TDP-43 ALS: Pathways of Protein Spread and Motor Neuron Decline

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that primarily affects motor neurons, leading to muscle weakness and eventual paralysis. A key pathological feature of ALS is the mislocalization and aggregation of the TDP-43 protein, which disrupts essential cellular processes. Understanding how TDP-43 spreads and contributes to motor neuron decline is critical for developing targeted therapies.

Research suggests that TDP-43 pathology propagates through interconnected neural networks, exacerbating disease progression. Its role in various cellular functions makes its dysfunction particularly damaging to motor neurons.

TDP-43’s Role In RNA Metabolism

TDP-43, a DNA/RNA-binding protein encoded by the TARDBP gene, regulates transcription, splicing, transport, and stability of RNA molecules. It is predominantly localized in the nucleus, where it binds to UG-rich sequences in pre-mRNA to modulate alternative splicing. Studies have identified thousands of RNA targets influenced by TDP-43, including those involved in synaptic function, cytoskeletal integrity, and stress response. Disruptions in these processes can have widespread consequences, particularly in neurons, which rely on precise RNA homeostasis for survival.

One of TDP-43’s most well-characterized functions is its involvement in alternative splicing. Genome-wide analyses have shown that TDP-43 depletion leads to aberrant exon inclusion or exclusion in numerous transcripts, affecting proteins critical for neuronal health. For instance, TDP-43 regulates the splicing of STMN2, a gene necessary for axonal maintenance and regeneration. Loss of TDP-43 function results in the misprocessing of STMN2 mRNA, producing truncated, nonfunctional proteins that impair axonal repair. This disruption is particularly detrimental in motor neurons, which have long axons requiring continuous maintenance.

Beyond splicing, TDP-43 ensures the proper transport of mRNAs to distal axonal and dendritic compartments, where local translation supports synaptic plasticity. It associates with ribonucleoprotein granules that shuttle mRNAs to these regions. In ALS patient-derived neurons, TDP-43 pathology correlates with defective RNA transport and reduced expression of proteins necessary for synaptic stability.

TDP-43 also regulates RNA stability and degradation by interacting with microRNAs and components of the RNA decay machinery. It prevents the accumulation of aberrant or unnecessary mRNAs that could disrupt cellular homeostasis. Dysregulation of this function has been linked to toxic RNA species, which exacerbate neuronal stress. For example, TDP-43 modulates the stability of mRNAs encoding proteins involved in stress granule dynamics, and its dysfunction can lead to persistent stress granule formation, a feature commonly observed in ALS-affected neurons.

TDP-43 Aggregation In ALS

A hallmark of ALS is the cytoplasmic aggregation of TDP-43, which disrupts its nuclear functions and leads to widespread cellular dysfunction. Under normal conditions, TDP-43 shuttles between the nucleus and cytoplasm to regulate RNA metabolism, but in ALS, it becomes mislocalized and forms insoluble inclusions. These aggregates are ubiquitinated and phosphorylated, suggesting that cellular degradation pathways attempt to clear them, albeit ineffectively. Post-mortem analyses of ALS patient spinal cords consistently reveal TDP-43 inclusions in motor neurons, indicating their role in disease pathology.

TDP-43 aggregation involves both loss-of-function and gain-of-toxicity effects. When sequestered in cytoplasmic inclusions, its nuclear role in RNA processing is compromised, leading to transcriptional dysregulation. Simultaneously, misfolded TDP-43 in the cytoplasm interferes with protein homeostasis and cellular stress responses. Studies using ALS patient-derived cells and animal models show that TDP-43 aggregates disrupt proteostasis by sequestering essential RNA-binding proteins, impairing ribonucleoprotein granule dynamics, and overwhelming protein degradation pathways. This results in unchecked accumulation of misfolded proteins, exacerbating neuronal dysfunction.

A defining characteristic of TDP-43 aggregation is its prion-like behavior, where misfolded proteins template the misfolding of native TDP-43, promoting pathology spread. Structural studies have identified low-complexity domains within TDP-43 that facilitate self-assembly, making it prone to aggregation under cellular stress. Experimental models show that TDP-43 aggregates can be transmitted between cells, possibly through extracellular vesicles or direct cell-to-cell contact. This aligns with the observation that TDP-43 pathology follows a spatiotemporal pattern in ALS, beginning in focal regions of the motor system before spreading.

TDP-43 inclusions actively disrupt cellular function by impairing organelle dynamics and intracellular trafficking. Mitochondrial dysfunction is a well-documented consequence, with studies showing that mislocalized TDP-43 interacts with mitochondrial membranes, leading to defects in energy production and increased oxidative stress. Additionally, TDP-43 inclusions interfere with the endoplasmic reticulum (ER), disrupting protein folding and triggering ER stress responses. These disturbances contribute to neurodegeneration by creating an environment where motor neurons cannot maintain homeostasis, making them vulnerable to apoptosis.

