TMEM106B coding variant is protective and deletion detrimental in a mouse model of tauopathy.

TMEM106B is a risk modifier of multiple neurological conditions, where a single coding variant and multiple non-coding SNPs influence the balance between susceptibility and resilience. Two key questions that emerge from past work are whether the lone T185S coding variant contributes to protection, and if the presence of TMEM106B is helpful or harmful in the context of disease. Here, we address both questions while expanding the scope of TMEM106B study from TDP-43 to models of tauopathy. We generated knockout mice with constitutive deletion of TMEM106B, alongside knock-in mice encoding the T186S knock-in mutation (equivalent to the human T185S variant), and crossed both with a P301S transgenic tau model to study how these manipulations impacted disease phenotypes. We found that TMEM106B deletion accelerated cognitive decline, hind limb paralysis, tau pathology, and neurodegeneration. TMEM106B deletion also increased transcriptional correlation with human AD and the functional pathways enriched in KO:tau mice aligned with those of AD. In contrast, the coding variant protected against tau-associated cognitive decline, synaptic impairment, neurodegeneration, and paralysis without affecting tau pathology. Our findings reveal that TMEM106B is a critical safeguard against tau aggregation, and that loss of this protein has a profound effect on sequelae of tauopathy. Our study further demonstrates that the coding variant is functionally relevant and contributes to neuroprotection downstream of tau pathology to preserve cognitive function.

Doxycycline for transgene control disrupts gut microbiome diversity without compromising acute neuroinflammatory response.

The tetracycline transactivator (tTA) system provides controllable transgene expression through oral administration of the broad-spectrum antibiotic doxycycline. Antibiotic treatment for transgene control in mouse models of disease might have undesirable systemic effects resulting from changes in the gut microbiome. Here we assessed the impact of doxycycline on gut microbiome diversity in a tTA-controlled model of Alzheimer's disease and then examined neuroimmune effects of these microbiome alterations following acute LPS challenge. We show that doxycycline decreased microbiome diversity in both transgenic and wild-type mice and that these changes persisted long after drug withdrawal. Despite the change in microbiome composition, doxycycline treatment had minimal effect on basal transcriptional signatures of inflammation the brain or on the neuroimmune response to LPS challenge. Our findings suggest that central neuroimmune responses may be less affected by doxycycline at doses needed for transgene control than by antibiotic cocktails at doses used for experimental microbiome disruption.

The TMEM106B T186S coding variant increases neurite arborization and synaptic density in primary hippocampal neurons.

The lysosomal protein TMEM106B was identified as a risk modifier of multiple dementias including frontotemporal dementia and Alzheimer's disease. The gene comes in two major haplotypes, one associated with disease risk, and by comparison, the other with resilience. Only one coding polymorphism distinguishes the two alleles, a threonine-to-serine substitution at residue 185 (186 in mouse), that is inherited in disequilibrium with multiple non-coding variants. Transcriptional studies suggest synaptic, neuronal, and cognitive preservation in human subjects with the protective haplotype, while murine in vitro studies reveal dramatic effects of TMEM106B deletion on neuronal development. Despite this foundation, the field has not yet resolved whether coding variant is biologically meaningful, and if so, whether it has any specific effect on neuronal phenotypes. Here we studied how loss of TMEM106B or expression of the lone coding variant in isolation affected transcriptional signatures in the mature brain and neuronal structure during development in primary neurons. Homozygous expression of the TMEM106B T186S variant in knock-in mice increased cortical expression of genes associated with excitatory synaptic function and axon outgrowth, and promoted neurite branching, dendritic spine density, and synaptic density in primary hippocampal neurons. In contrast, constitutive TMEM106B deletion affected transcriptional signatures of myelination without altering neuronal development in vitro. Our findings show that the T186S variant is functionally relevant and may contribute to disease resilience during neurodevelopment.

TMEM106B coding variant is protective and deletion detrimental in a mouse model of tauopathy.

TMEM106B is a risk modifier for a growing list of age-associated dementias including Alzheimer’s and frontotemporal dementia, yet its function remains elusive. Two key questions that emerge from past work are whether the conservative T185S coding variant found in the minor haplotype contributes to protection, and whether the presence of TMEM106B is helpful or harmful in the context of disease. Here we address both issues while extending the testbed for study of TMEM106B from models of TDP to tauopathy. We show that TMEM106B deletion accelerates cognitive decline, hindlimb paralysis, neuropathology, and neurodegeneration. TMEM106B deletion also increases transcriptional overlap with human AD, making it a better model of disease than tau alone. In contrast, the coding variant protects against tau-associated cognitive decline, neurodegeneration, and paralysis without affecting tau pathology. Our findings show that the coding variant contributes to neuroprotection and suggest that TMEM106B is a critical safeguard against tau aggregation.

