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Skip to search form Skip to main content. We also propose a paradigm that TLR2 signaling regulates the magnitude of the host Th1 response leading to either M. View on Springer. Save to Library. Create Alert. Similar Papers. Figures and Topics from this paper.
Citations Publications citing this paper. Everman , Michael J. McNamara , Luiz E. References Publications referenced by this paper. Bax-dependent regulation of Bak by voltage-dependent anion channel 2. Apoptosis of renal tubular cells in Shiga-toxin-mediated hemolytic uremic syndrome. Apoptosis of renal cortical cells in the hemolytic-uremic syndrome: in vivo and in vitro studies.
Hemolytic uremic syndrome; pathogenesis, treatment, and outcome. Richard L. Siegler , Robert S. Defining and assaying RNAi in mammalian cells. Konrad E. Huppi , Scott E. Cells lacking Mss4 are not viable, and inactivation of the enzyme results in enlarged cells with rounded shape. This phenotype is very reminiscent of the defects seen in mutants of actin binding proteins 40 , Indeed, actin organization is severely impaired in mss4 ts mutants at nonpermissive temperature 40 , 65 , Interestingly, mss4 ts and stt4 ts mutants have overlapping but not identical phenotypes, suggesting that not all Mss4 functions are mediated by Pkc1 Remarkably, some but not all temperature-sensitive calmodulin Cmd1 alleles confer defects in actin organization that can be rescued by Mss4 Inhibition of sphingolipid synthesis also interferes with the signaling function of Mss4 and releases the protein from the PM , further supporting a connection between PtdIns 4,5 P 2 synthesis and sphingolipid metabolism.
Phosphoinositides: Tiny Lipids With Giant Impact on Cell Regulation
The yeast phosphatidylcholine-specific PLD, Spo14 is also regulated by the PtdIns 4,5 P 2 generated by Mss4 , and Mss4 was also found to have a role in late stages of secretion probably at the fusion step. Finally, a fraction of Mss4 also localizes to the nucleus, and temperature-sensitive mutants of Mss4 have been isolated that either accumulate in the nucleus or remain PM-associated The functional significance of the nuclear Mss4 is not clear at this point, but it is worth noting that Mss4 can be phosphorylated by the yeast casein kinase I orthologs, and phosphorylation is required for PM binding and optimal functions From this description it is clear that Mss4 controls PtdIns 4,5 P 2 synthesis in the context of a number of diverse biological functions, all of which are interfaced with unique regulatory molecular complexes.
These functionally distinct roles have been diversified during evolution and are assumed by the various isoforms of the mammalian PIP 5-kinases. Aditional forms of and amino acid forms have been found in human and another kDa form in the rat Since the exon structure differs between the rodent and human genes, and different splice forms have similar apparent molecular weights, the designation of these splice forms becomes somewhat complicated , Both the and kDa forms localize to the PM, but the kDa form also localizes to focal adhesions via interaction with talin , The interaction with talin is controlled by a balance of Tyr and Ser phosphorylations at the COOH terminus of the kDa form of the kinase , A similarly important role of the enzyme was found in establishing adherent junctions via regulation of E-cadherin trafficking and for EGF-driven directional migration Stimulation of the enzyme by PtdOH has also been well documented, but it needs further studies to clarify if this regulation occurs in a physiological setting.
The lack of this enzyme also impairs anchoring the membrane to the cytoskeleton in megakaryocytes and platelets As most reviews do, I will also follow the human nomenclature. In fact, overexpression of any of the type I PIP kinases affects actin dynamics and increases membrane activity, but the effects depend on the enzyme, the expression level, and the cell type. There have been reports, however, on changes specific to particular isoforms. It has been suggested that PtdIns 4,5 P 2 is required for the actin polymerization during the pedestal formation to engulf and initiate ingestion of the phagocytic particle , but PtdIns 4,5 P 2 needs to be eliminated for closure of the phagocytic cup , These studies clearly showed that distinct PtdIns 4,5 P 2 pools delivered by distinct kinases had functionally unique roles in a highly orchestrated process of membrane remodeling.
