Profilin 1 with the amyotrophic lateral sclerosis associated mutation T109M displays unaltered actin binding and does not affect the actin cytoskeleton

Using ALS to understand profilin 1's diverse roles in cellular physiology

Profilin is an actin monomer-binding protein whose role in actin polymerization has been studied for nearly 50 years. While its principal biochemical features are now well understood, many questions remain about how profilin controls diverse processes within the cell. Dysregulation of profilin has been implicated in a broad range of human diseases, including neurodegeneration, inflammatory disorders, cardiac disease, and cancer. For example, mutations in the profilin 1 gene (PFN1) can cause amyotrophic lateral sclerosis (ALS), although the precise mechanisms that drive neurodegeneration remain unclear. While initial work suggested proteostasis and actin cytoskeleton defects as the main pathological pathways, multiple novel functions for PFN1 have since been discovered that may also contribute to ALS, including the regulation of nucleocytoplasmic transport, stress granules, mitochondria, and microtubules. Here, we will review these newly discovered roles for PFN1, speculate on their contribution to ALS, and discuss how defects in actin can contribute to these processes. By understanding profilin 1's involvement in ALS pathogenesis, we hope to gain insight into this functionally complex protein with significant influence over cellular physiology.

PFN1 Inhibits Myogenesis of Bovine Myoblast Cells via Cdc42-PAK/JNK

Myoblast differentiation is essential for the formation of skeletal muscle myofibers. Profilin1 (Pfn1) has been identified as an actin-associated protein, and has been shown to be critically important to cellular function. Our previous study found that PFN1 may inhibit the differentiation of bovine skeletal muscle satellite cells, but the underlying mechanism is not known. Here, we confirmed that PFN1 negatively regulated the myogenic differentiation of bovine skeletal muscle satellite cells. Immunoprecipitation assay combined with mass spectrometry showed that Cdc42 was a binding protein of PFN1. Cdc42 could be activated by PFN1 and could inhibit the myogenic differentiation like PFN1. Mechanistically, activated Cdc42 increased the phosphorylation level of p2l-activated kinase (PAK), which further activated the phosphorylation activity of c-Jun N-terminal kinase (JNK), whereas PAK and JNK are inhibitors of myogenic differentiation. Taken together, our results reveal that PFN1 is a repressor of bovine myogenic differentiation, and provide the regulatory mechanism.

The Amyotrophic Lateral Sclerosis M114T PFN1 Mutation Deregulates Alternative Autophagy Pathways and Mitochondrial Homeostasis

Mutations in profilin 1 (PFN1) have been identified in rare familial cases of Amyotrophic Lateral Sclerosis (ALS). PFN1 is involved in multiple pathways that could intervene in ALS pathology. However, the specific pathogenic role of PFN1 mutations in ALS is still not fully understood. We hypothesized that PFN1 could play a role in regulating autophagy pathways and that PFN1 mutations could disrupt this function. We used patient cells (lymphoblasts) or tissue (post-mortem) carrying PFN1 mutations (M114T and E117G), and designed experimental models expressing wild-type or mutant PFN1 (cell lines and novel PFN1 mice established by lentiviral transgenesis) to study the effects of PFN1 mutations on autophagic pathway markers. We observed no accumulation of PFN1 in the spinal cord of one E117G mutation carrier. Moreover, in patient lymphoblasts and transfected cell lines, the M114T mutant PFN1 protein was unstable and deregulated the RAB9-mediated alternative autophagy pathway involved in the clearance of damaged mitochondria. In vivo, motor neurons expressing M114T mutant PFN1 showed mitochondrial abnormalities. Our results demonstrate that the M114T PFN1 mutation is more deleterious than the E117G variant in patient cells and experimental models and suggest a role for the RAB9-dependent autophagic pathway in ALS.

