Structure of a VirD4 coupling protein bound to a VirB type IV secretion machinery.

Type IV secretion (T4S) systems are versatile bacterial secretion systems mediating transport of protein and/or DNA T4S systems are generally composed of 11 VirB proteins and 1 VirD protein (VirD4). The VirB1-11 proteins assemble to form a secretion machinery and a pilus while the VirD4 protein is responsible for substrate recruitment. The structure of VirD4 in isolation is known; however, its structure bound to the VirB1-11 apparatus has not been determined. Here, we purify a T4S system with VirD4 bound, define the biochemical requirements for complex formation and describe the protein-protein interaction network in which VirD4 is involved. We also solve the structure of this complex by negative stain electron microscopy, demonstrating that two copies of VirD4 dimers locate on both sides of the apparatus, in between the VirB4 ATPases. Given the central role of VirD4 in type IV secretion, our study provides mechanistic insights on a process that mediates the dangerous spread of antibiotic resistance genes among bacterial populations.

Chlamydia exploits filopodial capture and a macropinocytosis-like pathway for host cell entry.

Pathogens hijack host endocytic pathways to force their own entry into eukaryotic target cells. Many bacteria either exploit receptor-mediated zippering or inject virulence proteins directly to trigger membrane reorganisation and cytoskeletal rearrangements. By contrast, extracellular C. trachomatis elementary bodies (EBs) apparently employ facets of both the zipper and trigger mechanisms and are only ~400 nm in diameter. Our cryo-electron tomography of C. trachomatis entry revealed an unexpectedly diverse array of host structures in association with invading EBs, suggesting internalisation may progress by multiple, potentially redundant routes or several sequential events within a single pathway. Here we performed quantitative analysis of actin organisation at chlamydial entry foci, highlighting filopodial capture and phagocytic cups as dominant and conserved morphological structures early during internalisation. We applied inhibitor-based screening and employed reporters to systematically assay and visualise the spatio-temporal contribution of diverse endocytic signalling mediators to C. trachomatis entry. In addition to the recognised roles of the Rac1 GTPase and its associated nucleation-promoting factor (NPF) WAVE, our data revealed an additional unrecognised pathway sharing key hallmarks of macropinocytosis: i) amiloride sensitivity, ii) fluid-phase uptake, iii) recruitment and activity of the NPF N-WASP, and iv) the localised generation of phosphoinositide-3-phosphate (PI3P) species. Given their central role in macropinocytosis and affinity for PI3P, we assessed the role of SNX-PX-BAR family proteins. Strikingly, SNX9 was specifically and transiently enriched at C. trachomatis entry foci. SNX9-/- cells exhibited a 20% defect in EB entry, which was enhanced to 60% when the cells were infected without sedimentation-induced EB adhesion, consistent with a defect in initial EB-host interaction. Correspondingly, filopodial capture of C. trachomatis EBs was specifically attenuated in SNX9-/- cells, implicating SNX9 as a central host mediator of filopodial capture early during chlamydial entry. Our findings identify an unanticipated complexity of signalling underpinning cell entry by this major human pathogen, and suggest intriguing parallels with viral entry mechanisms.

One Face of Chlamydia trachomatis: The Infectious Elementary Body.

The lifestyle of Chlamydiae is unique: the bacteria alternate between two morphologically distinct forms, an infectious non-replicative elementary body (EB), and a replicative, non-infectious reticulate body (RB). This review focuses on recent advances in understanding the structure and function of the infectious form of the best-studied member of the phylum, the human pathogen Chlamydia trachomatis. Once considered as an inert particle of little functional capacity, the EB is now perceived as a sophisticated entity that encounters at least three different environments during each infectious cycle. We review current knowledge on its composition and morphology, and emerging metabolic activities. These features confer resistance to the extracellular environment, the ability to penetrate a host cell and ultimately enable the EB to establish a niche enabling bacterial survival and growth. The bacterial and host molecules involved in these processes are beginning to emerge.

Membrane contact sites between pathogen-containing compartments and host organelles.

Intracellular pathogens survive and replicate within specialised membrane-bound compartments that can be considered as pseudo-organelles. Using the obligate intracellular bacterium Chlamydia as an illustrative example, we consider the modes of lipid transport between pathogen-containing compartments and host organelles, including the formation of static membrane contact sites. We discuss how lipid scavenging can be mediated via the reprogramming of cellular transporters at these interfaces and describe recent data suggesting that pathogen effectors modulate the formation of specific membrane contacts. Further study of these emerging mechanisms is likely to yield new insights into the cell biology of lipid transport and organelle communication, which highlights potential new targets and strategies for future therapeutics. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

A Chlamydia effector recruits CEP170 to reprogram host microtubule organization.

