Kinetochores are multiprotein complexes that link chromosomes to microtubules and are essential for chromosome segregation during cell divisions. In this issue, Alves Domingos et al. (https://doi.org/10.1083/jcb.202311147) show that the conserved KNL-1/Knl1 protein of the Knl1/Mis12/Ndc80 (KMN) outer kinetochore complex postmitotically regulates F-actin to shape somatosensory dendrites.

Kinetochores are multiprotein assemblies that mediate attachment of spindle microtubules to chromosomes and are essential for cell divisions (1, 2). They comprise two major protein complexes: the inner kinetochore complex, which is comprised of several subcomplexes, and the outer kinetochore, which is composed of three subcomplexes (Fig. 1 A). These include the Mis12 complex, a hub that regulates and coordinates interactions between the inner and outer kinetochores, as well as the interaction of the Ndc80 complex as the primary microtubule receptor and the Knl1 complex (Fig. 1 A) (1, 2). The outer kinetochore complex is therefore also termed the Knl1/Mis12/Ndc80 (KMN) complex. The KMN complex is regulated by binding of additional proteins and complexes to Knl1, including the protein phosphatase 1 (PP1) and the spindle assembly checkpoint, among others (1, 2). The overall structure and function of the kinetochore is conserved from yeast to man, although differences exist in the composition of some subcomplexes (1, 2).

Recent work in the fruit fly Drosophila melanogaster and in the small nematode Caenorhabditis elegans showed that proteins of the KMN complex serve unexpected postmitotic functions during synaptic, axonal, and dendrite development (5, 6). These studies revealed that several proteins of the KMN kinetochore complex were localized outside of the nucleus to the periphery, including in synapses, axons, and dendrites. For example, staining of different proteins of the KMN complex in C. elegans and Drosophila revealed staining in axons and sensory dendrites (5, 6). Genetically removing components of the KMN complex in flies or worms resulted in abnormal neurite growth, including of dendrites. These mechanisms appeared conserved in cultured mouse hippocampal neurons, suggesting evolutionary conservation of the postmitotic functions of the KMN complex in neural patterning (5). However, the mechanisms by which the KMN complex regulated axon or dendrite morphogenesis remained largely elusive.

The work by Alves Domingos et al. (4) is now beginning to shed light on this process using the PVD mechanosensory neurons in C. elegans, which have recently emerged as an excellent model to study conserved mechanisms of dendrite morphogenesis (3). Born postembryonically, the pair of PVD neurons elaborate stereotyped dendritic trees, which can be easily observed by live imaging (Fig. 1 B). A primary dendrite emanates in both anterior and posterior directions from the cell body to then branch into regularly spaced orthogonal secondary dendrites. These form orthogonal tertiary dendrites from which quaternary dendrites emerge to create candelabra-like structures, also called “menorahs” (Fig. 1 B) (3).

Alves Domingos et al. found a reporter for KNL-1/Knl1 localized to the PVD cell body and along dendrites, including at branch points. To circumvent the essential functions of kinetochore proteins, they elegantly degraded components of the KMN complex only in postmitotic PVD somatosensory neurons (4). These studies showed that PVD neurons lacking components of the three subcomplexes of the KMN network, including the KNL-1/Knl1, Mis12, and Ndc80 complexes, displayed characteristic defects in dendrite morphogenesis, including an excess of quaternary branches and increased membrane protrusions (Fig. 1 C). Conversely, overexpression of KNL-1/Knl1 resulted in reduced numbers of quaternary branches in PVD. Through time-lapse imaging and careful morphometric analyses, the authors further detected defects in contact-mediated self-avoidance of tertiary dendrites of PVD (Fig. 1 C) (4). Finally, animals in which KNL-1/Knl1 was degraded in PVD neurons showed premature dendrite degeneration. In accordance with the observed morphological PVD defects, animals in which the KMN complex was degraded in PVD neurons also displayed defects in PVD-mediated behaviors (4). Behavioral defects included abnormalities in proprioception and the sensation of harsh touch, both functions served by PVD neurons (7, 8).

