Gregory Mitchell Kelly

Progressive ankylosis homolog (ANKH) is a transmembrane protein essential for regulating bone mineralization through the export of nucleoside triphosphates, primarily adenosine triphosphate (ATP), and citrate into the extracellular matrix. Exported ATP is hydrolyzed by ectonucleotide pyrophosphatase/phosphodiesterase 1 into AMP and inorganic pyrophosphate (PPi). Progressive ankylosis homolog is widely expressed across tissues, where it limits ectopic mineralization of tissues and other roles. Its functional role is particularly prominent in mineralizing cells such as osteoblasts and chondrocytes that maintain the balance between mineral formation and inhibition required for healthy skeletal function. Mutations in ANKH cause 2 main mineralization disorders: craniometaphyseal dysplasia, characterized by progressive craniofacial bone thickening, and calcium pyrophosphate deposition disease (CPPD), marked by crystal deposits causing arthritis-like joint symptoms. Functionally, ANKH operates within a coordinated regulatory axis with ectonucleotide pyrophosphatase/phosphodiesterase 1 and tissue-nonspecific alkaline phosphatase (TNAP) that governs extracellular PPi homeostasis. Although TNAP-mediated PPi hydrolysis contributes negligibly to extracellular inorganic phosphate (Pi) levels, this process remains essential for clearing PPi to prevent excessive inhibition of mineral deposition, thereby preserving the Pi/PPi ratio required for physiological mineralization. Emerging evidence also implicates ANKH in citrate export, influencing bone matrix composition, and mechanical integrity. Further, ANKH expression and function are regulated by multiple signaling pathways including Wnt, tumor necrosis factor-alpha, and FGF, as well as by post-transcriptional modifications, although these regulatory mechanisms remain poorly characterized. Current mouse models, primarily knock-in lines carrying craniometaphyseal dysplasia-associated mutations, only partially recapitulate human phenotypes while comparable models for CPPD-associated mutations remain unavailable. Addressing these gaps by elucidating mutation-specific mechanisms, signaling networks, and developing improved animal models will be critical to advance targeted therapies for ANKH-related mineralization disorders and improve patient outcomes.

Craniometaphyseal Dysplasia (CMD) is a rare skeletal disorder that can result from mutations in the ANKH gene. This gene encodes progressive anksylosis (ANK), which is responsible for transporting inorganic pyrophosphate (PPi) and ATP from the intracellular to the extracellular environment, where PPi inhibits bone mineralization. When ANK is dysfunctional, as in patients with CMD, the passage of PPi to the extracellular environment is reduced, leading to excess mineralization, particularly in bones of the skull. Zebrafish may serve as a promising model to study the mechanistic basis of CMD. Here, we provide a detailed analysis of the zebrafish Ankh paralogs, Ankha and Ankhb, in terms of their phylogenic relationship with ANK in other vertebrates as well as their spatiotemporal expression patterns during zebrafish development. We found that a closer evolutionary relationship exists between the zebrafish Ankhb protein and its human and other vertebrate counterparts, and stronger promoter activity was predicted for ankhb compared to ankha. Furthermore, we noted distinct temporal expression patterns, with ankha more prominently expressed in early development stages, and both paralogs also being expressed at larval growth stages. Whole-mount in situ hybridization was used to compare the spatial expression patterns of each paralog during bone development, and both showed strong expression in the craniofacial region as well as the notochord and somites. Given the substantial overlap in spatiotemporal expression but only subtle patterning differences, the exact roles of these genes remain speculative. In silico analyses predicted that Ankha and Ankhb have the same function in transporting PPi across the membrane. Nevertheless, this study lays the groundwork for functional analyses of each ankh paralog and highlights the potential of using zebrafish to find possible targeted therapies for CMD.

