InteractiveFly: GeneBrief

preli-like: Biological Overview | References

Dynamic morphological changes in mitochondria depend on the balance of fusion and fission in various eukaryotes, and are crucial for mitochondrial activity. Mitochondrial dysfunction has emerged as a common theme that underlies numerous neurological disorders, including neurodegeneration. However, how this abnormal mitochondrial activity leads to neurodegenerative disorders is still largely unknown. This study shows that the Drosophila mitochondrial protein Preli-like (Prel), a member of the conserved PRELI/MSF1 family, contributes to the integrity of mitochondrial structures, the activity of respiratory chain complex IV and the cellular ATP level. When Prel function was impaired in neurons in vivo, the cellular ATP level decreased and mitochondria became fragmented and sparsely distributed in dendrites and axons. Notably, the dendritic arbors were simplified and downsized, probably as a result of breakage of proximal dendrites and progressive retraction of terminal branches. By contrast, abrogation of the mitochondria transport machinery per se had a much less profound effect on the arbor morphogenesis. Interestingly, overexpression of Drob-1 (Debcl), a Drosophila Bax-like Bcl-2 family protein, in the wild-type background produced dendrite phenotypes that were reminiscent of the prel phenotype. Moreover, expression of the Drob-1 antagonist Buffy in prel mutant neurons substantially restored the dendritic phenotype. These observations suggest that Prel-dependent regulation of mitochondrial activity is important for both growth and prevention of breakage of dendritic branches (Tsubouchi, 2009).

Mitochondria are important for multiple cellular events such as ATP production, Ca²⁺ regulation, axonal and dendritic transport of organelles, and the release and re-uptake of neurotransmitters at synapses (Detmer, 2007). Mitochondria in most healthy cells exist as tubules of variable size and undergo dynamic morphological changes that depend on the balance of fusion and fission. This fusion-fission cycle ensures mixing of metabolites and mitochondrial DNA and influences organelle shape and bioenergetics functionality (Chan, 2006b; Okamoto, 2005). The dynamin-related GTPases have been shown to have central roles in the fusion-fission dynamics of mammalian mitochondria. The mitofusins (MFNs) are proteins localized at the mitochondrial outer membrane that are required for the fusion of mitochondria (Santel, 2006), whereas OPA1 in the inner membrane mediates the fusion (Olichon, 2006). The key component of the fission machinery is dynamin-related protein 1 (Drp1) (Labrousse, 1999; Smirnova, 2001; Tsubouchi, 2009 and references therein).

Dysfunction of mitochondria is highly connected to neurodegenerative diseases, and abrogation of the fusion machinery is an early and causal event in neurodegeneration (Chan, 2006a; Knott, 2008; Lin, 2006). Mutations in MFN2 cause the autosomal dominant disease Charcot-Marie-Tooth (CMT) type 2A, a peripheral neuropathy of long motor and sensory neurons; and Purkinje neurons in the mouse model have aberrant mitochondrial distribution, ultrastructure and electron transport activity. Mutations in OPA1 cause autosomal dominant optic atrophy (ADOA), the most commonly inherited form of optic nerve degeneration. However, it is still largely unknown as to how the abnormal mitochondrial morphology leads to neurodegenerative disorders (Tsubouchi, 2009).

Other consequences of impaired mitochondrial fusion-fission dynamics in the nervous system model have also been studied. In Drosophila, observation of opa1 and drp1 mutants has revealed impaired mitochondrial fusion-fission dynamics in the nervous system. An eye-specific homozygous mutation of opa1 causes rough and glossy eye phenotypes in adult flies, suggesting that an increase in apoptosis is occurring (Yarosh, 2008). Mutations in drp1 result in elongated mitochondria that are mostly absent from the presynapses (Verstreken, 2005). It has been reported that dendritic mitochondria are more metabolically active than axonal mitochondria (Overly, 1996) and that the dendritic distribution of mitochondria and their activity are essential and limiting for the development and morphological plasticity of dendritic spines in cultured hippocampal neurons (Li, 2004). In Drosophila loss of mitochondrial complex II activity causes degeneration of photoreceptors and disruption of mitochondrial protein translation severely affects the maintenance of terminal arborization of dendrites (Chihara, 2007; Mast, 2008). Nevertheless, it is not yet well understood how proper mitochondrial morphology, distribution and activity contribute to the formation and maintenance of dendritic arbors. This study addressed this question by using Drosophila dendritic arborization (da) neurons. Individually identified da neurons are classified into classes I-IV in order of increasing field size and arbor complexity, and they produce dendritic arbors of stereotypic patterns in a two-dimensional manner between the epidermis and muscles (Tsubouchi, 2009).

