Susceptibility to cell death induced by mutant SV40 T-antigen correlates with Purkinje neuron functional development

Cyclin-dependent kinase 4/6 inhibitors require an arcuate-to-paraventricular hypothalamus melanocortin circuit to treat diet-induced obesity

The arcuate nucleus (ARC) of the hypothalamus comprises two antagonistic neuron populations critical for energy balance, namely, the anorexigenic pro-opiomelanocortin (POMC) and the orexigenic agouti-related peptide (AgRP) neurons that act as agonists and antagonists, respectively, for neurons expressing the type IV melanocortin receptor (MC4R) (Andermann ML and Lowell BB. Neuron 95: 757-778, 2017). MC4R activation increases energy expenditure and decreases food intake during positive energy balance states to prevent diet-induced obesity (DIO). Work from our group identified aberrant neuronal cell cycle events both as a novel biomarker and druggable target in the ARC for the treatment of DIO, demonstrating pharmacological restoration of retinoblastoma protein function in the ARC using cyclin-dependent kinase 4/6 (CDK4/6) inhibitors could treat DIO in mice by increasing lipid oxidation to selectively decrease fat mass. However, the role of CDK4/6 inhibitors on food intake was not examined. Four-week-old Mc4r-lox^TB mice were continuously administered high-fat diet (60% kcal fat). At 8 wk of age, animals were administered 60 mg/kg abemaciclib orally or a saline control and monitored every 2 wk for fat mass changes by MRI. At 11 wk of age, all animals were injected bilaterally in the paraventricular hypothalamus with AAV8 serotype virus expressing a Cre-mCherry and monitored for another 5 wk. Restoration of Mc4r expression in the paraventricular hypothalamic nucleus (PVN/PVH) reduced food intake in hyperphagic obese mice when given CDK4/6 inhibitor therapy. The reduced food intake was responsible for reduced fat mass in mice treated with abemaciclib. These results indicate that targeting POMC neurons could be an effective strategy in treating diet-related obesity.NEW & NOTEWORTHY We have defined some of the necessary components to prevent high-fat diet-induced obesity at the molecular and cellular level. Within POMC neurons, the retinoblastoma protein must remain active and prevented from phosphoinactivation by cyclin-dependent kinases. The downstream neurons within the PVH must also properly express MC4R for the circuit to appropriately regulate feeding behavior.

Cyclin-dependent kinase 4 is a preclinical target for diet-induced obesity

When obesity is caused by consumption of a high-fat diet, the tumor suppressor pRb is phosphoinactivated in the neurons of the mediobasal hypothalamus, a brain area critical for energy-balance regulation. However, the functional relevance of pRb phosphoinactivation in the mediobasal hypothalamus to diet-induced obesity remains unknown. Here, we show that inhibiting pRb phosphorylation in the mediobasal hypothalamus can prevent and treat diet-induced obesity in mice. Expressing an unphosphorylable pRb nonselectively in the mediobasal hypothalamus or conditionally in anorexigenic POMC neurons inhibits diet-induced obesity. Intracerebroventricular delivery of US Food and Drug Administration–approved (FDA-approved) cyclin-dependent kinase 4 (CDK4) inhibitor abemaciclib inhibits pRb phosphorylation in the mediobasal hypothalamus and prevents diet-induced obesity. Oral administration of abemaciclib at doses approved for human use reduces fat mass in diet-induced obese mice by increasing lipid oxidation without significantly reducing lean mass. With analysis of recent literature identifying CDK4 as the most abundantly expressed neuronal CDK in the mediobasal hypothalamus, our work uncovers CDK4 as the major kinase for hypothalamic pRb phosphoinactivation and a highly effective central antiobesity target. As three CDK4/6 inhibitors have recently received FDA approval for life-long breast cancer therapy, our study provides a preclinical basis for their expedient repurposing for obesity management.

Inhibiting pRb phosphorylation in the mediobasal hypothalamus can prevent and treat diet-induced obesity in mice.

ATM and the epigenetics of the neuronal genome

Ataxia-telangiectasia (A-T) is a neurodegenerative syndrome caused by the mutation of the ATM gene. The ATM protein is a PI3kinase family member best known for its role in the DNA damage response. While repair of DNA damage is a critical function that every CNS neuron must perform, a growing body of evidence indicates that the full range of ATM functions includes some that are unrelated to DNA damage yet are essential to neuronal survival and normal function. For example, ATM participates in the regulation of synaptic vesicle trafficking and is essential for the maintenance of normal LTP. In addition ATM helps to ensure the cytoplasmic localization of HDAC4 and thus maintains the histone 'code' of the neuronal genome by suppressing genome-wide histone deacetylation, which alters the message and protein levels of many genes that are important for neuronal survival and function. The growing list of ATM functions that go beyond its role in the DNA damage response offers a new perspective on why individuals with A-T express such a wide range of neurological symptoms, and suggests that not all A-T symptoms need to be understood in the context of the DNA repair process.

