The identification of progranulin mutations by Baker and colleagues is a major advance in our understanding of frontal temporal dementia (FTD). The work by both Baker and Cruts and their colleagues shows that loss of progranulin function is a major cause of FTD, at least in some populations. These findings are remarkable for several reasons: first, this is the first simple loss-of-function autosomal dominant disease; second, it suggests that the genetic linkage of two FTD loci with similar clinical features, but different pathologies, close to the same locus was just a confusing coincidence. Third, it will undoubtedly spawn a huge amount of effort to define the limits of the phenotype and to elucidate its precise function in the CNS. It will also be interesting to see whether other diseases with ubiquitin inclusions will share related pathogenic mechanisms.

View all comments by John Hardy

These studies are spectacular advances in FTD research that open up new avenues for understanding mechanisms of FTLD-U. Notably, since progranulin proteins, or derivatives thereof, were not found in the ubiquitin inclusions of these FTLD-U disorders, it will be important to identify the ubiquitinated disease protein(s) that form these hallmark lesions of FTLD-U.

View all comments by Virginia Lee
View all comments by John Trojanowski

Both of these papers represent a significant discovery of a novel mutation on progranulin, a protein with no known CNS function. It is a known growth factor in vasculo and tumorigenesis, and it may turn out to have nerve growth factor properties as well; therefore, it is reasonable to postulate that a molecular deficit caused by its mutation could produce neurodegenerative disease such as frontotemporal dementia (FTD). We published the first chromosome 17-linked ubiquitin-positive family from Ontario in 2000 and the first intranuclear ubiquitin-positive inclusions in this and other families (1,2), but these genetic teams deserve credit for finding the mutation.

What is extraordinary is that progranulin is very close to the tau gene on chromosome 17, the known culprit in the mutated form in FTD linked to 17. How the two different genes interact, if at all, to cause a very similar illness is yet to be determined. The relationship of progranulin mechanisms to chromosome 9-linked cases and the valosin mutation with FTD and myopathy also deserves attention.

References:
1. Kertesz A, Kawarai T, Rogaeva E, St George-Hyslop P, Poorkaj P, Bird TD, Munoz DG. Familial frontotemporal dementia with ubiquitin-positive, tau-negative inclusions. Neurology. 2000 Feb 22;54(4):818-27. Abstract

2. Woulfe J, Kertesz A, Munoz DG. Frontotemporal dementia with ubiquitinated cytoplasmic and intranuclear inclusions. Acta Neuropathol 2001;102:94-102. Abstract

View all comments by Andrew Kertesz

From a clinical perspective, the identification of TDP-43 protein represents a major breakthrough in our understanding of both frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). The TDP-43 is the mystery protein that is associated with the ubiquitin-positive inclusions that are commonly found in many patients with FTLD and in most, if not all, patients with ALS.

This finding is particularly important because several recent papers suggest that patients who have FTLD with ubiquitin inclusions at autopsy (FTLD-U) account for approximately 50 percent of all autopsy-confirmed FTLD cases (1-3). The remaining majority of FTLD cases are associated with the tau protein, but other neuropathological diagnoses exist. The finding that possibly one-half of all FTLD patients may have ubiquitin-positive neuropathology means that any breakthroughs in the biology of this protein could potentially translate into helping a large proportion of FTLD patients.

In addition, the finding that the TDP-43 protein is also found in patients with ALS further supports...  Read more

From a clinical perspective, the identification of TDP-43 protein represents a major breakthrough in our understanding of both frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). The TDP-43 is the mystery protein that is associated with the ubiquitin-positive inclusions that are commonly found in many patients with FTLD and in most, if not all, patients with ALS.