This study shows that Preli (protein of relevant evolutionary and lymphoid interest)-like (Prel), a Drosophila mitochondrial protein of the conserved PRELI/MSF1 family (Dee, 2005), contributes to the integrity of mitochondrial structure and activity, and to the morphogenesis of dendritic arbors. Mutant prel class IV neurons simplified and downsized their dendritic arbors, and showed breakages of their major branches without detectable signs of apoptosis. Furthermore, genetic interactions were observed between Prel and the Drosophila Bax-like Bcl-2 family proteins Drob-1 (also known as Debcl) and Buffy. All of these observations suggest that Prel-dependent control of mitochondrial activity has a pivotal role in the development and maintenance of dendritic arbors (Tsubouchi, 2009).

A screening was conducted by using the Gene Search (GS) system to hunt for genes that control complex morphology of dendritic arbors of the class IV neuron at larval stages. One of the aims of this system is to drive overexpression or misexpression of genes neighboring the GS-vector insertion site. Out of 3000 GS lines screened, focus was placed on one, GS9160, and its candidate gene, preli-like (prel), which is conserved throughout eukaryotes. The predicted product of prel is 236 amino acids in length and a member of the PRELI/MSF1 family in Drosophila (Dee, 2005). The closest human homolog is PRELI; and its amino acid sequence shows 46% identity to that of Preli-like through the entire length. In yeast, Ups1p, a member of this family, is localized in mitochondria and regulates their shape (Sesaki, 2006). To confirm whether the Prel protein is localized to mitochondria in Drosophila cells, lysates of Drosophila Schneider 2 (S2) cells were fractionated, and both endogenous and exogenously expressed Prel proteins were shown to be predominantly detected in the mitochondria-enriched fraction but hardly found in the cytoplasm. Under the light microscope, the expressed Prel was mostly colocalized with the mitochondrial marker Mitotracker Orange, and it appeared to be associated with the cristae when observed by immunoelectron microscopy (Tsubouchi, 2009).

The role of Prel in shaping mitochondria in S2 cells was examined by both light and transmission electron microscopy. More than 80% of the control S2 cells had a filamentous mitochondrial network, and parallel, accordion-like folds of cristae structures were observed in each mitochondrion. Both knockdown of prel and its exogenous expression caused significant fragmentation of mitochondria. At the ultrastructural level, prel-knockdown cells had many mitochondria with lower electron density. Those mitochondria took on a round shape, having an abnormally expanded matrix and only a few thin cristae. It is known that OPA1, one of the mitochondrial dynamin-related GTPases, is important for both fusion of inner membranes and maintenance of the structure of cristae. This study showed that knockdown of opa1 in S2 cells affected the mitochondrial structure in a very similar manner to that found with knockdown of prel (Tsubouchi, 2009).

Thus, both prel loss-of-function and its overexpression abrogates mitochondrial structures and activity in S2 cells and also in da neurons. Apparently an appropriate expression level of Prel is required for controlling mitochondrial shape, structure of the cristae, the activity of respiratory chain complex IV and the cellular ATP level. What then is the exact molecular function of Prel (Tsubouchi, 2009)?

