Tetraspanin Cd9b and Cxcl12a/Cxcr4b have a synergistic effect on the control of collective cell migration

Collective cell migration is essential for embryonic development and homeostatic processes. During zebrafish development, the posterior lateral line primordium (pLLP) navigates along the embryo flank by collective cell migration. The chemokine receptors, Cxcr4b and Cxcr7b, as well as their cognate ligand, Cxcl12a, are essential for this process. We corroborate that knockdown of the zebrafish cd9 tetraspanin orthologue, cd9b, results in mild pLL abnormalities. Through generation of CRISPR and TALEN mutants, we show that cd9a and cd9b function partially redundantly in pLLP migration, which is delayed in the cd9b single and cd9a; cd9b double mutants. This delay led to a transient reduction in neuromast numbers. Loss of both Cd9a and Cd9b sensitized embryos to reduced Cxcr4b and Cxcl12a levels. Together these results provide evidence that Cd9 modulates collective cell migration of the pLLP during zebrafish development. One interpretation of these observations is that Cd9 contributes to more effective chemokine signalling.


Introduction
Cells can migrate individually or in groups, the latter is known as collective cell migration. During this process, cells exhibit coordinated behaviour, group polarisation and maintain cellcell contacts [1]. This mode of migration is employed during embryonic development in the morphogenesis of multiple organ systems and is also important for effective immune responses. This mechanism contributes to several diseases including metastatic cancer and rheumatoid arthritis [2].
The zebrafish lateral line consists of a series of mechanosensory organs (neuromasts), which are distributed along the lateral surface of the zebrafish body and connected to the central nervous system by afferent axons [3]. This arrangement of innervated neuromasts is achieved by the collective migration of a placodal primordium of approximately 100 cells, arising just posterior to the otic placode [4]. This posterior lateral line primordium (pLLP), then migrates along the horizontal myoseptum to the tip of the tail. During migration, clusters of cells are deposited from the trailing end of the pLLP, which ultimately differentiate into neuromasts. Primordium migration is directed by expression of the chemokine Cxcl12a along the horizontal myoseptum, which is received by two chemokine receptors expressed within the primordium [5][6][7]. Cxcr4b is expressed in the leading two thirds of the primordium, where ligand binding induces Gβ1 signalling and actin polymerisation to promote a migratory phenotype [8]. Cxcr7 is expressed in the anterior-most third and functions as a ligand sink, allowing the formation of a local Cxcl12a gradient. This results in directed migration of the primordium along the Cxcl12a pathway towards the caudal fin [9,10]. There is evidence that the membrane environment of chemokine receptors, including Cxcr4 and Cxcr7, strongly influences their signalling properties. This includes homodimerisation, oligomerisation and heteromerisation with other membrane receptors [11]. In addition, chemokine receptors have been linked to membrane microdomains where the lipid environment strongly modulates function [12].
Tetraspanins are a large family of small integral membrane proteins, which have been shown to organise neighbouring membrane proteins into complexes called tetraspanin enriched microdomains. This is often referred to as the tetraspanin web, as different interactions build to form dynamic signalling networks that often induce similar functional outcomes [13]. Thus, tetraspanins are associated with a wide variety of cellular functions including signalling and cell migration [14]. For example, the tetraspanin CD9 is downregulated in many human cancers including lung, breast and ovarian, and reduced CD9 expression is related to colon cancer metastasis [15]. In particular, CD9 has been shown to regulate, and be regulated by, CXCL12-CXCR4 signalling [16][17][18]. CD9 was found in close proximity to CXCR4 on the membrane of B acute lymphoblastic leukaemia cells in vitro, and enhanced their CXCL12 dependent migration [18]. If regulation of CXCL12/CXCR4 signalling by CD9 is conserved, we hypothesised Cd9 would be strongly expressed in the zebrafish pLLP, where it might modulate Cxcr4 signalling and thus pLLP migration. Indeed microarray experiments have localised the zebrafish cd9b paralogue to the migrating primordium, whilst a morpholino (MO) experiment indicated loss of cd9b altered primordium structure at 36 hours post fertilisation (hpf) and reduced neuromast number at 48 hpf [2].
In this study we aimed to investigate further the role of zebrafish CD9 orthologues in Cxcr4b-mediated pLLP migration, through use of genetic knockouts. We show expression of both cd9a and cd9b paralogues in the primordium and confirmed the abnormal pLL phenotype seen previously with cd9b knockdown. The cd9b mutants showed delayed pLLP migration but did not fully replicate cd9b morphant pLL phenotypes. We therefore generated cd9a; cd9b double knockouts (cd9 dKOs). These showed both reduced migration of the pLLP and increased sensitivity to reduced levels of Cxcr4b and Cxcl12a, supporting a role for CD9 in regulation of Cxcr4 signalling.

