|Home | About | Journals | Submit | Contact Us | Français|
Progranulin mutations result in frontotemporal dementia, but the underlying pathophysiology has remained largely unexplained. New data by Geschwind and colleagues uncovered that the Wnt/FZD2 signaling pathway is an early and critical contributor to disease pathology.
Frontotemporal dementia (FTD) is a neurodegenerative disorder, characterized by progressive behavioral, cognitive, emotional, social, language and personality deterioration during adult life (van Swieten and Heutink, 2008). The disease can arise as a result of mutations in many genes, including microtubule-associated protein tau (MAPT), progranulin (GRN), charged multivescicular body protein 2B (CHMP2B), and valosin-containing protein (VCP) (Neumann et al., 2009). Mutations in MAPT and in GRN, both located on chromosome 17q21, account for 50–60% of cases of familial FTD. While the causality of the GRN mutations vis-à-vis FTD has been well-replicated, limited progress has been made in understanding the molecular events by which reduced GRN levels give rise to disease symptoms. The study by Geschwind and colleagues in this issue (Rosen et al., 2011) exploits an impressive cascade of logical and comprehensive experiments, and represents the first significant breakthrough in this regard.
Progranulin (also known as acrogranin and epithelin precursor) is a 593 amino acid secreted glycoprotein that is composed of 7.5 tandem repeats of a 12-cysteine granulin motif with the consensus sequence, and the gene is expressed across a wide variety of tissues, including the brain (Bhandari et al., 1992). Progranulin was first identified as a gene that was over-expressed in epithelial tumors and involved in wound healing and inflammation, and did not attract the attention of neuroscientists for more than a decade: GRN mutations were first linked to FTD in 2006 by linkage analyses and positional cloning (Baker et al., 2006). GRN mutations lead to haploinsuficency (Ahmed et al., 2007), whereby GRN levels are reduced by approximately 50%, leading to ubiquitin positive TDP-43 inclusions in both neurons and glia, but in the absence of tau pathology (Neumann et al., 2009).
To address the changes associated with GRN deficiency, the team led by Geschwind started by developing an in vitro model using primary human neural stem cells (hNPC) in which shRNA was used to diminish GRN levels. Thus, GRN knockdown led to robust gene expression changes in the hNPCs, including enrichment in genes related to cell cycling and ubiquination. In addition, in GRN-inhibited neural progenitor cultures, they observed increased pyknotic nuclei and activated CASP3 staining, suggestive of increased apoptosis in this setting. Furthermore, immunostaining for neuronal and glial markers showed that GRN down-regulation in vitro led to reduced neuronal survival, mimicking the hallmark neuronal death observed in FTD patients.
To further elucidate the mechanisms underlying physiological changes in response to GRN downregulation, the authors tried to uncover the responsible transcript network. Using Illumina DNA microarrays, they analyzed the expression profile of GRN-inactivated hNPCs, and found that numerous members of the Wnt signaling pathway showed dysregulation of transcription, which they validated with qPCR. The pattern of dysregulation indicated increased activity in the Wnt pathway (Takahashi-Yanaga and Sasaguri, 2007) in GRN-inactivated cells. A follow up, custom-designed data mining with weighted gene co-expression network analysis (WGCNA) (Zhang and Horvath, 2005) was employed. WGCNA allows the identification of modules of co-expressed genes, and here it revealed that alteration in mitochondrial function is a primary effect of GRN deficiency, providing further support that mitochondrial and protein degradation pathways dysfunctions are a critical part of FTD pathophysiology (David et al., 2005; Zhang et al., 2009).
In an effort to seek further confirmation of their findings on diseased brain tissue, the authors performed WGCNA and Gene Ontology data mining of a previously published postmortem microarray dataset from patients with sporadic FTD, and GRN+ FTD, and matched controls. The overall results confirmed that the GRN-inhibited hNPC findings were highly concordant with the postmortem data from FTD subjects. Furthermore, gene expression data from cerebellum, cortex, and hippocampus of 6-week old GRN knockout mice revealed that frizzled homolog 2 - Fzd2 (a receptor that mediates Wnt signaling) upregulation was one of the most consistently up-regulated genes. Importantly, this upregulation occurred well before the appearance of neuropathological alterations or overt neurodegeneration in the brains of mutant mice.
The overall results prove, beyond any doubt, that the GRN+ FTD pathology is at least in part mediated through dysregulation of the Wnt signaling pathway, and that these changes are in place before the onset of neurodegenerative changes (Figure 1). Furthermore, their results imply that the mitochondrial and protein degradation pathways are a first consequence of the GRN-mediated Wnt signaling deficit, and that the inflammatory, synaptic and other associated changes represent downstream evolution of the disease. Finally it is also important to point out that their innovative use of human primary neuronal progenitors, postmortem data, transgenic mouse models and superb data mining strategies are an extremely powerful combination of research tools. Yet, regardless of the wealth of the presented data, a number of questions remain unanswered.
