Anosmin-1 stimulates outgrowth and branching of developing Purkinje axons

Fibroblast growth factor signaling in axons: from development to disease

The fibroblast growth factor (FGF) family regulates various and important aspects of nervous system development, ranging from the well-established roles in neuronal patterning to more recent and exciting functions in axonal growth and synaptogenesis. In addition, FGFs play a critical role in axonal regeneration, particularly after spinal cord injury, confirming their versatile nature in the nervous system. Due to their widespread involvement in neural development, the FGF system also underlies several human neurological disorders. While particular attention has been given to FGFs in a whole-cell context, their effects at the axonal level are in most cases undervalued. Here we discuss the endeavor of the FGF system in axons, we delve into this neuronal subcompartment to provide an original view of this multipurpose family of growth factors in nervous system (dys)function. Video Abstract.

Extracellular matrix protein anosmin-1 overexpression alters dopaminergic phenotype in the CNS and the PNS with no pathogenic consequences in a MPTP model of Parkinson's disease

The development and survival of dopaminergic neurons are influenced by the fibroblast growth factor (FGF) pathway. Anosmin-1 (A1) is an extracellular matrix protein that acts as a major regulator of this signaling pathway, controlling FGF diffusion, and receptor interaction and shuttling. In particular, previous work showed that A1 overexpression results in more dopaminergic neurons in the olfactory bulb. Prompted by those intriguing results, in this study, we investigated the effects of A1 overexpression on different populations of catecholaminergic neurons in the central (CNS) and the peripheral nervous systems (PNS). We found that A1 overexpression increases the number of dopaminergic substantia nigra pars compacta (SNpc) neurons and alters the striosome/matrix organization of the striatum. Interestingly, these numerical and morphological changes in the nigrostriatal pathway of A1-mice did not confer an altered susceptibility to experimental MPTP-parkinsonism with respect to wild-type controls. Moreover, the study of the effects of A1 overexpression was extended to different dopaminergic tissues associated with the PNS, detecting a significant reduction in the number of dopaminergic chemosensitive carotid body glomus cells in A1-mice. Overall, our work shows that A1 regulates the development and survival of dopaminergic neurons in different nuclei of the mammalian nervous system.

Long non-coding RNA MSTRG.5970.28 regulates proliferation and apoptosis of goose follicle granulosa cells via the miR-133a-3p/ANOS1 pathway

The development of follicles in the ovaries is a critical determinant of poultry egg production. There are existing studies on the follicular development patterns in poultry, but the specific regulatory mechanisms still need further study. In a previous study, we identified long non-coding RNA (lncRNA) MSTRG.5970.28, anosmin 1 (ANOS1), and its predicted target miR-133a-3p that may be associated with goose ovary development. However, the function of MSTRG.5970.28 in goose granulosa cells and its regulatory mechanisms affecting granulosa cell proliferation and apoptosis have not been reported. In the present study, MSTRG.5970.28 and miR-133a-3p overexpression and interference vectors were constructed. Combined with reverse-transcription real-time quantitative PCR (RT-qPCR), a dual luciferase activity assay, Cell Counting Kit-8 (CCK-8), and flow cytometric analysis, we investigated the role of the MSTRG.5970.28-miR-133a-3p-ANOS1 axis in goose follicular granulosa cells and the associated regulatory mechanisms. MSTRG.5970.28 was found to be localized in the cytoplasm and its expression was influenced by reproductive hormones. The targeting relationship among MSTRG.5970.28, ANOS1, and miR-133a-3p were verified by a dual luciferase activity assay. CCK-8 and apoptosis assays showed that MSTRG.5970.28 inhibited the proliferation and promoted apoptosis of goose granulosa cells. The regulatory role of miR-133a-3p on granulosa cell proliferation and apoptosis was opposite to MSTRG.5970.28. We found that the proliferative and apoptotic effects of granulosa cells caused by MSTRG.5970.28 overexpression were attenuated by miR-133a-3p. MSTRG.5970.28 functions as a competitive endogenous RNA that regulates ANOS1 expression by sponging miR-133a-3p and thus exerts regulatory functions in granulosa cells. In sum, the present study identified lncRNA MSTRG.5970.28 as associated with goose ovary development, which affects the expression of ANOS1 by targeting miR-133a-3p, thereby influencing the proliferation and apoptosis of goose granulosa cells.