Morpholino injection
Antisense morpholino oligonucleotides (MO) were obtained from GeneTools LLC and re-suspended in MilliQ H 2 O to give a stock concentration of 1 mM and injected in one-cell stage embryos. A Flaming/Brown micropipette puller was used to create micro-injection needles from borosilicate glass capillary tubes (0.5 mm inner diameter, Sutter). The PV800 Pneumatic PicoPump, as part of the micro-injection jig, was set up to release the required amount of injection material by adjusting the air pressure and air expulsion time. For the knockdown of cd9b two MOs were designed, a translation blocker with the following sequence (cd9b MO1): 5'-tttatgaggagaaacccaagactga-3' and a splice site blocker (cd9b i2e3) with the following sequence: 5'-aacccctgaacacagagaaacaaca-3', whilst the published mismatch MO was used 5'-tttccctgctgcttatacagcgatg -3' [20]. For knockdown of cxcr4b and cxcl12a, the following sequences were used respectively, 5'-aatgatgctatc gtaaaattccat-3'and 5'-ttgagatccatgtttgcagtgtgaa-3' [21].

Zebrafish mutant production
cd9b mutants were created from WT embryos using transcription activator-like effector nucleases (TALEN) and maintained on an WT background. TALEN (ZGene Biotech Inc., Taiwan) were provided in a pZGB4L vector, targeting the cd9b sequence 5'-ttgctctttatcttca-3'. Injected embryos were outcrossed and sequenced to identify mutations. A frameshifting deletion mutation (c.42_49del) was selected that caused premature termination within the first transmembrane domain (cd9b pg15 allele). CRISPR-Cas9 was used to create cd9a mutants using the gRNA sequence 5'-gagtgtatatcctcattgcgg-3', which targeted the 3rd exon encoding part of the second transmembrane domain. cd9 dKO mutants were created by injecting the above cd9a gRNA and Cas9 RNA into cd9b pg15 embryos. These fish were screened for germline transmission by sequencing and backcrossed to cd9b pg15 mutants. An indel mutation deleting 4bp and inserting 8bp (c.180_187delinsTCGCTATTGTAT; cd9a la61 ) generated a frameshift mutation resulting in a premature stop codon in exon 3, which was predicted to truncate the protein before the large extracellular domain. Heterozygous fish of the same genotype were incrossed and adult F2 fish were genotyped to identify homozygous cd9b pg15 ; cd9a la61 (cd9 dKO).

