TDP-43 regulates mRNA metabolism in the nucleus and occurs in the cytoplasm mainly in the form of aggregates in amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), and other neurodegenerative disorders. Researchers have debated whether this RNA binding protein performs any physiological role outside the nucleus, and now a new study suggests that it does. In the February 5 Neuron, J. Paul Taylor, St. Jude Children’s Research Hospital, Memphis, Tennessee, and colleagues report that TDP-43 helps move mRNA-containing granules along axons to nerve terminals. Pathogenic mutations in TDP-43 hold back this traffic. “This paper defines a novel function for TDP-43, and provides a mechanism by which mutations could cause dysfunction,” said Aaron Gitler, Stanford University School of Medicine, California, who was not involved in the study.

TDP-43, or TAR DNA-binding protein 43, is best known for its hand in DNA transcription and RNA splicing in the nucleus. However, scientists have also seen it binding to proteins responsible for the transport of mRNAs in the cytoplasm, and it has been spotted along dendrites and axons all the way out to their tips (see Freibaum et al., 2010Narayanan et al., 2012). This hints that TDP-43 helps ferry certain mRNAs to the far reaches of neurons, where they can be translated rapidly in situ in response to neuronal activity (see Liu-Yesucevitz et al., 2011). However, no one has observed TDP-43 behave in this fashion. First authors Nael Alami, Rebecca Smith, and colleagues set out to explore that possibility by comparing the behavior of wild-type and mutant versions of the human protein in cells from flies, mice, and people. 

In motor neurons from flies and primary cortical neurons from mice, the researchers fluorescently tagged overexpressed wild-type human TDP-43 or one of two ALS-causing mutant forms—M337V or A315T. Most of the wild-type TDP-43 stayed in the nucleus, though a steady trickle of cytoplasmic granules containing the protein extended down to the tips of the axon (see image below). In contrast, mutant forms of the protein built up in and around the cell body; little to none of it made it very far along the axon. Live imaging revealed that granules containing mutant TDP-43 moved more haltingly down the axon than the native form. Forward motion was particularly sluggish, and a higher percentage of the granules moved backward. Interestingly, mitochondria moved equally well down the axon in all mice, suggesting the TDP-43 mutants did not impair axonal transport in general. 

Going Places: A complex containing TDP-43 (red) moves along the axon (white outline) of a mouse cortical neuron. [Image courtesy of Neuron, Alami et al., 2014 Figure 2A]

To test whether TDP-43 mutations limit transport of mRNAs, Alami and colleagues created an “mRNA beacon.” This is a fluorescent reporter that allowed them to follow transcripts of Neurofilament-L (Nefl), a well-known TDP-43 binding partner. Most previous attempts to label mRNA required genetic modifications or chemical treatment that killed the cells. In this case, the researchers were able to measure transcripts in live cells by adding a fluorescent oligonucleotide that bound specifically to the Nefl transcript. The beacon tracked together with Green Fluorescent Protein-labeled TDP-43. In mouse cortical neurons, Nefl-positive granules containing TDP-43 moved forward, on average, while those without TDP-43 moved back toward the nucleus, suggesting that the RNA-binding protein helps drive anterograde motion of the granules. The researchers next tracked the movement of human Nefl transcript in motor neurons cultured from the induced pluripotent stem cells of three ALS patients. These cells expressed endogenous levels of TDP-43 with the M337V, A315T, or G298S mutations. The scientists saw that anterograde motion of Nefl was impaired compared with that in cells from healthy people. 

The work sheds light on yet another way in which TDP-43 dysfunction could lead to ALS and FTLD, according to the authors. “We think that TDP-43 is responsible for the movements of its mRNA partners along the axon. This might explain why motor neurons, which have especially long axons, are selectively affected in ALS,” said Alami. He and colleagues plan to test if these trafficking alterations translate to problems with protein synthesis at the synapse, neural development, or neurodegeneration. 

In addition, Alami’s group plans to make other mRNA beacons to track transcripts of FUS and granulin, which are associated with ALS, to see if TDP-43 mutations affect their transport. The scientists also want to know whether disease-causing mutations in other RNA-binding proteins disrupt delivery of mRNA along the axon. 