Novel factor in olfactory ensheathing cell-astrocyte crosstalk: Anti-inflammatory protein α-crystallin B

Astrocytes are key players in CNS neuroinflammation and neuroregeneration that may help or hinder recovery, depending on the context of the injury. Although pro-inflammatory factors that promote astrocyte-mediated neurotoxicity have been shown to be secreted by reactive microglia, anti-inflammatory factors that suppress astrocyte activation are not well-characterized. Olfactory ensheathing cells (OECs), glial cells that wrap axons of olfactory sensory neurons, have been shown to moderate astrocyte reactivity, creating an environment conducive to regeneration. Similarly, astrocytes cultured in medium conditioned by cultured OECs (OEC-CM) show reduced nuclear translocation of nuclear factor kappa-B (NFκB), a pro-inflammatory protein that induces neurotoxic reactivity in astrocytes. In this study, we screened primary and immortalized OEC lines to identify these factors and discovered that Alpha B-crystallin (CryAB), an anti-inflammatory protein, is secreted by OECs via exosomes, coordinating an intercellular immune response. Our results showed that: (a) OEC exosomes block nuclear NFκB translocation in astrocytes while exosomes from CryAB-null OECs could not; (b) OEC exosomes could be taken up by astrocytes, and (c) CryAB treatment suppressed neurotoxicity-associated astrocyte transcripts. Our results indicate CryAB, as well as other factors secreted by OECs, are potential agents that can ameliorate, or even reverse, the growth-inhibitory environment created by neurotoxic reactive astrocytes following CNS injuries.

A switch in pdgfrb+ cell-derived ECM composition prevents inhibitory scarring and promotes axon regeneration in the zebrafish spinal cord

In mammals, perivascular cell-derived scarring after spinal cord injury impedes axonal regrowth. In contrast, the extracellular matrix (ECM) in the spinal lesion site of zebrafish is permissive and required for axon regeneration. However, the cellular mechanisms underlying this interspecies difference have not been investigated. Here, we show that an injury to the zebrafish spinal cord triggers recruitment of pdgfrb⁺ myoseptal and perivascular cells in a PDGFR signaling-dependent manner. Interference with pdgfrb⁺ cell recruitment or depletion of pdgfrb⁺ cells inhibits axonal regrowth and recovery of locomotor function. Transcriptional profiling and functional experiments reveal that pdgfrb⁺ cells upregulate expression of axon growth-promoting ECM genes (cthrc1a and col12a1a/b) and concomitantly reduce synthesis of matrix molecules that are detrimental to regeneration (lum and mfap2). Our data demonstrate that a switch in ECM composition is critical for axon regeneration after spinal cord injury and identify the cellular source and components of the growth-promoting lesion ECM.

Novel Microdeletion in the X Chromosome Leads to Kallmann Syndrome, Ichthyosis, Obesity, and Strabismus

Background

A large deletion in Xp22.3 can result in contiguous gene syndromes, including X-linked ichthyosis (XLI) and Kallmann syndrome (KS), presenting with short stature, chondrodysplasia punctata, intellectual disability, and strabismus. XLI and KS are caused by the deletion of STS and ANOS1, respectively.

Method

Two KS patients with XLI were screened to identify possible pathogenic mutations using whole exome sequencing. The clinical characteristics, molecular genetics, treatment outcomes, and genotype–phenotype association for each patient were analyzed.

Results

We identified a novel 3,923 kb deletion within the Xp22.31 region (chrX: 5810838–9733877) containing STS, ANOS1, GPR143, NLGN4X, VCX-A, PUDP, and PNPLA4 in patient 1, who presented with KS, XLI, obesity, hyperlipidemia, and strabismus. We identified a novel 5,807 kb deletion within the Xp22.31-p22.33 regions (chrX: 2700083–8507807) containing STS, ANOS1, and other 24 genes in patient 2, who presented with KS, XLI, obesity, and strabismus. No developmental delay, abnormal speech development, or autistic behavior were noticed in either patient.

Conclusion

We identified two novel microdeletions in the X chromosome leading to KS and XLI. These findings contribute to the understanding of the molecular mechanisms that drive contiguous gene syndromes. Our research confirmed that the Kallmann-Ichthyosis phenotype is caused by microdeletions at the chromosome level.

