40 years of the homeobox: either it is wrong or it is quite interesting

Many genes that regulate development share a 180 bp DNA sequence, called the homeobox, encoding a 60 amino acid DNA-binding domain ( McGinnis et al., 1984c; Scott and Weiner, 1984). Because the homeobox is long enough to hybridize to related, but different, genes, it has been a powerful tool for discovering developmental regulators. This year is the 40th anniversary of the first homeobox report. Here, I describe work carried out at Indiana University that led to the discovery of the homeobox. The accompanying Perspective from McGinnis and Levine describes the independent discovery made at the Biozentrum in Basel ( McGinnis and Levine, 2024). At the time, the competition was lively but, as we all met each other - and realized that no one cares more about your work than competitors - we fortunately became friends and have enjoyed many years of following and respecting each other's work.

'First tonight, the contentious new code telling nurses to say, 'sorry for being white': Mental health nurses' beliefs about their Code of Conduct and cultural safety for Aboriginal and Torres Strait Islander Peoples

The Nursing and Midwifery Board of Australia's Code of Conduct for Nurses sets out the professional behaviour and conduct expectations for nurses in all practice settings. The publication of a revised version in 2018, which included expectations related to culturally safe and respectful practice and Aboriginal and Torres Strait Islander Peoples' health, caused reverberations beyond the profession of nursing. A controversy that the changes required nurses to verbally apologize for being white before their interactions with Aboriginal and Torres Strait Islander people gained the attention of the mainstream media. This interpretation, which came from outside nursing, was disputed by the Board. Challenged by these events, the authors were interested in understanding the actual impacts of the changes from the perspectives of nurses in practice. This research, carried out nearly three years after publication, has focused specifically on the speciality of mental health nurses in this context. The objective of this research was to undertake a social analysis focused on the impact that changes in the Code have had on the culture of mental health nursing utilizing a qualitative methodology. Eight mental health nurses were interviewed. The research found that there was little evidence of any impact on mental health nursing practice. Many of the participants were unaware of the amendments to the Code, whilst those nurses who were aware did not perceive that it had led to any real change within mental health nursing or service delivery.

Mental health nursing practice and Aboriginal and Torres Strait Islander people: an integrative review

Background: For nearly thirty years, significant concerns have been raised about the public-provided mental health services for Aboriginal and Torres Strait Islander peoples. Staff have been identified as having little understanding of Indigenous culture, and this had resulted in inappropriate treatment. In attempting to understand what specialist knowledge exists to guide mental health nursing practice with Aboriginal and Torres Strait Islander peoples, the authors have turned to published peer-reviewed literature.Methods: The approach chosen to explore this area was an integrative review. This provided a method to identify, analyse, and synthesise a wide range of literature.Results: The available evidence points to the need that treatment planning must be focused on the promotion of social and emotional wellbeing and not simply the treatment of symptoms. It also emphasises the importance of cultural safety informed by awareness and understanding of social, cultural and historical factors that can impact the health and treatment of Aboriginal and Torres Strait Islander peoples. Within the literature, staff reported difficulty in understanding how knowledge about social and emotional wellbeing could translate into practice. Nurses working in mental health contexts reported not feeling adequately prepared for, or confident in this area of practice.Conclusions: There is a paucity of current literature on mental health nursing practice for Aboriginal and Torres Strait Islander peoples, with the literature available not providing clear guidance for effective and meaningful practice.

Common Regulatory Targets of NFIA, NFIX and NFIB during Postnatal Cerebellar Development

Transcriptional regulation plays a central role in controlling neural stem and progenitor cell proliferation and differentiation during neurogenesis. For instance, transcription factors from the nuclear factor I (NFI) family have been shown to co-ordinate neural stem and progenitor cell differentiation within multiple regions of the embryonic nervous system, including the neocortex, hippocampus, spinal cord and cerebellum. Knockout of individual Nfi genes culminates in similar phenotypes, suggestive of common target genes for these transcription factors. However, whether or not the NFI family regulates common suites of genes remains poorly defined. Here, we use granule neuron precursors (GNPs) of the postnatal murine cerebellum as a model system to analyse regulatory targets of three members of the NFI family: NFIA, NFIB and NFIX. By integrating transcriptomic profiling (RNA-seq) of Nfia- and Nfix-deficient GNPs with epigenomic profiling (ChIP-seq against NFIA, NFIB and NFIX, and DNase I hypersensitivity assays), we reveal that these transcription factors share a large set of potential transcriptional targets, suggestive of complementary roles for these NFI family members in promoting neural development.

Granule neuron precursor cell proliferation is regulated by NFIX and intersectin 1 during postnatal cerebellar development

Cerebellar granule neurons are the most numerous neuronal subtype in the central nervous system. Within the developing cerebellum, these neurons are derived from a population of progenitor cells found within the external granule layer of the cerebellar anlage, namely the cerebellar granule neuron precursors (GNPs). The timely proliferation and differentiation of these precursor cells, which, in rodents occurs predominantly in the postnatal period, is tightly controlled to ensure the normal morphogenesis of the cerebellum. Despite this, our understanding of the factors mediating how GNP differentiation is controlled remains limited. Here, we reveal that the transcription factor nuclear factor I X (NFIX) plays an important role in this process. Mice lacking Nfix exhibit reduced numbers of GNPs during early postnatal development, but elevated numbers of these cells at postnatal day 15. Moreover, Nfix^-/- GNPs exhibit increased proliferation when cultured in vitro, suggestive of a role for NFIX in promoting GNP differentiation. At a mechanistic level, profiling analyses using both ChIP-seq and RNA-seq identified the actin-associated factor intersectin 1 as a downstream target of NFIX during cerebellar development. In support of this, mice lacking intersectin 1 also displayed delayed GNP differentiation. Collectively, these findings highlight a key role for NFIX and intersectin 1 in the regulation of cerebellar development.

Developmental phosphoproteomics identifies the kinase CK2 as a driver of Hedgehog signaling and a therapeutic target in medulloblastoma

A major limitation of targeted cancer therapy is the rapid emergence of drug resistance, which often arises through mutations at or downstream of the drug target or through intrinsic resistance of subpopulations of tumor cells. Medulloblastoma (MB), the most common pediatric brain tumor, is no exception, and MBs that are driven by sonic hedgehog (SHH) signaling are particularly aggressive and drug-resistant. To find new drug targets and therapeutics for MB that may be less susceptible to common resistance mechanisms, we used a developmental phosphoproteomics approach in murine granule neuron precursors (GNPs), the developmental cell of origin of MB. The protein kinase CK2 emerged as a driver of hundreds of phosphorylation events during the proliferative, MB-like stage of GNP growth, including the phosphorylation of three of the eight proteins commonly amplified in MB. CK2 was critical to the stabilization and activity of the transcription factor GLI2, a late downstream effector in SHH signaling. CK2 inhibitors decreased the viability of primary SHH-type MB patient cells in culture and blocked the growth of murine MB tumors that were resistant to currently available Hh inhibitors, thereby extending the survival of tumor-bearing mice. Because of structural interactions, one CK2 inhibitor (CX-4945) inhibited both wild-type and mutant CK2, indicating that this drug may avoid at least one common mode of acquired resistance. These findings suggest that CK2 inhibitors may be effective for treating patients with MB and show how phosphoproteomics may be used to gain insight into developmental biology and pathology.

