Latest articles published on Molecular Neurodegeneration

Our staff editors continue to share exciting, interesting, and thought-provoking reading material in the recommended articles series.

This week, we would like to share several latest articles published on Molecular Neurodegeneration.

Title: The promise of microRNA-based therapies in Alzheimer’s disease: challenges and perspectives
Authors: Hannah Walgrave, Lujia Zhou, Bart De Strooper, Evgenia Salta
Type: Perspective of Molecular Neurodegeneration
Abstract:
Multi-pathway approaches for the treatment of complex polygenic disorders are emerging as alternatives to classical monotarget therapies and microRNAs are of particular interest in that regard. MicroRNA research has come a long way from their initial discovery to the cumulative appreciation of their regulatory potential in healthy and diseased brain. However, systematic interrogation of putative therapeutic or toxic effects of microRNAs in (models of) Alzheimer’s disease is currently missing and fundamental research findings are yet to be translated into clinical applications. Here, we review the literature to summarize the knowledge on microRNA regulation in Alzheimer’s pathophysiology and to critically discuss whether and to what extent these increasing insights can be exploited for the development of microRNA-based therapeutics in the clinic.
Access this article: https://doi.org/10.1186/s13024-021-00496-7

Title: Characterization of mitochondrial DNA quantity and quality in the human aged and Alzheimer’s disease brain
Authors: Hans-Ulrich Klein, Caroline Trumpff, Hyun-Sik Yang, Annie J. Lee, Martin Picard, David A. Bennett, Philip L. De Jager
Type: Research Article of Molecular Neurodegeneration
Abstract:
Background
Mitochondrial dysfunction is a feature of neurodegenerative diseases, including Alzheimer’s disease (AD). Changes in the mitochondrial DNA copy number (mtDNAcn) and increased mitochondrial DNA mutation burden have both been associated with neurodegenerative diseases and cognitive decline. This study aims to systematically identify which common brain pathologies in the aged human brain are associated with mitochondrial recalibrations and to disentangle the relationship between these pathologies, mtDNAcn, mtDNA heteroplasmy, aging, neuronal loss, and cognitive function.
Methods
Whole-genome sequencing data from n = 1361 human brain samples from 5 different regions were used to quantify mtDNAcn as well as heteroplasmic mtDNA point mutations and small indels. Brain samples were assessed for 10 common pathologies. Annual cognitive test results were used to assess cognitive function proximal to death. For a subset of samples, neuronal proportions were estimated from RNA-seq profiles, and mass spectrometry was used to quantify the mitochondrial protein content of the tissue.
Results
mtDNAcn was 7–14% lower in AD relative to control participants. When accounting for all 10 common neuropathologies, only tau was significantly associated with lower mtDNAcn in the dorsolateral prefrontal cortex. In the posterior cingulate cortex, TDP-43 pathology demonstrated a distinct association with mtDNAcn. No changes were observed in the cerebellum, which is affected late by pathologies. Neither age nor gender was associated with mtDNAcn in the studied brain regions when adjusting for pathologies. Mitochondrial content and mtDNAcn independently explained variance in cognitive function unaccounted by pathologies, implicating complex mitochondrial recalibrations in cognitive decline. In contrast, mtDNA heteroplasmy levels increased by 1.5% per year of life in the cortical regions, but displayed no association with any of the pathologies or cognitive function.
Conclusions
We studied mtDNA quantity and quality in relation to mixed pathologies of aging and showed that tau and not amyloid-β is primarily associated with reduced mtDNAcn. In the posterior cingulate cortex, the association of TDP-43 with low mtDNAcn points to a vulnerability of this region in limbic-predominant age-related TDP-43 encephalopathy. While we found low mtDNAcn in brain regions affected by pathologies, the absence of associations with mtDNA heteroplasmy burden indicates that mtDNA point mutations and small indels are unlikely to be involved in the pathogenesis of late-onset neurodegenerative diseases.
Access this article: https://doi.org/10.1186/s13024-021-00495-8

