Deletion of protein kinase D1 in osteoprogenitor cells results in decreased osteogenesis in vitro and reduced bone mineral density in vivo

Effect of deletion of the protein kinase PRKD1 on development of the mouse embryonic heart

Congenital heart disease (CHD) is the most common congenital anomaly, with an overall incidence of approximately 1% in the United Kingdom. Exome sequencing in large CHD cohorts has been performed to provide insights into the genetic aetiology of CHD. This includes a study of 1891 probands by our group in collaboration with others, which identified three novel genes-CDK13, PRKD1, and CHD4, in patients with syndromic CHD. PRKD1 encodes a serine/threonine protein kinase, which is important in a variety of fundamental cellular functions. Individuals with a heterozygous mutation in PRKD1 may have facial dysmorphism, ectodermal dysplasia and may have CHDs such as pulmonary stenosis, atrioventricular septal defects, coarctation of the aorta and bicuspid aortic valve. To obtain a greater appreciation for the role that this essential protein kinase plays in cardiogenesis and CHD, we have analysed a Prkd1 transgenic mouse model (Prkd1em1) carrying deletion of exon 2, causing loss of function. High-resolution episcopic microscopy affords detailed morphological 3D analysis of the developing heart and provides evidence for an essential role of Prkd1 in both normal cardiac development and CHD. We show that homozygous deletion of Prkd1 is associated with complex forms of CHD such as atrioventricular septal defects, and bicuspid aortic and pulmonary valves, and is lethal. Even in heterozygotes, cardiac differences occur. However, given that 97% of Prkd1 heterozygous mice display normal heart development, it is likely that one normal allele is sufficient, with the defects seen most likely to represent sporadic events. Moreover, mRNA and protein expression levels were investigated by RT-qPCR and western immunoblotting, respectively. A significant reduction in Prkd1 mRNA levels was seen in homozygotes, but not heterozygotes, compared to WT littermates. While a trend towards lower PRKD1 protein expression was seen in the heterozygotes, the difference was only significant in the homozygotes. There was no compensation by the related Prkd2 and Prkd3 at transcript level, as evidenced by RT-qPCR. Overall, we demonstrate a vital role of Prkd1 in heart development and the aetiology of CHD.

Prkd1 regulates the formation and repair of plasma membrane disruptions (PMD) in osteocytes

We and others have seen that osteocytes sense high-impact osteogenic mechanical loading via transient plasma membrane disruptions (PMDs) which initiate downstream mechanotransduction. However, a PMD must be repaired for the cell to survive this wounding event. Previous work suggested that the protein Prkd1 (also known as PKCμ) may be a critical component of this PMD repair process, but the specific role of Prkd1 in osteocyte mechanobiology had not yet been tested. We treated MLO-Y4 osteocytes with Prkd1 inhibitors (Go6976, kbNB 142-70, staurosporine) and generated an osteocyte-targeted (Dmp1-Cre) Prkd1 conditional knockout (CKO) mouse. PMD repair rate was measured via laser wounding and FM1-43 dye uptake, PMD formation and post-wounding survival were assessed via fluid flow shear stress (50 dyn/cm2), and in vitro osteocyte mechanotransduction was assessed via measurement of calcium signaling. To test the role of osteocyte Prkd1 in vivo, Prkd1 CKO and their wildtype (WT) littermates were subjected to 2 weeks of unilateral axial tibial loading and loading-induced changes in cortical bone mineral density, geometry, and formation were measured. Prkd1 inhibition or genetic deletion slowed osteocyte PMD repair rate and impaired post-wounding cell survival. These effects could largely be rescued by treating osteocytes with the FDA-approved synthetic copolymer Poloxamer 188 (P188), which was previously shown to facilitate membrane resealing and improve efficiency in the repair rate of PMD in skeletal muscle myocytes. In vivo, while both WT and Prkd1 CKO mice demonstrated anabolic responses to tibial loading, the magnitude of loading-induced increases in tibial BMD, cortical thickness, and periosteal mineralizing surface were blunted in Prkd1 CKO as compared to WT mice. Prkd1 CKO mice also tended to show a smaller relative difference in the number of osteocyte PMD in loaded limbs and showed greater lacunar vacancy, suggestive of impaired post-wounding osteocyte survival. While P188 treatment rescued loading-induced increases in BMD in the Prkd1 CKO mice, it surprisingly further suppressed loading-induced increases in cortical bone thickness and cortical bone formation. Taken together, these data suggest that Prkd1 may play a pivotal role in the regulation and repair of the PMD response in osteocytes and support the idea that PMD repair processes can be pharmacologically targeted to modulate downstream responses, but suggest limited utility of PMD repair-promoting P188 in improving bone anabolic responses to loading.