Craniometaphyseal Dysplasia (CMD) is a rare skeletal disorder that can result from mutations in the ANKH gene. This gene encodes progressive ankylosis (ANK), which is responsible for transporting inorganic pyrophosphate (PPi) and ATP from the intracellular to the extracellular environment, where PPi inhibits bone mineralization. When ANK is dysfunctional, as in patients with CMD, the passage of PPi to the extracellular environment is reduced, leading to excess mineralization, particularly in bones of the skull. Zebrafish may serve as a promising model to study the mechanistic basis of CMD. Here we provide a detailed analysis of the zebrafish ankh paralogs, ankha and ankhb, in terms of their phylogenic relationship with ANKH in other vertebrates as well as their spatiotemporal expression patterns during zebrafish development. We found a closer evolutionary relationship exists between the zebrafish ankhb protein and its human and other “higher” vertebrate counterparts. Furthermore, we noted distinct temporal expression patterns with ankha more prominently expressed in early development stages, and ankhb expression at larval growth stages. Whole mount in situ hybridization was used to compare spatial expression patterns of each paralog during bone development. Both paralogs showed strong expression in the craniofacial region as well as the notochord and somites, with only subtle patterning differences. Given the substantial overlap in spatiotemporal expression of ankha and ankhb , the exact roles of these genes remain speculative. However, this study lays the groundwork for functional analyses of each ankh paralog and the potential of using zebrafish to find possible targeted therapies for CMD.

Hedgehog signaling is essential for vertebrate development, however, less is known about the negative regulators that influence this pathway. Using the mouse P19 embryonal carcinoma cell model, Suppressor of Fused (SUFU), a negative regulator of the Hedgehog pathway, was investigated during retinoic acid-induced neural differentiation. We found Hedgehog signaling increased activity in the early phase of differentiation but was reduced during terminal differentiation of neurons and astrocytes. This early increase in pathway activity was required for neural differentiation, however, it alone was not sufficient to induce neural lineages. SUFU, which regulates signaling at the level of Gli, remained relatively unchanged during differentiation, but its loss through CRISPR-Cas9 gene editing resulted in ectopic expression of Hedgehog target genes. Interestingly, these SUFU-deficient cells were unable to differentiate towards neural lineages without retinoic acid, and when directed towards these lineages they showed delayed and decreased astrocyte differentiation; neuron differentiation was unaffected. Ectopic activation of Hh target genes in SUFU-deficient cells remained throughout retinoic acid-induced differentiation and this was accompanied by the loss of Gli3, despite the presence of the Gli3 message. Thus, the study indicates the proper timing and proportion of astrocyte differentiation requires SUFU, likely acting through Gli3, to reduce Hh signaling during late-stage differentiation.

1. Wnt and Hh are known signalling pathways involved in neural differentiation and recent work has shown the cell cycle regulator, Never in Mitosis Kinase 2 (Nek2) is able to regulate both pathways. Despite its known function in pathway regulation, few studies have explored Nek2 within embryonic development. The P19 embryonal carcinoma cell model was used to investigate Nek2 and neural differentiation through CRISPR knockout and overexpression studies. Loss of Nek2 reduced cell proliferation in the undifferentiated state and during directed differentiation, while overexpression increased cell proliferation. Despite these changes in proliferation rates, Nek2 deficient cells maintained pluripotency markers after neural induction while Nek2 overexpressing cells lost these markers in the undifferentiated state. Nek2 deficient cells lost the ability to differentiate into both neurons and astrocytes, although Nek2 overexpressing cells enhanced neuron differentiation at the expense of astrocytes. Hh and Wnt signaling were explored, however there was no clear connection between Nek2 and these pathways causing the observed changes to differentiation phenotypes. Mass spectrometry was also used during wildtype and Nek2 knockout cell differentiation and we identified reduced electron transport chain components in the knockout population. Immunoblotting confirmed the loss of these components and additional studies showed cells lacking Nek2 were exclusively glycolytic. Interestingly, hypoxia inducible factor 1α was stabilized in these Nek2 knockout cells despite culturing them under normoxic conditions. Since neural differentiation requires a metabolic switch from glycolysis to oxidative phosphorylation, we propose a mechanism where Nek2 prevents HIF1α stabilization, thereby allowing cells to use oxidative phosphorylation to facilitate neuron and astrocyte differentiation.