Title: Synaptic dysfunction of Aldh1a1 neurons in the ventral tegmental area causes impulsive behaviors
Authors: Xinyan Li, Wenting Chen, Xian Huang, Wei Jing, Tongmei Zhang, Quntao Yu, Hongyan Yu, Hao Li, Qing Tian, Yumei Ding, Youming Lu
Type: Research Article of Molecular Neurodegeneration
Abstract:
Background
Aldh1a1 neurons are a subtype of gamma-aminobutyric acid (GABA) inhibitory neurons that use Aldh1a1 rather than glutamate decarboxylase (GAD) as an enzyme for synthesizing GABA transmitters. However, the behaviors and circuits of this newly identified subtype of inhibitory interneurons remain unknown.
Methods
We generated a mutant mouse line in which cyclization recombination enzyme (CRE) was expressed under the control of the Aldh1a1 promotor (Aldh1a1-CRE mice). Using this mutant strain of mice together with the heterozygous male Alzheimer’s disease (AD) related model mice (APPswe/PSEN1dE9, or AD mice) and a genetically modified retrograde and anterograde synaptic tracing strategy, we have studied a specific synaptic circuit of Aldh1a1 neurons with system-level function and disease progression in AD mice.
Results
We demonstrate that Aldh1a1 neurons encode delay of gratification that measures self-control skills in decision making by projecting inhibitory synapses directly onto excitatory glutamate neurons in the intermediate lateral septum (EGNIS) and receiving synaptic inputs from layer 5b pyramidal neurons in the medial prefrontal cortex (L5PN). L5PN → Aldh1a1 synaptic transmission undergoes long-term potentiation (LTP). Pathway specific inhibition by either genetic silencing presynaptic terminals or antagonizing postsynaptic receptors impairs delay of gratification, resulting in the impulsive behaviors. Further studies show that reconstitution of Aldh1a1-deficient neurons with the expression of exogenous Aldh1a1 (eAldh1a1) restores Aldh1a1 → EGNIS synaptic transmission and rescues the impulsive behaviors in AD mice.
Conclusions
These results not only identify a specific function and circuit of Aldh1a1 neurons but also provide a cellular point of entry to an important but understudied synaptic mechanism for the induction of impulsive behaviors at an early stage of AD.
Access this article: https://doi.org/10.1186/s13024-021-00494-9

Title: METTL3-dependent RNA m⁶A dysregulation contributes to neurodegeneration in Alzheimer’s disease through aberrant cell cycle events
Authors:  Fanpeng Zhao, Ying Xu, Shichao Gao, Lixia Qin, Quillan Austria, Sandra L. Siedlak, Kinga Pajdzik, Qing Dai, Chuan He, Wenzhang Wang, James M. O’Donnell, Beisha Tang, Xiongwei Zhu
Type: Research Article of Molecular Neurodegeneration
Abstract:
Background
N6-methyladenosine (m⁶A) modification of RNA influences fundamental aspects of RNA metabolism and m⁶A dysregulation is implicated in various human diseases. In this study, we explored the potential role of RNA m⁶A modification in the pathogenesis of Alzheimer disease (AD).
Methods
We investigated the m⁶A modification and the expression of m⁶A regulators in the brain tissues of AD patients and determined the impact and underlying mechanism of manipulated expression of m⁶A levels on AD-related deficits both in vitro and in vivo.
Results
We found decreased neuronal m⁶A levels along with significantly reduced expression of m⁶A methyltransferase like 3 (METTL3) in AD brains. Interestingly, reduced neuronal m⁶A modification in the hippocampus caused by METTL3 knockdown led to significant memory deficits, accompanied by extensive synaptic loss and neuronal death along with multiple AD-related cellular alterations including oxidative stress and aberrant cell cycle events in vivo. Inhibition of oxidative stress or cell cycle alleviated shMettl3-induced apoptotic activation and neuronal damage in primary neurons. Restored m⁶A modification by inhibiting its demethylation in vitro rescued abnormal cell cycle events, neuronal deficits and death induced by METTL3 knockdown. Soluble Aβ oligomers caused reduced METTL3 expression and METTL3 knockdown exacerbated while METTL3 overexpression rescued Aβ-induced synaptic PSD95 loss in vitro. Importantly, METTL3 overexpression rescued Aβ-induced synaptic damage and cognitive impairment in vivo.
Conclusions
Collectively, these data suggested that METTL3 reduction-mediated m⁶A dysregulation likely contributes to neurodegeneration in AD which may be a therapeutic target for AD.
Access this article: https://doi.org/10.1186/s13024-021-00484-x

