IMPDH2 filaments protect from neurodegeneration in AMPD2 deficiency.

Metabolic dysregulation is one of the most common causes of pediatric neurodegenerative disorders. However, how the disruption of ubiquitous and essential metabolic pathways predominantly affect neural tissue remains unclear. Here we use mouse models of a childhood neurodegenerative disorder caused by AMPD2 deficiency to study cellular and molecular mechanisms that lead to selective neuronal vulnerability to purine metabolism imbalance. We show that mouse models of AMPD2 deficiency exhibit predominant degeneration of the hippocampal dentate gyrus, despite a general reduction of brain GTP levels. Neurodegeneration-resistant regions accumulate micron-sized filaments of IMPDH2, the rate limiting enzyme in GTP synthesis, while these filaments are barely detectable in the hippocampal dentate gyrus. Furthermore, we show that IMPDH2 filament disassembly reduces GTP levels and impairs growth of neural progenitor cells derived from individuals with human AMPD2 deficiency. Together, our findings suggest that IMPDH2 polymerization prevents detrimental GTP deprivation, opening the possibility of exploring the induction of IMPDH2 assembly as a therapy for neurodegeneration.

Genomes of the Orestias pupfish from the Andean Altiplano shed light on their evolutionary history and phylogenetic relationships within Cyprinodontiformes.

BACKGROUND: To unravel the evolutionary history of a complex group, a comprehensive reconstruction of its phylogenetic relationships is crucial. This requires meticulous taxon sampling and careful consideration of multiple characters to ensure a complete and accurate reconstruction. The phylogenetic position of the Orestias genus has been estimated partly on unavailable or incomplete information. As a consequence, it was assigned to the family Cyprindontidae, relating this Andean fish to other geographically distant genera distributed in the Mediterranean, Middle East and North and Central America. In this study, using complete genome sequencing, we aim to clarify the phylogenetic position of Orestias within the Cyprinodontiformes order. RESULTS: We sequenced the genome of three Orestias species from the Andean Altiplano. Our analysis revealed that the small genome size in this genus (~ 0.7 Gb) was caused by a contraction in transposable element (TE) content, particularly in DNA elements and short interspersed nuclear elements (SINEs). Using predicted gene sequences, we generated a phylogenetic tree of Cyprinodontiformes using 902 orthologs extracted from all 32 available genomes as well as three outgroup species. We complemented this analysis with a phylogenetic reconstruction and time calibration considering 12 molecular markers (eight nuclear and four mitochondrial genes) and a stratified taxon sampling to consider 198 species of nearly all families and genera of this order. Overall, our results show that phylogenetic closeness is directly related to geographical distance. Importantly, we found that Orestias is not part of the Cyprinodontidae family, and that it is more closely related to the South American fish fauna, being the Fluviphylacidae the closest sister group. CONCLUSIONS: The evolutionary history of the Orestias genus is linked to the South American ichthyofauna and it should no longer be considered a member of the Cyprinodontidae family. Instead, we submit that Orestias belongs to the Orestiidae family, as suggested by Freyhof et al. (2017), and that it is the sister group of the Fluviphylacidae family, distributed in the Amazonian and Orinoco basins. These two groups likely diverged during the Late Eocene concomitant with hydrogeological changes in the South American landscape.

Case 311: Generalized Lymphatic Anomaly.

HISTORY: A 14-year-old boy presented with asthenia, low back pain, and abdominal distention. The onset of symptoms was slow and progressive over a few months. The patient had no contributing past medical history. At physical examination, all vital signs were normal. Only pallor and positive fluid wave test results were noted; there was no lower limb edema, mucocutaneous lesions, or palpable lymph node enlargement. Laboratory work-up revealed a decreased hemoglobin concentration of 9.3 g/dL (normal range, 12-16 g/dL) and a decreased hematocrit level of 29.8% (normal range, 37%-45%), but all other laboratory values were normal. Contrast-enhanced CT of the chest, abdomen, and pelvis was performed.

