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The Leech Homeobox Gene Lox4 May Determine Segmental Differentiation of Identified Neurons

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The Leech Homeobox Gene Lox4 May Determine Segmental Differentiation of Identified Neurons The Journal of Neuroscience, August 1995, 15(8): 5551-5559 The Leech Homeobox Gene Lox4 May Determine Segmental Differentiation of Identified Neurons Victoria Y. Wong, Gabriel 0. Aisemberg, Wen-Biao Gan, and Eduardo R. Macagno Department of Biological Sciences, Columbia University, New York, New York 10027 We cloned and characterized a new leech homeobox gene, pression and function of regulatory proteins involved in con- Lox4, a homolog of the Drosophila genes Ultrabithorax and trolling early development in the nervous system. abdominal-A. Lox4 has a complex and dynamic pattern of The leech Hirudo medicinalis provides a uniquely advanta- expression within a series of segmentally homologous geous system to study these problems at the level of single, neurons. These include a pair of specialized motor neurons identified neurons. Leeches have a simple CNS comprising a of one segmental ganglion, the rostra1 penile evertors ventral nerve cord with 32 segmentalneuromeres and a supra- (RPEs), and their segmental homologs in other midbody esophagealganglion of nonsegmentalorigin. Four neuromeres ganglia. During gangliogenesis, Lox4 was expressed within fuse to form the head ganglion and seven to form the tail gan- this series of neurons in three different temporal patterns: glion. The remaining 21 neuromeresform the midbody ganglia, (1) it was never expressed in the RPE homologs of ganglia which have about 400 neuronseach (Macagno, 1980). Many of l-3; (2) it was expressed in the RPEs during gangliogene- these neuronscan be reliably identified in the embryo and are sis, but was turned off when these neurons started to dif- amenableto experimental manipulation at early developmental ferentiate after gangliogenesis; and (3) it was expressed in stages.They differentiate in very similar early embryonic en- the RPE homologs of segments 4-5 and 7-21 during gan- vironments, yet many segmentallyhomologous neurons exhibit gliogenesis and the subsequent period of axonogenesis. distinct, segment-specificpatterns of differentiation (Glover and We found that these neurons have three distinct peripheral Mason, 1986; Gao and Macagno, 1987a,b; Jellies et al., 1987; axonal morphologies that correlate with the three temporal Stewart et al., 1991; Jellies et al., 1992; reviewed in French and patterns of expression. Our results suggest that the hom- Kristan, 1994). Although a role for cell-cell interactions during eobox gene Lox4 may determine neuronal identities within development has been demonstratedfor someof theseneurons, this series of segmental homologs. it is likely that certain fates and pathways of differentiation are [Key words: neuron, axon, homeodomain, Hirudo medi- initially determinedby specific, cell-autonomousfactors that in cinalis, Hex genes, CNS, morphology] turn bias the responsesto environmental signals. The Hox/HOM genesand other related homeobox-containing There is more cellular diversity within the nervous systemthan genescode for transcription factors which are highly conserved in any other system in an animal. Although essentialfor under- among a wide variety of species(reviewed in Kappen and Rud- standing nervous systemdevelopment and pathology, the mech- dle, 1993; Krumlauf, 1994). In addition to their early function anismsthat generatethis diversity are extremely complex and, in determining anteroposterior(A-P) positional information, the as a consequence,poorly understood. A useful model for ad- Hox/HOM genes are later expressedat high levels in the ner- dressingsome basic aspectsof this issueis the regional differ- vous system (reviewed in Doe and Scott, 1988; Kessel and Gruss, 1990; Boncinelli et al., 1993), where they are likely to entiation in invertebratesof segmentallyrepeated neurons. These neuronsderive from precursorslocated in similar cellular envi- be involved in the determination of neuronal identities. ronments, but neverthelessacquire unique identities. Although Previous work in this laboratory hasdetermined that leechHex/ both cell-cell interactions and autonomous factors appear to HOM genesare expressedin segmentallyrepeated subsets of neu- contribute to their different developmentalfates, the specific in- rons within their particular A-P domains(Wysocka-Diller et al., fluence of the environment on neuronal identity dependslargely 1989; Aisemberg et al., 1993, 1995; Aisemberg and Macagno, on the intrinsic properties of an individual cell. To gain a better 1994).We presenthere the characterizationof Lo,&, a leechHex/ understandingof the function of autonomousfactors in neuronal HOM gene that displays a complex and dynamic pattern of ex- differentiation, a fruitful approach has been to examine the ex- pressionin the CNS and offers a very interestingmodel for the study of differentiation within a seriesof segmentallyhomologous neurons.The results we present here suggestthat Lox4 plays a Received Feb. 16, 1995; revised Apr. 14, 1995; accepted Apr. 20, 1995. key role in the specificationof three different cell fates amongat We thank Laura Wolszon for her critical reading of the manuscript, Tim least one group of segmentallyhomologous neurons. Gershon for removing the male organ from experimental embryos, Mike Ni- tabach for immunizing the rabbits and for helpful discussions, and Nik Necles Materials and Methods for photographic work. We also thank Marty-Shankland for his help with the initial in sine hybridization experiments and Anne Duggan for useful comments Animals. Embryosobtained from our colony of Hirudo medicinalis on an earlier version of the manuscript. Lastly, we thank J.