Activity disruption causes degeneration of entorhinal neurons in a mouse model of Alzheimer's circuit dysfunction.

Neurodegenerative diseases are characterized by selective vulnerability of distinct cell populations; however, the cause for this specificity remains elusive. Here, we show that entorhinal cortex layer 2 (EC2) neurons are unusually vulnerable to prolonged neuronal inactivity compared with neighboring regions of the temporal lobe, and that reelin + stellate cells connecting EC with the hippocampus are preferentially susceptible within the EC2 population. We demonstrate that neuronal death after silencing can be elicited through multiple independent means of activity inhibition, and that preventing synaptic release, either alone or in combination with electrical shunting, is sufficient to elicit silencing-induced degeneration. Finally, we discovered that degeneration following synaptic silencing is governed by competition between active and inactive cells, which is a circuit refinement process traditionally thought to end early in postnatal life. Our data suggests that the developmental window for wholesale circuit plasticity may extend into adulthood for specific brain regions. We speculate that this sustained potential for remodeling by entorhinal neurons may support lifelong memory but renders them vulnerable to prolonged activity changes in disease.

Neurodegenerative conditions cause irreversible damage to the brain and have a devastating impact on quality of life. However, these diseases start gradually, meaning that the entire brain is not affected at once. For example, the initial signs of Alzheimer's disease appear only in specific areas. One of the first brain regions to degenerate in Alzheimer's is the entorhinal cortex. In healthy individuals, entorhinal neurons send electrical signals to the hippocampus, a part of the brain important for memory and learning. During Alzheimer's, hippocampal neurons also die off, leading to 'shrinkage' of this brain region and, ultimately, the memory problems that are a hallmark of the disease. Many neurons in the developing brain require electrical input from other cells to survive ­ in other words, if they do not belong to an 'active circuit', they are eliminated. This is crucial for the connection between the entorhinal cortex and the hippocampus, where the circuit's development and maintenance require carefully controlled electrical activity. Abnormal electrical activity is also an early sign of diseases like Alzheimer's, but how this relates to degeneration is still poorly understood. By investigating these questions, Zhao, Grunke, Wood et al. uncovered a potential relationship between electrical activity and degeneration in the adult brain, long after the circuit between the hippocampus and the entorhinal cortex had matured. Mice were genetically engineered so that their entorhinal cortex would carry a protein designed to silence electrical signaling. The communication between the entorhinal cortex and the hippocampus could therefore be shut off by activating the protein with an injected drug. Remarkably, within just a few days of silencing, cells from the entorhinal cortex started to die off. Zhao, Grunke, Wood et al. went on to show that different silencing methods yielded the same results ­ in other words, the degeneration of cells from the entorhinal cortex was not linked to a particular method. This vulnerability to electrical inactivity was also unique to the entorhinal cortex: when neighboring parts of the brain were silenced, the nerve cells in these areas did not die as readily. Interestingly, in one of their experiments, Zhao, Grunke, Wood et al. found that electrical activity of neighboring nerve cells participated in killing the silenced neurons, suggesting that nerve cells in these brain areas might compete to survive. Overall, this work highlights a direct link between electrical activity and nerve cell degeneration in a part of the brain severely affected by Alzheimer's. In the future, Zhao, Grunke, Wood et al. hope that these results will pave the way to a better understanding of the biological mechanisms underpinning such neurodegenerative diseases.

Gene therapy using Aß variants for amyloid reduction.

Numerous aggregation inhibitors have been developed with the goal of blocking or reversing toxic amyloid formation in vivo. Previous studies have used short peptide inhibitors targeting different amyloid ß (Aß) amyloidogenic regions to prevent aggregation. Despite the specificity that can be achieved by peptide inhibitors, translation of these strategies has been thwarted by two key obstacles: rapid proteolytic degradation in the bloodstream and poor transfer across the blood-brain barrier. To circumvent these problems, we have created a minigene to express full-length Aß variants in the mouse brain. We identify two variants, F20P and F19D/L34P, that display four key properties required for therapeutic use: neither peptide aggregates on its own, both inhibit aggregation of wild-type Aß in vitro, promote disassembly of pre-formed fibrils, and diminish toxicity of Aß oligomers. We used intraventricular injection of adeno-associated virus (AAV) to express each variant in APP/PS1 transgenic mice. Lifelong expression of F20P, but not F19D/L34P, diminished Aß levels, plaque burden, and plaque-associated neuroinflammation. Our findings suggest that AAV delivery of Aß variants may offer a novel therapeutic strategy for Alzheimer's disease. More broadly our work offers a framework for identifying and delivering peptide inhibitors tailored to other protein-misfolding diseases.