Genome sequencing and transcriptomic analysis of the Andean killifish Orestias ascotanensis reveals adaptation to high-altitude aquatic life.

Orestias ascotanensis (Cyprinodontidae) is a teleost pupfish endemic to springs feeding into the Ascotan saltpan in the Chilean Altiplano (3,700 m.a.s.l.) and represents an opportunity to study adaptations to high-altitude aquatic environments. We have de novo assembled the genome of O. ascotanensis at high coverage. Comparative analysis of the O. ascotanensis genome showed an overall process of contraction, including loss of genes related to G-protein signaling, chemotaxis and signal transduction, while there was expansion of gene families associated with microtubule-based movement and protein ubiquitination. We identified 818 genes under positive selection, many of which are involved in DNA repair. Additionally, we identified novel and conserved microRNAs expressed in O. ascotanensis and its closely-related species, Orestias gloriae. Our analysis suggests that positive selection and expansion of genes that preserve genome stability are a potential adaptive mechanism to cope with the increased solar UV radiation to which high-altitude animals are exposed to.

Case 311.

HISTORY: A 14-year-old boy presented with asthenia, low back pain, and abdominal distention. The onset of symptoms was slow and progressive over a few months. The patient had no contributing past medical history. At physical examination, all vital signs were normal. Only pallor and positive fluid wave test results were noted; there was no lower limb edema, mucocutaneous lesions, or palpable lymph node enlargement. Laboratory work-up revealed a decreased hemoglobin concentration of 9.3 g/dL (normal range, 12-16 g/dL) and a decreased hematocrit level of 29.8% (normal range, 37%-45%), but all other laboratory values were normal. Contrast-enhanced CT of the chest, abdomen, and pelvis was performed (Figs 1-3).

Phylogeny of Telmatobius marmoratus complex (Anura, Telmatobiidae) reveals high cryptic diversity in the Andean Altiplano.

Telmatobius is the most diverse group of anurans in the Andean Altiplano (highlands) Morphologically, these amphibians have a generally conserved morphology but in turn present large intraspecific variation, which has led to a complex taxonomy and systematics. T. marmoratus has the widest distribution of the genus and forms a complex composed of at least two Telmatobius species. Partial systematic studies based on molecular evidence reveal the existence of three lineages with a complex spatial distribution. However, these studies did not include the entire distribution of T. marmoratus. Our study aims to reassess the current systematic scenario including the complete distribution of the complex. For this, we used a multilocus approach based on mitochondrial (16S, Cytb) and nuclear (RAG1-1, BFIB) DNA sequences to build a phylogenetic hypothesis based on Bayesian inference, maximum likelihood and maximum parsimony. Subsequently, we performed single-locus (ABGD and PTP) and multilocus (STACEY) species delimitation analyses to verify the diversity of nominal species within the complex. The analyses suggest seven non-sibling lineages and 6-10 candidate species within the marmoratus complex. Only one of the two lineages restricted to the central northern plateau correspond to T. marmoratus sensu stricto. South-central marbled water frogs belong to completely new lineages closer to T. gigas and T. culeus, evidencing the polyphyletic condition of the marmoratus complex. The findings of several sympatric lineages in some localities reveal a complex history of ancient water connections in south-central Altiplano.

The E6/E7 oncogenes of human papilloma virus and estradiol regulate hedgehog signaling activity in a murine model of cervical cancer.

Human papilloma virus oncogenes and estradiol are major etiologic factors associated with cervical cancer. In order to understand the mechanism by which these two factors promote carcinogenesis, the role of the Hedgehog (Hh) signaling pathway was evaluated during the normal growth of cervical epithelium and in the presence of E6/E7 oncogenes and exogenous estradiol. Hh signaling activity was determined in live animals (i.e., Gli-Luc reporter levels) during the estrous cycle and was found to be higher in the cervical area during the major growth phases, proestrus-estrus, in comparison to the diestrus phase. The same pattern was observed in transgenic mice expressing the E6/E7 oncogenes, though with notably higher levels than in control mice. Adding estradiol also markedly increased Gli activity in the cervix and the skin. In agreement with the correlation between high bioluminescence and tissue growth in different context, cervical cell proliferation was reduced upon Hh signaling inhibition in mice. Treatment with itraconazole, a putative novel Hh inhibitor, at an early stage of cervical carcinogenesis, did not decrease Hh signaling but it did reduce growth. Therefore, Hh signaling likely contributes to cervical carcinogenesis and itraconazole is effective to reduce growth but by a mechanism involving additional signaling pathways.