Protein kinase D3 conditional knockout impairs osteoclast formation and increases trabecular bone volume in male mice

Studies using kinase inhibitors have shown that the protein kinase D (PRKD) family of serine/threonine kinases are required for formation and function of osteoclasts in culture. However, the involvement of individual protein kinase D genes and their in vivo significance to skeletal dynamics remains unclear. In the current study we present data indicating that protein kinase D3 is the primary form of PRKD expressed in osteoclasts. We hypothesized that loss of PRKD3 would impair osteoclast formation, thereby decreasing bone resorption and increasing bone mass. Conditional knockout (cKO) of Prkd3 using a murine Cre/Lox system driven by cFms-Cre revealed that its loss in osteoclast-lineage cells reduced osteoclast differentiation and resorptive function in culture. Examination of the Prkd3 cKO mice showed that bone parameters were unaffected in the femur at 4 weeks of age, but consistent with our hypothesis, Prkd3 conditional knockout resulted in 18 % increased trabecular bone mass in male mice at 12 weeks and a similar increase at 6 months. These effects were not observed in female mice. As a further test of our hypothesis, we asked if Prkd3 cKO could protect against bone loss in a ligature-induced periodontal disease model but did not see any reduction in bone destruction in this system. Together, our data indicate that PRKD3 promotes osteoclastogenesis both in vitro and in vivo.

Genome-wide association study implicates novel loci and reveals candidate effector genes for longitudinal pediatric bone accrual

BACKGROUND

Bone accrual impacts lifelong skeletal health, but genetic discovery has been primarily limited to cross-sectional study designs and hampered by uncertainty about target effector genes. Here, we capture this dynamic phenotype by modeling longitudinal bone accrual across 11,000 bone scans in a cohort of healthy children and adolescents, followed by genome-wide association studies (GWAS) and variant-to-gene mapping with functional follow-up.

RESULTS

We identify 40 loci, 35 not previously reported, with various degrees of supportive evidence, half residing in topological associated domains harboring known bone genes. Of several loci potentially associated with later-life fracture risk, a candidate SNP lookup provides the most compelling evidence for rs11195210 (SMC3). Variant-to-gene mapping combining ATAC-seq to assay open chromatin with high-resolution promoter-focused Capture C identifies contacts between GWAS loci and nearby gene promoters. siRNA knockdown of gene expression supports the putative effector gene at three specific loci in two osteoblast cell models. Finally, using CRISPR-Cas9 genome editing, we confirm that the immediate genomic region harboring the putative causal SNP influences PRPF38A expression, a location which is predicted to coincide with a set of binding sites for relevant transcription factors.

CONCLUSIONS

Using a new longitudinal approach, we expand the number of genetic loci putatively associated with pediatric bone gain. Functional follow-up in appropriate cell models finds novel candidate genes impacting bone accrual. Our data also raise the possibility that the cell fate decision between osteogenic and adipogenic lineages is important in normal bone accrual.

Telangiectasia-ectodermal dysplasia-brachydactyly-cardiac anomaly syndrome is caused by de novo mutations in protein kinase D1

BACKGROUND

We describe two unrelated patients who display similar clinical features including telangiectasia, ectodermal dysplasia, brachydactyly and congenital heart disease.