The obligate intracellular bacterial pathogen Chlamydia trachomatis deploys virulence effectors to subvert host cell functions enabling its replication within a specialized membrane-bound compartment termed an inclusion. The control of the host cytoskeleton is crucial for Chlamydia uptake, inclusion biogenesis and cell exit. Here, we demonstrate how a Chlamydia effector rearranges the microtubule (MT) network by initiating organization of the MTs at the inclusion surface. We identified an inclusion-localized effector that is sufficient to interfere with MT assembly, which we named inclusion protein acting on MTs (IPAM). We established that IPAM recruits and stimulates the centrosomal protein 170 kDa (CEP170) to hijack the MT organizing functions of the host cell. We show that CEP170 is essential for chlamydial control of host MT assembly, and is required for inclusion morphogenesis and bacterial infectivity. Together, we demonstrate how a pathogen effector reprograms the host MT network to support its intracellular development.

Pathogen-host reorganization during Chlamydia invasion revealed by cryo-electron tomography.

Invasion of host cells is a key early event during bacterial infection, but the underlying pathogen-host interactions are yet to be fully visualized in three-dimensional detail. We have captured snapshots of the early stages of bacterial-mediated endocytosis in situ by exploiting the small size of chlamydial elementary bodies (EBs) for whole-cell cryo-electron tomography. Chlamydiae are obligate intracellular bacteria that infect eukaryotic cells and cause sexually transmitted infections and trachoma, the leading cause of preventable blindness. We demonstrate that Chlamydia trachomatis LGV2 EBs are intrinsically polarized. One pole is characterized by a tubular inner membrane invagination, while the other exhibits asymmetric periplasmic expansion to accommodate an array of type III secretion systems (T3SSs). Strikingly, EBs orient with their T3SS-containing pole facing target cells, enabling the T3SSs to directly contact the cellular plasma membrane. This contact induces enveloping macropinosomes, actin-rich filopodia and phagocytic cups to zipper tightly around the internalizing bacteria. Once encapsulated into tight early vacuoles, EB polarity and the T3SSs are lost. Our findings reveal previously undescribed structural transitions in both pathogen and host during the initial steps of chlamydial invasion.

Chlamydiae assemble a pathogen synapse to hijack the host endoplasmic reticulum.

Chlamydiae are obligate intracellular bacterial pathogens that replicate within a specialized membrane-bound compartment, termed an 'inclusion'. The inclusion membrane is a critical host-pathogen interface, yet the extent of its interaction with cellular organelles and the origin of this membrane remain poorly defined. Here we show that the host endoplasmic reticulum (ER) is specifically recruited to the inclusion, and that key rough ER (rER) proteins are enriched on and translocated into the inclusion. rER recruitment is a Chlamydia-orchestrated process that occurs independently of host trafficking. Generation of infectious progeny requires an intact ER, since ER vacuolation early during infection stalls inclusion development, whereas disruption post ER recruitment bursts the inclusion. Electron tomography and immunolabelling of Chlamydia-infected cells reveal 'pathogen synapses' at which ordered arrays of chlamydial type III secretion complexes connect to the inclusion membrane only at rER contact sites. Our data show a supramolecular assembly involved in pathogen hijack of a key host organelle.

What can a morphometric study of unoperated children teach us about the natural history of metopic synostosis?