Different phosphorylation patterns regulate α1D-adrenoceptor signaling and desensitization.

Human α1D-adrenoceptors (α1D-ARs) are a group of the seven transmembrane-spanning proteins that mediate many of the physiological and pathophysiological actions of adrenaline and noradrenaline. Although it is known that α1D-ARs are phosphoproteins, their specific phosphorylation sites and the kinases involved in their phosphorylation remain largely unknown. Using a combination of in silico analysis, mass spectrometry and site directed mutagenesis, we identified distinct α1D-AR phosphorylation patterns during noradrenaline- or phorbol ester-mediated desensitizations. We found that the G protein coupled receptor kinase, GRK2, and conventional protein kinases C isoforms α/ß, phosphorylate α1D-AR during these processes. Furthermore, we showed that the phosphorylated residues are located in the receptor's third intracellular loop (S300, S323, T328, S331, S332, S334) and carboxyl region (S441, T442, T477, S486, S492, T507, S515, S516, S518, S543) and are conserved among orthologues but are not conserved among the other human α1-adrenoceptor subtypes. Additionally, we found that phosphorylation in either the third intracellular loop or carboxyl tail was sufficient to regulate calcium signaling desensitization. By contrast, mutations in either of these two domains significantly altered mitogen activated protein kinase (ERK) pathway and receptor internalization, suggesting that they have differential regulatory mechanisms. Our data provide new insights into the functional repercussions of these posttranslational modifications in signaling outcomes and desensitization.

Assesment and interpretation of negative forelimb allometry in the evolution of non-avian Theropoda.

BACKGROUND: The origin of birds is marked by a significant decrease in body size along with an increase in relative forelimb size. However, before the evolution of flight, both traits may have already been related: It has been proposed that an evolutionary trend of negative forelimb allometry existed in non-avian Theropoda, such that larger species often have relatively shorter forelimbs. Nevertheless, several exceptions exist, calling for rigorous phylogenetic statistical testing. RESULTS: Here, we re-assessed allometric patterns in the evolution of non-avian theropods, for the first time taking into account the non-independence among related species due to shared evolutionary history.We confirmed a main evolutionary trend of negative forelimb allometry for non-avian Theropoda, but also found support that some specific subclades (Coelophysoidea, Ornithomimosauria, and Oviraptorosauria) exhibit allometric trends that are closer to isometry, losing the ancestral negative forelimb allometry present in Theropoda as a whole. CONCLUSIONS: Explanations for negative forelimb allometry in the evolution of non-avian theropods have not been discussed, yet evolutionary allometric trends often reflect ontogenetic allometries, which suggests negative allometry of the forelimb in the ontogeny of most non-avian theropods. In modern birds, allometric growth of the limbs is related to locomotor and behavioral changes along ontogeny. After reviewing the evidence for such changes during the ontogeny of non-avian dinosaurs, we propose that proportionally longer arms of juveniles became adult traits in the small-sized and paedomorphic Aves.

Noradrenaline, oxymetazoline and phorbol myristate acetate induce distinct functional actions and phosphorylation patterns of α1A-adrenergic receptors.

