Cell type-specific expression, regulation and compensation of CDKL5 activity in mouse brain

CDKL5 is a brain-enriched serine/threonine kinase, associated with a profound developmental and epileptic encephalopathy called CDKL5 deficiency disorder (CDD). To design targeted therapies for CDD, it is essential to determine where CDKL5 is expressed and is active in the brain and test if compensatory mechanisms exist at cellular level. We generated conditional Cdkl5 knockout mice in excitatory neurons, inhibitory neurons and astrocytes. To assess CDKL5 activity, we utilized a phosphospecific antibody for phosphorylated EB2, a well-known substrate of CDKL5. We found that CDKL5 and EB2 pS222 were prominent in excitatory and inhibitory neurons but were not detected in astrocytes. We observed that approximately 15-20% of EB2 pS222 remained in Cdkl5 knockout brains and primary neurons. Surprisingly, the remaining phosphorylation was modulated by NMDA and PP1/PP2A in neuronal CDKL5 knockout cultures, indicating the presence of a compensating kinase. Using a screen of candidate kinases with highest homology to the CDKL5 kinase domain, we found that CDKL2 and ICK can phosphorylate EB2 S222 in HEK293T cells and in primary neurons. We then generated Cdkl5/Cdkl2 dual knockout mice to directly test if CDKL2 phosphorylates EB2 in vivo and found that CDKL2 phosphorylates CDKL5 substrates in the brain. This study is the first indication that CDKL2 could potentially replace CDKL5 functions in the brain, alluding to novel therapeutic possibilities.

Epilepsy-linked kinase CDKL5 phosphorylates voltage-gated calcium channel Cav2.3, altering inactivation kinetics and neuronal excitability

Developmental and epileptic encephalopathies (DEEs) are a group of rare childhood disorders characterized by severe epilepsy and cognitive deficits. Numerous DEE genes have been discovered thanks to advances in genomic diagnosis, yet putative molecular links between these disorders are unknown. CDKL5 deficiency disorder (CDD, DEE2), one of the most common genetic epilepsies, is caused by loss-of-function mutations in the brain-enriched kinase CDKL5. To elucidate CDKL5 function, we looked for CDKL5 substrates using a SILAC-based phosphoproteomic screen. We identified the voltage-gated Ca2+ channel Cav2.3 (encoded by CACNA1E) as a physiological target of CDKL5 in mice and humans. Recombinant channel electrophysiology and interdisciplinary characterization of Cav2.3 phosphomutant mice revealed that loss of Cav2.3 phosphorylation leads to channel gain-of-function via slower inactivation and enhanced cholinergic stimulation, resulting in increased neuronal excitability. Our results thus show that CDD is partly a channelopathy. The properties of unphosphorylated Cav2.3 closely resemble those described for CACNA1E gain-of-function mutations causing DEE69, a disorder sharing clinical features with CDD. We show that these two single-gene diseases are mechanistically related and could be ameliorated with Cav2.3 inhibitors.

Loss of NDR1/2 kinases impairs endomembrane trafficking and autophagy leading to neurodegeneration

Autophagy is essential for neuronal development and its deregulation contributes to neurodegenerative diseases. NDR1 and NDR2 are highly conserved kinases, implicated in neuronal development, mitochondrial health and autophagy, but how they affect mammalian brain development in vivo is not known. Using single and double Ndr1/2 knockout mouse models, we show that only dual loss of Ndr1/2 in neurons causes neurodegeneration. This phenotype was present when NDR kinases were deleted both during embryonic development, as well as in adult mice. Proteomic and phosphoproteomic comparisons between Ndr1/2 knockout and control brains revealed novel kinase substrates and indicated that endocytosis is significantly affected in the absence of NDR1/2. We validated the endocytic protein Raph1/Lpd1, as a novel NDR1/2 substrate, and showed that both NDR1/2 and Raph1 are critical for endocytosis and membrane recycling. In NDR1/2 knockout brains, we observed prominent accumulation of transferrin receptor, p62 and ubiquitinated proteins, indicative of a major impairment of protein homeostasis. Furthermore, the levels of LC3-positive autophagosomes were reduced in knockout neurons, implying that reduced autophagy efficiency mediates p62 accumulation and neurotoxicity. Mechanistically, pronounced mislocalisation of the transmembrane autophagy protein ATG9A at the neuronal periphery, impaired axonal ATG9A trafficking and increased ATG9A surface levels further confirm defects in membrane trafficking, and could underlie the impairment in autophagy. We provide novel insight into the roles of NDR1/2 kinases in maintaining neuronal health.