Phosphodiesterase 4D acts downstream of Neuropilin to control Hedgehog signal transduction and the growth of medulloblastoma

Alterations in Hedgehog (Hh) signaling lead to birth defects and cancers including medulloblastoma, the most common pediatric brain tumor. Although inhibitors targeting the membrane protein Smoothened suppress Hh signaling, acquired drug resistance and tumor relapse call for additional therapeutic targets. Here we show that phosphodiesterase 4D (PDE4D) acts downstream of Neuropilins to control Hh transduction and medulloblastoma growth. PDE4D interacts directly with Neuropilins, positive regulators of Hh pathway. The Neuropilin ligand Semaphorin3 enhances this interaction, promoting PDE4D translocation to the plasma membrane and cAMP degradation. The consequent inhibition of protein kinase A (PKA) enhances Hh transduction. In the developing cerebellum, genetic removal of Neuropilins reduces Hh signaling activity and suppresses proliferation of granule neuron precursors. In mouse medulloblastoma allografts, PDE4D inhibitors suppress Hh transduction and inhibit tumor growth. Our findings reveal a new regulatory mechanism of Hh transduction, and highlight PDE4D as a promising target to treat Hh-related tumors.

DOI:http://dx.doi.org/10.7554/eLife.07068.001

A communication system in cells called the Hedgehog signaling pathway plays an essential role in the formation of tissues and organs in animal embryos. The activity of the pathway is carefully controlled during development and if Hedgehog signaling is disrupted it can lead to developmental defects and particular types of cancer. Some of these cancers can be treated with a drug called vismodegib, which targets a particular molecule in the Hedgehog signaling pathway. However, tumor cells can become resistant to this drug, so researchers are hoping to find new therapies that target other aspects of the signaling pathway.

Hedgehog signaling promotes the division of brain cells called granule neuron precursor cells (or GNP cells for short). If the signaling pathway is over-active it can trigger the GNP cells to divide more than they should. This can lead to medulloblastoma, which is the most common type of brain tumor that affects children. Proteins called Neuropilins—which bind to molecules known as Semaphorins—promote Hedgehog signaling and the formation of medulloblastoma, but it was not clear how this works.

Here Ge et al. studied the role of Neuropilin in cultured cells and in the cerebellum of mice. The experiments show that Semaphorin 3 promotes the accumulation of an enzyme called PDE4D at the cell membrane. PDE4D interacts with Neuropilin and blocks the activity of another enzyme that normally inhibits Hedgehog signaling. In mice that lack Neuropilin and Semophorin 3, the GNP cells are less able to divide, which leads to abnormal development of the cerebellum.

Further experiments show that drugs that target PDE4D inhibit both the Hedgehog pathway and the growth of tumors that are resistant to vismodegib treatment. Ge et al.'s findings uncover a new way in which Hedgehog signaling is regulated and highlight a potential new strategy for treating medulloblastoma and other similar tumors. Current PDE4D inhibitors are associated with severe side effects, so the next challenge is to develop new drugs that have fewer side effects.

DOI:http://dx.doi.org/10.7554/eLife.07068.002

Multiple Surface Regions on the Niemann-Pick C2 Protein Facilitate Intracellular Cholesterol Transport

The cholesterol storage disorder Niemann-Pick type C (NPC) disease is caused by defects in either of two late endosomal/lysosomal proteins, NPC1 and NPC2. NPC2 is a 16-kDa soluble protein that binds cholesterol in a 1:1 stoichiometry and can transfer cholesterol between membranes by a mechanism that involves protein-membrane interactions. To examine the structural basis of NPC2 function in cholesterol trafficking, a series of point mutations were generated across the surface of the protein. Several NPC2 mutants exhibited deficient sterol transport properties in a set of fluorescence-based assays. Notably, these mutants were also unable to promote egress of accumulated intracellular cholesterol from npc2(-/-) fibroblasts. The mutations mapped to several regions on the protein surface, suggesting that NPC2 can bind to more than one membrane simultaneously. Indeed, we have previously demonstrated that WT NPC2 promotes vesicle-vesicle interactions. These interactions were abrogated, however, by mutations causing defective sterol transfer properties. Molecular modeling shows that NPC2 is highly plastic, with several intense positively charged regions across the surface that could interact favorably with negatively charged membrane phospholipids. The point mutations generated in this study caused changes in NPC2 surface charge distribution with minimal conformational changes. The plasticity, coupled with membrane flexibility, probably allows for multiple cholesterol transfer routes. Thus, we hypothesize that, in part, NPC2 rapidly traffics cholesterol between closely appositioned membranes within the multilamellar interior of late endosomal/lysosomal proteins, ultimately effecting cholesterol egress from this compartment.

AMP-Activated Protein Kinase Directly Phosphorylates and Destabilizes Hedgehog Pathway Transcription Factor GLI1 in Medulloblastoma

The Hedgehog (Hh) pathway regulates cell differentiation and proliferation during development by controlling the Gli transcription factors. Cell fate decisions and progression toward organ and tissue maturity must be coordinated, and how an energy sensor regulates the Hh pathway is not clear. AMP-activated protein kinase (AMPK) is an important sensor of energy stores and controls protein synthesis and other energy-intensive processes. AMPK is directly responsive to intracellular AMP levels, inhibiting a wide range of cell activities if ATP is low and AMP is high. Thus, AMPK can affect development by influencing protein synthesis and other processes needed for growth and differentiation. Activation of AMPK reduces GLI1 protein levels and stability, thus blocking Sonic-hedgehog-induced transcriptional activity. AMPK phosphorylates GLI1 at serines 102 and 408 and threonine 1074. Mutation of these three sites into alanine prevents phosphorylation by AMPK. This leads to increased GLI1 protein stability, transcriptional activity, and oncogenic potency.

Single-molecule imaging of Hedgehog pathway protein Smoothened in primary cilia reveals binding events regulated by Patched1

Accumulation of the signaling protein Smoothened (Smo) in the membrane of primary cilia is an essential step in Hedgehog (Hh) signal transduction, yet the molecular mechanisms of Smo movement and localization are poorly understood. Using ultrasensitive single-molecule tracking with high spatial/temporal precision (30 nm/10 ms), we discovered that binding events disrupt the primarily diffusive movement of Smo in cilia at an array of sites near the base. The affinity of Smo for these binding sites was modulated by the Hh pathway activation state. Activation, by either a ligand or genetic loss of the negatively acting Hh receptor Patched-1 (Ptch), reduced the affinity and frequency of Smo binding at the base. Our findings quantify activation-dependent changes in Smo dynamics in cilia and highlight a previously unknown step in Hh pathway activation.

