Transcriptomic evaluation of tau and TDP-43 synergism shows tauopathy predominance and reveals potential modulating targets

Role of tau versus TDP-43 pathology on medial temporal lobe atrophy in aging and Alzheimer's disease

Hippocampal atrophy on magnetic resonance imaging is an important biomarker in Alzheimer's disease (AD). While hippocampal atrophy was thought to result from tau tangles in AD, different neuropathologies can lead to hippocampal atrophy, especially TAR DNA-binding protein 43 (TDP-43) pathology. In this narrative review, we evaluate existing studies on the relative contribution of tau and TDP-43 pathology to medial temporal lobe (MTL) atrophy. We report a clear association of both tau and TDP-43 neuropathology with MTL atrophy, even after correcting for other neuropathologies. Next, we discuss a potential synergism between tau and TDP-43 and the relative timing of the effects of both neuropathologies. Finally, avenues for future research will be discussed. A better understanding of the interplay between tau and TDP-43 neuropathologies and their effect on atrophy will help with the development of more specific biomarkers for limbic-predominant age-related TDP-43 encephalopathy and pinpointing of the optimal timing for testing anti-tau and anti-TDP-43 treatments in trials. HIGHLIGHTS: Both tau and TAR DNA-binding protein 43 (TDP-43) pathology contribute to medial temporal lobe atrophy. There is a positive association between tau and TDP-43 and potentially a synergism. It is unclear if tau and TDP-43 have an additive or synergistic effect on atrophy. The relative timing of the tau and TDP-43 effects on atrophy remains unclear. Clarifying the interplay between tau and TDP-43 will help improve magnetic resonance imaging biomarkers.

Co-Aggregation of TDP-43 with Other Pathogenic Proteins and Their Co-Pathologies in Neurodegenerative Diseases

Pathological aggregation of a specific protein into insoluble aggregates is a common hallmark of various neurodegenerative diseases (NDDs). In the earlier literature, each NDD is characterized by the aggregation of one or two pathogenic proteins, which can serve as disease-specific biomarkers. The aggregation of these specific proteins is thought to be a major cause of or deleterious result in most NDDs. However, accumulating evidence shows that a pathogenic protein can interact and co-aggregate with other pathogenic proteins in different NDDs, thereby contributing to disease onset and progression synergistically. During the past years, more than one type of NDD has been found to co-exist in some individuals, which may increase the complexity and pathogenicity of these diseases. This article reviews and discusses the biochemical characteristics and molecular mechanisms underlying the co-aggregation and co-pathologies associated with TDP-43 pathology. The TDP-43 aggregates, as a hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), can often be detected in other NDDs, such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and spinocerebellar ataxia type 2 (SCA2). In many cases, TDP-43 is shown to interact and co-aggregate with multiple pathogenic proteins in vitro and in vivo. Furthermore, the co-occurrence and co-aggregation of TDP-43 with other pathogenic proteins have important consequences that may aggravate the diseases. Thus, the current viewpoint that the co-aggregation of TDP-43 with other pathogenic proteins in NDDs and their relevance to disease progression may gain insights into the patho-mechanisms and therapeutic potential of various NDDs.

Limbic-predominant age-related TDP-43 encephalopathy (LATE-NC): Co-pathologies and genetic risk factors provide clues about pathogenesis

Limbic-predominant age-related TDP-43 encephalopathy neuropathologic change (LATE-NC) is detectable at autopsy in more than one-third of people beyond age 85 years and is robustly associated with dementia independent of other pathologies. Although LATE-NC has a large impact on public health, there remain uncertainties about the underlying biologic mechanisms. Here, we review the literature from human studies that may shed light on pathogenetic mechanisms. It is increasingly clear that certain combinations of pathologic changes tend to coexist in aging brains. Although "pure" LATE-NC is not rare, LATE-NC often coexists in the same brains with Alzheimer disease neuropathologic change, brain arteriolosclerosis, hippocampal sclerosis of aging, and/or age-related tau astrogliopathy (ARTAG). The patterns of pathologic comorbidities provide circumstantial evidence of mechanistic interactions ("synergies") between the pathologies, and also suggest common upstream influences. As to primary mediators of vulnerability to neuropathologic changes, genetics may play key roles. Genes associated with LATE-NC include TMEM106B, GRN, APOE, SORL1, ABCC9, and others. Although the anatomic distribution of TDP-43 pathology defines the condition, important cofactors for LATE-NC may include Tau pathology, endolysosomal pathways, and blood-brain barrier dysfunction. A review of the human phenomenology offers insights into disease-driving mechanisms, and may provide clues for diagnostic and therapeutic targets.