On the other hand, the effects of PIP5K on ezrin membrane localization were found to be Rac1 dependent The existence of a nuclear PI system has been appreciated and studied for quite some time see sect. X , but the exact processes that are regulated by the PI lipids and their modifying enzymes have been elusive. Star-PAP was found strongly stimulated by PtdIns 4,5 P 2 and responsible for the control of the expression of selected group of genes Interestingly, the platelets of these mice showed a defect in thrombin-induced InsP 3 increases and aggregation that could be overcome at high agonist concentrations This effect is explained by a defect in the stability of the actin cytoskeleton.
Although the type II form was the first member cloned from the PIP kinase family , , the role s of these enzymes has remained largely elusive. Structurally, they are highly homologous to the type I enzymes Figure 4 but, as pointed out above, they use PtdIns5P as substrate and convert it to PtdIns 4,5 P 2 Understanding the function of the kinases is closely coupled to understanding the distribution, the pathway of production, and function s of the minor PI species PtdIns5P.
Notably, in other reports, the expressed enzyme was found ER-associated The enzyme is also expressed in nervous tissues in neurons, again associated with endomembranes The yeast ortholog, named Fab1, was first described based on a genetic screen for defects in nuclear segregation The cloning of the kDa protein revealed a strong sequence similarity with the first cloned type II PIPK within its putative catalytic region Although FAB1 is not an essential gene, fab1 null or temperature-sensitive mutant cells grow slowly and show defective spindle orientation resulting in haploid and binucleated daughter cells Fab1 null cells also develop huge vacuoles and display a block in the transport of certain hydrolyses such as CPY to the vacuole It is speculated that the chromosomal segregation defect is secondary to the vacuolar defect possibly because of steric interference from the enlarged vacuoles That Fab1 is a PtdIns3 P 5-kinase was revealed when it was found that osmotic stress led to a rapid increase in the level of a novel phospholipid, PtdIns 3,5 P 2 , in yeast , and it was shown that this increase was dependent on functional Fab1 Simultaneously, it was demonstrated that Fab1 deletion or Fab1 mutant cells lacked PtdIns 3,5 P 2 Finally, in vitro Fab1 was shown to phosphorylate PtdIns3P to PtdIns 3,5 P 2 , , and consistent with its roles in vacuole physiology, Fab1 was localized to the vacuolar membrane , , Interestingly, deletion of two other genes, Vac7 and Vac14, also eliminates detectable PtdIns3P 5-kinase activity , , , suggesting that they are upstream regulators of Fab1.
Quite to the contrary, it decreases both the basal and osmotic stress-induced increase in the level of this lipid , This curious finding is due to the fact that Vac14 acts as a scaffold that holds Fig4 and Fab1 in a signaling complex that also contains Vac7 and Atg18 , and removal of Fig4 leads to the destabilization of this complex with impaired Fab1 activity Functionally speaking, all these proteins via the control of PtdIns 3,5 P 2 have an important role in the multivesicular body MVB sorting pathway , This pathway is also important in autophagy via connection to the Atg18 protein see more on this in sect.
PIKfyve shows many similarities to Fab1 both structurally and functionally. It also has a PtdIns3P recognizing FYVE domain and interacts with several proteins that are homologues of the yeast proteins found in complex with Fab1. Intriguingly, the presence of Sac3 in the complex not only ensures that the kinase is balanced, but curiously, it also keeps PIKfyve in a highly active conformation The biological advantage of this unique phenomenon that maintains a high turnover rate of PtdIns 3,5 P 2 is not fully understood.
These vacuoles represent swollen endocytic compartments and are associated with defects in fluid phase endocytosis and in retrograde transport from the endosomes to the TGN Importantly, while the trafficking and degradation of transferrin and EGF receptors were found unaffected by knockdown studies , , acute pharmacological blockade of PIKfyve did affect the degradation of EGF receptors with their accumulation in the membrane of the enlarged vesicles , This latter finding also indicates a defect in the fusion of prelysosomal compartments and MVB with lysosomes. PIKfyve-depleted or -inhibited cells also fail to properly acidify their endocytic compartments , but it is not clear whether PtdIns 3,5 P 2 directly or indirectly affects the activity of the V-ATPases.
Other recent studies indicated that PIKfyve, together with some of the myotubularin phosphatases that remove the 3-phosphate from PtdIns 3,5 P 2 see sect. VI , are important for PtdIns5P generation in mammalian cells , , One of these studies also found that PtdIns5 P generated via this pathway is a regulator of cell migration PIKfyve inactivation in Caenorhabditis elegans and Drosophila also leads to swollen endocytic compartments.