-L. Picard and K. were maintainedat 23°Cwithin their cocoonsuntil embryonicday 8 Janeway. This work was supported by NIH Grant HD-20954. (E8), and in sterileartificial springwater (0.5 gm/liter InstantOcean, Correspondence should be addressed to Gabriel 0. Aisemberg at the above AquariumSystems, Sarrebourg, France) afterwards. Under these con- address. ditions, embryonic development is completed in about 30 d. Copyright 0 1995 Society for Neuroscience 0270-6474/95/155551-09$05.00/O Cloning and sequencing. The Lox4 homeobox was isolated by PCR 5552 Wong et al. - Lox4 May Determine Neuronal Differentiation amplification from genomic DNA. A reaction mixture of 100 p,l con- 260. A probe derived from this cDNA clone, which was as- tained 10 mu Tris-HCl, pH 8.3, 50 mu KCl, 3 mu MgCl,, 200 p,M sumed to contain the entire coding region of Lox4, detected a dGTP, 200 JLM dCTP, 200 PM dATP 200 PM dTTP, 0.8 pg of each oligonucleotide primer, 5 pg of leech DNA, and 2.5 U of Taq DNA transcript of about 3.4 kb in both ElO-El3 embryos and ju-, polymerase (Perkin-Elmer Cetus, Norwalk, CT). After 1 min at 94”C, venile leeches(Fig. 1B). the sample was subjected to 10 cycles of 94°C 35°C and 72°C (1 min The Lox4 protein contained an Antennapedia- (Antp-) class at each temperature), followed by 25 cycles in which the annealing homeodomain(for review, seeBiirglin, 1993). The recognition temperature was increased from 35°C to 50°C and a final 5 min exten- sion at 72°C. The sequences of the primers were GAA/GC/TTGGAA/ helix (helix 3) of Lox4 was identical to the one of Antp-class GAAA/GGAA/GTT for the 5’ primer and CT/GCCT/GG/ATTT/CTGG/ homeodomains(Fig. 1C). In addition, Lox4 containeda YPWM AAACCA for the 3’ primer. The amplified homeobox fragments of 120 motif that is found upstreamof the homeodomainin most Antp- bp were cloned into the vector pIBI31 (International Biotechnologies, classgenes (McGinnis and Krumlauf, 1992). The highestdegree Inc., New Haven, CT). Individual clones were sequenced with the Se- of homeodomainhomology to Lox4 occurred in Hox genesof quenase kit (U.S. Biochemicals, Cleveland, OH). The Lox4 fragment was used as a probe to screen, at high stringency, groups 6 and 7 (Scott, 1992), with a 52-53/60 amino acid iden- a XZap (Stratagene, La Jolla, CA) cDNA library of late Hirudo embryos tity. (a generous gift of Dr. James Boulter, The Salk Institute). Two positive The Lox4 homeobox also showedhigh homology to the hom- clones were isolated from about 1 X lo6 plaques. They were plaque- eoboxes of leech Lox2 (Wysocka-Diller et al., 1989), and Dro- purified and excised in vivo according to the manufacturer’s instructions, and found to contain inserts of 2.4 and 1.7 kb. A series of nested de- sophila Ultrabithorax (Ubx; Kornfeld et al., 1988) and abdomi- letions of the longest Lox4 cDNA clone were generated using the Erase- nal-A (abd-A; Karch et al., 1990), with 52/60, 51/60, and 52/60 a-Base system (Promega, Madison, WI) and both strands of this Lox4 amino acid identities, respectively. The homology of Lox4 to cDNA clone were sequenced. Lox2, Ubx, and abd-A was found to extend to a sequencead- RNA analysis. PolyA+ RNA was purified by oligo(dT)-cellulose chro- matography from juvenile (lo-30 d after hatching) and embryonic jacent to the C-end of the homeodomain(Fig. 1C). This previ- (ElO-E13) leeches, separated in an agarose gel containing formalde- ously describedconserved motif (Wysocka-Diller et al., 1989) hyde (about 5 pg of polyA+ RNA per lane), transferred onto a nylon has not been found in any other arthropod or leech homeobox membrane, and hybridized at high stringency to a 32P-labeled antisense genes. Within this 8-amino acid-long region, Lox4 had seven riboprobe synthesized from the 2.4 kb Lox4 cDNA clone, as described amino acid identities to Ubx, and four to abd-A. This high de- in Aisemberg and Macagno (1994). For in situ hybridization, a digox- igenin-labeled Lox4 riboprobe was used to probe whole Hirudo embry- gree of similarity in and downstream from the homeodomain os under essentially the same conditions as described in Nardelli-Hae- suggeststhat Lox2 and Lox4 are leech homologsof the arthropod fliger and Shankland (1992). genesUbx and abd-A. Generation qf a Lox4 antibody and antibody stuininR of embryos. A Lox2 and Lox4 shared additional homology within the up- fragment containing the entire &x4 open reading frame was amplified streamregion adjacent to the homeodomain(about 50% homol- using PCR and subcloned into the BamHI site located upstream of the glutathione S-transferase gene (GST) in the pGEX-2T vector (Smith ogy within a 20 amino acid stretch).
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