Barry W. McColl

BACKGROUND: Adult hippocampal neurogenesis is altered after cerebral ischemia. Although stroke increases newborn neuron production, many cells display aberrant morphological and positional features that may impair functional integration and contribute to long-term cognitive deficits. Given the clinical heterogeneity of ischemic stroke and limited translational success of preclinical studies relying on single models, it remains unclear whether poststroke neurogenic alterations are conserved across experimental paradigms. This study aimed to identify common and model-specific features of hippocampal neurogenesis across focal ischemia models. METHODS: We conducted a multicenter, multimodel analysis within the Leducq-funded Stroke-Impact Transatlantic Network of Excellence using permanent and transient middle cerebral artery occlusion paradigms, including distal middle cerebral artery occlusion under normoxic or hypoxic conditions (distal middle cerebral artery occlusion+hypoxia), and filament-based transient middle cerebral artery occlusion, across 6 sites. Adult C57BL/6J mice were analyzed at 3 days, 7 days, and 2 months after ischemia, sham, or naïve conditions. Hippocampal proliferation (Ki67) and neuroblasts (DCX [doublecortin]) were quantified; morphological maturation of newborn neurons was assessed through high-resolution analyses of dendritic architecture and somatodendritic polarity. RESULTS: Across all stroke models, ischemia induced a robust bilateral increase in hippocampal proliferation, most pronounced at 3 days and still elevated at 7 days, returning to baseline by 2 months. Neuroblast density was similarly increased at 7 days, particularly in the ipsilateral hippocampus, but normalized over time. Despite recovery in cell number, long-term analyses revealed a consistent reduction in apical dendrite length and increased proportion of neurons with aberrant features, including ectopic positioning, polarity defects, and abnormal lateral growth, across models and centers. CONCLUSIONS: Aberrant hippocampal neurogenesis represents a robust hallmark of poststroke pathology in mice, independent of ischemia type or surgical approach, despite known differences in the spatial distribution of primary injury across models. Our findings underscore the importance of considering structural quality, and not only quantity, of newborn neurons when evaluating poststroke plasticity and developing therapeutic strategies.

Following intracerebral haemorrhage (ICH), secondary injury contributes to negative outcomes. Here, we describe inter-dependent injurious and adaptive-protective pathways that form a therapeutically modifiable ICH response. In perihaematomal tissue from post-mortem ICH patients, transcriptomic analysis pointed to concurrent perihaematomal activation of the cytoprotective stress-induced transcription factor NRF2 and activation of interferon responses, with mononuclear myeloid cells (microglia and monocyte-derived cells, MMCs) and astrocytes contributing to this signature. In vitro, microglia-astrocyte-neuron co-culture studies revealed that microglial NRF2 activation was induced by clot-conditioned medium and could influence the interferon pathway in both microglia themselves as well as astrocytes. In vivo, MMC-specific deletion of NRF2 inhibited NRF2 target-gene induction, exacerbated interferon pathway activation following ICH, and impaired functional recovery. Pharmacological NRF2 activation suppressed interferon pathway induction post-ICH, and direct interferon pathway antagonism rescued neurological recovery deficits caused by MMC-specific NRF2 deficiency. Thus, augmenting NRF2- or inhibiting interferon-signalling may improve outcomes after ICH.

Macrophages localize in sub-tissular niches associated with their ontogeny and activity. In the intestine, a paradigm has emerged that long-lived macrophages are present in the muscular layer, while highly monocyte-replenished populations are found in the lamina propria (LP). Whether long-lived macrophages are restricted in such a simplified manner has not been well explored. Moreover, the impact of specific gut-associated factors on macrophage identity across intestinal tissue layers is unknown. We generated scRNA-seq data from WT and Ccr2−/− mice to identify phenotypic features of long-lived macrophage populations in distinct intestinal layers and identified CD163 as a marker to distinguish submucosal/muscularis (S/M) from LP macrophages. Challenging the emerging paradigm, long-lived macrophages were found in the LP and S/M, with distinct transcriptomes and responsiveness to proinflammatory stimuli. Employing transgenic mice, we demonstrate a critical role for TGF-β signalling in maintaining the identity of long-lived LP but not S/M macrophages and that macrophage-derived TGF-β1 is required to instruct intestinal macrophage identity after development.

Cognitive decline is a significant long-term consequence of stroke with no available treatments. To aid in therapy development, we sought to achieve robust detection of cognitive performance after stroke in a multi-site design. Ischemic stroke was induced in adult and middle-aged male C57BL/6 mice utilizing permanent distal middle cerebral artery occlusion (dMCAO), dMCAO with hypoxia or transient MCAO by filament insertion. Cognitive outcomes were assessed by Novel Object Recognition (NOR) and Barnes Maze (BM) tests prior to surgery, and during sub-acute (1–2 weeks) and chronic (8 weeks) phases post-stroke. Histology and immunostaining were used to assess infarct size, tissue damage and neuronal loss, and plasma neurofilament light chain levels were quantified. We did not detect a cognitive deficit using NOR, but detected significantly worse performance in dMCAO mice with the BM. Multi-site BM validation will be a key next step. Overall, our study highlights common challenges in detecting post-stroke cognitive impairment within the pre-clinical stroke community, and underscores the complexities of designing and executing these studies, particularly as applied to a multi-site structure. We provide recommendations and suggest important considerations for future stroke cognition studies, whether operating as an individual lab or a multi-site group.