In LNCaP cells that stably express α1A-adrenergic receptors, oxymetazoline increased intracellular calcium and receptor phosphorylation, however, this agonist was a weak partial agonist, as compared to noradrenaline, for calcium signaling. Interestingly, oxymetazoline-induced receptor internalization and desensitization displayed greater effects than those induced by noradrenaline. Phorbol myristate acetate induced modest receptor internalization and minimal desensitization. α1A-Adrenergic receptor interaction with ß-arrestins (colocalization/coimmunoprecipitation) was induced by noradrenaline and oxymetazoline and, to a lesser extent, by phorbol myristate acetate. Oxymetazoline was more potent and effective than noradrenaline in inducing ERK 1/2 phosphorylation. Mass spectrometric analysis of immunopurified α1A-adrenergic receptors from cells treated with adrenergic agonists and the phorbol ester clearly showed that phosphorylated residues were present both at the third intracellular loop and at the carboxyl tail. Distinct phosphorylation patterns were observed under the different conditions. The phosphorylated residues were: a) Baseline and all treatments: T233; b) noradrenaline: S220, S227, S229, S246, S250, S389; c) oxymetazoline: S227, S246, S381, T384, S389; and d) phorbol myristate acetate: S246, S250, S258, S351, S352, S401, S402, S407, T411, S413, T451. Our novel data, describing the α1A-AR phosphorylation sites, suggest that the observed different phosphorylation patterns may participate in defining adrenoceptor localization and action, under the different conditions examined.

Hidden and cryptic species reflect parallel and correlated evolution in the phylogeny of the genus Callyntra (Coleoptera: Tenebrionidae) of Central Chile.

The origin of cryptic species has traditionally been associated with events of recent speciation, genetic constraints, selection of an adaptive character, sexual selection and/or convergent evolution. Species of the genus Callyntra inhabit coastal terraces, mountain slopes, and peaks; their elytral designs are associated with each of these habitats. However, cryptic species have been described within each of these habitats; the taxonomy of this group has been problematic, thus establishing the phylogenetic relationships in this group is fundamental to clarify the systematics and evolutionary patterns of Callyntra. We reconstructed the phylogeny of this group using two mitochondrial genes (COI, 16S) and one nuclear gene (Mp20). We also performed species delimitation using PTP based methods (PTP, mlPTP, bPTP) and GMYC, and evaluated the evolution of the elytral design related to habitat preference. The results showed a tree with five clades, that together with the different methods of species delimitation recovered the described species and suggested at least five new species. The elytral design and habitat preference showed phylogenetic signals. We propose a new classification based on monophyletic groups recovered by phylogenetic analyses. We also suggest that parallel evolution in different habitats and later stasis in the elytral design would be the cause of the origin of cryptic species in this group from central Chile.

Development and characterization of 22 polymorphic microsatellites of the Andean frog Telmatobius chusmisensis (Anura, Telmatobius) and cross amplification in seven Chilean species of the genus.

The genus Telmatobius Wiegmann, 1834 is composed of a wide variety of species and is one of the most diverse native components of the high-altitude Andean environments. The species of the genus present in Chile are considered as endangered, critically endangered or data deficient. We isolated and evaluated 44 microsatellites in 80 individuals of 8 species of Telmatobius present in Chile, obtaining 22 polymorphic microsatellite loci for Telmatobius chusmisensis. The cross-amplification test was successful in all other species tested. For the first time, microsatellite markers are described for Telmatobius. The description of these primers will be useful for further genetic studies for T. chusmisensis and other species of the same genus; allowing further analyses of population structuring, dispersal patterns, recent demographic history and population effective size. This information is also significant to undertake conservation actions for the species of the genus Telmatobius, since most species have conservation issues.

Protein Kinase C Activation Promotes α1B-Adrenoceptor Internalization and Late Endosome Trafficking through Rab9 Interaction. Role in Heterologous Desensitization.