G1 arrest and differentiation can occur independently of Rb family function

Repression of E2F target genes is required for cell cycle arrest in Rb family (Rb, p107, and p130)-deficient cells.

The ability of progenitor cells to exit the cell cycle is essential for proper embryonic development and homeostasis, but the mechanisms governing cell cycle exit are still not fully understood. Here, we tested the requirement for the retinoblastoma (Rb) protein and its family members p107 and p130 in G0/G1 arrest and differentiation in mammalian cells. We found that Rb family triple knockout (TKO) mouse embryos survive until days 9–11 of gestation. Strikingly, some TKO cells, including in epithelial and neural lineages, are able to exit the cell cycle in G0/G1 and differentiate in teratomas and in culture. This ability of TKO cells to arrest in G0/G1 is associated with the repression of key E2F target genes. Thus, G1 arrest is not always dependent on Rb family members, which illustrates the robustness of cell cycle regulatory networks during differentiation and allows for the identification of candidate pathways to inhibit the expansion of cancer cells with mutations in the Rb pathway.

Nuclear ataxias

Historically basic neuroscience research has made several important contributions to the cell biology of the nucleus, in particular the elucidation of nuclear structures and compartments. As research progressed towards elucidating the mechanism of neurological disease at the cellular and molecular levels, it is now providing insight into the importance and basis of coordination of nuclear pathways within the nucleus and with other cellular compartments. Ataxias, lethal neurodegenerative diseases that are distinguished by a progressive loss of motor coordination, stem from disruption of nuclear function.

Cdk5 nuclear localization is p27-dependent in nerve cells: implications for cell cycle suppression and caspase-3 activation

Initiation of a cell cycle in an adult neuron leads to cell death, placing great importance on the mechanisms that normally suppress the neuronal cell cycle. We have previously shown that the cyclin-dependent kinase Cdk5 is an important part of this process, but only when it is present in the nucleus. We report here that Cdk5 nuclear localization relies on its binding to the cyclin-dependent kinase inhibitor p27. Cdk5 has no intrinsic nuclear localization signal; in the absence of p27, two weak nuclear export signals that bind CRM1 cause it to shuttle to the cytoplasm. When a neuron is subjected to stress, such as exposure to beta-amyloid, the Cdk5-p27 interaction is lost, reducing Cdk5 levels in the nucleus and depriving the neuron of a major cell cycle suppression mechanism. Caspase-3 is activated within hours, but death is not immediate; elevated levels of cytoplasmic Cdk5 appear to retard neuronal death by a mechanism that may involve Bcl2. These data suggest a model in which Cdk5 exerts a double protective function in neurons: chronically suppressing the cell cycle when located in the nucleus and transiently delaying cell death in the cytoplasm.

Cell cycle inhibition and retinoblastoma protein overexpression prevent Purkinje cell death in organotypic slice cultures

Purkinje cells are vulnerable to a number of physical, chemical, and genetic insults during development and maturity. Normal development of these cells depends on the cell-cell interactions between granule and astroglial cell populations. Apoptotic death in Purkinje neurons had been shown to be associated with cell cycle activation, and new DNA synthesis is associated with Purkinje cell death in staggerer and lurcher mutant mice. Here using an in vitro organotypic slice culture model from 9 (P9) and 4 days (P4) old postnatal rats we show that the cyclin dependent kinase (cdk) inhibitors (roscovitine, olomoucine, and flavopiridol) protect the Purkinje cells from cell death. The results are more pronounced in the cerebellar sections from P4 rats. Analysis of Purkinje neurons in sections from P4 rats after 1 week of culturing showed that while there were very limited calbindin positive neurons in the untreated sections the cdk inhibitor treated sections had a notably higher number. Although treatment with cdk inhibitors inhibited Purkinje cell loss significantly, the morphology of these neurons was abnormal, with stunted dendrites and axons. Since the retinoblastoma protein (Rb) is the major pocket protein involved in determining the differentiated state of neurons we examined the effect of over-expressing Rb in the organotypic cultures. Rb overexpression significantly inhibited the Purkinje cell death and these neurons maintained their normal morphology. Thus our studies show that the cell death in Purkinje neurons observed in organotypic cultures is cell cycle dependent and the optimal survival requires Rb.

Cell cycle regulation in the postmitotic neuron: oxymoron or new biology?

