 Journal of Biological Chemistry online.
The third paper that touches on neurodegeneration induced by polyglutamine proteins does so from a different angle. In a short article in today’s Molecular Cell, Howard Hughes Investigator Nancy Bonini and colleagues at the University of Pennsylvania in Philadelphia report that microRNAs can protect against the neurotoxicity of polyQ expanded proteins. Using Drosophila, first author Julide Bilen and colleagues identified a specific microRNA, bantam, which functions to prevent neurodegeneration downstream of both polyQ and tau proteins. The results make a place for microRNAs in modulating neurodegeneration and in the process open up another potential pathway to therapeutic manipulation.—Pat McCaffrey.
Sardi PS, Murtie J, Koirala S, Patten BA, Corfas G. Presenilin-dependent ErbB4 nuclear signaling regulates the timing of astrogenesis in the developing brain. Cell. 2006 Oct 6; 127:185-197. Abstract
Schlessinger J, Lemmon MA. Nuclear signaling by receptor tyrosine kinases: The first robin of spring. Cell. 2006 Oct 6; 127:45-48. Abstract
Cui L, Jeong H, Borovecki F, Parkhurst CN, Tanese N, Krainc D. Transcriptional repression of PGC-1a by mutant huntingtin leads to mitochondrial dysfunction and neurodegeneration. Cell. 2006 Oct 6; 127:59-69. Abstract
Cornett J, Smith L, Friedman M, Shin JY, Li XJ, Li SH. Context-dependent dysregulation of transcription by mutant huntingtin. J Biol Chem. 2006 Sep 29; [Epub ahead of print] Abstract
Bilen J, Liu N, Burnett BG, Pittman RN, Bonini N. MicroRNA pathways modulate polyglutamine-induced neurodegeneration. Molecular Cell. 2006 Oct 6; 24:157-163. Abstract