The molecular complex of ciliary and golgin protein is crucial for skull development

Ciliary and non-ciliary functions of Rab34 during craniofacial bone development

The primary cilium is a hair-like projection that controls cell development and tissue homeostasis. Although accumulated studies identify the molecular link between cilia and cilia-related diseases, the underlying etiology of ciliopathies has not been fully understood. In this paper, we determine the function of Rab34, a small GTPase, as a key regulator for controlling ciliogenesis and type I collagen trafficking in craniofacial development. Mechanistically, Rab34 is required to form cilia that control osteogenic proliferation, survival, and differentiation via cilia-mediated Hedgehog signaling. In addition, Rab34 is indispensable for regulating type I collagen trafficking from the ER to the Golgi. These results demonstrate that Rab34 has both ciliary and non-ciliary functions to regulate osteogenesis. Our study highlights the critical function of Rab34, which may contribute to understanding the novel etiology of ciliopathies that are associated with the dysfunction of RAB34 in humans.

A novel perspective of calvarial development: the cranial morphogenesis and differentiation regulated by dura mater

There are lasting concerns on calvarial development because cranium not only accommodates the growing brain, but also safeguards it from exogenous strikes. In the past decades, most studies attributed the dynamic expansion and remodeling of cranium to the proliferation of osteoprecursors in cranial primordium, and the proliferation of osteoprogenitors at the osteogenic front of cranial suture mesenchyme. Further investigations identified series genes expressed in suture mesenchymal cells as the markers of the progenitors, precursors and postnatal stem cells in cranium. However, similar to many other organs, it is suggested that the reciprocal interactions among different tissues also play essential roles in calvarial development. Actually, there are increasing evidence indicating that dura mater (DM) is indispensable for the calvarial morphogenesis and osteogenesis by secreting multiple growth factors, cytokines and extracellular matrix (ECM). Thus, in this review, we first briefly introduce the development of cranium, suture and DM, and then, comprehensively summarize the latest studies exploring the involvement of ECM in DM and cranium development. Eventually, we discussed the reciprocal interactions between calvarium and DM in calvarial development. Actually, our review provides a novel perspective for cranium development by integrating previous classical researches with a spotlight on the mutual interplay between the developing DM and cranium.

Enhanced BMP signaling in Cathepsin K-positive tendon progenitors induces heterotopic ossification

Heterotopic ossification (HO) is abnormal bone growth in soft tissues that results from injury, trauma, and rare genetic disorders. Bone morphogenetic proteins (BMPs) are critical osteogenic regulators which are involved in HO. However, it remains unclear how BMP signaling interacts with other extracellular stimuli to form HO. To address this question, using the Cre-loxP recombination system in mice, we conditionally expressed the constitutively activated BMP type I receptor ALK2 with a Q207D mutation (Ca-ALK2) in Cathepsin K-Cre labeled tendon progenitors (hereafter "Ca-Alk2:Ctsk-Cre"). Ca-Alk2:Ctsk-Cre mice were viable but they formed spontaneous HO in the Achilles tendon. Histological and molecular marker analysis revealed that HO is formed via endochondral ossification. Ectopic chondrogenesis coincided with enhanced GLI1 production, suggesting that elevated Hedgehog (Hh) signaling is involved in the pathogenesis of HO. Interestingly, focal adhesion kinase, a critical mediator for the mechanotransduction pathway, was also activated in Ca-Alk2:Ctsk-Cre mice. Our findings suggest that enhanced BMP signaling may elevate Hh and mechanotransduction pathways, thereby causing HO in the regions of the Achilles tendon.

Thalamic Neuron Resilience during Osmotic Demyelination Syndrome (ODS) Is Revealed by Primary Cilium Outgrowth and ADP-ribosylation factor-like protein 13B Labeling in Axon Initial Segment