Anosmin-1 activates vascular endothelial growth factor receptor and its related signaling pathway for olfactory bulb angiogenesis

Anosmin-1 is a secreted glycoprotein encoded by the ANOS1 gene, and its loss of function causes Kallmann syndrome (KS), which is characterized by anosmia and hypogonadism due to olfactory bulb (OB) dysfunction. However, the physiological function of anosmin-1 remains to be elucidated. In KS, disordered angiogenesis is observed in OB, resulting in its hypoplasia. In this study, we examined the involvement of anosmin-1 in angiogenic processes. Anosmin-1 was detected on the vessel-like structure in OB of chick embryos, and promoted the outgrowth of vascular sprouts as shown by assays of OB tissue culture. Cell migration, proliferation, and tube formation of endothelial cells were induced by treatment with anosmin-1 as well as vascular endothelial growth factor-A (VEGF-A), and further enhanced by treatment with both of them. We newly identified that anosmin-1 activated VEGF receptor-2 (VEGFR2) by binding directly to it, and its downstream signaling molecules, phospholipase Cγ1 (PLCγ1) and protein kinase C (PKC). These results suggest that anosmin-1 plays a key role in the angiogenesis of developing OB through the VEGFR2–PLCγ1–PKC axis by enhancing the VEGF function.

ANOS1: a unified nomenclature for Kallmann syndrome 1 gene (KAL1) and anosmin-1

It is accepted that confusion regarding the description of genetic variants occurs when researchers do not use standard nomenclature. The Human Genome Organization Gene Nomenclature Committee contacted a panel of consultants, all working on the KAL1 gene, to propose an update of the nomenclature of the gene, as there was a convention in the literature of using the ‘KAL1’ symbol, when referring to the gene, but using the name ‘anosmin-1’ when referring to the protein. The new name, ANOS1, reflects protein name and is more transferrable across species.

Endocrine disorders and the cerebellum: from neurodevelopmental injury to late-onset ataxia

Hormonal disorders are a source of cerebellar ataxia in both children and adults. Normal development of the cerebellum is critically dependent on thyroid hormone, which crosses both the blood-brain barrier and the blood-cerebrospinal fluid barrier thanks to specific transporters, including monocarboxylate transporter 8 and the organic anion-transporting polypeptide 1C1. In particular, growth and dendritic arborization of Purkinje neurons, synaptogenesis, and myelination are dependent on thyroid hormone. Disturbances of thyroid hormone may also impact on cerebellar ataxias of other origin, decompensating or aggravating the pre-existing ataxia manifesting with motor ataxia, oculomotor ataxia, and/or Schmahmann syndrome. Parathyroid disorders are associated with a genuine cerebellar syndrome, but symptoms may be subtle. The main conditions combining diabetes and cerebellar ataxia are Friedreich ataxia, ataxia associated with anti-GAD antibodies, autoimmune polyglandular syndromes, aceruloplasminemia, and cerebellar ataxia associated with hypogonadism (especially Holmes ataxia/Boucher-Neuhäuser syndrome). The general workup of cerebellar disorders should include the evaluation of hormonal status, including thyroid-stimulating hormone and free thyroxine levels, and hormonal replacement should be considered depending on the laboratory results. Cerebellar deficits may be reversible in some cases.

Feedback regulation of RTK signaling in development

Precise regulation of the amplitude and duration of receptor tyrosine kinase (RTK) signaling is critical for the execution of cellular programs and behaviors. Understanding these control mechanisms has important implications for the field of developmental biology, and in recent years, the question of how augmentation or attenuation of RTK signaling via feedback loops modulates development has become of increasing interest. RTK feedback regulation is also important for human disease research; for example, germline mutations in genes that encode RTK signaling pathway components cause numerous human congenital syndromes, and somatic alterations contribute to the pathogenesis of diseases such as cancers. In this review, we survey regulators of RTK signaling that tune receptor activity and intracellular transduction cascades, with a focus on the roles of these genes in the developing embryo. We detail the diverse inhibitory mechanisms utilized by negative feedback regulators that, when lost or perturbed, lead to aberrant increases in RTK signaling. We also discuss recent biochemical and genetic insights into positive regulators of RTK signaling and how these proteins function in tandem with negative regulators to guide embryonic development.