The molecular function of the Preli family is largely unknown in multicellular organisms. It has been recently shown that Ups1p, the yeast homologue of Prel, regulates the level of cardiolipin (CL), a phospholipid of mitochondrial membranes (Osman, 2009; Tamura, 2009). CL is known to be located predominantly in the mitochondria and has diverse mitochondrial functions including stabilization of the respiratory chain supercomplex (Joshi, 2009). These reports imply that the reduction in the complex IV activity and the ATP level in the prel-knockdown cells could be attributed to the altered phospholipid composition. Further biochemical study is required to measure the complex IV activity and phospholipid composition in various genetic backgrounds including Buffy or Drob1 overexpression, which might help to find evidence for a molecular pathway that includes Prel and Buffy (Tsubouchi, 2009).

However, it has also been proposed that loss of Ups1p affects the function of the yeast OPA1 homolog Mgm1p, which is important for both fusion of inner membranes and maintenance of the structure of the cristae (Chan, 2006b; Cipolat, 2006; Frezza, 2006; Meeusen, 2006; Okamoto, 2005). Mgm1p is imported into mitochondria, and its function is regulated by proteolytic cleavage. This import and cleavage pattern is altered, and the mitochondria fragmented, in the yeast ups1p mutant (Sesaki, 2006; Tamura, 2009). Knockdown of Drosophila opa1 in S2 cells affected the structure and activity of mitochondria similarly to prel knockdown, suggesting that Drosophila Prel is also required for organizing the mitochondrial inner membrane structure in concert with Drosophila Opa1. However, positive evidence could not be provided for any functional linkage between these two molecules. Neither prel knockdown nor its overexpression strongly affected the cleavage pattern of endogenous Opa1 in S2 cells. A better understanding of Prel function in the protein import into mitochondria requires further study. It should be noted that there seems to be a bidirectional relationship between mitochondrial shape and bioenergetics, as suggested by the fact that a decrease in the ATP level can also stimulate mitochondrial fragmentation (Knott, 2008). One possible interpretation of the current data might be that Prel is primarily required for maintaining the normal level of intracellular ATP and controls mitochondrial shape indirectly (Tsubouchi, 2009).

Mitochondria are abundant in regions of intense energy consumption, such as muscles, sperm and neurons; and OPA1 expression is high in the retina, brain, testis, heart and skeletal muscle. The autosomal dominant disease Charcot-Marie-Tooth (CMT) type 2A phenotype due to MFN2 mutations might reflect the extreme cell geometry, as shown by the fact that long peripheral nerves are particularly sensitive to perturbations produced by MFN2-mediated mitochondrial dysfunction (Santel, 2006). Class IV neurons, which develop expansive and complicated dendritic arbors, probably consume the highest amount of ATP; thus they are very vulnerable to a loss or reduction in the prel-dependent mitochondrial function. The differential Gal4 expression of the trap line and the cis-element fusion line suggests that Prel is expressed most strongly in class IV neurons among the four subclasses (Tsubouchi, 2009).

It had speculated that the severe dendritic phenotype of the prel class IV neurons could be primarily due to the misdistribution of mitochondria. However, attempts to correlate the mitochondrial localization and the dendritic phenotype suggested that such a view might be naive. Loss of function of milt dramatically reduced the mitochondrial density in neuronal processes; nevertheless, the dendritic phenotype of the milt mutant neuron was much less profound than that of the prel neurons. Similarly, milt mutant eyes are indistinguishable from the wild-type eyes in their external and photoreceptor morphology in spite of the paucity of mitochondria in photoreceptor axon terminals (Stowers, 2002). These observations imply the possibility that the visible misdistribution of mitochondria per se might not necessarily lead to the severe morphological abnormality of dendritic arbors or axons (Tsubouchi, 2009).

How can these observations be interpreted? Overexpression of prel diminished the ATP level in vivo, where the local ATP concentration within the cell might fall below a threshold that is necessary for dendritic growth and maintenance. However, mitochondria that remain in the cell body of the milt neuron might maintain their ATP-producing activity, and at least a subpopulation of synthesized ATP molecules might diffuse a long distance to reach the distal region in the dendritic arbor. These hypotheses would be testable if the ATP level in the cell body and its diffusion inside dendritic branches could be visualized and measured quantitatively in various genetic backgrounds (Tsubouchi, 2009).