Biochemical characterization of actin assembly mechanisms with ALS-associated profilin variants

Eight separate mutations in the actin-binding protein profilin-1 have been identified as a rare cause of amyotrophic lateral sclerosis (ALS). Profilin is essential for many neuronal cell processes through its regulation of lipids, nuclear signals, and cytoskeletal dynamics, including actin filament assembly. Direct interactions between profilin and actin monomers inhibit actin filament polymerization. In contrast, profilin can also stimulate polymerization by simultaneously binding actin monomers and proline-rich tracts found in other proteins. Whether the ALS-associated mutations in profilin compromise these actin assembly functions is unclear. We performed a quantitative biochemical comparison of the direct and formin mediated impact for the eight ALS-associated profilin variants on actin assembly using classic protein-binding and single-filament microscopy assays. We determined that the binding constant of each profilin for actin monomers generally correlates with the actin nucleation strength associated with each ALS-related profilin. In the presence of formin, the A20T, R136W, Q139L, and C71G variants failed to activate the elongation phase of actin assembly. This diverse range of formin-activities is not fully explained through profilin-poly-L-proline (PLP) interactions, as all ALS-associated variants bind a formin-derived PLP peptide with similar affinities. However, chemical denaturation experiments suggest that the folding stability of these profilins impact some of these effects on actin assembly. Thus, changes in profilin protein stability and alterations in actin filament polymerization may both contribute to the profilin-mediated actin disruptions in ALS.

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).

Mutant Profilin1 Aggregation in Amyotrophic Lateral Sclerosis: An in Vivo Biochemical Analysis

INTRODUCTION

Profilin1 (PFN1) is a ubiquitously expressed protein known for its function as a regulator of actin polymerization and dynamics. A recent discovery linked mutant PFN1 to Amyotrophic Lateral Sclerosis (ALS), which is a fatal and progressive motor neuron disease. We have also demonstrated that Gly118Val mutation in PFN1 is a cause of ALS, and the formation of aggregates containing mutant PFN1 may be a mechanism for motor neuron death. Hence, we were interested in investigating the aggregation of PFN1 further and searching for co-aggregated proteins in our mouse model overexpressing mutant PFN1.

METHODS

We investigated protein aggregation in several tissues of transgenic and notransgenic mice using western blotting. To further understand the neurotoxicity of mutant PFN1, we conducted a pull-down assay using an insoluble fraction of spinal cord lysates from hPFN1G118V transgenic mice. For this assay, we expressed His6-tagged PFN1WT and PFN1G118V in E. coli and purified these proteins using the Ni-NTA column.

RESULTS

In this study, we demonstrated that mutant PFN1 forms aggregate in the brain and spinal cord of hPFN1G118V mice, while WT-PFN1 remains soluble. Among these tissues, spinal cord lysates were found to have PFN1 bands at higher molecular weights recognized with anti-PFN1. Moreover, the pull-down assay using His6-PFN1G118V showed that Myelin Binding Protein (MBP) was present in the insoluble fraction.

CONCLUSION

Our analysis of PFN1 aggregation in vivo revealed further details of mutant PFN1 aggregation and its possible complex formation with other proteins, providing new insights into the ALS mechanism.

Mutation of PFN1 Gene in an Early Onset, Polyostotic Paget-like Disease

CONTEXT

Paget disease of bone (PDB) is a metabolic bone disease whose genetic cause remains unknown in up to 50% of familial patients.

OBJECTIVE

Our aim was to investigate the underlying genetic defect in a large pedigree with a severe, early onset, autosomal dominant form of PDB across 3 generations.

METHODS

Whole exome sequencing was performed in affected and unaffected family members, and then mutation screening was replicated in a sample of PDB patients with early-onset, polyostotic PDB.

RESULTS

We identified a frameshift D107Rfs*3 mutation in PFN1 (encoding for profilin 1, a highly conserved regulator of actin-polymerization and cell motility) causing the truncation of the C-terminal part of the protein. The mutation was also detected in a 17-year-old asymptomatic family member who upon biochemical and radiological analyses was indeed found to be affected. Sequencing of the entire PFN1 coding region in unrelated PDB patients identified the same mutation in 1 patient. All mutation carriers had a reduced response to bisphosphonates, requiring multiple zoledronate infusions to control bone pain and achieve biochemical remission over a long term. In vitro osteoclastogenesis in peripheral blood mononuclear cells (PBMCs) from mutation carriers showed a higher number of osteoclasts with PDB-like features. A similar phenotype was observed upon PFN1 silencing in murine bone marrow-derived monocytes, suggesting that the frameshift PFN1 mutation confers a loss of function in profilin 1 activity that induces PDB-like features in the osteoclasts, likely due to enhanced cell motility and actin ring formation.

CONCLUSIONS

Our findings indicate that PFN1 mutation causes an early onset, polyostotic PDB-like disorder.