Title: Nuclear dynamics and stress responses in Alzheimer’s disease
Authors: Artemis Iatrou, Eric M. Clark, Yanling Wang
Type: Review of Molecular Neurodegeneration
Abstract:
In response to extracellular and intracellular stressors, the nucleus and nuclear compartments undergo distinct molecular changes to maintain cell homeostasis. In the context of Alzheimer’s disease, misfolded proteins and various cellular stressors lead to profound structural and molecular changes at the nucleus. This review summarizes recent research on nuclear alterations in AD development, from the nuclear envelope changes to chromatin and epigenetic regulation and then to common nuclear stress responses. Finally, we provide our thoughts on the importance of understanding cell-type-specific changes and identifying upstream causal events in AD pathogenesis and highlight novel sequencing and gene perturbation technologies to address those challenges.
Access this article: https://doi.org/10.1186/s13024-021-00489-6

Title: Adult-onset CNS myelin sulfatide deficiency is sufficient to cause Alzheimer’s disease-like neuroinflammation and cognitive impairment
Authors: Shulan Qiu, Juan Pablo Palavicini, Jianing Wang, Nancy S. Gonzalez, Sijia He, Elizabeth Dustin, Cheng Zou, Lin Ding, Anindita Bhattacharjee, Candice E. Van Skike, Veronica Galvan, Jeffrey L. Dupree, Xianlin Han
Type: Research Article of Molecular Neurodegeneration
Abstract:
Background
Human genetic association studies point to immune response and lipid metabolism, in addition to amyloid-beta (Aβ) and tau, as major pathways in Alzheimer’s disease (AD) etiology. Accumulating evidence suggests that chronic neuroinflammation, mainly mediated by microglia and astrocytes, plays a causative role in neurodegeneration in AD. Our group and others have reported early and dramatic losses of brain sulfatide in AD cases and animal models that are mediated by ApoE in an isoform-dependent manner and accelerated by Aβ accumulation. To date, it remains unclear if changes in specific brain lipids are sufficient to drive AD-related pathology.
Methods
To study the consequences of CNS sulfatide deficiency and gain insights into the underlying mechanisms, we developed a novel mouse model of adult-onset myelin sulfatide deficiency, i.e., tamoxifen-inducible myelinating glia-specific cerebroside sulfotransferase (CST) conditional knockout mice (CST^fl/fl/Plp1-CreERT), took advantage of constitutive CST knockout mice (CST^−/−), and generated CST/ApoE double knockout mice (CST^−/−/ApoE^−/−), and assessed these mice using a broad range of methodologies including lipidomics, RNA profiling, behavioral testing, PLX3397-mediated microglia depletion, mass spectrometry (MS) imaging, immunofluorescence, electron microscopy, and Western blot.
Results
We found that mild central nervous system (CNS) sulfatide losses within myelinating cells are sufficient to activate disease-associated microglia and astrocytes, and to increase the expression of AD risk genes (e.g., Apoe, Trem2, Cd33, and Mmp12), as well as previously established causal regulators of the immune/microglia network in late-onset AD (e.g., Tyrobp, Dock, and Fcerg1), leading to chronic AD-like neuroinflammation and mild cognitive impairment. Notably, neuroinflammation and mild cognitive impairment showed gender differences, being more pronounced in females than males. Subsequent mechanistic studies demonstrated that although CNS sulfatide losses led to ApoE upregulation, genetically-induced myelin sulfatide deficiency led to neuroinflammation independently of ApoE. These results, together with our previous studies (sulfatide deficiency in the context of AD is mediated by ApoE and accelerated by Aβ accumulation) placed both Aβ and ApoE upstream of sulfatide deficiency-induced neuroinflammation, and suggested a positive feedback loop where sulfatide losses may be amplified by increased ApoE expression. We also demonstrated that CNS sulfatide deficiency-induced astrogliosis and ApoE upregulation are not secondary to microgliosis, and that astrogliosis and microgliosis seem to be driven by activation of STAT3 and PU.1/Spi1 transcription factors, respectively.
Conclusion
Our results strongly suggest that sulfatide deficiency is an important contributor and driver of neuroinflammation and mild cognitive impairment in AD pathology.
Access this article: https://doi.org/10.1186/s13024-021-00488-7