In situ hybridisation
Embryos were raised at 28˚C in petri dishes containing E3 solution. The E3 was changed daily, and any dead embryos removed. At 30-32 hpf, embryos were anaesthetised using tricaine and dechorionated before returned to fresh E3 solution. At the relevant timepoint embryos were fixed overnight at 4˚C using 4% (w/v) paraformaldehyde (Sigma-Aldrich, UK) in phosphate buffered saline (PBS). Embryos were washed in PBS/0.05% (v/v) Tween 20 (PBST), then put through a methanol/PBS series using 30%, 60% and 100% (v/v) methanol before being stored in 100% methanol (Sigma-Aldrich) at -20˚C. In situ hybridisation (ISH) was carried out as described by [22], except for the embryo digestion with proteinase K, for which 30-32 hpf embryos were digested with 10 mg/ml proteinase K at 20˚C for 22 min. Primers used for PCR generation of the in situ probes are given in Table 1 below. The protocol was conducted with the embryos in 1.5 ml microfuge tubes for the first two days, after which they were held in 12-well plates for staining before transferring back to microfuge tubes for storage. Stained embryos were stored in the dark in 80% (v/v) glycerol.

Time lapse imaging
Time-lapse recording was performed using an inverted Zeiss LSM700 Confocal microscope. All larvae were anaesthetised using 0.02% tricaine and then embedded on their side in 1% low melting agarose (Lonza) on a glass bottom dish (MalTEK Corporation) and covered with E3 supplemented with 0.02% tricaine. Separate Z-stack images covering the depth of the horizontal myoseptum were taken at specified intervals over a specified period and assembled into a final movie at a specified frame rate.

Statistics
Data distribution was first assessed for normality using a D'Agostino-Pearson omnibus K2 normality test on the experimental residuals, as well as creating a histogram of residuals. For normally distributed data, an ANOVA with Dunnet's or Holms-Sidak multiple comparisons tests were used. For non-normally distributed data non-parametric tests, the Mann-Whitney U test or Kruskal-Wallis with Dunn's multiple comparisons test, were used.

Both cd9 zebrafish paralogues are expressed in the lateral line
Zebrafish possess two Cd9 paralogues, Cd9a [NP_997784] and Cd9b [NP_998593] which show 60% and 59% identity to human CD9 respectively and 63% identity to each other using multiple sequence comparison by log-expectation (MUSCLE) [23]. Whole mount in situ hybridisation (WISH) using probes against cd9a and cd9b demonstrated expression of both paralogues in the migrating primordium at 36 hpf (Fig 1A-1D). Expression of both cd9a and cd9b was also observed in recently deposited neuromasts and was retained in neuromasts until at least 5 dpf (Fig 1G-1L).

cd9b morphant phenotype
To evaluate if there was a role for either paralogue in pLLP migration, we initially focused on cd9b as its role in the primordium had been previously reported [2]. Two different MOs were designed to target cd9b; cd9b MO1 was a translation blocking MO which targeted the 5' UTR of cd9b RNA and cd9b i2e3 was a splice blocking MO designed against the intron 2-exon 3 splice site. These MOs were injected independently into 1-cell stage embryos to ensure that they produced the same phenotype. Embryos injected with either cd9b MO1 or cd9b i2e3, but not the mismatch or uninjected embryos, showed a significant decrease in neuromasts deposited (Fig 2). For embryos injected with cd9b MO1, there was a decrease in the percentage of trunk length between the first and last neuromasts ( Fig 2I). This suggests that the primordium is stalling prematurely or migrating more slowly. This recapitulates and expands on the work by Gallardo et al., 2010 which suggested a role for Cd9b in lateral line development [2].