Upon agonist stimulation, α1B-adrenergic receptors couple to Gq proteins, calcium signaling and protein kinase C activation; subsequently, the receptors are phosphorylated, desensitized, and internalized. Internalization seems to involve scaffolding proteins, such as ß-arrestin and clathrin. However, the fine mechanisms that participate remain unsolved. The roles of protein kinase C and the small GTPase, Rab9, in α1B-AR vesicular traffic were investigated by studying α1B-adrenergic receptor-Rab protein interactions, using Förster resonance energy transfer (FRET), confocal microscopy, and intracellular calcium quantitation. In human embryonic kidney 293 cells overexpressing Discosoma spp. red fluorescent protein (DsRed)-tagged α1B-ARs and enhanced green fluorescent protein--tagged Rab proteins, pharmacological protein kinase C activation mimicked α1B-AR traffic elicited by nonrelated agents, such as sphingosine 1-phosphate (i.e., transient α1B-AR-Rab5 FRET signal followed by a sustained α1B-AR-Rab9 interaction), suggesting brief receptor localization in early endosomes and transfer to late endosomes. This latter interaction was abrogated by blocking protein kinase C activity, resulting in receptor retention at the plasma membrane. Similar effects were observed when a dominant-negative Rab9 mutant (Rab9-GDP) was employed. When α1B-adrenergic receptors that had been mutated at protein kinase C phosphorylation sites (S396A, S402A) were used, phorbol ester-induced desensitization of the calcium response was markedly decreased; however, interaction with Rab9 was only partially decreased and internalization was observed in response to phorbol esters and sphingosine 1-phosphate. Finally, Rab9-GDP expression did not affect adrenergic-mediated calcium response but abolished receptor traffic and altered desensitization. Data suggest that protein kinase C modulates α1B-adrenergic receptor transfer to late endosomes and that Rab9 regulates this process and participates in G protein-mediated signaling turn-off.

Novel Structural Approaches to Study GPCR Regulation.

BACKGROUND: Upon natural agonist or pharmacological stimulation, G protein-coupled receptors (GPCRs) are subjected to posttranslational modifications, such as phosphorylation and ubiquitination. These posttranslational modifications allow protein-protein interactions that turn off and/or switch receptor signaling as well as trigger receptor internalization, recycling or degradation, among other responses. Characterization of these processes is essential to unravel the function and regulation of GPCR. METHODS: In silico analysis and methods such as mass spectrometry have emerged as novel powerful tools. Both approaches have allowed proteomic studies to detect not only GPCR posttranslational modifications and receptor association with other signaling macromolecules but also to assess receptor conformational dynamics after ligand (agonist/antagonist) association. RESULTS: this review aims to provide insights into some of these methodologies and to highlight how their use is enhancing our comprehension of GPCR function. We present an overview using data from different laboratories (including our own), particularly focusing on free fatty acid receptor 4 (FFA4) (previously known as GPR120) and α1A- and α1D-adrenergic receptors. From our perspective, these studies contribute to the understanding of GPCR regulation and will help to design better therapeutic agents.

Glutamate-dependent translational control through ribosomal protein S6 phosphorylation in cultured bergmann glial cells.

Glutamate (Glu) the main excitatory neurotransmitter of the central nervous system regulates gene expression at different levels through the activation of specific membrane receptors and transporters expressed in neurons and glia cells. A membrane to nucleus signaling cascade triggered by this neurotransmitter has been described in cultured cerebellar Bergmann glia cells isolated from chick embryos. Furthermore, it has also been described that Glu receptors activation is linked to a modulation of [(35)S]-methionine incorporation into newly synthesized polypeptides. In order to gain insight into the signal transduction cascades that participate in this effect, in the present study we characterized the phosphorylation of a critical component of the translational machinery, namely the ribosomal protein S6. The phosphorylation sites in rpS6 have been mapped to five clustered residues, Ser235, Ser236, Ser240, Ser244 and Ser247. Nevertheless, Ser236 phosphorylation is the primary phosphorylation site. The kinases responsible of this modification are p70(S6K) and p90(RSK). rpS6 phosphorylation increases the affinity of 40s subunit for mRNAs and thus facilitates translational initiation. Glutamate exposure of cultured cerebellar Bergmann glia cells results in a time- and dose-dependent increase in rpS6 phosphorylation. This effect is mainly observed at cytoplasm, and involves the phosphoinositol-3 kinase/protein kinase B pathway. Our results favor the notion of a continuous neuronal signaling to glia cells that regulates the proteome of these cells not only at the transcriptional level but also at the level of protein synthesis.