This also resulted in a decrease rather than an increase in PtdIns 3,5 P 2 levels and caused a prominent defect at the late-endosome to lysosome transition and probably in many other trafficking steps responsible for the pigmentation defect and a progressive neurodegeneration Pathogenic mutations in the human FIG4 gene have also been identified in a novel form of autosomal recessive Charcot-Marie-Tooth disorder, CMT4J, characterized by motor and sensory neuropathy and in a small percentage of patients with amyotrophic lateral sclerosis ALS Similarly, a Vac14 gene-trap mouse showed early newborn lethality associated with rapid neurodegeneration with massive vacuolization and cell death in certain brain areas.
Since the development of these animals appears to be normal and their other organs are not as severely affected, the lack of Vac14 only compromises but does not completely eliminate PIKfyve function It is a fascinating question why only certain neurons mainly after birth are so dependent on PtdIns 3,5 P 2 function. In parallel studies it was discovered that type I PtdIns kinases could phosphorylate the 3-position of PtdIns and that the PI 3-kinase activity associated with oncogenes and stimulated growth factor receptors primarily produced PtdIns 3,4,5 P 3 and, in fact, was a PtdIns 4,5 P 2 3-kinase These early reports have marked the beginning of the explosion of the PI 3-kinase field that unraveled the existence of several classes of PI 3-kinase enzymes and their regulatory adaptor proteins Figure 5.
These are larger proteins — kDa with relatively little known about their cellular regulation and functions. However, because of the still lingering uncertainty about their in vivo substrate preference, these enzymes will be discussed here. Figure 5. The family of PI 3-kinase enzymes. Their association is mediated by an interaction between the very NH 2 -terminal pbinding region p85BR of the catalytic chains with the inter-SH2 iSH2 region of the regulatory subunits.
These enzymes are also called class IA enzymes. These enzymes can phosphorylate PtdIns and PtdIns4P in vitro, but their in vivo substrate preference is still debated. The enzymes most likely form PtdIns3P in the cell. These enzymes associate with a larger regulatory protein Vps15p in yeast and p in mammalian cells not shown.
Class I PI 3-kinase activation leads to the production of PtdIns 3,4,5 P 3 in the PM, and this molecule is the ultimate initiator of a great variety of cellular signaling responses and cascades of molecular events. Activated Akt phosphorylates a whole range of substrates to regulate energy metabolism, survival, cell cycle progression, or differentiation depending on the tissue and the signaling context under which it is activated. These pathways will be detailed in section IX. PtdIns 3,4,5 P 3 gradients have a major role in the migrational response of cells to chemotactic stimuli , a response not only serving phagocytic cells, but also being critical for morphogenic patterning, axon extension, blood vessel formation, and for invasion and metastasis by cancer cells.
The contribution of the individual isoforms and their regulatory subunits to these processes during development and to the functions of various cells and tissues is only beginning to unfold with the help of genetic deletion of the individual enzymes in whole animals or in selected tissues and with the availability of subtype-specific PI 3-kinase inhibitors. Class I PI 3-kinases control several cellular processes such as cell energetics and metabolism or immune cell functions, and they are also key players in cancer development, propagation, and metastasis.
These important roles have propelled PI 3-kinase research into ever-increasing heights. In light of the incredible progress made in this research area, it seems ironic that PI 3-kinases were initially deemed nondruggable targets because of their fundamental roles in supporting almost every aspect of a cell's life. Contrast this with the fact that several subtype-specific PI 3-kinase inhibitors are now in development and some are being tested in clinical trials. Due to the enormity of the PI 3-kinase literature, its coverage in this review will be inevitably limited and focused only on fundamentals.
Several excellent reviews have summarized various aspects of this topic in greater details in recent years 75 , , , , , , , , , , This group of catalytic domains is also highly conserved from yeast to human, although yeast has only a class III PI 3-kinase, Vps The domain architecture of these enzymes is shown in Figure 5. During this process, the p85 subunit also gets tyrosine phosphorylated and renders the catalytic subunit active via molecular rearrangements, the details of which are just beginning to unfold thanks to elegant structural studies , These findings also highlight the complex changes that can indirectly result from a complete deletion of a protein in a knockout animal.
