Purkinje cell loss and motor coordination defects in profilin1 mutant mice

Regulation of actin filament assembly and disassembly in growth cone motility and axon guidance

Directed outgrowth of axons is fundamental for the establishment of neuronal networks. Axon outgrowth is guided by growth cones, highly motile structures enriched in filamentous actin (F-actin) located at the axons' distal tips. Growth cones exploit F-actin-based protrusions to scan the environment for guidance cues, and they contain the sensory apparatus to translate guidance cue information into intracellular signaling cascades. These cascades act upstream of actin-binding proteins (ABP) and thereby control assembly and disassembly of F-actin. Spatiotemporally controlled F-actin dis-/assembly in growth cones steers the axon towards attractants and away from repellents, and it thereby navigates the axon through the developing nervous system. Hence, ABP that control F-actin dynamics emerged as critical regulators of neuronal network formation. In the present review article, we will summarize and discuss current knowledge of the mechanisms that control remodeling of the actin cytoskeleton in growth cones, focusing on recent progress in the field. Further, we will introduce tools and techniques that allow to study actin regulatory mechanism in growth cones.

Transient reduction in dendritic spine density in brain-specific profilin1 mutant mice is associated with behavioral deficits

Actin filaments form the backbone of dendritic spines, the postsynaptic compartment of most excitatory synapses in the brain. Spine density changes affect brain function, and postsynaptic actin defects have been implicated in various neuropathies. It is mandatory to identify the actin regulators that control spine density. Based on previous studies, we hypothesized a role for the actin regulator profilin1 in spine formation. We report reduced hippocampal spine density in juvenile profilin1 mutant mice together with impairments in memory formation and reduced ultrasonic communication during active social behavior. Our results, therefore, underline a previously suggested function of profilin1 in controlling spine formation and behavior in juvenile mice.

Specificity and Redundancy of Profilin 1 and 2 Function in Brain Development and Neuronal Structure

Profilin functions have been discussed in numerous cellular processes, including actin polymerization. One puzzling aspect is the concomitant expression of more than one profilin isoform in most tissues. In neuronal precursors and in neurons, profilin 1 and profilin 2 are co-expressed, but their specific and redundant functions in brain morphogenesis are still unclear. Using a conditional knockout mouse model to inactivate both profilins in the developing CNS, we found that threshold levels of profilin are necessary for the maintenance of the neuronal stem-cell compartment and the generation of the differentiated neurons, irrespective of the specific isoform. During embryonic development, profilin 1 is more abundant than profilin 2; consequently, modulation of profilin 1 levels resulted in a more severe phenotype than depletion of profilin 2. Interestingly, the relevance of the isoforms was reversed in the postnatal brain. Morphology of mature neurons showed a stronger dependence on profilin 2, since this is the predominant isoform in neurons. Our data highlight redundant functions of profilins in neuronal precursor expansion and differentiation, as well as in the maintenance of pyramidal neuron dendritic arborization. The specific profilin isoform is less relevant; however, a threshold profilin level is essential. We propose that the common activity of profilin 1 and profilin 2 in actin dynamics is responsible for the observed compensatory effects.

Profilin Isoforms in Health and Disease - All the Same but Different

Profilins are small actin binding proteins, which are structurally conserved throughout evolution. They are probably best known to promote and direct actin polymerization. However, they also participate in numerous cell biological processes beyond the roles typically ascribed to the actin cytoskeleton. Moreover, most complex organisms express several profilin isoforms. Their cellular functions are far from being understood, whereas a growing number of publications indicate that profilin isoforms are involved in the pathogenesis of various diseases. In this review, we will provide an overview of the profilin family and "typical" profilin properties including the control of actin dynamics. We will then discuss the profilin isoforms of higher animals in detail. In terms of cellular functions, we will focus on the role of Profilin 1 (PFN1) and Profilin 2a (PFN2a), which are co-expressed in the central nervous system. Finally, we will discuss recent findings that link PFN1 and PFN2a to neurological diseases, such as amyotrophic lateral sclerosis (ALS), Fragile X syndrome (FXS), Huntington's disease and spinal muscular atrophy (SMA).