Summary: Synovial joints harbor a protective lining layer, consisting of fibroblasts and macrophages, which form an epithelial-like barrier. In inflamed joints, lining macrophages regulate both early inflammatory cell influx and resolution. Despite these critical functions, it is currently unknown at what stage during development the synovial macrophage lining is established, and which signals drive this process. Here, we use a combination of genetic models and in vivo perturbations, single cell transcriptomics and imaging to delineate the process of lining formation in mice. We find that the synovial lining is immature at birth and becomes established within the first 3 weeks of life. In this window, the lining is gradually populated with macrophages that originate from fetal sources, proliferate and acquire the lining-specific transcriptional identity. In contrast, monocytes contribute only minimally to the developing lining, and their input remains limited in healthy adulthood. We identify CSF1 and TGFβ as key signals in this process, which also involves mechanosensing through PIEZO1. Our study thus identifies the early postnatal window as a critical period for lining macrophage development, with potential lifelong impact on joint health and disease.

Cognitive decline is a significant long-term consequence of stroke and has no available treatments. To aid in therapy development, we sought to achieve robust detection of cognitive performance after stroke in a multi-site design. Ischemic stroke was induced in adult and middle-aged male C57BL/6J mice utilizing three well-established models: distal middle cerebral artery occlusion (dMCAO), dMCAO with hypoxia and transient MCAO. Cognitive outcomes were assessed via Novel Object Recognition (NOR) and Barnes Maze (BM) tests prior to surgery, and during sub-acute (1-2 weeks) and chronic (8 weeks) phases post-stroke. Histology and immunostaining were used to assess infarct size, tissue damage and neuronal loss, and plasma neurofilament light was quantified. We did not detect a reliable cognitive deficit after stroke using NOR but saw a promising signal from BM (single site tested only). Overall, our study highlights the often-encountered challenges in detecting post-stroke cognitive impairment within the pre-clinical stroke community, as well as a number of complexities in the design and execution of pre-clinical stroke cognition studies, particularly as applied to a multi-site structure. We provide recommendations and suggest important aspects of stroke cognition studies to consider in the future, whether operating as an individual lab or a multi-site group.

Introduction: Triggering receptor expressed on myeloid cells-2 (TREM2) variants have been identified as risk factors for neurodegenerative disease, including Alzheimer’s disease. TREM2 is a cell surface receptor on microglia that regulates homeostatic and immunomodulatory functions, including phagocytosis of apoptotic debris and the resolution of damage-associated inflammation. It remains unclear how TREM2 may mediate an influence on neurodegenerative disease, particularly in relation to key neuropathological hallmarks such as neuronal loss and proteinopathy.Methods: We used the ME7 prion disease model to assess the role of TREM2 in the progression and pathology of neurodegenerative disease. Prion diseases are characterised by the accumulation of misfolded prion proteins and provide a highly tractable platform to determine if TREM2 has disease-modifying effects.Results: Trem2^−/− and wild-type (WT) mice were inoculated intracerebrally with mouse-passaged ME7 scrapie prions, and their effects on CNS disease pathogenesis were determined. Although the accumulation of prion disease-specific PrP was similar in the brains of mice from each group, the severity of neuropathology was increased in Trem2^−/− mice. Morphometric analysis of the microglia also indicated blunted disease-induced reactivity in the brains of infected Trem2^−/− mice compared to wild-type (WT) controls. Expression of genes involved in myelination was reduced in prion-infected Trem2^−/− mice compared to infected WT mice.Conclusion: We conclude that during brain infection with prions, TREM2 supports microglial reactive changes associated with resilience to neuronal loss independently of affecting misfolded PrP deposition. These data imply that TREM2 status may be an important influence on the downstream response to CNS proteinopathy, which alters the susceptibility of neurons and brain tissue to proteinopathy-induced degenerative changes.

Microglial endolysosomal (dys)function is strongly implicated in neurodegenerative disease. Transcriptomic studies show that a microglial state characterised by a set of genes involved in endolysosomal function is induced in both mouse Alzheimer’s disease (AD) models and human AD brain, and that the emergence of this state is emphasised in females. Cst7 (encoding cystatin F) is among the most highly upregulated genes in these microglia. However, despite such striking and robust upregulation, the function of Cst7 in neurodegenerative disease is not understood. Here, we crossed Cst7^-/- mice with the App^NL-G-F mouse to test the role of Cst7 in a model of amyloid-driven AD. Surprisingly, we found that Cst7 plays a sexually dimorphic role regulating microglia in this model. In females, Cst7^-/-App^NL-G-F microglia had greater endolysosomal gene expression, lysosomal burden, and amyloid beta (Aβ) burden in vivo and were more phagocytic in vitro. However, in males, Cst7^-/-App^NL-G-F microglia were less inflammatory and had a reduction in lysosomal burden but had no change in Aβ burden. Overall, our study reveals functional roles for one of the most commonly upregulated genes in microglia across disease models, and the sex-specific profiles of Cst7^-/--altered microglial disease phenotypes. More broadly, the findings raise important implications for AD including crucial questions on sexual dimorphism in neurodegenerative disease and the interplay between endolysosomal and inflammatory pathways in AD pathology.