GFAP (Glial Fibrillary Acidic Protein) as a Biomarker in Motor Neuron Disease
Most PLS and ALS biomarker research focuses on neurofilaments — proteins released when neurons die. GFAP is different. It is not a neuronal protein. It comes from astrocytes — the glial cells that support, nourish, and communicate with neurons. When neurons are damaged, astrocytes respond: they activate, proliferate, and release GFAP into the surrounding tissue. In ALS, this process — called astrogliosis — is measurable in the blood. In PLS, no one has yet published systematic GFAP data. But the biology suggests it should be elevated, and understanding why GFAP matters adds an important dimension to the biomarker picture.
What GFAP is and where it comes from
Glial fibrillary acidic protein is a cytoskeletal protein found predominantly in astrocytes, the star-shaped glial cells that perform critical support functions in the brain and spinal cord. Astrocytes maintain the blood-brain barrier, regulate ion balance in the extracellular space, provide metabolic support to neurons, and participate in synaptic signaling. When neurons are injured or dying, astrocytes undergo a reactive process called astrogliosis — they change shape, proliferate, and upregulate GFAP production. GFAP is then released into the CSF and, at lower concentrations, into the blood.
GFAP measures something different from neurofilaments. NfL and pNfH reflect axonal injury — how much neuronal structure is breaking down. GFAP reflects the glial response to that injury — the neuroinflammatory and reactive component of neurodegeneration. These two processes often occur together, but they are not identical, and they do not always track together. A patient could have significant glial activation with modest axonal loss, or vice versa. Measuring both provides a more complete picture.
GFAP in ALS: the established evidence
ALS is not a pure motor neuron disease — it involves significant glial pathology, including reactive astrogliosis in the motor cortex and spinal cord. This means GFAP elevation is expected in ALS, and the data confirm it.
A 2025 Scientific Reports study examining serum NfL and GFAP simultaneously in ALS patients found that serum GFAP in ALS patients averaged 104.42 ± 37.31 pg/mL versus 57.71 ± 11.64 pg/mL in healthy controls (p < 0.001) — approximately a 1.8-fold elevation. Serum GFAP correlated positively with ALS disease duration (r = 0.668, p = 0.018), suggesting that the longer the disease has been active, the more cumulative astrogliosis has occurred.
An important finding is the disease specificity of GFAP's associations. While NfL primarily reflects motor neuron loss and correlates most strongly with functional decline in the motor domain, GFAP is particularly associated with non-motor features of ALS — including cognitive impairment, behavioral changes, and the degree of demyelination more broadly. This makes GFAP particularly relevant to the ~50% of ALS patients who develop cognitive or behavioral changes, and potentially to the understanding that PLS is not a pure motor disease (extra-motor pathology is increasingly documented in PLS, as the Scirocco 2025 review notes).
The 2025 Frontiers Molecular Biosciences review of emerging ALS biomarkers positions GFAP as part of a multi-marker panel strategy — not as a replacement for NfL, but as a complementary marker that captures a different biological dimension. Combined NfL + GFAP measurement provides more information than either alone.
GFAP in PLS: no published data, but clear rationale
No study has published systematic GFAP measurements in a PLS cohort as of 2026. This is a gap in the field. However, the biology makes clear that GFAP elevation in PLS should be expected — the question is whether it is elevated to the same degree as in ALS (unlikely, given PLS's slower degeneration) and whether it provides prognostic information in PLS independently of NfL.
The rationale for expecting GFAP elevation in PLS:
- PLS involves progressive degeneration of Betz cells (upper motor neurons) and their corticospinal axons. Astrocytes in the motor cortex and along the corticospinal tract will respond to this degeneration with reactive astrogliosis.
- Post-mortem pathology in PLS shows reactive astrogliosis in the motor cortex — the histological correlate of elevated GFAP. The serum marker should reflect this.
- The extra-motor pathology documented in PLS by neuroimaging (frontotemporal, cerebellar, thalamic involvement in a subset of patients) may involve glial activation in those regions, which NfL might not capture but GFAP could.
Whether PLS GFAP levels will be in the range detectable by current serum assays — given that PLS degeneration is slower than ALS — is not known. In some slowly progressive conditions, GFAP elevation may be modest enough to overlap with age-related variation, limiting its clinical utility as a standalone marker. This is one reason why panel-based approaches (combining GFAP with NfL and pNfH) are likely more informative than single-marker strategies.
GFAP and non-motor features
One specific application of GFAP in PLS that deserves attention is the non-motor domain. A growing body of evidence shows that PLS is not purely a motor disease: neuroimaging reveals frontotemporal, cerebellar, and thalamic involvement in some patients, and cognitive or behavioral changes — while less common than in ALS — do occur in PLS. Given that GFAP in ALS specifically tracks with cognitive impairment and non-motor pathology, GFAP measurements in PLS could potentially help identify patients with broader CNS involvement beyond the motor system.
This is speculative for PLS, but it represents one of the more compelling scientific reasons to include GFAP in PLS biomarker studies: not just as an additional motor biomarker, but as a potential marker of the non-motor disease burden that functional scales do not capture.
Technical considerations
Serum GFAP is measured by immunoassay, with Simoa and Lumipulse platforms offering the highest sensitivity. GFAP is age-sensitive — it rises with age in healthy individuals — and reference ranges must be age-adjusted for meaningful interpretation, similar to NfL. Blood-based GFAP measurement has been validated across multiple neurological conditions (traumatic brain injury, Alzheimer's disease, multiple sclerosis) as well as ALS, making it a well-characterized assay in general. The PLS-specific gap is in the disease-specific interpretation of elevated values.
How this connects
GFAP fits into the multi-marker panel described on the multi-biomarker panel page, alongside NfL and pNfH. The broader biomarker landscape in PLS is synthesized on the Biomarker Research hub. For how biomarkers are currently used in the diagnostic process, see Diagnosis. The non-motor features of PLS that GFAP might track are discussed in the context of prognosis on the Prognosis page.
Key studies referenced
Correlation analysis of serum neurofilament light chain and glial fibrillary acidic protein
with ALS disease parameters. Scientific Reports. 2025.
Emerging biomarkers in amyotrophic lateral sclerosis: from pathogenesis to clinical applications.
Frontiers in Molecular Biosciences. 2025.
Blood diagnostic and prognostic biomarkers in amyotrophic lateral sclerosis.
Neural Regeneration Research. [review article].