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Tetraspanin Cd9b and Cxcl12a/Cxcr4b have a synergistic effect on the control of collective cell migration cd9b mutant phenotype To confirm this MO result, we created a TALEN mediated knockout of cd9b. A cd9b TALEN pair was designed by ZgeneBio using the program "TEL Effector Nucleotide Targeter 2.0". The TALEN pair was designed to target Exon 1 and predicted to cut in the 1st transmembrane domain ( Fig 3A). The TALEN pair was injected and after 72 hpf a proportion of embryos were genotyped. Injected embryos showed clear mosaicism around the TALEN cut site so embryos were raised to create mosaic adult F0s. F0s transmitting a mutation to offspring were outcrossed with WT fish and the resulting F1 offspring were raised. To create a cd9b homozygous mutant line with a single mutant allele, F1s with a c.42_49del mutation were selected, incrossed and the resulting offspring raised to adulthood. This allele (cd9b pg15 ) was used as it had the largest deletion and caused the earliest nonsense stop codon. The 8bp deletion in the cd9b allele leads to a frameshift in exon 1, changing codon 15 from TTT (Phe) to CAA (Glu), then 22 aberrant amino acids followed by a stop codon (p.Phe15GlufsTer22) ( Fig 3B). cd9b pg15 homozygous mutants are viable and showed normal development. Loss of cd9b in situ signal in the cd9b mutants suggested strong nonsense mediated decay (NMD) of the mutated allele (S1 Fig). Due to the phenotypes seen in cd9b morphants, it was expected that a lateral line phenotype would be seen in cd9b mutants with fewer neuromasts deposited. However, no significant difference was found in the number of neuromasts at 52 hpf (Fig 3). As cd9b is expressed throughout pLL development, migration of the primordium as well as lateral line structure was assessed at 36 hpf. Although the pLL shows no structural abnormalities in cd9b mutants at 52 hpf, it is possible that cd9b mutants show a lateral line phenotype earlier in development

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and have recovered by 52 hpf. At 36 hpf, cd9b mutants show delayed primordium migration, with the percentage of trunk traversed by the primordium reduced in mutants (Fig 3).

Generation of a cd9a mutant
To ascertain a role for Cd9a, we targeted the cd9a gene using CRISPR/Cas9. A gRNA was designed to target cd9a in exon 3 ( Fig 4A) and injected along with RNA encoding Cas9 into

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Tetraspanin Cd9b and Cxcl12a/Cxcr4b have a synergistic effect on the control of collective cell migration WT embryos to generate cd9a KOs. After screening for mosaicism, the CRISPR/Cas9 injected embryos were raised to maturity and screened for germline transmission of the cd9a mutation. F0s were outcrossed with WT fish to generate F1 cd9a single heterozygous mutants carrying an indel mutant allele (c.183_186delinsAT; cd9a ic62 ). This indel led to a frameshift mutation, changing codon 62 from ATT (Iso) to TGC (Cys), then generating 54 aberrant amino acids followed by a premature stop codon before the EC2 domain (p.Iso62Cysfs54Ter) (Fig 4B and  4C). Heterozygous cd9a +/ic62 adults were in-crossed to produce the F2 generation. Adult F2s were viable and fertile and were genotyped to identify homozygous cd9a ic62 KOs. Loss of cd9a in situ signal in the cd9a mutants suggested strong NMD of the mutated allele (S2 Fig). The effect of cd9a KO on pLL development was investigated but overall, there were no major defects. No significant difference was found in the number of neuromasts deposited at 48 hpf or primordium migration at 36 hpf (Fig 4F-4K).

Generation of a cd9a; cd9b double mutants
Due to the redundant nature of tetraspanins, and cd9a having a similar mRNA expression pattern to cd9b, it was speculated that functional redundancy between Cd9a and Cd9b might be masking stronger phenotypes in the single mutants. Hence, we generated cd9a; cd9b double mutants. To do this, the same cd9a gRNA as above (Fig 4) and cas9 mRNA were injected into cd9b pg15 embryos. These fish were screened for mutation of cd9a as above and then backcrossed to cd9b pg15 mutants. An indel mutation in exon 3 was detected (c.180_184delinsTCGC TATTGTAT) that was predicted to lead to a frameshift mutation and an early stop codon, which truncated the protein before the EC2 domain (p.Leu61Alafs9Ter) (Fig 5A-5D). Transheterozygous fish were crossed and adult F2s were genotyped to identify doubly homozygous cd9b pg15 ; cd9a la61 individuals, from now on referred to as cd9 dKOs. Adult cd9 dKOs were viable and fertile and loss of cd9a and cd9b in situ signal suggested strong NMD of the mutated alleles (S3 Fig). It was expected that with both paralogues knocked out, the lateral line phenotype would now be as severe as cd9b morphants with fewer neuromasts deposited. However, no significant difference was found in the number of neuromasts at 48 hpf (Fig 5). Primordium migration was significantly delayed in cd9 dKO embryos at 36 hpf (Fig 5). This matches the phenotype shown by the cd9b pg15 single KOs. To see if the knockout of cd9a la61 had an additional effect on the primordium delay, embryos from cd9b pg15 KOs were compared to those from cd9 dKOs (S4 Fig). The distance the primordium had migrated at 36 hpf was measured and no difference was seen between cd9b pg15 single KO and cd9b dKO.
In order to further investigate the migration and organisation of the primordium in cd9 dKO embryos they were crossed into the cldnb:gfp transgenic line. Under the claudin b promoter GFP is expressed in the lateral line primordium and newly deposited neuromasts [5]. This allowed observation of the primordium migration in real time. Primordium migration