The regulation of proteins through the addition and removal of O-linked β-N-acetylglucosamine (O-GlcNAc) plays a role in many signaling events, specifically in stem cell pluripotency and the regulation of differentiation. However, these post-translational modifications have not been explored in extraembryonic endoderm (XEN) differentiation. Of the plethora of proteins regulated through O-GlcNAc, we explored galectin-3 as a candidate protein known to have various intracellular and extracellular functions. Based on other studies, we predicted a reduction in global O-GlcNAcylation levels and a distinct galectin expression profile in XEN cells relative to embryonic stem (ES) cells. By conducting dot blot analysis, XEN cells had decreased levels of global O-GlcNAc than ES cells, which reflected a disbalance in the expression of genes encoding O-GlcNAc cycle enzymes. Immunoassays (Western blot and ELISA) revealed that although XEN cells (low O-GlcNAc) had lower concentrations of both intracellular and extracellular galectin-3 than ES cells (high O-GlcNAc), the relative secretion of galectin-3 was significantly increased by XEN cells. Inducing ES cells toward XEN in the presence of an O-GlcNAcase inhibitor was not sufficient to inhibit XEN differentiation. However, global O-GlcNAcylation was found to decrease in differentiated cells and the extracellular localization of galectin-3 accompanies these changes. Inhibiting global O-GlcNAcylation status does not, however, impact pluripotency and the ability of ES cells to differentiate to the XEN lineage.

Metabolism plays a crucial role for cell survival and function; however, recent evidence has implicated it in regulating embryonic development. In the embryo, the inner cell mass undergoes orchestrated cellular divisions resulting in the formation of pluripotent epiblast stem cells and primitive endoderm cells. However, both lineages can be captured in vitro as embryonic stem (ES) cells and extraembryonic endoderm (XEN) cells. Concomitantly, changes in the metabolic profile occurs during development, and are well documented in the embryonic lineages. However, a comprehensive multi-omic analysis of these features in XEN cells remains lacking. We observed that mouse XEN cells exhibited high sensitivity to glycolytic inhibition in addition to maintaining elevated intra- and extracellular lactate levels in vitro. Extraembryonic endoderm cells maintain high lactate levels by increased LDHA activity, and re-routing pyruvate away from the mitochondria resulting in reduced mitochondrial activity due to disruptions in electron transport chain stoichiometry. Importantly, exogenous lactate supplementation or promoting intracellular lactate accumulation enhances XEN differentiation in vitro. These results highlight how lactate contributes to XEN differentiation in vitro and may serve to enhance reprogramming efficiency of cells used for regenerative medicine.

Hedgehog signaling is essential for vertebrate development, however, less is known about the negative regulators that influence this pathway. Using the mouse P19 embryonal carcinoma cell model, Suppressor of Fused (SUFU), a negative regulator of the Hedgehog pathway, was investigated during retinoic acid-induced neural differentiation. We found Hedgehog signaling was activated in the early phase of differentiation but was inactive during terminal differentiation of neurons and astrocytes. Early activation was required for neural differentiation however, it alone was not sufficient to induce neural lineages. SUFU, which regulates signaling at the level of GLI, remained relatively unchanged during differentiation, but its loss through CRISPR-Cas9 gene editing resulted in ectopic expression of Hedgehog target genes. Interestingly, these SUFU-deficient cells were unable to differentiate without retinoic acid, and when used they showed delayed and decreased astrocyte differentiation; neuron differentiation was unaffected. Ectopic activation of Hh target genes in SUFU-deficient cells remained throughout retinoic acid-induced differentiation and this was accompanied by the loss of GLI3, despite the presence of the Gli3 message. Thus, the study indicates the proper timing and proportion of astrocyte differentiation requires SUFU, and its normal regulation of GLI3 to maintain Hh signaling in an inactive state.

Summary: Metabolism plays a crucial role for cell survival and function; however, recent evidence has implicated it in regulating embryonic development. The inner cell mass undergoes orchestrated cellular divisions resulting in the formation of embryonic stem cells and extraembryonic endoderm (XEN) cells. Concomitantly, changes in the metabolic profile occurs during development and are well-documented in the embryonic lineages. However, a comprehensive multi-omics analysis of these features in XEN cells remains lacking. We observed that feeder-free XEN cells exhibited high sensitivity to glycolytic inhibition in addition to maintaining elevated intra- and extracellular lactate levels. XEN cells maintain high lactate levels by increased LDHA activity and re-routing pyruvate away from the mitochondria. Importantly, exogenous lactate supplementation or promoting intracellular lactate accumulation enhances XEN differentiation in vitro . Our results highlight how lactate contributes to XEN differentiation in the mammalian embryo and may serve to enhance reprogramming efficiency of cells used for regenerative medicine. Highlights: Feeder-free XEN cells exhibit high sensitivity to glycolytic inhibition Distinct transcriptomic, proteomic and metabolomic profile exists between feeder-free ES and XEN cells Elevated intracellular and extracellular lactate is observed in feeder-free XEN cells Lactate enhances feeder-free XEN differentiation in vitro