Abstract A01: YAP1 drives ependymoma-like tumor formation in the brain

Ependymoma is the third most common pediatric brain cancer. YAP1 gene fusions have been observed in a subset of pediatric ependymomas. Here we show that increased YAP1 activity has a causative role in ependymoma-like tumor formation. YAP1 protein is strongly expressed and localized in the nucleus in a subset of human ependymomas. In the developing brain, YAP1 expression is normally found in subventricular zone neural progenitor cells, and NEX/NeuroD6-Cre induced conditional expression of active nuclear YAP1 (YAP1-5SAnls) is sufficient to drive brain tumor formation. YAP1 drives tumor formation by maintaining a nestin positive neural stem cell-like specification and preventing hippocampal neuronal differentiation. Dual conditional deletion of LATS1 and LATS2 kinases in the NEX-Cre lineage also generates similar tumors. Genetic rescue experiments in mice show that either YAP1 or its homolog TAZ can drive tumor formation downstream of LATS1/2 kinases. YAP1/TAZ activity causes tumors that display histologic and molecular characteristics of ependymoma, including ultrastructural features microvilli and tight junctions. Our results show that disruption of YAP1/TAZ activity in neuronal precursor cells leads to ependymoma-like tumors.

Citation Format: Noreen Eder, Marie-Charlotte Dolmart, Suzanne Claxton, Jennifer Cotton, Jun-Hao Mao, Bram Snijders, Federico Roncaroli, Barry Thompson, Sila Ultanir. YAP1 drives ependymoma-like tumor formation in the brain [abstract]. In: Proceedings of the AACR Special Conference on the Hippo Pathway: Signaling, Cancer, and Beyond; 2019 May 8-11; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(8_Suppl):Abstract nr A01.

YAP1/TAZ drives ependymoma-like tumour formation in mice

YAP1 gene fusions have been observed in a subset of paediatric ependymomas. Here we show that, ectopic expression of active nuclear YAP1 (nlsYAP5SA) in ventricular zone neural progenitor cells using conditionally-induced NEX/NeuroD6-Cre is sufficient to drive brain tumour formation in mice. Neuronal differentiation is inhibited in the hippocampus. Deletion of YAP1's negative regulators LATS1 and LATS2 kinases in NEX-Cre lineage in double conditional knockout mice also generates similar tumours, which are rescued by deletion of YAP1 and its paralog TAZ. YAP1/TAZ-induced mouse tumours display molecular and ultrastructural characteristics of human ependymoma. RNA sequencing and quantitative proteomics of mouse tumours demonstrate similarities to YAP1-fusion induced supratentorial ependymoma. Finally, we find that transcriptional cofactor HOPX is upregulated in mouse models and in human YAP1-fusion induced ependymoma, supporting their similarity. Our results show that uncontrolled YAP1/TAZ activity in neuronal precursor cells leads to ependymoma-like tumours in mice.

Chemical genetic identification of GAK substrates reveals its role in regulating Na+/K+-ATPase

Novel GAK phosphorylation targets are identified using chemical genetic methods. One of the substrates is the α subunit of the Na⁺/K⁺-ATPase, phosphorylation of which is necessary for its surface trafficking from endosomes. Conserved functions of NAK family kinases are described.