Cd9 interacts with the Cxcr4b/Cxcl12a pathway in the migrating zebrafish lateral line primordium
To investigate if cd9 had an effect on cxcr4b expression in the primordium, cxcr4b WISH was performed but no perturbation of cxcr4b expression was seen in the cd9b KOs (S6 Fig). Quantification showed no significant difference between the expression of cxcr4b in WT and cd9b KOs. To investigate if Cd9 was affecting migration in the zebrafish primordium through the Cxcr4b/Cxcl12a pathway, an experiment using MOs was conducted. Cxcr4b and cxcl12a MOs induce premature stalling of the primordium at concentrations of 1.5 mM and 0.5 mM respectively [21]. 100 μM of cxcr4b MO was found to be the highest concentration injected into WT embryos that did not induce a phenotype. However, this concentration of MO resulted in a further delay of the primordium in the cd9 dKOs (Fig 7A). For cxcl12a the highest concentration of MO that did not induce a phenotype in WT embryos was determined to be 12.5 μM but this had no further effect on the primordium migration in cd9 dKOs (S7 Fig). If the cxcl12a MO concentration was raised to 25 μM then the primordium in both WT and cd9 dKOs was delayed although the primordium in cd9 dKOs was significantly more delayed ( Fig 7B).

Discussion
In this study we investigated the role of Cd9 in the migration of the pLLP in zebrafish development. We first verified previous data showing cd9b expression in the primordium at 30 hpf [2]. We also noticed cd9b was expressed throughout recently deposited neuromasts at 48 hpf and 5 dpf. Our MO knockdown recapitulated previous results from Gallardo et al., 2010. cd9b morphants showed fewer neuromasts deposited, which suggested a role for Cd9b in the development of the lateral line. A homozygous cd9b mutant was created using TALENs but surprisingly, the morphant phenotype was not recapitulated in cd9b homozygous mutants. This lack of phenotype could be due to several reasons. Firstly, the phenotype seen in cd9b morphants could be due to off target effects of the MOs and not due to cd9b knockdown. However, the fact that two different MOs and researchers induced the same phenotype suggests it was not the lack of specificity [2]. Secondly, the Cd9b truncated protein could have some residual function, although ISH results showed a downregulation of mRNA suggesting NMD was occurring. Also, the cd9b mutation aborts the normal sequence after aa15, before the EC2 domain, so any translated protein would be expected to have minimal function. Finally, the cd9b morphant phenotypes may be specific but tetraspanin redundancy could rescue the phenotype in cd9b mutants. Tetraspanins are well known for their redundancy within the tetraspanin family and mouse knockouts of single tetraspanins often appear healthy and viable with mild phenotypes, whereas double tetraspanin knockout mice often show increased numbers and severity of phenotypes [24][25][26][27][28][29][30][31]. In zebrafish, this redundancy may be amplified due to the occurrence of a fish-specific whole genome duplication in teleost fish. This means that many tetraspanins, for which there is one mammalian ortholog, have two paralogs in zebrafish [32][33][34][35][36]. This is true for CD9, as mammals have a single CD9 whereas zebrafish have Cd9a and Cd9b. We demonstrated cd9a to also be expressed in the pLL in a similar pattern to cd9b. Furthermore, NMD of mutated genes, as we have seen in both our mutants, has been shown to invoke an upregulation of closely related genes as a genetic compensation mechanism [37].

