Reelin, a guidance signal for the regeneration of the entorhino-hippocampal path

Induced Pluripotent Stem Cell-Derived Neural Culture Model of Alzheimer’s Disease: A 3D Organoid Approach

Alzheimer’s disease (AD) is a progressive neurologic disorder that impacts a diverse population of older adults. As three-dimensional (3D) models are powerful tools for advancing AD studies, the authors have been developed AD cortical organoids to enable the observation of AD pathology at the cellular, tissue, and organ levels. For creating the model, APPSwe/Ind (APP) and PSEN1 (PS1) mutant genes were transfected into mouse induced pluripotent stem cells (iPSCs) following which the iPSC lines that expressed mutant APP and PS1 proteins were obtained. Then, using modified serum-free suspended embryoid body culture, AD cerebral organoids were made successfully at various ages. The AD model can show AD’s biochemical and pathological alterations, such as overexpressions of Aβ40 and Aβ42 and a decrease of GABAergic interneurons. The proposed model has the potential for implementation in many biomedical applications, including AD drug screening, stem cell transplant, and neuronal tissue engineering.

Lipid Peroxidation Induced ApoE Receptor-Ligand Disruption as a Unifying Hypothesis Underlying Sporadic Alzheimer's Disease in Humans

BACKGROUND

Sporadic Alzheimer's disease (sAD) lacks a unifying hypothesis that can account for the lipid peroxidation observed early in the disease, enrichment of ApoE in the core of neuritic plaques, hallmark plaques and tangles, and selective vulnerability of entorhinal-hippocampal structures.

OBJECTIVE

We hypothesized that 1) high expression of ApoER2 (receptor for ApoE and Reelin) helps explain this anatomical vulnerability; 2) lipid peroxidation of ApoE and ApoER2 contributes to sAD pathogenesis, by disrupting neuronal ApoE delivery and Reelin-ApoER2-Dab1 signaling cascades.

METHODS

In vitro biochemical experiments; Single-marker and multiplex fluorescence-immunohistochemistry (IHC) in postmortem specimens from 26 individuals who died cognitively normal, with mild cognitive impairment or with sAD.

RESULTS

ApoE and ApoER2 peptides and proteins were susceptible to attack by reactive lipid aldehydes, generating lipid-protein adducts and crosslinked ApoE-ApoER2 complexes. Using in situ hybridization alongside IHC, we observed that: 1) ApoER2 is strongly expressed in terminal zones of the entorhinal-hippocampal 'perforant path' projections that underlie memory; 2) ApoE, lipid aldehyde-modified ApoE, Reelin, ApoER2, and the downstream Reelin-ApoER2 cascade components Dab1 and Thr19-phosphorylated PSD95 accumulated in the vicinity of neuritic plaques in perforant path terminal zones in sAD cases; 3) several ApoE/Reelin-ApoER2-Dab1 pathway markers were higher in sAD cases and positively correlated with histological progression and cognitive deficits.

CONCLUSION

Results demonstrate derangements in multiple ApoE/Reelin-ApoER2-Dab1 axis components in perforant path terminal zones in sAD and provide proof-of-concept that ApoE and ApoER2 are vulnerable to aldehyde-induced adduction and crosslinking. Findings provide the foundation for a unifying hypothesis implicating lipid peroxidation of ApoE and ApoE receptors in sAD.

Imaging genomics discovery of a new risk variant for Alzheimer's disease in the postsynaptic SHARPIN gene

Molecular mechanisms underlying Alzheimer's disease (AD) are difficult to investigate, partly because diagnosis lags behind the insidious pathological processes. Therefore, identifying AD neuroimaging markers and their genetic modifiers may help study early mechanisms of neurodegeneration. We aimed to identify brain regions of the highest vulnerability to AD using a data-driven search in the AD Neuroimaging Initiative (ADNI, n = 1,100 subjects), and further explored genetic variants affecting this critical brain trait using both ADNI and the younger UK Biobank cohort (n = 8,428 subjects). Tensor-Based Morphometry (TBM) and Independent Component Analysis (ICA) identified the limbic system and its interconnecting white-matter as the most AD-vulnerable brain feature. Whole-genome analysis revealed a common variant in SHARPIN that was associated with this imaging feature (rs34173062, p = 2.1 × 10^-10 ). This genetic association was validated in the UK Biobank, where it was correlated with entorhinal cortical thickness bilaterally (p = .002 left and p = 8.6 × 10^-4 right), and with parental history of AD (p = 2.3 × 10^-6 ). Our findings suggest that neuroanatomical variation in the limbic system and AD risk are associated with a novel variant in SHARPIN. The role of this postsynaptic density gene product in β1-integrin adhesion is in line with the amyloid precursor protein (APP) intracellular signaling pathway and the recent genome-wide evidence.