This finding is particularly important because several recent papers suggest that patients who have FTLD with ubiquitin inclusions at autopsy (FTLD-U) account for approximately 50 percent of all autopsy-confirmed FTLD cases (1-3). The remaining majority of FTLD cases are associated with the tau protein, but other neuropathological diagnoses exist. The finding that possibly one-half of all FTLD patients may have ubiquitin-positive neuropathology means that any breakthroughs in the biology of this protein could potentially translate into helping a large proportion of FTLD patients.

In addition, the finding that the TDP-43 protein is also found in patients with ALS further supports the overlap between FTLD and ALS. Future research on the TDP-43 protein will likely also benefit ALS patients and help us understand how these two very different clinical phenotypes are related.

References:
1. Lipton AM, White CL 3rd, Begio EH. Frontotemporal lobar degeneration with motor neuron disease-type inclusions predominates in 76 cases of frontotemporal degeneration. Acta Neuropathol (Berl). 2004 Nov;108(5):379-85. Abstract

2. Johnson JK, Diehl J, Mendez MF, Neuhaus J, Shapira JS, Forman M, Chute DS, Roberson ED, Pace-Savitsky C, Neumann M, Chow TW, Rosen HJ, Forstl H, Kurz A, Miller BL.. Frontotemporal lobar degeneration: demographic characteristics of 353 patients. Archives of Neurology. 2005;62:925-930. Abstract

3. Forman MS, Farmer J, Johnson JK, Clark CM, Arnold SE, Coslett HB, Chatterjee A, Hurtig HI, Karlawish JH, Rosen HJ, Van Deerlin V, Lee V M-Y, Miller BL, Trojanowski JQ, & Grossman M. (2006). Frontotemporal dementia: Clinicopathological correlations. Annals of Neurology. 2006;59:952-962. Abstract

View all comments by Julene K. Johnson

In this paper, Drs. Lee and Trojanowski and colleagues have at long last identified the mystery protein hiding within the ubiquitinated inclusions that characterize certain histological forms of frontotemporal lobar degeneration (FTLD), termed FTLD-U. This task has challenged neuroscientists for well over a decade, with all prior attempts at identification using immunohistochemical or biochemical methods proving fruitless. The culprit protein is a TAR DNA-binding protein, known as TDP-43. This protein is present within all the ubiquitinated structures in FTLD-U, viz., the neuronal cytoplasmic inclusions, the neuronal intranuclear inclusions, and the neuritic changes, though whether this is the sole component of these structures (other than ubiquitin) remains uncertain. Some previous studies reported the presence of p62 protein within neuronal cytoplasmic inclusions, but such findings have been inconsistent. Moreover, Lee and Trojanowski have shown that the ubiquitinated neuronal cytoplasmic inclusions seen within spinal and cranial nerve nuclear motor neurons in motor neuron...  Read more

In this paper, Drs. Lee and Trojanowski and colleagues have at long last identified the mystery protein hiding within the ubiquitinated inclusions that characterize certain histological forms of frontotemporal lobar degeneration (FTLD), termed FTLD-U. This task has challenged neuroscientists for well over a decade, with all prior attempts at identification using immunohistochemical or biochemical methods proving fruitless. The culprit protein is a TAR DNA-binding protein, known as TDP-43. This protein is present within all the ubiquitinated structures in FTLD-U, viz., the neuronal cytoplasmic inclusions, the neuronal intranuclear inclusions, and the neuritic changes, though whether this is the sole component of these structures (other than ubiquitin) remains uncertain. Some previous studies reported the presence of p62 protein within neuronal cytoplasmic inclusions, but such findings have been inconsistent. Moreover, Lee and Trojanowski have shown that the ubiquitinated neuronal cytoplasmic inclusions seen within spinal and cranial nerve nuclear motor neurons in motor neuron disease (amyotrophic lateral sclerosis) also contain TDP-43.

This is an immensely important study with huge implications for neurobiology.