OBJECTIVE: Outcomes of surgical repair of trigonocephaly are well reported in the literature, but there is a paucity of information on the natural history of unoperated children. The authors evaluated a group of unoperated children with metopic synostosis to describe the natural change in head shape over time. METHODS: A database was screened for scans of children with unoperated trigonocephaly (2010-2021). Multisuture cases and those with a metopic ridge were excluded. Three-dimensional surface scans (3D stereophotogrammetry/CT) were used for morphological analysis. Nine previously published parameters were used: frontal angle (FA30°), anteroposterior (AP) volume ratio (APVR), AP area ratio (APAR), AP width ratios 1 and 2 (APWR1 and APWR2), and 4 AP diagonal ratios (30° right APDR [rAPDR30], 30° left APDR [lAPDR30], 60° right APDR [rAPDR60], and 60° left APDR [lAPDR60]). RESULTS: Ninety-seven scans were identified from a cohort of 316 patients with a single metopic suture, in which the male-to-female ratio was 2.7:1. Ages at the time of the scan ranged from 9 days to 11 years and were stratified into 4 groups: group 1, < 6 months; group 2, 6-12 months; group 3, 1-3 years; and group 4, > 3 years. Significant improvements were detected in 5 parameters (APVR, APAR, APWR1, rAPDR30, and lAPDR30) over time, whereas no significant differences were found in FA30, APWR2, rAPDR60, and lAPDR60 between age groups. CONCLUSIONS: Forehead shape (surface area and volume), as well as narrowing and anterolateral contour at the frontal points, differed significantly over time without surgery. However, forehead angulation, narrowing, and anterolateral contour at temporal points did not show significant differences. This knowledge will aid in surgical and parental decision-making.

Monobloc and bipartition in craniofacial surgery.

BACKGROUND: The frontofacial monobloc advancement with osteogenic distraction is increasingly used as a surgical treatment for children with complex craniosynostosis-associated syndromes. However, the subfrontal osteotomy cuts to free the facial skeleton from the skull base require extradural retraction of the frontal lobes. The purpose of this study was to determine the frequency and degree of radiologically identifiable frontal lobe changes and whether any such changes affected the patients' outcome. METHODS: The clinical records and preoperative and postoperative computed tomography imaging of all patients undergoing monobloc frontofacial distraction advancement (with or without bipartition) were reviewed. A retrospective medical notes review was undertaken to assess any patient or surgically related factors that might predispose to frontal lobe changes and evaluate outcome from surgery. Where available, magnetic resonance imaging scans were reviewed to compare outcome with that on computed tomography. RESULTS: Fifty cases were identified as suitable for the study. Eighteen patients (36%) had no frontal lobe changes. Thirty-two cases (64%) did have changes that appeared related to the position of maximum retraction during subfrontal osteotomy cuts. There were no changes in the incidence/extent of these changes over time or of any link to the patients' diagnosis, age at surgery, phenotype severity, surgery type, or any surgical or postoperative adverse events. We found no evidence that these changes were responsible for neurologic problems (eg, epilepsy) or reduced cognitive function. CONCLUSIONS: This study reveals a high incidence of frontal lobe changes demonstrable on neuroimaging following the frontofacial monobloc procedure reflecting the retraction points during surgery. Although no postoperative disability was reported, it is clearly important to consider more detailed neuropsychologic testing and review current surgical techniques to ensure that such changes are kept to a minimum.

Chlamydia trachomatis Slc1 is a type III secretion chaperone that enhances the translocation of its invasion effector substrate TARP.

Bacterial type III secretion system (T3SS) chaperones pilot substrates to the export apparatus in a secretion-competent state, and are consequently central to the translocation of effectors into target cells. Chlamydia trachomatis is a genetically intractable obligate intracellular pathogen that utilizes T3SS effectors to trigger its entry into mammalian cells. The only well-characterized T3SS effector is TARP (translocated actin recruitment protein), but its chaperone is unknown. Here we exploited a known structural signature to screen for putative type III secretion chaperones encoded within the C. trachomatis genome. Using bacterial two-hybrid, co-precipitation, cross-linking and size exclusion chromatography we show that Slc1 (SycE-like chaperone 1; CT043) specifically interacts with a 200-amino-acid residue N-terminal region of TARP (TARP¹â»²°°). Slc1 formed homodimers in vitro, as shown in cross-linking and gel filtration experiments. Biochemical analysis of an isolated Slc1-TARP¹â»²°° complex was consistent with a characteristic 2:1 chaperone-effector stoichiometry. Furthermore, Slc1 was co-immunoprecipitated with TARP from C. trachomatis elementary bodies. Also, coexpression of Slc1 specifically enhanced host cell translocation of TARP by a heterologous Yersinia enterocolitica T3SS. Taken together, we propose Slc1 as a chaperone of the C. trachomatis T3SS effector TARP.

Estimated Cost of Transcarotid Arterial Revascularization Compared With Carotid Endarterectomy and Transfemoral Carotid Stenting.