Dysfunction of mitochondria correlates with neurodegenerative diseases and is an early and causal event in neurodegeneration. The results of this study imply that the evolutionally conserved Prel might prevent neurodegeneration in other animal species. When Prel function was impaired, branches in the proximal region of the arbor were degraded, and terminal branches were eliminated at the mature larval stage; and in adult flies, overall arbors retracted. These phenotypes are reminiscent of breakage of neurite branches within and near amyloid deposits in the brain of a transgenic mouse model of Alzheimer disease, which is speculated to occur through mitochondrial dysfunction, oxidative stress and calcium deregulation. The branch destruction of the prel mutant neuron could also be due to a 'physical' reason. Transport of various cargos might be impaired when ATP is limited, leading to a defect in mechanical strength of the membrane. Such fragile branches could be ruptured during larval locomotion (Tsubouchi, 2009).

It has been intensively studied whether the mutant proteins that are associated with hereditary neurodegenerative diseases affect mitochondrial function. This study has provided cellular and genetic evidence that Prel is a novel target of such research. Future studies should be directed towards further characterization of the Prel protein by using both fly and vertebrate systems to clarify its function in mitochondria and its involvement in mechanisms that prevent the regression of dendritic arbors (Tsubouchi, 2009).

The genetic interactome of prohibitins: coordinated control of cardiolipin and phosphatidylethanolamine by conserved regulators in mitochondria

Prohibitin ring complexes in the mitochondrial inner membrane regulate cell proliferation as well as the dynamics and function of mitochondria. Although prohibitins are essential in higher eukaryotes, prohibitin-deficient yeast cells are viable and exhibit a reduced replicative life span. This study defines the genetic interactome of prohibitins in yeast using synthetic genetic arrays, and identifies 35 genetic interactors of prohibitins (GEP genes) required for cell survival in the absence of prohibitins. Proteins encoded by these genes include members of a conserved protein family, Ups1 and Gep1, which affect the processing of the dynamin-like GTPase Mgm1 and thereby modulate cristae morphogenesis. Ups1 and Gep1 regulate the levels of cardiolipin and phosphatidylethanolamine in mitochondria in a lipid-specific but coordinated manner. Lipid profiling by mass spectrometry of GEP-deficient mitochondria reveals a critical role of cardiolipin and phosphatidylethanolamine for survival of prohibitin-deficient cells. It is proposed that prohibitins control inner membrane organization and integrity by acting as protein and lipid scaffolds (Osman, 2009).

Ups1p and Ups2p antagonistically regulate cardiolipin metabolism in mitochondria

Cardiolipin, a unique phospholipid composed of four fatty acid chains, is located mainly in the mitochondrial inner membrane (IM). Cardiolipin is required for the integrity of several protein complexes in the IM, including the TIM23 translocase, a dynamic complex which mediates protein import into the mitochondria through interactions with the import motor presequence translocase-associated motor (PAM). This study reports that two homologous intermembrane space proteins, Ups1p and Ups2p, control cardiolipin metabolism and affect the assembly state of TIM23 and its association with PAM in an opposing manner. In ups1Delta mitochondria, cardiolipin levels were decreased, and the TIM23 translocase showed altered conformation and decreased association with PAM, leading to defects in mitochondrial protein import. Strikingly, loss of Ups2p restored normal cardiolipin levels and rescued TIM23 defects in ups1Delta mitochondria. Furthermore, synthetic growth defects were observed in ups mutants in combination with loss of Pam17p, which controls the integrity of PAM. These findings provide a novel molecular mechanism for the regulation of cardiolipin metabolism (Tamura, 2009).