Human antigen R-regulated mRNA metabolism promotes the cell motility of migrating mouse neurons

Neocortex development during embryonic stages requires the precise control of mRNA metabolism. Human antigen R (HuR) is a well-studied mRNA-binding protein that regulates mRNA metabolism, and it is highly expressed in the neocortex during developmental stages. Deletion of HuR does not impair neural progenitor cell proliferation or differentiation, but it disturbs the laminar structure of the neocortex. We report that HuR is expressed in postmitotic projection neurons during mouse brain development. Specifically, depletion of HuR in these neurons led to a mislocalization of CDP⁺ neurons in deeper layers of the cortex. Time-lapse microscopy showed that HuR was required for the promotion of cell motility in migrating neurons. PCR array identified profilin 1 (Pfn1) mRNA as a major binding partner of HuR in neurons. HuR positively mediated the stability of Pfn1 mRNA and influenced actin polymerization. Overexpression of Pfn1 successfully rescued the migration defects of HuR-deleted neurons. Our data reveal a post-transcriptional mechanism that maintains actin dynamics during neuronal migration.

Summary: Maintaining actin dynamics is crucial for cell motility. Post-transcriptional regulation plays a pivotal role in supporting actin dynamics during neuronal migration.

Profilin1-Dependent F-Actin Assembly Controls Division of Apical Radial Glia and Neocortex Development

Neocortex development depends on neural stem cell proliferation, cell differentiation, neurogenesis, and neuronal migration. Cytoskeletal regulation is critical for all these processes, but the underlying mechanisms are only poorly understood. We previously implicated the cytoskeletal regulator profilin1 in cerebellar granule neuron migration. Since we found profilin1 expressed throughout mouse neocortex development, we here tested the hypothesis that profilin1 is crucial for neocortex development. We found no evidence for impaired neuron migration or layering in the neocortex of profilin1 mutant mice. However, proliferative activity at basal positions was doubled in the mutant neocortex during mid-neurogenesis, with a drastic and specific increase in basal Pax6+ cells indicative for elevated numbers of basal radial glia (bRG). This was accompanied by transiently increased neurogenesis and associated with mild invaginations resembling rudimentary neocortex folds. Our data are in line with a model in which profilin1-dependent actin assembly controls division of apical radial glia (aRG) and thereby the fate of their progenies. Via this mechanism, profilin1 restricts cell delamination from the ventricular surface and, hence, bRG production and thereby controls neocortex development in mice. Our data support the radial cone hypothesis” claiming that elevated bRG number causes neocortex folds.

Developmental Expression of Mutant PFN1 in Motor Neurons Impacts Neuronal Growth and Motor Performance of Young and Adult Mice

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease with limited treatment and no cure. Mutations in profilin 1 were identified as a cause of familial ALS (fALS) in 2012. We investigated the functional impact of mutant profilin 1 expression in spinal cords during mouse development. We developed a novel mouse model with the expression of profilin 1 C71G under the control of the Hb9 promoter, targeting expression to α-motor neurons in the spinal cord during development. Embryos of transgenic mice showed evidence of a significant reduction of brachial nerve diameter and a loss of Mendelian inheritance. Despite the lack of transgene expression, adult mice presented with significant motor deficits. Transgenic mice had a significant reduction in the number of motor neurons in the spinal cord. Further analysis of these motor neurons in aged transgenic mice revealed reduced levels of TDP-43 and ChAT expression. Although profilin 1 C71G was only expressed during development, adult mice presented with some ALS-associated pathology and motor symptoms. This study highlights the effect of profilin 1 during neurodevelopment and the impact that this may have in later ALS.

Stability of an aggregation-prone partially folded state of human profilin-1 correlates with aggregation propensity

A set of missense mutations in the gene encoding profilin-1 has been linked to the onset of familial forms of ALS (fALS), also known as Lou Gehrig's disease. The pathogenic potential of these mutations is linked to the formation of intracellular inclusions of the mutant proteins and correlates with the mutation-induced destabilization of its native, fully folded state. However, the mechanism by which these mutations promote misfolding and self-assembly is yet unclear. Here, using temperature-jump and stopped-flow kinetic measurements, we show that, during refolding, WT profilin-1 transiently populates a partially folded (PF) state endowed with hydrophobic clusters exposed to the solvent and with no detectable secondary structure. We observed that this conformational state is marginally stable at neutral pH but becomes significantly populated at mildly acidic pH. Interestingly, the fALS-associated mutations did not cause a change in the refolding mechanism of profilin-1, but induced a stabilization of the PF state. In the presence of preformed profilin-1 aggregates, the PF state, unlike the unfolded and folded states, could interact with these aggregates via nonspecific hydrophobic interactions and also increase thioflavin-T fluorescence, revealing its amyloidogenic potential. Moreover, in the variants tested, we found a correlation between conformational stability of PF and aggregation propensity, defining this conformational state as an aggregation-prone folding intermediate. In conclusion, our findings indicate that mutation-induced stabilization of a partially folded state can enhance profilin-1 aggregation and thereby contribute to the pathogenicity of the mutations.