A murine osmotic demyelinating syndrome (ODS) model was developed through chronic hyponatremia, induced by desmopressin subcutaneous implants, followed by precipitous sodium restoration. The thalamic ventral posterolateral (VPL) and ventral posteromedial (VPM) relay nuclei were the most demyelinated regions where neuroglial damage could be evidenced without immune response. This report showed that following chronic hyponatremia, 12 h and 48 h time lapses after rebalancing osmolarity, amid the ODS-degraded outskirts, some resilient neuronal cell bodies built up primary cilium and axon hillock regions that extended into axon initial segments (AIS) where ADP-ribosylation factor-like protein 13B (ARL13B)-immunolabeled rod-like shape content was revealed. These AIS-labeled shaft lengths appeared proportional with the distance of neuronal cell bodies away from the ODS damaged epicenter and time lapses after correction of hyponatremia. Fine structure examination verified these neuron abundant transcriptions and translation regions marked by the ARL13B labeling associated with cell neurotubules and their complex cytoskeletal macromolecular architecture. This necessitated energetic transport to organize and restore those AIS away from the damaged ODS core demyelinated zone in the murine model. These labeled structures could substantiate how thalamic neuron resilience occurred as possible steps of a healing course out of ODS.

Enhanced BMP signaling leads to enlarged nasal cartilage formation in mice

Bone morphogenetic proteins (BMPs) are required for craniofacial bone development. However, it remains elusive how BMP signaling regulates craniofacial cartilage development. To address this question, we utilized a genetic system to enhance BMP signaling via one of BMP type I receptors ALK2 in a chondrocyte-specific manner (hereafter Ca-Alk2:Col2-Cre) in mice. Ca-Alk2:Col2-Cre mice died shortly after birth due to severe craniofacial abnormalities including cleft palate, defective tongue, and shorter mandible formation. Histological analysis revealed that these phenotypes were attributed to the extensive chondrogenesis. Compared with controls, enhanced SOX9 and RUNX2 production were observed in nasal cartilage of Ca-Alk2:Col2-Cre mice. To reveal the mechanisms responsible for enlarged nasal cartilage, we examined Smad-dependent and Smad-independent BMP signaling pathways. While the Smad-independent BMP signaling pathway including p38, ERK, and JNK remained silent, the Smad1/5/9 was highly phosphorylated in Ca-Alk2:Col2-Cre mice. Interestingly, Ca-Alk2:Col2-Cre mice showed enhanced S6 kinase phosphorylation, a readout of mammalian target of rapamycin complex 1 (mTORC1). These findings may suggest that enhanced Smad-dependent BMP signaling positively regulates the mTOR pathway and stimulates chondrocytes toward hypertrophic differentiation, thereby leading to enlarged nasal cartilage formation in mice.

The factory, the antenna and the scaffold: the three-way interplay between the Golgi, cilium and extracellular matrix underlying tissue function

The growth and development of healthy tissues is dependent on the construction of a highly specialised extracellular matrix (ECM) to provide support for cell growth and migration and to determine the biomechanical properties of the tissue. These scaffolds are composed of extensively glycosylated proteins which are secreted and assembled into well-ordered structures that can hydrate, mineralise, and store growth factors as required. The proteolytic processing and glycosylation of ECM components is vital to their function. These modifications are under the control of the Golgi apparatus, an intracellular factory hosting spatially organised, protein-modifying enzymes. Regulation also requires a cellular antenna, the cilium, which integrates extracellular growth signals and mechanical cues to inform ECM production. Consequently, mutations in either Golgi or ciliary genes frequently lead to connective tissue disorders. The individual importance of each of these organelles to ECM function is well-studied. However, emerging evidence points towards a more tightly linked system of interdependence between the Golgi, cilium and ECM. This review examines how the interplay between all three compartments underpins healthy tissue. As an example, it will look at several members of the golgin family of Golgi-resident proteins whose loss is detrimental to connective tissue function. This perspective will be important for many future studies looking to dissect the cause and effect of mutations impacting tissue integrity.

Creating morphological diversity in reptilian temporal skull region: A review of potential developmental mechanisms

Reptilian skull morphology is highly diverse and broadly categorized into three categories based on the number and position of the temporal fenestrations: anapsid, synapsid, and diapsid. According to recent phylogenetic analysis, temporal fenestrations evolved twice independently in amniotes, once in Synapsida and once in Diapsida. Although functional aspects underlying the evolution of tetrapod temporal fenestrations have been well investigated, few studies have investigated the developmental mechanisms responsible for differences in the pattern of temporal skull region. To determine what these mechanisms might be, we first examined how the five temporal bones develop by comparing embryonic cranial osteogenesis between representative extant reptilian species. The pattern of temporal skull region may depend on differences in temporal bone growth rate and growth direction during ontogeny. Next, we compared the histogenesis patterns and the expression of two key osteogenic genes, Runx2 and Msx2, in the temporal region of the representative reptilian embryos. Our comparative analyses suggest that the embryonic histological condition of the domain where temporal fenestrations would form predicts temporal skull morphology in adults and regulatory modifications of Runx2 and Msx2 expression in osteogenic mesenchymal precursor cells are likely involved in generating morphological diversity in the temporal skull region of reptiles.

