LSm4 Associates with the Plasma Membrane and Acts as a Co-factor in Cell Volume Regulation

Water channel aquaporin 4 (AQP4) plays a key role in the regulation of water homeostasis in the central nervous system (CNS). It is predominantly expressed in astrocytes lining blood–brain and blood–liquor boundaries. AQP4a (M1), AQP4c (M23), and AQP4e, present in the plasma membrane, participate in the cell volume regulation of astrocytes. The function of their splicing variants, AQP4b and AQP4d, predicted to be present in the cytoplasm, is unknown. We examined the cellular distribution of AQP4b and AQP4d in primary rat astrocytes and their role in cell volume regulation. The AQP4b and AQP4d isoforms exhibited extensive cytoplasmic localization in early and late endosomes/lysosomes and in the Golgi apparatus. Neither isoform localized to orthogonal arrays of particles (OAPs) in the plasma membrane. The overexpression of AQP4b and AQP4d isoforms in isoosmotic conditions reduced the density of OAPs; in hypoosmotic conditions, they remained absent from OAPs. In hypoosmotic conditions, the AQP4d isoform was significantly redistributed to early endosomes, which correlated with the increased trafficking of AQP4-laden vesicles. The overexpression of AQP4d facilitated the kinetics of cell swelling, without affecting the regulatory volume decrease. Therefore, although they reside in the cytoplasm, AQP4b and AQP4d isoforms may play an indirect role in astrocyte volume changes.

Download Full-text

Autocrine signaling involved in cell volume regulation: The role of released transmitters and plasma membrane receptors

Journal of Cellular Physiology ◽

10.1002/jcp.21406 ◽

2008 ◽

Vol 216 (1) ◽

pp. 14-28 ◽

Cited By ~ 27

Author(s):

Rodrigo Franco ◽

Mihalis I. Panayiotidis ◽

Lenin D. Ochoa de la Paz

Keyword(s):

Plasma Membrane ◽

Cell Volume ◽

Volume Regulation ◽

Cell Volume Regulation ◽

Membrane Receptors ◽

Autocrine Signaling ◽

Plasma Membrane Receptors

Download Full-text

Functional significance of the cell volume for detecting sperm membrane changes and predicting freezability in dog semen

Reproduction ◽

10.1530/rep.1.00296 ◽

2004 ◽

Vol 128 (6) ◽

pp. 829-842 ◽

Cited By ~ 31

Author(s):

Anna M Petrunkina ◽

Barbara Gröpper ◽

Anne-Rose Günzel-Apel ◽

Edda Töpfer-Petersen

Keyword(s):

Plasma Membrane ◽

Cell Volume ◽

Volume Regulation ◽

Sperm Quality ◽

Cell Volume Regulation ◽

Calcium Ionophore ◽

Induced Response ◽

Ionophore A23187 ◽

Sperm Membrane ◽

Membrane Changes

Due to the similarity of plasma membrane changes induced by capacitation and cryopreservation, the parameters describing sperm response to capacitating conditions can be used for evaluating the cryopreservation response in many animal systems. In dog sperm, the response of the total sperm population to ionophore treatment has been shown to be an indication of the freezability of semen samples. Another sperm functional characteristic decisive for cryopreservability is cell volume regulation, due to the generation of essential osmotic gradients across the plasma membrane during the freeze-thaw cycles. In the present study, cryopreservation-induced changes in the membrane functional integrity were examined by monitoring the osmotically induced response of cell volume and the response to an ionophore in live cell populations. Cell volume measurements were performed on Percoll-washed suspensions of freshly diluted and frozen-thawed dog spermatozoa. The proportion of live acrosome-reacted cells was evaluated by flow cytometry after incubation under capacitating conditions in the presence of the calcium ionophore, A23187. During freezing-thawing, significant membrane changes occurred related to the disturbance of volume control ability and the loss of a proportion of live acrosome-reacted cells (P < 0.05). There were significant differences between individuals with respect to the degree of functional and structural membrane changes after thawing. Significant correlations were found between acrosomal integrity and functional membrane integrity. When assessed in freshly diluted semen, these parameters correlated with those of frozen-thawed semen samples, pointing to the similarities between mechanisms of cryopreservation-related changes and those mechanisms that mediate changes in membrane permeabilities and in cell volume regulation. The detection of changes in the sperm plasma membrane by monitoring the sperm cell volume represents a simple, rapid and sensitive method to estimate sperm quality after the cryopreservation procedure. The individual variability in response to osmotic stress or to calcium ionophore treatment appears to reflect the subtle differences in the sperm membrane functionality which are crucial for the prediction of cryopreservability.

Download Full-text