Since the kinetochore complex links chromosomes to the microtubules of the spindles during cell divisions, an obvious question was whether microtubule polarity or dynamics are perturbed in PVD dendrites of animals in which KNL-1/Knl1 was compromised in PVD neurons. Surprisingly, no changes were observed in microtubule polarity of PVD dendrites, consistent with similar observations by the same group in another set of sensory neurons in C. elegans (9). While no apparent changes in the overall dynamics of the microtubules were observed in KNL-1/Knl1 mutants, the authors could detect an increase in microtubule growth dynamics at the microtubule plus end of primary dendrites (4).

Dendrite self-avoidance of PVD sensory dendrites was known to be dependent on tightly regulated actin dynamics (10, 11). Therefore, the authors next investigated the localization of F-actin in PVD neurons. Compromising KNL-1/Knl1 in PVD neurons lead to increased amounts of F-actin in the PVD cell body and dendritic branches (Fig. 1 D), suggesting that KNL-1/Knl1 and, by inference the KMN complex, is involved in regulating F-actin dynamics. Consistent with this interpretation, the increase in actin and the associated phenotypes in KMN mutants required formin, a regulator of unbranched F-actin. Similarly, pharmacologically compromising F-actin with the toxin latrunculin-A in animals lacking KNL-1/Knl1 was able to suppress the increased number of quaternary branches and the self-avoidance defects, as well as the accumulation of excessive dendritic F-actin in PVD dendrites.

Alves Domingos et al. finally also employed a gain of function approach. Surprisingly, animals in which the KNL-1/Knl1 protein was forced to localize to the plasma membrane also showed increases in the amount of F-actin at the periphery of the cell body. This increase in F-actin was dependent on specific protein domains in KNL-1/Knl1. For example, the domain of KNL-1/Knl1 that interacts with the PP1 was required for the formation of the actin clusters, suggesting a role for the regulatory phosphatase in this process. In contrast, the interaction with the spindle assembly checkpoint complex was not required for the formation of actin clusters. Similarly, the interaction of KNL-1/Knl1 domains that interact with Ndc80, which is the microtubule receptor of the KMN network, were dispensable. Therefore, KNL-1/Knl1 and possibly the KMN complex may function by novel mechanisms to regulate F-actin.

Taken together, the studies by Alves Domingos et al. reveal that, unexpectedly, the KMN complex is required for morphogenesis of somatosensory dendrites. Specifically, the KNL-1/Knl1 protein does so by regulating F-actin. Important questions remain for the future. What is the role, if any, of the inner kinetochore complex in dendrite morphogenesis? Are other components of the KMN complex also required for regulating actin dynamics, and if so, which ones? What are the binding partners of the KMN complex, and is it anchored along the dendrites or at specific subcellular locations? What is the functional significance of the KMN complex in regulating microtubule end dynamics? Could the KMN complex be mediating the dynamics or interplay between actin and microtubules, and how is the process itself regulated? Could KNL-1/Knl1 utilize more than one mechanism to regulate actin? Lastly, what are the roles of the PP1 in this process, and what other regulatory factors may be involved? Many of these questions can now be directly addressed.

Work in the author’s laboratory is supported by grants from the National Institute of Health (R01NS096672, R21NS081505, and R01NS129992).

Author contributions: H.E. Bülow: visualization and writing—original draft, review, and editing.

1
Musacchio
,
A.
, and
A.
Desai
.
2017
.
Biology
.
2
Cheeseman
,
I.M.
2014
.
Cold Spring Harb. Perspect. Biol.
3
Heiman
,
M.G.
, and
H.E.
Bülow
.
2024
.
Genetics
.
4
Alves Domingos
,
H.
, et al
.
2025
.
J. Cell Biol.
6
Cheerambathur
,
D.K.
, et al
.
2019
.
Dev. Cell
.
7
Way
,
J.C.
, and
M.
Chalfie
.
1989
.
Genes Dev.
8
Albeg
,
A.
, et al
.
2011
.
Mol. Cell Neurosci.
9
Ouzounidis
,
V.R.
, et al
.
2024
.
Mol. Biol. Cell
.
11
Sundararajan
,
L.
, et al
.
2019
.
PLoS Genet.

Author notes

Disclosures: The author has completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.

This article is distributed under the terms as described at https://rupress.org/pages/terms102024/.