METHODS

We performed trio whole exome sequencing and functional analysis using in vitro kinase assays with recombinant proteins.

RESULTS

We identified two different de novo mutations in protein kinase D1 (PRKD1, NM_002742.2): c.1774G>C, p.(Gly592Arg) and c.1808G>A, p.(Arg603His), one in each patient. PRKD1 (PKD1, HGNC:9407) encodes a kinase that is a member of the protein kinase D (PKD) family of serine/threonine protein kinases involved in diverse cellular processes such as cell differentiation and proliferation and cell migration as well as vesicle transport and angiogenesis. Functional analysis using in vitro kinase assays with recombinant proteins showed that the mutation c.1808G>A, p.(Arg603His) represents a gain-of-function mutation encoding an enzyme with a constitutive, lipid-independent catalytic activity. The mutation c.1774G>C, p.(Gly592Arg) in contrast shows a defect in substrate phosphorylation representing a loss-of-function mutation.

CONCLUSION

The present cases represent a syndrome, which associates symptoms from several different organ systems: skin, teeth, bones and heart, caused by heterozygous de novo mutations in PRKD1 and expands the clinical spectrum of PRKD1 mutations, which have hitherto been linked to syndromic congenital heart disease and limb abnormalities.

Monomethylfumarate protects against ovariectomy-related changes in body composition

Osteoporosis, low bone mass that increases fracture susceptibility, affects approximately 75 million individuals in the United States, Europe and Japan, with the number of osteoporotic fractures expected to increase by more than 3-fold over the next 50 years. Bone mass declines with age, although the mechanisms for this decrease are unclear. Aging enhances production of reactive oxygen species, which can affect bone formation and breakdown. The multiple sclerosis drug Tecfidera contains dimethylfumarate, which is rapidly metabolized to monomethylfumarate (MMF); MMF is thought to function through nuclear factor erythroid-derived-2-like-2 (Nrf2), a transcription factor activated by oxidative stress which induces the expression of endogenous anti-oxidant systems. We hypothesized that MMF-elicited increases in anti-oxidants would inhibit osteopenia induced by ovariectomy, as a model of aging-related osteoporosis and high oxidative stress. We demonstrated that MMF activated Nrf2 and induced anti-oxidant Nrf2 target gene expression in bone marrow-derived mesenchymal stem cells. Sham-operated or ovariectomized adult female mice were fed chow with or without MMF and various parameters monitored. Ovariectomy produced the expected effects, decreasing bone mineral density and increasing body weight, fat mass, bone marrow adiposity and serum receptor activator of nuclear factor-kappa-B ligand (RANKL) levels. MMF decreased fat but not lean mass. MMF improved trabecular bone microarchitecture after adjustment for body weight, although the unadjusted data showed few differences; MMF also tended to increase adjusted cortical bone and to reduce bone marrow adiposity and serum RANKL levels. Because these results suggest the possibility that MMF might be beneficial for bone, further investigation seems warranted.

Protein kinase D1 conditional null mice show minimal bone loss following ovariectomy

We previously found that 3- and 6-month-old male mice with conditional ablation of protein kinase D1 (PRKD1) in osteoprogenitor cells (expressing Osterix) exhibited reduced bone mass. Others have demonstrated similar effects in young female PRKD1-deficient mice. Here we examined the bone resorptive response of adult female floxed control and conditional knockout (cKO) mice undergoing sham surgery or ovariectomy (OVX). Femoral and tibial bone mineral density (BMD) values were significantly reduced upon OVX in control, but not cKO, females compared to the respective sham-operated mice. Micro-CT analysis showed that OVX significantly increased trabecular number and decreased trabecular spacing in cKO but not control mice. Finally, in control mice serum levels of a marker of bone resorption (pyridinoline crosslinks) and the osteoclast activator RANKL significantly increased upon OVX; however, no such OVX-induced increase was observed in cKO mice. Our results suggest the potential importance of PRKD1 in response to estrogen loss in bone.