INTRODUCTION: Alternative splicing of the human MAPT gene generates six brain‐specific TAU isoforms. Imbalances in the TAU isoform ratio can lead to neurodegenerative diseases, underscoring the need for precise control over TAU isoform balance. Tauopathies, characterized by intracellular aggregates of hyperphosphorylated TAU, exhibit extensive neurodegeneration and can be classified by the TAU isoforms present in pathological accumulations.METHODS: A comprehensive review of TAU and related dementia syndromes literature was conducted using PubMed, Google Scholar, and preprint server.RESULTS: While TAU is recognized as key driver of neurodegeneration in specific tauopathies, the contribution of the isoforms to neuronal function and disease development remains largely elusive.DISCUSSION: In this review we describe the role of TAU isoforms in health and disease, and stress the importance of comprehending and studying TAU isoforms in both, physiological and pathological context, in order to develop targeted therapeutic interventions for TAU‐associated diseases.Highlights: MAPT splicing is tightly regulated during neuronal maturation and throughout life. TAU isoform expression is development‐, cell‐type and brain region specific. The contribution of TAU to neurodegeneration might be isoform‐specific. Ineffective TAU‐based therapies highlight the need for specific targeting strategies.

Amyloid-β peptide (Aβ) forms metastable oligomers >50 kDa, termed AβOs, that are more effective than Aβ amyloid fibrils at triggering Alzheimer’s disease-related processes such as synaptic dysfunction and Tau pathology, including Tau mislocalization. In neurons, Aβ accumulates in endo-lysosomal vesicles at low pH. Here, we show that the rate of AβO assembly is accelerated 8,000-fold upon pH reduction from extracellular to endo-lysosomal pH, at the expense of amyloid fibril formation. The pH-induced promotion of AβO formation and the high endo-lysosomal Aβ concentration together enable extensive AβO formation of Aβ42 under physiological conditions. Exploiting the enhanced AβO formation of the dimeric Aβ variant dimAβ we furthermore demonstrate targeting of AβOs to dendritic spines, potent induction of Tau missorting, a key factor in tauopathies, and impaired neuronal activity. The results suggest that the endosomal/lysosomal system is a major site for the assembly of pathomechanistically relevant AβOs.

In the adult human brain, six isoforms of the microtubule-associated protein TAU are expressed, which result from alternative splicing of exons 2, 3, and 10 of the MAPT gene. These isoforms differ in the number of N-terminal inserts (0N, 1N, 2N) and C-terminal repeat domains (3R or 4R) and are differentially expressed depending on the brain region and developmental stage. Although all TAU isoforms can aggregate and form neurofibrillary tangles, some tauopathies, such as Pick’s disease and progressive supranuclear palsy, are characterized by the accumulation of specific TAU isoforms. The influence of the individual TAU isoforms in a cellular context, however, is understudied. In this report, we investigated the subcellular localization of the human-specific TAU isoforms in primary mouse neurons and analyzed TAU isoform-specific effects on cell area and microtubule dynamics in human SH-SY5Y neuroblastoma cells. Our results show that 2N-TAU isoforms are particularly retained from axonal sorting and that axonal enrichment is independent of the number of repeat domains, but that the additional repeat domain of 4R-TAU isoforms results in a general reduction of cell size and an increase of microtubule counts in cells expressing these specific isoforms. Our study points out that individual TAU isoforms may influence microtubule dynamics differentially both by different sorting patterns and by direct effects on microtubule dynamics.

INTRODUCTION: Human tau protein, composed of six brain‐specific isoforms, is a major driver of Alzheimer's disease (AD). The role of its isoforms however remains unclear and human AD models are scarce.METHODS: We generated human MAPT– (tau–) knockout (KO) induced pluripotent stem cells (iPSC) using CRISPR/Cas9, differentiated these into glutamatergic neurons, and assessed isoform‐specific functions of tau in these neurons. We used omic‐ approaches, live‐cell imaging, subcompartmental analysis, and lentivirus‐based reintroduction of specific tau isoforms to investigate isoform‐mediated neuronal dysfunction in an AD model.RESULTS: Tau KO human iPSC‐derived neurons showed decreased neurite outgrowth and axon initial segment length and, notably, resisted amyloid beta oligomer (AβO)–induced neuronal activity reduction. Introducing the 1N4R‐tau isoform, but not other isoforms, confers AβO vulnerability and increases KxGS phosphorylation of tau, without altering neuronal activity or microtubule modifications.DISCUSSION: While tau KO impacts neuronal development and activity, tau‐KO also confers resistance against AβO insult. 1N4R‐tau likely mediates AβO‐induced and phosphorylated tau toxicity, representing a novel prime therapeutic target for AD.Highlights: Tau knockout alters neurite growth and axon initial segment formation in human neurons. Tau isoforms show differential axonal localization in human neurons. Tau depletion protects against amyloid beta oligomer (AβO)–mediated neurotoxicity. 1N4R tau mediates AβO‐induced toxicity in human neurons.

Epigenetic mechanisms are emerging key players for the regulation of brain function, synaptic activity, and the formation of neuronal engrams in health and disease. As one important epigenetic mechanism of transcriptional control, DNA methylation was reported to distinctively modulate synaptic activity in excitatory and inhibitory cortical neurons in mice. Since DNA methylation signatures are responsive to neuronal activity, DNA methylation seems to contribute to the neuron’s capacity to adapt to and integrate changing activity patterns, being crucial for the plasticity and functionality of neuronal circuits. Since most studies addressing the role of DNA methylation in the regulation of synaptic function were conducted in mice or murine neurons, we here asked whether this functional implication applies to human neurons as well. To this end, we performed calcium imaging in human induced pluripotent stem cell (iPSC)-derived excitatory cortical neurons forming synaptic contacts and neuronal networks in vitro. Treatment with DNMT1 siRNA that diminishs the expression of the DNA (cytosine-5)-methyltransferase 1 (DNMT1) was conducted to investigate the functional relevance of DNMT1 as one of the main enzymes executing DNA methylations in the context of neuronal activity modulation. We observed a lowered proportion of actively firing neurons upon DNMT1-knockdown in these iPSC-derived excitatory neurons, pointing to a correlation of DNMT1-activity and synaptic transmission. Thus, our experiments suggest that DNMT1 decreases synaptic activity of human glutamatergic neurons and underline the relevance of epigenetic regulation of synaptic function also in human excitatory neurons.