BDNF/trkB Induction of Calcium Transients through Cav2.2 Calcium Channels in Motoneurons Corresponds to F-actin Assembly and Growth Cone Formation on β2-Chain Laminin (221)

Spontaneous Ca²⁺ transients and actin dynamics in primary motoneurons correspond to cellular differentiation such as axon elongation and growth cone formation. Brain-derived neurotrophic factor (BDNF) and its receptor trkB support both motoneuron survival and synaptic differentiation. However, in motoneurons effects of BDNF/trkB signaling on spontaneous Ca²⁺ influx and actin dynamics at axonal growth cones are not fully unraveled. In our study we addressed the question how neurotrophic factor signaling corresponds to cell autonomous excitability and growth cone formation. Primary motoneurons from mouse embryos were cultured on the synapse specific, β2-chain containing laminin isoform (221) regulating axon elongation through spontaneous Ca²⁺ transients that are in turn induced by enhanced clustering of N-type specific voltage-gated Ca²⁺ channels (Ca_v2.2) in axonal growth cones. TrkB-deficient (trkBTK^-/-) mouse motoneurons which express no full-length trkB receptor and wildtype motoneurons cultured without BDNF exhibited reduced spontaneous Ca²⁺ transients that corresponded to altered axon elongation and defects in growth cone morphology which was accompanied by changes in the local actin cytoskeleton. Vice versa, the acute application of BDNF resulted in the induction of spontaneous Ca²⁺ transients and Ca_v2.2 clustering in motor growth cones, as well as the activation of trkB downstream signaling cascades which promoted the stabilization of β-actin via the LIM kinase pathway and phosphorylation of profilin at Tyr129. Finally, we identified a mutual regulation of neuronal excitability and actin dynamics in axonal growth cones of embryonic motoneurons cultured on laminin-221/211. Impaired excitability resulted in dysregulated axon extension and local actin cytoskeleton, whereas upon β-actin knockdown Ca_v2.2 clustering was affected. We conclude from our data that in embryonic motoneurons BDNF/trkB signaling contributes to axon elongation and growth cone formation through changes in the local actin cytoskeleton accompanied by increased Ca_v2.2 clustering and local calcium transients. These findings may help to explore cellular mechanisms which might be dysregulated during maturation of embryonic motoneurons leading to motoneuron disease.

Profilin1 biology and its mutation, actin(g) in disease

Profilins were discovered in the 1970s and were extensively studied for their significant physiological roles. Profilin1 is the most prominent isoform and has drawn special attention due to its role in the cytoskeleton, cell signaling, and its link to conditions such as cancer and vascular hypertrophy. Recently, multiple mutations in the profilin1 gene were linked to amyotrophic lateral sclerosis (ALS). In this review, we will discuss the physiological and pathological roles of profilin1. We will further highlight the cytoskeletal function and dysfunction caused by profilin1 dysregulation. Finally, we will discuss the implications of mutant profilin1 in various diseases with an emphasis on its contribution to the pathogenesis of ALS.

Systemic inflammation in early neonatal mice induces transient and lasting neurodegenerative effects

Background

The inflammatory mediator lipopolysaccharide (LPS) has been shown to induce acute gliosis in neonatal mice. However, the progressive effects on the murine neurodevelopmental program over the week that follows systemic inflammation are not known. Thus, we investigated the effects of repeated LPS administration in the first postnatal week in mice, a condition mimicking sepsis in late preterm infants, on the developing central nervous system (CNS).

Methods

Systemic inflammation was induced by daily intraperitoneal administration (i.p.) of LPS (6 mg/kg) in newborn mice from postnatal day (PND) 4 to PND6. The effects on neurodevelopment were examined by staining the white matter and neurons with Luxol Fast Blue and Cresyl Violet, respectively. The inflammatory response was assessed by quantifying the expression/activity of matrix metalloproteinases (MMP), toll-like receptor (TLR)-4, high mobility group box (HMGB)-1, and autotaxin (ATX). In addition, B6 CX3CR1^gfp/+ mice combined with cryo-immunofluorescence were used to determine the acute, delayed, and lasting effects on myelination, microglia, and astrocytes.