These studies surprisingly found that all these mice showed increased insulin sensitivity and hypoglycemia. Moreover, the MEF cells isolated from these animals showed enhanced rather than decreased PtdIns 3,4,5 P 3 response after stimulation. Most of these mutations are located in regions where the catalytic domain interacts with the iSH2 domain.
Host Innate Immune Response to Mycobacterium tuberculosis
These mutations probably weaken the interaction between the p85 and p subunits, therefore releasing the catalytic domain from the inhibitory effect exerted by the iSH2 domain. A unique role of the p85 regulatory subunits in stabilization of the PTEN phosphatase has been reported recently as well as mutations within these subunits causing destabilization and loss of PTEN in endometrial cancers Later studies using different strategies, either replacing the enzyme with a catalytically inactive KR form or deleting exons critical for catalytic activity , unexpectedly showed that these animals were viable and developed to adulthood.
However, heterozygous mice in either case were born at a sub-Mendelian ratio, and some of the early embryos were small and moribund. This suggested a noncatalytic function of the protein that was attributed to a scaffolding role supporting clathrin-mediated endocytosis Analysis of MEF cells from these animals revealed unexpected enzymatic roles of the kinase in mediating signals from certain GPCRs such as the sphingosine 1-phosphate receptor or the LPA receptor The two p85 regulatory subunits were also found to be required but redundant in B-cell development It was also observed that PI3K inhibition blocked chemotaxis and oxidative burst in neutrophils, and the GPCR-stimulated degranulation in basophilic leukemia cells 52 , , However, these enzymes are also found in lower levels in other tissues such as the heart or the exocrine pancreas , More recently, the development and testing of subtype-specific PI 3-kinase inhibitors also shed light on the unique functions of this kinase in physiology and disease.
One of the best examples of the complementary roles of the two enzymes is mast cells. This explains why inactivation of either enzyme impairs mast cell-mediated allergic responses and attenuates passive anaphylaxis 30 , Class II PI 3-kinases are larger proteins — kDa that have highly homologous domains with other PI 3-kinases, such as the Ras binding domain, the helical and catalytic domains, but also have some distinctive features, such as a PX and C2 domains at their COOH termini.
They are most dissimilar at their NH 2 termini that differ in lengths but all contain proline-rich segments Figure 5. Since relatively little is known about the physiology of these enzymes, the three isoforms will be discussed together.
These studies suggested a role of these kinases in clathrin-mediated endocytosis and trafficking. As pointed out above, it is increasingly evident that class II PI 3-kinases make PtdIns3P as their signaling molecule , , If the enzyme indeed makes primarily PtdIns3P, it is likely that it does so in very specific molecular contexts that differ from those regulated by the class III Vps34 enzymes.
This has initiated a discussion as to what extent class II enzymes contribute to the bulk amount of PtdIns3P found in cells , Inactivation of the enzyme's ortholog in C. Ectopic expression of the PI3KC2 ortholog or its kinase-dead version in fly epithelium caused pattern formation defects Initial interest toward the inositol lipid phosphatases in the early s paled compared with the excitement for lipid kinases and PLC enzymes, and only a few groups pursued research in this direction.
Therefore, identification of the Ins 1,4,5 P 3 5-phosphatase that showed no lithium sensitivity and the lithium sensitive inositol monophosphatase s were top priorities in the mid s. However, Phil Majerus' group had already focused on every form of inositol phosphate or -lipid phosphatases from early on and identified several enzymes that acted upon various inositol phosphate isomers and some on the lipids themselves see Ref.
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The interest level was significantly raised when the gene causing the X-linked human disease Oculo-Cerebro-Renal Syndrome of Lowe OCRL was mapped and found to have significant homology to an already known kDa inositol polyphosphate 5-phosphatase Further momentum was gained when a PtdIns 4,5 P 2 5-phosphatase associated with presynaptic membranes and named synaptojanin Snj was described and cloned These studies helped disseminate the notion that inositol lipid dephosphorylation was an important regulator of inositol lipid signaling and that it was essential to determine whether a phosphatase acted on lipids or on soluble inositol phosphates in its natural cellular location.