Anosmin-1 over-expression increases adult neurogenesis in the subventricular zone and neuroblast migration to the olfactory bulb

New subventricular zone (SVZ)-derived neuroblasts that migrate via the rostral migratory stream are continuously added to the olfactory bulb (OB) of the adult rodent brain. Anosmin-1 (A1) is an extracellular matrix protein that binds to FGF receptor 1 (FGFR1) to exert its biological effects. When mutated as in Kallmann syndrome patients, A1 is associated with severe OB morphogenesis defects leading to anosmia and hypogonadotropic hypogonadism. Here, we show that A1 over-expression in adult mice strongly increases proliferation in the SVZ, mainly with symmetrical divisions, and produces substantial morphological changes in the normal SVZ architecture, where we also report the presence of FGFR1 in almost all SVZ cells. Interestingly, for the first time we show FGFR1 expression in the basal body of primary cilia in neural progenitor cells. Additionally, we have found that A1 over-expression also enhances neuroblast motility, mainly through FGFR1 activity. Together, these changes lead to a selective increase in several GABAergic interneuron populations in different OB layers. These specific alterations in the OB would be sufficient to disrupt the normal processing of sensory information and consequently alter olfactory memory. In summary, this work shows that FGFR1-mediated A1 activity plays a crucial role in the continuous remodelling of the adult OB.

The effect of glia-glia interactions on oligodendrocyte precursor cell biology during development and in demyelinating diseases

Oligodendrocyte precursor cells (OPCs) originate in specific areas of the developing central nervous system (CNS). Once generated, they migrate towards their destinations where they differentiate into mature oligodendrocytes. In the adult, 5-8% of all cells in the CNS are OPCs, cells that retain the capacity to proliferate, migrate, and differentiate into oligodendrocytes. Indeed, these endogenous OPCs react to damage in demyelinating diseases, like multiple sclerosis (MS), representing a key element in spontaneous remyelination. In the present work, we review the specific interactions between OPCs and other glial cells (astrocytes, microglia) during CNS development and in the pathological scenario of MS. We focus on: (i) the role of astrocytes in maintaining the homeostasis and spatial distribution of different secreted cues that determine OPC proliferation, migration, and differentiation during CNS development; (ii) the role of microglia and astrocytes in the redistribution of iron, which is crucial for myelin synthesis during CNS development and for myelin repair in MS; (iii) how microglia secrete different molecules, e.g., growth factors, that favor the recruitment of OPCs in acute phases of MS lesions; and (iv) how astrocytes modify the extracellular matrix in MS lesions, affecting the ability of OPCs to attempt spontaneous remyelination. Together, these issues demonstrate how both astroglia and microglia influence OPCs in physiological and pathological situations, reinforcing the concept that both development and neural repair are complex and global phenomena. Understanding the molecular and cellular mechanisms that control OPC survival, proliferation, migration, and differentiation during development, as well as in the mature CNS, may open new opportunities in the search for reparative therapies in demyelinating diseases like MS.

Olfactory system and demyelination

Within the central nervous system, the olfactory system represents one of the most exciting scenarios since it presents relevant examples of long-life sustained neurogenesis and continuous axonal outgrowth from the olfactory epithelium with the subsequent plasticity phenomena in the olfactory bulb. The olfactory nerve is composed of nonmyelinated axons with interesting ontogenetic interpretations. However, the centripetal projections from the olfactory bulb are myelinated axons which project to more caudal areas along the lateral olfactory tract. In consequence, demyelination has not been considered as a possible cause of the olfactory symptoms in those diseases in which this sense is impaired. One prototypical example of an olfactory disease is Kallmann syndrome, in which different mutations give rise to combined anosmia and hypogonadotropic hypogonadism, together with different satellite symptoms. Anosmin-1 is the extracellular matrix glycoprotein altered in the X-linked form of this disease, which participates in cell adhesion and migration, and axonal outgrowth in the olfactory system and in other regions of the central nervous system. Recently, we have described a new patho-physiological role of this protein in the absence of spontaneous remyelination in multiple sclerosis. In the present review, we hypothesize about how both main and satellite neurological symptoms of Kallmann syndrome may be explained by alterations in the myelination. We revisit the relationship between the olfactory system and myelin highlighting that minor histological changes should not be forgotten as putative causes of olfactory malfunction.

cGMP signaling and branching of sensory axons in the spinal cord

Axonal branching is essential for neurons to establish contacts to different targets. It therefore provides the physical basis for the integration and distribution of information within the nervous system. During embryonic and early postnatal development, several axonal branching modes may be distinguished that might be regulated by activities of the growth cone or by the axon shaft. The various forms of axonal branching are dependent on intrinsic components and are regulated by extrinsic factors that activate specific signaling systems. This article focuses on components implicated in cyclic guanosine monophosphate signaling that regulate axon bifurcation – a specific form of branching – within the spinal cord in animal models. This cascade is composed of the ligand CNP, the guanylyl cyclase Npr2 and the cyclic guanosine monophosphate-dependent kinase I. In the absence of one of these components, axons of dorsal root ganglion neurons do not form T-shaped branches when entering the spinal cord, while collateral (interstitial) branching, another branching mode of the same type of the neuron, is not affected. It will be important to analyze human patients with mutations in the corresponding genes to get insights into the pathophysiological effects of impaired sensory axon branching in the spinal cord.