Firstly, it pinpoints a key biochemical constituent in the pathogenesis of FTLD-U and motor neuron disease (MND), and one which previous work would never have regarded as a likely candidate protein. Secondly, although an association between FTLD and MND had long been known on account of some cases showing defined clinical features of both disorders, sharing pathological features of both disorders, and families being known where some members had FTLD, others MND, and others the combined disorder, it was never clear whether this association was coincidental or causal. Now we can see causality, and the implication that FTLD and MND are part and parcel of the same disease spectrum will have major ramifications for understanding pathogenesis, and eventual treatment. Thirdly, the finding of TDP-43 pathological changes in FTLD patients with mutations in the newly identified progranulin (PGRN) gene, who typically show FTLD-U pathological changes, firmly brings together a causal relationship in these two fundamental proteins in driving the pathogenesis of the disorder, and opens up untapped vistas of neurobiological research.

Therefore, in rapid time, two major (protein) pieces in the jigsaw puzzle of FTLD have been identified. The challenge now will be to fit the pieces around these and eventually identify the linking processes that bring these together into the fuller picture. Nonetheless, it is clear that even within FTLD-U there are different histological and clinical phenotypes, and it will be necessary to dissect out biochemical or other factors that might determine where the TDP-43 pathological changes take place in the brain to produce the clinical phenotype. That is, why is it that in some patients the most common clinical manifestation of FTLD-U, frontotemporal dementia, is present in association with bilateral involvement of the frontal and temporal lobes, yet in others only the temporal lobes are affected—producing semantic dementia—and in others the left hemisphere is preferentially affected to give progressive non-fluent aphasia. Also, what determines whether TDP-43 changes will be in the brainstem and spinal cord to give MND, or in the cerebral cortex to give FTLD? Lastly, in all this flurry of excitement, it should not be forgotten that tauopathy is still a major cause of FTLD, and it is not immediately apparent how pathological changes in the expression or function of tau might link in with progranulin and TDP-43. Clearly, changes in all three molecules can produce the same disorder of FTLD either separately or collectively: it is not possible to unequivocally discriminate FTD patients with MAPT mutations from those with PGRN mutations, or others without mutations in either. Interrelationships within this Bermuda triangle of tau, progranulin, and TDP-43 will need to be addressed.

The identification of TDP-43 as a (major/sole) component of the ubiquitinated protein of FTLD and MND, in conjunction with the identification of mutations in PGRN, have opened up huge new fields within the neurobiology of neurodegenerative disease with tentacles that may stretch far wider than these two disorders themselves. Whether there is a role for either or both of these proteins in other disorders like Alzheimer disease and Parkinson disease remains to be seen. The gauntlet has been cast down—it is up to the neuroscience community to pick this up and address these issues. What is certain is that there will be a major change in the focus of neurobiological research as groups worldwide seek to investigate the implications of changes in proteins such as progranulin and TDP-43 in terms of health and disease. We can look forward within the near future to major advances in our understanding of how the brain works in respect of these molecules and why neurodegenerative disease occurs when they fail to function properly. Maybe even a treatment for neurodegenerative disease may come a little closer.

View all comments by David M.A. Mann

Neumann, Sampathu, Kwong, and colleagues have resolved a long-standing issue in the research field of frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). These authors have identified TDP-43 as a major component of ubiquitin-positive inclusions that characterize these disorders. They first extracted a fraction from the patients' brains using monoclonal antibodies and then analyzed it by mass spectrometry. Their findings have greatly facilitated the understanding of the molecular pathogenesis of FTLD and ALS.

Independently, we have also found TDP-43 as a component of the inclusions in FTLD [1]. Following electrophoresis of the sarkosyl-insoluble brain extracts from FTLD, Alzheimer disease (AD) and dementia with Lewy bodies (DLB), we have done exhaustive analyses by mass spectrometry. Following identification of each molecule that is more abundant in FTLD than AD/DLB, we have studied FTLD brain samples immunochemically and immunohistochemically. The antibodies to TDP-43 have immuno-stained neuronal inclusions and dystrophic neurites in the...  Read more

Neumann, Sampathu, Kwong, and colleagues have resolved a long-standing issue in the research field of frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). These authors have identified TDP-43 as a major component of ubiquitin-positive inclusions that characterize these disorders. They first extracted a fraction from the patients' brains using monoclonal antibodies and then analyzed it by mass spectrometry. Their findings have greatly facilitated the understanding of the molecular pathogenesis of FTLD and ALS.