Objectives Transcarotid arterial revascularization (TCAR) is associated with a lower risk of stroke or death than transfemoral carotid artery stenting (TF-CAS). TCAR infers a lower risk of cranial nerve injury and a similar risk of myocardial infarction (MI) than carotid endarterectomy (CEA). There have been no comparative studies on the cost of TCAR, TF-CAS, and CEA, which may have important implications for institutional support for the new modality to address carotid artery stenosis. Our aim was to compare the estimated cost profiles of TCAR, TF-CAS, and CEA. Methods A review was performed on Medicare patients who underwent TCAR, TF-CAS, or CEA between January 1, 2020, and December 31, 2020. Demographics, comorbidities, operative details, and postoperative complications were reviewed. Acute stroke presentations and elective procedures were included. Cost data were obtained from the hospital's finance department. Quantitative variables were compared using analysis of variance, and categorical variables were compared using the chi-square analysis. Results In total, 21 TCAR, 97 TF-CAS, and 26 CEA patients were initially identified. After removing the non-Medicare patients, 17 TCAR, 57 TF-CAS, and 13 CEA patients were included in the analysis. In-hospital stroke, MI, and mortality included three deaths in TF-CAS patients. At 30 days, the stroke rates for TCAR, TF-CAS, and CEA groups were 0%, 1.8%, and 0%, respectively. The payments for TCAR, TF-CAS, and CEA were $15,400 ± 2,100, $23,400 ± 11,800 and $14,300 ± 5,700 (p=0.001), respectively. The estimated costs for TCAR, TF-CAS, and CEA were $10,500 ± 3,300, $13,800 ± 14,300, and $12,400 ± 6,000 (p=0.575), respectively. The profit margins for TCAR, TF-CAS, and CEA were $5,100 ± 3,100, $9,600 ± 12,100, and $1,900 ± 6,400 (p=0.032), respectively. There was no significant difference in American Society of Anesthesiologists (ASA) scores (p=0.635) or age (p=0.485) among the three groups. The length of hospital stay was not significantly different (p=0.107). The TF-CAS maintained the highest profit margin (p<0.001) when matched for the same diagnosis-related code (without complications or comorbidities). Urgency classification within the TF-CAS group included 45 elective, four urgent, and eight emergent cases. The profit margin was significantly higher for the elective group than for the emergent group (p=0.002) but not different for elective versus urgent (p=0.503) or urgent versus emergent (p=0.102). All patients who underwent TCAR and CEA were elective. Conclusion The hospital reimbursement and profit margins are higher for TF-CAS than for TCAR. With the increasing data now demonstrating similar outcomes with TF-CAS and CEA, further research is required to examine the long-term cost-effectiveness of TCAR and how this will compare to TF-CAS.

Phosphorylation of the enteropathogenic E. coli receptor by the Src-family kinase c-Fyn triggers actin pedestal formation.

Enteropathogenic Escherichia coli (EPEC) causes diarrhoeal disease worldwide. Pathogen adherence to host cells induces reorganization of the actin cytoskeleton into 'pedestal-like' pseudopods beneath the extracellular bacteria. This requires two bacterial virulence factors that mimic a ligand-receptor interaction. EPEC delivers its own receptor, the translocated intimin receptor (Tir), into the target cell plasma membrane, which is phosphorylated on interaction with the bacterial surface protein intimin. Tir phosphorylated on Tyr 474 (ref. 4) binds the cellular adaptor Nck, triggering actin polymerization. Nevertheless, despite its critical role, the mechanism of Tir Tyr 474 phosphorylation remains unknown. Here, by artificially uncoupling Tir delivery and activity, we show that Tir phosphorylation and Nck-dependent pedestal formation require the Src-family kinase (SFK) c-Fyn. SFK inhibitors prevent Tyr 474 phosphorylation, and cells lacking c-fyn are resistant to pedestal formation. c-Fyn exclusively phosphorylates clustered Tir in vitro, and kinase knockdown suppresses Tir phosphorylation and pedestal formation in cultured cells. These results identify the transient interaction with host c-Fyn as a pivotal link between bacterial Tir and the cellular Nck-WASP-Arp2/3 cascade, illuminating a tractable experimental system in which to dissect tyrosine kinase signalling.

Chlamydia Uses K+ Electrical Signalling to Orchestrate Host Sensing, Inter-Bacterial Communication and Differentiation.

