Exosome secretion kinetics are controlled by temperature

Based on: Mahmood A, Otruba Z, Weisgerber AW, et al. Exosome Secretion Kinetics are Controlled by Temperature. Biophysical Journal. 2023.

Introduction Exosomes have rapidly emerged as one of the most discussed topics in regenerative medicine, oncology, neurology, and biomarker research. These nanoscale extracellular vesicles serve as biological messengers, carrying proteins, lipids, RNA, and other signaling molecules between cells.

While much attention has focused on the therapeutic potential of exosomes, a fundamental question has remained incompletely understood:

How are exosomes actually released from cells, and what factors regulate that process?

A landmark 2023 study by Mahmood and colleagues provides important mechanistic insights. Using advanced live-cell imaging and computational modeling, the authors demonstrated that exosome secretion is highly temperature-dependent. Their findings reveal that temperature influences not only how frequently exosomes are released but also how rapidly they disperse after secretion.

These observations have important implications for laboratory research, exosome manufacturing, regenerative medicine, and potentially our understanding of disease physiology. Understanding Exosome Biology Before examining the study, it is useful to review how exosomes are generated.

Exosomes are small extracellular vesicles approximately 30–100 nanometers in diameter. They originate inside cells within structures known as multivesicular endosomes (MVEs).

The process occurs in three major steps:

1. Formation of intraluminal vesicles inside MVEs

2. Transport of MVEs toward the plasma membrane

3. Fusion of the MVE with the plasma membrane, releasing intraluminal vesicles as exosomes

Once released, exosomes can travel locally or systemically and influence recipient cells through transfer of bioactive cargo.

Exosomes have been implicated in:

• Tissue repair and regeneration

• Immune modulation

• Tumor progression

• Neurodegenerative diseases

• Angiogenesis

• Stem cell signaling
  

Because of these diverse roles, understanding the kinetics of exosome release is critical.

The Clinical Problem Previous studies established that exosomes are released when MVEs fuse with the cell membrane. However, researchers did not fully understand:

• How rapidly exosomes leave the fusion site

• Whether all exosomes are immediately released

• What controls secretion dynamics

• How environmental conditions influence release
  

Temperature was suspected to play a role because most cellular trafficking processes depend on membrane fluidity, enzyme activity, and cytoskeletal function.Yet direct evidence was limited.

This study sought to answer these questions through real-time observation of individual exosome release events.

Fig. 1. (a) Schematic of exosome biogenesis. Multivesicular endosomes (MVEs) form and encapsulate exosomes. MVEs either fuse with the plasma membrane to release exosomes into the extracellular space or fuse with lysosomes for degradation. In contrast, microvesicles are generated by direct budding from the plasma membrane. Exosomes carry proteins, DNA, RNA, and membrane-associated proteins characteristic of their cell of origin (Colombo et al., 2014).

---

How the Researchers Studied Exosome Release

The investigators used human A549 cells and employed Total Internal Reflection Fluorescence (TIRF) microscopy, a technique capable of visualizing events occurring directly at the cell membrane.

To track exosome secretion, they used a fluorescent reporter called CD63-pHluorin.

CD63 is a well-established exosome marker.

The attached pH-sensitive fluorescent protein remains non-fluorescent inside acidic intracellular compartments. Once an MVE fuses with the plasma membrane and is exposed to extracellular pH, the probe fluoresces brightly, allowing individual secretion events to be visualized in real time.

The researchers then examined secretion behavior across four temperatures:

• 23°C

• 27°C

• 32°C

• 37°C

This enabled precise quantification of how temperature affects exosome release dynamics.

Fig.2. MVE fusion events are visualized using CD63-pHluorin. (A) Diagram depicting the assay used to detect MVE fusion events. CD63- pHluorin is quenched when in an acidic vesicle. Once the MVE fuses, the pH increases and pHluorin emits green fluorescence. (B) Membrane fusion is observable in live A549 cells using TIRFM. A maximum projection of a difference movie with potential fusion events circled in green. The purple arrow marks the event in the example in (C–E). (C) The intensity profile of a single fusion event, where 0 s is defined as the onset of fusion. The intensity shown is the average intensity within a 0.76 mm diameter circle and normalized to the maximum intensity. (D) Montage of a single fusion event. (E) Montage of a difference movie of a single fusion event. Green box is 0 s. Scale bar, 2 mm (cell AM1587).