Independently, we have also found TDP-43 as a component of the inclusions in FTLD [1]. Following electrophoresis of the sarkosyl-insoluble brain extracts from FTLD, Alzheimer disease (AD) and dementia with Lewy bodies (DLB), we have done exhaustive analyses by mass spectrometry. Following identification of each molecule that is more abundant in FTLD than AD/DLB, we have studied FTLD brain samples immunochemically and immunohistochemically. The antibodies to TDP-43 have immuno-stained neuronal inclusions and dystrophic neurites in the hippocampus and the temporal cortex in FTLD, and skein-like inclusions in the spinal cord in FTLD and ALS. Immunoblotting of the sarkosyl-insoluble fraction has shown abnormal changes in TDP-43 including hyperphosphorylation, fragment formation, and smear-like staining, all of which are similar to abnormal tau in AD and suggest a central role for the formation of abnormal aggregates. These findings are comparable with those by Lee's group. This is not surprising, since both groups have employed principally the same polyclonal antibody which is the only commercially available rabbit polyclonal.

In addition, however, we have found TDP-43-positive glial inclusions in the spinal cord in FTLD and ALS. These inclusions were also positive for tau. The distribution of glial inclusions was consistent with the degenerating areas, suggesting that glial abnormalities are involved in the pathological processes of ALS and FTLD. A difference between our results and theirs is the TDP-43-positive staining of some, but not all, tau-positive structures including Pick bodies and neurofibrillary tangles. The significance of these findings remains to be established, since immunoblot analysis did not show any abnormality in TDP-43 in Pick disease and Alzheimer disease. Our paper will appear shortly in Biochem Biophys Res Commun [1].

In the case of tau and α-synuclein, detection of abnormally modified molecules has revealed far more extensive pathology than that seen by ubiquitin immunohistochemistry. While lesions immunohistochemically labeled for TDP-43 are a little more numerous than those labeled for ubiquitin, the difference is far less than that we have experienced for tau and α-synuclein immunohistochemistry. This may be a point that remains to be cleared up. Another issue that is open for further investigations is to prove, by protein chemistry, the ubiquitination of TDP-43.

In any event, it has to be emphasized that two different approaches have come to the same conclusion, establishing with certainty that TDP-43 is the major component of the inclusions in FTLD and ALS. This further strengthens the hypothesis that these disorders are part of a clinicopathological spectrum that shares similar pathogenesis, and suggests the possibility that TDP-43 may be a common therapeutic target for these disorders. It is now necessary to investigate the relationship of TDP-43 to other molecules that have been reported to be associated with familial FTD, FTD with motor neuron disease, or ALS. Such molecules include progranulin, charged multivesicular body protein 2B (CHMP2B), valosin-containing protein, dynactin, and an unidentified protein in familial disease linked to chromosome 9.

References:
1. T. Arai, M. Hasegawa, H. Akiyama, K. Ikeda, T. Nonaka, H. Mori, D. Mann, K. Tsuchiya, M. Yoshida, Y. Hashizume, T. Oda, TDP-43 is a component of ubiquitin-positive tau-negative inclusions in frontotemporal lobar degeneration and amyotrophic lateral sclerosis, Biochem Biophys Res Commun, in press

View all comments by Tetsuaki Arai

The manuscript by Rademakers and colleagues provides evidence that increased binding of miR-659 to the 3’UTR of the GRN gene could underlie an important risk for TDP-43-positive frontotemporal dementia (FTLD-U). These data bring strong clinical support for the role of microRNAs in neurodegenerative disorders in humans. These results are consistent with a loss of function of the GRN gene in the disease, further linking gene dosage effects in neurodegenerative disorders (as seen, e.g., with APP in Alzheimer disease and SNCA in Parkinson disease).