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Following this, tetraspanin cd9a was targeted for knockout using CRISPR technology. Cd9a ic62 homozygous mutants and cd9b pg15 cd9a la61 double homozygous mutants were generated. Cd9a KO embryos displayed a normal lateral line phenotype with normal primordium migration and neuromast deposition. Initial analysis of cd9 dKO embryos using the same measurements revealed that the number of neuromasts deposited and spacing were normal yet the distance of primordium migration at 36 hpf was significantly delayed. It was expected that knockout of cd9a would result in a stronger pLL phenotype but cd9 dKOs showed a similar phenotype to the cd9b KOs. Furthermore, when primordium migration of cd9b single KO was compared to that of the cd9 dKOs there was no difference. These results suggest that the different phenotypes seen between the cd9b morphants and mutants are not due to compensation by cd9a. It remains possible that there is compensation by another tetraspanin [38]. Close relations to CD9 are the tetraspanins CD81 and CD63, which have been shown previously to substitute for CD9 in some circumstances. Overexpression of CD81 partially rescued the infertility phenotype seen in CD9 KO mice [39] and it was recently demonstrated that CD9 deletion in human melanoma cells was quickly compensated by CD63 expression upregulation [40]. It would be interesting to perform RNA-seq or SWATH-MS on the cd9 dKOs to identify upregulated RNA and proteins respectively [41].
Videos generated of cd9 dKOs with fluorescent claudin b showed primordium migration and internal organisation were normal. Measurements of the primordium after DAPI stain also revealed the primordium were similar sizes between mutants and WT at 36 hpf. Overall, it seemed the delay in migration at 36 hpf in the cd9 dKO embryos was not due to alterations in the primordium organisation. Therefore, the cd9 dKO phenotype appeared to be a migratory defect that could either be due to a change in the onset of migration, a change in the speed of migration or a combination of the two. CD9 has previously been reported to promote CXCR4b/CXCL12a signalling in mammalian cells so we decided to investigate a potential interference with this chemokine signalling pathway [16][17][18]. Results from the WISH analysis and quantification showed no obvious perturbation of cxcr4b expression in the cd9 KOs. To postulate if there was some interaction between Cd9 and the Cxcr4b/Cxcl12a signalling pathway a MO experiment was performed. It was theorised that a primordium lacking Cd9 would be more susceptible to disruption by sub-functional doses of either cxcr4b or cxcl12a MO than WT. Injection with a sub-functional concentration of 100 μM cxcr4b MO demonstrated that the cd9 dKO larvae were more sensitive to MO treatment compared to WT with significant