First, how is GRN exactly regulating the Wnt signaling pathway? Non-canonical Wnt signaling pathways driven by AP1, cJun, and NFAT did not show significant changes in the current study, and the exact relationship between GRN – Wnt signaling is an intriguing topic of further investigations. Assessing the role of genes like Tcf7l2, a key mediator of canonical Wnt signaling, might be fruitful, as dnTcf7l2 (a truncated Tcf7l2 isoform) cannot bind beta-catenin and therefore acts as a potent dominant-negative Wnt antagonist. Such experiments might help to map out the pathway between GRN and Wnt and their regulators, and provide knowledge-based targets for drug design.
Second, GRN haploinsufficiency is present in the brain from early embryonic life. Why is the effect of the GRN reduction most prominent and progressive in the 6th and 7th decade of life, and what are the compensatory mechanisms that “burn out” by late adulthood? Clearly, GRN+ FTD has two phases: a latent, and asymptomatic phase, when the molecular pathophysiology progresses over time, but cellular adaptational mechanisms can compensate for the detrimental effects of GRN haploinsufficiency. Over time the compensatory mechanisms fail, cellular damage accumulates, and FTD pathology and symptoms evolve. The compensatory mechanisms that keep the disease “in check” for half a century are poorly understood, and it is not known if this compensation is mediated through a Wnt-dependent signaling pathway. However, it is very likely that this part of the protective-adaptational response will involve additional, non-Wnt dependent processes (Kumar-Singh, 2011), potentially including growth factor-related signaling cascades for endogenous neuroprotection (Saragovi et al., 2009).
Current FTD drug discovery approaches are targeting pathways of TDP-43 and tau, with a rationale that the new drugs should either prevent formation or increase clearance of these protein aggregates (Trojanowski et al., 2008). So, could modulation of the Wnt signaling pathway achieve this goal? Regardless of the enticing findings of the current study, there is no clear-cut answer to this question, and one can only be cautiously optimistic. Wnt/β-catenin signaling is widespread in the whole body (from brain to bone and muscle), and it is conceivable that systemic modulation of the Wnt pathway might result in numerous and potentially serious side effects (Takahashi-Yanaga and Sasaguri, 2007). In addition, the in vitro cell line and in vivo mouse models might not fully recapitulate the critical features of the human disease. Finally, the most beneficial effect of Wnt pathway modulation would be expected during the latent phase of the disease: any beneficial effect of Wnt modulation could be diminished by the time that the diagnosis is made and/or the inflammatory and degenerative changes arise.
Given that disease pathophysiology encompasses both neuronal and glial changes, what is the relationship between these two deficits? Previous studies indicated that GRN-deficient macrophages and microglia were cytotoxic to hippocampal cells in vitro, and that GRN-deficient hippocampal slices were hypersusceptible to deprivation of oxygen and glucose (Yin et al., 2010). Thus, while the present results by Rosen et al. argue for a strong neuronal pathology in response to reduced GRN levels, early contribution of glial dysfunction to the FTD pathology cannot be excluded. Both glia and neurons express GRN from early development (Ahmed et al., 2007), and microglia lacking GRN may become activated, triggering neuronal-glial interactions that can further accelerate neuronal degeneration and cell death.
Finally, what is the convergence of the early-activated molecular pathways between the various forms of FTD, and more widely, across all dementias? The findings from Rosen et al., as well as previous reports suggest that the molecular pathophysiology, regardless of the genetic cause, might share significant molecular commonalities between the various forms of early onset dementias. To underscore this point, it was suggested almost a decade ago that drugs that both inhibit the cell cycle and rescue Wnt activity could provide novel Alzheimer’s disease therapeutics (Caricasole et al., 2003). Thus, the accumulating evidence suggests that the effect of various FTD-causing mutations and other dementias converge on a few, common intracellular pathways including, but not limited to Wnt signaling. Using converging approaches across hNPC, transgenic animal models and human postmortem brains, we should attempt to decipher the earliest commonalities between the transcriptome/signaling disturbances across various forms of early-onset dementias. Consistent data mining with WGCNA (Zhang and Horvath, 2005) could be crucial for a success of such an effort, as over the last several years WGCNA has arisen as a very powerful, function-based network analysis tool.
A great study always opens up new research avenues and highlights the most important, missing knowledge. The current study is no exception to this rule, and the findings of Rosen et al. indicate a clear path to the most intriguing future experiments – and hopefully, provide us with a good foundation for development of long-awaited, efficacious therapies for early-onset dementias.
Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.