Prokaryotic communities coordinate quorum behaviour in response to external stimuli to control fundamental processes including inter-bacterial communication. The obligate intracellular bacterial pathogen Chlamydia adopts two developmental forms, invasive elementary bodies (EBs) and replicative reticulate bodies (RBs), which reside within a specialised membrane-bound compartment within the host cell termed an inclusion. The mechanisms by which this bacterial community orchestrates different stages of development from within the inclusion in coordination with the host remain elusive. Both prokaryotic and eukaryotic kingdoms exploit ion-based electrical signalling for fast intercellular communication. Here we demonstrate that RBs specifically accumulate potassium (K+) ions, generating a gradient. Disruption of this gradient using ionophores or an ion-channel inhibitor stalls the Chlamydia lifecycle, inducing persistence. Using photobleaching approaches, we establish that the RB is the master regulator of this [K+] differential and observe a fast K+ exchange between RBs revealing a role for this ion in inter-bacterial communication. Finally, we demonstrate spatio-temporal regulation of bacterial membrane potential during RB to EB differentiation within the inclusion. Together, our data reveal that Chlamydia harnesses K+ to orchestrate host sensing, inter-bacteria communication and pathogen differentiation.

Deciphering interplay between Salmonella invasion effectors.

Bacterial pathogens have evolved a specialized type III secretion system (T3SS) to translocate virulence effector proteins directly into eukaryotic target cells. Salmonellae deploy effectors that trigger localized actin reorganization to force their own entry into non-phagocytic host cells. Six effectors (SipC, SipA, SopE/2, SopB, SptP) can individually manipulate actin dynamics at the plasma membrane, which acts as a 'signaling hub' during Salmonella invasion. The extent of crosstalk between these spatially coincident effectors remains unknown. Here we describe trans and cisbinary entry effector interplay (BENEFIT) screens that systematically examine functional associations between effectors following their delivery into the host cell. The results reveal extensive ordered synergistic and antagonistic relationships and their relative potency, and illuminate an unexpectedly sophisticated signaling network evolved through longstanding pathogen-host interaction.

Repetitive N-WASP-binding elements of the enterohemorrhagic Escherichia coli effector EspF(U) synergistically activate actin assembly.

Enterohemorrhagic Escherichia coli (EHEC) generate F-actin-rich adhesion pedestals by delivering effector proteins into mammalian cells. These effectors include the translocated receptor Tir, along with EspF(U), a protein that associates indirectly with Tir and contains multiple peptide repeats that stimulate actin polymerization. In vitro, the EspF(U) repeat region is capable of binding and activating recombinant derivatives of N-WASP, a host actin nucleation-promoting factor. In spite of the identification of these important bacterial and host factors, the underlying mechanisms of how EHEC so potently exploits the native actin assembly machinery have not been clearly defined. Here we show that Tir and EspF(U) are sufficient for actin pedestal formation in cultured cells. Experimental clustering of Tir-EspF(U) fusion proteins indicates that the central role of the cytoplasmic portion of Tir is to promote clustering of the repeat region of EspF(U). Whereas clustering of a single EspF(U) repeat is sufficient to bind N-WASP and generate pedestals on cultured cells, multi-repeat EspF(U) derivatives promote actin assembly more efficiently. Moreover, the EspF(U) repeats activate a protein complex containing N-WASP and the actin-binding protein WIP in a synergistic fashion in vitro, further suggesting that the repeats cooperate to stimulate actin polymerization in vivo. One explanation for repeat synergy is that simultaneous engagement of multiple N-WASP molecules can enhance its ability to interact with the actin nucleating Arp2/3 complex. These findings define the minimal set of bacterial effectors required for pedestal formation and the elements within those effectors that contribute to actin assembly via N-WASP-Arp2/3-mediated signaling pathways.

Clustering transfers the translocated Escherichia coli receptor into lipid rafts to stimulate reversible activation of c-Fyn.

Enteropathogenic Escherichia coli (EPEC) mimic a ligand-receptor interaction to induce 'pedestal-like' pseudopodia on mammalian cells, providing a tractable system to study tyrosine kinase signalling to the actin cytoskeleton. EPEC delivers its own receptor (Tir), which is engaged by a bacterial surface ligand (intimin). When Tir delivery and activity are uncoupled, intimin-induced Tir clustering stimulates Tir(Y474) phosphorylation by the Src-family kinase (SFK) c-Fyn, triggering actin polymerization and pedestal formation. How c-Fyn specifically targets Tir and is regulated remains unknown. We show that clustering transfers Tir into cholesterol-rich detergent-resistant microdomains (DRMs), a signal prompting transient c-Fyn accumulation at bacterial adhesion sites. Co-clustering of Tir(Y474) and c-Fyn in DRMs rapidly stimulates robust kinase activation both by induced c-Fyn(Y531) dephosphorylation to unlock the inactive state and by reciprocal c-Fyn(Y417) autophosphorylation to promote activity. After signal induction, c-Fyn dissipates and the resting state restored by Csk-dependent phosphorylation of c-Fyn(Y531). These data illustrate a sophisticated mechanism evolved by a pathogen effector to reversibly regulate SFKs, and resolve early interactions at a model receptor initiating tyrosine kinase signalling.

