Specialized roles of neurofilament proteins in synapses: Relevance to neuropsychiatric disorders

Highlights

• Distinctive assemblies of neurofilament subunits are integral components of synapses.

• Synaptic neurofilament proteins are a sizable population in the CNS.

• NF subunits modulate neurotransmission and behavior via interactions with receptors.

• Alterations of NF subunits may contribute to neuropsychiatric dysfunction.

Abstract

Neurofilaments are uniquely complex among classes of intermediate filaments in being composed of four subunits (NFL, NFM, NFH and alpha-internexin in the CNS) that differ in structure, regulation, and function. Although neurofilaments have been traditionally viewed as axonal structural components, recent evidence has revealed that distinctive assemblies of neurofilament subunits are integral components of synapses, especially at postsynaptic sites. Within the synaptic compartment, the individual subunits differentially modulate neurotransmission and behavior through interactions with specific neurotransmitter receptors. These newly uncovered functions suggest that alterations of neurofilament proteins not only underlie axonopathy in various neurological disorders but also may play vital roles in cognition and neuropsychiatric diseases. Here, we review evidence that synaptic neurofilament proteins are a sizable population in the CNS and we advance the concept that changes in the levels or post-translational modification of individual NF subunits contribute to synaptic and behavioral dysfunction in certain neuropsychiatric conditions.

Introduction

Neurofilaments (NFs), the intermediate filaments of mature neurons, are among the most abundant proteins in brain. Unlike the intermediate filaments of other cell types, which are usually homopolymers, NFs in the CNS are hetero-polymers composed of NFL, NFM, NFH and alpha-internexin subunits (Yuan et al., 2006). Although structurally distinctive, these four NF subunits share a basic tripartite domain structure consisting of a conserved central α-helical rod region, a short variable head domain at the amino-terminal end and a tail of highly variable length at the C-terminal end. The short head domain is rich in serine and threonine residues and contains consensus sites for O-linked glycosylation and phosphorylation (Yuan et al., 2012a). The central rod domain, which is relatively conserved among intermediate filament family members, contains long stretches of hydrophobic heptad repeats favoring formation of α-helical coiled-coil dimers. The C-terminal domains contain glutamic- and lysine-rich stretches of varying length that mainly establish the size range (58–200 kDa on SDS gels) of the four NF subunits. Although all intermediate filament types serve roles as structural scaffolds, NF subunit heterogeneity also confers specialized structural properties to NF, in axons where the filaments are extremely long and are often arranged in parallel with uniform spacing conferred by the long C-terminal tails of NFM and NFH extending perpendicularly from the filament core (Rao et al., 2003, Rao et al., 2002). These unique space filling properties of NF facilitate their well-established role in caliber expansion of large-diameter myelinated axons of peripheral nerves, which is critical for effective nerve conduction (Zhu et al., 1997). NFs also extensively cross-link with other cytoskeletal elements along axons to form a large metabolically stable stationary NF network (Nixon and Logvinenko, 1986, Yuan et al., 2015a, Yuan et al., 2009) that is critical to axon caliber expansion (Friede and Samorajski, 1970, Hoffman et al., 1987, Ohara et al., 1993) and organelle distribution along axons (Rao et al., 2011).

Besides these unique structural and functional features, NF subunits are distinguished from other intermediate filament proteins by the complex regulation of their head and tail domains by phosphorylation, especially those of NFM and NFH, which involves actions of multiple protein kinases and phosphatases at many polypeptide sites (Pant and Veeranna, 1995). Although certain phosphorylation events are known to control tail extension and subunit assembly and slow turnover, the purpose of such complex and dynamically changing phosphate topography on NF subunits (de Waegh et al., 1992, Nixon and Sihag, 1991, Pant and Veeranna, 1995) has remained puzzling, given the mainly static structural support roles ascribed to NF. Both the complex hetero-polymeric structure and dynamically changing phosphate topography of NF proteins suggests that individual NF proteins might serve additional biological roles although there has been relatively little exploration of this issue until recently.

NF gene mutations are well recognized as causes of several neurological disorders mainly involving degeneration of peripheral nerve fibers in accordance with the prominent function of NF in supporting large-diameter myelinated axons (Brownlees et al., 2002). Notably, however, NF proteins are abundant in grey matter CNS regions as well as white matter (Chan et al., 1997) but influence caliber expansion much less dramatically in most populations of CNS axons (Dyakin et al., 2010). These observations and evidence that NF subunits can be axonally transported in various minimally assembled forms (including as heterodimers) suggest a broader distribution and range of assembly forms of NF subunits within CNS neurons, including substantial populations in synapses. In light of these findings, alterations of a particular NF subunit as seen in specific brain regions in psychiatric and neuropsychiatric disorders (Cairns et al., 2003, Clinton et al., 2004, Garcia-Sevilla et al., 1997) may reflect an alteration within synapses which influences the clinical phenotype.

Psychiatric diseases, affecting an estimated 54 million Americans yearly, cause mild to severe disturbances in thought or behavior usually in the absence of known changes in axonal integrity. Nevertheless, changes in levels and phosphorylation of NF subunits have consistently been noted in certain psychiatric disorders although the location of these changes within neurons is poorly understood. Psychiatric diseases prominently involve alterations of synaptic transmission. The highly specialized composition of synapses includes not only the well characterized vesicular and protein receptor machinery supporting neurotransmission but also a specialized cytoskeleton important for delivering, inserting, and recycling synaptic components. Like other domains in a neuron, the cytoskeleton in synapses is composed of microtubules, actin filaments, the spectrin-rich membrane skeleton, and as recently shown, NF assemblies (Yuan et al., 2015b, Yuan et al., 2015c). Evidence is also emerging that the cytoskeleton, and especially the NF scaffold, acts as a docking platform to organize the topography of organelles within different neuronal compartments. Rearrangements of this topography are dynamically coordinated at least in part by cellular signals regulating the phosphorylation state of the binding partners (Perrot and Julien, 2009, Rao et al., 2011, Styers et al., 2004, Yuan et al., 2015b). Such evidence suggests a range of possibilities for understanding the newly recognized roles of individual NF subunits in modulating synaptic function. In this review, we consider the properties of synaptic NF assemblies in the CNS and, in this context, raise the possibility that certain changes in levels or phosphorylation of NF subunits reported in neuropsychiatric disorders (Table 1) disrupt synaptic signaling or, in some cases, reflect adaptive or maladaptive responses to these synaptic disruptions.