Adult CNS neurons are typically described as permanently postmitotic but there is probably nothing permanent about the neuronal cell cycle arrest. Rather, it appears that these highly differentiated cells must constantly keep their cell cycle in check. Relaxation of this vigilance leads to the initiation of a cell cycle and entrance into an altered and vulnerable state, often leading to death. There is evidence that neurons which are at risk of neurodegeneration are also at risk of re-initiating a cell cycle process that involves the expression of cell cycle proteins and DNA replication. Failure of cell cycle regulation might be a root cause of several neurodegenerative disorders and a final common pathway for others.

Signal molecules and receptors in the differential development of cerebellum lobules. Acute effects of cisplatin on nitric oxide and glutamate systems in Purkinje cell population

Three functionally correlated parameters, nitric oxide (NO), glutamate and NMDA receptors were analyzed through enzymehistochemical and immunohistochemical reactions. A single injection of cisplatin (cisPt) was administered to 10-day-old rats in order to study how Purkinje cells differentiation may be early changed by a mild injury due to the drug during postnatal cerebellar histogenesis. In comparison with age-matched control rats, a correlated decreasing expression of nitric oxide synthase (NOS), glutamate and NMDAR1 was observed in the Purkinje cells of lobules VI-VIII 6 h after the treatment. Moreover, at 24 h after cisPt, the expression of glutamate, NMDAR1 and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPHd) reactivity was further decreased. In the same period, the ionotropic receptor GluR2 evidenced a less developed dendrite of Purkinje neurons in the top of lobules. In addition, the metabotropic receptor mGluR1alpha revealed unstained areas in the molecular layer, which was entirely stained in control rats; on PD11 this altered pattern was observed in all the lobules and in both the outer and the inner parts. Findings show the importance of NO-glutamate interactions via NMDAR1 in the crucial phases of Purkinje cells differentiation and their involvement on Purkinje neurons dendrite branching as demonstrated by the patterns of the other glutamate receptors. Changes were discussed in relation to an important critical event of Purkinje cell differentiation, i.e. regression of perisomatic spines and elimination of climbing fiber synapses on the somata. Finally, lobules VI-VIII appear to be the most vulnerable ones when cisplatin treatment is administered at 10 days of life, which demonstrates that at this stage some critical developmental changes occur in these lobules and that slower/damaged dendritic tree development is different in the outer versus the inner regions of the lobules.

Genetic targeting of cerebellar Purkinje cells: history, current status and novel strategies

This review is an account of developments in the field of transgenic and gene targeting approaches with special emphasis on the cerebellar Purkinje cell. A critical discussion of the available genetic tools is provided. As genetic engineering of the mouse is still a rapidly moving field, we felt it appropriate to include some ideas on novel strategies for refined genetic manipulations.

A null mutation in inositol polyphosphate 4-phosphatase type I causes selective neuronal loss in weeble mutant mice

Weeble mutant mice have severe locomotor instability and significant neuronal loss in the cerebellum and in the hippocampal CA1 field. Genetic mapping was used to localize the mutation to the gene encoding inositol polyphosphate 4-phosphatase type I (Inpp4a), where a single nucleotide deletion results in a likely null allele. The substrates of INPP4A are intermediates in a pathway affecting intracellular Ca(2+) release but are also involved in cell cycle regulation through binding the Akt protooncogene; dysfunction in either may account for the neuronal loss of weeble mice. Although other mutations in phosphoinositide enzymes are associated with synaptic defects without neuronal loss, weeble shows that Inpp4a is critical for the survival of a subset of neurons during postnatal development in mice.

The developmental biology of brain tumors

Tumors of the central nervous system (CNS) can be devastating because they often affect children, are difficult to treat, and frequently cause mental impairment or death. New insights into the causes and potential treatment of CNS tumors have come from discovering connections with genes that control cell growth, differentiation, and death during normal development. Links between tumorigenesis and normal development are illustrated by three common CNS tumors: retinoblastoma, glioblastoma, and medulloblastoma. For example, the retinoblastoma (Rb) tumor suppressor protein is crucial for control of normal neuronal differentiation and apoptosis. Excessive activity of the epidermal growth factor receptor and loss of the phosphatase PTEN are associated with glioblastoma, and both genes are required for normal growth and development. The membrane protein Patched1 (Ptc1), which controls cell fate in many tissues, regulates cell growth in the cerebellum, and reduced Ptc1 function contributes to medulloblastoma. Just as elucidating the mechanisms that control normal development can lead to the identification of new cancer-related genes and signaling pathways, studies of tumor biology can increase our understanding of normal development. Learning that Ptc1 is a medulloblastoma tumor suppressor led directly to the identification of the Ptc1 ligand, Sonic hedgehog, as a powerful mitogen for cerebellar granule cell precursors. Much remains to be learned about the genetic events that lead to brain tumors and how each event regulates cell cycle progression, apoptosis, and differentiation. The prospects for beneficial work at the boundary between oncology and developmental biology are great.