A Systematic and Comprehensive Review on Disease-Causing Genes in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder and is characterized by degeneration and axon loss from the upper motor neuron, that descends from the lower motor neuron in the brain. Over the period, assorted outcomes from medical findings, molecular pathogenesis, and structural and biophysical studies have abetted in providing thoughtful insights underlying the importance of disease-causing genes in ALS. Consequently, numerous mechanisms were proposed for the pathogenesis of ALS, considering protein mutations, aggregation, and misfolding. Besides, the answers to the majority of ALS cases that happen to be sporadic still remain obscure. The application in discovering susceptibility factors in ALS contemplating the genetic factors is to be further dissevered in the future years with innovation in research studies. Hence, this review targets in revisiting the breakthroughs on the disease-causing genes related with ALS.

Profilin2a-phosphorylation as a regulatory mechanism for actin dynamics

Profilin is a major regulator of actin dynamics in multiple specific processes localized in different cellular compartments. This specificity is not only meditated by its binding to actin but also its interaction with phospholipids such as phosphatidylinositol (4,5)-bisphosphate (PIP₂ ) at the membrane and a plethora of proteins containing poly-L-proline (PLP) stretches. These interactions are fine-tuned by posttranslational modifications such as phosphorylation. Several phospho-sites have already been identified for profilin1, the ubiquitously expressed isoform. However, little is known about the phosphorylation of profilin2a. Profilin2a is a neuronal isoform important for synapse function. Here, we identified several putative profilin2a phospho-sites in silico and tested recombinant phospho-mimetics with regard to their actin-, PLP-, and PIP₂ -binding properties. Moreover, we assessed their impact on actin dynamics employing a pyrene-actin polymerization assay. Results indicate that distinct phospho-sites modulate specific profilin2a functions. We could identify a molecular switch site at serine residue 71 which completely abrogated actin binding-as well as other sites important for fine-tuning of different functions, for example, tyrosine 29 for PLP binding. Our findings suggest that differential profilin2a phosphorylation is a sensitive mechanism for regulating its neuronal functions. Moreover, the dysregulation of profilin2a phosphorylation may contribute to neurodegeneration.

Activation of Cofilin Increases Intestinal Permeability via Depolymerization of F-Actin During Hypoxia in vitro

Mechanical barriers play a key role in maintaining the normal function of the intestinal mucosa. The barrier function of intestinal epithelial cells is significantly damaged after severe hypoxia. However, the molecular mechanisms underlying this hypoxia-induced damage are still not completely clear. Through the establishment of an in vitro cultured intestinal epithelial cell monolayer model (Caco-2), we treated cells with hypoxia or drugs [jasplakinolide or latrunculin A (LatA)] to detect changes in the transepithelial electrical resistance (TER), the expression of the cellular tight junction (TJ) proteins zonula occludens-1 (ZO-1) and occludin, the distribution of F-actin, the ratio of F-actin/G-actin content, and the expression of the cofilin protein. The results showed that hypoxia and drug treatment could both induce a significant reduction in the TER of the intestinal epithelial cell monolayer and a significant reduction in the expression of the ZO-1 and occludin protein. Hypoxia and LatA could cause a significant reduction in the ratio of F-actin/G-actin content, whereas jasplakinolide caused a significant increase in the ratio of F-actin/G-actin content. After hypoxia, cofilin phosphorylation was decreased. We concluded that the barrier function of the intestinal epithelial cell monolayer was significantly damaged after severe burn injury. The molecular mechanism might be that hypoxia-induced F-actin depolymerization and an imbalance between F-actin and G-actin through cofilin activation resulted in reduced expression and a change in the distribution of cellular TJ proteins.

The Peripheral Nervous System in Amyotrophic Lateral Sclerosis: Opportunities for Translational Research

Although amyotrophic lateral sclerosis (ALS) has been considered as a disorder of the motor neuron (MN) cell body, recent evidences show the non-cell-autonomous pathogenic nature of the disease. Axonal degeneration, loss of peripheral axons and destruction of nerve terminals are early events in the disease pathogenic cascade, anticipating MN degeneration, and the onset of clinical symptoms. Therefore, although ALS and peripheral axonal neuropathies should be differentiated in clinical practice, they also share damage to common molecular pathways, including axonal transport, RNA metabolism and proteostasis. Thus, an extensive evaluation of the molecular events occurring in the peripheral nervous system (PNS) could be fundamental to understand the pathogenic mechanisms of ALS, favoring the discovery of potential disease biomarkers, and new therapeutic targets.