The Eya1 phosphatase promotes Shh signaling during hindbrain development and oncogenesis

Sonic hedgehog (Shh) signaling is critical in development and oncogenesis, but the mechanisms regulating this pathway remain unclear. Although protein phosphorylation clearly affects Shh signaling, little is known about phosphatases governing the pathway. Here, we conducted a small hairpin RNA (shRNA) screen of the phosphatome and identified Eya1 as a positive regulator of Shh signaling. We find that the catalytically active phosphatase Eya1 cooperates with the DNA-binding protein Six1 to promote gene induction in response to Shh and that Eya1/Six1 together regulate Gli transcriptional activators. We show that Eya1, which is mutated in a human deafness disorder, branchio-oto-renal syndrome, is critical for Shh-dependent hindbrain growth and development. Moreover, Eya1 drives the growth of medulloblastoma, a Shh-dependent hindbrain tumor. Together, these results identify Eya1 and Six1 as key components of the Shh transcriptional network in normal development and in oncogenesis.

A rapid and simple method for DNA engineering using cycled ligation assembly

DNA assembly techniques have developed rapidly, enabling efficient construction of complex constructs that would be prohibitively difficult using traditional restriction-digest based methods. Most of the recent methods for assembling multiple DNA fragments in vitro suffer from high costs, complex set-ups, and diminishing efficiency when used for more than a few DNA segments. Here we present a cycled ligation-based DNA assembly protocol that is simple, cheap, efficient, and powerful. The method employs a thermostable ligase and short Scaffold Oligonucleotide Connectors (SOCs) that are homologous to the ends and beginnings of two adjacent DNA sequences. These SOCs direct an exponential increase in the amount of correctly assembled product during a reaction that cycles between denaturing and annealing/ligating temperatures. Products of early cycles serve as templates for later cycles, allowing the assembly of many sequences in a single reaction. To demonstrate the method’s utility, we directed the assembly of twelve inserts, in one reaction, into a transformable plasmid. All the joints were precise, and assembly was scarless in the sense that no nucleotides were added or missing at junctions. Simple, efficient, and low-cost cycled ligation assemblies will facilitate wider use of complex genetic constructs in biomedical research.

Wing tips: The wing disc as a platform for studying Hedgehog signaling

Hedgehog (Hh) signal transduction is necessary for the development of most mammalian tissues and can go awry and cause birth defects or cancer. Hh signaling was initially described in Drosophila, and much of what we know today about mammalian Hh signaling was directly guided by discoveries in the fly. Indeed, Hh signaling is a wonderful example of the use of non-vertebrate model organisms to make basic discoveries that lead to new disease treatment. The first pharmaceutical to treat hyperactive Hh signaling in Basal Cell Carcinoma was released in 2012, approximately 30 years after the isolation of Hh mutants in Drosophila. The study of Hh signaling has been greatly facilitated by the imaginal wing disc, a tissue with terrific experimental advantages. Studies using the wing disc have led to an understanding of Hh ligand processing, packaging into particles for transmission, secretion, reception, signal transduction, target gene activation, and tissue patterning. Here we describe the imaginal wing disc, how Hh patterns this tissue, and provide methods to use wing discs to study Hh signaling in Drosophila. The tools and approaches we highlight form the cornerstone of research efforts in many laboratories that use Drosophila to study Hh signaling, and are essential for ongoing discoveries.

Twelve tips for teaching with ultrasound in the undergraduate curriculum

With ever increasing use of imaging as a diagnostic tool in medicine, medical schools are being urged to further integrate imaging into their curricula. Ultrasound is one such way of doing so-enabling students to bridge the gap between form and function, medical school and clinical practice. As a non-invasive imaging technique, with low risk when compared to other methods of imaging, ultrasound is ideal for integration into basic science and clinical teaching. The twelve tips given in this article offer advice on the practicalities of running a successful ultrasound imaging session in an appropriate environment, promoting safety and curriculum integration.

Neuropilin-2 contributes to tumorigenicity in a mouse model of Hedgehog pathway medulloblastoma

The Hedgehog (Hh) signaling pathway has been implicated in the most common childhood brain tumor, medulloblastoma (MB). Given the toxicity of post-surgical treatments for MB, continued need exists for new, targeted therapies. Based upon our finding that Neuropilin (Nrp) transmembrane proteins are required for Hh signal transduction, we investigated the role of Nrp in MB cells. Cultured cells derived from a mouse Ptch (+/-) ;LacZ MB (Med1-MB), effectively modeled the Hh pathway-related subcategory of human MBs in vitro. Med1-MB cells maintained constitutively active Hh target gene transcription, and consistently formed tumors within one month after injection into mouse cerebella. The proliferation rate of Med1-MBs in culture was dependent upon Nrp2, while reducing Nrp1 function had little effect. Knockdown of Nrp2 prior to cell implantation significantly increased mouse survival, compared to transfection with a non-targeting siRNA. Knocking down Nrp2 specifically in MB cells avoided any direct effect on tumor vascularization. Nrp2 should be further investigated as a potential target for adjuvant therapy in patients with MB.

Engineered knottin peptide enables noninvasive optical imaging of intracranial medulloblastoma

Central nervous system tumors carry grave clinical prognoses due to limited effectiveness of surgical resection, radiation, and chemotherapy. Thus, improved strategies for brain tumor visualization and targeted treatment are critically needed. We demonstrate that mouse cerebellar medulloblastoma (MB) can be targeted and illuminated with a fluorescent, engineered cystine knot (knottin) peptide that binds with high affinity to αvβ3, αvβ5, and α5β1 integrin receptors. This integrin-binding knottin peptide, denoted EETI 2.5F, was evaluated as a molecular imaging probe in both orthotopic and genetic models of MB. Following tail vein injection, fluorescence arising from dye-conjugated EETI 2.5F was localized to the tumor compared with the normal surrounding brain tissue, as measured by optical imaging. The imaging signal intensity correlated with tumor volume. Due to its unique ability to bind to α5β1 integrin, EETI 2.5F showed superior in vivo and ex vivo brain tumor imaging contrast compared with other engineered integrin-binding knottin peptides and with c(RGDfK), a well-studied integrin-binding peptidomimetic. Next, EETI 2.5F was fused to an antibody fragment crystallizable (Fc) domain (EETI 2.5F-Fc) to determine if a larger integrin-binding protein could also target intracranial brain tumors. EETI 2.5F-Fc, conjugated to a fluorescent dye, illuminated MB following i.v. injection and was able to distribute throughout the tumor parenchyma. In contrast, brain tumor imaging signals were not detected in mice injected with EETI 2.5F proteins containing a scrambled integrin-binding sequence, demonstrating the importance of target specificity. These results highlight the potential of using EETI 2.5F and EETI 2.5-Fc as targeted molecular probes for brain tumor imaging.