The next huge development of the phosphatase field was marked by the realization that the tumor suppressor gene PTEN phosphatase and tensin homolog located on chromosome 10 was actually a PtdIns 3,4,5 P 3 3-phosphatase and not only a protein tyrosine phosphatase as previously believed and that it countered the actions of PI 3-kinases , , Fortuitously, another group of enzymes initially thought to be phosphotyrosine phosphatases, called the myotubularins, were also discovered by identification of a gene mutated in human X-linked centromyotubular myopathy So, in the case of the phosphatases, human genetics played a pivotal role in unearthing several of the enzymes, and the usually pioneering yeast studies can be credited with the first identification of only one of the phosphatases, namely Sac1, with connection to actin polymerization , inositol lipids and which was later proven to be a phosphoinositide monophosphatase The structure of the 5-phosphatase domain of one of the S.
In addition to their catalytic domain, 5-phosphatases contain several additional protein motifs that determine their cellular localization and interactions with other proteins Figure 6. Figure 6. The inositol lipid 5-phosphatase family. Two of these, synaptojanin-1 and synaptojanin-2, exist in multiple splice forms differing in their respective COOH termini only the two main forms are shown.
The characteristic feature of these enzymes is the presence of a Sac1 homology domain upstream of their conserved 5-phosphatase domains. OCRL and INPP5B are also very similar to one another, with a whole set of domains providing multiple interactions in addition to the catalytic 5-ptase domain.
They have an NH 2 -terminal SH2 domain and C2 and proline-rich sequences downstream of the 5-phosphatase domain. As described above, synaptojanin-1 Synj1 was discovered by the De Camilli group as a 5-phosphatase that functions in synaptic vesicle exocytosis and recycling A highly homologous sister enzyme, synaptojanin-2 SYNJ2 , is more widely expressed and also exists in three splice forms differing in their COOH termini On the basis of these findings, it is believed that the Sac1-domain takes the 5-phosphatase product mainly PtdIns4P and further dephosphorylates it to PtdIns.
Indeed, it has been shown that the activities of both the 5-phosphatase domain and the Sac1 domain of SYNJ1 were required to support synaptic vesicle recycling SYNJ1 knockout mice develop and are born seemingly normally but die shortly after birth. This also causes poor recycling of vesicles into a fusion competent synaptic vesicle pool SYNJ1 associates via its COOH-terminal domain with several proteins enriched in the nerve terminal such as amphyphysin and with proteins that play important roles in the endocytic pathways, such as endophilin, intersectin, syndapin, and Eps15 SYNJ1 is constitutively phosphorylated and undergoes activity-dependent dephosphorylation by calcineurin, which, in turn, affects both its catalytic activity and interactions with other proteins , , These results suggest nonredundant functions of the SYNJ phosphatases in controlling PtdIns 4,5 P 2 levels in the membranes of vesicles along the endocytic pathway.
It is an intriguing possibility that membrane deformations during mitochondrial fission and fusion are regulated by this enzyme in a manner similar to what occurs during endocytosis. An extra copy of SYNJ1 due to a trisomic arrangement found in the Ts65Dn mice, a model for Down's syndrome, is associated with neurological defects and PtdIns 4,5 P 2 dysregulation, and these aberrations are corrected by restoring SYNJ1 to disomy Since SYNJ1 is located on human Chr21 that is trisomic in Down's syndrome patients, it is an attractive possibility that SYNJ1 is a major factor in the neurological dysfunctions found in these patients Recent studies also linked SYNJ2 to hearing and cochlear function.
In a mouse strain, called Mozart , which exhibits progressive hearing loss, the causative mutation was mapped to a critical region within the phosphatase domain of the Synj2 gene NK Whether this gene is affected in human hereditary hearing disorders has yet to be investigated. SYNJ2 deleted mice have not been reported yet in the literature. Interestingly, a mammalian type II 5-phosphatase kDa phosphatase, or Inpp5B that does not contain a Sac1 domain still can restore viability in triple Sjl inactivated cells or alleviate the phenotypic changes observed in double-deleted strains The significance of this finding is that a functional Sac1 domain may not be necessary for the functions of the yeast Sjl proteins.
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This conclusion was supported by further reports that Inp52 or Inp53 enzymes with mutated, inactive Sac1 but not inactive 5-phosphatase domains were able to restore the viability of triple Sjl inactivated strains. Mutations in key residues within the Sac1 domain already render this domain of the Inp51 protein inactive.