Evolution and Conservation on Top of the World: Phylogeography of the Marbled Water Frog (Telmatobius marmoratus Species Complex; Anura, Telmatobiidae) in Protected Areas of Chile.

The Andean Altiplano has served as a complex setting throughout its history, driving dynamic processes of diversification in several taxa. We investigated phylogeographic processes in the Telmatobius marmoratus species complex occurring in this region by studying the geographic patterns of genetic variability, genealogies, and historical migration, using the cytochrome b (cyt-b) gene as a marker. DNA sequences from Telmatobius gigas and Telmatobius culeus, Bolivian species with an uncertain taxonomic status, were also included. Additionally, we evaluated the phylogenetic diversity (PD) represented within Chilean protected areas and the complementary contribution from unprotected populations. Phylogenetic reconstructions from 148 cyt-b sequences revealed 4 main clades, one of which corresponded to T. culeus. T. gigas was part of T. marmoratus clade indicating paraphyletic relationships. Haplotypes from Chilean and Bolivian sites were not reciprocally monophyletic. Geographic distribution of lineages, spatial Bayesian analysis, and migration patterns indicated that T. marmoratus displays a weaker geographic structure than expected based on habitat distribution and physiological requirements. Demographic and statistical phylogeography analyses pointed out to a scenario of recent population expansion and high connectivity events of a more recent age than the post Last Glacial Maximum, probably associated to more humid events in Altiplano. PD of T. marmoratus populations within protected areas represents 55.6% of the total estimated PD. The unprotected populations that would contribute the most to PD are Caquena and Quebe (21%). Recent evolutionary processes and paleoclimatic changes, potentially driving shifts in habitat connectivity levels and population sizes, could explain the phylogeographic patterns recovered herein.

Population structure and microscale morphological differentiation in a freshwater snail from the Chilean Altiplano.

BACKGROUND: The diversity and population genetic structure of many species have been shaped by historical and contemporary climatic changes. For the species of the South American Altiplano, the historical climatic changes are mainly related to the wet events of great magnitude and regional influence that occurred during the Pleistocene climatic oscillations (PCOs). In contrast, contemporary climate changes are associated with events of lesser magnitude and local influence related to intensifications of the South American Summer Monsoon (SASM). Although multiple studies have analyzed the effect of PCOs on the genetic patterns of highland aquatic species, little is known about the impact of contemporary climate changes in recent evolutionary history. Therefore, in this study, we investigated the change in population structure and connectivity using nuclear and mitochondrial markers throughout the distribution range of Heleobia ascotanensis, a freshwater Cochliopidae endemic to the Ascotán Saltpan. In addition, using geometric morphometric analyses, we evaluated the concomitance of genetic divergence and morphological differentiation. RESULTS: The mitochondrial sequence analysis results revealed the presence of highly divergent co-distributed and geographically nested haplotypes. This pattern reflects an extension in the distribution of groups that previously would have differentiated allopatrically. These changes in distribution would have covered the entire saltpan and would be associated with the large-scale wet events of the PCOs. On the other hand, the microsatellite results defined five spatially isolated populations, separated primarily by geographic barriers. Contemporary gene flow analyses suggest that post-PCO, climatic events that would have connected all populations did not occur. The morphometric analyses results indicate that there is significant morphological differentiation in the populations that are more isolated and that present the greatest genetic divergence. CONCLUSIONS: The contemporary population structure and morphological variation of H. ascotanensis mainly reflect the post-PCO climatic influence. Although both markers exhibit high genetic structuring, the microsatellite and morphology results show the preponderant influence of fragmentation in recent evolutionary history. The contemporary genetic pattern shows that in species that have limited dispersal capabilities, genetic discontinuities can appear rapidly, erasing signs of historical connectivity.

Crosstalk of growth factor receptors at plasma membrane clathrin-coated sites.

