By: Geoffrey Feld, Ph.D.
Geocyte LLC & Alamar Biosciences collaborator 

A Numbers Game: 217, 4, and 50

When it comes to detecting circulating biomarkers for neurodegenerative diseases, consider the numbers: 217, 4, and 50.

217 refers to p-Tau217, a tau protein phosphorylated at threonine 217 that closely tracks with amyloid pathology in Alzheimer’s disease (AD). Measuring this biomarker in biofluids, including cerebrospinal fluid (CSF) and blood, marks a convenient shift from relying on costly and limited-access tau PET imaging.

However, the concentration of p-Tau217 in CSF is typically around 4 picograms per milliliter (pg/ml)—already near the lower detection limit of standard ELISA-based assays (Triana-Baltzer, 2020). In plasma, the concentration drops even further—50 times lower than in CSF (Triana-Baltzer, 2021). Detecting such ultra-low levels in blood requires technologies sensitive enough to measure femtograms per milliliter (fg/ml). Moreover, increasing evidence suggests that assays specific for brain-derived tau isoforms are necessary to distinguish the CNS and peripheral origins of p-tau. Clearly, more advanced and multiplexed detection methods are essential to make biofluid-based biomarker testing a practical reality in neurodegenerative disease research and care.

The Case for Blood-Based Proteomics in AD

AD and other neurodegenerative diseases are typically diagnosed through clinical symptoms and neuroimaging. However, overlapping phenotypes across dementias and the high cost of PET scans have driven demand for fluid-based biomarkers that can more accurately and affordably distinguish AD from other cognitive impairments caused by different pathologies (Schumacher-Schuh, 2022).

Surrogate biomarkers for plaque formation—such as phosphorylated tau (p-Tau217, p-Tau181, p-Tau231) and amyloid beta (Aß40, Aß42)—can be measured in CSF. But CSF extraction is an invasive procedure. Plasma and serum, routinely collected in clinical settings, represent ideal sample types for early detection, disease monitoring, and assessing response to therapy. Furthermore, advances in dried blood spot technologies are enabling community-based screening and at-home collection—long considered the holy grail for AD diagnostics.

Brain-Derived p-Tau Targets: A New Era in Multiplexing

While tau and its hyperphosphorylated forms are promising markers of neurodegeneration, tau proteins are expressed outside the brain in the peripheral nervous system. As such, teasing out the source of tau may be necessary for accurate disease diagnoses. Brain-derived (BD) tau isoforms lack an exon 4a insert, making them shorter (Gonzalez-Ortiz, 2024), and are often referred to as low-molecular-weight tau. Most assays utilize antibodies targeting the N-terminal epitope, which detect both peripheral and brain-derived tau without differentiating between the tissue sources (see Figure 1).

Alamar has developed a suite of antibodies that specifically bind to low-molecular-weight p-Tau isoforms, including totalTau-BD, p-Tau217-BD, p-Tau231-BD, and p-Tau181-BD. These four BD-tau isoforms are now included in the NULISAseq CNS Disease Panel 120, enabling researchers to measure the CNS-derived levels of p-Tau more accurately.

BD-total tau has emerged as a powerful marker for amyloid-associated neurodegeneration in AD (Gonzalez-Ortiz, 2024). As the only product on the market that includes BD-total tau and the three primary brain-derived phosphorylated tau isoforms, the NULISAseq CNS Disease Panel 120 presents researchers with a unique toolset to explore neurodegenerative biology. Read about the first unpublished results of the assays from neuroscientists Jonathan Schott at University College London, Henrik Zetterberg at University of Gothenburg, and Gil Rabinovici at UCSF in this recent Alzforum news article.