Cyclin G–associated kinase (GAK) is a ubiquitous serine/threonine kinase that facilitates clathrin uncoating during vesicle trafficking. GAK phosphorylates a coat adaptor component, AP2M1, to help achieve this function. GAK is also implicated in Parkinson's disease through genome-wide association studies. However, GAK's role in mammalian neurons remains unclear, and insight may come from identification of further substrates. Employing a chemical genetics method, we show here that the sodium potassium pump (Na⁺/K⁺-ATPase) α-subunit Atp1a3 is a GAK target and that GAK regulates Na⁺/K⁺-ATPase trafficking to the plasma membrane. Whole-cell patch clamp recordings from CA1 pyramidal neurons in GAK conditional knockout mice show a larger change in resting membrane potential when exposed to the Na⁺/K⁺-ATPase blocker ouabain, indicating compromised Na⁺/K⁺-ATPase function in GAK knockouts. Our results suggest a modulatory role for GAK via phosphoregulation of substrates such as Atp1a3 during cargo trafficking.

Chemical genetic identification of CDKL5 substrates reveals its role in neuronal microtubule dynamics

Loss‐of‐function mutations in CDKL5 kinase cause severe neurodevelopmental delay and early‐onset seizures. Identification of CDKL5 substrates is key to understanding its function. Using chemical genetics, we found that CDKL5 phosphorylates three microtubule‐associated proteins: MAP1S, EB2 and ARHGEF2, and determined the phosphorylation sites. Substrate phosphorylations are greatly reduced in CDKL5 knockout mice, verifying these as physiological substrates. In CDKL5 knockout mouse neurons, dendritic microtubules have longer EB3‐labelled plus‐end growth duration and these altered dynamics are rescued by reduction of MAP1S levels through shRNA expression, indicating that CDKL5 regulates microtubule dynamics via phosphorylation of MAP1S. We show that phosphorylation by CDKL5 is required for MAP1S dissociation from microtubules. Additionally, anterograde cargo trafficking is compromised in CDKL5 knockout mouse dendrites. Finally, EB2 phosphorylation is reduced in patient‐derived human neurons. Our results reveal a novel activity‐dependent molecular pathway in dendritic microtubule regulation and suggest a pathological mechanism which may contribute to CDKL5 deficiency disorder.

MST3 kinase phosphorylates TAO1/2 to enable Myosin Va function in promoting spine synapse development

Mammalian Sterile 20 (Ste20)-like kinase 3 (MST3) is a ubiquitously expressed kinase capable of enhancing axon outgrowth. Whether and how MST3 kinase signaling might regulate development of dendritic filopodia and spine synapses is unknown. Through shRNA-mediated depletion of MST3 and kinase-dead MST3 expression in developing hippocampal cultures, we found that MST3 is necessary for proper filopodia, dendritic spine, and excitatory synapse development. Knockdown of MST3 in layer 2/3 pyramidal neurons via in utero electroporation also reduced spine density in vivo. Using chemical genetics, we discovered thirteen candidate MST3 substrates and identified the phosphorylation sites. Among the identified MST3 substrates, TAO kinases regulate dendritic filopodia and spine development, similar to MST3. Furthermore, using stable isotope labeling by amino acids in culture (SILAC), we show that phosphorylated TAO1/2 associates with Myosin Va and is necessary for its dendritic localization, thus revealing a mechanism for excitatory synapse development in the mammalian CNS.

Conditional deletion of neurogenin-3 using Nkx2.1iCre results in a mouse model for the central control of feeding, activity and obesity

The ventral hypothalamus acts to integrate visceral and systemic information to control energy balance. The basic helix-loop-helix transcription factor neurogenin-3 (Ngn3) is required for pancreatic β-cell development and has been implicated in neuronal development in the hypothalamus. Here, we demonstrate that early embryonic hypothalamic inactivation of Ngn3 (also known as Neurog3) in mice results in rapid post-weaning obesity that is associated with hyperphagia and reduced energy expenditure. This obesity is caused by loss of expression of Pomc in Pomc- and Cart-expressing (Pomc/Cart) neurons in the arcuate nucleus, indicating an incomplete specification of anorexigenic first order neurons. Furthermore, following the onset of obesity, both the arcuate and ventromedial hypothalamic nuclei become insensitive to peripheral leptin treatment. This conditional mouse mutant therefore represents a novel model system for obesity that is associated with hyperphagia and underactivity, and sheds new light upon the roles of Ngn3 in the specification of hypothalamic neurons controlling energy balance.