Genetic dissection of a cell-autonomous neurodegenerative disorder: lessons learned from mouse models of Niemann-Pick disease type C

Understanding neurodegenerative disease progression and its treatment requires the systematic characterization and manipulation of relevant cell types and molecular pathways. The neurodegenerative lysosomal storage disorder Niemann-Pick disease type C (NPC) is highly amenable to genetic approaches that allow exploration of the disease biology at the organismal, cellular and molecular level. Although NPC is a rare disease, genetic analysis of the associated neuropathology promises to provide insight into the logic of disease neural circuitry, selective neuron vulnerability and neural-glial interactions. The ability to control the disorder cell-autonomously and in naturally occurring spontaneous animal models that recapitulate many aspects of the human disease allows for an unparalleled dissection of the disease neurobiology in vivo. Here, we review progress in mouse-model-based studies of NPC disease, specifically focusing on the subtype that is caused by a deficiency in NPC1, a sterol-binding late endosomal membrane protein involved in lipid trafficking. We also discuss recent findings and future directions in NPC disease research that are pertinent to understanding the cellular and molecular mechanisms underlying neurodegeneration in general.

Kif3a is necessary for initiation and maintenance of medulloblastoma

Medulloblastoma (MB) cells arise from granule neuron precursors (GNPs) that have lost growth control. During normal development, GNPs divide in response to Sonic hedgehog (SHH), a ligand that binds to the patched (PTCH) receptor on GNPs. If one copy of the Ptch gene is lost, as in human Gorlin's syndrome and in Ptch(+/-) mice, MBs may form. Proper transduction of the SHH signal critically depends on primary cilia. Loss of primary cilia results in improper signal reception and failure to properly activate SHH target genes. KIF3a, part of a kinesin motor, is required for formation of primary cilia. Here, we use tamoxifen-induced ablation of Kif3a in GNPs of postnatal Ptch(+/-) mouse cerebella to show that KIF3a is necessary for MB formation. To investigate the importance of primary cilia in established tumors, we deleted Kif3a from cultured cells and from tumor cell grafts. The loss of Kif3a from established tumors led to their growth arrest and regression. MBs behave as if they are addicted to the presence of primary cilia. These results underscore the potential utility of agents that disrupt cilia for the treatment of Hh pathway-related MBs.

The chromatin remodeling factor Chd1l is required in the preimplantation embryo

During preimplantation development, the embryo must establish totipotency and enact the earliest differentiation choices, processes that involve extensive chromatin modification. To identify novel developmental regulators, we screened for genes that are preferentially transcribed in the pluripotent inner cell mass (ICM) of the mouse blastocyst. Genes that encode chromatin remodeling factors were prominently represented in the ICM, including Chd1l, a member of the Snf2 gene family. Chd1l is developmentally regulated and expressed in embryonic stem (ES) cells, but its role in development has not been investigated. Here we show that inhibiting Chd1l protein production by microinjection of antisense morpholinos causes arrest prior to the blastocyst stage. Despite this important function in vivo, Chd1l is non-essential for cultured ES cell survival, pluripotency, or differentiation, suggesting that Chd1l is vital for events in embryos that are distinct from events in ES cells. Our data reveal a novel role for the chromatin remodeling factor Chd1l in the earliest cell divisions of mammalian development.

Complement is dispensable for neurodegeneration in Niemann-Pick disease type C

Background

The immune system has been implicated in neurodegeneration during development and disease. In various studies, the absence of complement (that is, C1q deficiency) impeded the elimination of apoptotic neurons, allowing survival. In the genetic lysosomal storage disease Niemann-Pick C (NPC), caused by loss of NPC1 function, the expression of complement system components, C1q especially, is elevated in degenerating brain regions of Npc1^-/- mice. Here we test whether complement is mediating neurodegeneration in NPC disease.

Findings

In normal mature mice, C1q mRNA was found in neurons, particularly cerebellar Purkinje neurons (PNs). In Npc1^-/- mice, C1q mRNA was additionally found in activated microglia, which accumulate during disease progression and PN loss. Interestingly, C1q was not enriched on or near degenerating neurons. Instead, C1q was concentrated in other brain regions, where it partially co-localized with a potential C1q inhibitor, chondroitin sulfate proteoglycan (CSPG). Genetic deletion of C1q, or of the downstream complement pathway component C3, did not significantly alter patterned neuron loss or disease progression. Deletion of other immune response factors, a Toll-like receptor, a matrix metalloprotease, or the apoptosis facilitator BIM, also failed to alter neuron loss.

Conclusion

We conclude that complement is not involved in the death and clearance of neurons in NPC disease. This study supports a view of neuroinflammation as a secondary response with non-causal relationship to neuron injury in the disease. This disease model may prove useful for understanding the conditions in which complement and immunity do contribute to neurodegeneration in other disorders.

Interactions between Type III receptor tyrosine phosphatases and growth factor receptor tyrosine kinases regulate tracheal tube formation in Drosophila

The respiratory (tracheal) system of the Drosophila melanogaster larva is an intricate branched network of air-filled tubes. Its developmental logic is similar in some ways to that of the vertebrate vascular system. We previously described a unique embryonic tracheal tubulogenesis phenotype caused by loss of both of the Type III receptor tyrosine phosphatases (RPTPs), Ptp4E and Ptp10D. In Ptp4E Ptp10D double mutants, the linear tubes in unicellular and terminal tracheal branches are converted into bubble-like cysts that incorporate apical cell surface markers. This tube geometry phenotype is modulated by changes in the activity or expression of the epidermal growth factor receptor (Egfr) tyrosine kinase (TK). Ptp10D physically interacts with Egfr. Here we demonstrate that the Ptp4E Ptp10D phenotype is the consequence of the loss of negative regulation by the RPTPs of three growth factor receptor TKs: Egfr, Breathless and Pvr. Reducing the activity of any of the three kinases by tracheal expression of dominant-negative mutants suppresses cyst formation. By competing dominant-negative and constitutively active kinase mutants against each other, we show that the three RTKs have partially interchangeable activities, so that increasing the activity of one kinase can compensate for the effects of reducing the activity of another. This implies that SH2-domain downstream effectors that are required for the phenotype are likely to be able to interact with phosphotyrosine sites on all three receptor TKs. We also show that the phenotype involves increases in signaling through the MAP kinase and Rho GTPase pathways.