Individual or double mutant Sjl strains display functional defects that still suggest some specialization among the three Sjl proteins. The complicated phenotypes of the inactivation of the three Inp51—53 proteins in various combinations are due to several factors. First, the 5-phosphatase domains of the three enzymes control specific but perhaps overlapping pools of PtdIns 4,5 P 2 and because of this the severity of the functional defects do not always correlate with overall PtdIns 4,5 P 2 elevations.
Second, the Sac1 phosphatase activity of Inp52 and Inp53 also contributes to the control of specific PtdIns4P and PtdIns3P pools and in most of these locations the Sac1 phosphatase has a redundant function with the Sjl enzymes OCRL patients present with congenital cataract, renal tubular acidosis, aminoaciduria, and mental retardation However, some patients with mutations of the same gene can also present with Dent's disease with milder symptoms that are mostly confined to the kidney An attempt to make a mouse model of OCRL was complicated by the fact that OCRL deletion in mice does not reproduce the phenotypes associated with the human disease, apparently because of the overlapping function s of the homologous kDa 5-phosphatase Inpp5b Combined inactivation of both genes, on the other hand, manifests in early embryonic lethality The difference between humans and mice in the ability of the 75 kDa phosphatase to compensate for OCRL functions must lie within the 75 kDa phosphatase itself, which differs between the two species in expression level and splicing.
This mouse model should greatly facilitate the progress in understanding the molecular pathology of OCRL and Dent's disease. Not surprisingly, it has been difficult to show increased levels of total PtdIns 4,5 P 2 in cells obtained from OCRL patients, although in some cases such increases have been reported Other studies showed only indirect signs of aberrant PtdIns 4,5 P 2 levels, such as altered F-actin distribution and dynamics in fibroblasts obtained from OCRL patients Localized functions of OCRL rely on the recruitment and regulation of the protein to different membranes and, therefore, the domain structure and multiple interactions of the protein with various regulatory proteins have attracted significant attention.
OCRL has two spliceoforms that only differ in a small eight-amino acid stretch within the Rho-GAP-like domain close to the clathrin binding site; the longer form binds clathrin with higher affinity Figure 6. However, somewhat surprisingly, a prominent role of OCRL in the early steps of the endocytic pathway has been also described , Interaction of OCRL with another Rab protein, Rab35, connects the phosphatase with abscission, the last step in cytokinesis The structural details of the various domains of OCRL and their possible roles in activating and orienting of the phosphatase domain have begun to emerge based on the crystal structures of these domains with possible associated proteins , , , , Taking all these data into consideration, the OCRL protein plays multiple roles in endocytic trafficking by its interaction with clathrin and Rab proteins It has been more difficult to connect the molecular details and cellular functions of OCRL to the defects underlying the pathophysiology of the disease.
The similarity of the kidney symptoms of OCRL patients to those of Fanconi syndrome led to the suggestion that the large LDL receptor family protein megalin, which recycles between the apical membrane and the TGN in proximal tubule cells, displays aberrant trafficking in OCRL patients , However, one study found no signs of aberrant megalin trafficking in human and canine renal epithelial cells after siRNA-mediated knockdown of OCRL, even though there was increased secretion of lysosomal hydrolyses Recent studies also suggested that OCRL is important for the assembly and functions of the primary cilium, and this may be an important clue to understand the renal pathology of the disease , , The enzyme, also called kDa type II 5-phosphatase, is widely expressed, especially enriched in the kidney, lung, and testes Although initial studies claimed mitochondrial localization , subsequent studies found the enzyme to be associated with the Golgi and ER exit sites , and Rab proteins also affect the localization of the enzyme , The mouse has three splice variants of Inpp5B as , , and kDa proteins, the expression of which are tissue and developmental stage specific Homozygous mice deficient in Inpp5B are viable and appear normal, but do have testicular degeneration and defective sperm functions that manifest after sexual maturation The sperm of these mice show reduced motility and reduced ability to fertilize eggs , In addition, there is a defect in the apical endocytosis in Sertoli cells causing swollen vacuoles This 5-phosphatase, also called proline-rich inositol polyphosphate 5-phosphatase PIPP , was identified due to the presence of consensus 5-phosphatase sequences The enzyme is widely expressed and contains proline-rich regions both at the NH 2 and COOH terminus and several protein-binding motifs Overexpression of the enzyme in PC12 cells decreased PtdIns 3,4,5 P 3 levels, Akt activation, and neurite outgrowth Recent studies identified collapsing response mediator protein 2 CRMP2 as an INPP5J interacting partner and showed that the two proteins have opposing roles in regulation of nerve cell polarization, axon selection, and neurite elongation Importantly, higher INPP5J expression levels have been correlated with better prognosis and disease outcome in breast cancer , This enzyme is enriched in skeletal muscle and kidney hence the name skeletal muscle and kidney enriched inositol polyphosphate phosphatase and was identified also because of the presence of the two conserved regions found in 5-phosphatases These 5-phosphatases were named because of their NH 2 -terminal SH2-domains.