Efficient, inducible Cre-recombinase activation in vascular endothelium

In recent years, gene-targeting studies in mice have elucidated many molecular mechanisms in vascular biology. However, it has been difficult to apply this approach to the study of postnatal animals because mutations affecting the vasculature are often embryonically lethal. We have therefore generated transgenic mice that express a tamoxifen-inducible form of Cre recombinase (iCreER(T2)) in vascular endothelial cells using a phage artificial chromosome (PAC) containing the Pdgfb gene (Pdgfb-iCreER mice). This allows the genetic targeting of the vascular endothelium in postnatal animals. We tested efficiency of tamoxifen-induced iCre recombinase activity with ROSA26-lacZ reporter mice and found that in newborn animals recombination could be achieved in most capillary and small vessel endothelial cells in most organs including the central nervous system. In adult animals, recombination activity was also widespread in capillary beds of skeletal muscle, heart, skin, and gut but not in the central nervous system where only a subpopulation of endothelial cells was labeled. We also tested recombination efficiency in a subcutaneous tumor model and found recombination activity in all detectable tumor blood vessels. Thus, Pdgfb-iCreER mice are a valuable research tool to manipulate endothelial cells in postnatal mice and study tumor angiogenesis.

Mash1 is required for generic and subtype differentiation of hypothalamic neuroendocrine cells

The neuroendocrine hypothalamus regulates a number of critical biological processes and underlies a range of diseases from growth failure to obesity. Although the elucidation of hypothalamic function has progressed well, knowledge of hypothalamic development is poor. In particular, little is known about the processes underlying the neurogenesis and specification of neurons of the ventral nuclei, the arcuate and ventromedial nuclei. The proneural gene Mash1 is expressed throughout the basal retrochiasmatic neuroepithelium and loss of Mash1 results in hypoplasia of both the arcuate and ventromedial nuclei. These defects are due to a failure of neurogenesis and apoptosis, a defect that can be rescued by ectopic Ngn2 under the control of the Mash1 promoter. In addition to its role in neurogenesis, analysis of Mash1(-/-), Mash1(+/-), Mash1(KINgn2/KINgn2), and Mash1(KINgn2/+) mice demonstrates that Mash1 is specifically required for Gsh1 expression and subsequent GHRH expression, positively regulates SF1 expression, and suppresses both tyrosine hydroxylase (TH) and neuropeptide Y (NPY) expression. Although Mash1 is not required for propiomelanocortin (POMC) expression, it is required for normal development of POMC(+) neurons. These data demonstrate that Mash1 is both required for the generation of ventral neuroendocrine neurons as well as playing a central role in subtype specification of these neurons.

Periodic Delta-like 4 expression in developing retinal arteries

During vascular development, Notch signalling plays important roles in cell-cell communication and cell fate decisions. We studied expression of Notch 1-4 and its ligand Delta-like 4 (Dll4) in the developing retinal vasculature. Dll4 mRNA is strongly expressed in endothelial cells at the very tips of growing vessels ('tip cells') and also in arteries, where it is expressed in a segmented 'tiger's tail' pattern. This implies that developing retinal arteries contain different types of endothelial cells, Dll4-positive and Dll4-negative. The Dll4-positive stripes do not correspond to any obvious morphological property of the vascular network but correlate to some extent with the distribution of platelet derived growth factor B (PDGF-B) mRNA. However, PDGF-B expression is neither as artery-specific nor as clearly segmented as Dll4. Possible target cells for Dll4 signalling are retinal astrocytes (Notch1 positive), arterial pericytes (Notch3 positive) or arterial endothelial cells themselves (Notch4 positive). However, there is no clear reciprocity of Notch and Dll4 expression that allows identification of the interacting cells. Nevertheless, Dll4 stripes are a novel property of immature arteries, the origin and function of which remain to be explained.

