Sperm Differentiation: The Role of Trafficking of Proteins

Abstract

Sperm differentiation encompasses a complex sequence of morphological changes that takes place in the seminiferous epithelium. In this process, haploid round spermatids undergo substantial structural and functional alterations, resulting in highly polarized sperm. Hallmark changes during the differentiation process include the formation of new organelles, chromatin condensation and nuclear shaping, elimination of residual cytoplasm, and assembly of the sperm flagella. To achieve these transformations, spermatids have unique mechanisms for protein trafficking that operate in a coordinated fashion. Microtubules and filaments of actin are the main tracks used to facilitate the transport mechanisms, assisted by motor and non-motor proteins, for delivery of vesicular and non-vesicular cargos to specific sites. This review integrates recent findings regarding the role of protein trafficking in sperm differentiation. Although a complete characterization of the interactome of proteins involved in these temporal and spatial processes is not yet known, we propose a model based on the current literature as a framework for future investigations.

Keywords: acrosome; manchette; protein trafficking; sperm differentiation.

Figure 1

Schematic representation of the processes involved in remodeling of nuclear architecture during sperm differentiation. Illustrations represent mouse spermatids. (A) Sertoli cells influence nuclear remodeling by an ectoplasmic specialization that encircles more than one-third of the spermatid head and generates external pressure. (B) The manchette, a scaffolding of microtubules and actin filaments, encircles two-thirds of the remaining area of the nucleus. It connects intimately with the nuclear lamina and generates internal forces by a zipper-like movement. (C) Replacement of histones by transition proteins and then protamines generates the last twists for chromatin condensation. (D) The acroplaxome, which covers more than one-third of the head, modulates the external forces and anchors the acrosome to the nucleus. Additionally, the (E) nuclear membrane restructures its molecular composition, redistributing proteins in order to adjust the flexibility of the membrane to allow for external and internal forces. Red arrows indicate the direction of forces.

Figure 2

Schematic representation of the Golgi transport mechanism. Several proteins have been identified to play a role in acrosome biogenesis and vesicle transport. Golgi transport involves several steps including protein trafficking from the reticulum to the Golgi apparatus, vesicle transport to the acrosome (Acr) via microtubules and F-actin tracks, vesicle fusion, interaction with the acroplaxome, interaction with the nuclear envelop (NE), and interaction with the acrosomal matrix.

Figure 3

Model illustrating the intramanchette transport mechanism. Illustration represents a mouse spermatid. The manchette is a transitory organelle surrounding the elongating spermatid nucleus. It consists of bundles of microtubules connected to a perinuclear ring and filaments of actin intercalated between the microtubules. Proteins are transported on these tracks to specific intracellular sites during the process of sperm differentiation. Some proteins form large complexes that can transport vesicular as well as non-vesicular cargos.

Figure 4

Interactome of proteins involved in intramanchette transport. The Ingenuity Pathway Analysis (IPA), software from QIAGEN Inc., was used for protein interaction analysis. Proteins known to localize in the manchette were analyzed. Only proteins with validated protein–protein binding were included in the analysis. The tool “Path Explorer” in the IPA software was used to generate the interactome. Gray lines are interactions found by the software in IPA databases. Blue lines indicate known interaction validated from current literature by IP, co-IP, yeast two-hybrid screen, tubulin-binding assay, and affinity column purification. Species were limited to human, mouse, and rat.

Figure 5

Interactome of proteins involved in Golgi transport. The Ingenuity Pathway Analysis (IPA), software from QIAGEN Inc., was used for protein interaction analysis. Proteins known to participate in Golgi transport were analyzed. Only proteins with validated protein–protein binding were included in the analysis. The tool “Path Explorer” in the IPA software was used to generate the interactome. Gray lines are interactions found by the software in IPA databases. Blue lines indicate known interaction validated from current literature by IP, co-IP, yeast two-hybrid screen, tubulin-binding assay, and affinity column purification. Species were limited to human, mouse, and rat.

Figure 6

Examples of stochastically optical reconstruction microscopy (STORM) super-resolution imaging in mouse sperm cells. (A) Representative color-coded 3D STORM reconstruction of the actin cytoskeleton in the flagellum of mouse sperm. Actin was labeled with phalloidin-AlexaFluor 647. The midpiece and the principal piece (PP) are indicated for clarity. Scale bar: 5 μm. Imaged by Xinran Xu and Maria G. Gervasi [223]. The actin structure in the midpiece is found to be arranged in a double helix. (B) Zoom of the midpiece structure. Scale bar: 500 nm. (C) Cross section of the midpiece structure. (D) Representative 3D STORM reconstruction of actin-associated protein adducin in the mouse sperm flagellum. (E) Representative 3D STORM reconstruction of the actin cytoskeleton in the head of non-capacitated mouse sperm. Imaged by Xinran Xu and Mariano Buffone [228]. The color bar on the right shows the axial localization of this reconstruction.