Neuronal and epithelial cell rescue resolves chronic systemic inflammation in the lipid storage disorder Niemann-Pick C

Chronic systemic inflammation is thought to be a major contributor to metabolic and neurodegenerative diseases. Since inflammatory components are shared among different disorders, targeting inflammation is an attractive option for mitigating disease. To test the significance of inflammation in the lipid storage disorder (LSD) Niemann-Pick C (NPC), we deleted the macrophage inflammatory gene Mip1a/Ccl3 from NPC diseased mice. Deletion of Ccl3 had been reported to delay neuronal loss in Sandhoff LSD mice by inhibiting macrophage infiltration. For NPC mice, in contrast, deleting Ccl3 did not retard neurodegeneration and worsened the clinical outcome. Depletion of visceral tissue macrophages also did not alter central nervous system (CNS) pathology and instead increased liver injury, suggesting a limited macrophage infiltration response into the CNS and a beneficial role of macrophage activity in visceral tissue. Prevention of neuron loss or liver injury, even at late stages in the disease, was achieved through specific rescue of NPC disease in neurons or in liver epithelial cells, respectively. Local epithelial cell correction was also sufficient to reduce the macrophage-associated pathology in lung tissue. These results demonstrate that elevated inflammation and macrophage activity does not necessarily contribute to neurodegeneration and tissue injury, and LSD defects in immune cells may not preclude an appropriate inflammatory response. We conclude that inflammation remains secondary to neuronal and epithelial cell dysfunction and does not irreversibly contribute to the pathogenic cascade in NPC disease. Without further exploration of possible beneficial roles of inflammatory mediators, targeting inflammation may not be therapeutically effective at ameliorating disease severity.

Neuropilins are positive regulators of Hedgehog signal transduction

The Hedgehog (Hh) pathway is essential for vertebrate embryogenesis, and excessive Hh target gene activation can cause cancer in humans. Here we show that Neuropilin 1 (Nrp1) and Nrp2, transmembrane proteins with roles in axon guidance and vascular endothelial growth factor (VEGF) signaling, are important positive regulators of Hh signal transduction. Nrps are expressed at times and locations of active Hh signal transduction during mouse development. Using cell lines lacking key Hh pathway components, we show that Nrps mediate Hh transduction between activated Smoothened (Smo) protein and the negative regulator Suppressor of Fused (SuFu). Nrp1 transcription is induced by Hh signaling, and Nrp1 overexpression increases maximal Hh target gene activation, indicating the existence of a positive feedback circuit. The regulation of Hh signal transduction by Nrps is conserved between mammals and bony fish, as we show that morpholinos targeting the Nrp zebrafish ortholog nrp1a produce a specific and highly penetrant Hh pathway loss-of-function phenotype. These findings enhance our knowledge of Hh pathway regulation and provide evidence for a conserved nexus between Nrps and this important developmental signaling system.

Abstract 932: Mechanisms of hedgehog target gene selection in development and cancer

Hedgehog (Hh) signaling is fundamental to the control of differentiation and growth. During development of the cerebellum, Purkinje neurons emit Sonic hedgehog (Shh), a potent mitogen for adjacent granule neuron precursors (GNPs). GNPs respond to Shh by altering the processing, location and modification of Gli transcription factors that activate or repress target genes. Mutations in human or mouse patched1, which encodes the Shh receptor, promote transformation of GNP cells into medulloblastomas (MBs), the most common childhood malignant brain tumor. We used chromatin immunoprecipitation (ChIP) and high-throughput sequencing to identify locations of Gli1 binding in the chromatin of murine GNPs and MB cells. This led us to Gli-responsive transcriptional enhancers. Combining ChIP data with gene expression data we identified putative target genes that are directly regulated by Shh. Dramatic changes in targeting of Gli1, and target gene expression, occur when cerebellum precursor cells become cancer cells. We will investigate the mechanisms of target gene selection, the connections between Hh target genes and the cell cycle and the roles of target genes in normal development and tumorigenesis. Specific Aim 1: Investigate how Gli transcription factors coordinate to regulate gene expression in cerebellum development and tumorigenesis. Our ChIP data led to many novel target genes and well-established targets like Ptch1, Gli1, and N-myc. 132 genes are consistent targets in normal and tumor cell types. Remarkably, despite the close relation between GNPs and MB cells, many putative target genes are specific to one cell type or the other. We will determine the mechanism of selective recognition of enhancer elements in the two cell types. Specific Aim 2: Determine how chromatin modifiers influence Gli-regulated gene expression. We have identified histone modifications that correlate with the regulatory changes for specific target genes in GNPs vs. MBs. We will investigate the mechanistic importance of these changes in Hh target gene specification. Specific Aim 3: Investigate interactions of Gli proteins with other transcription factors. Computational analyses of the DNA regions bound by Gli1 revealed evidence for two types of transcription factors that may work in parallel, or collaborate, with Gli1 protein. We will investigate their roles in target gene control. Specific Aim 4: Define contributions of Gli targets to GNP development and tumorigenesis. We will investigate selected target genes that mediate the connection between Hh signaling and the cell cycle, in the context of GNPs and tumors. The planned studies have direct importance for understanding developmental and tumorigenic roles of the Hedgehog pathway in many tissues and organs. Discovering genes that are directly regulated by Hh signaling will lead to new ways to intervene errors in signal transduction lead to birth defects or cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 932. doi:10.1158/1538-7445.AM2011-932

The cholesterol trafficking protein NPC1 is required for Drosophila spermatogenesis

Niemann-Pick C (NPC) disease is a lethal neurodegenerative disorder affecting cellular sterol trafficking. Besides neurodegeneration, NPC patients also exhibit other pleiotropic conditions, indicating that NPC protein is required for other physiological processes. Previous studies indicated that a sterol shortage that in turn leads to a shortage of steroid hormones (for example, ecdysone in Drosophila) is likely to be the cause of NPC disease pathology. We have shown that mutations in Drosophila npc1, one of the two NPC disease-related genes, leads to larval lethal and male infertility. Here, we reported that npc1 mutants are defective in spermatogenesis and in particular in the membrane-remodeling individualization process. Interestingly, we found that ecdysone, the steroid hormone responsible for the larval lethal phenotype in npc1 mutants, is not required for individualization. However, supplying 7-dehydrocholesterol can partially rescue the male infertility of npc1 mutants, suggesting that a sterol shortage is responsible for the spermatogenesis defects. In addition, the individualization defects of npc1 mutants were enhanced at high temperature, suggesting that the sterol shortage may lead to temperature-sensitive defects in the membrane-remodeling process. Together, our study reveals a sterol-dependent, ecdysone-independent mechanism of NPC1 function in Drosophila spermatogenesis.