Oxygen modifies artery differentiation and network morphogenesis in the retinal vasculature

The mechanisms that control differentiation of immature blood vessels into either arteries or veins are not well understood. Because oxygen tension in arteries is higher than in veins, oxygen has the potential to be an instructive signal for artery/vein (AV) differentiation. We test this hypothesis by exposing newborn mice to moderate hypoxia (10% atmospheric oxygen) and studying AV differentiation in the developing retinal vasculature. Forming retinal arteries fail to express the artery-specific markers Delta-like 4 (Dll4) and EphrinB2 during hypoxia. However, other aspects of AV differentiation are retained such as high levels of alpha smooth muscle actin in arterial mural cells and vein-specific expression of the msr/apj gene. The capillary network between arteries and veins is denser, and capillaries expressing the venous marker msr/apj are found in territories normally occupied by arterial capillaries. Thus, it appears that high oxygen in arterial blood is required for arterial expression of Dll4 and EphrinB2, which could be involved in cell-cell repulsion pathways that dictate the normal segregation of arteries and veins.

Role of arteries in oxygen induced vaso-obliteration

In mice the retinal vasculature develops in the first postnatal week by spreading from the optic nerve head towards the retinal periphery. During this growth period, exposure to hyperoxia causes vaso-obliteration of capillaries in the retinal center but not in peripheral regions. High oxygen levels lead to downregulation of vascular endothelial growth factor (VEGF), an important survival factor for vascular endothelial cells, which could explain the vaso-obliteration caused by hyperoxia. However, it is not clear why only capillaries in the center of the retina are affected. We therefore investigated how capillary obliteration correlates with VEGF mRNA distribution by in situ hybridization in retinal whole mount preparations. In mouse pups reared under normoxic conditions VEGF mRNA was detectable across the entire vascular network but was virtually absent in the immediate vicinity of arteries. This was true along developing retinal arteries but also around the optic nerve head through which the entire arterial blood supply for the retinal and hyaloid vasculature passes. In these areas capillaries were absent, resulting in so-called capillary free zones. Exposure to hyperoxia caused an expansion of areas with low VEGF mRNA which correlated with capillary obliteration in these regions. Combined capillary obliteration around the optic nerve head and along retinal arteries lead to a large capillary free zone in the center of the retina. Thus, our observations suggest that hyperoxia affects the retinal vasculature by reducing VEGF mRNA levels near arteries and causing a widening of capillary free zones.

Resistance exercise training reduces hypertriglyceridemia in HIV-infected men treated with antiviral therapy

Hypertriglyceridemia, peripheral insulin resistance, and trunk adiposity are metabolic complications recently recognized in people infected with human immunodeficiency virus (HIV) and treated with highly active antiretroviral therapy (HAART). These complications may respond favorably to exercise training. Using a paired design, we determined whether 16 wk of weight-lifting exercise increased muscle mass and strength and decreased fasting serum triglycerides and adipose tissue mass in 18 HIV-infected men. The resistance exercise regimen consisted of three upper and four lower body exercises done for 1-1.5 h/day, 4 days/wk for 64 sessions. Dual-energy X-ray absorptiometry indicated that exercise training increased whole body lean mass 1.4 kg (P = 0.005) but did not reduce adipose tissue mass (P = NS). Axial proton-magnetic resonance imaging indicated that thigh muscle cross-sectional area increased 5-7 cm(2) (P < 0.005). Muscle strength increased 23-38% (P < 0.0001) on all exercises. Fasting serum triglycerides were decreased at the end of training (281-204 mg/dl; P = 0.02). These findings imply that resistance exercise training-induced muscle hypertrophy may promote triglyceride clearance from the circulation of hypertriglyceridemic HIV-infected men treated with antiviral therapy.