A direct role for SNX9 in the biogenesis of filopodia.

Filopodia are finger-like actin-rich protrusions that extend from the cell surface and are important for cell-cell communication and pathogen internalization. The small size and transient nature of filopodia combined with shared usage of actin regulators within cells confounds attempts to identify filopodial proteins. Here, we used phage display phenotypic screening to isolate antibodies that alter the actin morphology of filopodia-like structures (FLS) in vitro. We found that all of the antibodies that cause shorter FLS interact with SNX9, an actin regulator that binds phosphoinositides during endocytosis and at invadopodia. In cells, we discover SNX9 at specialized filopodia in Xenopus development and that SNX9 is an endogenous component of filopodia that are hijacked by Chlamydia entry. We show the use of antibody technology to identify proteins used in filopodia-like structures, and a role for SNX9 in filopodia.

Direct modulation of the host cell cytoskeleton by Salmonella actin-binding proteins.

Invasive Salmonella trigger their own uptake into non-phagocytic eukaryotic cells by delivering virulence proteins that stimulate signaling pathways and remodel the actin cytoskeleton. It has recently emerged that Salmonella encodes two actin-binding proteins, SipC and SipA, which together efficiently nucleate actin polymerization and stabilize the resulting supramolecular filament architecture. Therefore, Salmonella might directly initiate actin polymerization independently of the cellular Arp2/3 complex early in the cell entry process. This is an unprecedented example of a direct intervention strategy to facilitate entry of a pathogen into a target cell. Here, we discuss the Salmonella actin-binding proteins and how they might function in combination with entry effectors that stimulate Rho GTPases. We propose that membrane-targeted bacterial effector proteins might trigger actin polymerization through diverse mechanisms during cell entry by bacterial pathogens.

Exploiting pathogenic Escherichia coli to model transmembrane receptor signalling.

Many microbial pathogens manipulate the actin cytoskeleton of eukaryotic target cells to promote their internalization, intracellular motility and dissemination. Enteropathogenic and enterohaemorrhagic Escherichia coli, which both cause severe diarrhoeal disease, can adhere to mammalian intestinal cells and induce reorganization of the actin cytoskeleton into 'pedestal-like' pseudopods beneath the extracellular bacteria. As pedestal assembly is triggered by E. coli virulence factors that mimic several host cell-signalling components, such as transmembrane receptors, their cognate ligands and cytoplasmic adaptor proteins, it can serve as a powerful model system to study eukaryotic transmembrane signalling. Here, we consider the impact of recent data on our understanding of both E. coli pathogenesis and cell biology, and the rich prospects for exploiting these bacterial factors as versatile tools to probe cellular signalling pathways.

Evolution of tonsillar ectopia associated with frontal encephalocoele.

INTRODUCTION: Chiari I malformation has been traditionally considered a congenital malformation. However, there is growing clinical evidence suggesting that it is an acquired phenomenon as also exemplified by this case. CASE: Fetal magnetic resonance imaging (MRI) at 28th week gestation revealed a frontonasal encephalocoele with no hindbrain abnormalities. Post-natal MRI of brain and cervical spine of the 7-week-old infant showed the presence of tonsillar ectopia in the absence of hydrocephalus. The normally developed cerebellar tonsil has herniated through the foramen magnum during the third trimester and neonatal period. DISCUSSION: We hypothesise that the presence of the encephalocoele resulted in dampening expansile forces, produced by the growing brain as well as the cerebrospinal fluid pulsation required to stimulate of the skull growth. As a result, cranial growth is diminished producing a small posterior fossa. The subsequent growth spurt of the cerebellum at the end of the third trimester and during the neonatal period has resulted in cerebellar tonsillar ectopia consequent upon the state of cephalocranial disproportion.