Role of lipid metabolism in smoothened derepression in hedgehog signaling

The binding of Hedgehog (Hh) to its receptor Patched causes derepression of Smoothened (Smo), resulting in the activation of the Hh pathway. Here, we show that Smo activation is dependent on the levels of the phospholipid phosphatidylinositol-4 phosphate (PI4P). Loss of STT4 kinase, which is required for the generation of PI4P, exhibits hh loss-of-function phenotypes, whereas loss of Sac1 phosphatase, which is required for the degradation of PI4P, results in hh gain-of-function phenotypes in multiple settings during Drosophila development. Furthermore, loss of Ptc function, which results in the activation of Hh pathway, also causes an increase in PI4P levels. Sac1 functions downstream of STT4 and Ptc in the regulation of Smo membrane localization and Hh pathway activation. Taken together, our results suggest a model in which Ptc directly or indirectly functions to suppress the accumulation of PI4P. Binding of Hh to Ptc derepresses the levels of PI4P, which, in turn, promotes Smo activation.

A septin diffusion barrier at the base of the primary cilium maintains ciliary membrane protein distribution

In animal cells, the primary cilium transduces extracellular signals through signaling receptors localized in the ciliary membrane, but how these ciliary membrane proteins are retained in the cilium is unknown. We found that ciliary membrane proteins were highly mobile, but their diffusion was impeded at the base of the cilium by a diffusion barrier. Septin 2 (SEPT2), a member of the septin family of guanosine triphosphatases that form a diffusion barrier in budding yeast, localized at the base of the ciliary membrane. SEPT2 depletion resulted in loss of ciliary membrane protein localization and Sonic hedgehog signal transduction, and inhibited ciliogenesis. Thus, SEPT2 is part of a diffusion barrier at the base of the ciliary membrane and is essential for retaining receptor-signaling pathways in the primary cilium.

Learning from Jekyll to control Hyde: Hedgehog signaling in development and cancer

The Hedgehog (Hh) cascade controls cell proliferation, differentiation and patterning of tissues during embryogenesis but is largely suppressed in the adult. The Hh pathway can become reactivated in cancer. Here, we assimilate data from recent studies to understand how and when the Hh pathway is turned on to aid the neoplastic process. Hh signaling is now known to have a role in established tumors, enabling categorization of tumors based on the role Hh signaling plays in their growth. This categorization has relevance for prognosis and targeted therapeutics. In the first category, abnormal Hh signaling initiates the tumor. In the second category, Hh signaling helps maintain the tumor. In the third category, Hh signaling is implicated but its role is not yet defined.

Not lost in space: trafficking in the hedgehog signaling pathway

Compartmentalization within cells provides spatial organization of signaling pathways and ensures the specificity of signaling. In vertebrates, the primary cilium, a tiny microtubule-based protrusion present on most cells, is essential for organizing events during Hedgehog signal transduction. When cells are stimulated with Hedgehog ligands, proteins in the pathway move in and out of the cilia. Protein kinase A (PKA), which is implicated in diverse cellular processes including protein trafficking, is a component of the Hedgehog signaling pathway. PKA has been localized near primary cilia, at a location suitable for regulating the localization of other proteins in the pathway.

The output of Hedgehog signaling is controlled by the dynamic association between Suppressor of Fused and the Gli proteins

The transcriptional program orchestrated by Hedgehog signaling depends on the Gli family of transcription factors. Gli proteins can be converted to either transcriptional activators or truncated transcriptional repressors. We show that the interaction between Gli3 and Suppressor of Fused (Sufu) regulates the formation of either repressor or activator forms of Gli3. In the absence of signaling, Sufu restrains Gli3 in the cytoplasm, promoting its processing into a repressor. Initiation of signaling triggers the dissociation of Sufu from Gli3. This event prevents formation of the repressor and instead allows Gli3 to enter the nucleus, where it is converted into a labile, differentially phosphorylated transcriptional activator. This key dissociation event depends on Kif3a, a kinesin motor required for the function of primary cilia. We propose that the Sufu-Gli3 interaction is a major control point in the Hedgehog pathway, a pathway that plays important roles in both development and cancer.

Lateral transport of Smoothened from the plasma membrane to the membrane of the cilium

A distinct pathway for ligand-stimulated lateral trafficking of the hedgehog protein Smoothened into cilia is described.

The function of primary cilia depends critically on the localization of specific proteins in the ciliary membrane. A major challenge in the field is to understand protein trafficking to cilia. The Hedgehog (Hh) pathway protein Smoothened (Smo), a 7-pass transmembrane protein, moves to cilia when a ligand is received. Using microscopy-based pulse-chase analysis, we find that Smo moves through a lateral transport pathway from the plasma membrane to the ciliary membrane. Lateral movement, either via diffusion or active transport, is quite distinct from currently studied pathways of ciliary protein transport in mammals, which emphasize directed trafficking of Golgi-derived vesicles to the base of the cilium. We anticipate that this alternative route will be used by other signaling proteins that function at cilia. The path taken by Smo may allow novel strategies for modulation of Hh signaling in cancer and regeneration.

Tail wags dog: primary cilia and tumorigenesis

Aberrant activation of the Hedgehog (Hh) signaling pathway contributes to many forms of cancer. Primary cilia are Hh signal transduction centers. Two papers in a recent issue of Nature Medicine (Han et al., 2009; Wong et al., 2009) show that mutating cilia can increase or reduce the rates of tumorigenesis depending on how the Hh pathway is disrupted.

Case studies of ends-out gene targeting in Drosophila

Ends-in and ends-out gene replacement approaches have been successfully used to disrupt Drosophila genes involved in a variety of biological processes. These methods combine double-strand breaks and homologous recombination to replace a targeted chromosome region with a designed DNA sequence. Unfortunately, these methods require large numbers of single animal crosses, making them both time consuming and labor intensive. Here, we designed a single complete targeting vector for use in a mass crossing ends-out gene targeting study. Importantly, our gene targeting method included a balancer chromosome to block endogenous homologous chromosome pairing and to promote pairing between the foreign targeting DNA fragment and the targeted chromosome. This technique provided successful and efficient gene replacement, greatly facilitating the gene knockout procedure.

Rab35 controls actin bundling by recruiting fascin as an effector protein

Actin filaments are key components of the eukaryotic cytoskeleton that provide mechanical structure and generate forces during cell shape changes, growth, and migration. Actin filaments are dynamically assembled into higher-order structures at specified locations to regulate diverse functions. The Rab family of small guanosine triphosphatases is evolutionarily conserved and mediates intracellular vesicle trafficking. We found that Rab35 regulates the assembly of actin filaments during bristle development in Drosophila and filopodia formation in cultured cells. These effects were mediated by the actin-bundling protein fascin, which directly associated with active Rab35. Targeting Rab35 to the outer mitochondrial membrane triggered actin recruitment, demonstrating a role for an intracellular trafficking protein in localized actin assembly.

Bortezomib reverses a post-translational mechanism of tumorigenesis for patched1 haploinsufficiency in medulloblastoma

BACKGROUND

Tumor initiation has been attributed to haploinsufficiency at a single locus for a large number of cancers. Patched1 (Ptc1) was one of the first such loci, and Ptc1 haploinsufficiency has been asserted to lead to medulloblastoma and rhabdomyosarcoma in mice.