Fig. 3. Potential fusion events can be categorized as (A) fusion, (B) motion, or (C) docking. (A–C) Show single events, scale bar ¼ 2 mm. (D) Docking (red) is noted by the lack of fluorescence loss in the intensity trace. The slope of a line fit from the maximum intensity to 1 s later is substantially lower than moving or fusing vesicles (n ¼ 15 events each). (E) The moving vesicles can be separated from fusing vesicles based on the diffusion coefficient from tracking analyses (n ¼ 15 vesicles each). (F) Fusion events should also show fluorescence expanding in time as CD63-pHluorin exosomes and CD63- pHluorin on the MVE membrane leave the fusion site. (G) The diameter as a function of time for fusion events (error bars are mean 5 SE, n ¼ 53). (H) The diameter of the fluorescence signal at the onset of fusion reflects the diameter of the MVE. The black dashed line is the diffraction limit of the microscope, measured using a yellow-green FluoSphere (d ¼ 200 nm).

---

Major Finding #1: Higher Temperatures Increase Exosome Secretion Frequency

One of the clearest observations was that exosome release events occurred more frequently at higher temperatures.

As temperature decreased:

• Fusion events became less common

• Exosome secretion slowed

• Cellular release activity declined
  

At physiological temperature (37°C), cells secreted exosomes significantly more often than at room temperature or below.

This finding aligns with what is known about membrane trafficking, where vesicle transport and fusion machinery generally function more efficiently under physiological conditions.

Why This Matters

Many laboratories process cells at room temperature during harvesting, transport, or experimental preparation.

This study suggests that even moderate temperature reductions may substantially alter exosome biology, potentially affecting:

• Exosome yield

• Biomarker measurements

• Experimental reproducibility

• Therapeutic manufacturing outcomes

Major Finding #2: Lower Temperatures Slow Exosome Release The study revealed an additional layer of complexity.

Temperature influenced not only how often exosomes were released but also how rapidly they dispersed after secretion.

When researchers tracked fluorescence loss at secretion sites, they observed:

• Faster signal decay at 37°C

• Slower signal decay at lower temperatures
  

This indicates that exosome-associated material remains near the fusion site longer when temperature decreases.

In practical terms:

Warm cells release exosomes more efficiently and clear secretion sites more rapidly.

Cooler cells retain exosome-associated material longer.

Major Finding #3: Exosome Release Is Not an All-or-Nothing Process

Perhaps the most intriguing finding was that exosome release appeared incomplete: although fluorescence was expected to fully dissipate following secretion, a residual signal frequently persisted at fusion sites. This suggested that some exosomes do not immediately disperse into the extracellular space after release. To account for this unexpected observation, the authors developed computational models to better explain the post-fusion retention behavior.

Fig. 4. CD63-pHluorin fusion events are not lysosomes or trafficking vesicles. (A) CD63-pHluorin was expressed in A549 cells and imaged with TIRFM; an average of five frames (0.85 s) is shown starting with the first frame where fusion was observed within the pink circle. (B) Magic Red labels lysosomes, (C)a single fusion event CD63-pHluorin, and (D) Magic Red. The region is marked by a pink circle in (A). (E) CD63-pHluorin intensity during fusion (average of n ¼ 17 events, 7 cells). (F) Magic Red intensity in time during CD63 fusion events (mean5 SE). The onset of CD63-pHluorin fusion is at 0 s. (G) Brefeldin A (BfA) (5 mg/mL for 60 min) was used to block ER to Golgi membrane trafficking. DMSO-treated cells (n ¼ 25 events, 8 cells) and BfA-treated cells (n ¼ 30 events, 9 cells) both had observable fusion events and the half time to reach the plateau was not significantly different (t-test, p ¼ 0.44). (H) Small EVs were collected from A549 cells after 24 and 48 h incubation and precipitated with ExoQuick-TC and blotted on a slot blot for the presence of CD63.

Fig. 5. Multiple kinetic modes are observed in MVE fusion events. (A) A single fusion event (black dots) fit to a one-component (red), or a twocomponent (blue) fit. (B) Sample fusion event thatis fit well with a one-component decay. (C) Average time course of 120 fusion events. (D) Pie chart showing percent of one- vs. two- component decay profiles. (E) Average fast (blue) and slow (light blue) decay constants from a biexponential fit and a single exponential fit (red), wheret1/2¼ ln(2)/k, was calculated usingthe rate constants from fits of individual fusion events. The mobility of CD63-pHluorin on the plasma membrane was measured by FRAP and an expected t1/2 was calculated based on the diffusion coefficient and circle size (white) (mean 5 SE). (F) Percent of the fast component in decay curves for fusion events at 37
C (mean 5 SE). The gray band represents the portion of CD63 present on the endosomal membrane (30–34%) based on EM data (39). (G) Plateau for decay curves (median 5 95% CI). All data were taken at 37
C.