Results

LPS administration led to acute body and brain weight loss as well as overt structural changes in the brain such as cerebellar hypoplasia, neuronal loss/shrinkage, and delayed myelination. The impaired myelination was associated with alterations in the proliferation and differentiation of NG2 progenitor cells early after LPS administration, rather than with excessive phagocytosis by CNS myeloid cells. In addition to disruptions in brain architecture, a robust inflammatory response to LPS was observed. Quantification of inflammatory biomarkers revealed decreased expression of ATX with concurrent increases in HMGB1, TLR-4, and MMP-9 expression levels. Acute astrogliosis (GFAP⁺ cells) in the brain parenchyma and at the microvasculature interface together with parenchymal microgliosis (CX3CR1⁺ cells) were also observed. These changes preceded the migration/proliferation of CX3CR1⁺ cells around the vessels at later time points and the subsequent loss of GFAP⁺ astrocytes.

Conclusion

Collectively, our study has uncovered a complex innate inflammatory reaction and associated structural changes in the brains of neonatal mice challenged peripherally with LPS. These findings may explain some of the neurobehavioral abnormalities that develop following neonatal sepsis.

Profilin isoforms modulate astrocytic morphology and the motility of astrocytic processes

The morphology of astrocytic processes determines their close structural association with synapses referred to as the ‘tripartite synapse’. Concerted morphological plasticity processes at tripartite synapses are supposed to shape neuronal communication. Morphological changes in astrocytes as well as the motility of astrocytic processes require remodeling of the actin cytoskeleton. Among the regulators of fast timescale actin-based motility, the actin binding protein profilin 1 has recently been shown to control the activity-dependent outgrowth of astrocytic processes.

Here, we demonstrate that cultured murine astrocytes in addition to the ubiquitous profilin 1 also express the neuronal isoform profilin 2a. To analyze the cellular function of both profilins in astrocytes, we took advantage of a shRNA mediated isoform-specific downregulation. Interestingly, consistent with earlier results in neurons, we found redundant as well as isoform-specific functions of both profilins in modulating cellular physiology. The knockdown of either profilin 1 or profilin 2a led to a significant decrease in cell spreading of astrocytes. In contrast, solely the knockdown of profilin 2a resulted in a significantly reduced morphological complexity of astrocytes in both dissociated and slice culture astrocytes. Moreover, both isoforms proved to be crucial for forskolin-induced astrocytic stellation. Furthermore, forskolin treatment resulted in isoform-specific changes in the phosphorylation level of profilin 1 and profilin 2a, leading to a PKA-dependent phosphorylation of profilin 2a. In addition, transwell assays revealed an involvement of both isoforms in the motility of astrocytic processes, while FRAP analysis displayed an isoform-specific role of profilin 1 in the regulation of actin dynamics in peripheral astrocytic processes. Taken together, we suggest profilin isoforms to be important modulators of astrocytic morphology and motility with overlapping as well as isoform-specific functions.

Profilin1 activity in cerebellar granule neurons is required for radial migration in vivo

Neuron migration defects are an important aspect of human neuropathies. The underlying molecular mechanisms of such migration defects are largely unknown. Actin dynamics has been recognized as an important determinant of neuronal migration, and we recently found that the actin-binding protein profilin1 is relevant for radial migration of cerebellar granule neurons (CGN). As the exploited brain-specific mutants lacked profilin1 in both neurons and glial cells, it remained unknown whether profilin1 activity in CGN is relevant for CGN migration in vivo. To test this, we capitalized on a transgenic mouse line that expresses a tamoxifen-inducible Cre variant in CGN, but no other cerebellar cell type. In these profilin1 mutants, the cell density was elevated in the molecular layer, and ectopic CGN occurred. Moreover, 5-bromo-2'-deoxyuridine tracing experiments revealed impaired CGN radial migration. Hence, our data demonstrate the cell autonomous role of profilin1 activity in CGN for radial migration.

The central mechanisms of secretin in regulating multiple behaviors

Secretin (SCT) was firstly discovered as a gut peptide hormone in stimulating pancreatic secretion, while its novel neuropeptide role has drawn substantial research interests in recent years. SCT and its receptor (SCTR) are widely expressed in different brain regions, where they exert multiple cellular functions including neurotransmission, gene expression regulation, neurogenesis, and neural protection. As all these neural functions ultimately can affect behaviors, it is hypothesized that SCT controls multiple behavioral paradigms. Current findings support this hypothesis as SCT-SCTR axis participates in modulating social interaction, spatial learning, water and food intake, motor coordination, and motor learning behaviors. This mini-review focuses on various aspects of SCT and SCTR in hippocampus, hypothalamus, and cerebellum including distribution profiles, cellular functions, and behavioral phenotypes to elucidate the link between cellular mechanisms and behavioral control.