PROCEDURE

To study the role of Ptc1 in cerebellar tumor development and to create a preclinical therapeutic platform, we have generated a conditional Ptc1 haploinsufficiency model of medulloblastoma by inactivating Ptc1 in Pax7-expressing cells of the cerebellum.

RESULTS

These mice developed exclusively medulloblastoma. We show that despite the presence of transcription of Ptc1, Ptc1 protein is nearly undetectable or absent in tumors. Our results suggest that Ptc1 loss of function is complete, but achieved at the protein level rather than by the classic genetic two-hit mechanism or a strict half-dosage genetic haploinsufficiency mechanism. Furthermore, we found that bortezomib, a 26S proteasome inhibitor, had a significant anti-tumor activity in vitro and in vivo, which was accompanied by restoration of Ptc1 protein and downregulation of the hedgehog signaling pathway. The same effect was seen for both human and mouse medulloblastoma tumor cell growth.

CONCLUSIONS

These results suggest that proteasome inhibition is a potential new therapeutic approach in medulloblastoma.

Manipulation of an innate escape response in Drosophila: photoexcitation of acj6 neurons induces the escape response

Background

The genetic analysis of behavior in Drosophila melanogaster has linked genes controlling neuronal connectivity and physiology to specific neuronal circuits underlying a variety of innate behaviors. We investigated the circuitry underlying the adult startle response, using photoexcitation of neurons that produce the abnormal chemosensory jump 6 (acj6) transcription factor. This transcription factor has previously been shown to play a role in neuronal pathfinding and neurotransmitter modality, but the role of acj6 neurons in the adult startle response was largely unknown.

Principal Findings

We show that the activity of these neurons is necessary for a wild-type startle response and that excitation is sufficient to generate a synthetic escape response. Further, we show that this synthetic response is still sensitive to the dose of acj6 suggesting that that acj6 mutation alters neuronal activity as well as connectivity and neurotransmitter production.

Results/Significance

These results extend the understanding of the role of acj6 and of the adult startle response in general. They also demonstrate the usefulness of activity-dependent characterization of neuronal circuits underlying innate behaviors in Drosophila, and the utility of integrating genetic analysis into modern circuit analysis techniques.

Insulin-like growth factor 2 is required for progression to advanced medulloblastoma in patched1 heterozygous mice

Medulloblastoma (MB) can arise in the cerebellum due to genetic activation of the Sonic Hedgehog (Shh) signaling pathway. During normal cerebellum development, Shh spurs the proliferation of granule neuron precursors (GNP), the precursor cells of MB. Mutations in the Shh receptor gene patched1 (ptc1+/-) lead to increased MB incidence in humans and mice. MB tumorigenesis in mice heterozygous for ptc1+/- shows distinct steps of progression. Most ptc1+/- mice form clusters of preneoplastic cells on the surface of the mature cerebellum that actively transcribe Shh target genes. In approximately 15% of mice, these preneoplastic cells will become fast-growing, lethal tumors. It was previously shown that the loss of function of insulin-like growth factor 2 (igf2) suppresses MB formation in ptc1+/- mice. We found that igf2 is not expressed in preneoplastic lesions but is induced as these lesions progress to more advanced MB tumors. Igf2 is not required for formation of preneoplastic lesions but is necessary for progression to advanced tumors. Exogenous Igf2 protein promoted proliferation of MB precursor cells (GNP) and a MB cell line, PZp53(MED). Blocking igf2 signaling inhibited growth of PZp53(MED) cells, implicating igf2 as a potential clinical target.

A homozygous mutation in human PRICKLE1 causes an autosomal-recessive progressive myoclonus epilepsy-ataxia syndrome

Progressive myoclonus epilepsy (PME) is a syndrome characterized by myoclonic seizures (lightning-like jerks), generalized convulsive seizures, and varying degrees of neurological decline, especially ataxia and dementia. Previously, we characterized three pedigrees of individuals with PME and ataxia, where either clinical features or linkage mapping excluded known PME loci. This report identifies a mutation in PRICKLE1 (also known as RILP for REST/NRSF interacting LIM domain protein) in all three of these pedigrees. The identified PRICKLE1 mutation blocks the PRICKLE1 and REST interaction in vitro and disrupts the normal function of PRICKLE1 in an in vivo zebrafish overexpression system. PRICKLE1 is expressed in brain regions implicated in epilepsy and ataxia in mice and humans, and, to our knowledge, is the first molecule in the noncanonical WNT signaling pathway to be directly implicated in human epilepsy.

A nucleostemin family GTPase, NS3, acts in serotonergic neurons to regulate insulin signaling and control body size

Growth and body size are regulated by the CNS, integrating the genetic developmental program with assessments of an animal's current energy state and environmental conditions. CNS decisions are transmitted to all cells of the animal by insulin/insulin-like signals. The molecular biology of the CNS growth control system has remained, for the most part, elusive. Here we identify NS3, a Drosophila nucleostemin family GTPase, as a powerful regulator of body size. ns3 mutants reach <60% of normal size and have fewer and smaller cells, but exhibit normal body proportions. NS3 does not act cell-autonomously, but instead acts at a distance to control growth. Rescue experiments were performed by expressing wild-type ns3 in many different cells of ns3 mutants. Restoring NS3 to only 106 serotonergic neurons rescued global growth defects. These neurons are closely apposed with those of insulin-producing neurons, suggesting possible communication between the two neuronal systems. In the brains of ns3 mutants, excess serotonin and insulin accumulate, while peripheral insulin pathway activation is low. Peripheral insulin pathway activation rescues the growth defects of ns3 mutants. The findings suggest that NS3 acts in serotonergic neurons to regulate insulin signaling and thus exert global growth control.

ISWI regulates higher-order chromatin structure and histone H1 assembly in vivo

Imitation SWI (ISWI) and other ATP-dependent chromatin-remodeling factors play key roles in transcription and other processes by altering the structure and positioning of nucleosomes. Recent studies have also implicated ISWI in the regulation of higher-order chromatin structure, but its role in this process remains poorly understood. To clarify the role of ISWI in vivo, we examined defects in chromosome structure and gene expression resulting from the loss of Iswi function in Drosophila. Consistent with a broad role in transcriptional regulation, the expression of a large number of genes is altered in Iswi mutant larvae. The expression of a dominant-negative form of ISWI leads to dramatic alterations in higher-order chromatin structure, including the apparent decondensation of both mitotic and polytene chromosomes. The loss of ISWI function does not cause obvious defects in nucleosome assembly, but results in a significant reduction in the level of histone H1 associated with chromatin in vivo. These findings suggest that ISWI plays a global role in chromatin compaction in vivo by promoting the association of the linker histone H1 with chromatin.