The Three-Component Model of Exosome Release

To reproduce their experimental data, the researchers determined that three distinct processes must occur after MVE fusion:

1. Free Exosomes

Some exosomes detach rapidly and diffuse away from the release site.

These likely contribute to long-distance intercellular communication.

2. CD63 Membrane Diffusion

CD63 proteins incorporated into the endosomal membrane become part of the plasma membrane following fusion and diffuse laterally. This contributes to signal dispersion.

3. Tethered Exosomes

A proportion of exosomes remain attached to the cell surface after release.

These tethered exosomes move slowly and account for persistent fluorescence at secretion sites.

Importantly, models lacking tethered exosomes could not accurately reproduce the experimental observations.

A Paradigm Shift: Not All Exosomes Immediately Enter Circulation

Historically, exosome secretion has often been conceptualized as a simple release event.

This study challenges that assumption.

The data suggest that after secretion:

• Some exosomes rapidly disperse

• Others remain transiently attached to the cell membrane

• Surface retention may influence local signaling
  

The authors estimated that tethered exosomes exhibit extremely slow movement, supporting the concept of post-secretion membrane attachment. This has important biological implications. Cells may regulate signaling not only by controlling exosome production but also by controlling how long exosomes remain attached to the cell surface.

---

Relevance to Regenerative Medicine

Interest in exosome-based therapeutics has grown substantially over the past decade.

Many regenerative medicine strategies aim to harness exosomes for:

• Tissue repair

• Musculoskeletal healing

• Neuroregeneration

• Wound healing

• Immune modulation
  

The present study suggests that manufacturing and handling conditions could significantly influence exosome characteristics.

Potential considerations include:

Cell Culture Conditions

Temperature changes during production may alter:

• Secretion rates

• Exosome concentration

• Functional properties
  

Storage and Processing

Cold handling could potentially affect release kinetics and measured exosome yield.

Product Standardization

Understanding secretion dynamics may become important for developing reproducible exosome therapeutics and quality-control protocols.

Key Takeaways for Physicians

The work by Mahmood and colleagues provides one of the most detailed examinations of exosome secretion dynamics to date.

The major clinical and scientific insights include:

• Exosome secretion is strongly temperature-dependent.

• Higher temperatures increase both the frequency and speed of exosome release.

• Lower temperatures slow secretion and prolong retention at release sites.

• Exosome release is not complete; a subset of exosomes remains tethered to the cell surface.

• Surface retention may represent an important regulatory mechanism for cell-to-cell communication.

• These findings have implications for exosome therapeutics, regenerative medicine, biomarker development, and cancer biology.

Conclusion

Exosomes are increasingly recognized as central mediators of intercellular communication and promising therapeutic tools. However, translating exosome science into clinical practice requires a detailed understanding of how these vesicles are produced and released.

Mahmood et al. demonstrate that temperature is a fundamental regulator of exosome secretion kinetics. Their findings show that physiological temperature promotes more frequent and faster exosome release, while lower temperatures reduce secretion and increase retention of exosomes at the cell surface.

Perhaps most importantly, the study reveals that exosome secretion is not simply a release event but a dynamic process involving both freely diffusing and membrane-tethered vesicles. This new understanding may influence future approaches to exosome-based diagnostics, regenerative therapies, and disease modeling.

As exosome research moves closer to clinical application, studies such as this provide the mechanistic foundation necessary for safe, reproducible, and effective therapeutic development. Reference:

1. Mahmood, A., Otruba, Z., Weisgerber, A. W., Palay, M. D., Nguyen, M. T., Bills, B. L., & Knowles, M. K. (2023). Exosome secretion kinetics are controlled by temperature. Biophysical Journal, 122(7), 1301–1314. https://doi.org/10.1016/j.bpj.2023.02.025

   
   

---

Incorporare regenerative aesthetic medicine, PRP, Exosome and advanced cellular therapies into your clinical practice

IFAAS Masterclass (Hands-On)​

The Regenerative Medicine Masterclass - PRP, Exosomes & Next-Generation Cellular Therapies

26 September, 2026 - Sydney, Australia  - [Register Now]

---

More Upcoming Global Events

Global Event Calendar