A novel family of mitochondrial proteins is represented by the Drosophila genes slmo, preli-like and real-time

Mitochondria play essential roles in development and disease. The characterisation of mitochondrial proteins is therefore of particular importance. The slowmo (slmo) gene of Drosophila has been shown to encode a novel type of mitochondrial protein, and is essential in the developing central nervous system. The Slmo protein contains a conserved PRELI/MSF1pprime domain, found in proteins from a wide variety of eukaryotic organisms. However, the function of the proteins of this family is currently unknown. In this study, the evolutionary relationships between members of the PRELI/MSF1pprime family are described, and the first analysis of two novel Drosophila genes predicted to encode proteins of this type is presented. The first of these, preli-like (prel), is expressed ubiquitously during embryonic development, while the second, real-time (retm), is expressed dynamically in the developing gut and central nervous system. retm encodes a member of a novel conserved subclass of larger PRELI/MSF1pprime domain proteins, which also contain the CRAL-TRIO motif thought to mediate the transport of small hydrophobic ligands. Evidence is provided that, like Slmo, both the Prel and Retm proteins are localised to the mitochondria, indicating that the function of the PRELI/MSF1pprime domain is specific to this organelle (Dee, 2005).

Search PubMed for articles about Drosophila Prel

Chan, D. C. (2006a). Mitochondria: Dynamic organelles in disease, aging, and development. Cell 125: 1241-1252. PubMed ID: 16814712

Chan, D. C. (2006b). Mitochondrial fusion and fission in mammals. Annu. Rev. Cell Dev. Biol. 22: 79-99. PubMed ID: 16704336

Chihara, T., Luginbuhl, D. and Luo, L. (2007). Cytoplasmic and mitochondrial protein translation in axonal and dendritic terminal arborization. Nat. Neurosci. 10: 828-837. PubMed ID: 17529987

Cipolat, S., Rudka, T., Hartmann, D., Costa, V., Serneels, L., Craessaerts, K., Metzger, K., Frezza, C., Annaert, W., D'Adamio, L. et al. (2006). Mitochondrial rhomboid PARL regulates cytochrome c release during apoptosis via OPA1-dependent cristae remodeling. Cell 126: 163-175. PubMed ID: 16839884

Dee, C. T. and Moffat, K. G. (2005). A novel family of mitochondrial proteins is represented by the Drosophila genes slmo, preli-like and real-time. Dev. Genes Evol. 215(5): 248-54. PubMed ID: 15700158

Detmer, S. A. and Chan, D. C. (2007). Functions and dysfunctions of mitochondrial dynamics. Nat. Rev. Mol. Cell Biol. 8: 870-879. PubMed ID: 17928812

Frezza, C., Cipolat, S., Martins de Brito, O., Micaroni, M., Beznoussenko, G. V., Rudka, T., Bartoli, D., Polishuck, R. S., Danial, N. N., De Strooper, B. et al. (2006). OPA1 controls apoptotic cristae remodeling independently from mitochondrial fusion. Cell 126: 177-189. PubMed ID: 16839885

Joshi, A. S., Zhou, J., Gohil, V. M., Chen, S. and Greenberg, M. L. (2009). Cellular functions of cardiolipin in yeast. Biochim. Biophys. Acta 1793: 212-218. PubMed ID: 18725250

Knott, A. B., Perkins, G., Schwarzenbacher, R. and Bossy-Wetzel, E. (2008). Mitochondrial fragmentation in neurodegeneration. Nat. Rev. Neurosci. 9: 505-518. PubMed ID: 18568013

Labrousse, A. M., Zappaterra, M. D., Rube, D. A. and van der Bliek, A. M. (1999). C. elegans dynamin-related protein DRP-1 controls severing of the mitochondrial outer membrane. Mol. Cell 4: 815-826. PubMed ID: 10619028

Li, Z., Okamoto, K., Hayashi, Y. and Sheng, M. (2004). The importance of dendritic mitochondria in the morphogenesis and plasticity of spines and synapses. Cell 119: 873-887. PubMed ID: 15607982

Lin, M. T. and Beal, M. F. (2006). Mitochondrial dysfunction and oxidative stress in neurodegenerative diseases. Nature 443: 787-795. PubMed ID: 17051205

Mast, J. D., Tomalty, K. M., Vogel, H. and Clandinin, T. R. (2008). Reactive oxygen species act remotely to cause synapse loss in a Drosophila model of developmental mitochondrial encephalopathy. Development 135: 2669-2679. PubMed ID: 18599508

