Specific Features of Neuronal Size and Shape Are Regulated by Tropomyosin Isoforms

Abstract

Spatially distinct populations of microfilaments, characterized by different tropomyosin (Tm) isoforms, are present within a neuron. To investigate the impact of altered tropomyosin isoform expression on neuronal morphogenesis, embryonic cortical neurons from transgenic mice expressing the isoforms Tm3 and Tm5NM1, under the control of the ␤-actin promoter, were cultured in vitro. Exogenously expressed Tm isoforms sorted to different subcellular compartments with Tm5NM1 enriched in filopodia and growth cones, whereas the Tm3 was more broadly localized. The Tm5NM1 neurons displayed significantly enlarged growth cones accompanied by an increase in the number of dendrites and axonal branching. In contrast, Tm3 neurons displayed inhibition of neurite outgrowth. Recruitment of Tm5a and myosin IIB was observed in the peripheral region of a significant number of Tm5NM1 growth cones. We propose that enrichment of myosin IIB increases filament stability, leading to the enlarged growth cones. Our observations support a role for different tropomyosin isoforms in regulating interactions with myosin and thereby regulating morphology in specific intracellular compartments. INTRODUCTIONThe actin cytoskeleton plays an essential role in the structural changes that initially establish neuronal shape and subsequently contribute to the morphological differentiation of neurons. Actin filaments have been implicated in the initial sprouting of neurites, whereas microtubules strengthen and support the new extensions (Smith, 1988(Smith, , 1994.Tropomyosin (Tm) isoforms, integral components of actin microfilaments, form coiled-coil head-to-tail dimers that bind along the major groove of actin polymers (Phillips et al., 1979). Tms are derived from four highly conserved genes known as the ␣Tm fast , ␤Tm, ␥Tm (Tm5NM), and ␦Tm genes that, via alternative splicing, give rise to Ͼ40 isoforms (LeesMiller and Helfman, 1991;Dufour et al., 1998;Cooley and Bergtrom, 2001). Despite the well understood function of Tms in muscle where, together with the troponin complex, they regulate contraction in a calcium-dependent manner, little is known about their role in nonmuscle cells. In vitro studies have implicated Tms in the stabilization of the actin cytoskeleton by protecting actin filaments from the severing action of gelsolin (Ishikawa et al., 1989) and the depolymerizing action of ADF/cofilin (Bernstein and Bamburg, 1982). Gene transfection studies have demonstrated that tropomyosin isoforms can regulate the organization of actin filaments in transformed cells (Prasad et al., 1993;Boyd et al., 1995;Gimona et al., 1996) and the neuroepithelial cell line B35 (Bryce et al., 2003).In neurons, a strict repertoire of Tm isoforms is known to be expressed. These include TmBr3, Tm5a and Tm5b from the ␣Tm fast gene; multiple products from the ␥Tm gene; and Tm4 from the ␦Tm gene. Most interestingly, Tm isoforms have been previously shown to be spatially and temporally regulated, identifying distinct subcellular compartments. The Tm5a and Tm5b isoforms are enric...

Lysine-Specific Demethylase 1 Is Strongly Expressed in Poorly Differentiated Neuroblastoma: Implications for Therapy

“…At this time, 28 genes were significantly induced at least 1.5-fold and 29 genes were significantly repressed at least 1.5-fold. Among the 28 induced genes, four are markers of cytoskeletal remodeling (TNS1, TPM1, DNM2, DNAL4), indicating differentiation, and three (TPM1, DNM2, and SHANK2) are functionally linked to neurite dynamics and synaptic trafficking (19)(20)(21). TaqMan quantitative reverse transcription-PCR confirmed the expression changes detected via microarray analysis for LSD1, DNAL4, DNM2, TNS1, and TPM1 ( Supplementary Fig.…”

Tropomyosin Isoform Expression Regulates the Transition of Adhesions To Determine Cell Speed and Direction

“…B35 cell clones overexpressing Tm5NM1 (B35/Tm5NM1) and Tm3 (B35/Tm3) have been previously described (3,9,34). ␥-Tm knockout mouse lines and primary mouse embryo fibroblasts (MEF/ Tm5NM1 Ϫ/Ϫ ) have also been described (35; T. Fath et al, unpublished data).…”

“…At the molecular level, actin filaments are highly dynamic, and this aspect of actin polymer biology provides an important control mechanism by which cells can organize filaments into structures with distinct properties. Tropomyosins are a multi-isoform family of actin-associating proteins that confer isoform-specific regulation of diverse actin filaments (3,16,34,35). The interdependence of integrin adhesions and actin filaments suggests that expression of actinassociated proteins such as the tropomyosins may represent a mechanism for the regulation of adhesion dynamics that determine cell migration.…”

“…The cytoskeletal tropomyosin Tm5NM1 is a broadly distributed isoform (37) that alters cell shape (34), localizes to and promotes stress fibers that are resistant to actin depolymerizing drugs (9), enhances myosin IIA activation and recruitment to stress fibers, and inhibits cell migration (3). Therefore, we hypothesized that Tm5NM1 expression might determine cell migration by coordinating actin-dependent transition toward a predominance of focal adhesions and fibrillar adhesions.…”

Tropomyosin – master regulator of actin filament function in the cytoskeleton

“…This isoform promotes increased axon length, growth cone size and dendritic branching, whereas expression of the non-neuronal Tpm1.7 (also known as Tm3) in neurons attenuates neurogenesis (Schevzov et al, 2005). By contrast, knockout of Tpm3.1 and Tpm3.2 (also known as Tm5NM2) resulted in a decrease in the size of growth cones with an increased rate of lamellipodia protrusions (Fath et al, 2010).…”