Chromatin-remodeling factors such as ISWI play a role in transcription and other nuclear processes by altering the structure and positioning of nucleosomes (the protein–DNA complexes that organize chromatin). Recent studies have suggested that chromatin-remodeling factors can also influence higher-order chromatin structure, but how they do this is not well understood. Using Drosophila melanogaster as a model organism, we investigated the role of ISWI in gene expression and the regulation of chromosome structure in higher eukaryotes. Loss of ISWI alters the expression of a large number of genes. The loss of ISWI function also causes dramatic alterations in higher-order chromatin structure—including the decondensation of mitotic and polytene chromosomes—accompanied by a striking reduction in the amount of the linker histone H1 associated with chromatin. Based on these findings, we propose that ISWI plays a global role in chromosome compaction by promoting the association of a linker histone with chromatin.

ISWI is a chromatin-remodeling factor that recruits linker histone H1 into chromatin, thus altering the structure and positioning of nucleosomes and the degree of chromatin compaction.

Patching the gaps in Hedgehog signalling

The Hedgehog (Hh) pathway plays central roles in animal development and stem-cell function. Defects in Hh signalling lead to birth defects and cancer in humans. The first and often genetically damaged step in this pathway is the interaction between two membrane proteins - Patched (Ptc), encoded by a tumour suppressor gene, and Smoothened (Smo), encoded by a proto-oncogene. Recent work linking Hh signalling to sterol metabolites and protein-trafficking events at the primary cilium promises to shed light on the biochemical basis of how Patched inhibits Smoothened, and to provide new avenues for cancer treatment.

Drosophila Niemann-Pick type C-2 genes control sterol homeostasis and steroid biosynthesis: a model of human neurodegenerative disease

Mutations in either of the two human Niemann-Pick type C (NPC) genes, NPC1 and NPC2, cause a fatal neurodegenerative disease associated with abnormal cholesterol accumulation in cells. npc1a, the Drosophila NPC1 ortholog, regulates sterol homeostasis and is essential for molting hormone (20-hydroxyecdysone; 20E) biosynthesis. While only one npc2 gene is present in yeast, worm, mouse and human genomes, a family of eight npc2 genes (npc2a-h) exists in Drosophila. Among the encoded proteins, Npc2a has the broadest expression pattern and is most similar in sequence to vertebrate Npc2. Mutation of npc2a results in abnormal sterol distribution in many cells, as in Drosophila npc1a or mammalian NPC mutant cells. In contrast to the ecdysteroid-deficient, larval-lethal phenotype of npc1a mutants, npc2a mutants are viable and fertile with relatively normal ecdysteroid level. Mutants in npc2b, another npc2 gene, are also viable and fertile, with no significant sterol distribution abnormality. However, npc2a; npc2b double mutants are not viable but can be rescued by feeding the mutants with 20E or cholesterol, the basic precursor of 20E. We conclude that npc2a functions redundantly with npc2b in regulating sterol homeostasis and ecdysteroid biosynthesis, probably by controlling the availability of sterol substrate. Moreover, npc2a; npc2b double mutants undergo apoptotic neurodegeneration, thus constituting a new fly model of human neurodegenerative disease.

Patched1 regulates hedgehog signaling at the primary cilium

Primary cilia are essential for transduction of the Hedgehog (Hh) signal in mammals. We investigated the role of primary cilia in regulation of Patched1 (Ptc1), the receptor for Sonic Hedgehog (Shh). Ptc1 localized to cilia and inhibited Smoothened (Smo) by preventing its accumulation within cilia. When Shh bound to Ptc1, Ptc1 left the cilia, leading to accumulation of Smo and activation of signaling. Thus, primary cilia sense Shh and transduce signals that play critical roles in development, carcinogenesis, and stem cell function.

Asymmetric distribution of prickle-like 2 reveals an early underlying polarization of vestibular sensory epithelia in the inner ear

Vestibular hair cells have a distinct planar cell polarity (PCP) manifest in the morphology of their stereocilia bundles and the asymmetric localization of their kinocilia. In the utricle and saccule the hair cells are arranged in an orderly array about an abrupt line of reversal that separates fields of cells with opposite polarity. We report that the putative PCP protein Prickle-like 2 (Pk2) is distributed in crescents on the medial sides of vestibular epithelial cells before the morphological polarization of hair cells. Despite the presence of a line of polarity reversal, crescent position is not altered between hair cells of opposite polarity. Frizzled 6 (Fz6), a second PCP protein, is distributed opposite Pk2 along the lateral side of vestibular support cells. Similar to Pk2, the subcellular localization of Fz6 does not differ between cells located on opposite sides of the line of reversal. In addition, in Looptail/Van Gogh-like2 mutant mice Pk2 is distributed asymmetrically at embryonic day 14.5 (E14.5), but this localization is not coordinated between adjacent cells, and the crescents subsequently are lost by E18.5. Together, these results support the idea that a conserved PCP complex acts before stereocilia bundle development to provide an underlying polarity to all cells in the vestibular epithelia and that cells on either side of the line of reversal are programmed to direct the kinocilium in opposite directions with respect to the polarity axis defined by PCP protein distribution.

Thirty-one flavors of Drosophila rab proteins

Rab proteins are small GTPases that play important roles in transport of vesicle cargo and recruitment, association of motor and other proteins with vesicles, and docking and fusion of vesicles at defined locations. In vertebrates, >75 Rab genes have been identified, some of which have been intensively studied for their roles in endosome and synaptic vesicle trafficking. Recent studies of the functions of certain Rab proteins have revealed specific roles in mediating developmental signal transduction. We have begun a systematic genetic study of the 33 Rab genes in Drosophila. Most of the fly proteins are clearly related to specific vertebrate proteins. We report here the creation of a set of transgenic fly lines that allow spatially and temporally regulated expression of Drosophila Rab proteins. We generated fluorescent protein-tagged wild-type, dominant-negative, and constitutively active forms of 31 Drosophila Rab proteins. We describe Drosophila Rab expression patterns during embryogenesis, the subcellular localization of some Rab proteins, and comparisons of the localization of wild-type, dominant-negative, and constitutively active forms of selected Rab proteins. The high evolutionary conservation and low redundancy of Drosophila Rab proteins make these transgenic lines a useful tool kit for investigating Rab functions in vivo.

Assessing the conservation of mammalian gene expression using high-density exon arrays

Microarray data from multiple species have been used to study evolutionary constraints on gene expression. Expression measurements from conventional microarray platforms such as the 3' expression arrays are strongly affected by platform-dependent probe effects that may introduce apparent but misleading discrepancies between species. In this manuscript, we assess the conservation of mammalian gene expression in adult tissues using data from a high-density exon array platform. The exon arrays have more than 6 million probes on a single array targeting all exons in a genome. We find that, unlike 3' array data, gene expression measurements from exon arrays reveal patterns of gene expression that are highly conserved between humans and mice in multiple tissues. Our analysis provides strong evidence for widespread stabilizing selection pressure on transcript abundance during mammalian evolution.