A proline- and glutamine-rich protein promotes apoptosis in neuronal cells

During development, excess neurons are eliminated by programmed cell death. Similarly, conditionally immortalized (SV40-Tts) rat hippocampal and septal cells undergo cell death following differentiation with several factors such as fibroblast growth factor, constitutively activated Raf-1, or phorbol esters. The mechanism by which cell death occurs has not been identified. Using RNA differential display, we have identified and characterized a novel immediate early gene (denoted PQR for proline- and glutamine-rich) induced during differentiation of both rat hippocampal and septal cell lines. The 44-kDa PQR protein, rich in PQ, PH, and QQ repeats, is homologous to a murine protein (TDAG51) required for Fas-mediated apoptosis in T cells. To determine whether PQR acts as a mediator of apoptosis in neuronal cells, the hippocampal H19-7 cells were microinjected with either a plasmid expressing PQR cDNA or an antibody against PQR. Microinjection of differentiating H19-7 cells with a neutralizing antibody against PQR increased the number of surviving cells by 50%. Transient expression of PQR in both differentiating and nondifferentiating H19-7 cells decreased the number of surviving cells by 35-50%; this reduction was reversed by microinjection of PQR antibody. Finally, levels of Fas transcripts are not increased in the neuronal cells, indicating that the mechanism of action differs from that in T cells. These results demonstrate that PQR can be induced by growth factors and differentiating agents and can itself induce apoptosis in hippocampal H19-7 cells. Furthermore, these data suggest that PQR can function more generally as a mediator of apoptosis and provide a possible mechanism for induction of programmed cell death during neuronal development.

The transcription factor E2F-1 in SV40 T antigen-induced cerebellar Purkinje cell degeneration

Transgenic targeting of SV40 large T antigen (Tag) expression to murine cerebellar Purkinje cells induces these normally postmitotic neurons to undergo DNA synthesis and apoptosis. It has been proposed that these effects of Tag are due to the binding of Tag to pRb, which leads to the release and activation of the transcription factor E2F. Here it is reported that E2F and CDC2, the protein product of a gene regulated by E2F, were detectable in the Purkinje cell nuclei of Tag expressing transgenic animals. To directly test whether E2F-1 is part of the mechanism of Tag-induced Purkinje cell degeneration, transgenic mice that overexpress E2F-1 specifically in cerebellar Purkinje cells were generated. Although E2F-1 itself did not affect Purkinje cells, it did accelerate Tag-induced ataxia and Purkinje cell loss, suggesting that E2F-1 can contribute to the mechanism of Tag-induced Purkinje cell degeneration.

Cerebellar vascular and synaptic responses in normal mice and in transgenics with Purkinje cell dysfunction

We used transgenic mice with Purkinje cell dysfunction (PO3 line) to study the role of these neurons in the increase in cerebellar blood flow (BFcrb) produced by stimulation of the cerebellar parallel fibers (PF). Mice (age 8-10 wk) were anesthetized (halothane) and artificially ventilated. Arterial pressure and end-tidal CO2 were monitored continuously. Arterial blood gases were measured. The PF were stimulated electrically (100 microA, 30 Hz; 40 s), and the increases in BFcrb were monitored by a laser-Doppler flow probe. First, we characterized the increases in BFcrb and the field potentials produced by PF stimulation in normal mice. PF stimulation evoked the typical field potentials and increased BFcrb by 60 +/- 4% (100 microA, 30 Hz; n = 10). The increases in BFcrb were attenuated by the broad-spectrum glutamate receptor antagonist kynurenate (-84 +/- 3%; P < 0.05 analysis of variance; n = 5), by the DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (-62 +/- 6%; P < 0.05; n = 5), and by the nitric oxide synthase inhibitor N omega-nitro-L-arginine (-46 +/- 7%; P < 0.05; n = 5). In PO3 transgenic mice, the increases in BFcrb produced by PF stimulation were reduced (P < 0.001) at every stimulus intensity and frequency tested (residual increase at 100 microA, 30 Hz: 19 +/- 2%; n = 6). The field potentials evoked by PF stimulation also were abnormal in that they lacked the late negative wave (n = 6), a finding consistent with lack of depolarization of Purkinje cells. The residual flow response in the transgenics was abolished by N omega-nitro-L-arginine (n = 5; P > 0.05). Ultrastructural studies showed that the density of PF-Purkinje cell synapses is reduced in PO3 mice, whereas the morphology of molecular layer interneurons (stellate cells) is normal. The findings suggest that Purkinje cells are responsible for a sizable component of the flow response whereas molecular layer interneurons mediate the remainder of the response. The study provides evidence that mouse mutants with spontaneous or genetically engineered cerebellar abnormalities could be useful to study the cellular and molecular correlates of functional hyperemia in the central nervous system.