Molecular control of axon branching

Axon branching is a complex morphological process, the regulation of which we are just beginning to understand. Many factors known to be important for axon growth and guidance have emerged as key regulators of axon branching. The extrinsic factors implicated in axon branching include traditional axon guidance cues such as the slits, semaphorins, and ephrins; neurotrophins such as BDNF; the secreted glycoprotein Wnt; the extracellular matrix protein anosmin-1; and certain transmembrane cell adhesion molecules--as well as sensory experience and neuronal activity. Although less is known about the intracellular control of axon branching, in recent years significant advances have been made in this area. Kinases and their regulators, Rho GTPases and their regulators, transcription factors, ubiquitin ligases, and several microtubule and actin-binding proteins are now implicated in the control of axon branching. It is likely that many more branching regulators remain to be discovered, as do the links between extrinsic cues and intracellular signaling proteins in the control of axon branching.

The WFDC1 gene: role in wound response and tissue homoeostasis

The present evaluates the key features of the WFDC1 [WAP (whey acidic protein) four disulfide core 1] gene that encodes ps20 (20 kDa prostate stromal protein), a member of the WAP family. ps20 was first characterized as a growth inhibitory activity that was secreted by fetal urogenital sinus mesenchymal cells. Purified ps20 exhibited several activities that centre on cell adhesion, migration and proliferation. The WFDC1 gene was cloned, contained seven exons, and was mapped to chromosome 16q24, suggesting that it may function as a tumour suppressor; however, direct evidence of this has not emerged. In vivo, ps20 stimulated angiogenesis, although expression of WFDC1/ps20 was down-regulated in the reactive stroma tumour microenvironment in prostate cancer. WFDC1 expression is differential in other cancers and inflammatory conditions. Recent studies point to a role in viral infectivity. Although mechanisms of action are not fully understood, WFDC1/ps20 is emerging as a secreted matricellular protein that probably affects response to micro-organisms and tissue repair homoeostasis.

Biochemical dissection of Anosmin-1 interaction with FGFR1 and components of the extracellular matrix

Anosmin-1, defective in Kallmann's syndrome, participates in the adhesion, migration and differentiation of different cell types in the CNS. Although not fully understood, the mechanisms of action of Anosmin-1 involve the interaction with different proteins, being the interaction with fibroblast growth factor receptor 1 (FGFR1) and the modulation of its signalling the best studied to date. Using glutathione-S-transferase pull-down assays we demonstrate that the FnIII.3 (Fibronectin-like type III) domain and the combination whey acidic protein-FnIII.1, but not each of them individually, interact with FGFR1. The interaction of the whey acidic protein-FnIII.1 domains is substantially reduced when the cysteine-rich region is present, suggesting a likely regulatory role for this domain. The introduction in FnIII.3 of any of the two missense mutations found in Kallmann's syndrome patients, E514K and F517L, abolished the interaction with FGFR1, what suggests an important role for these residues in the interaction. Interestingly, the chemoattraction of Anosmin-1 on rat neuronal precursors (NPs) via FGFR1 is retained by the N-terminal region of Anosmin-1 but not by FnIII.3 alone, and is lost in proteins carrying either one of the missense mutations, probably because of a highly reduced binding capacity to FGFR1. We also describe homophilic interaction Anosmin-1/Anosmin-1 via the FnIII repeats 1 and 4, and the interaction of FnIII.1 and FnIII.3 with Fibronectin and of FnIII.3 with Laminin.

Keratinocyte-derived anosmin-1, an extracellular glycoprotein encoded by the X-linked Kallmann syndrome gene, is involved in modulation of epidermal nerve density in atopic dermatitis

BACKGROUND

Epidermal nerve density is increased in atopic dermatitis (AD), suggesting that the hyperinnervation is partly responsible for abnormal itch perception. It is probably controlled by axonal guidance molecules produced by keratinocytes. An extracellular matrix glycoprotein anosmin-1 encoded by KAL1 has chemoattractive or chemorepulsive effects on different neuronal types.

OBJECTIVE

This study was performed to investigate the roles of anosmin-1 in skin innervation.