Meeusen, S., DeVay, R., Block, J., Cassidy-Stone, A., Wayson, S., McCaffery, J. M. and Nunnari, J. (2006). Mitochondrial inner-membrane fusion and crista maintenance requires the dynamin-related GTPase Mgm1. Cell 127: 383-395. PubMed ID: 17055438

Okamoto, K. and Shaw, J. M. (2005). Mitochondrial morphology and dynamics in yeast and multicellular eukaryotes. Annu. Rev. Genet. 39: 503-536. PubMed ID: 16285870

Olichon, A., Guillou, E., Delettre, C., Landes, T., Arnaune-Pelloquin, L., Emorine, L. J., Mils, V., Daloyau, M., Hamel, C., Amati-Bonneau, P. et al. (2006). Mitochondrial dynamics and disease, OPA1. Biochim. Biophys. Acta 1763: 500-509. PubMed ID: 16737747

Osman, C., Haag, M., Potting, C., Rodenfels, J., Dip, P. V., Wieland, F. T., Brugger, B., Westermann, B. and Langer, T. (2009). The genetic interactome of prohibitins: coordinated control of cardiolipin and phosphatidylethanolamine by conserved regulators in mitochondria. J. Cell Biol. 184: 583-596. PubMed ID: 19221197

Overly, C. C., Rieff, H. I. and Hollenbeck, P. J. (1996). Organelle motility and metabolism in axons vs dendrites of cultured hippocampal neurons. J. Cell Sci. 109: 971-980. PubMed ID: 8743944

Santel, A. (2006). Get the balance right: mitofusins roles in health and disease. Biochim. Biophys. Acta 1763: 490-499. PubMed ID: 16574259

Sesaki, H., Dunn, C. D., Iijima, M., Shepard, K. A., Yaffe, M. P., Machamer, C. E. and Jensen, R. E. (2006). Ups1p, a conserved intermembrane space protein, regulates mitochondrial shape and alternative topogenesis of Mgm1p. J. Cell Biol. 173: 651-658. PubMed ID: 16754953

Smirnova, E., Griparic, L., Shurland, D. L. and van der Bliek, A. M. (2001). Dynamin-related protein Drp1 is required for mitochondrial division in mammalian cells. Mol. Biol. Cell 12: 2245-2256. PubMed ID: 11514614

Stowers, R. S., Megeath, L. J., Gorska-Andrzejak, J., Meinertzhagen, I. A., Schwarz, T. L. (2002). Axonal transport of mitochondria to synapses depends on Milton, a novel Drosophila protein. Neuron 36(6): 1063-1077. PubMed ID: 12495622

Tamura, Y., Endo, T., Iijima, M. and Sesaki, H. (2009). Ups1p and Ups2p antagonistically regulate cardiolipin metabolism in mitochondria. J. Cell Biol. 185(6): 1029-45. PubMed ID: 19506038

Tsubouchi, A., (2009). Mitochondrial protein Preli-like is required for development of dendritic arbors and prevents their regression in the Drosophila sensory nervous system. Development 136(22): 3757-66. PubMed ID: 19855018

Verstreken, P., Ly, C. V., Venken, K. J., Koh, T. W., Zhou, Y. and Bellen, H. J. (2005). Synaptic mitochondria are critical for mobilization of reserve pool vesicles at Drosophila neuromuscular junctions. Neuron 47: 365-378. PubMed ID: 16055061

Yarosh, W., Monserrate, J., Tong, J. J., Tse, S., Le P. K., Nguyen, K., Brachmann, C. B., Wallace, D. C. and Huang, T. (2008). The molecular mechanisms of OPA1-mediated optic atrophy in Drosophila model and prospects for antioxidant treatment. PLoS Genet. 4: e6. PubMed ID: 18193945

date revised: 16 May 2010

Home page: The Interactive Fly © 2009 Thomas Brody, Ph.D.

The Interactive Fly resides on the
Society for Developmental Biology's Web server.