Progesterone receptor expression in cajal-retzius cells of the developing rat dentate gyrus: Potential role in hippocampus-dependent memory

The development of medial temporal lobe circuits is critical for subsequent learning and memory functions later in life. The present study reports the expression of progesterone receptor (PR), a powerful transcription factor of the nuclear steroid receptor superfamily, in Cajal-Retzius cells of the molecular layer of the dentate gyrus of rats. PR was transiently expressed from the day of birth through postnatal day 21, but was absent thereafter. Although PR immunoreactive (PR-ir) cells did not clearly express typical markers of mature neurons, they possessed an ultrastructural morphology consistent with neurons. PRir cells did not express markers for GABAergic neurons, neuronal precursor cells, nor radial glia. However, virtually all PR cells co-expressed the calcium binding protein, calretinin, and the glycoprotein, reelin, both reliable markers for Cajal-Retzius neurons, a transient population of developmentally critical pioneer neurons that guide synaptogenesis of perforant path afferents and histogenesis of the dentate gyrus. Indeed, inhibition of PR activity during the first two weeks of life impaired adult performance on both the novel object recognition and object placement memory tasks, two behavioral tasks hypothesized to describe facets of episodic-like memory in rodents. These findings suggest that PR plays an unexplored and important role in the development of hippocampal circuitry and adult memory function.

Characterization of hippocampal Cajal-Retzius cells during development in a mouse model of Alzheimer's disease (Tg2576)

Cajal-Retzius cells are reelin-secreting neurons in the marginal zone of the neocortex and hippocampus. The aim of this study was to investigate Cajal-Retzius cells in Alzheimer's disease pathology. Results revealed that the number of Cajal-Retzius cells markedly reduced with age in both wild type and in mice over-expressing the Swedish double mutant form of amyloid precursor protein 695 (transgenic (Tg) 2576 mice). Numerous reelin-positive neurons were positive for activated caspase 3 in Tg2576 mice, suggesting that Cajal-Retzius neuronal loss occurred via apoptosis in this Alzheimer's disease model. Compared with wild type, the number of Cajal-Retzius cells was significantly lower in Tg2576 mice. Western blot analysis confirmed that reelin levels were markedly lower in Tg2576 mice than in wild-type mice. The decline in Cajal-Retzius cells in Tg2576 mice was found to occur concomitantly with the onset of Alzheimer's disease amyloid pathology and related behavioral deficits. Overall, these data indicated that Cajal-Retzius cell loss occurred with the onset and development of Alzheimer's disease.

Sorting nexin 17 regulates ApoER2 recycling and reelin signaling

ApoER2 is a member of the low density-lipoprotein receptor (LDL-R) family. As a receptor for reelin, ApoER2 participates in neuronal migration during development as well as synaptic plasticity and survival in the adult brain. A previous yeast two-hybrid screen showed that ApoER2 is a binding partner of sorting nexin 17 (SNX17) - a cytosolic adaptor protein that regulates the trafficking of several membrane proteins in the endosomal pathway, including LRP1, P-selectin and integrins. However, no further studies have been performed to investigate the role of SNX17 in ApoER2 trafficking and function. In this study, we present evidence based on GST pull-down and inmunoprecipitation assays that the cytoplasmic NPxY endocytosis motif of ApoER2 interacts with the FERM domain of SNX17. SNX17 stimulates ApoER2 recycling in different cell lines including neurons without affecting its endocytic rate and also facilitates the transport of ApoER2 from the early endosomes to the recycling endosomes. The reduction of SNX17 was associated with accumulation of an ApoER2 carboxy-terminal fragment (CTF). In addition, in SNX17 knockdown cells, constitutive ApoER2 degradation was not modified, whereas reelin-induced ApoER2 degradation was increased, implying that SNX17 is a regulator of the receptor's half-life. Finally, in SNX17 silenced hippocampal and cortical neurons, we underscored a positive role of this endosomal protein in the development of the dendritic tree and reelin signaling. Overall, these results establish the role of SNX17 in ApoER2 trafficking and function and aid in identifying new links between endocytic trafficking and receptor signaling.

Contributions of VLDLR and LRP8 in the establishment of retinogeniculate projections

BACKGROUND

Retinal ganglion cells (RGCs), the output neurons of the retina, project to over 20 distinct brain nuclei, including the lateral geniculate nucleus (LGN), a thalamic region comprised of three functionally distinct subnuclei: the ventral LGN (vLGN), the dorsal LGN (dLGN) and the intergeniculate leaflet (IGL). We previously identified reelin, an extracellular glycoprotein, as a critical factor that directs class-specific targeting of these subnuclei. Reelin is known to bind to two receptors: very-low-density lipoprotein receptor (VLDLR) and low-density lipoprotein receptor-related protein 8 (LRP8), also known as apolipoprotein E receptor 2 (ApoER2). Here we examined the roles of these canonical reelin receptors in retinogeniculate targeting.