Mechanisms Of Protein Spread

The propagation of TDP-43 pathology across neural circuits suggests mechanisms beyond cell-autonomous toxicity, implicating intercellular transfer of misfolded protein species. In vitro and in vivo models indicate that TDP-43 aggregates move between neurons, facilitating disease progression. One proposed route involves extracellular vesicles, such as exosomes, which encapsulate misfolded TDP-43 and transport it to neighboring cells. These vesicles, secreted as part of normal cellular communication, may inadvertently serve as conduits for disease propagation. Studies have detected TDP-43 within exosomal fractions from ALS patient cerebrospinal fluid, supporting this hypothesis.

Beyond vesicular transport, direct cell-to-cell transmission may contribute to TDP-43 spread through mechanisms such as tunneling nanotubes—thin cytoplasmic extensions that facilitate molecular exchange between adjacent neurons. These structures have been observed in neurodegenerative disease models, where they enable the passage of protein aggregates, organelles, and RNA species. Experimental work using ALS-derived cells shows that TDP-43 inclusions transfer via these nanotubes, leading to misfolded protein accumulation in recipient cells. Motor neurons, which are highly interconnected, may be particularly susceptible to this progressive dissemination.

Once inside a new host cell, misfolded TDP-43 acts as a template, inducing the misfolding and aggregation of native TDP-43. This prion-like behavior accelerates disease progression by systematically compromising neuronal populations. Structural studies of TDP-43 fibrils reveal that specific regions of the protein’s low-complexity domain promote self-assembly, making it prone to forming stable aggregates. Experimental models demonstrate that synthetic TDP-43 fibrils, introduced into cultured neurons, seed endogenous aggregate formation. This seeding mechanism aligns with the anatomical progression of ALS, where pathology spreads in a predictable pattern, often starting in the motor cortex or spinal cord before advancing to other regions.

Influences On Motor Neuron Dysfunction

Motor neurons are particularly vulnerable to disruptions in cellular homeostasis, and the presence of misfolded TDP-43 exacerbates their susceptibility to dysfunction. These large, highly specialized cells rely on efficient RNA processing, protein turnover, and intracellular transport to maintain their long axons and intricate synaptic connections. When TDP-43 aggregates accumulate, they interfere with these processes, leading to widespread disruptions in axonal integrity and synaptic signaling. Studies show that motor neurons from ALS patients exhibit reduced axonal transport efficiency, impairing the delivery of essential proteins and organelles to distal regions. This deficit compromises synaptic stability, weakening neuromuscular junctions and contributing to progressive muscle atrophy.

Beyond transport deficits, TDP-43 pathology disrupts the regulation of stress granules—cytoplasmic aggregates that form in response to cellular stress to temporarily halt translation. Under normal conditions, these granules disassemble once the stressor is resolved, but in ALS, persistent TDP-43 mislocalization leads to chronic stress granule accumulation. This prolonged sequestration of RNA-binding proteins and translation factors further impairs cellular function, reducing the ability of motor neurons to mount adaptive responses. Stress granule dysfunction has been linked to an increased propensity for apoptosis, as neurons struggle to recover from metabolic and oxidative imbalances.

Comparisons To Other Neurodegenerative Disorders

TDP-43 pathology in ALS shares similarities with protein misfolding diseases such as Alzheimer’s, Parkinson’s, and frontotemporal dementia (FTD), where aberrant protein aggregation leads to progressive neuronal degeneration. In ALS, TDP-43 inclusions predominantly affect motor neurons, whereas in FTD, the same protein aggregates in cortical and limbic regions, contributing to cognitive and behavioral impairments. The overlap between ALS and FTD is further underscored by genetic mutations in TARDBP, GRN, and C9orf72, which are associated with both conditions. This genetic and pathological continuum suggests that TDP-43 dysfunction may drive neurodegeneration across a spectrum of disorders rather than being confined to a single disease.

Other proteinopathies, such as Alzheimer’s and Parkinson’s, exhibit prion-like propagation mechanisms akin to TDP-43 spread in ALS. In Alzheimer’s, misfolded tau proteins disseminate through neural networks, exacerbating cognitive decline, while in Parkinson’s, α-synuclein aggregates propagate similarly, affecting dopaminergic neurons. Despite these parallels, the selective vulnerability of motor neurons to TDP-43 pathology in ALS remains a distinguishing feature. Unlike tau and α-synuclein, TDP-43 regulates RNA metabolism, and its dysfunction disrupts RNA processing in a manner uniquely detrimental to motor neuron survival. Understanding these disease-specific vulnerabilities could inform therapeutic strategies aimed at mitigating TDP-43 toxicity and preventing neurodegeneration.