The interkinetic envelope: A guardian or just a freeloader?

In the meiotic cell division program, two divisions follow one round of DNA duplication, which ultimately halves the chromosome number during gamete formation. In oocytes in interkinesis (the stage between the first and second meiotic divisions), the lack of chromosome decondensation and genome replication led to the assumption that the nuclear envelope does not reassemble at this stage (1). However, a study combining live imaging with serial block-face scanning electron microscopy of Caenorhabditis elegans oocytes reveals the formation of a transient structure called the interkinetic envelope around condensed chromosomes. Although this is not a true nuclear envelope since it does not fully compartmentalize the genome, it shares a double-membrane structure and contains certain inner nuclear membrane- and lamina-associated proteins (2); see Fig. 1 for a schematic comparison.

The interkinetic envelope assembles asymmetrically, initiating on the external chromosome surface and more prominently around the chromosome masses assigned to the second meiotic division. This pattern parallels the sequential disassembly of meiotic spindle pole microtubules before central spindle microtubules in C. elegans oocytes form (3); this suggests a coordinated interplay between microtubule disassembly and interkinetic envelope formation.

A distinguishing feature of the interkinetic envelope is its lack of continuity with the ER; in contrast, the postmitotic nuclear envelope forms through the incorporation of ER membranes (4). This feature likely explains the absence of outer nuclear membrane proteins in the interkinetic envelope. Reasons for this discontinuity with the ER remain unclear but may involve an as-yet-unidentified physical barrier that prevents fusion with ER-derived membranes. Small membrane fragments—presumably not ER derived—are located near to the chromosomes appear to contribute to interkinetic envelope assembly. These fragments may originate from nuclear envelope remnants that persist after oocyte diakinesis. The lipid composition of the interkinetic envelope is unknown but may determine its dynamic properties and directly influence its lipid–protein composition (5). Unlike the nuclear envelope, the interkinetic envelope does not fully seal, which allows condensed chromosomes to remain in direct contact with the cytoplasm and enables the unrestricted exchange of components. The lack of a complete seal may be explained by the absence of ESCRT-III complex proteins, such as CHMP-7 (human CHMP7) and VPS-32 (human CHMP4).

Nucleoporins with membrane-binding domains appear to play a nonconventional role in interkinetic envelope assembly, as distinct from their canonical function in nuclear pore formation. Unlike in interphase, nucleoporins are not consistently co-recruited in their usual subcomplexes. For example, NPP-21 (human TPR) is absent despite the presence of the nuclear pore basket protein NPP-7 (human NUP153), and the inner ring nucleoporin NPP-19 (human NUP35) localizes to the interkinetic envelope without NPP-8 (human NUP155), highlighting that unique nucleoporin recruitment patterns occur in this context. The interkinetic envelope also lacks the nucleoporins that are essential for nuclear pore assembly (such as NPP-8) and does not form functional fully assembled nuclear pore complexes. The absence of such nucleoporins is consistent with the transient nature of the interkinetic envelope, which exists only during the brief transition between meiosis I and II when nuclear pore–mediated trafficking between the nucleus and cytoplasm is not needed. The nucleoporin MEL-28 (human ELYS) helps to recruit nucleoporins with potential membrane-binding domains to the interkinetic envelope, which may facilitate membrane assembly. MEL-28’s role in nucleoporin recruitment could involve regulation of the phosphorylation state of essential components via the phosphatase PP1, which can dock onto chromosomes via MEL-28/ELYS during postmitotic-meiotic nuclear envelope reassembly (6). In the absence of MEL-28, small membrane fragments are still present but do not associate with chromosomes, possibly because of a spatial barrier formed by ectopic microtubules. Nocodazole-mediated microtubule destabilization in absence of MEL-28 allowed membrane fragments to associate with the chromosomes destined for meiosis II; however, it remains unclear whether these fragments can form an interkinetic envelope comparable with the wild type.

The key regulator of postmitotic nuclear envelope reassembly, barrier to autointegration factor 1 (BAF-1), binds to condensed chromosomes and brings LEM (LAP2, emerin, and MAN1) domain nuclear lamina–associated proteins to the chromosomes and cross-links them to create a mechanically rigid chromosome surface (7). Depletion of BAF-1 compromised the structural integrity of the interkinetic envelope, leading to fenestration and the presence of unwanted membrane fragments between chromosomes. BAF-1 depletion also prevented LEM-2 and EMR-1 (human emerin) association with chromosomes. Interestingly, overloading chromatin with ectopic BAF-1 also caused a fenestrated interkinetic envelope but without ectopic interchromosomal membrane fragments, indicating that these phenotypes are distinct. However, the observation that both the absence and over-recruitment of LEM-2 result in the same fenestrated phenotype suggests that precise regulation of LEM-domain protein localization is critical for proper interkinetic envelope assembly.

In summary, El Mossadeq and colleagues present a salient detailed cytological analysis of the interkinetic envelope that forms around the chromosome masses destined to participate in the second meiotic division. They identify the main regulators of this process as the BAF-1–LEM-domain protein module and the nucleoporin MEL-28/ELYS, along with other nuclear membrane nucleoporins. But what is the functional significance of the interkinetic envelope? Due to the highly transient nature of the interkinetic envelope and the universal roles of the main players, it is difficult to specifically interfere with formation of the interkinetic envelope without disturbing other aspects of cell division. BAF-1 depletion accelerated anaphase of the first meiotic division and MEL-28 depletion caused the chromosomes disposed for meiosis II to become more loosely spread. These results are consistent with a role for the interkinetic envelope in preventing chromosomes from joining polar body–containing chromosome groups or becoming stray micronuclei. Furthermore, BAF-1–mediated “chromosome cohesion” could prevent unwanted cellular material from interfering with the chromosomes destined to undergo meiosis II. Imprecise chromosome partitioning generates an unbalanced chromosome content in oocytes, which is detrimental to the offspring. Alternatively, although incompletely sealed, the interkinetic envelope could provide a barrier that protects chromosomes from exposure to potentially harmful cytoplasmic components, such as organelles, oxidative enzymes, nucleases, or enzymes, that pose a threat to genome integrity.

Finally, condensed chromosomes may attract nuclear membrane vesicles and associated proteins for recycling during the ensuing embryonic divisions, as previously shown for the inner nuclear membrane protein SUN-1 (8). Of course, all of the suggested functions are not mutually exclusive.

Is this a C. elegans–specific phenomenon? This study provides an incentive to hunt for the existence of an interkinetic envelope in other organisms that undergo open or semi-open nuclear division. Identifying an experimental system where interfering with the interkinetic envelope is less challenging should help to determine its definitive function. This could be contributing to the fidelity of chromosome segregation or protecting genome integrity in the offspring.