RESULTS

To assess the roles of VLDLR and LRP8 in retinogeniculate targeting, we used intraocular injections of fluorescently conjugated cholera toxin B subunit (CTB) to label all RGC axons in vivo. Retinogeniculate projections in mutant mice lacking either VLDLR or LRP8 appeared similar to controls; however, deletion of both receptors resulted in dramatic defects in the pattern of retinal innervation in LGN. Surprisingly, defects in vldlr(-/-);lrp8(-/-) double mutant mice were remarkably different than those observed in mice lacking reelin. First, we failed to observe retinal axons exiting the medial border of the vLGN and IGL to invade distant regions of non-retino-recipient thalamus. Second, an ectopic region of binocular innervation emerged in the dorsomedial pole of vldlr(-/-);lrp8(-/-) mutant dLGN. Analysis of retinal projection development, retinal terminal sizes and LGN cytoarchitecture in vldlr(-/-);lrp8(-/-) mutants, all suggest that a subset of retinal axons destined for the IGL are misrouted to the dorsomedial pole of dLGN in the absence of VLDLR and LRP8. Such mistargeting is likely the result of abnormal migration of IGL neurons into the dorsomedial pole of dLGN in vldlr(-/-);lrp8(-/-) mutants.

CONCLUSIONS

In contrast to our expectations, the development of both the LGN and retinogeniculate projections appeared dramatically different in mutants lacking either reelin or both canonical reelin receptors. These results suggest that there are reelin-independent functions of VLDLR and LRP8 in LGN development, and VLDLR- and LRP8-independent functions of reelin in class-specific axonal targeting.

Reelin is required for class-specific retinogeniculate targeting

Development of visual system circuitry requires the formation of precise synaptic connections between neurons in the retina and brain. For example, axons from retinal ganglion cells (RGCs) form synapses onto neurons within subnuclei of the lateral geniculate nucleus (LGN) [i.e., the dorsal LGN (dLGN), ventral LGN (vLGN), and intergeniculate leaflet (IGL)]. Distinct classes of RGCs project to these subnuclei: the dLGN is innervated by image-forming RGCs, whereas the vLGN and IGL are innervated by non-image-forming RGCs. To explore potential mechanisms regulating class-specific LGN targeting, we sought to identify differentially expressed targeting molecules in these LGN subnuclei. One candidate targeting molecule enriched in the vLGN and IGL during retinogeniculate circuit formation was the extracellular matrix molecule reelin. Anterograde labeling of RGC axons in mutant mice lacking functional reelin (reln(rl/rl)) revealed reduced patterns of vLGN and IGL innervation and misrouted RGC axons in adjacent non-retino-recipient thalamic nuclei. Using genetic reporter mice, we further demonstrated that mistargeted axons were from non-image-forming, intrinsically photosensitive RGCs (ipRGCs). In contrast to mistargeted ipRGC axons, axons arising from image-forming RGCs and layer VI cortical neurons correctly targeted the dLGN in reln(rl/rl) mutants. Together, these data reveal that reelin is essential for the targeting of LGN subnuclei by functionally distinct classes of RGCs.

Axon guidance and neuronal migration research in China

Proper migration of neuronal somas and axonal growth cones to designated locations in the developing brain is essential for the assembly of functional neuronal circuits. Rapid progress in research of axon guidance and neuronal migration has been made in the last twenty years. Chinese researchers began their exploration in this field ten years ago and have made significant contributions in clarifying the signal transduction of axon guidance and neuronal migration. Several unique experimental approaches, including the migration assay of single isolated neurons in response to locally delivered guidance cues, have been developed by Chinese neuroscientists to investigate the molecular machinery underlying these guidance events.

The synaptic remodeling between regenerated perforant pathway and granule cells in slice culture

In order to understand the synaptic remodeling in the course of axonal regeneration, the synaptic remodeling of the perforant path in hippocampus was investigated in the present study with entorhino-hippocampal coculture, DiI DiOlistic assay and transmission electron microscopy. The results showed that the regeneration of the perforant pathway occurred in entorhino-hippocampal slice coculture, and putative synaptic contacts formed between the regenerated fibers and dendritic spines of granule cells. Ultrastructural analysis confirmed the formation of new synaptic contacts. In conclusion, the synaptic formation implicated in the neuroregeneration could integrate into the network in CNS.