Cellular communication is regulated at the plasma membrane by the interactions of receptor, adhesion, signaling, exocytic, and endocytic proteins. Yet, the composition and control of these nanoscale complexes in response to external cues remain unclear. Here, we use high-resolution and high-throughput fluorescence imaging to map the localization of growth factor receptors and related proteins at single clathrin-coated structures across the plasma membrane of human squamous HSC3 cells. We find distinct protein signatures between control cells and cells stimulated with ligands. Clathrin sites at the plasma membrane are preloaded with some receptors but not others. Stimulation with epidermal growth factor induces a capture and concentration of epidermal growth factor-, fibroblast growth factor-, and low-density lipoprotein-receptors (EGFR, FGFR, and LDLR). Regulatory proteins including ubiquitin ligase Cbl, the scaffold Grb2, and the mechanoenzyme dynamin2 are also recruited. Disrupting FGFR or EGFR individually with drugs prevents the recruitment of both EGFR and FGFR. Our data reveals novel crosstalk between multiple unrelated receptors and regulatory factors at clathrin-coated sites in response to stimulation by a single growth factor, EGF. This behavior integrates growth factor signaling and allows for complex responses to extracellular cues and drugs at the plasma membrane of human cells.

IMPDH2 filaments protect from neurodegeneration in AMPD2 deficiency.

Metabolic dysregulation is one of the most common causes of pediatric neurodegenerative disorders. However, how the disruption of ubiquitous and essential metabolic pathways predominantly affect neural tissue remains unclear. Here we use mouse models of AMPD2 deficiency to study cellular and molecular mechanisms that lead to selective neuronal vulnerability to purine metabolism imbalance. We show that AMPD deficiency in mice primarily leads to hippocampal dentate gyrus degeneration despite causing a generalized reduction of brain GTP levels. Remarkably, we found that neurodegeneration resistant regions accumulate micron sized filaments of IMPDH2, the rate limiting enzyme in GTP synthesis. In contrast, IMPDH2 filaments are barely detectable in the hippocampal dentate gyrus, which shows a progressive neuroinflammation and neurodegeneration. Furthermore, using a human AMPD2 deficient neural cell culture model, we show that blocking IMPDH2 polymerization with a dominant negative IMPDH2 variant, impairs AMPD2 deficient neural progenitor growth. Together, our findings suggest that IMPDH2 polymerization prevents detrimental GTP deprivation in neurons with available GTP precursor molecules, providing resistance to neurodegeneration. Our findings open the possibility of exploring the involvement of IMPDH2 assembly as a therapeutic intervention for neurodegeneration.

Altered lipid homeostasis is associated with cerebellar neurodegeneration in SNX14 deficiency.

Dysregulated lipid homeostasis is emerging as a potential cause of neurodegenerative disorders. However, evidence of errors in lipid homeostasis as a pathogenic mechanism of neurodegeneration remains limited. Here, we show that cerebellar neurodegeneration caused by Sorting Nexin 14 (SNX14) deficiency is associated with lipid homeostasis defects. Recent studies indicate that SNX14 is an interorganelle lipid transfer protein that regulates lipid transport, lipid droplet (LD) biogenesis, and fatty acid desaturation, suggesting that human SNX14 deficiency belongs to an expanding class of cerebellar neurodegenerative disorders caused by altered cellular lipid homeostasis. To test this hypothesis, we generated a mouse model that recapitulates human SNX14 deficiency at a genetic and phenotypic level. We demonstrate that cerebellar Purkinje cells (PCs) are selectively vulnerable to SNX14 deficiency while forebrain regions preserve their neuronal content. Ultrastructure and lipidomic studies reveal widespread lipid storage and metabolism defects in SNX14-deficient mice. However, predegenerating SNX14-deficient cerebella show a unique accumulation of acylcarnitines and depletion of triglycerides. Furthermore, defects in LD content and telolysosome enlargement in predegenerating PCs suggest lipotoxicity as a pathogenic mechanism of SNX14 deficiency. Our work shows a selective cerebellar vulnerability to altered lipid homeostasis and provides a mouse model for future therapeutic studies.