Changes in biophysical characteristics of PFN1 due to mutation causing amyotrophic lateral sclerosis

Single amino acid mutations in profilin 1 (PFN1) have been found to cause amyotrophic lateral sclerosis (ALS). Recently, we developed a mouse model for ALS using a PFN1 mutation (glycine 118 to valine, G118V), and we are now interested in understanding how PFN1 becomes toxically lethal with only one amino acid substitution. Therefore, we studied mutation-related changes in the PFN1 protein and hypothesized that such changes significantly disturb its structure. Initially, we expressed and studied the purified PFN1^WT and PFN1^G118V proteins from bacterial culture. We found that the PFN1^G118V protein has a different mean residue ellipticity, as measured by far-UV circular dichroism, accompanied by a spectral shift. The intrinsic fluorescence of PFN1^G118V showed a small fluctuation in maximum fluorescence absorption and intensity. Moreover, we examined the time course of PFN1 aggregation using SDS-PAGE, western blotting, and MALDI-TOF/TOF and found that compared with PFN1^WT, PFN1^G118V had an increased tendency to form aggregates. Dynamic light scattering data confirmed this, showing a larger size distribution for PFN1^G118V. Our data explain why PFN1^G118V tends to aggregate, a phenotype that may be the basis for its neurotoxicity.

Dysregulation of a novel miR-1825/TBCB/TUBA4A pathway in sporadic and familial ALS

Genetic and functional studies suggest diverse pathways being affected in the neurodegenerative disease amyotrophic lateral sclerosis (ALS), while knowledge about converging disease mechanisms is rare. We detected a downregulation of microRNA-1825 in CNS and extra-CNS system organs of both sporadic (sALS) and familial ALS (fALS) patients. Combined transcriptomic and proteomic analysis revealed that reduced levels of microRNA-1825 caused a translational upregulation of tubulin-folding cofactor b (TBCB). Moreover, we found that excess TBCB led to depolymerization and degradation of tubulin alpha-4A (TUBA4A), which is encoded by a known ALS gene. Importantly, the increase in TBCB and reduction of TUBA4A protein was confirmed in brain cortex tissue of fALS and sALS patients, and led to motor axon defects in an in vivo model. Our discovery of a microRNA-1825/TBCB/TUBA4A pathway reveals a putative pathogenic cascade in both fALS and sALS extending the relevance of TUBA4A to a large proportion of ALS cases.

Cucurbitacin E Inhibits Proliferation and Migration of Intestinal Epithelial Cells via Activating Cofilin

The proliferation and migration of intestinal epithelial cell is important to the barrier integrity of intestinal epithelium. Cucurbitacin E (CuE) is one of the tetracyclic triterpenoids extracted from the cucurbitaceae that has been shown to inhibit cancer cell growth, tumor angiogenesis and inflammatory response. Nevertheless, the role of Cucurbitacin E in regulating the proliferation and migration of intestinal epithelial cells remain unclear. In this study, the human intestinal epithelial cell line Caco-2 was treated with CuE and the effects of CuE on cell cycle, proliferation, migration and actin dynamics in Caco-2 cells were investigated successively. We found that CuE significantly inhibited the cell proliferation and migration, inducing the cell cycle arrest in G2/M phase and disrupting the actin dynamic balance in Caco-2 cells. Finally, we showed that CuE inhibited cofilin phosphorylation by suppressing the phosphorylation of both LIM kinase (LIMK)1 and LIMK2 in vitro, resulting in the activation of cofilin, which is closely associated with cell proliferation and migration. Therefore, our studies provided the first evidence that CuE inhibited the proliferation and migration of intestinal epithelial cells via activating cofilin, and CuE is a potential candidate in intestinal disease therapy.

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.

Profilin Directly Promotes Microtubule Growth through Residues Mutated in Amyotrophic Lateral Sclerosis