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retardation in primordium migration in the dKO. This suggests that there is some interaction between Cd9 and Cxcr4b within the primordium that promotes migration. Injection with cxcl12a MO was more ambiguous as at a sub-functional concentration of 12.5 μM neither the WT nor the cd9 dKO primordium migration was perturbed. When the dose of cxcl12a MO was increased to 25 μM a migratory delay was induced in both genotypes but the delay seen in the cd9 dKO primordium was significantly worse. Despite disruption in the presence of the MO, the primordium was still able to eventually migrate the full length in the cd9 dKO embryos, suggesting that Cd9 is not essential for maintaining Cxcr4b/Cxcl12a signalling but may play a regulatory or buffering role. A further experiment to check for interaction between Cd9 and Cxcl12a/Cxcr4b in the primordium would be to use immunohistochemistry to investigate co-localisation. Unfortunately, there are no tetraspanin, Cxcl12a or Cxcr4b antibodies available in zebrafish. However, two plasmids are available that encode cxcr4b-egfp or cxcl12avenus constructs [42]. An interesting experiment would be to inject embryos with these plasmids and visualise the distribution of fluorescent versions of the proteins in the cd9 dKOs.
We mentioned that the migratory defect could be due to a change in the onset of migration. This could be further investigated by performing claudin b ISHs at the onset and earliest stages of development of the posterior lateral line. On the other hand, to try and determine if Cd9b has a role in the speed of migration, ISHs could also be performed at various time points throughout primordium migration and speed could be calculated in a similar method to previous studies [43,44].
We focused on a connection between Cd9 and Cxcr4b/Cxcl12 signalling in this paper because of previous research linking these proteins. However, Cd9, like other tetraspanins, contributes to cellular processes by organising molecules within the plasma membrane and there are many other signalling pathways involved in primordium development [45,46]. The fundamental four are FGF, Wnt, Notch and chemokine signalling pathways and their specific and coordinated expression ensures the regular morphogenesis and migration of the pLL [47]. Even though the organisation of the primordium at high resolution appears normal, analysis of the expression of these crucial signalling molecules, through WISH or immunohistochemistry, during primordium development could reveal subtle changes in their organisation that may be affecting migration. Whilst Fgf signalling functions to determine rosette formation and deposition, Wnt signalling drives proliferation in the leading zone [46]. Proliferation analysis with BrdU incorporation could show if there are any changes in cell proliferation within the primordium, specifically in the leading edge. CD9 has also been demonstrated to downregulate expression of Wnt signalling pathways in cell culture [48,49]. Another important component of collective cell migration is cell adhesion so migratory cells can pull insensitive cells in the same direction. Within the primordium cadherins mediate cell-cell adhesion between primordium cells and are required for directed and robust migration [50]. Cadherin-dependent intercellular adhesion regulated by tetraspanins plays an important role in tumour invasion and metastasis [51,52]. CD9 has been associated with keratinocyte motility by regulating E-cadherin-mediated cell-cell contacts [53].
The use of computational models has been a valued tool to integrate analysis of the lateral line primordium acquired through microscopic, cellular, molecular and genetic analysis [47,54]. These models allow researchers to edit parameters and observe their influences on the migrating primordium. This could help us choose the most appropriate avenue to explore next by checking what modifications are able to regenerate the phenotype we observe in the cd9 dKO embryos.
In conclusion, we propose that within the primordium Cd9b is functioning to compartmentalise signalling molecules, like Cxcr4b, to orchestrate and amplify signalling upon ligand binding but is not essential. This would explain why only subtle differences are seen between WT and cd9 dKOs.
Supporting information S1 Fig. cd9b is significantly decreased in cd9b KO  The height was measured three times at equal points along the primordium and then averaged. The width of primordium was measured from the two furthest points along the middle of the primordium. Width was then divided by height to generate a ratio. The area was circled using the freehand selection and measured. Cells were counted using the multipoint tool on Image J software. Significance was assessed using an unpaired t test, p = <0.05, N = minimum 11. Bars show mean +/-SD. (TIF) S6 Fig. cxcr4b expression is not altered in the pLLP of cd9 dKO embryos at indicated stages. A-D: Representative images of cxcr4b ISH in the primordium of (a,c) WT and (b,d) cd9 dKO mutants at time shown. Scale bar: 50 μm. E-F: Graphs showing measurements of (e) length and (f) height of cxcr4b expression in the primordium of WT and cd9 KO embryos at indicated stages. Length measurements were taken along the middle of the embryo between the two furthest points of expression within the primordium. Height measurements were taken between the two highest points of expression within the primordium. Significance was assessed using an unpaired T test. N = minimum 13 for 24 hpf and 11 for 30 hpf. Bars show mean +/-SD.