METHODS

Rat dorsal root ganglion (DRG) neurones were cultured in conditioned medium from control or KAL1-overexpressing cells for neurite outgrowth assay. KAL1 expression in cultured epidermal keratinocytes or human skin was examined by quantitative RT-PCR (qRT-PCR). Anosmin-1 distribution in normal and atopic skin was examined immunohistochemically. The effects of calcium concentrations and cytokines on KAL1 expression in cultured normal human epidermal keratinocytes (NHEK) were analysed by qRT-PCR.

RESULTS

Neurite outgrowth in cultured DRG neurones was inhibited by conditioned medium from KAL1-overexpressing cells, while it was rescued by addition of recombinant fibroblast growth factor receptor 1 for capturing anosmin-1. KAL1 transcripts were expressed in cultured keratinocytes or in normal skin. Anosmin-1 was strongly expressed in the basal cell layer of normal skin, but decreased in atopic skin, concomitant with increases of epidermal nerve fibres. KAL1 expression was downregulated during keratinocyte differentiation. The expression was also upregulated by interleukin-4 (IL-4), IL-13 or transforming growth factor (TGF)-beta1. TGF-beta1 acted synergistically with IL-13 to enhance KAL1 expression, while interferon-gamma inhibited its expression.

CONCLUSION

Anosmin-1 produced by epidermal keratinocytes in response to calcium concentrations or cytokines may modulate epidermal nerve density in AD.

Wiring Olfaction: The Cellular and Molecular Mechanisms that Guide the Development of Synaptic Connections from the Nose to the Cortex

Within the central nervous system, the olfactory system fascinates by its developmental and physiological particularities, and is one of the most studied models to understand the mechanisms underlying the guidance of growing axons to their appropriate targets. A constellation of contact-mediated (laminins, CAMs, ephrins, etc.) and secreted mechanisms (semaphorins, slits, growth factors, etc.) are known to play different roles in the establishment of synaptic interactions between the olfactory epithelium, olfactory bulb (OB) and olfactory cortex. Specific mechanisms of this system (including the amazing family of about 1000 different olfactory receptors) have been also proposed. In the last years, different reviews have focused in partial sights, specially in the mechanisms involved in the formation of the olfactory nerve, but a detailed review of the mechanisms implicated in the development of the connections among the different olfactory structures (olfactory epithelium, OB, olfactory cortex) remains to be written. In the present work, we afford this systematic review: the different cellular and molecular mechanisms which rule the formation of the olfactory nerve, the lateral olfactory tract and the intracortical connections, as well as the few data available regarding the accessory olfactory system. These mechanisms are compared, and the implications of the differences and similarities discussed in this fundamental scenario of ontogeny.

Distinct modes of neuritic growth in purkinje neurons at different developmental stages: axonal morphogenesis and cellular regulatory mechanisms

Background

During development, neurons modify their axon growth mode switching from an elongating phase, in which the main axon stem reaches the target territory through growth cone-driven extension, to an arborising phase, when the terminal arbour is formed to establish synaptic connections. To investigate the relative contribution of cell-autonomous factors and environmental signals in the control of these distinct axon growth patterns, we examined the neuritogenesis of Purkinje neurons in cerebellar cultures prepared at elongating (embryonic day 17) or arborising (postnatal day zero) stages of Purkinje axon maturation.

Methodology/Principal Findings

When placed in vitro, Purkinje cells of both ages undergo an initial phase of neurite elongation followed by the development of terminal ramifications. Nevertheless, elongation of the main axon stem prevails in embryonic Purkinje axons, and many of these neurons are totally unable to form terminal branches. On the contrary, all postnatal neurites switch to arbour growth within a few days in culture and spread extensive terminal trees. Regardless of their elongating or arborising pattern, defined growth features (e.g. growth rate and tree extension) of embryonic Purkinje axons remain distinct from those of postnatal neurites. Thus, Purkinje neurons of different ages are endowed with intrinsic stage-specific competence for neuritic growth. Such competence, however, can be modified by environmental cues. Indeed, while exposure to the postnatal environment stimulates the growth of embryonic axons without modifying their phenotype, contact-mediated signals derived from granule cells specifically induce arborising growth and modulate the dynamics of neuritic elongation.

Conclusions/Significance

Cultured Purkinje cells recapitulate an intrinsically coded neuritogenic program, involving initial navigation of the axon towards the target field and subsequent expansion of the terminal arborisation. The execution of this program is regulated by environmental signals that modify the growth competence of Purkinje cells, so to adapt their endogenous properties to the different phases of neuritic morphogenesis.