Profilin is an abundant actin monomer-binding protein with critical actin regulatory roles in vivo [1, 2]. However, profilin also influences microtubule dynamics in cells, which may be mediated in part through its interactions with formins that in turn bind microtubules [3, 4]. Specific residues on human profilin-1 (PFN1) are mutated in patients with amyotrophic lateral sclerosis (ALS) [5, 6]. However, the observation that some ALS-linked PFN1 mutants fail to alter cellular actin organization or dynamics [5-8] or in vitro actin-monomer affinity [9] has been perplexing, given that profilin is best understood as an actin regulator. Here, we investigated direct effects of profilin on microtubule dynamics and whether ALS-linked mutations in PFN1 disrupt such functions. We found that human, fly, and yeast profilin homologs all directly enhance microtubule growth rate by several-fold in vitro. Microtubule stimulatory effects were unaffected by mutations in the canonical actin- or poly-proline-binding sites of profilin. Instead, microtubule activities depended on specific surface residues on profilin mutated in ALS patients. Furthermore, microtubule effects were attenuated by increasing concentrations of actin monomers, suggesting competition between actin and microtubules for binding profilin. Consistent with these biochemical observations, a 2-fold increase in the expression level of wild-type PFN1, but not the ALS-linked PFN1 mutants, increased microtubule growth rates in cells. Together, these results demonstrate that profilin directly enhances the growth rate of microtubules. They further suggest that ALS-linked mutations in PFN1 may perturb cellular microtubule dynamics and/or the coordination between the actin and microtubule cytoskeletons, leading to motor neuron degeneration.

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.

Biophysical analysis of three novel profilin-1 variants associated with amyotrophic lateral sclerosis indicates a correlation between their aggregation propensity and the structural features of their globular state

Profilin-1 is a small protein involved in actin-mediated cytoskeleton rearrangement. Recently, mutations of profilin-1 have been associated with familial amyotrophic lateral sclerosis. It was previously reported that pathogenic mutations of profilin-1 increase the aggregation propensity of this protein, leaving its function unaffected. However, it is not clear if the mutations act by decreasing the conformational stability or by promoting structural perturbations of the folded state of this protein. In this work we have purified three novel profilin-1 mutants that were recently discovered and have investigated their conformational stability, structural features and aggregation behaviour in vitro. Analysis of the data obtained with the three novel variants, and a global statistical analysis with all profilin-1 mutants so far characterised, indicate significant correlations between aggregation propensity and structural perturbations of the folded state, rather than its conformational stability, in this group of mutants.

The Actin Cytoskeleton in SMA and ALS: How Does It Contribute to Motoneuron Degeneration?

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are neurodegenerative diseases with overlapping clinical phenotypes based on impaired motoneuron function. However, the pathomechanisms of both diseases are largely unknown, and it is still unclear whether they converge on the molecular level. SMA is a monogenic disease caused by low levels of functional Survival of Motoneuron (SMN) protein, whereas ALS involves multiple genes as well as environmental factors. Recent evidence argues for involvement of actin regulation as a causative and dysregulated process in both diseases. ALS-causing mutations in the actin-binding protein profilin-1 as well as the ability of the SMN protein to directly bind to profilins argue in favor of a common molecular mechanism involving the actin cytoskeleton. Profilins are major regulat ors of actin-dynamics being involved in multiple neuronal motility and transport processes as well as modulation of synaptic functions that are impaired in models of both motoneuron diseases. In this article, we review the current literature in SMA and ALS research with a focus on the actin cytoskeleton. We propose a common molecular mechanism that explains the degeneration of motoneurons for SMA and some cases of ALS.

Mutant Profilin1 transgenic mice recapitulate cardinal features of motor neuron disease

The recent identification of profilin1 mutations in 25 familial ALS cases has linked altered function of this cytoskeleton-regulating protein to the pathogenesis of motor neuron disease. To investigate the pathological role of mutant profilin1 in motor neuron disease, we generated transgenic lines of mice expressing human profilin1 with a mutation at position 118 (hPFN1G118V). One of the mouse lines expressing high levels of mutant human PFN1 protein in the brain and spinal cord exhibited many key clinical and pathological features consistent with human ALS disease. These include loss of lower (ventral horn) and upper motor neurons (corticospinal motor neurons in layer V), mutant profilin1 aggregation, abnormally ubiquitinated proteins, reduced choline acetyltransferase (ChAT) enzyme expression, fragmented mitochondria, glial cell activation, muscle atrophy, weight loss, and reduced survival. Our investigations of actin dynamics and axonal integrity suggest that mutant PFN1 protein is associated with an abnormally low filamentous/globular (F/G)-actin ratio that may be the underlying cause of severe damage to ventral root axons resulting in a Wallerian-like degeneration. These observations indicate that our novel profilin1 mutant mouse line may provide a new ALS model with the opportunity to gain unique perspectives into mechanisms of neurodegeneration that contribute to ALS pathogenesis.

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.