These proteins function as part of multi-protein complexes, and their cell- and tissue-specific functions are probably dictated by their binding partners. Initially, the SHIP proteins were believed to have a restricted substrate recognition only able to dephosphorylate the 5-position in PtdIns 3,4,5 P 3 [and Ins 1,3,4,5 P 4 and perhaps other soluble higher inositol phosphates] , Recent studies, however, suggested that they can also hydrolyze PtdIns 4,5 P 2 and control clathrin-mediated endocytosis The enzymatic activity of SHIP1 does not appear to be regulated by Tyr phosphorylation or protein-protein interaction , but it is enhanced by binding of the product PtdIns 3,4 P 2 to the C2 domain This could contribute to the well-documented negative effects of cAMP-mobilizing signals on PI 3-kinase signaling in immune cells Some meroterpenoid compounds are also able to activate SHIP1 and exert strong anti-inflammatory effects that could represent a means of controlling PI 3-kinase downstream effectors in hematopoietic cells , , Consistent with its expression pattern and enzymatic functions, SHIP1 deficiency results in enhanced PI 3-kinase signaling affecting cells and organs with hematopoietic cell involvement B and T cells, macrophages, osteoclasts, mast cells, neutrophil cells, and platelets.
Ship1 knockout mice develop a chronic myelogenic leukemia CML -like disease with myeloid proliferation, splenomegaly, and vast myeloid infiltration of the lungs causing premature death , The significantly elevated PtdIns 3,4,5 P 3 levels induce a hyperproliferative response of hematopoietic progenitors to a variety of growth factors and increased Akt activation in mature neutrophils and mast cells Since PtdIns 3,4,5 P 3 has a pivotal role in amplifying polarization during chemotaxis both in phagocytic cells , and lower organisms, such as Dictyostelium , there is a massive dysregulation of neutrophil polarization in Ship1 null mice The more ubiquitously expressed SHIP2 enzyme serves as a negative regulator of insulin signaling.
Additionally, transgenic mouse, overexpressing SHIP2, showed signs of mild insulin resistance A mouse strain lacking Ship2 and the neighboring Phox2a gene shows insulin hypersensitivity and severe hypoglycemia and has early neonatal mortality Curiously, another study reported no such problems with a SHIP2 knockout strain and, to the contrary, reported a resistance of these mice to weight gain in a high-fat diet A recent study identified Ser phosphorylation of SHIP2 that regulates its nuclear localization in nuclear speckles in a cell cycle-dependent manner.
Understanding the relationship between these novel features of SHIP2 and the phenotypic changes listed above will require further investigations. INPP5E is located mostly in the Golgi and partially in the PM , but its distribution could be more specialized in specific cell types. In one of these studies, mutations clustering within the 5-phosphatase domain of INPP5E were identified in patients with Joubert syndrome These patients present with specific midbrain-hindbrain malformation, and variably with retinodystrophy, nephronophthisis, liver cirrhosis, and polydactyly and are considered as an emerging group of ciliopathies In this study, INPP5E was found to localize to the primary cilium in cells of the affected organs and the mutations caused destabilization of the cilia during stimulation The other study found that disruption of the Inpp5e gene in mice caused multiorgan failure associated with defects in primary cilia This latter study also found destabilization of cilia in response to growth factor stimulation and identified a mutation in the human INPP5E gene that affected cilia localization in a family with MORM syndrome.
The physical targeting of INPP5E to the cilia was recently shown to be dependent on a protein complex that included phosphodiesterase 6D, the Arf-like protein, ARL13B, and the centrosomal protein, CEP, all of which had been linked to ciliopathies These interesting discoveries should facilitate further studies to better understand the role of phosphoinositides in ciliary function and how INPP5E controls this process.
The phosphoinositide 3-phosphatases are comprised of two major groups.