Wilfried Rossoll, Emory University, Atlanta, has previously reported that TDP-43 is transported down the axons of motoneurons (Fallini et al., 2012). He called this new study exciting for taking a rare look at the axonal function of TDP-43 and finding similar effects in several disease models. It will be important to see whether this transport defect applies to TDP-43 proteinopathies that involve mutations in other proteins and in sporadic forms of ALS, he added. Gitler, who penned an accompanying Preview in Neuron, noted that this mRNA transport problem is likely one of several defects that arise when TDP-43 is mutated. Genetic disruptions also cause loss of nuclear function and toxic aggregation in the cytoplasm. It is unclear which contributes most to disease, he said.—Gwyneth Dickey Zakaib

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  1. The pathogenetic mechanisms whereby TDP-43 alterations lead to ALS and FTLD are far from being clear. Defining these processes is challenging since TDP-43 is a "multitasking" protein, implicated in different aspects of RNA metabolism.

    Thus, before evaluating possible therapies targeting TDP-43, most researchers' efforts are still devoted to characterizing TDP-43's pathophysiological functions.

    Earlier observations by other groups suggested that neuromuscular junction alteration was one of the possible consequences of the lack of functional TDP-43. However, there was only indirect evidence for putative axonal spreading of TDP-43.

    In this context, the work now published by the group coordinated by Dr. Taylor further addressed this issue in three different models: in vivo in Drosophila, in mouse cortical neurons, and in human stem cell-derived motor neurons from ALS patients.

    The authors were able to demonstrate that TDP-43 is implicated in the anterograde axonal transport of specific mRNAs from the soma to distal axonal compartments, including the neuromuscular junctions.

    Till now, TDP-43 functions have been mostly limited to the nucleus, with some incursions in the cytoplasm through shuttling processes.

    This work now lends support to a novel "cytoplasmic" role for this nuclear factor, specifically related to the axonal transport.

    The importance of this new function for TDP-43 goes along with the observation that this activity seems to be conserved through evolution (not only in the "next of kin" mouse but also in the "distantly related" fruit fly). This is consistent with the previously characterized splicing-related functions of the human and Drosophila TDP-43 orthologs.

    In addition, it is intriguing that two TDP-43 mutations (M337V and A315T) seem to alter the TDP-43 "cargo" function. In this respect, it should be noted that previous works have shown that these amino acid changes cause neurotoxicity in different animal models (including Drosophila) without a clear demonstration that they alter the splicing-related function of TDP-43, or its ability to interact with other proteins or to form aggregates.

    However, it should be kept in mind that the number of patients affected by ALS (and other neurodegenerative diseases) who carry TDP-43 mutations is limited and that age of onset of the disease in patients is extremely variable, suggesting that other endogenous and exogenous factors might contribute to prime/modulate TDP-43 proteinopathies and trigger the onset of neurodegenerative disorders.

    Nevertheless, the involvement of TDP-43 in axonal protein transport is an important milestone in our knowledge of its functions. In the future it will be important to understand the "mission" of this transport and identify additional factors involved, as well as the targets of this transport.

    Overall, Dr. Taylor's group has carried out a very insightful piece of work that will allow us to shed further light on the puzzling pathogenesis of ALS and other TDP-43 related diseases.

    View all comments by Maurizio Romano

References

Alzpedia Citations

  1. TDP-43

Paper Citations

  1. . Global analysis of TDP-43 interacting proteins reveals strong association with RNA splicing and translation machinery. J Proteome Res. 2010 Feb 5;9(2):1104-20. PubMed.
  2. . Identification of RNA bound to the TDP-43 ribonucleoprotein complex in the adult mouse brain. Amyotroph Lateral Scler Frontotemporal Degener. 2012 Oct 24; PubMed.
  3. . Local RNA translation at the synapse and in disease. J Neurosci. 2011 Nov 9;31(45):16086-93. PubMed.
  4. . The ALS disease protein TDP-43 is actively transported in motor neuron axons and regulates axon outgrowth. Hum Mol Genet. 2012 Aug 15;21(16):3703-18. PubMed.

Further Reading

Papers

  1. . Evolutionarily-conserved hnRNP A/B proteins functionally interact with human and Drosophila TAR DNA-binding protein 43 (TDP-43). J Biol Chem. 2014 Feb 3; PubMed.

News

  1. TDP-43 Routes Mapped in Alzheimer’s, Frontotemporal Dementia
  2. Stress Relief: Anti-Stress Granule Therapy Saves ALS Models
  3. Québec: Teasing Out the Function of TDP-43

Primary Papers

  1. . Axonal Transport of TDP-43 mRNA Granules Is Impaired by ALS-Causing Mutations. Neuron. 2014 Feb 5;81(3) PubMed.
  2. . TDP-43 in ALS: Stay on Target…Almost There. Neuron. 2014 Feb 5;81(3) PubMed.