I think Amber Dance did a fantastic job reviewing the highlights of this paper. I would like to discuss additional issues with regard to certain technical and mechanistic aspects of these findings, which could be taken into account when interpreting the data.

First, miR-659, located on chromosome 22 in humans, seems to be relatively very weakly expressed in adult brain (with cycle threshold [Ct] values of approximately 32 as measured by qRT-PCR). Therefore, whether endogenous miR-659 levels are sufficient to regulate GRN levels...  Read more

The manuscript by Rademakers and colleagues provides evidence that increased binding of miR-659 to the 3’UTR of the GRN gene could underlie an important risk for TDP-43-positive frontotemporal dementia (FTLD-U). These data bring strong clinical support for the role of microRNAs in neurodegenerative disorders in humans. These results are consistent with a loss of function of the GRN gene in the disease, further linking gene dosage effects in neurodegenerative disorders (as seen, e.g., with APP in Alzheimer disease and SNCA in Parkinson disease).

I think Amber Dance did a fantastic job reviewing the highlights of this paper. I would like to discuss additional issues with regard to certain technical and mechanistic aspects of these findings, which could be taken into account when interpreting the data.

First, miR-659, located on chromosome 22 in humans, seems to be relatively very weakly expressed in adult brain (with cycle threshold [Ct] values of approximately 32 as measured by qRT-PCR). Therefore, whether endogenous miR-659 levels are sufficient to regulate GRN levels in vivo remains speculative. Mechanistically, one must envisage that regulation of GRN mRNA by miR-659 occurs in a cell-autonomous fashion. One possibility, not shown here, is that miR-659 is expressed in specific cell types, such as the granular cell layer of the cerebellum where GRN protein is decreased (it should be noted that the qRT-PCR for miR-659 was performed on whole tissues). In my opinion, this would strongly strengthen the biological significance of the proposed mode of regulation.

Here, the authors use basic, but widely accepted in vitro systems to validate their hypothesis. First, artificial overexpression of miR-659 (at a concentration of 12 nM) in human M17 neuroblastoma cells leads to decreased expression of endogenous GRN protein levels (note that inverse experiments using antisense oligonucleotides to block endogenous miR-659 was not performed, possibly due to the extremely low levels of this microRNA in these cells). Whether GRN mRNA levels are affected in these conditions is not shown. Then, additional studies were conducted in mouse Neuro2A cells using luciferase-based constructs containing the GRN 3’UTR. In these latter experiments, functional effects on GRN expression are seen with the mutant TT construct at concentrations starting at 5 pM of exogenous miR-659. Again from a mechanistic point of view, it would be interesting to see whether the “increased” binding (i.e., increased sequence complementarity) of miR-659 to the mutant TT allele causes an siRNA effect (thus degradation of mRNA). It should be noted, however, that, in affected patients, GRN mRNA (from total tissue sections) is not affected.

Interestingly, the predicted target site (more particularly the “seed” sequence) for miR-659 in the GRN 3’UTR is only conserved in humans, and is not found in other mammals including mouse and dog (e.g., see www.targetscan.org). Similarly, miR-659 is, at least for now, only found in humans. Interestingly, the GRN 3’UTR is quite short (approximately 300 bp in length). In comparison, the BACE1 and APP 3’UTRs, which equally have functional microRNA target sites, are approximately 4,000 bp and 2,000 bp in length, respectively.

Overall, these findings provide novel and important clues into the development of FTLD-U. In addition, this study contributes to the potential role of microRNA pathways in the development of neurodegenerative disorders in human. I agree that relatively few patients were analyzed here to make definitive conclusions with regard to the biological relevance of these findings.

View all comments by Sebastien S. Hebert