Export of a misprocessed GPI-anchored protein from the endoplasmic reticulum in vitro in an ATP- and cytosol-dependent manner

Strict quality control mechanisms within the mammalian endoplasmic reticulum act to prevent misfolded and unprocessed proteins from entering post-endoplasmic reticulum (ER) compartments. Following translocation into the ER lumen via the Sec61p translocon, nascent polypeptide chains fold and are modified in an environment that contains numerous chaperones and other folding mediators. Recently it has emerged that polypeptides failing to acquire the native state are re-exported from the ER to the cytosol for ultimate degradation by the proteasome ubiquitin system, apparently mediated again via Sec61p. Substrates for this degradation pathway include proteins destined to become glycosyl phosphatidylinositol (GPI)-anchored, but which fail to be processed and retain the C-terminal GPI signal peptide. In order to characterise this process we have used a model GPI-anchored mutant protein, prepro mini human placental alkaline phosphatase (PLAP) W179, which cannot be processed efficiently on account of being a poor substrate for the transamidase which cleaves the GPI signal peptide and adds the GPI anchor in a coupled reaction. In vitro transcription, translation and translocation into canine pancreatic microsomes resulted in ER-targeting signal sequence cleavage and formation of prominiPLAP in the ER lumen. We were able to show that prominiPLAPW179 could be exported from the microsomes in a time-dependent manner and that release requires both ATP and cytosol. Export was not supported by GTP, indicating a biochemical distinction from glycopeptide export which we showed recently requires GTP hydrolysis. The process was not affected by redox, unlike several other GPI-anchored model proteins. These data demonstrate that misprocessed proteins can be exported in vitro from mammalian microsomes, facilitating identification of factors involved in this process.

Accelerated bone mineral loss in HIV-infected patients receiving potent antiretroviral therapy

BACKGROUND

The use of highly active antiretroviral therapy (HAART) has been associated with multiple metabolic complications whose pathogenesis is poorly understood at the present time.

METHODS

We performed a cross-sectional analysis of whole-body, lumbar spine (L1-L4) and proximal femur bone mineral density in 112 male subjects (HIV-infected patients on HAART that included a protease inhibitor, HIV-infected patients not receiving a protease inhibitor and healthy seronegative adults) using dual energy x-ray absorptiometry.

RESULTS

Men receiving protease inhibitors had a higher incidence of osteopenia and osteoporosis according to World Health Organization definitions: relative risk = 2.19 (95% confidence interval 1.13-4.23) (P = 0.02). Subjects receiving protease inhibitors had greater central: appendicular adipose tissue ratios than the other two groups (P < 0.0001). There was no relationship between the central: appendicular fat ratio and the lumbar spine or proximal femur bone mineral density t- or z- scores, suggesting that osteoporosis and body fat redistribution are independent side effects of HAART.

CONCLUSIONS

Osteopenia and osteoporosis are unique metabolic complications associated with protease inhibitor-containing potent antiretroviral regimens, that appear to be independent of adipose tissue maldistribution.

Refractory amiodarone-associated thyrotoxicosis: an indication for thyroidectomy

BACKGROUND

Tasmania is an area of endemic iodine deficiency. Amiodarone is a class III anti-arrhythmic drug that is widely used for the management of ventricular and supraventricular tachydysrhythmias. Individuals from areas of endemic iodine deficiency appear more likely to manifest hyperthyroidism following amiodarone therapy, whereas hypothyroidism is a more frequent complication in iodine-replete communities.

METHODS

Cases series. The clinical and biochemical response to medical and surgical management of five consecutive Tasmanian patients presenting with severe type-II amiodarone-associated thyrotoxicosis was reviewed.

RESULTS

Five patients were identified. Combinations of antithyroid therapy including propylthiouracil, lithium carbonate, dexamethasone and cholestyramine were used. Thyroidectomy was required in two cases (40%) due to severe unremitting thyrotoxicosis despite combined drug regimens. Anaesthesia and total thyroidectomy were undertaken without complication despite the presence of severe hyperthyroidism at the time of surgery. In both cases thyroid histopathology demonstrated degenerative and destructive follicular lesions with multinuclear cell infiltrate and focal fibrosis.

CONCLUSION

Amiodarone-associated thyrotoxicosis may be severe and refractory to medical therapy. Despite the potential risks of anaesthesia associated with uncontrolled thyrotoxicosis, thyroidectomy should be considered in the setting of life-threatening thyrotoxicosis.