Enhanced BMP signaling through ALK2 attenuates keratinocyte differentiation

Accumulated studies have suggested that bone morphogenetic proteins (BMPs) are critical for skin development. However, it remains elusive how BMP signaling via ALK2 (aka ACVR1), one of the important BMP type I receptors, regulates keratinocyte differentiation. To address this question, we utilized a genetic system that enhances BMP signaling via ALK2 in an epidermis-specific manner in mice (hereafter ca-Alk2:K14-Cre). Ca-Alk2:K14-Cre mice displayed a sticky and hairless skin phenotype with a thinner epidermis incapable of differentiating. Although cellular proliferation and survival were comparable between wild-type and ca-Alk2:K14-Cre mice, skin differentiation was severely hampered in ca-Alk2:K14-Cre mice. To uncover the mechanism of altered keratinocyte differentiation, we performed a transcriptome analysis. As a result, we found that the expression levels of cell cycle inhibitor p21 were increased in ca-Alk2:K14-Cre mice. Our findings suggest that aberrant BMP signaling via ALK2 positively regulates p21 expression that attenuates keratinocyte differentiation, and further highlights the critical role of BMP signaling in skin development.

In vivo characterization of Drosophila golgins reveals redundancy and plasticity of vesicle capture at the Golgi apparatus

The Golgi is the central sorting station in the secretory pathway and thus the destination of transport vesicles arriving from the endoplasmic reticulum and endosomes and from within the Golgi itself. Cell viability, therefore, requires that the Golgi accurately receives multiple classes of vesicle. One set of proteins proposed to direct vesicle arrival at the Golgi are the golgins, long coiled-coil proteins localized to specific parts of the Golgi stack. In mammalian cells, three of the golgins, TMF, golgin-84, and GMAP-210, can capture intra-Golgi transport vesicles when placed in an ectopic location. However, the individual golgins are not required for cell viability, and mouse knockout mutants only have defects in specific tissues. To further illuminate this system, we examine the Drosophila orthologs of these three intra-Golgi golgins. We show that ectopic forms can capture intra-Golgi transport vesicles, but strikingly, the cargo present in the vesicles captured by each golgin varies between tissues. Loss-of-function mutants show that the golgins are individually dispensable, although the loss of TMF recapitulates the male fertility defects observed in mice. However, the deletion of multiple golgins results in defects in glycosylation and loss of viability. Examining the vesicles captured by a particular golgin when another golgin is missing reveals that the vesicle content in one tissue changes to resemble that of a different tissue. This reveals a plasticity in Golgi organization between tissues, providing an explanation for why the Golgi is sufficiently robust to tolerate the loss of many of the individual components of its membrane traffic machinery.

Disruption of Trip11 in cranial neural crest cells is associated with increased ER and Golgi stress contributing to skull defects in mice

BACKGROUND

Absence of Golgi microtubule-associated protein 210 (GMAP210), encoded by the TRIP11 gene, results in achondrogenesis. Although TRIP11 is thought to be specifically required for chondrogenesis, human fetuses with the mutation of TRIP11 also display bony skull defects where chondrocytes are usually not present. This raises an important question of how TRIP11 functions in bony skull development.

RESULTS

We disrupted Trip11 in neural crest-derived cell populations, which are critical for developing skull in mice. In Trip11 mutant skulls, expression levels of ER stress markers were increased compared to controls. Morphological analysis of electron microscopy data revealed swollen ER in Trip11 mutant skulls. Unexpectedly, we also found that Golgi stress increased in Trip11 mutant skulls, suggesting that both ER and Golgi stress-induced cell death may lead to osteopenia-like phenotypes in Trip11 mutant skulls. These data suggest that Trip11 plays pivotal roles in the regulation of ER and Golgi stress, which are critical for osteogenic cell survival.

CONCLUSION

We have recently reported that the molecular complex of ciliary protein and GMAP210 is required for collagen trafficking. In this paper, we further characterized the important role of Trip11 being possibly involved in the regulation of ER and Golgi stress during skull development. This article is protected by copyright. All rights reserved.